Determination of the mechanical response of polymeric materials with dimensions less than 100 nm is a continuing challenge. Here we describe a novel membrane (“nano-bubble”) inflation method we have developed for the purpose of making measurements of the creep response of ultrathin polymer films and show two major findings. The first is that the material dynamics as measured by the creep response of the membranes depends dramatically on film thickness. For example, in polystyrene films, the dynamics is accelerated so much that the glass transition temperatureT_g of a 11 nm thick film is reduced by approximately 50 K relative to the macroscopic T_g [1]. Furthermore, we have discovered that the nominal rubbery plateau in ultrathin films is stiffened by upwards of two orders of magnitude relative to the macroscopic state and the rate of stiffening (stiffening index S) correlates with the shape of the segmental relaxation in accordance with a recent model proposed by Ngai, Prevosto and Grassia[2]. We have elaborated this finding further and observe a strong correlation with the fragility index m that is related to glass formation according to the Angell categorization [3] of super cooled liquids. These results will be discussed in terms of current understanding of the impact of nanoconfinement on the glass transition behavior of polymers. In addition to being able to characterize the creep response of the ultrathin polymer films, we have also succeeded in adapting the bubble inflation method to make measurements on a graphene/polymer nano-sandwich structure and show that the method can be used to not only extract the stiffness of the graphene inner layer of the composite but that the method can be used to extract the interfacial shear strength of the polymer-graphene couple [4].
[1] P.A. O&’Connell, S.A. Hutcheson and G.B. McKenna, Journal of Polymer Science: Part B: Polymer Physics, 46, 1952-1965 (2008).
[2] K.L. Ngai, D. Prevosto and L. Grassia, Journal of Polymer Science: Part B: Polymer Physics, 51, 214-224 (2013).
[3] C.A. Angell, Journal of Non-Crystalline Solids, 73, 1-17 (1985).
[4] X. Li, J. Warzywoda and G.B. McKenna, Polymer, 55, 4976-4982 (2014).

The measurement of thin layer properties is important for both fundamental and technological reasons. Measurement of stress versus strain mechanics in one of the fundamental tenets upon which materials engineering is based. Direct measurements of elastic moduli and plasticity in nanometer thick polymeric glass layers is non-trivial, and one must often turn to indirect probes or small scale mechanical measurements that require complex interpretation. We introduce a novel strain-gradient free nanoscale compression test for thin soft layers on high stiffness substrates, which permits direct extraction of stress versus strain mechanics of the layer in elastic and plastic states from a single loading curve. Confined layer compression testing is built upon the compression of a precisely aligned flat probe into the layer forming a confined, stationary volume of highly compressed material in a well-defined, invariant geometry. Due to natural confinement from the surrounding material, we show that a state of uniaxial strain is created beneath the probe under small axial strains. Under a uniaxial strain stress state lateral strains within the film are zero throughout the entire volume under the probe. By this methodology we are able to directly probe via load-displacement measurement the stress-strain relationship in uniaxial flows under both reversible elastic and irreversible plastic conditions. Such nanoscale uniaxial strain testing is in some sense the complementary antithesis of nanopillar compression testing that has become so valuable for mechanical testing of size effects in hard materials. However, nanoscale uniaxial strain testing is suitable for in situ testing of both soft materials and continuous coatings on solid supporting substrates, in both elastic and plastic states. Confined compression measurements via instrumented nanoindentation apparatus or force microscopy apparatus provide new means to investigate mechanics and physics of thin films of structured or free volume materials such as glasses and biomaterials, including the plasticity of molecular materials, non-equilibrium states and behavior under the presence of high shear stress and hydrostatic pressure.

Secreted proteins experience multiple collisions with other biomolecules. To address how a single protein copes with collisions requires to measure its mechanical response in physiological-like conditions. Force microscopy methods have been applied to study the mechanical properties^1,2 of proteins. However, most of the studies involving folded proteins are performed on two dimensional arrays³, and characterizing the nanomechanical response of a single, isolated protein remains a challenge. Here, we develop a force microscopy method to measure the stress-strain curve of a single antibody pentamer by applying forces in the 20 to 300 pN range. Elastic recovery is shown for up to 0.4 compressive strains. We report the Young modulus and the yield strength of the protein&’s central region, respectively, 3.1 MPa and 2.1 pN/nm². The yield strength explains the capability of the protein to sustain multiple collisions without any loss of biological functionality. It also indicates the upper forces that could be used in force microscopy for the non-invasive imaging of β-sheet domains.
[1]Martinez-Martin , D., Herruzo, E. T., Dietz, C., Gomez-Herrero, J. & Garcia, R. Noninvasive protein structural flexibility mapping by bimodal dynamic AFM. Phys. Rev. Lett. 106, 198101 (2011).
[2]Dong, M.D. & Sahin, O. Determination of protein structural flexibility by microsecond force microscopy. Nature Nanotech.4, 514-517 (2009).
[3]Medalsy, I.D.; Muller, D.J. Nanomechanical properties of proteins and membranes depend on loading rate and electrostatic interactions. ACS Nano.2013,7, 2642-2650.

The Atomic Force Microscope (AFM) is an ideal tool for probing the nanometer-scale mechanical response of a free interface, with industrial applications in the design and quality control of nano-composite materials. Recent developments with dynamic AFM have enabled quantitative determination of tip-surface force with unprecedented resolution, speed and accuracy. The dominant paradigm for understanding these tip-surface forces builds upon contact mechanics, where force is considered to be a function of tip position, arising from elastic deformation in the contact volume. However, fundamental scaling arguments suggest that surface forces should dominate over volume forces at the nano-scale, especially with soft materials. Furthermore, many applications desire an understanding of the viscous, or energy-dissipating mechanical response of soft nano-fillers and their interphase to the surrounding matrix.
To understand both the viscous and elastic nature of surface forces we must go beyond contact mechanics with its assumption of quasi-static force equilibrium, and consider the tip-surface interaction as a dynamic two-body problem. We describe a multifrequency measurement technique and analysis method for AFM which, similar to Dynamic Mechanical Analysis (DMA) extracts the force that is in phase with, and quadrature to the harmonic tip motion [1]. We present measurements on several soft polymer blends that clearly show the significant effect of viscous forces arising from motion of the soft material surface. We introduce a new type of dynamic interaction model that takes in to account surface motion, allowing for a correct extraction of the elastic stiffness and viscous damping constants of the soft material interface.
[1] D. Platz et al. Nature Comm. 4, 1360 (2013).

Nanoindentation techniques have recently been adapted for the study of hydrated materials, including biological materials and hydrogels. There are unique challenges associated with testing hydrated materials in commercial instruments not designed for this application. Some key results from recent works using nanoindentation to evaluate hydrated materials including soft biological tissues, bulk hydrogels and thin hydrogel layers will be reviewed. Both natural and synthetic hydrogels have been characterized using indentation and nanoindentation across a wide range of experimental length-scales. The material response is shown to be greatly dependent on the chemical bonding within the hydrogel, i.e. whether the network is physically or chemically cross-linked. Hydrogels in particular are an attractive system for studying structure-properties relationships, as the water fraction can be systematically varied for a single polymer, and different polymers with the same water fraction can be compared. Based upon knowledge of the properties of each individual component, composite hydrogels can be created to mimic the overall response of complex biological materials to create multi-component tissue engineering scaffolds.

The application of polymeric materials in engineering is rapidly increasing, in both bulk and coating form, in technologies from automotive to plastic electronics to medical applications. Determining the mechanical properties is important for structural applications, high throughput screening of materials and understanding of polymer physics. Nanoindentation is a convenient means to determine mechanical properties but the response is often difficult to interpret. Unlike metal or ceramic materials, the mechanical response of polymers can depend on strain, strain rate, strain state and load history. The test method, contact geometry and load cycle can also strongly influence the measured response. In indentation, assumptions about the contact area derived from displacement measurements may not hold, and the induced strain gradients can lead to variable time-dependent responses within the strained volume, making determination of material parameters more problematic. It is often difficult to obtain values equivalent to tensile, flexure or dynamic test methods, with measured indentation modulus and hardness values often too high, leading to the supposition of size effects.
Here we present the results of an inter-comparison study of indentation methods on a range of engineering polymers. Different indentation geometries and test methods are compared to tensile and dynamic data from the same materials. The results show that the choice of indenter geometry, load cycle and analysis method have a large influence on the measured mechanical property values obtained. The results imply that confinement of the polymer within the high pressure beneath the indenter can significantly change the elastic and flow properties, creating apparent size effects. However, comparative values for time-dependent elastic properties can be obtained using appropriate geometries and analysis methods. The work reported here formed part of the European ‘MeProVisc&’ project, providing background data for the planned ISO 14577 part 5, instrumented indentation measurement of time dependent materials.

There is a rapidly growing interest in using plate-like nano-fillers such as graphene to reinforce polymers. It is also known that the spatial orientation of the reinforcing elements in polymer-based composites plays a vital role in controlling mechanical properties. There is, however, no generally-accepted way of quantifying the spatial orientation at the nanoscale of plate-like fillers in nanocomposites.
It was found in our previous study that the intensity of scattering of the Raman band is dependent on the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane and it was demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) could be used to quantify the spatial orientation of the graphene. Based on this approach, polarized Raman spectroscopy has been used to quantify, as an example, the level of spatial orientation of graphene oxide (GO) flakes in different nanocomposites using a variety of polymeric matrices. It is demonstrated further how it is possible to relate the spatial orientation of nanoplatelets to the mechanical properties of the in the nanocomposites and, from the stress-induced Raman band shifts, to stress transfer to the reinforcement. In particular, it has been possible to determine the well-known Krenchel orientation factor for these plate-like fillers directly from the experimental polarized Raman data.
Apart from the quantification method for the spatial orientation of nanoplatelets in the nanocomposites, another significant finding of this study is that the Krenchel factor for 3D randomly-oriented nanoplatelets is 8/15. This means that random orientation of fillers such as graphene should reduce the Young&’s modulus of the nanocomposites by less than a factor of 2 compared with the fully-aligned material. Compared to the reduction in the modulus of a factor of 5 going from aligned to 3D randomly-oriented fibres and nanotubes, it means that better levels of reinforcement should be achievable with misaligned nanoplatelets compared with, for example, nanotubes and there is less need to ensure accurate alignment of nanoplatelets in composites. Beyond just graphene and GO, this approach should be more widely applicable to the determination of the orientation of other nanoplatelet fillers for which well-defined Raman spectra can be obtained. Moreover, the effect of spatial orientation upon the mechanical properties of composites predicted in terms of the Krenchel orientation factor will be valid for all types of 2D platelet reinforcement.

An alternative approach to nanoscale electromechanical coupling is to exploit the flexoelectric effect, which is a linear coupling of strain gradient and the electric field. Flexoelectricity is functionally different from piezoelectricity, which in contrast is a linear coupling of strain to the electric field, in two significant ways. First, it lifts the restriction of the material structure being non-centrosymmetric, making it a universal effect whose magnitude depends upon the strength of the strain gradient and the geometrical arrangements of the molecular moieties. Second, the dependence on the strain gradient makes flexoelectricity a promising approach to nanoscale functionality and devices.
We report the results of a study on the dependence of the flexoelectric response on thickness in the ultrathin films of polar and non-polar polymers of vinylidene fluoride (VDF). The flexoelectric response was measured using a bent cantilever method and the results were corrected for the contribution from the interfacial electrode oxide layer. The results indicate enhancement of the response with decreasing film thickness in polar and non-polar polymers by up to a factor of 40 as compared to the bulk values, reaching such enhanced values in films only 10 nm thick. These results are consistent with a model accounting for interfacial contributions, and underline how large electromechanical coupling can be produced at the nanoscale.

Dentin is a naturally occurring nano-composite material (found in teeth) consisting of a collagen matrix, mineralized hydroxyapatite, and anisotropic tubule structures. This natural structure is of interest for biomimetic applications due it&’s remarkable mechanical properties, derived largely from the strength of the mineralized component combined with the unique fracture behavior dictated by the tubule features. An understanding of the damage evolution in such a naturally occurring structure can possibly aid in the design of man-made materials with similar properties. To that end, this work will present an in situ nanoindentation study of a sample of elephant dentin, performed in a commercial nanoscale 3D X-ray microscope. The X-ray microscope system provides spatial resolution on the tens of nanometer scale for samples ranging from tens to hundreds of microns in size, enabling new scientific investigations by serving a middle ground between the length scales of 3D in situ techniques possible with higher resolution TEM and somewhat coarser resolution classical micro-CT. The 3D data is collected by performing tomographic data acquisition by rotation and imaging of the sample through a range of angular orientations. A new load stage designed specifically for manipulation of samples in this system was implemented, allowing progressive indentation steps to induce fracture in the dentin sample under increasing load. In this study, 3D imaging was performed at multiple states of applied load to monitor the crack initiation and growth processes at the nanoscale, and gain insight into the connections between the novel microstructure, crack shielding mechanisms, and the material&’s fracture toughness. Visualization and quantitative analysis will be presented to help understand the process.

The damage evolution and structural failure in trees and other plants are mainly originated from the plastic yielding in the wood cell wall at the microstructural level, which consists of cellulose fibers embedded in a matrix of hemicellulose and either lignin or pectin¹. Understanding the mechanical behavior of wood cell wall at the plastic regime is critical to the investigation of the fracture characteristics of trees at macro-scale. In this research work, the wood cell wall, which consists of cellulose fibers, hemicellulose chains and lignin macromolecules, is modeled at the mesoscale to investigate the mechanical responses under tensile deformation. By examining the stress-strain relationship, the mechanical behaviors of the wood cell wall at the plastic yielding range will be obtained, which will be compared with experimental measurements and theoretical predictions to provide a bottom-up description of micromechanics of the wood cell wall, and to explain the damage evolution and structural failure occurred at the larger scales. The wood cell wall investigated here can be applied to the construction of wood hierarchical structure as a basic modeling unit and such application can be further extended to other natural materials.
1. L. Köhler and H.-C. Spatz, Planta 215 (1), 33-40 (2002).

Bulk GaN substrate has attracted much attention because of high quality and low dislocation density. We previously reported the dislocation formation and movement in surface dimples nano-indented on the high quality bulk GaN, and also proposed the mechanism of r-plane (-1012) slip initiating plastic deformation, so called pop-in event, which is supported by molecular dynamics simulation [1,2]. However, by TEM analyses, it was difficult to confirm the clear evidence of r-plane slip initiating the plastic deformation because of the generation of several kinds of dislocation multiplications and dislocation loops as secondary and tertiary slips accompanying the r-plane slip. In this paper we present evidence of r-plane (-1012) slip mechanism in indented GaN surface by using precise nano-indentation with smaller radius indenter (~100 nm) and lower pop-in load (~400 mu;N), comparing with our previous studies, and TEM observation right after the plastic deformation.
Nano-indentation was precisely performed on the surface of c-plane and m-plane bulk GaN. We chose a Berkovich diamond indenter with smaller radius of ~100 nm. The maximum load of transition from elastic to plastic deformation (pop-in load) was observed at 400 mu;N which is relatively smaller than that of our previous studies.
Cross-sectional TEM observation was performed right after the pop-in event beneath the indented surface of the c-plane bulk GaN. The pyramidal dislocation line of r-plane slip was clearly observed which is inclined by 43 degree from c-plane surface. And the basal c-plane slip and the prism m-plane slip occurred as a secondary or tertiary slip system were not observed, which is previously observed in larger radius indenter and higher pop-in load.
From these results including the nano-indentation for the m-plane bulk GaN, we confirm the mechanism for initiating plastic deformation as follows: i) indentation stress caused small-scale shuffles of atoms at the center of the diamond tip beneath the contact surface, ii) a metastable boundary arose locally along the r-plane, and iii) plastic deformation was initiated via the r-plane slip. The characterization by TEM analyses agrees well with the initial stage of plastic deformation with r-plane dislocation predicted by the molecular dynamics simulation.
[1] Masaki Fujikane, Toshiya Yokogawa, Shijo Nagao, and Roman Nowak, Jpn. J. Appl. Phys. 52, 08JJ01 (2013).
[2] Toshiya Yokogawa, Masaki Fujikane, Shijo Nagao, and Roman Nowak, 2015 MRS Spring Meeting & Exhibit, San Francisco, FF2.09, April 07, 2015

High Entropy alloys (HEAs) are solid solution alloys containing five or more principal elements in equal or near equal atomic percent (at %). In this study, the Al_0.7CoCrFeNi HEA was synthesized by vacuum arc melting and homogenized at 1250 ⁰C for 50 hours. The microstructure shows the presence of 2 phases. The BCC (A2+B2) and the FCC phase. Nano-pillars were fabricated from the single crystal Al_0.7CoCrFeNi HEA in the [001] of the BCC (A2+B2) phase and [324] of the FCC phase having diameters ranging from 400 nm to 2 mu;m using the focused ion beam and compressed using the Hysitron Nanoindenter. High yield strengths of about 2.2 GPa was observed in the BCC (A2+B2) phase and about 1.2 GPa for the FCC phase. Size effect occurs in both phases, the smaller pillars having the highest strength values. Comparing the BCC phase of the HEA with pure BCC metals shows a reduced size dependence with increased yield strength but the FCC phase in comparism with pure FCC metals shows similar size dependence with increased yield strength. This is attributed to the difference in lattice resistance of the two phases.

During the manufacturing process of electronic devices, semiconductor materials are subject to handling and contact with surfaces that could lead to plastic deformation or even fracture. Thus, an understanding of the mechanical deformation mechanisms of semiconductor materials is important. This can be achieved using nanoindentation techniques that allow the study of mechanical deformation mechanisms in nanoscale. Studies have been performed on the mechanical properties of gallium nitride (GaN) using nanoindentation with sperical tips with radius of curvature between 4 mu;m and 5 mu;m, but the early stages of plastic deformation and the dynamics of the deformation process are still unclear. In this work, we present a nanoindentation study using a conespherical tip with a radius of curvatore of 260 nm, which is approximatelly ~ 20 times smaller than the radius of curvature of the tips used in the literature. Therefore, our nanoindentations were able to produce highly-localized stress fields allowing a discussion on the incipient plasticity of GaN at the nanoscale. We report the deformation mechanism of GaN thin films with a [0001] orientation and the optical properties associated with the plastic deformation resulting from stress fields produced by nanoindentation. The residual indentation pits were studied using atomic force microscopy, transmission electron microscopy, and cathodoluminescence spectroscopy. The incipient plasticity was observed to be initiated by metastable atomic-scale slip events that occur as the crystal conforms to the shape of the nanoscale tip. Dislocation locking leads to pop-in events associated with the introduction of large volumetric material displacements at the indentation site. A large number of non-radiative defects were observed directly below the indentation. Regions under tensile strain extending from the nanoindentation were also observed. An insight of a plastic deformation process based on dislocation nucleation under highly-localized stress field is presented.

Micro-cantilever deflection testing has been used to determine fracture toughness at magnesium aluminate spinel bicrystal interfaces. Focused ion beam milling was used for micro beam fabrication and prenotch preparation. In the present study, the micro cantilever beams were fabricated along the bi-crystal grain boundary of magnesium aluminate spinel doped with either Ytterbium or Europium. The boundary location was confirmed using Electron Backscatter Diffraction after milling and a prenotch aligned with the interface was cut using a lower ion beam current. An in-situ nano mechanical test system, the Hysitron PI-85 Picoindenter, was used to perform the test and record the loading and displacement. A 3D finite element model of the micro cantilever beam was built in ANSYS APDL. A dedicated solver called Frac3D was utilized to calculate the finite specimen width correction term and estimate the fracture intensity factor. High resolution electron microscopy was used to identify the structure and chemistry of the boundaries. By combining the microscopy and mechanical testing results, a direct correlation between different complexion types and grain boundary mechanical behavior is to be established.

Deformation twinning is a critical component of HCP Mg&’s deformation mechanisms. Recently, anomalies were detected in an important {10-12}<10-1-1> twinning system[1]. This experimental result hinted that deformation twinning in Mg could be fundamentally different from traditional models for cubic metals. Theoretically, this topic has also been controversial and unsettled: shear-controlled or shuffling-controlled issue [2][3]. In this study, we computed atomistic pathways of deformation twinning with First-principles calculation, and quantified the pathways by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxtaposition of both. If one can obtain the same saddle point by continuously varying the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. With this way, we find the {10-12} <10-1-1> twinning of Mg is shuffling-controlled, that is, the reaction coordinate is dominated by non-affine displacement instead of strain. This has important consequences on the temperature and strain-rate dependences of {10-12} <10-1-1> deformation twinning due to the reduced importance of long-range elastic interactions. Thus, the room-temperature deformation twinning in magnesium is markedly and qualitatively different from traditional understanding.
[1] B. Yu, et al. Nature Comm. 5 (2014) 3297
[2] B. Li & E. Ma, Phys. Rev. Lett. 103 (2009) 035503.
[3]A. Serra, D. J. Bacon and R. C. Pond, Phys. Rev. Lett. 104 (2010) 029603.

Indenter angle effects on the cracking behavior of fused quartz were studied using nanoindentation experiments with different triangular pyramidal indenters with the centerline-to-face angles ranging from 35.3° to 85.0°. The indentation loads were varied from 10 to 500 mN to examine both the initial stages of cracking as well as the propagation of well-developed cracks with lengths much greater than size of the contact impression. Images of the indentations were obtained in a high-resolution scanning electron microscope (SEM) to observe the general features of the cracking and measure the indentation sizes and crack lengths for each indenter. The observations are used to examine the mechanisms of crack nucleation and crack growth in this classical brittle material.

