Symposium MB6 : Cyclic Deformation and Fracture at the Nanoscale

This work investigated the effects of two extreme normalized stress gradients (η = 17 vs 36%.μm^-1) on the fully-reversed bending, high- and very-high-cycle fatigue behavior (fatigue life and fatigue crack propagation curves) of 20-μm-thick, electroplated Ni microbeams (with and without a thin Au coating), in humid air environments. The results highlight the significant challenges in predicting the bending fatigue life of microbeams subjected to extreme stress gradients, which was measured to be three order of magnitudes larger for η = 36%.μm^-1 at a stress amplitude of ~450 MPa. The fatigue life is dominated by the ultraslow growth of microstructurally small cracks, which is a strong function of the normalized stress gradient. For η = 17%.μm^-1, the crack growth rates are initially about one order of magnitude larger than for η = 36%.μm^-1 and, in contrast to the larger stress gradient microbeams, do not decrease with increasing crack size. This singular behavior results in low Basquin and Coffin-Manson exponents (in absolute value) compared to η = 0. As a result, the fatigue endurance limit increases from 35 to 50% of the tensile strength for η increasing from 17 to 36%.μm^-1, compared to 30% in the absence of stress gradients. The environmental effects are discussed based on energy dispersive spectroscopy measurements. The presence of a thin Au coating also results in significantly larger fatigue lives, thanks to a delay in extrusion formation and fatigue crack nucleation in the Ni microbeam.

Fatigue issues of metals at small scales have drawn extensive attention due to not only the long-term service reliability of small-scale metallic components in micro/nano-devices, but also the fundamental fatigue mechanisms of the materials at small scales. It has been demonstrated that the confinement of the length scale on typical dislocation structures (dislocation walls and cells) strongly suppresses cyclic strain localization, leading to the enhancement of fatigue strength. With further decreasing the length scale of the materials down to submicrons and even nanometers, fatigue damage behavior is closely related to the grain boundary behavior. To understand the relationship between the fatigue performance and the grain boundary behavior, in this talk we will present the systematic investigation on fatigue properties and grain growth behavior of the nanocrystalline gold films with a thickness ranging from 20 to 930 nm. In thicker films (≥ 90 nm), abnormal grain growth happened along fatigue cracks and in regions away from the cracks under the high applied load amplitude, while uniform grain growth occurred in the regions away from the cracks under the low applied load amplitude. When decreasing the film thickness to several nanometers, grain growth tended to be locally uniform regardless of the applied load amplitude. The abnormally grown grains were observed to reorient to gains with the twin and low-angle misorientations. The fatigue damage mechanism and the scaling effect of the grain boundary instability of the gold films under cyclic loading were discussed.

Shape memory alloys (SMA) undergo a reversible thermo-elastic martensitic transformation (MT) characterized by a crystallographic shearing of the atomic lattice, which can be thermally stimulated or induced by the application of an external stress. The stress-induced MT is known as superelastic effect, characterized by a complete recovery of the deformation when withdrawing the stress. During this process, a certain amount of the applied mechanical energy is dissipated, giving place to a mechanical damping, which is particularly high at nano-scale.
Nowadays, the current device miniaturization tendency leads to the need for developing micro and nano electro-mechanical systems (MEMS and NEMS) with an optimum reliability even in extremely noisy environments. Then, SMA can be used to damp the mechanical vibrations at nano-scale improving the MEMS cycle of life and their reliability. In order to assess the potential of SMA for its incorporation to MEMS, the superelastic behaviour at nano-scale has been studied during long-term cycling in Cu-Al-Ni micropillars. Several arrays of micropillars were milled by focused ion beam (FIB) technique on [100] oriented Cu-Al-Ni single crystals. All pillars from the array were tested in an instrumented nanoindenter with a 2mm radius sphero-conical diamond indenter at room temperature. The superelastic response was tested over hundreds cycles on all pillars and over several thousands cycles on some randomly selected pillars. The results were analysed from its load-depth curves individually, and comparatively among pillars array. Recoverable and reproducible superelastic behaviour has been obtained during thousands cycling tests on Cu-Al-Ni micro-pillars, and the evolution of characteristic magnitudes during cycling, critical stress and mechanical damping are presented and discussed. These promising results open the door for designing potential applications doing use of 3D dampers of Cu-Al-Ni SMA, which could be integrated in MEMS technology.