Si pillars fabricated by focused ion beam (FIB) had been reported to have a critical size of 310-400 nm, below which their deformation behavior would experience a brittle-to-ductile transition at room temperature [1]. However, the potential effects from the high energy ion bombardment have been overlooked so far. Here, we demonstrated that the amorphous Si (a-Si) shell introduced during the FIB fabrication process was much softer and more ductile than the crystalline Si (c-Si), which can effectively inhibit the crack propagation in the c-Si core. Hence, the reported size-dependent brittle-to-ductile transition was actually stemmed from the imperfect sample fabrication method. Once the a-Si shell was crystallized by thermal treatment, Si pillars would behave brittle again with their modulus comparable to their bulk counterpart, showing that the crystalline Si is inherently brittle no matter in bulk or in submicron size. We also developed an effective analytical core-shell model to predict the modulus of as-FIBed Si pillars and derive the modulus of both crystalline Si core and a-Si shell, which fits well with the experiment results.
References
[1]Fredrik Östlund, Karolina Rzepiejewska-Malyska, Klaus Leifer, Lucas M. Hale, Yuye Tang, Roberto Ballarini, William W. Gerberich, and Johann Michler, Adv. Funct. Mater. 19, 2439 (2009).

There has been much research on highly nanotwinned Cu due to its potential for simultaneous high strength and ductility. Single element systems have limited engineering applications, though, and introducing alloying elements allows for a wide range of new materials with lower stacking fault energies which are more likely to produce fully twinned microstructures. In this study, the tensile properties of fully nanotwinned Cu-based binary alloys containing different alloying elements were determined with a custom-built small-scale tensile tester utilizing digital image correlation to generate in-situ strain maps. The effects of varying microstructure are examined for different alloy systems in order to understand their mechanical behavior. Yield strengths between 800 and 1500 MPa and strains to failure between 2 and 6% were achieved with varying twin thickness, grain size, and composition. Samples were synthesized by magnetron sputtering with stacking fault energies ranging from 6 to 45 mJ/m².

We report the crystallite-size-dependency of the compressibility of nanoceria under hydrostaticpressure for a wide variety of crystallite diameters and comment on the size-based trends indicating an extremum near 33 nm. Uniform nano-crystals of ceria were synthesized by basic precipitation from cerium (III) nitrate. Size-control was achieved by adjusting mixing time and, for larger particles, a subsequent annealing temperature. The nano-crystals were characterized bytransmission electron microscopy and standard ambient x-ray diffraction (XRD). Compressibility, or its reciprocal, bulk modulus, was measured with high-pressure XRD at LBL-ALS, using helium, neon, or argon as the pressure-transmitting medium for all samples. As crystallite size decreased below 100 nm, the bulk modulus first increased, and then decreased, achieving a maximum near a crystallite diameter of 33 nm. We review earlier work and examine several possible explanations for the peaking of bulk modulus at an intermediate crystallite size.

Flexible display is promising electronic devices which can act bending motion in anywhere. For some time past, it has been concerned from numerous manufacturer to apply flexible technology in e-paper, mobile phone and consumer electronics. Even if flexible display has many potentials in industry fields, it still has been issues to resolve yet. Traditionally, semiconductors and glass materials have been used as substrate. But these materials have trouble with manufacturing a flexible devices owing to material property such as brittleness, rigidity and weight.
Polymer (polycarbonate, PET, etc.) regarded as complementary materials instead of silicon and glass because device which is composed of the plastic material can be curved deeply as bendable display. Al-doped-ZnO (AZO) thin film was well known as low-cost transparent conductive oxide film instead of indium tin oxide (ITO). And Al-doped-ZnO is one of the most widely investigated electrode materials. Previous reports demonstrated that resistivity of their AZO film exhibited 2.2*10^-3 #8486;cm when 2 at% of Zn atoms were replaced by Al atoms.¹ However, not only electrical property but also mechanical property of AZO thin film as electrode is very significant in flexible devices.
Reports of mechanical property of AZO film are not enough, relatively. Especially, mechanical property such as strength, stiffness can be crucial factor to predict reliability, life time of device under environmental loading conditions. In this research, we fabricated AZO on polyimide film using atomic layer deposition (ALD) process. ALD is a thin film deposition technique allowing low process temperature, uniformity and controllability of thickness. Moreover, separated injection of the precursors in a reaction chamber allows use of highly reactive precursors and control of reaction time for each step. This facilitate to make thin film at low temperature and can be key of application of flexible display.
And electric-micro tensile test was performed to compare of the performance of the conductive oxide film deposited on polyimide substrate under process conditions such as deposition temperature, %Al doping ratio, O₂ plasma. As tensile force was loaded on specimen, electrical resistance & stress value was measured under %strain in DAQ system. Especially, it brought the result that elastic modulus of O₂ plasma treated-PI deposited with AZO was increased up to 20% then before. And in situ LFM (Lateral force microscopy) was performed to investigate interaction between thin film and substrate. An x-ray diffraction (XRD) was employed to reveal the structure of AZO using Cu Κα radiation. The surface morphology was analyzed by using the scanning electron microscope (SEM). To quantify the bonding strength, x-ray photoelectron spectroscopy (XPS) was used in this experiments.
Reference
J. W. Elam , D. Routkevitch , S. M. George , J. Electrochem. Soc. 2003,150 , G339

The use of nanoindentation to test through a range of strain rates is a growing field, with applications to both beginning, e.g. metal forming, and end of lifetime, e.g. impact deformation in many materials. The use of dynamic indentation methodology, superimposing an oscillating AC load over the DC quasistatic load, can be used to measure properties such as Hardness and Modulus as a function of time and depth. However, several assumptions need to be made to do so reliably. One such requirement is that the material response be such that the area function remains valid. For many materials, including the nanocrystalline copper tested here, that assumption is violated when the amount of pile-up varies with the strain rate. Theoretical strain rate pile-ups are compared to the experimental result, using both in and ex situ testing. A simple method for correcting the hardness as a function of the strain rate is presented. Additionally, a MEMS based transducer is used to test at strain rates approaching 10, to further extend the range of nanoindentation strain rate testing.

A good understanding of the true plastic stresses and strains are of vital importance in studying metal forming processes and in the fracture mechanics field. The information extracted from the tensile test has been used traditionally to derive the flow curve and to compute plastic properties of the metals like the strength coefficient and the strain hardening exponent. However, knowledge of the plastic behavior beyond necking is uncertain due to the triaxial state of the stresses generated in the sample, and therefore, the flow curve does not replicate the uniaxial condition sought during the tensile test. In this research, a correction method for the post-necking section of the flow curve is proposed by linking nanoindentation measurements, performed along the tensile axis, to the flow curve. Three bulk materials were studied, steel G101800, copper C11000 and stainless steel S30400. The true flow curve after the onset of necking was corrected utilizing the nanohardness data converted to true stress values through the constraint factor C proposed by Tabor. It was observed that this constraint factor is not constant for all materials as reported in various preceding works. The corrected section of the curves showed a strain hardening behavior different to that before the onset of necking since the converted nanohardness to flow stress revealed the uniaxial character of the deformation, in this manner, separating out the effects of triaxiality in the uniaxial true flow curve.

Multimillion-atom reactive molecular dynamics simulations are performed to study self-healing of cracks in Al₂O₃ by SiO₂ nanoparticles and by SiC/SiO₂ core shell nanoparticles embedded in the Al₂O₃ matrix. These simulations are carried out at 300 K and 1800 K for three different system sizes: 400Å*300Å*100Å, 600Å*300Å*150Å and 1600Å*800Å*400Å containing approximately 1.2 million to 60 million atoms. Brittle fracture is observed in alumina at 300K, while void nucleation and coalescence are observed to cause ductile fracture at 1800 K. Silica is found to be highly effective in healing cracks in the Al₂O₃ matrix at high temperatures. In addition, silica causes strain hardening and gives higher fracture toughness to alumina. Detailed analysis of atomistic mechanisms shows that crack healing is due to the flow SiO₂ into the crack, which stops the crack growth. Results will be presented for (1) stress distributions in alumina with and without SiO₂ and SiC/SiO₂ core shell nanoparticles, (2) size and shape of cavities in the damage zone, and (3) atomic diffusivities of silicon and oxygen during healing of cracks and damage cavities in alumina.
This work was supported by Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, Grant #DE-FG02-04ER-46130. Some of the simulations were performed on the 262144-core IBM Blue Gene/Q supercomputer at Argonne National Laboratory under the DOE INCITE program.

HCP materials like magnesium and magnesium alloys due to their high strength to weight ratio have been deemed as promising candidates for next generation armor materials. A fundamental understanding of the deformation and failure response of the material during high strain rate deformation and shock loading is required in order to develop the capability to withstand ballistic/blast impact. The mechanical response of these materials is primarily determined by the active deformation modes operating (slip, twinning, etc.) as well as the micromechanisms (voids, shear bands, etc.) related to failure. Of particular importance is the role of twinning in Mg due to the limited number of slip systems in HCP metals. The aim of this study is to identify the contribution and dependence of different atomic scale deformation mechanisms like basal, prismatic, pyramidal slip and twinning on the loading conditions (strain rates, shock pressures etc.), microstructure of the system (loading orientation, grain size) and temperature.
Molecular dynamics (MD) simulations allow the characterization and investigation of the evolution of defect structures at the atomic scales during deformation and failure. Large-scale MD simulations are therefore carried out to investigate the deformation and failure micromechanisms in nanocrystalline Mg with grain sizes up to 100 nm under uniaxial strain and shock loading conditions. The evolution of twinning during deformation, the mechanisms for nucleation, growth, and coalescence of voids, and the variation of the spall strength as a function of strain rate and grain size in comparison to that for single crystal Mg will be presented.

The protectiveness and performance of passive corrosion films as effective barrier layers depends greatly on their mechanical integrity under corrosion conditions. This mechanical stability is particularly important for two-dimensional layered surface films like mackinawite (Fe_1+xS), which are known to fail by delamination, leading to exposure of the bare metal and subsequent catastrophic localized corrosion [1,2].
In an effort to explain the mechanism behind this facile failure, we show, by analogy to three-dimensional passive films [3,4], how hydrogen point defects generated by cathodic proton reduction can weaken the interlayer van der Waals bonding in mackinawite leading to delamination. Specifically, we use density functional theory calculations to identify dominant hydrogen defects in mackinawite based on their formation energies and quantify their effect on important mechanical metrics like binding energies, elastic moduli and tensile and shear strengths by mapping the stress-strain behavior of mackinawite crystals containing hydrogen point defects.
We find that hydrogen is most detrimental to film passivity and protectiveness when it is present as molecular H₂ interstitials in the interlayer region. This configuration results in up to 80% degradation in the mechanical strength of mackinawite crystal and therefore increases the chances of passive film failure by delamination.
References
[1] D.W. Shoesmith, P. Taylor, M.G. Bailey, D.G. Owen, Journal of the Electrochemical Society 127 (1980) 1007.
[2] X.M. Dong, Q.C. Tian, Q.A. Zhang, Corrosion Engineering Science and Technology 45 (2010) 181.
[3] S.N. Rashkeev, K.W. Sohlberg, S. Zhuo, S.T. Pantelides, The Journal of Physical Chemistry C 111 (2007) 7175.
[4] M. Youssef, B. Yildiz, Physical Chemistry Chemical Physics 16 (2014) 1354.

Dislocations in fcc and hcp metals usually dissociate into a planar shape, and their slip is confined in the corresponding slip planes. Cross-slip usually requires transformation of the planar dislocation core into a linear, perfect dislocations, which requires some activation energy. Using an extensive DFT calculations, we have found a notable exception to this conventional view. The pyramidal screw dislocation in Mg consists of two partial dislocations connected by a stacking fault, and the stacking fault can migrate perpendicular to the plane by atom shuffling, enabling the dislocation to cross-slip without transforming into a perfect dislocation.

The mechanical properties of bulk metallic glasses, such as high strength and low ductility, are both of fundamental scientific interest and technological importance. It is now well recognized that the plastic deformation of metallic glasses below their glass transition temperature involves irreversible rearrangements of small clusters of atoms. In the present study, molecular dynamics simulations are performed to investigate the plastic response of a model glass to a local shear transformation in a quiescent system. The deformation of the material is induced by a spherical inclusion that is gradually strained into an ellipsoid of the same volume and then reverted back into the sphere. We show that the number of cage-breaking events increases with increasing strain amplitude of the shear transformation. The results of numerical simulations indicate that the density of cage jumps is larger in the cases of weak damping or slow shear transformation. Remarkably, we also found that, for a given strain amplitude, the peak value of the density profiles is a function of the ratio of the damping coefficient and the time scale of the shear transformation.

Metal-organic frameworks (MOFs) are a new class of porous materials with important applications ranging from gas sequestration to medicine. Understanding the processes of deformation of MOFs beyond the elastic limit is critical for several applications and processing where they are subjected to significant stresses. In this study, mechanisms of plastic deformation of metal-organic framework-5 (MOF-5) are investigated using large-scale molecular dynamics simulations. Atomistic-level details from the simulations of uniaxial compression along [001], [101], and [111] directions reveal that structural collapse of MOF-5 during compression occurs by slips along <100> directions on {001} planes due to the flexibility of the connection between Zn-O clusters and 1,4-benzenedicarboxylate (BDC) ligands. A dramatic decrease in stress of the system occurring during plastic deformation corresponds to the collapse of extensive free volume of the crystal structure. In addition to resolved shear stress, compressive stress normal to the active slip plane is essential for driving the plastic deformation depending on the orientation and this explains the anisotropic behaviors of this material. Plastic deformation of MOF-5 shows very interesting behaviors of strain localization that significantly affects the mechanical properties. Structural collapse propagates on the active slip planes along the directions perpendicular to slip directions and this is reminiscent of the glide of screw dislocation. The process exhibits interesting interaction between collapsing regions due to the effects of stress relieving by the volume shrinkage.

Nanoindentation testing has a “typical” protocol for execution of experiments and for data analysis, but there are many alternatives available. The choice of experimental parameters and technique for the analysis of data can depend on the stiffness of the material, its hydration state, the material constitutive response, and the presence or absence of pores in the material. Systematic experiments have been undertaken to examine a wide range of choices for nanoindentation experiments, particularly in the context of biological materials. Samples have been tested in dry and rehydrated conditions. Spherical and Berkovich indenter probes have been utilized, including spheres with radii across several orders of magnitude. Data analysis has been considered within elastic-plastic (Oliver-Pharr), viscoelastic and viscoelastic-plastic and poroelastic frameworks. Results suggest that the hydration state, probe geometry and the limitations and assumptions of each analysis method significantly influence the measured mechanical properties in materials such as bone. This systematic study has been carried out to illustrate that the discrepancies in the nanoindentation properties found in the literature should not be attributed only to the differences between the materials themselves, but also to the testing and analysis protocols.

Understanding the molecular network structure of polymer matrix composite and its relationship with mechanical performances can lead to a complete understanding of failure mechanisms. This work investigates the nanoscopic nature of PMC matrix as well as interphases in terms of topography, chemical mapping/bonding, thermal and mechanical properties to find a bridge between nanoscopic, microscopic and macroscopic mechanical properties. It is guided by ongoing simulations aimed at an understanding of bond scission, interphase structure and mechanical properties at the nanoscale level via a multiscale computational approach. DGEBA epoxy resins with known chemical structures are processed in-house with varying molecular weight, controlled stoichiometric ratios (resin and cross-linker) and degrees of cure, both in the presence and absence of carbon fiber. AFM-IR is used to investigate the chemical structure at ~ 100 nm resolution, which is calibrated by controlled experimental characterization at varying conditions using FTIR (mid-IR and near-IR) in bulk. The chemical mapping results show that there is a variation in the chemical network structure between the matrix and the interphase as well as within the matrix. Control experiments using spectroscopy and thermal analysis confirm that this variability in the three-dimensional network is due to differences in cross-linker density and degree of cure. To understand if these local chemical variations have any influence on the property, thermal AFM was used to investigate the glass transition temperature and modulus mapping was done by Peak-force tapping AFM on ultramicrotome surfaces. It is confirmed that these network structure variations resulted in changes in local thermal and mechanical properties.

Here we define a set of standard for providing data originating from atomic force microscopy AFM force measurements to be used to compare between sample properties, parameters or more generally, heterogeneity with a minimum number of points and conclusively. We exemplify our case by employing sapphire as a model system because it presents a homogeneous surface, that is, where the force of adhesion F_AD should be homogeneous even in the nanoscale. After setting the standards, we select a more challenging model system consisting of copolymer to show that approximately 100 suffice to conclusively establish the presence of nanoscale heterogeneity with a resolution of 10pN in F_AD. Employing the work of adhesion W_AD as an alternative parameter requires approximately ~200 data points to arrive at the same conclusion. . Our findings reinforce the relevance of multi-parametric models and analysis in the field of nanoscale force spectroscopy.
Keywords: nanoscale, adhesion, work, force

The thermally-induced nanostructure evolution in the thermotropic copolyester based on 1,3-hydroxybenzoic acid (B) and 2,4-hydroxynaphthoic acid (N) was investigated by small-angle X-ray scattering (SAXS). Heat treatments were carried out on extruded tapes at temperatures close to the solid-to-nematic transition (T_s→n) for up to five hours, under dry air conditions. Wide-angle X-ray scattering (WAXS) showed that the as-received tape exhibited fiber-like macromolecular orientation along the extrusion axis, along well defined crystalline reflections, denoting crystalline order. On the other hand, SAXS elucidated a highly anisotropic nanovoid morphology elongated along the extrusion axis over 17 nm and aspect ratios (axial/lateral dimension) in excess of 15. Upon heat treatment the crystalline reflections sharpened suggesting better molecular packing, and the degree of crystallinity increased. At the same time, SAXS showed that the lateral dimension of the nanovoids significantly decreased and the aspect ratio dramatically increased up to nearly 30, suggesting better molecular packing. There was also sharpening of azimuthal intensity around the 110 equatorial reflection, thus indicating an increase of molecular alignment along the extrusion axis. The quality of molecular alignment was quantified by the order parameter P₂. The thermally-induced molecular and nanometer scale changes correlated with a significant increase of mechanical Young&’s modulus, E, along the extrusion axis.

Complex nanostructures with semiconductor quantum dots (SQDs) and metallic nanoparticles (MNP) assembled in a close vicinity of each other provide a possibility for hybridization of the local intrinsic excitations, namely excitons in SQD and plasmons in MNP. Such hybridization gives rise to interparticle energy transfer, enhancement of local electromagnetic field and light-matter interaction with numerous practical applications if the host material, fabrication technology and target structures are compatible with those common for device production.
In this paper we show a possibility to self-organize InAs SQDs and SbAs MNPs in a close vicinity to each other in the GaAs matrix. For that we develop a combined process of the molecular beam epitaxy (MBE), in which self-organization of the InAs SQDs was achieved in the Stranski-Krastanow mode, whereas self-organization of the SbAs MNPs was realized by the low-temperature MBE with the subsequent anneal. Coupling of the InAs SQDs and SbAs MNPs was revealed by transmission electron microscopy. The distance between the SQD and MNP arrays was 10 nm. This phenomenon originates from local strain-stress fields surrounding both InAs SQDs and SbAs MNPs. We analyze the specific feature of the mechanical fields and their impact on the self-organization processes in our system.

Natural composites, like Nacre and wood, have attracted a lot of attention lately in the field of biomimetic materials given their unique combination of light-weight along with high strength and toughness. If the structure of natural composites could be successfully transferred to the conventionally man-made materials which are made of ceramics and polymer, it would benefit a wide area of fields, such as transportation and construction, biomedical implants and sensors. Inspired by unique microstructure of nacre, we present a new method to fabricate a material through layer-by-layer (LBL) deposition technique using an organic polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] and an inorganic material (alumina platelets). The platelets are transferred to a silicon wafer substrate by dip-coating and a layer of organic polymer [P (VDF-TrFE)] is produced by spin-coating, resulting in strong hydrogen bonds formed between the two kinds of material. After repeating these two steps in succession several times, the 4-layer films are fabricated with a thickness of a few tens of microns. The nacre-inspired structure material composed by [P (VDF-TrFE)] and alumina platelets enhanced the mechanical properties overall when compared to the individual components in a very similar manner to nacre. The transfer of piezo-electrical properties of the copolymer P(VDF-TrFE) is another focus of this study. P(VDF-TrFE) shows the different behaviors after annealing under different temperature. Both the mechanical properties and piezo-electrical properties of the P(VDF-TrFE) are strongly dependent on the film&’s morphology and its fabrication process and that is the reason why we want the co-polymer [P(VDF-TrFE)] to be annealed. The Crystallization of [P(VDF-TrFE)]can be changed under different temperature , and that lead to morphology of the P(VDF-TrFE) film changed. This property was studied to see how it affected the nano-composite material and if there were any enhancements in annealing. Also the nacre-inspired structure material could be applied under different temperature based on this property. The resulting nacre-inspired material fabricated using our method is not only suitable for the small devices; it also satisfies the requirement for the large-scale applications. It is a simple time-saving and energy-efficient method to produce this type material.

Tunable gauge characteristics are important for wearable and conformal electronics. This report describes a gauge sensor device which is fabricated by assembling and printing molecularly linked thin films of gold nanoparticles on flexible microelectrodes with unusually high and anisotropic gauge factors. A sharp difference in gauge factors up to two to three orders of magnitude between bending perpendicular (B perp; ) and parallel (B || ) to the current flow directions is observed. The origin of the unusual high and anisotropic gauge factors is analyzed in terms of nanoparticle size, interparticle spacing, interparticle structure, and other parameters, and by considering the theoretical aspects of electron conduction mechanism and percolation pathway. A critical range of resistivity where a very small change in strain and the strain orientation is identified to impact the percolation pathway in a significant way, leading to the high and anisotropic gauge factors. The gauge anisotropy stems from molecular and nanoscale fi ne tuning of interparticle properties of molecularly linked nanoparticle assembly on flexible microelectrodes. The implication for the design of gauge sensors for highly sensitive detection of deformation in complex sensing environment or on complex curved surfaces such as wearable electronics and skin sensors will be discussed.

Sea sponge spicules possess an unusual combination of effective failure strain and optical light propagation properties due to their micro- and nanoscale hierarchical structure. We present measurements of the elasto-optic properties of Tethya aurantia and Euplectella aspergillum sea sponge spicules using Brillouin laser light scattering. Brillouin scattering is a powerful, non-invasive technique that can be used for determination of material elastic constants, sound velocities, and refractive indices. We non-invasively determine the full elastic stiffness tensor of these materials and show that spicules have optical propagation similar to that of man-made glass fibers. These measurements help us understand the interplay of flexibility, strength, and material microstructure for future functional biomimicry.

#8203;The unique properties of bamboo come from the natural composite structure of fibers that consists mainly of cellulose nanofibrils in a matrix of intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Here, we have used atomistic simulations to study the effect of water on the mechanical properties of and adhesive interactions between these materials in bamboo fibers. With this aim, the relationships between water content and elastic moduli and the adhesion energies between these materials and cellulose nanofibrils were investigated. The results imply that the densities of hemicellulose, lignin and LCC decrease with increasing water content. However, in low water content (around 1%) lignin and LCC maintain the density. The elastic moduli of lignin and LCC show an increase at low water content (around 1%) and decrease at higher water content, whereas the hemicellulose elastic modulus constantly decreases. This suggests that at the early stage of introducing water in these systems, water molecules enhance the structure of LCC and lignin and increase the modulus of elasticity while maintaining the density. For hemicellulose, however, the effect is always regressive. This behavior was found to be associated with the water molecules interference in the hydrogen bond energies of these materials.

This talk will be based on recent publication on In Situ Atomic-Scale Observation of Twinning Dominated Deformation in Nanoscale Body-Centred Cubic Tungsten, Nature Materials, doi:10.1038/nmat4228, March 2015 by Jiangwei Wang, Zhi Zeng, Christopher R. Weinberger, Ze Zhang, Ting Zhu and Scott X. Mao. Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. In metallic nanostructures, plastic deformation requires higher stresses than those needed in their bulk counterparts, resulting in the smaller is stronger&’ phenomenon. Such high stresses are thought to favour twinning over dislocation slip. Deformation twinning has been well documented in face-centred cubic (FCC) nanoscale crystals. However, it remains unexplored in bodycentred cubic (BCC) nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy and atomistic simulations, we show that twinning is the dominant deformation mechanism in nanoscale bi-crystals of BCC tungsten. Such deformation twinning is found to be pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC bi-crystals. Our work provides direct observations of deformation twinning as well as new insights into the deformation mechanism in BCC nanostructures.

Polycrystalline Cu and CrN thin films were sputter deposited sequentially onto Si(001) substrates to form specimens of CrN/Cu bilayers on Si, in which Cu thin films were confined between elastic-brittle Si and CrN. Cylindrical micro-pillars were fabricated from CrN/Cu/Si(001) specimens through Ga⁺ focused ion beam (FIB) milling, in which the interfaces were either inclined at 45° to the pillar axial direction or normal to it. Load-displacement curves were measured through axial compression loading of CrN/Cu/Si micro-pillars, and together with post-compression morphological observations, demonstrated clearly the occurrence of extensive plasticity within the Cu interlayers confined between Si and CrN. Measured plastic flow stresses showed a marked dependence on the thickness of the Cu interlayer, exhibiting significant mechanical size effects. Detailed structural characterizations of the sputter deposited CrN and Cu thin films were carried out with techniques of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM). A simple strain gradient plasticity model was applied to the present experimental situation, and comparision between experimental data and model output will be given. The present experimental protocol affects significant plasticity in metal thin films in simple deformation geometries, results from which offer new experimental test cases for non-local plasticity theories.
References:
1. K. Chen, Y. Mu, W.J. Meng, A new experimental approach for evaluating the mechanical integrity of interfaces between hard coatings and substrates, MRS Comm. 4, 19-23 (2014).
2. Y. Mu, K. Chen, W. J. Meng, Thickness dependence of flow stress of Cu thin films in confined shear plastic flow, MRS Comm. 4, 129-133 (2014).
3. Y. Mu, J. W. Hutchinson, W. J. Meng, Micro-pillar measurements of plasticity in confined Cu thin films, Extreme Mechanics Letters 1, 62-69 (2014).

Nanoporous (np) metals formed by dealloying are expected to have high strengths, because their solid component, the ligaments with diameter at nm-scale, may be as strong as similar-sized metal nanowires (with GPa scale strength). However, previous mechanical testing of bulk np Au samples (prepared by dealloying Au-Ag alloys) found that their strength are much lower than theoretical predictions, raising a question as to whether the np metals are intrinsically soft. Here we present that the theoretical high strength can be approached in np metals when the dealloying condition was optimized. By minor-alloying of Pt to suppress the coarsening of np structure, and by dealloying (AuPtAg parent alloy) at a lowest possible potential to prevent crack-formation, we prepared millimeter-sized np Au-Pt with strength of 345 MPa. This value is 700% larger than the strength of fully dense parent alloy and is much higher than that of any previous np metals. We will also show that a slight coarsening can severely degrade the strength of these np AuPt samples - it explains the low strength of previous np Au samples, which have more or less experienced coarsening. Our macroscopic np AuPt samples couple high strength, small structure size (~5 nm), and large surface-area-to-volume ratio (~10⁸ m^-1), which are promising for many applications, such as actuation.

Displacive deformation usually plays a dominant role in the plasticity of crystalline solids at room temperature. Here we report in situ quantitative transmission electron microscope deformation tests of single crystal Sn samples. We found that when the sample size was reduced from 450 nm down to 130 nm, diffusional deformation replaces displacive plasticity as the dominant deformation mechanism at room temperature. At the same time, the strength-size relationship changed from “smaller is stronger” to “smaller is much weaker”. The effective surface diffusivity calculated based on our experimental data matches well with that reported in literature for boundary diffusion. The observed change in the deformation mode arises from the sample size-dependent competition between the Hall-Petch-like strengthening of displacive processes and Coble diffusion softening processes. Our findings have important implications for the stability and reliability of nanoscale devices such as metallic nanogaps.

Most of our knowledge on the mechanical behavior of metals has been derived from conventional uniaxial testing. However production and application of metallic components involve often methods that include strain path changes. The effect of a sudden change in strain path on the operating deformation mechanisms, the evolving microstructures and their mechanical behavior is less well investigated. In-situ deformation studies of such strain path changes during X-Ray Diffraction (XRD) or during observation in a scanning electron microscopy (SEM) can give valuable insights on deformation mechanisms and microstructures.
While miniaturized uniaxial tensile and/or compression deformation set-ups that can be used in-situ during synchrotron radiation and/or in SEM are nowadays relatively common and known to provide valuable insights on deformation mechanisms and microstructures, miniaturized multi-axial deformation devices allowing to change the strain path during deformation are still rare.
Here we present a newly developed miniaturized biaxial tensile machine allowing operation of the two deformation axes independently on a sample that has been previously cut out from a bulk metallic material. The machine can be used in a synchrotron beamline (for transmission or reflection x-ray diffraction) or in a SEM environment for in-situ studies. The characteristics of the miniaturized device and the developed sample preparation technique using picosecond laser ablation will be discussed in detail. First in-situ studies will be presented including in-situ SEM and in-situ XRD analyses performed at the MS and the MicroXAS beamlines of the Swiss Light Source (SLS) on nanocrystalline and ultrafine grained metallic microstructures. The research is performed within the ERC advanced grant MULTIAX (339245)

Driven by the ongoing miniaturization, new exciting material behavior at small length scales was encountered. In the (sub-) micron range, pronounced size effects come into play and material properties are subject to change. This was a driving force for the development of miniaturized testing techniques that enable determination of local plastic properties of materials. While static testing techniques are well established, there were only few efforts regarding the determination of fracture properties in miniaturized systems. Moreover, when working with thin films, residual stresses are a permanent prime concern that needs to be addressed.
In this presentation, we studied the influence of residual stresses on the fracture properties of thin films. We will demonstrate the quantitative capability and precision of recent developments regarding miniaturized determination of residual stresses and stress gradients by a FIB based layer removal method, as well as consecutive miniaturized fracture testing of stressed Cu, W, and W-Cu-W multilayers, respectively, in situ in the SEM. Subsequently, for a detailed data analysis we introduce an accompanying finite element based modeling approach. The possibilities and benefits of these approaches will be demonstrated by examining the deformation and fracture properties of the above mentioned thin films. Moreover, possible limitations of commonly used data analysis approaches are addressed, and possible extensions are discussed

We report on the mechanical behavior of nanostructured W/Cu (3 nm/1 nm grain sizes) thin films deposited on Kapton® under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (Saint-Aubin, France) [1]. In situ tensile tests were carried out combining 2D synchrotron x-ray diffraction (XRD) and digital-image correlation (DIC) techniques. In the elastic domain, the results show that the strain measurements (in the crystalline film by XRD and the substrate by DIC) match to within 10^-4. This result demonstrates the full transmission of strains in the elastic domain through the film-substrate interface and thus a good adhesion of the thin film to the substrate although no adhesion layer was used. Then, we have been able to extract the yield surface of W-Cu nanocomposites and demonstrate its brittle behavior [1] The elastic limit of the nanostructured W/Cu thin films was determined at the bifurcation point between strains obtained by XRD and DIC at ~0.50 % (depending on load ratio). After bifurcation the film elastic strain still increases linearly up to an applied load of ~100 N (i.e. a corresponding strain of ~0.67%, depending on load ratio) and then saturates. Deformation mechanisms such as strain localisation and film fragmentation are proposed and discussed in view of the behavior of monolithic Cu and W films obtained under similar conditions [2].
[1] S. Djaziri, D. Faurie, P.O. Renault, E. Le Bourhis, P. Goudeau, G. Geandier, D. Thiaudière, Acta Mater. (2013) 61, 5067.
[2] S. Djaziri, P.-O. Renault, E. Le Bourhis, Ph. Goudeau, D. Faurie, C. Mocuta, D. Thiaudière
J. Appl. Phys. (2014) 116, 093504

α-Fe single crystal nanopillars with different diameters were fabricated through focused ion beam (FIB) and then in situ tested using uniaxial microcompression methodology inside a SEM. Besides the well-established tenet of “smaller is stronger”, we demonstrated that the strain rate sensitivity of small-scaled single-crystal α-Fe decreased about 12 times as the pillar diameter decreased from 1000nm to 200nm. We propose that the observed phenomena are stemmed from the diminishing mobility difference between edge and screw dislocations which resulted from the increase of the external applied stress, i.e. the decrease of the sample size. This in turn renders the behavior of dislocations and the strain rate sensitivity of BCC metals similar to that of FCC metals.

The mechanical failure of amorphous materials is a complex and longstanding problem, and to this day the deformation mechanisms occurring in such materials remain poorly understood. With increased use in applications including microelectromechanical systems, and energy generation and storage devices, a better understanding of the mechanisms by which the materials deform and break is desired. Disordered nanoparticle packings are one useful class of amorphous materials; they have significant potential for usage in solar cells and other energy applications, where film integrity is important for electrical conductivity, and they also have applications in electrochemistry and optical coatings [1]. Importantly, they can also serve as a larger-scale model representation of disordered atomic systems like metallic glasses, whose structure and failure modes are extremely difficult to observe at the atomic level. In this study, quantitative nanoindentation techniques using atomic force microscopy are employed to investigate the mechanical behavior of thin film nanoparticle packings where the film thickness is varied. Mechanical properties such as the elastic modulus and hardness can be evaluated at the level of a single nanoparticle, allowing spatial inhomogeneities in the film structure to be mapped. The elastic modulus is measured to be 9.3 ± 3.6 GPa for a 100 nm thick film. The significant spread seen on any given sample is attributed to local variations in mechanical properties arising from the heterogeneous film structure. For loads below 500 nN on such films, outliers with very high moduli appear; these are never seen for higher loads or on thicker samples. The dependence of elastic properties on film thicknesses and load is attributed to mechanical communication between the nanoparticles being probed and the substrate. Topography scans before and after indentations allow for individual nanoparticle displacements to be resolved with nanometer accuracy. This resolution is significantly finer than the nanoparticle radius, which is unheard of for atomic displacements in metallic glasses. The hardness of the 100 nm film is measured to be 1.03 ± 0.51 GPa. Plastic indentations are accompanied by highly inhomogeneous nanoparticle displacements in the vicinity of the indent, in contrast to the displacement fields expected for crystals. The magnitudes of nanoparticle displacements after inelastic indentations are closely correlated with the measured mechanical properties. These relationships have implications for the spatial distribution of mechanical properties in other disordered packings including metallic glasses, for which so-called “soft spots” have been hypothesized but never directly observed. To illustrate this, the variance in the mechanical properties of soft spots as opposed to non-soft spots is estimated.
[1] Lee, D., Rubner, M. F., & Cohen, R. E. (2006). All-nanoparticle thin-film coatings. Nano Letters, 6(10), 2305-12. doi:10.1021/nl061776m

Nanoporous metals have been the subject of exciting studies in various fields such as catalysis, sensing, MEMS and biomedical applications, because of their large surface-area-to-volume ratio. The dimensions of the interconnected ligaments which create the nanoporous structure are typically below 100 nm. Despite the inherent ductility of metals, samples become brittle when the cell size is reduced to this scale, which hinders the development of direct applications.
Using a combination of free and electrochemical dealloying steps, bulk nanoporous gold (np-Au) samples of millimeter-scale dimensions were created from gold-silver alloys. This method led to crack-free nanoporous structures with negligible residual presence of the sacrificial element (silver), as verified by scanning electron microscopy and energy dispersive x-ray spectroscopy. The mechanical properties of np-Au were determined by tension and compression testing on a custom-built system, and by nanoindentation. Although np-Au specimens were extremely brittle at the macroscopic scale, observation of the fracture surface after testing revealed extensive plastic deformation and necking of the individual ligaments prior to rupture.
Classic scaling relations which link the mechanical properties of a porous metal to its dense counterpart do not take the cell size into consideration. Because the cells in nanoporous metals are of nanometer dimensions, the relations need to account for the change in mechanical properties of the individual ligaments. Based on the measurements realized on np-Au, a modified scaling relation will be presented. In addition, the deformation behavior of the cells under load will be discussed, based on some observations and 3D images of the deformed nanoporous structure, reconstructed from serial sectioning and imaging in a focused ion beam/scanning electron microscope.

A majority of refractory high-entropy alloys suffer from brittleness and limited formability at ambient temperature, but studies of their fracture properties are scarce. Here, we have conducted in-situ micro-cantilever tests to investigate the fracture behavior of a typical refractory high-entropy alloy, Nb₂₅Mo₂₅Ta₂₅W₂₅. The results show that the fracture toughness and strength of bi-crystal specimens are one order lower than those of single-crystal ones, suggesting that brittle intergranular fracture is a major fracture mode and strengthening grain boundaries is critical.

In situ nanomechanics is an emerging field that investigates the mechanical properties and deformation mechanisms of nanostructured materials. The study of in situ nanomechanics is often conducted by integrating the real-time mechanical testing inside an electron microscope and the mechanics modeling with atomic resolution. It provides a powerful approach to visualize the intrinsic nanomechanical behavior of materials - seeing is believing. In this talk, I will present recent in situ nanomechanics studies from my group, including the electrode degradation in nanoscale lithium-ion batteries (Nature Nanotechnology, 7, 749, 2012); deformation-induced stacking fault tetrahedra in FCC nanocrystals (Nature Communications, 4, 2340, 2013); fracture toughness of graphene (Nature Communications, 5, 3782, 2014); and twinning-dominated deformation in BCC nanowires (Nature Materials, 14, 594, 2015). The in situ nanomechanics studies provide new insights that cannot be offered by traditional mechanics studies. Ultimately, the in situ nanomechanics research aims to enable the design of nanostructured materials to realize their latent mechanical strength to the full.

It is now well-established that introducing nanoscale coherent twin boundaries (CTBs) is an efficient way to simultaneously increase strength and ductility in face-centered-cubic metals. Specific details on dislocation-CTB interactions have been studied extensively in the past using atomistic simulations, experiments and continuum theory, and are well understood when CTBs are considered to be perfect. However, recent nanodiffraction mapping experiments have revealed the existence of short segments of incoherent twin boundary (ITB) forming intrinsic kink-like twin boundary defects in the structure of nanotwinned copper, which contradicts the perfect CTB assumption and could play a major role in plastic deformation. This presentation will focus on a molecular dynamics simulation study of the effects of intrinsic kink-like twin boundary defects on plasticity and strengthening mechanisms in nanotwinned copper. Using simple bicrystal models, it is found that kink-like defects have a profound impact on screw dislocation - coherent twin boundary interactions, resulting in significant strengthening behavior. The dislocation products of a screw dislocation intersecting two different types of kink defects oriented at either 0 degree or 60 degrees with respect to the dislocation line, are studied at atomic scale to explain this phenomenon.

Extreme value theory is applied to the critical stress of the first plastic event which occurs in discrete intermittent plasticity. This could be the first pop-in during nano-indentation, or a displacement jump in a micro-deformation experiment. It is found that when the same deformation is repeated many times, the average of this first critical stress is related to the deforming crystal volume via an exponentially truncated power law. The present work demonstrates this trend and the expected Weibull fluctuations around it, for the experimental nano-indentation data of Morris et al Phys. Rev. Lett. 106, 165502 (2011), and for dislocation dynamics simulations suggesting a quite general phenomenon is at play. The identified truncated power law is found to uniquely define the underlying master distribution of critical stresses present in the deforming crystal, and also the density of discrete plastic events available to the system, thus providing a procedure via a deformation experiment to characterize a material's microstructure prior to loading.

The experiment results indicated that Al-TiN multilayers have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers thicknesses are in the range of nanometers, and showed a pronounced size effect in these mechanical properties, not only depending on the layer thickness but also on the layer thickness ratio. Using atomistic simulations, Atomistically-informed Dislocation Dynamics (AIDD) simulator, and three-dimensional crystal elastic-plastic model (3DCEPM), we characterize interface structure and dislocation nucleation and study the effect of layer thickness and layers thickness ratio on mechanical properties. The high strain-hardening rate is ascribed to the accumulation of dislocations at interface and the load transfer related to the layer thickness ratio. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.

Metal nanowires are usually strong but not able to maintain a high plastic flow due to the lack of strain hardening in them. In this talk, I would like to report a new category of metal nanowires with hollow interior, metal nanotubes, to exhibit a combination of ultrahigh strength and extraordinary plastic flow. It was revealed based on tensile deformations through molecular dynamics simulations that by controlling the diameter, wall thickness and axial orientation, ultrahigh plastic flow stress of more than 2 GPa could be maintained for up to ~ 60% tensile strain in Au nanotubes, whereas the solid Au nanowires of similar size yielded at tensile strain of less than 5%, after which the stress dropped immediately below 1 GPa. Particularly, the yield strength in [1 1 1] Au nanotubes was found to be up to 60% higher than the corresponding solid Au nanowire, which approaches the theoretical ideal strength in Au. A universal trend of surface reconstruction to the energetically favourable close-packed {1 1 1} orientation was found in Au nanotubes regardless of the initial orientation, which may be responsible for the unique plasticity in Au nanotubes with extremely thin walls.

Magnesium has recently drawn increasing interest as a lightweight material for applications in transportation and aerospace industries, driven by energy efficiency. However, low strength due primarily to easy dislocation glide on the basal plane and limited formability due to its low symmetry HCP structure have restricted broader application of wrought Mg. It is for this reason that cast alloys still account for more than 99% of Mg alloys used today. To achieve more isotropic deformation, a variety of Mg alloys have been developed and significant improvement has been observed. However, there is still a lack of fundamental understanding of the effects of alloying and, specifically, how solutes alter dislocation mobility. This limits property prediction, parameterization of constitutive models, and materials design for performance improvement and cost reduction.
To further the understanding of the solute effect on plastic deformation, dislocation mobility was studied using atomistic simulation methods with semi-empirical MEAM models for Mg alloys. A simple shear was applied on the slip plane and the critical resolved shear stress (CRSS) of Mg/Al alloys for <a> dislocation was estimated for basal, prismatic and pyramidal I planes, at temperatures from 0K to 500K, and with solute concentrations from 0 to 7 at.%. Solute hardening and three possible solute softening mechanisms were identified. Solute hardening for in-plane dislocation glide on the basal plane follows Labusch statistics and its temperature dependence was found to be consistent with existing thermal-activation theories. When the lattice resistance becomes high and thermal fluctuation is low, as for screw dislocations on the basal plane at 0K, solute atoms aid the formation of double-kinks and reduce CRSS. Out-of-plane glide with jogs formed through solute assisted climb was observed for edge dislocations on prismatic and pyramidal I planes, and softening due to solute addition was observed on the pyramidal I plane. Another out-of-plane glide mechanism involving cross-slip, the locking-unlocking mechanism, was observed for screw dislocations on the prismatic plane, where solute softening in yield stress was achieved by the assistance of solute atoms for unlocking. The mobility of <c+a> dislocations was also studied on pyramidal I and pyramidal II planes for both Mg/Al and Mg/Y alloys. Similar softening and hardening effects were observed and the difference between the Al alloy and the Y alloy was analyzed.

Surface modification and functionalization are of fundamental importance in tuning physical and mechanical properties of two dimensional materials, which are supported by various substrates. Our first-principles calculations show that hexagon boron nitride (h-BN) nanosheets on Cu substrates exhibit ferromagnetic, antiferromagnetic, or ferrimagnetic properties with ozone molecules or oxygenminus;hydrogen groups bonded on its B atoms, depending on the adsorption density and configuration of these functional groups. We also reveal possible hydroxylation of h-BN monolayer on Ni substrate by surface O adatom induced spontaneous dissociation of water molecule. The metal substrates play an important catalytic role in enhancing chemical reactivity of O adatoms on h-BN with water molecules through transferring additional charges to them. Further study shows that surface oxidation and fluorination could remarkably reduce energy corrugation of interlayer sliding in commensurate graphene/h-BN heterogeneous structures as interlayer charge transfer. A registry index model is established to describe the interlayer sliding friction in graphene/h-BN system.

Dislocation shielding is an important aspect in understanding the toughening mechanism of graphene. Combining atomistic simulations and continuum modeling, we studied the effects of dislocation shielding of a mode-I nanocrack in monolayer graphene. Different crack/dislocation geometries were considered and the influence of shielding on the threshold stress intensity for crack propagation was quantified. Excellent agreement between simulation results and linear-elastic fracture mechanics (LEFM) predictions were achieved. In addition, we noted that the dislocation shielding effect scales as 1/r_R when r_R is larger than ~20 Å while scales as 1/r_R^1/2 for smaller r_R, where r_R is the separation between crack tip and dislocation. The scaling (for large r_R) is a direct manifestation of stress field of edge dislocation in graphene. Besides the effects on crack initiation, dislocations also have a significant impact on crack propagation, and may trigger phenomena such as crack bridging, crack deflection, and crack pinning, depending on the details of the interplay between the crack and dislocation, to further toughen graphene. Our work presents the first systematic study on crack-dislocation interactions at nanoscale in graphene.

As a graphene-like two dimensional material, multilayer MoS₂ is a promising candidate in electronics, optoelectronics, and catalytic applications. Strain engineering of layered MoS₂ entails a thorough understanding of the deformation behavior, which is hard to capture experimentally, whereas various computational methods are well-suited to probe the underlying mechanism on the atomistic level. In this paper, classic molecular dynamics (MD) simulations are performed to investigate the deformation response of multilayer MoS₂ at high strain rates using the REBO potential. The variations in local stresses and strains during deformation of multilayered MoS₂ structures as predicted using the REBO potential is validated using density functional theory (DFT) calculations for nanoscale dimensions of the layers. The local stresses and strains during deformation of the micron-sized MoS₂ layers are observed to vary due to the presence of edges in multilayered MoS₂ sheets. The effect of strain rate, temperature and size of the MoS₂ layers on the stress-strain response will be discussed. Notably, a new metallic phase is observed to form at around 11% tensile strain in all cases considered. Edge effect is observed to be dominant at small dimensions and is found to be negligible at large dimensions.

Ni₃Al exhibits superior mechanical properties at elevated temperature due to its long-range order L1₂ superlattice. However, the pristine Ni₃Al is very brittle due to the notorious intergranular fracture and the covalent-like atomic bonding. The brittle nature is a long-standing unsolved bottleneck of Ni₃Al, which hinders its vast applications as structural materials. In this study, we use extensive atomistic simulations to highlight the unique role of nanotwinning in the mechanical properties of Ni₃Al, which is based on an atomic-scale cracking model. It is quite surprising to find a &’smaller is stronger&’, and simultaneously &’smaller is tougher&’ phenomenon in the nanotwinned Ni₃Al. The strength and toughness are both improved with decreasing twin size. Besides intergranular ductile fracture, we also notice a novel quasi-brittle fracture mechanism in pristine Ni₃Al as nucleating twinning partials from crack tip. In contrast, dislocation avalanches serve as a crack blunting mechanism which leads to ductile fracture pattern of nanotwinned Ni₃Al. Finally, a transition fracture mechanism from dislocation nucleation to shear localization is observed in nanotwinned Ni₃Al as twin becomes extremely small, and subsequent energetics analysis is provided to rationalize the transition. Our atomistic simulations agree with enhanced noticeable elongation and improved strength of Ni₃Al containing disorder lamellar nanotwins after high pressure torsion.

The discovery of atomically thin materials has raised hopes of new applications ranging from flexible electronics to wearable sensors. Commonly employed production methods that rely on the exfoliation of bulk materials yield thin films of overlapping flakes. Charge transport in these inhomogeneous films proceeds by percolation which is sensitive to variations in the film morphology. This behavior represents both challenges and opportunities as exemplified by graphene-based percolative strain gauges. Their operation is based on the high sensitivity of transport to strain-induced changes of flake overlap which results in a high and tunable gauge factors. Whereas high strain sensitivity is an attractive feature of strain sensors it has to be minimized for flexible conductors and wearable electronic devices.
Despite significant research in the formation and application of 2D thin films, the issue of strain sensitive transport, which is affecting reliability and operation, has not been addressed.
We here present our work to identify the factors that are controlling changes in percolative transport upon mechanical deformation. For this purpose we simulated the percolative transport using large-scale three-dimensional resistor networks. Overlapping flakes of arbitrary shapes can be considered in our simulation tool. Fast solver techniques enable the comparison of transport under different amounts of uniaxial strain.
Using this novel tool, we characterize the influence of individual-flake parameters on the films response to strain. An increased flake-flake contact resistance and a decreasing flake size were found to enhance the sensitivity of transport to deformation. Experimental characterization of transport in percolative films supports our findings and highlights the potential of our simulation tool in providing guidelines for enhancing the sensitivity of strain sensors and improving the tolerance to deformation in flexible electronic devices.

The replacement of manganese with various transition metals (such as Ni and Fe) on the 16d sublattice of cubic LiMn₂O₄ (LMO) spinel has been explored recently as a means of improving its energy density and rate capability as a Li-ion battery (LIB) cathode material. The phase behavior of these spinel cathode microparticles and nanoparticles is linked with their mechanical degradation during repeated cycling, including active particle fracture, which in turn is widely thought to exacerbate capacity fade and impedance growth. Accurate mechanical properties of these materials are essential for the modeling and prediction of fracture and mechanics-related aging mechanisms. Here, we use instrumented grid nanoindentation to determine Young&’s elastic modulus E and hardness H for fields of LMO, LiMn_1.5Ni_0.5O₄ (LMNO), and LiFe_0.08Mn_1.5Ni_0.42O₄ spinel microparticles embedded in a relatively compliant polymer matrix. The pseudorandom distributions of particle-, matrix-, and boundary-dominated E measurements are analyzed to determine the extent of the finite particle size effects, and a statistical model is developed to extract the Young&’s modulus from these distributions. We find that E measured at the nanoscale increases by up to 30% from both the Ni and Ni/Fe doping, which follows the decrease in lattice parameter and increased oxidization state of Mn. This result demonstrates how changes in transition metal occupancy can significantly affect mechanical behavior for the spinel family of LIB cathode materials.

Mechanical properties of BCC refractory metal nanowires have been extensively studied as the high melting temperature and strength make them good candidates for nanostructured applications. However, the nanowire configuration cannot facilitate a high plastic flow of the material under tensile deformation, as manifested by significant drop in stress value right after yielding. Recently, a new configuration of hollow nanowire, or nanotube, is proposed in attempt to resolve this problem in FCC metals by introducing decent strain hardening character into the material. In this study, we report extraordinary high plastic flow stress of BCC nanotubes revealed by using molecular dynamics simulations. Specifically, it is found that when the wall thickness of <100>/{110} cylindrical Nb nanotubes is reduced to a certain value, the material can sustain an ultrahigh plastic flow stress up to 1.5 times the tensile strength of its nanowire counterpart. This is in contrast to previously reported case of Au where the FCC nanotube has to sacrifice its strength in order to realize plastic flow. The intriguing phenomenon can be attributed to the emergence of planer defects that impede dislocation motion when nanotube surfaces reorient to the relatively closely-packed {110} atomic planes. Our work suggests that BCC refractory metal nanotube can be engineered to have superior mechanical properties than BCC nanowire, and thus an alternative material solution for high-strength nanostructured systems.

The Oliver & Pharr method for analyzing load vs. displacement data from a nanoindentation experiment assumes that the sample can be treated as an isotropic homogeneous half-space. However, in many practical situations, samples are small and embedded in a dissimilar matrix material. The minimum size of an indentation is limited by the sharpness of the indenter tip and the quality of the sample surface. Thus, the matrix may be expected to influence the calculated hardness and modulus values. In this work we present a method to correct for the influence of the surrounding matrix and demonstrate that it can be used to accurately calculate the indentation modulus and hardness of a small particle, independent of the properties of the matrix. Examples include pure and composite synthetic calcite crystals (with bio-inspired applications) embedded in a polymer matrix, and individual mineral grains naturally embedded in organic-rich shale.

Graphene is characterized by a unique combination of outstanding electrical, mechanical, thermal and optical properties [1], which make it an ideal candidate for a variety of applications [2]. However, in order to translate its enormous potential in high performance yet reliable devices, it is necessary to have a deep understanding of its mechanical behavior, thus developing ad-hoc experimental set-up. In fact, the standardized methods and equipment commonly considered for materials mechanical characterization are designed for macro-sized components, and are not effective for manipulation and testing of 2D materials, like single graphene layers [3]. As a consequence, many challenges have to be overcome, like availability of freestanding samples, which is a necessary condition for performance of direct tensile tests. Thus, it should not surprise that up to date no data are available for graphene strength measured through a tensile test.
In the present paper we address such main issue through the design of a novel device, which can perform direct tensile tests on samples, like single atomic layers, initially deposited onto a substrate. In order to demonstrate the validity of the present design, the device is first applied for the mechanical characterization of micro and nanospecimens, like aluminum microwires (18 µm diameter) and ultra thin films (800 nm thickness). The results derived from our device are then compared to those obtained through a commercial nanotensile testing machine, showing good agreement. Finally, the device is applied for tensile testing of a single graphene layer, providing results in good agreement with the predictions of atomistic simulations and nano indentation membrane experiments, see review [4].
References
[1] A K Geim, K S Novoselov (2007) “The rise of graphene”, Nature Materials, 6, 183-191.
[2] Zhang, J., Ryu, S., Pugno, N., Wang, Q., Tu, Q., Buehler, M. and Zhao, X. (2013) “Multifunctionality and control of the crumpling and unfolding of large-area graphene”, Nature Materials, 12, 321-325.
[3] Pantano, M. F., Espinosa, H. D. and Pagnotta, L. (2012) “Mechanical characterization of materials at small length scales, Journal of Mechanical Science and Technology, 26 (2), 545-561.
[4] Ferrari, A. et al. (2015) “Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems”, Nanoscale, 7, 4598-4810.

Contact Resonance Atomic Force Microscopy (CR-AFM) is a useful technique to map the mechanical properties of ultra-thin layered materials. Contrast in CR-AFM imaging arises from resonance frequency differences when the cantilever is in contact with different surface regions [1]. Here we establish CR-AFM as a powerful technique to study the out-of-plane mechanical properties of substrate-supported graphene, and to characterize graphene-graphene and graphene-substrate interfaces. We chose few-layer graphene (FLG, i.e., mono-, bi-, or trilayer) on SiC as a model 2D material system due to its well-defined surfaces and clean interfaces. We show that CR-AFM can clearly distinguish regions of different graphene layer numbers based on the different mechanical properties of the layered material system. We develop a multi-scale method that combines ab initio and continuum layered material modeling [2, 3] to deconvolute and quantify the stiffness contribution from each interface, from each graphene layer, and from the SiC substrate. The calculated AFM-sample contact stiffness values agree remarkably well with the experimental measurements. Finally, we show that CR-AFM can detect subsurface defects in oxygen intercalated monolayer graphene on SiC (0001) [4].
References
[1] U. Rabe, "Atomic Force Acoustic Microscopy," in Applied Scanning Probe Methods II, B. Bhushan and H. Fuchs, Eds., ed: Springer Berlin Heidelberg, 2006, pp. 37-90.
[2] F. L. Degertekin, "Hertzian contact Lamb wave sensors," 1997.
[3] G. G. Yaralioglu, F. L. Degertekin, K. B. Crozier, and C. F. Quate, "Contact stiffness of layered materials for ultrasonic atomic force microscopy," Journal of Applied Physics, vol. 87, pp. 7491-7496, 2000.
[4] Q. Tu, B. Lange, Z. Parlak, J.M.J. Lopes, S. Zauscher, V. Blum, "Nanomechanical Subsurface Structure Fingerprint for Graphene Based Nanostructures: Contact Resonance AFM and Theory", in preparation

Here we discuss the characterization and modeling of a relatively new class of embedded artificial hair sensors that take advantage of the mechanical properties of structural S2 glass microfibers as the hair element and the electromechanical properties of self-aligned carbon nanotube arrays to rapidly transduce small changes in force or displacement into changes in resistance. CNTs grow radially from the surface of the glass fiber, self-positioning and mechanically supporting it within a glass microcapillary pore. As the hair bends outside the pore, the nanotubes inside the pore can be compressed against the metalized capillary resulting in a decrease in resistance. The hair sensors were subjected to point loads with a laboratory scale setup and to the distributed loads within an air flow. A plane wave tube was used to perturb the hair at acoustic frequencies in order to probe their dynamic response. We show that both the quasi-static and vibrational performance of the sensors can be analytically modeled by treating the nanotube array support as a partial elastic foundation. We find the load from the support on the hair within the pore can be assumed to be linearly proportional to the local deflection of the hair, and by comparing the models to the experimental results the stiffness-per-length proportionality constant is determined to be about 6.5 MN/m². While other hair sensor designs suffer from reduction in bandwidth due to mechanical coupling of the hair by the transducer, our models indicate that the stiffness of the distributed CNT array supporting the S2 fiber is high relative to the low mass of the fiber. As a result the resonance frequencies of the hair are maximized almost as if the hair is fixed within the pore, so we observe adjustable first-resonance bandwidths of a few hundred Hz to over 10 kHz as the length of the hair is varied. The stiffness is low enough, however, to allow the fiber to necessarily deflect near the opening of the pore. The resistance decreases with deflection of the hair and is observed to saturate when the nanotube array at the opening of the pore directly beneath the fiber is strained by 7 to 10%. Considering sensors of the same design but with nanotube arrays of differing morphology or alternate nanomaterials, the bandwidth shouldn&’t be affected even by decreasing the support stiffness by 100X but the sensitivity could be increased by a factor of 25. By adjusting their range and sensitivity, the artificial hair sensors can be considered for monitoring the air flow across aircraft skins over a wide range of air speeds or for fine detection of mechanical deformations within structures.

Two dimensional materials including graphene, silicene and MoS2 and so forth represent ideal materials composed of a single layer of atoms organized in a lattice form. They exhibit intriging mechanical, thermal and electrical properties, making them perfect candidates for condensed engineeraing applications. It is useful to study their material behaviors on the substrate that may involve nonlinear complex deformations including wrinkle, crumple, fracture and exfoliation. However, it is difficult for experiments to quantitatively measure the governing factor in these mechanical behaviors because of the difficulties to access such extreme small size, preparing the samples and controlling the boundary conditions noninvasively, while limits in knowledge about the mechanics of these 2D materials on substrates significantly impede an evaluation of their mechanical robustness in engineering applications as well as using mechanical force to functionalize them. Here, we use multiscale computational method from simulations based on density functional theory to ones based on continuum mechanics to investigate how mechanical properties of these 2D materials are affected by interacting with different substrates. For example, we study the changes in surface area induced by wrinkles. We find that the high specific surface area of the single piece of graphene can only be affected up to 2% regardless of loading conditions, geometry, and defects. We investigate possible ways of peeling silicene solely by mechanical force from substrate and find that the peeling at a 45° angle with the substrate is the most efficient one to detach silicene without early fracture. With the multiscale simulation methods developed in this work, we provide important insight into the deformation of several 2D materials as well as efficient tools for understanding their mechanical behaviors on different substrates.

Electro-Mechanical devices are now commonplace in variety of commercial and consumer applications. With commercialization, the push towards making devices smaller follows. Besides the ability to enhance the packing density of devices, miniaturization also improves the resolution of these sensors^1-3. However, there are significant challenges associated with miniaturization including well established actuation and detection techniques failing radically at nanometer scale and limited dynamic range of the devices due to onset of nonlinearity at much smaller displacement amplitudes⁴. The reduction in dynamic range is particularly severe in devices that are only a few atomic layers thick⁵. In our work we report an increase of 25dB in dynamic range by changing the strain of graphene sheet. Strain in graphene sheet is changed largely by changing the temperature of the device. A partial tuning of strain is also seen with back-gate action. We attribute the increase in dynamic range to the decrease in cubic nonlinearity coefficient⁶ at lower temperatures. Our experimental results matches well with earlier reported works⁷. The resulting mass resolution is 100yoctogram which is an order of magnitude better than the previously reported values^5,8.
References:
1. Hanay, M. S. et al. Single-protein nanomechanical mass spectrometry in real time. Nat. Nanotechnol.7, 602-608 (2012).
2. Li, M., Tang, H. X. & Roukes, M. L. Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very high-frequency applications. Nat. Nanotechnol.2, 114-120 (2007).
3. Chaste, J. et al. A nanomechanical mass sensor with yoctogram resolution. Nat. Nanotechnol.7, 301-304 (2012).
4. Ekinci, K. L. Electromechanical transducers at the nanoscale: actuation and sensing of motion in nanoelectromechanical systems (NEMS). Small Weinh. Bergstr. Ger.1, 786-797 (2005).
5. Chen, C. et al. Performance of monolayer graphene nanomechanical resonators with electrical readout. Nat. Nanotechnol.4, 861-867 (2009).
6. Postma, H. W. C., Kozinsky, I., Husain, A. & Roukes, M. L. Dynamic range of nanotube- and nanowire-based electromechanical systems. Appl. Phys. Lett.86, 223105 (2005).
7. Wang, Z. & Feng, P. X.-L. Dynamic range of atomically thin vibrating nanomechanical resonators. Appl. Phys. Lett.104, 103109 (2014).
8. Kumar, M. & Bhaskaran, H. Ultrasensitive Room-Temperature Piezoresistive Transduction in Graphene-Based Nanoelectromechanical Systems. Nano Lett.15, 2562-2567 (2015).

Hybrid nanomaterials are an interesting class of materials that can find applications in diverse fields owing to their multifunctionality tailored at the interface of the constituents. Graphene has attracted a great deal of attention attributed to extraordinary physical (electronic, mechanical, thermal, optical and electrochemical) properties useful for a gamut of technologies. Likewise, pi-conjugated polymers serve as playground for preparing not only supramolecular nanostructures but also forming hybrids and nanocomposites. This work is centered at the design and development of ‘smart&’ hybrid nanocomposites from functional nanobuilding blocks of graphene and its derivatives and electrochemically polymerized conducting polymers (polypyrrole and poylaniline ) using layer-by-layer approach. Here we investigate their structural and physical properties using complementary techniques including scanning and transmission electron microscopy, atomic force microscopy, room temperature electrical property and resonance Raman spectroscopy with mapping, to determine surface morphology, surface roughness / uniformity / (in)homogeneity, nanoscale (amorphous versus semicrystalline) structure through diffraction, and elastic modulus to establish microscopic structure- processing-property correlations. The experimental findings elucidate the optimized properties of synthesized hybrids for space and energy industries.

High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photo-piezotransistors. Here, we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS₂ devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS₂ devices can be actively modulated by the piezoelectric charge polarization induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type MoS₂ strain/force sensor built using a monolayer MoS₂ triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS₂ piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical-electronic nanodevices.

First-principles density functional theory (DFT) calculations were performed to investigate the effects of B and N doping on the fracture behaviors of a series of symmetric graphene grain boundaries (STGBs). B and N atoms were introduced to exothermic adsorption sites along STGBs. The doping was shown to generally enhance the fracture strength of STGBs. The observed strengthening can be attributed to the effects of dopants on stretching energies of critical bonds and energetics of the dopants, where the stretching energy is obtained via the generalized von Karman equation. In addition, we showed that N doping can deflect the fracture propagation to further increase the fracture toughness of the STGBs. Furthermore, the doping may also collaborate with prestrain to promote new crack propagation route that is perpendicular (or at a large angle) to the grain boundary. Our results provide important insights on defect engineering and strengthening of polycrystalline graphene.

Nano-crystalline dispersed amorphous Ag-Cu alloy thin films were synthesized using DC magnetron sputtering. The thin films showed a very unique structure, consisting of homogeneously dispersed 2-10 nm crystallites in an amorphous matrix. The size and the volume fraction of the nanocrystallites was found to increase with increasing film thickness, resulting in the increase of elastic modulus of the composite thin-films. However, increasing nanocrystallite size and volume fraction deteriorated the electrical properties of the thin films. The film containing lowest volume fraction of nanocrystallites exhibited very high conductivity of ~ 5.09 x 10⁷ S/m. Furthermore, the mechanical and electrical characteristics of the amorphous-crystalline interface dominated thin-film was explained by ultra-violet photoemission valance band study.

As compared to a film(or substrate), interface has a poor physical properties. Therefore, breakage occurs preferentially when the stress is applied from the outside. For this reason, the number of test methods have been developed by a number of researchers. However, the scratch test, full test, such as commercially available test methods has significant limitations, which can be summarized as follows: The effect of the thin film and the substrate deformation energy acts as a main factor. And the accuracy has dropped because of the need to measure the delamination point(area) using an high resolution optical measuring device. In order to overcome the conventional testing method, a new model proposed using the indentation. If the indentation test performed on thin film sample, it can obtain a load-displacement curve containing the effect of the film/substrate/interface. In this study, the adhesion was evaluated by a quantitative analysis of the resultant curve. In order to analyze the curve, it was cited an expanded hardness concept and plastic zone which generated under the indenter is constrained at the interface. Based on these results, defines the interfacial constraint parameter and formula was derived. Using the indentation test method were evaluated for adhesion of the samples made with a variety of materials and processes, the model was validated through comparison with other experiments.

Graphene and carbon nanotubes have extraordinary mechanical properties.
Intrinsic defects such as local non-hexagonal reconstructions or grain boundaries, however, significantly affect the material characteristics.
On the one hand defects reduce the stability by lowering the ultimate tensile strength. On the other hand, dangling bonds at such defect sites show high chemical reactivity, whose selectivity can even be fine-tuned by applying an external stress.
Maximizing the materials strength in the presence of defect is therefore highly desirable.
Here, we address the properties of defects in graphene from first-principles on the level of full-potential density functional theory, and assess doping as one strategy to strengthen such materials. We carefully disentangle the global and local effect of doping by comparing results from the virtual crystal approximation with those from local substitution of chemical species, in order to gain a detailed understanding of the breaking and stabilization mechanisms.
We find that n-type doping or local substitution with electron rich species increases the ultimate tensile strength significantly. In particular, it can stabilize the defects beyond the ultimate tensile strength of the pristine material. We therefore propose that doping should be a key strategy to strengthen graphenic material.

We report on the nanoscale elastic hardness and modulus distribution in SiC of a reaction bonded SiC/Si (RBSC) ceramic matrix composites (CMCs) anticipated for advanced applications including those for extreme environments and semiconductor wafer processing platforms due to its high hardness, high modulus and structural stability. The RBSC CMCs consisting of 80 vol% α-SiC were fabricated with liquid Si infiltration. Samples were imaged using an Auriga 60 focused ion beam and scanning electron microscope (FIB/SEM) after in-situ polishing by FIB using Ga ion beam to minimize the relief at grain boundaries and reaction interfaces. In addition to a major component SiC from the green body, unreacted silicon and reaction formed cubic SiC structure were detected in the RBSC CMC by SEM and further confirmed by a JEM-2010F TEM.
To understand how the hardness and modulus distribute in nanoscale within the SiC grain that has experienced the thermal shock, inter-diffusion and reactions and how the nanoscale properties may affect the bulk mechanical behavior. Nano-mechanical properties of selected SiC grain were obtained by nanoindentation. 2-D hardness maps suggested a distribution of 32.4 to 43.6 GPa while localized moduli of the SiC altered between 240.5 to 331.7 GPa correspondingly. It was found that when the cubic SiC formed epitaxically to α-SiC, an enhancement of elastic modulus and hardness occurred in the adjacent α-SiC. Further analysis revealed that microstructural features arising from diffusion and reactions were responsible for the nano-mechanical characteristics.

Nowadays in materials science there is a great interest for nanostructured and architectured materials in order to build very hard materials with low density. The metal foams are one of the candidate but they may have a quite low stability in temperature. Biener et al. [Nano Lett. 2011] showed that a thin layer of a hard amorphous oxide can stabilize the foam in temperature. The goal of our study is the investigation of the mechanical and temperature stability of metallic foams covered by a thin layer of hard material, such as silicon.
To understand the onset of plasticity in such architectured materials, we used molecular dynamics simulation. We first identify a potential able to correctly model the mechanical properties of a metal and silicon. We focused on the MEAM potentials for gold and silicon that we have optimized in order to get better mechanical properties than with the previous parametrizations. In this work, we will present the improvement obtained with our fitted potential for bulk silicon and bulk gold, and we will show the ability of this potential to correctly model Si-Au interfaces. Our first results obtained on the stability of the Si-Au interface under temperature and mechanical stress will be also presented.

We performed a series of molecular dynamics (MD) simulations of uniaxial compression of several types of metals including Cu, Ni, Al and Ta with average grain size of 5, 10, 15 and 20 nm. The uniaxial compression was applied at a constant engineering strain rate until the model reached 50% reduction in the loading direction. The simulation results obtained for all types of materials revealed grain growth in those with small average grain sizes such as 5 and 10 nm while those with big grain sizes (15 and 20 nm) demonstrated grain refinement.
While in the past grain growth has been mainly claimed to be caused by grain boundary motion, it was observed in our MD simulations that grain rotation induced coalescence of neighbouring grains dominated the grain growth in those metals with grain sizes of 5 and 10 nm. On the other hand, grain refinement during SPD in metals with large average grain sizes can be explained by the formation of dislocations. At the early stage of deformation, a very high density of dislocations emerged which led to the formation of intragranular structures. By increasing the deformation strain, these intragranular structures gradually evolved into grain boundaries. Overall, both grain growth and grain refinement exist during SPD, but the metals with relatively small grain sizes (lower than 10 nm) are more prone to grain growth while those with relatively large grain sizes are prone to grain refinement.

Unlike face-centered cubic (fcc) and body-centered cubic (bcc) metals, hexagonal close packed (hcp) metals do not have sufficient slip systems at room temperature to accommodate plastic deformation by slip alone. As a result, deformation twinning plays an important role during plastic deformation of hcp metals. Here the room temperature plastic response of a-axis oriented magnesium microcrystals are reported. Single crystal Mg micropillars ranging in size from 2 mu;m to 20 mu;m were fabricated using focused ion beam (FIB) milling, then tested using an in-situ SEM nanoindentation system. The effect of crystal size on the transformation in deformation mechanisms is reported. Electron backscatter diffraction (EBSD) analysis of the deformed pillars indicates that the deformation is governed by twinning, followed by excessive basal slip in the twinned region. The stress required for twin propagation was found to increase with decreasing sample size. An anomalous increase in strain hardening was observed for larger microcrystals, which is induced by twin-twin interactions. The results are compared to deformed bulk Mg single crystals to further shed light on the hardening induced by twin-twin interaction.

The generation of non-basal dislocations near the c-axis direction and the introduction of twin boundaries are the fundamental processes for enhancing plasticity in highly anisotropic hcp magnesium materials. Using the molecular dynamics method, we have successfully generated a c+a dislocation from the obstacle set in the perfect magnesium crystal by extending the material in the c-axis direction. A structure of the stacking fault is found to be corrugated and is extended indefinitely in the perfect crystal. The introduction of a twin boundary in the system enables the extended c+a dislocation to emit from the source and glide in the system. The interaction of a c+a dislocation or a basal dislocation with a twin boundary is found to be manifested in various results. A dislocation is absorbed in the twin boundary, and, in some condition, is reemitted from the boundary. The results of these interaction processes are assessed by simulating the systems in various configurations.

Atomic scale modeling is one of the most widely used tools in studying plasticity mechanisms at the nanoscale. There are several issues that need to be addressed in order to compare the results of atomic scale modeling with real experiments.The most important issue is that of the time scale, i.e. the strain rate is about a few orders of magnitude faster in the simulations that in the experiments.In this report we review the strain rate and temperature effects observed in atomic scale modeling of nanoscale plasticity.We review the modeling of dislocation-obstacle interactions as well as nanoindentation.The most common effects of temperature and strain rate that are critical to stress and interaction mechanisms are demonstrated and discussed.
Research sponsored by the Office of Fusion Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.

Due to the high energy content, aluminum is added to propellants and explosives becoming an extra energy source. This aluminum can release additional energy during its explosive dispersal. The ignition of Al particles under shock is attributed to the melting behavior during the propagation of the shock waves. As a result, a fundamental understanding of the mechanisms as well as the thresholds for ignition under shock loading conditions has significant interest. This study aims to investigate of the links between the shock loading conditions (shock pressures, strain rates, temperature, etc.), the microstructure (Al particle size, density of pressed powder), and the melting behavior of the Al powder compacts. This study was conducted in two phases: The first phase was to establish a relationship between the aluminum particle size and the melting temperature and behavior using molecular dynamics (MD) simulations. The smaller the particle size the lower the melting temperature. The second phase was built upon this relationship in order to find a relationship between particle size of the Al powder compacts and required shock pressures to induce melting using MD simulations. The wave propagation behavior, the evolution of temperature, and the melting behavior for impact velocities of up to 2 km/s and for particle sizes ranging from 5 nm to 60 nm will be presented.

Chevron-notched specimens offer an attractive approach for fracture toughness testing at the microscale if stable crack growth can be achieved at the notch. Stable crack extension allows the fracture toughness measurement to be independent of the crack growth length. Based on the ASTM standard test specimen configuration, a fixed-fixed (i.e., double-supported) 3-point bending micro beam with a chevron notch in the middle has been manufactured using Focused Ion Beam milling. The specimen was made from single crystal magnesium aluminate spinel with the intended fracture plane aligned with a {100} direction. The crystal orientation was confirmed with Electron Backscatter Diffraction before the milling. The fracture test was carried out using a Hysitron PI85 Picoindenter, an in-situ nano mechanical test system. The indentation test has been conducted under a complex displacement control with loading, hold, and unloading segments so that a successful test result may contain multiple unloading curves, each of which represent the notched microbeam stiffness at a certain stage of the test as the crack propagates. A 3D model of the chevron notched micro beam has been built in ANSYS APDL using a large deflection nonlinear solver. A series model of different initial crack lengths has been built to simulate the crack propagation during the test. The simulation result has been used to estimate the crack growth length corresponding to the experiment result by comparing the stiffness curve. The goal is to measure the fracture toughness value of a single crystal magnesium aluminate spinel by calculating the energy release rate using the unloading stiffness curve.

Nanoporous silicon (np-Si) is an interesting material that has potential applications in photo-voltaics, memory devices, lithium-ion batteries and nano-electromechanical systems (NEMS) due to the effective surface area increase as the dimensions of the device get smaller. High purity np-Si films were produced by dealloying as-deposited precursor films in water. A variety of microstructures were created and milled with a focused ion beam (FIB) to create micropillars with an average diameter of 500 nm and height of 1 micron that consisted entirely of as-dealloyed material. In-situ micropillar compression was then carried out in a TEM to determine what role size effects may play in the brittle-ductile transition of this material. Upon removal of the load applied to the np-Si micropillars, it was observed that there was a large recovery of strain that was induced during deformation. The extent to which ligaments have deformed during deformation, the strain recovery upon removal of the load and the mechanism of deformation for a variety of microstructures will be presented.

Numerous studies on the mechanical properties and size effects of both face-centred cubic (fcc) and body-centred cubic (bcc) metals at the small scale have been reported. In recent years there have been a number of reports on the compression strength of metallic nanopillars of diameter < 1 µm. In all cases the strength of the pillars increases with decreasing sample diameter. The influence of diameter on plastic flow stress shows different behaviour with fcc and bcc metals. In both metals, the yield strength, y, is inversely proportional with some power of the pillar diameter, d. In most fcc metals the power law exponent, n, is very similar and lies in the range -1 < n < -0.6. However, bcc pillars show a less pronounced size effect with the exponent -0.45 < n < -0.22. The difference in behaviour between bcc and fcc pillars is attributed to the non-planar core structures of screw dislocations which cause high lattice friction and hence a slower mobility compared to the edge dislocations. Here, we aim to extend our understanding of the size dependent behaviour of bcc materials through a study of the mechanical properties and deformation mechanisms in iron nanopillars. In-situ compression tests have been carried out on iron (Fe) pillars using a picoindenter mounted in a SEM at ambient temperature. Iron pillars with diameters ranging from 250 nm to 5 µm were machined by focused ion beam (FIB) technique milling [001] oriented grains from a polycrystalline sheet. In addition Fe nanowires with diameters in the range of 30-800 nm were fabricated by electrodeposition into Polycarbonate (PC) and anodised aluminium oxide (AAO) templates. The electrodeposited nanowires are polycrystalline with mean grains size of approxiamtely 20 nm. The size dependence of the flow stress tested in compression will be reported for this range of sample diameters along with a limited number of tensile tests.

The strength hardening of polycrystalline and nano-crystalline metals in the large grain size range has been extensively studied and reviewed. The very high strength and hardness of nano-crystalline metals suggest that these benefits may be derived from nano-structuring at two structural length scales: grain size and grain boundary (GB). Investigations at different microstructure scales seem to demonstrate the occurrence of a wide range of physical mechanisms, which depend on the metallurgical state and solutes content [1]. To improve both aspects, nanostructured nickel and nickel-tungsten samples obtained by electrodeposition are explored. A careful characterization of the microstructure was performed by using different techniques (AFM, TEM, XRD, EBSD, ASTAR, DSC) and the composition and/or contamination of the nanostructured nickel samples were measured by X-ray fluorescence and hot extraction analysis [2]. A direct relation between the light elements (O, H, N, C) and crystallographic texture is first discussed. In a second step, the impact of light elements, crystallographic texture and grain boundary character on the Hall-Petch relationship is demonstrated. The dependence of grain size on flow stress is directly a consequence of solute content (solute strengthening) and evolutions of the internal stresses with grain size [3]. Additionally, the occurrence of two competing physical mechanisms, grain boundary shearing and dislocation emission at grain boundary is suggested to explain the experimental data. Both are impacted by grain-boundary character and solute content.
[2] N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall, J. of Alloys and Compounds, 609 (2014) 296-301.
[1] A. Godon, J. Creus, S. Cohendoz, E. Conforto, X. Feaugas, P. Girault, C. Savall, Scripta Mater., 62 (2010) 403-406.
[3] N. Shakibi Nia, J. Bourgon, P. Girault, S. Cohendoz, J. Creus, X. Feaugas, C. Savall, Mater. Sci. and Eng. A, (2015) in progress.

The strength of nanoporous gold (npg) has been shown to be strongly dependent on the length-scale of the gold ligaments, an observation consistent with countless micromechanical experiments that show the typical trend of “smaller is stronger”. Such size-dependent strength can be exploited in npg through targeted annealing in order to tailor the structures for specific applications. However, this approach to structural optimization requires adequate scaling laws for the prediction of strength as a function of structural length-scale, an approach reliant on the assumption of self-similarity of the coarsened structure. Here we present evidence from high-resolution 3D structural characterization that the condition of self-similarity may be sufficiently met for the prediction of mechanical response. By identifying representative volumes, a rich set of structural parameters is achieved, though many parameters may be superfluous for understanding the scaling behavior in mechanical response. Applying nanoindentation and microcompression testing to such representative volumes, a correlation of salient structural parameters to the mechanical properties is achieved, and discussed in terms of the limits of the Ashby-Gibson laws and the potential for optimizing strength of nanoporous metals.

Extensive research has been carried out on cyclic stress-strain response and final dislocation arrangements in cyclically deformed metals up to the occurrence of saturation. There is, however, a limited knowledge about the crystal rotation that the evolving dislocation ensembles cause at the early stages of cyclic deformation. This information is essential to validate novel cyclic-computational models that are currently being developed. Laue x-ray micro-diffraction has proven to be an effective technique to study dislocation ensembles and their evolution during in-situ mechanical testing [1].
We present a novel in-situ micro-mechanical testing system that allows deforming samples cyclically in shear mode. The system is installed at the MicroXAS beamline at the Swiss Light Source (SLS). The samples are designed based on the Miyauchi&’s geometry and prepared by using picosecond laser ablation [2]. At various stages of cyclic deformation, spatial resolved Laue diffraction patterns are recorded in transmission mode. In particular, we study the evolution of dislocation arrangements during the first cycles on single crystalline copper samples oriented for single slip and double slip (coplanar and collinear configurations). The samples are deformed at different strain amplitudes up to different number of cycles. The evolving dislocation microstructures are analyzed in terms of misorientation and spatial distribution. The experimental results aim to validate a low cycle fatigue modeling by crystal plasticity finite element method.
REFERENCES
[1] H. Van Swygenhoven, S. Van Petegem, JOM 62 (2010) 36
[2] A. Guitton, A. Irastorza-Landa, R. Broennimann, D. Grolimund, S. Van Petegem, H. Van Swygenhoven, Materials Letters (under review)

In this work, virtual x-ray and electron diffraction are used to analyze the deformation mechanisms observed in nanotwinned Cu. Multimillion-atom nanocrystalline Cu simulations are produced with varying nanotwinned concentration and texture. Virtual diffraction patterns are computed directly from snapshots of these simulations at different stages of tension and compression. The computed diffraction patterns reveal deviations in intensity, line broadening and peak smearing that are attributed to distortions of the atomic lattice. These lattice distortions arise from the significant volume fraction of grain boundaries, triple junctions, dislocations and other defects within the initial nanocrystalline structure as well as the increased distortion of the lattice from these groups to accommodate the deformation. The influence of these deformation mechanism groups on the smearing and broadening of diffraction patterns is investigated by computing diffraction patterns for each separately. These results are also related to other traditional per atom metrics for metallic atomistic simulation, e.g. centrosymmetry, common neighbor analysis, free volume, etc., in order to provide greater understanding to the deformation mechanisms.

Plasticity in crystalline materials confined to small volumes is thought to be a competition between the length scales imposed by the specimen or feature geometry (distance between free surfaces or interfaces) and those of the pre-existing microstructure (mean dislocation spacing). In the latter case, modifications to bulk crystal plasticity that incorporate discrete defect interactions with themselves and boundaries provide a basis for interpreting inelastic behavior. In the case of high crystalline quality FCC nanostructures absent of experimentally-observable defects, nucleation of new defects becomes predominant and manifests unique features of plastic deformation, including yield strengths near the theoretical limit, twinning-mediated plasticity, pronounced temperature dependence of yielding, and a propensity for strongly localized plasticity. Recently, the importance of diffusive plasticity mechanisms that either directly carry plastic deformation or facilitate displacive activities has also been highlighted. In this talk, experiments that directly measure the surface dislocation nucleation strengths in high quality noble FCC metallic nanowhiskers subjected to uniaxial tension will be presented. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered as characterized by atomic-scale activation volumes. Such a framework explains both the ultrahigh athermal strength as well as the temperature-dependent scatter quantified in our experiments. Modeling of the probabilistic nature of surface dislocation nucleation suggests activation energies consistent with surface self-diffusion as the rate-limiting step needed to promote displacive activity. In this context, approaches allowing for modification of the surface chemistry and structure of metallic nanostructures to either inhibit or enhance surface diffusion will be discussed. We also present experiments designed to stabilize the specimens against the large energy released during nucleation, which promotes extended plastic flow in otherwise rapidly localized configurations.

Single crystals loaded cyclically are known to develop a number of distinct dislocation microsturctural patterns such as cell structure and persistent slip bands (PSBs). In addition, cyclic hardening and softening are known to crucially depend the evolution of the dislocation microstructure and the dislocation density multiplication that takes place as the number of loading cycles increases. However, a clear understanding of the dislocation activity pathways that lead to such patterning is still missing. In this work, three-dimensional discrete dislocation dynamics method (DDD) is used to study the evolution of dislocation networks in Nickel single crystals. A physics based cross-slip mechanism, which has been found to be critical for the proper modeling of dislocation activity, are fully considered in the current simulations. First the effect of crystal size on the evolution of the dislocation microstructure in microcrystal ranging in size from 0.5 to 7.5 microns with initial dislocation densities in between 10¹¹ and 10¹⁴ m^-2 loaded in fully reversed cyclic loading at a constant total strain amplitude of 0.4%. A new metric for dislocation patterning is suggested and used to quantify the evolution of dislocation patterns. Larger crystals show a strong tendency for patterning dislocations into cell-like structures similar to those observed experimentally. Early fatigue cyclic hardening was also observed with larger crystals showing a higher hardening rate due to the fast multiplication of the dislocation. Second, the dislocation evolution in crystals with fully developed PSBs are subsequently investigated. Point defect generation and the spatio-temporal evolution of the point defect concentration is quantified as a function of the dislocation density in the PSB channels and walls. The results are discussed in view of a point defect diffusion model to study their migration rates to the surface or the bulk of the crystal.

Plastic deformation of crystalline materials is the result of the collective movement of dislocations, in response of their mutual interactions, external applied loading and interactions with boundaries such as free surfaces, interfaces or grain boundaries. The dislocation microstructures emerging from such dynamics are intrinsically heterogeneous and the way they affect the mechanical properties is a puzzling problem, especially at the micro- and nano- scales.
A reliable tool to model crystal plasticity at such scales is the Discrete-Continuum Model (DCM). The DCM is based on a coupling between 3D Dislocation Dynamics (DD) simulations and Finite Element (FE) method. In particular, the DD simulation code is in charge of the dislocation microstructure evolution while displacement field and boundary conditions (including surface and interface effects) are handled by the FE simulation code.
The DCM, which was proposed first in 1999 [1], has been significantly improved during the last years [2]. It is now possible to handle problems with very large number of dislocations (the performances of the DCM algorithm overcome the multipole algorithm gain with large number of segments), to use non-regular FE meshes, to precisely take into account the influence of finite or periodic boundary conditions, to consider isotropic and anisotropic elasticity.
Here, the new capabilities of the DCM are presented and illustrated with recent calculations made for Ni micro-samples and a SiGe nanostructure. With those examples, we show how the DCM is suitable to investigate the plastic properties of small volume objects. In particular, the calculation performed for the SiGe nanostructure highlights the attractive capability of running DD simulation in a full FE framework accounting for complex boundary conditions and using anisotropic elasticity.
[1] C. Lemarchand, B. Devincre, L. Kubin, and J.-L. Chaboche. In laquo; Multiscale Modelling of Materials raquo;, vol. 538, pp. 63-68. MRS, Warrendale, Pennsylvania, 1999.
[2] O. Jamond, R. Gatti, A. Roos, B. Devincre. Submitted to International Journal of Plasticity.

The prediction of crack growth is one of the most technologically important and scientifically intriguing problems in mechanics of materials. Yet, despite decades of research, a comprehensive understanding of the process has remained elusive. As a quintessential multiscale phenomenon, crack growth is both a chemical and mechanical process, involving interatomic bond breakage driven by long range mechanical stress fields. Thanks to growing supercomputing resources and novel concurrent multiscale modeling techniques that can accurately couple quantum and continuum mechanics modeling domains, crack tip processes in real environments are just now becoming accessible to powerful quantum chemistry approaches such as Kohn Sham Density Functional Theory. The majority of our work in this area has been focused on understanding how environment and impurities influence the behavior of cracks in aluminum and silicon carbide. In this talk, I will discuss our most exciting findings on this topic.

Nanocrystalline metals have become widely accepted as a technologically important class of materials owing particularly to their remarkable strength under ambient conditions. However, dislocation impingement upon grain boundaries under an applied stress produces nanovoids that considerably limit ductility. By adding amorphous layers to form a crystalline-amorphous nanolaminate structure, vast improvements in ductility have been achieved and attributed to a unique strain accommodation process where dislocations are absorbed at the amorphous-crystalline interfaces (ACIs). In this study, new columnar nanolaminate simulation cells were constructed that incorporated grain boundaries and ACIs in the same structure to more closely represent experimental materials. Molecular dynamics uniaxial tensile simulations were conducted on these structures with particular focus on the role of stress state in the deformation physics. The atomic slip vector was employed to characterize the coupling between shear transformation zone (STZ) dynamics and dislocation plasticity, and the distribution of plastic strain among the deformation mechanisms was quantified using a Green strain tensor analysis. Initially, plastic strain was accommodated within the amorphous layers with STZ activity preferentially located directly adjacent to the ACIs. Relative to nanolaminates free of grain boundaries, this enhanced slip in the columnar nanolaminates was further biased to amorphous atoms near the intersection of ACIs with grain boundary planes. Lattice dislocations, often involving leading partials and stacking faults, were first emitted from these regions of locally high shear strain, and the addition of grain boundaries significantly reduced the stresses required for global yielding via dislocation plasticity. The emission of trailing partials wasn&’t biased to the grain boundaries, but rather depended on the stress state at the ACIs. Full dislocation formation followed emission of these trailing partials, which represented the dominant strain accommodation process in the columnar nanolaminates. Absorption of these dislocations at ACIs suppressed strain accommodation at the grain boundaries, which eliminated the formation of deleterious stress concentrations that ultimately lead to grain boundary microcracking in nanocrystalline metals.

Contact between metal surfaces results already at small loads in plastic deformation of the surface asperities. Such asperities span various length scales, and at the micron-scale their plastic response is size dependent. Size dependent plasticity can be captured by discrete dislocation plasticity simulations [1, 2]. However, so far, only simulations for very simple surface geometries have been carried out using dislocation dynamics, e.g. two-dimensional surfaces with sinusoidal profile. The study of more complex three-dimensional surface geometries requires prohibitive computational time.
Our aim is to be able to address realistic three dimensional surfaces. To this end we here present a modified version of the discrete dislocation plasticity method [3] to solve contact problems. This method relies on Green&’s functions molecular dynamics [4] to track the change in true contact area during contact loading. The image fields are obtained using analytical solutions for an elastic layer with arbitrary surface geometry.
Preliminary results show that the results obtained by discrete dilsocation plasticity simulations for simple surfaces can be reproduced with this method at significantly lower computational costs.
References
[1] V. Deshpande, D. Balint, A. Needleman, and E. Van der Giessen, “Size effects in single asperity frictional contacts,” Modelling and Simulation in Materials Science and Engineering, vol. 15, no. 1, pp. 97-108, 2007.
[2] F. Sun, E. V. der Giessen, and L. Nicola, “Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study,” Wear, vol. 296, pp. 672 - 680, 2012.
[3] E. V. der Giessen and A. Needleman, “Discrete dislocation plasticity: a simple planar model,” Modelling and Simulation in Materials Science and Engineering, vol. 3, no. 5, pp. 689-735, 1995.
[4] C. Campañá and M. H. Müser, “Practical green&’s function approach to the simulation of elastic semi-infinite solids,” Physical Review B, vol. 74, p. 075420, Aug 2006.

Pristine defect-free single crystal graphene has a mechanical strength that approaches its theoretically determined intrinsic value. To utilize this strength in real world applications, it is necessary to develop methods of up scaling production of the material such that it maintains its high strength. Chemical vapor deposition (CVD) offers an industrially scalable method for growing large area sheets of graphene, however CVD grown graphene contains grain boundaries. Nanoindentation experiments on freestanding circular CVD graphene films demonstrate that grain boundaries reduce the strength of graphene. These results motivate the need to understand the mechanics of failure of the grain boundaries of CVD grown graphene. In this talk, we describe a multiscale model that simulates the experiments in order to rationalize the results from the experiments. The graphene grains are treated as a nonlinear and anisotropic elastic material with a continuum constitutive model based upon ab initio Density Functional Theory (DFT) calculations. The grain boundaries are treated a potential fracture edges via a cohesive zone model whose traction-separation behavior has been extracted from Molecular Dynamics (MD) simulations. Thus the constitutive behavior for both the grains as well as grain boundaries are multiscale and is implemented numerically in the context of the Finite Element Method using ABAQUS. We then simulate a large number of freestanding polycrystalline graphene with grain boundary structures consistent with experiment. We compare the histogram of the breaking strength from the simulations with that observed experimentally to determine the validity of the cohesive zone model. In addition, we determine the Weibull distribution parameters for graphene.

While bulk silicon is brittle at temperatures below 600K-700K, the compression of nanopillars has revealed that a decrease of one characteristic dimension below few hundreds of nanometers could change the mechanical behavior from brittle to ductile [1,2]. Such size effect can not be related to the initial defect density like in metals, because residual defects density is negligible in silicon nano-objects. Characterizing the key parameters controling the transition and the associated plasticity mechanisms is required to further developing the MEMS and NEMS technology or preventing the failure of microelectronic components based on the silicon strained technology. We have then performed molecular dynamics simulations in order to understand the mechanisms at the origin of cracks and dislocations nucleation in silicon nanopillars.
In this work, nanopillars up to 44 nm in diameter and height were investigated in compression and tension with a controled-displacement mode, at temperatures ranging from 1K to 600K. The atomic interactions in silicon were modeled using two different interatomic potentials, both fitted to reliably reproduce the ductile and brittle properties of bulk silicon.
Under compressive load and small sizes, simulations revealed a ductile behavior and the presence of large shear bands along slip planes, in agreement with experiments. In addition, the simulations suggested the formation of stacking fault defects in the anti-twining shear stress direction at the onset of plasticity, a result not yet confirmed by experiments. In tensile mode, the simulations revealed the nucleation of perfect dislocations from the surface, eventually leading to cavity opening when dislocations interact [3]. We first determined that the nanopillars height should be larger than 20 nm to allow for cavity opening, and second that the brittle to ductile transition is also controled by the diameter of the nanopillars, as observed experimentally in compression. The plastic deformation of large diameters nanopillars operates by cavity expansion leading to the brittle fracture, while thinner nanopillars yielded by dislocations gliding ultimately leading to ductile fracture. Finally, our simulations seem to confirm that the critical dimension leading to the brittle to ductile transition could increase with temperature, as experimentally presumed [2].
[1] F. Oestlund et al., Adv. Funct. Mat. 19 (2009) 2439.
[2] J. Rabier et al., Physica Status solidi c 10 (2013) 11.
[3] F. Abed El Nabi et al., Modelling Simul. Mater. Sci. Eng. 23 (2015) 025010.

In many systems such as whiskers, wires and pillars the reduction of the mean free path of dislocations below the micron scale produces significant increase of the mechanical strength. In small-grain polycrystals, the constraint to the motion of dislocation is due to grain boundaries (GB). By absorbing dislocations, GBs contribute to shut down dislocation activities [1]. At that point, other alternate plasticity mechanisms are needed.
Shear-migration coupling is one of them and is the focus of many theoretical and experimental studies. At variance from dislocation-based plasticity, the shear produced by a moving GB can result in different values, depending on a parameter called the coupling factor Beta. Recent results obtained by in-situ Transmission Electron Microscopy (TEM) in ultra fine-grained Aluminum, show that many deformation modes are activated, including shear migration coupling. The coupling factor can be measured experimentally using image correlation analysis and therefore confronted to what has been predicted by models such as the one from Cahn and Mishin [2]. Although solid statistical data are still missing, beta appears smaller than what has been predicted. A reason could lie in the atomic-scale mechanisms that guide the migration of GBs. The Cahn and Mishin model assumes collective motion of GB dislocations, while Rae and coworkers insist on the role of steps propagation [3]. High resolution imaging of bicrystals shows that steps decorate GBs and that the motion of imperfect steps could result in the migration of the GB associated with a shear. To take in account these observations we also proposed a geometrical model for the shear migration coupling of grain boundaries [4], based of the shuffling of atoms within extended cells around the GB. Finally, recent simulations show that step dislocations (disconnections) are probably the basic mechanism leading to grain boundary migration [5]. Those disconnections are found in non-ideal GBs and can be created from interactions between lattice dislocations and GBs [6]
References:
[1] F. Mompiou, D. Caillard, M. Legros, H. Mughrabi, Acta Mat. 60/8 (2012) 3402.
[2] J.W. Cahn, Y. Mishin, A. Suzuki, Acta Mat. 54/19 (2006) 4953.
[3] C.M.F. Rae, D.A. Smith, Philosophical Magazine 41/4 (1980) 477.
[4] F. Mompiou, D. Caillard, M. Legros, Acta Mat. 57/7 (2009) 2198.
[5] A.Rajabzadeh, F. Mompiou, M. Legros, N. Combe, Phys. Rev. Lett., (2013) 110,265507
[6] A.Rajabzadeh, F. Mompiou, N. Combe, M. Legros, D. A. Molodov, S. Lartigue-Korinek, Acta Mat. 2014

Severe plastic deformation (SPD) is widely used to process bulk nanocrystalline metals. The most accepted models to describe the microstructural evolution, e.g., the grain refinement, during SPD are based on dislocation activities and the influence of grain boundary motion has not been fully integrated to any of the existing models yet. However, due to the large volume fraction of grain boundaries in nanocrystalline metals, it is expected that grain boundary motion has significant influence on the microstructural evolution. In this work the interaction of grain boundary motion with cracks in nanocrystalline metals during shear deformation has been studied by atomistic simulations. It is shown that based on metal type, temperature and grain boundary structure different mechanisms, namely crack healing, grain boundary decohesion and sub-grain formation can happen, which could be used to tailor the overall microstructures in nanocrystalline metals.

Nanocrystalline (NC) metals have shown potential for improved wear response over their bulk counterparts, however the propensity for grain evolution under wear could limit their applicability for tribological applications. In this work, electroplated Ni-40at%W --- a binary NC alloy with demonstrated improvement in thermal stability over pure Ni --- was studied under wear as a function of applied force and number of cycles. Microstructural evolution in each wear track was then analyzed via cross-sectional transmission electron microscopy (TEM). Results indicate an increased propensity for grain coarsening under sliding contact with increasing contact force. At low contact force (1 mN), a low friction wear regime (friction coefficient below 0.3) is maintained up to 10000 cycles. However, at higher contact forces (100 mN and 1000 mN), a shift in wear regime from low to high friction occurs within a few hundred cycles. Cross-sectional TEM analysis shows that in the 1 mN case, only minimal grain coarsening occurs and is confined to a depth of 200 nm. In contrast, for applied forces of 100 and 1000 mN, significant grain coarsening is observed down to depths of 400 and 800 nm. We hypothesize that material softening due to grain coarsening gives rise to the observed shift to a high friction regime at higher contact forces.

The study of interactions between grain boundaries (GBs) and second phase precipitates is crucial to enhance our understanding of microstructural evolution in crystalline materials. We show both analytically and computationally that a planar symmetric GB contained within a second phase lamellar precipitate is unstable against long wavelength perturbations. The instability is mediated by the elastic interactions between the GB and compositional domain boundaries.
A simple relationship between the growth rate and wavelength of this instability is derived analytically for an elastically isotropic material in the case of a symmetrical GB centered in a second phase lamella infinite in the GB direction. This dispersion relation reveals that short wave-lengths are stabilized by the surface tension while larger wave-lengths are unstable due to elastic interactions. In particular, we show that the growth rate is maximum for a given wave-length, revealing the pattern-forming character of this instability.
In a second step, we derive a similar dispersion relation in the case of anisotropic elasticity for cubic symmetry. The anisotropy is shown to inhibit the instability by reducing the growth rate and increasing the wave-length corresponding to the maximum growth rate.
Simulations using both an elastically periodic model for dislocations [Geslin et al., Acta. Mater. 71, p.80-88, 2014] and amplitude equations derived from the phase-field-crystal model [Spatschek, Karma, PRB 81, 214201, 2010] confirm the key predictions of the linear stability analysis in both the isotropic and anisotropic cases and shed light on the subsequent nonlinear stages of this instability. In particular, we show that this instability can lead to the break-up of low angle GBs, thus changing their properties and mobilities.
Finally, simulations performed with a circular precipitate in the vicinity of the GB show that elastic interactions between the precipitate and the GB lead to a similar instability.

One-dimensional nanocrystalline metallic nanowires with extremely high mechanical strength are believed to be promising building blocks for a wide range of emerging applications in electronic, optical and nanoelectromechanical devices, etc. Metallic nanowires, however, experience severe degradation of mechanical behavior in the presence of minute impurities, such as oxygen, especially along the grain boundary. Various mechanisms have been proposed, whereas these phenomena are still poorly understood on an atomic level. To find solutions to these problems, it is crucial to develop an in-depth understanding of the deformation mechanisms of metallic nanowires, and degradation effects of oxygen along the grain boundary, which is hard to probe experimentally. A thorough investigation of the tensile deformation and failure mechanisms of pure nanocrystalline Ni and nanocrystalline Ni with varying levels of oxygen impurities is therefore carried out using molecular dynamics simulations. The evolution of defect structures and the mechanisms of failure are characterized for nanowires of 24 nm and 48 nm diameter. It is observed that segregation of varying levels of atomic oxygen impurities along the grain boundaries have a great impact on the deformation and failure modes of nickel nanowires. The effect of the concentration of oxygen impurities on the evolution of defect structures, densities and the micromechanisms of failure for the various nanocrystalline Ni nanowires will be discussed.

I will present our group main advances with respect to direct mechanical tests on diverse inorganic nanomaterials using piezo-driven “Nanofactory Instruments” sample holders compatible with a 300 kV field-emission high-resolution transmission electron microscope, HRTEM [1]. It is worth noting that the regarded in situ mechanical studies combined the highest spatial and temporal resolutions peculiar to HRTEM and unique possibilities of precise manipulations with an individual nanostructure, including its bending, stretching and peeling under an accurate control of all force-displacement parameters. Bending and tensile strength, Young&’s modulus or fracture toughness of carbon, boron nitride, dichalcogenide nanotubes and nanosheets, silicon and boron nanowires, and aluminum/boron nitride nanotube nanocomposites, and their peculiar deformation kinetics were revealed through informative nanomechanical tests by means of an atomic-force-microscope (AFM)-HRTEM setup [2-10]. Peeling of individual molybdenum disulfide atomic layers (while modeling the famous “Scotch-tape” technique of graphene-like nanosheets&’ isolation) was for the first time accomplished inside HRTEM under a precise control of energetics involved and stress-strain fields distribution [11].
The author is particularly grateful to many colleagues and coauthors, namely, D.M. Tang, M.S. Wang, X.L. Wei, X. Zhi, C.Y. Zhi, M. Yamaguchi, N. Kawamoto, F. Liu, D. Kvashnin, B.S. Sorokin, N. Berseneva, A. Krasheninnikov, Y. Huang, J. Lin, J. Zou, C.L. Ren, C. Liu, N. Fukata, M. Mitome, Y. Bando, R. Arenal, J. Garel, E. Joselevich, A. Zak, R. Tenne, I. Nikiforov, T. Dumitrica, P.M. Ajayan, J. Lou and B. Yakobson for their important contributions to the experimental and theoretical mechanical studies of advanced nanostructured materials at different stages of the “Nanoscale Mechanics” Project accomplishment within MANA-NIMS over the last decade.
References:
[1] Golberg D. et al. Adv. Mater. 24, 177 (2012).
[2] Wang M.S., Golberg D., Bando Y. Adv. Mater. 22, 4071 (2010).
[3] Wei X.L. et al.Adv. Mater. 22, 4895 (2010).
[4] Tang D.M. et al. ACS Nano 5, 7362 (2011).
[5] Nikiforov I. et al. Phys. Rev. Lett. 109, 025504 (2012).
[6] Tang D.M. et al. Nano Lett. 12, 1898 (2012)
[7] Yamaguchi M. et al. Acta Mater. 60, 6213 (2012).
[8] Tang D.M. et al. Nano Lett. 13, 1034 (2013).
[9] Liu F. et al.ACS Nano 7, 10112 (2013).
[10] Wei X.L. et al. Nano Lett. 15, 689 (2015).
[11] Tang D.M. et al.Nature Commun. 5, 3631 (2014).

Nano-sized metallic glasses have drawn extensive attention due to a frequently reported brittle-to-ductile transition taking place below some critical size, imparting distinctive mechanical properties at nanoscale compared to their bulk counterparts. However, the origin of this transition has yet to be unveiled due to the complications introduced by common fabrication techniques, such as focused ion beam milling, which tend to introduce defects and impurities, and may potentially modify the local structure of the metallic glasses. Here, we report an experimental approach to achieve ultrafast liquid quenching, which enables formation of metallic glasses with compositions far beyond the glass-formation zone identified by conventional vitrification techniques. Our methodology is a technological break-through in making nano-sale metallic glasses with tunable dimensions, offering unique possibilities to study their structure and property relationships. Combining in-situ transmission electron microscopy observation and atoms-to-continuum modeling, we studied the formation condition and the thermal stability of these novel metallic glasses. An investigation of the size dependent ductility of single-element Ta, Cu-Zr, and Pd based metallic glasses has been conducted, which may shed light on the relationship between mechanical behavior and structure of metallic glasses.

Coherent x-ray micro-diffraction was used to detect and count phase defects (stacking faults, SFs, left in the crystal after the glide of partial dislocations) preliminarily introduced by deformation of InSb single-crystalline micro-pillars. Diffraction patterns were recorded by scanning the coherent nanobeam along the pillars axis: peak splitting is observed in the diffraction pattern associated to the top region, in agreement with the presence of a few SFs located in the upper part of the deformed pillars. Simulations of coherent diffraction patterns were also performed considering SFs randomly distributed in the illuminated volume: they show that not only the number of defects but also the size of the defected volume influences the maximum intensity of the pattern, allowing for a precise counting of defects [Physical Review Letters, 111 (2013), 065503].
Recently, diffraction measurements were performed in-situ, during compression, to detect the first lattice defects, i.e. the first events of the plastic deformation appearing in InSb micro-pillars

Recently, there has been an increased interest in high temperature nanoindentation due to improved instrumentation and better experimental techniques. This has further extended the capability of nanoindentation based techniques to study temperature effects on the mechanical properties of small volumes of materials. However, the maximum test temperatures are often limited by the indenter tip material and the fastening techniques. Many factors including hardness, elastic modulus, thermal expansion coefficient, fracture toughness, chemical reactivity etc., need to be considered in selecting a material for a high temperature nanoindentation tip. Recently, several single crystal high temperature refractory materials have been identified for possible use as indenter tip materials such as TiC, NbC, VC, and ZrC. In this presentation, we present mechanical properties measured from nanoindentation on these single crystal materials to assess their suitability for use as an indenter tip. New techniques for fastening the tips are also discussed.

We present a new computational approach that couples a recently developed potential energy surface exploration technique with applied mechanical loading to study the deformation of atomistic systems at strain rates that are much slower, i.e. experimentally-relevant, as compared to classical molecular dynamics simulations, and at time scales on the order of seconds or longer. We then discuss the new insights into the plasticity of amorphous solids that are obtained using this new approach, with a particular emphasis on how the shear transformation zone characteristics, which are the amorphous analog to dislocations in crystalline solids, undergo a transition that is strain-rate and temperature-dependent. More generally, we will also discuss how the proposed approach predicts differences in deformation mechanisms in comparison to scaling the results of classical molecular dynamics simulations down to experimental strain rates.

Both carbon nanotubes (CNTs) and CNTs based hybrid carbon nanostructures have been considered as ideal reinforcements for light-weight and high-strength composites. Understanding the interfacial behaviors between CNTs and the matrix, and of junctions in hybrid carbon nanostructure is crucial for engineering the desired properties in composites. In the first part, in-situ pull-out experiments with a polymer derived ceramic (PDC) matrix were carried out with both pristine CNTs and CNTs coated with Al₂O₃ (CNT/Al₂O₃) or HfO₂ (CNT/HfO₂) produced by Atomic Layer Deposition (ALD), using micro-fabricated devices in a scanning electron microscope (SEM). This carefully designed comparative study makes it possible to better understand the interfacial interactions between CNTs and PDC matrices. The interfacial shear strength (IFSS) of CNT-PDC is 9.99±2.84 MPa, while the IFSS of (CNT/Al₂O₃)/PDC with ~3 nm Al₂O₃ coating thickness is 14.52±2.66 MPa, demonstrating a 45.3% improvement. The non-linear failure observed with the coated CNTs is also indicative of energy dissipation mechanisms that promote toughening in ceramic matrix composites. The improved properties in (CNT/Al₂O₃)/PDC are believed to originate from increased surface roughness, which leads to mechanical interlocking at the interface during the pull-out process. In the second part, CNTs/graphene hybrid carbon nanostructures have successfully been synthesized. The junction strength between CNTs and graphene is challenging to be characterized using current experimental methods. We have designed a custom-made nanomanipulator that enables us to pull out individual CNT bundle from graphene in a SEM. The junction strength is estimated to be ~4.94 ±1.28 GPa, which suggests very strong bonding between CNT bundle and graphene.

Hexagonal closed packed (HCP) metals with limited number of easy slip system exhibit limited formability and deform via multiple twinning modes. Twinning/detwinning plays a critical role in plastic cyclic deformation, characterized by multiple twin variants that interact with each other and form twin-twin junction. A recent cyclic study using in situ optical microscopy reveals a correlation between twin-twin junctions and mechanical hardening, but does not provide information on the mechanical properties of the individual twin-twin junctions. Here we explore the latter by performing in situ compression of micro-pillars containing a twin-twin junction (parent and two twin variants), a twin boundary (parent and one twin variant) and no twin boundary (just parent phase) on single crystal Mg. The loading direction is designed to activate basal slip, where the dislocation motion and interaction with twin-twin junctions is anticipated. Our results show that twin-twin junctions exhibit high yield strength and work-hardening behavior compared to its twin/parent or just parent counterpart.
This work was supported by DoE BES Office, USA.

The mechanical robustness of freestanding films (FSFs) is critical for many of their applications, e.g., sensors, wound dressing, and drug delivery. Accurately characterizing the mechanical properties of FSFs is essential to design these devices or systems. However, mechanical characterization of nanoscale FSFs is very challenging. The most common ways include bulge testing, microelectromechanical system (MEMS) tensile testing, and nanoindentation stretching. However, the bulge and MEMS tests require sophisticated nanofabrication processes to fabricate the samples to specific configurations and also cannot be easily applied to as-synthesized FSFs. Atomic force microscope (AFM) or nanoindenter have been used to mechanically characterize FSFs by indention stretching. Previous studies were based on measured displacement to which the inelasticity may contribute, resulting in possible significant underestimation of the FSFs&’ moduli.
In this study, we develop a new comprehensive methodology for characterizing the modulus, pretension, and strength of membrane based on spherical indentation using continuous stiffness measurement (CSM) technique, which can be easily applied without the need of any extra nanofabrication step to prepare the sample. While previous methodology requires that the membrane must be purely elastic, the new methodology can be accurately applied even in the case that it shows inelastic behavior. Here, the elastic stiffness, measured using CSM technique, is analyzed using a newly-derived simple analytical solution. This method was implemented to characterize our all-nanoparticle FSFs.
Here, 400nm-thick all-nanoparticle FSFs composed of 22nm-positively-charged and 7nm-negatively-charged silica nanoparticles were fabricated on a pH-responsive sacrificial polymer layer using Layer-by-Layer (LbL) assembly and sequentially released through an increase of pH to dissolve the polymer layer. This is the first study to fabricate all-nanoparticle FSFs through using sacrificial pH responsive polymer layer.
When the nanoparticle FSFs were indented using a spherical tip, two film deformation modes were identified: film stretching and film cutting. The two deformation modes can be used to determine two different sets of mechanical properties: (1) the tear strength by analyzing the cutting mode, and (2) the elastic modulus, pretension, and failure strength by analyzing the stretching mode. Based on the analysis, the silica nanoparticle FSFs have the following properties: elastic modulus: 14.6 GPa, pretension: 3.5%, and failure strength: 0.123 GPa. The tear strength and inelasticity of the FSFs were studied as well. The development of this new testing methodology would help us to more accurately testing the mechanical properties of FSFs, especially for the films showing time-dependent behavior.

Thermally stable nanocrystalline (nc) metals retain their high strength characteristics at high temperature. One such material is nc-NiW, where W acts to reduce grain boundary energy, thereby decreasing the driving force for grain growth. However, creep rate generally increases as grain size decreases. This work studies the creep behavior of freestanding thermally stable nc-NiW thin films of various grain sizes under high temperature and high tensile stress conditions. A self-aligned, self-actuating micromachined creep platform has been designed and is being fabricated to perform creep experiments under approximately constant load versus displacement conditions. The platform consists of a V-shaped thermal actuator, a load cell, a tensile specimen and nm-scale displacement gauges. Using various geometries, a wide range of loading conditions can be achieved on a single chip. The thermal actuators are fabricated using low creep Mo. They utilize a buckling instability to achieve approximately constant load, as modeled by finite element analysis. The platform is self-actuated at the experimental temperature due to coefficient of thermal expansion differences between the actuator and the substrate, allowing multiple creep experiments to be performed simultaneously. The fabrication process flow includes two mask levels, bias sputtering backfill, planarization, and electrodeposition of nc-NiW thin films. Creep experiments will be performed in an environmental probe station under vacuum using a boralectric heater stage and optical observation of in-plane displacement. Initial results of thermally stabilized nc Ni-W films will be presented for various grain sizes.

Recent advances in electronics have enabled nanomechanical measurements with very low noise levels (sub nanometer) at fast time constants (20 mu;s) and high data acquisition rates (100 KHz). These capabilities open the doors for a wide range of ultra-fast nanomechanical testing, for instance, indentation testing at very high strain rates. With an accurate dynamic model and an instrument with fast time constants, step load tests can be performed which enable access to indentation strain rates approaching ballistic levels (i.e. 4000 s^-1). Results from a novel testing technique that involves a combination of step load and constant load and hold tests that enable measurement of strain rate dependence of hardness spanning over seven orders of magnitude in strain rate will be presented. Stress exponents determined from the proposed method on several materials including tin, aluminum and polycarbonate will be presented and compared to the results from uniaxial testing. Given the significant inertial effects during fast testing, the dynamic contribution of the testing instrument to the overall response will also be discussed.

The modern materials engineer has access to a range of excellent high fidelity tools to explore the mechanical behaviour of materials, including recent developments with high spatial resolution digital image correlation (DIC) and high angular resolution electron backscatter diffraction (EBSD). These exciting techniques enable probing of the total strain & rotation (HR-DIC) and elastic strain & lattice rotation (HR-EBSD) at a local scale and with very high precision. In this talk we will focus on studying single slip during room temperature deformation of a Ni superalloy and explore the extent which we can access the deformation gradient, F, in its entirety and uncouple contributions from elastic strain gradients, slip and rigid body rotations. This clear demonstration outlines the wonderful capabilities of these techniques within our experimental toolbox to underpin fundamental mechanistic studies of deformation in polycrystalline materials and the role of microstructure, utilising full field measurement of components of F successfully and combining these with high fidelity modelling tools, necessary to underpin modelling real materials in two and three dimensions across a range of time and length scales.

Defect-free crystalline nanostructures reach strengths which are close to their ultimate shear strength since their deformation is controlled by dislocation nucleation at the surfaces. In this talk we examine how the size, shape and crystal structure effect dislocation nucleation-controlled plasticity. Earlier compression experiments of Au nanoparticles showed that they become stronger as they are smaller [1]. With large scale Molecular Dynamics (MD) simulations and Finite-Elements Analysis we show that the size effect arises from a size-dependent dislocation nucleation threshold at the nanoparticle&’s vertices. This model is generalized and a dislocation nucleation model is developed to study the size-dependent stresses at which FCC nanoparticles yield under compression. We discuss how the result depends on material properties, such as the stacking fault energy and elastic constants. MD simulation of Ni₃Al intermetallic nancubes under compression were also perofrmed. An analysis of the dislocation evolution in Ni₃Al nanocubes under compression shows that partial dislocations are nucleated at the vertices, shearing the nanoparticle with large complex stacking faults planes. Based upon these results, we propose that size effect in strength is suppressed in Ni₃Al nanocubes under compression since the stress concentration vanishes in this geometry. In Fe nanoparticles and nanowires (BCC structure) dislocations also nucleate at stress concentrators. However, dislocation pile-ups are formed and the strength is increasing with further dislocation nucleation. In result, smaller specimens are still stronger, but with a different size dependency than FCC nanoparticles. A dislocation-nucleation model is developed to explain this size dependency. The combined computational-experimental study provides us with insights on how to dislocation nucleation-controlled deformation can be tailored at the nanoscale.
[1] D. Mordehai, S-W. Lee, B. Backes, D.J. Srolovitz, W.D. Nix and E. Rabkin. Acta Mater. 59, 5202-5215 (2011).

The perfect graphene is believed to be the strongest material. However, the useful strength of large-area graphene with engineering relevance is determined by its fracture toughness, rather than the intrinsic strength that governs the uniform breaking of atomic bonds in perfect graphene. Here, we report the in situ tensile testing of suspended graphene using a nanomechanical device to measure the fracture toughness of graphene. During tensile loading, the cracked graphene samples fracture at a breaking stress substantially lower than the intrinsic strength of graphene. Our combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Also in this talk, we report the atomic structure and morphology of the grains and their boundaries in the CVD grown polycrystalline molybdenum disulfide atomic layers. The implications of the effects of defects such as grain boundaries on mechanical properties of two-dimensional atomic layers will be discussed.

Nanostructured materials are of interest for both fundamental scientific and applied research which can be ascribed to their outstanding mechanical properties, e.g. single crystalline Au nanowires can exhibit high strength (of the order of the ideal strength) and ductility. Recent studies point to the important role of defects for the strength of such nanowires. In order to develop a profound understanding of the role of defects for the deformation mechanisms of single crystalline Au nanowires we make use of a helium ion microscope to alter their structure on the nano-scale. It offers the possibility to modify nanostructures by altering the surface characteristics or creating defects, such as notches or holes, at much smaller length scales than a conventional focused ion beam employing Ga+ ions.
In-situ tensile tests of modified Au nanowires containing notches with different geometries and sizes were performed inside a scanning electron microscope. The experimental findings and their interpretation are supported by molecular dynamics simulations. The measured strength of the nanowires is not changed for notch sizes in the range of 25 nm and below pointing to a notch insensitive strength. Nevertheless, plastic deformation is initiated by the nucleation of Shockley partial dislocations at the notch. Upon further straining the plastic deformation zone extends over the whole length of the nanowire and is not localized at the notch resulting in large plastic strains of more than 10%. At this stage the nucleation, propagation and interaction of further partial dislocations results in the formation of unique defect morphologies, i.e. the nucleation of leading and trailing partial dislocations and the formation of non-twin boundaries. Final fracture of all nanowires occurred at the notches. Our experiments indicate that strain hardening can compensate for the local stress concentrations at the notch as is typical of bulk metals. Thus, strain hardening leads to stable plastic flow with extended strain-to-failure and promotes the notch insensitive ductility even at the nano-scale.

Heterostructures of two-dimensional materials, such as hexagonal boron nitride (h-BN) and graphene, are of increasing interest for nanoelectronics applications. As in conventional three-dimensional epitaxy, lattice mismatch strain in the h-BN/graphene system may be relieved by formation of interfacial misfit dislocations. Previous theoretical and experimental studies show that the precise structure of the interfacial defects, which is important for potential device applications, is sensitive to the nature of the remote boundary conditions, the potential for out-of-plane strain relaxation, and other details including curvature and atomic steps at the interface. Here we examine the full range of possible interfacial misfit dislocations in h-BN/graphene using a novel minima-hopping energy minimization technique. The method maintains complete generality with respect to BN stoichiometry at the interface, allowing for the study of localized core structures, delocalized and split core structures (i.e. partial dislocations), separation of the core from the interface, and partial relaxation of strain in the neighborhood of other atomistic features such as interfacial steps and arbitrarily curved interfaces. The technique is demonstrated with both empirical interatomic force fields and density functional theory energy calculations, and the results are shown to compare well with experimental data and other recent computational results available in the literature.

Bendable membranes (with bending rigidity comparable to their thermal energy) exhibit entropic effects in the form of out of plane fluctuations that bring out exotic mechanical properties such as size dependent elastic constant and negative thermal expansion coefficient [1].Graphene is the nature&’s thinnest elastic membrane. It is highly bendable, stiff and anharmonic. Therefore the above mentioned phenomena should apply to it.
In this work we measure, by means of indentation experiments, the dependence of the elastic modulus of graphene both as a function of controlled induced defects [2] and a as a function of external strain [3]. Our results support renormalization of the elastic constants of graphene at room temperature.
We experimentally observe that graphene stiffens up to the double of its initial value when low densities of carbon vacancies are induced. It also presents a substantial increase in Young's modulus at high external strains. We attribute these observations to the suppression of out of plane oscillations both by defects and strain.
References
W. Helfrich, Z. Naturforsch. 28, 693 (1973)
G. Lopez-Polin et al., Nature Physics 11, 26-31 (2015)
arXiv:1504.05521v1

Si nanowires exhibit electronic, photonic and even mechanical properties that can differ markedly from bulk Si and it is therefore interesting to examine if pressure-induced phased transformations in nanowires are also different to the bulk. When bulk Si is subjected to a confining pressure exceeding about 11 GPa, it undergoes a diamond cubic (dc) to metallic (β-Sn) transformation, and increasing pressure progressively transforms the material into increasingly denser metallic phases. On unloading, kinetic barriers inhibit a return to the dc phase and metastable rhombohedral (r8) and body centred cubic (bc8) Si phases result. Previously it has been shown that Si nanoparticles undergo the metallic transformation at significantly higher pressures than bulk Si whereas the few studies on nanowires are somewhat contradictory, variously reporting bulk-like behaviour under pressure or supressed transformations.
We have investigated VLS-grown Si nanowires of 80-150 nm diameter under pressure in a diamond anvil cell (DAC). In one set of experiments dispersed Si nanowires in a methanol-ethanol pressure medium were loaded into a DAC and Raman spectroscopy was used to monitor the pressure-induced changes with pressure. In a second set, a Ne gas pressure medium was used and phase changes monitored by in-situ X-ray diffraction (XRD) using Synchrotron radiation. Raman indicated lattice compaction of dc-Si with increasing pressure and a considerable suppression of the dc to metallic transformation was observed, whereby a significant portion of dc-Si remained at a pressure of 17 GPa, which was only fully transformed by 22 GPa. On unloading a clear (β-Sn)-Si Raman signal appeared at about 15 GPa and persisted until around 7 GPa, beyond which a transformation to amorphous Si (a-Si) occurred rather than to the r8 and bc8 phases that are characteristic of bulk Si. The XRD data showed a similar trend but the metallic phases were more easily identified. By 15 GPa some of the dc-Si had converted to the β-Sn phase but by 17 GPa much less dc-Si remained and the only metallic phases were sh-Si and possibly Imma-Si. On unloading, below 16 GPa the sh and possible Imma phases converted to the β-Sn phase which persisted to pressures as low as 8 GPa. Further unloading appeared to result in a-Si. Taken together the Raman and XRD data strongly suggest that the onset of phase transformations (from dc-Si to metallic Si as well as metallic Si to a-Si) are strongly retarded for nanowires in the range 80-150 nm. These results are consistent with the suggestion that it is progressively more difficult to nucleate new Si phases on both loading and unloading as the nanowire diameter is reduced.

Due to the size effect, brittle materials such as silicon (Si) could become quite flexible at the nanoscale. Here we further show that high quality VLS-grown single crystalline Si nanowires (with diameters around 100 nm) can be consistently stretched to above 10% elastic strain at room temperature, making them excellent candidates for flexible electronics, epidermal electronics, and many nano-bio interface applications. The ability to achieve such “ultra-strength” (approaching to the theoretical strength due to the extremely large elastic strain) was believed to be associated with the defect-scare single crystalline structure and surfaces, as well as the uniaxial tensile loading geometry. And we ruled out the electron beam effect, a common challenge for many existing nanoscale characterizations via in situ SEM/TEM, by performing additional measurements under an optical microscope in ambient environment. This result may indicates that other semiconductor nanowires could have similar large lattice strains, making their band-gap structures tunable for promising elastic strain engineering (ESE) applications.

Liquid phase exfoliation is a scalable manufacturing route for 2-dimensional materials such as graphene and transition metal dichalcogenides. However, the selection of appropriate solvents and surfactants used in during production is typically based upon empirical experimentation or simple solvation models. We have explored how nanoindentation of graphite immersed in a variety of solvents and surfactants can be used to identify appropriate exfoliation conditions and the underlying mechanism. This could potentially provide a quick and quantitative method for selecting solvents/surfactants for use in liquid phase exfoliation.
Highly oriented pyrolytic graphite (HOPG) was immersed in a variety of solvents and surfactants and then nanoindented. The load required for ‘pop-ins&’ to occur was found to depend on the solvent or surfactant used; the pop-in loads lower were found to be lower in the presence of solvents that are known promote exfoliation than the pop-in loads found when no solvent was used. The pop-ins are related to delamination of the basal plane around the indentation site and the associated stress-strain curves show evidence of kinking based deformation. SEM images confirm delamination along the graphite basal plane occurred as well as the formation of kink boundaries. The morphology of these pop-ins may give insight into the nature of the graphite-solvent or graphite-surfactant interactions. In order to recreate and study individual fracture events of the type that will happen in the bulk during the liquid phase exfoliation of graphite to graphene, individual graphite crystals were used in place of the HOPG. Again, variations in the stresses required for pop-ins to occur are seen between different solvents/surfactants as well as variations between different graphite powders tested in the same solvent. This may allow for the characterisation of graphite powders, in terms of how easily they can be exfoliated, using this technique.

We report on a computational study of the fracture properties of polycrystalline graphene (PCG) models upon tensile loading. Starting from an initial model generated with a liquid quench molecular dynamics simulation [1], a series of models was generated by a 25 ns long molecular dynamics simulation at 4000 K, during which the structure clearly evolves towards graphene (decrease of energy and hetero-ring fraction, increase of apparent grain sizes, etc...).
Simulations of the fracture properties, using the long range, cutoff free, SED-REBO potential [2], at different stages of the annealing process, unravel an unexpected somewhat paradoxical behavior. While the Young's modulus slightly increases with annealing time towards the values characteristic of graphene, fracture strain, stress and energy consistently decrease. A close look at the systems close to fracture reveals the microscopic origin of this paradox, often termed "pseudo Hall-Petch effect"[3-4]. At low annealing time, when the system remains highly disordered, several single-bond cracks can nucleate without fracture propagation. Conversely, in more ordered systems (high annealing time) the load transfer following a bond rupture is more local and quickly activates fracture propagation.
The evaluation
[1] Leyssale et al., App. Phys. Lett.95, 231912 (2009).
[2] Perriot et al., Phys. Rev. B88, 064101 (2013).
[3] Song et al. Nano Lett.13, 1829 (2013).
[4] Sha et al. Sci Rep.4, 5991 (2014).

Two-dimensional (2D) materials have been envisioned as wonder materials not only because of their fantastic properties, but also due to their unique role as model materials for fundamental research of 2D physics. In this talk, I will present the topological and geometrical effects of defects in modulating the structures and mechanical responses of 2D materials. We conclude that as the dimension is reduced, imperfections become vital for the material behaviors compared to those in the bulk. These effects have further implications in modulating the growth and assembly of graphene single-crystal domains into continuum films. The constructive aspect of defects in engineering 2D materials with designed pre-stress and geometry will also be discussed.

Imprinting or mechanical forming is an emerging technology for the fabrication of micron- to nano-scale metallic structures. While so far mainly polymeric materials have been considered for imprinting, the direct structuring of metals at the nanometer scale by stamping techniques has received only little attention. The potential of large-scale direct structuring of metals is obvious, though, since in principle feature sizes in the 10 nm range can be achieved, while classic lithography techniques reach their limits in the range of 100 nm.
In principle, imprinting is a simple process. A rigid mold is brought into contact with the sample and is then loaded with a compressive force for a certain time. But this simplicity is more apparent than real. There are several technological challenges regarding the imprinting process, such as the mold-sample alignment. Also the more fundamental questions of the mechanisms of deformation during imprinting need to be addressed. Since, for example, the yield strength and strain hardening of a metal depend on its microstructure, the size and quality of the patterned structures will be affected by the materials chosen.
On the laboratory scale, a nanoindenter equipped with a structured flat punch tip can be used to study the deformation mechanisms during imprinting. In our study, patterns were created in bulk copper and copper thin films of different thicknesses on Si substrates. The resulting pattern shape was analyzed using atomic force microscopy along with scanning electron and focused ion beam microscopy techniques. In this presentation, the applicability of the imprinting method for the direct mechanical forming of micro- and nanoscale metallic structures together with drawbacks and limitations will be discussed.

Nanoimprinting techniques are widely used in polymers, for achieving nanostructured surfaces by a simple imprinting process. The concept is applied here to metallic materials, which provide new challenges concerning the plastic flow of individual grains in micron and submicron sized cavities. A multiple length scale approach is employed in the experimental setup, where the grain size is systematically varied from micrometers to nanometers. The concept is based on the extraordinary properties of nanocrystalline and ultrafine-grained metals, combining both high strength and ductility. Coupled with the small grain size, these metals show promise of achieving excellent formability of extruded surface structures at small length scales.
Using a Nanoindenter with a flat punch tip, with a forming cavity, the flow of crystalline materials in submicron sized cavities was studied. The microstructure after imprinting was investigated in detail by FIB cross-sections and electron back scatter diffraction (EBSD), as well as by using finite element analysis (FEA) of the forming process. SX-Ni showed the smallest extrusion height together with a sinking-in of the formed region. This is accompanied by strong orientation gradients in the deformed volume. The UFG samples exhibited the best formability, with a subgrain formation inside and around the cavities. The plastic flow is confined to the surface and a pile-up formation occurs. For the nanocrystalline material only a slight elongation of the grains inside the cavity was found, yielding moreover a smooth and homogeneous extruded geometry. These findings can be explained by the grain size / cavity width ratio as well as the yield strength and the work hardening behaviour of the materials.

The phenomenon of thin film wrinkling on micro/nanometer scale has stimulated tremendous interest and many applications such as flexible electronics, nanofluidic channels, optical gratings, film modulus measurement, cell mechanic study, etc. Most wrinkling research to date are limited in 2D configurations. The formation of waviness due to thin film buckling on a planar surface is reasonably well understood. However, wrinkling formation on 3D surfaces is not widely studied. Most of the pioneering work on using wrinkling to create 3D patterns are theoretical modeling and few experimental studies demonstrating the 3D wrinkle formation have been reported. The majority of the reported 3D wrinkle research requires complex material or complicated structure preparations that are not easily achievable using conventional micro/nanofabrication methods.
Here we report an experimental method to create 3D wrinkles using a very simple strategy, with samples easily constructed using existing micro/nanofabrication techniques. PDMS micro ridges were prepared from replica-molding using a KOH-etched silicon mold. Then PDMS micro ridges were prestrained and deposited with Cr film. After Cr film deposition, prestrain on the PDMS sample was removed and 3D buckling was observed on the PDMS micro ridges due to buckling of the Cr film.
Our experimental results show that, because of the wrinkling occurs on two different planes intersecting along a ridge, the wrinkling patterns on the two planes coordinate with each other, forming two individual wrinkling waves with phase opposite to each other. The 3D structure also seems to constrain the wrinkling patterns more, leading to a consistently smaller wrinkling wavelength compared to the 2D wrinkling scenario.
Compared to 2D wrinkling, 3D wrinkling can provide additional advantages. First, 3D wrinkling can push the wrinkling wavelength to smaller dimensions therefor enable smaller resolution nanofabrication. Second, 3D wrinkling can increase the ratio of surface area to volume much further compared to 2D case, which can benefit many applications that requires large surface to volume ratio such as battery electrode, filters, catalyst, etc. Third, most biological applications require 3D environment, exploring the construction of 3D structures are very meaningful. Last but not the least, 3D wrinkling can be applied to create micro/nano wrinkles on the sidewall of the micro/nano pillars to increase water compelling capability for superhydrophobic applications.

Nanochannels can effectively be used for manipulating nanomaterials, such as DNA, proteins, and nanoparticles. Nanolithography-based methods have been widely used for the fabrication of nanochannels owing to their excellent patterning resolution, reproducibility, and flexibility. However, they often involve multistep processes with costly equipment and expertise, and therefore, are not readily accessible. Here, we present a facile method for fabricating nanochannels that exploits mechanical wrinkling/folding of a stiff skin formed on a soft polymer. A flat PDMS substrate is uniaxially stretched and treated with oxygen plasma for varying durations, resulting in formation of a stiff oxidized skin of varying thickness on the surface. Thereafter, the stretch is released to induce a compressive strain in the skin. At small strain levels, wrinkles appear on the substrate surface, forming open channels with a specific wavelength and amplitude. Further increase in the strain level triggers the transition from wrinkles to folds, creating well-defined closed channels with a diameter in the nanoscale range. The characteristic dimensions of the channels such as wavelength, amplitude, and diameter are tunable simply by changing the duration of oxygen plasma treatment and strain level. Furthermore, the configuration of the channels is switchable between open and closed by controlling the strain level. The tunable nanochannels allow for “on demand” trapping and release of gold nanoparticles, which could be used to construct a 3D platform for specific biomedical applications.

One of the great challenges of science is the manipulation of materials at the nanometer scale to achieve tailored properties. An effective path forward is conducting biomaterials research to adapt biological structures and drive novel design techniques. One promising example is found in ubiquitous diatoms, which are microscopic algae with intricate porous shell morphologies and features ranging from the micrometer to the nanometer scale. Diatom shells are viable examples for nanotemplates, drug delivery carriers, optical devices, and microfluidic systems. In some diatom species it is typical to find shells naturally deformed in patterns suggestive of structural instability phenomena. This occurrence opens the door to design and manufacturing techniques based on the potential micromanipulation of bio-inspired structures. An experimental study of instability in diatom shells would be too costly and cumbersome to be practical, hence the need for complementary methods that can drive novel design and fabrication techniques. In this work, the mechanical instability of diatom shells was investigated using the Finite Element Method (FEM) in combination with morphology and material properties obtained using high-resolution Scanning Electron Microscopy (SEM) and independent mechanical tests. The SEM images have allowed the classification of a series of deformation patterns observed frequently on the shells of centric diatom species, with specific attention given to Coscinodiscus sp. In order to elucidate the nature of these deformations, a three-dimensional diatom shell CAD model of a centric diatom species was created. The geometry was simplified to a single porous silica layer (instead of the hierarchical array found in nature) with nanopores to reduce the computational cost. This simplified domain was discretized using shear-deformable shell finite elements. Using this shell model, linear buckling and modal analyses of the diatom shell structure were performed in an attempt to correlate the deformation modes obtained with the geometries observed experimentally. These studies have led to conclude that these biostructures experience mechanical instability naturally. More specifically, it has been determined that these shell deformations are dynamic in nature and consistent with current theories on diatom morphogenesis. Additionally, FEM models were used to study the relation between diatom morphology and the onset of instability. In this diatom species, a clear quadratic correlation was found between the size of the nanopores and the critical buckling load (i.e. the onset of instability phenomena) as well as between the shell thickness and the critical buckling load. This research contributes to improving understanding of the mechanical response of biomaterials and represents a step toward innovative design and manufacturing process of bio-inspired microstructures.

3D printing is an additive manufacturing technique, wherein three-dimensional objects are created layer-by-layer with minimal material wastage. 3D printing is capable of creating complex, highly customized and net-shaped structures that are otherwise difficult or impossible to produce using conventional methods such as injection molding. Fused Deposition Modeling (FDM) is one of the most commonly used methods for 3D printing. It is based on micro-extrusion of thermoplastics through a nozzle held at elevated temperatures. In terms of materials, amorphous grade polylactic acid (PLA) is widely utilized in FDM because of its thermal properties and consequently ease of use during printing. In this presentation, we will report the incorporation of carbon nanotubes (CNTs) into PLA with a goal to accentuate the anisotropy in the mechanical and electrical properties of the 3D-printed composites. The orientation distribution of CNTs within the printed filaments has been characterized using both X-ray Diffraction (XRD) and scanning electron microscopy (SEM) after vibratome sectioning. The effects of infill direction and process conditions, including printing speeds and extrusion rates, on CNT orientation and the subsequent mechanical and dielectric properties of dog-bone-shaped coupons have been explored and modeled.

Understanding post-irradiation changes in mechanical properties is key in the development of advanced materials for future nuclear reactors (up to 100 displacements per atom as opposed 1-10dpa). Ion irradiations can be completed more cheaply and rapidly than neutron irradiation, accelerating material development. Nanoindentation is one of the key tools in understanding the nanoscale deformation of engineering materials. Recent developments in instrument design allows for the testing of materials from sub ambient temperatures to close to 1000#730;C. This now allows the testing of materials for nuclear applications at reactor relevant temperatures. In this work both nanoindentation and micro-cantilever bend experiments performed from -40#730;C to 950#730;C are used to study the plastic deformation of refractory metals.
The plastic deformation of single crystal molybdenum was studied was both nanoindentation and micro-cantilever bend experiments form -30#730;C to 950#730;C. The hardness was seen to rapidly decrease from -30 #730;C to asymp;150 #730;C before showing a reduction in rate of change from 150 #730;C to 950 #730;C. This is in good agreement with literature data. Micro-cantilever tests performed across a similar temperature range showed similar trend in yield stress to that seen in hardness values.
To understand the nano-mechanical properties of tungsten implanted with helium ions. Tests were carried out from 21#730;C to 750#730;C using a high temperature vacuum nanoindenter. The hardness of pure tungsten decreases strongly with increasing temperature, from ~6 GPa at 50#730;C to ~3 GPa at 250#730;C, after which it remains constant. He+ implantation to levels of 600 appm produces an increase in hardness of ~4 GPa at 50#730;C which decreases to ~2 GPa by 750#730;C. This decrease in hardness with temperature is thought to be due to the increased mobility of dislocations to bypass helium-vacancy clusters which cause the hardening effect.
In both the tungsten and molybdenum experiments drift rates at high temperature were at a similar level to those performed at room temperature and no oxidation of samples were observed. This demonstrates that high temperature in vacuum nanoindentation can be used to study reactive nuclear materials at reactor relevant temperatures.

Metamaterials derive unique optical properties from artificial structuring of plasmonic materials, while auxetics consist of artificial reconfigurable structures causing them to exhibit unique mechanical properties. In particular, auxetics possess a negative Poisson&’s ratio - that is a unique mechanical property causing them to expand laterally upon being stretched. Here we merge the fields of plasmonic metamaterials and auxetics by creating novel nanomaterials that simultaneously exhibit optical properties of metamaterials and mechanical properties of auxetics.
We demonstrate isotropic and anisotropic nanoauxetic metamaterials of about 100 nm thickness. The structures, which have micro- and nanoscale periodicity, have been fabricated by focused ion beam milling from an elastic dielectric membrane supporting a thin film of plasmonic metal. The nanomechanical metamaterials are based on isotropic auxetic stars as well as anisotropic re-entrant honeycomb designs. They exhibit optical resonances in the visible and near infrared parts of the spectrum that are characteristic for plasmonic metamaterials. Actuation using a 100 nm micromanipulator tip confirms lateral expansion upon axial stretching and lateral shrinking upon axial compression with Poisson&’s ratios down to -0.6 for the latter.
Nanoauxetic metamaterials are a novel class of mechanical nanomaterials promising a range of novel optomechanical properties such as tuneable reconfigurable metamaterials and two-dimensional gratings that retain isotropic optical properties (or fixed anisotropy) upon deformation. Careful design and structuring will enable materials with both tuneable optical and mechanical properties that can be chosen at will. The auxetic micro- and nanostructures exhibiting both plasmonic optical resonances and a negative Poisson&’s ratio - that we demonstrate here - are first examples of this.

A chip-size vertically aligned nanowire (NW) resonator arrays (VNRs) device has been fabricated with simple one-step lithography process by using grown self-assembled zinc oxide (ZnO) NW arrays. VNR has cantilever diameter of 50 nm, which breakthroughs smallest resonator record (>100 nm) functioning in atmosphere. A new atomic displacement sensing method by using atomic force microscopy is developed to effectively identify the resonance of NW resonator with diameter 50 nm in atmosphere. Sizeeffect and half-dimensional properties of the NW resonator have been systematically studied. Additionally, VNR has been demonstrated with the ability of detecting nanofilm thickness with subnanometer (<10minus;9 m) resolution.

The titanium carbide Ti₃C₂T_x is a member of the recently discovered family of two-dimensional (2D) materials known as MXenes. Capacitance values as high as 900 F cm^-3 were recently reported for intercalation of protons in Ti₃C₂T_xclays, higher than most carbon materials.[1] However, the intercalation process is poorly understood and in-situ techniques able to probe the cation storage mechanism and its dynamics are needed.
Here, in-situ atomic force microscopy (AFM) was used to monitor the strain developed in delaminated Ti₃C₂T_x electrodes during electrochemical intercalation of various cations. Interestingly, large and reversible volume contraction is measured when Li⁺, Na⁺ and Mg²⁺ ions are intercalated. The relative deformation amplitude strongly depends on the cation radius and electric charge ranging from almost no change to over 15% of shrinkage [2].
A Contact Resonance AFM study in Li+ containing electrolyte evidenced that the variation of the c-lattice parameter is associated with noticeable change of the elastic modulus values. Furthermore, contact resonance frequency mapping revealed that the cation intercalation preferentially occurs at the shallow sites of the MXene flakes.
These results are exciting because they shed light on the cation intercalation mechanism in the 2D structure of the carbide, and show for the first time that the actuation of Ti₃C₂ can be finely controlled by a proper electrolyte selection. Moreover, the dynamics of interactions between cations and the layers can be efficiently probed with an AFM tip. Since a variety of 2D transition metal carbides can be synthesized, MXenes offer the promise of exciting discoveries for electrochemical capacitors and actuators.
The experiments and sample preparation in this work were supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The facilities to perform the experiments were provided by the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
References
[1] M. Ghidiu et al., Conductive Two-Dimensional Titanium Carbide ‘Clay&’ with High Volumetric Capacitance, Nature, 2014, 516, 78-82
[2] J. Come et al., Controlling the Actuation Properties of MXene Paper Electrodes upon Cation Intercalation, Nano Energy, 2015 Accepted.

Recently emerging ultrathin two-dimensional carbon materials, including graphene and graphyne provide potentially game-changing membranes for water filtration. Here, by using multiscale computational modeling, including first-principles-based modeling and coarse-grained molecular dynamics simulations, we discover a changed water behavior at the nanoscale that is significantly distinct from its bulk state as water flows through two-dimensional carbon allotropes. We find that water exhibits a very high viscosity due to the cooperativity of water molecules that enhances the nonbonded H-bond interactions with the dense lattice of carbon structures, which renders flow significantly more viscous, with a resistance that is inversely proportional to the sixth power of the characteristic length of the nanopores. This is in contrast to a constant value as assumed in conventional knowledge. Our findings reveal how water molecules behave drastically different from their bulk state under extreme nanoconfinement conditions. These insights enable us to incorporate the size analysis of particles in variant untreated water into membrane design and we propose the design of more efficient devices with higher filtration throughput and greater mechanical resilience.

Mechanical testing of nanomaterials at the micro-scale provides deeper understanding of nanoscale mechanisms determining various macroscopic behaviors. Although many microscopic mechanical characterization methods have been introduced such as nanoindentation, atomic force microscopy, nano tensile testing, and micro-Raman spectroscopy, their effective strain rates have remained in the slow strain rate regime. Since materials with multiphase or hierarchical structures tend to show a large strain-rate dependency, achieving high strain rate (HSR) capability (up to 10⁹ s^-1) is crucial for studying mechanical properties of such nanomaterials at HSRs.
Since a micro-indenter deforming a specimen at a high speed (100-1,000 m/s) is conceptually nothing but a projectile, a miniaturized ballistic method is naturally considered for the HSR mechanical characterization at the micro-scale. In our approach, a single silica microsphere (< 10 mu;m dia.) is accelerated to a supersonic speed, indirectly powered by laser ablation. The speeding microsphere is employed as a high speed micro-indenter or a micro-impactor applying a localized HSR deformation onto a specimen. The motion of the micro-impactor is photographed by use of synchronized femtosecond pulses at a maximum frame rate equivalent to 80 million frames per second and the ultra-high-speed photograph provides accurate kinetic information of the micro-impactor before and after its impact. In this talk, I will introduce a recently developed micro-ballistic system and some application examples regarding nanostructured polymers,^[1] graphene,^[2] and metals. This demonstration may provide the inspiration for an effective and rapid means for the HSR characterization of various nanomaterials.
[1] J.-H. Lee et al. Nat. Commun.3, 1164 (2012).
[2] J.-H. Lee, P.E. Loya, J. Lou, E.L. Thomas, Science346, 1092 (2014).

A fundamental issue related to comparisons of microstructural and mechanical property changes is the efficiency of different microstructural features at impeding dislocation motion. This efficiency is often called the barrier strength, and has been estimated using a variety of elasticity-based models. Experimentally, it can be related to the dislocation dipole configuration formed when a dislocation attempts to pass though or around a defect. The accuracy of elasticity solutions is particularly limited for small defects (often those of greatest interest) and regions near the dislocation core (which is where the relevant interactions occur). With the advent of modern computers and computational tools, these atomistic interactions can be assessed directly using molecular dynamics simulations. The results of an extensive study the interaction between an edge dislocation and a broad variety of small, typically radiation-induced defects: bubbles, voids, dislocation loops and precipitates, will be described. The critical resolved shear stress is obtained directly from the simulations for each defect size and density. Since the shear modulus and Burgers vector are known, it is possible to calculate values for the obstacle strength which account for the local atomistic interactions involved. The values obtained are suitable for use in the dispersed barrier hardening model and should improve agreement between microstructural and mechanical property measurements

Zirconium is an important metal in nuclear industry owing to its high strength, resistance to corrosion and irradiation, high melting point. Mechanical and thermal properties of nanocrystalline Zirconium thin films as a function of Zr⁺ ion irradiation doses from 0 to 3E14 ions/cm² were investigated using transmission electron microscope (TEM) and infrared microscope, respectively. Results show that Young&’s modulus drops with increasing doses, specifically more than 50% with ion irradiation dose of 3E14 ions/cm². The fracture strain increases from 1.5% to 2.6% at 0 and 3E14 ions/cm², respectively. Thermal conductivity decreases from 20 W/mK at 0 ions/cm² to 13.6 W/mK at 3E14 ions/cm². TEM observation reveals that fringes in the microstructures increase with irradiation doses. We propose that ion irradiation knocks off the atoms in the target material thereby creating more defects and vacancies in the microstructures. This is responsible for the changes of mechanical and thermal properties. This hypothesis will be further confirmed by checking the cross section in TEM.

Transmission electron microscopy, energy-dispersive x-ray spectroscopy, Raman scattering spectroscopy and photoluminescence studies of structural and optical properties of axial silicon-germanium nanowire heterojunctions confirm that despite the 4% lattice mismatch they can be grown without structural defects. The lattice mismatch induced strain is reduced due to spontaneous SiGe intermixing at the Si/Ge heterointerface and lateral expansion of the Ge segment of the nanowire. However, the mismatch in Ge and Si coefficients of thermal expansion is responsible for significant thermally induced stress, which is found under intense laser excitation and explained by bending of the Si/Ge nanowire heterojunctions.

We have developed and demonstrated a technique that enables nanoscale modulation of tunneling gaps through electromechanical compression of molecular thin-films. In this approach, tunneling junctions are formed using few-nanometer-thin molecular films sandwiched between conductive contacts. By applying a voltage between the contacts, a force is provided to mechanically compress the molecular film which reduces the tunneling gap and leads to an exponential increase in the tunneling current. Once the voltage is removed, the molecular film will restore to its uncompressed state and the tunneling gap recovers to its original thickness. By modulating the mechanical properties of the molecular films through chemical synthesis and thin-film assembly, the electromechanical performance of the tunneling junction can be tuned to achieve the desired force control. This mechanism can be utilized as a nanoscale metrological tool to determine the mechanical and electrical properties of the molecular films, while also serving as a unique platform enabling a multitude of novel devices. In particular, we will present in more detail the application of the proposed technique in developing tunneling molecular electromechanical switches. In this design, a metal-molecule-graphene junction is utilized with a 3 nm-thick self-assembled monolayer of poly-ethylene glycol (PEG) forming the switching gap. Electromechanical modulation of a PEG nanogap with few MPa Young&’s modulus leads to about 6 orders of magnitude change in the current conduction with about 50% compression of the film. With operation in the sub-2 V regime and an abrupt switching behavior, these low-voltage devices can overcome the shortcomings of the current electromechanical switches.

The objective of this study is to characterize changes in the strain hardening coefficient of an oxide dispersion strengthened (ODS) alloy due to irradiation. It is well known that irradiation produces a supersaturation of defects, which alters the mechanical properties of a material. In order to engineer materials for use in advanced nuclear reactors, the long-term effects of irradiation on mechanical performance must be understood. However, high-dose neutron irradiation is often simulated using ion bombardment to prevent sample activation and reduce the cost and experiment time. Unfortunately, ion irradiation results in a shallow damage layer that prevents traditional bulk mechanical characterization methods from being utilized. A technique with high surface sensitivity is required to provide insight into the changes in yield stress, elastic modulus, and hardness. Nano-indentation experiments have thus become a powerful tool to analyze surface effects in ion-irradiated materials, but a thorough understanding of the plastic deformation that occurs during nano-indention is required to accurately interpret the results. In this work, a coupled experimental and modeling approach resulted in an understanding of the effects of irradiation on strain hardening in a model 9wt%Cr ODS alloy. Nano-indentation was performed on the alloy before and after irradiation, either with 5.0 MeV Fe⁺⁺ ions to 100 displacements per atom (dpa) at 400°C or with a fast neutron spectrum to 3 dpa at 500° C. The size and shape of the residual plastic zone beneath nano-indents were characterized using transmission electron microscopy coupled with Automated Crystal Orientation Mapping (ACOM-TEM) techniques. A finite element analysis model, using the spherical indenter approximation, was combined with the experimental results to calculate the effective strain hardening coefficient, n, that resulted from irradiation induced defects. Results indicate a 26% increase in strain hardening resulting from both irradiation conditions, but little differentiation between the ion and neutron irradiations.