Traditional dislocation plasticity is suppressed in nanocrystalline materials, giving way to more collective physical mechanisms such as grain boundary dislocation emission, grain rotation, and grain boundary sliding. In this talk, we present a combination of experiments and simulations that aim to uncover how cyclic plastic deformation (both uniaxial loading and during sliding contact) in these materials changes the network of grain boundaries. New techniques for tracking structural features at the nanoscale, such as transmission Kikuchi diffraction and quantitative analysis of atomistic models, are used to characterize and track mesoscale structural features like grain boundary character distribution and interfacial topology. As a whole, we find that grain structure can evolve to a lower energy state, often characterized by increased numbers of “special” boundaries and a disruption of random boundary percolation paths. Boundary faceting is also a common occurrence. Our results highlight the dynamic nature of nanocrystalline grain structures and provide a better understanding of cyclic plasticity in these materials.

In this fundamental study, the role of grain boundaries and their effect on dislocation motion under cyclic loading is investigated with in situ micro compression experiments. Therefore, micro columns are machined into selected grain boundaries of annealed nickel of high purity. The grain boundaries are selected based on the misorientation measured with electron backscatter diffraction and the inclination of the grain boundary revealed by cross sections. The pillars are cut directly at the grain boundaries and for comparison into the adjacent grains. Different experiments are performed with a variation of the sample geometry to investigate the influence of the grain boundary relative to the bicrystalline volume. The cyclic micro compression tests run to different numbers of cycles or to failure by a major slip event. To trigger dislocation motion the load is increased depending on the progress of displacement within a certain numbers of cycles. After the tests the local strains are calculated by digital image correlation of platinum patterns on the side of the pillar. In combination with the alignments of activated slip bands the results give further insight into the role of grain boundaries and their role in fatigue deformation.

Little experimental work has been carried out to investigate the fundamental mechanical properties of thin metallic films at high strains and cyclic loading. First, experimental results for the yield strength, fracture toughness and fatigue behavior of sputtered Cu and Ta/Cu film systems on polyimide substrates will be presented. The films have been tested by a unique synchrotron-based tensile testing technique. The thinnest films (<100 nm) show more and more brittle behavior and crack formation. The fracture toughness of the Cu films decreases whereas the fatigue lifetime and yield strength increase. This may be attributed to an increasing constraint on dislocation plasticity and may be corroborated by the observation that the strain hardening is strongly reduced for the thinnest films. Furthermore, results of electro-mechanical testing of printed and evaporated Ag films will be presented. Electrical conductivity and mechanical reliability during cyclic bending are investigated with respect to the inherently nanoporous microstructure, and are compared to those of evaporated Ag films of the same thickness. It is shown that there is an optimized nanoporous microstructure for inkjet-printed Ag films, which provides a high conductivity and improved reliability. It is argued that the nanoporous microstructure ensures connectivity within the particle network and at the same time reduces plastic deformation and the formation of fatigue damage.

In-situ nanoindentation and micropillar compression are increasingly used to assess the mechanical properties and deformation behavior of thin films, individual phases and small scale structures. Off late, these techniques have been extended to high temperatures and high strain rates. This presentation will report the development of a novel piezo-based indentation technique that allows accessing extremely high strain rates (>10⁵ s^-1) and high oscillation frequencies (up to 10 kHz). Validation data on standard reference materials like single crystal Si will be presented to demonstrate the stability of the developed instrument, experimental testing protocol and test results.


Micron scale, high cycle fatigue data on nanocrystalline nickel micropillars using this newly developed technique will be presented. Due to high oscillation frequencies, it is now possible to access 1-10 million cycles in a reasonably short span of time. The results will be discussed and compared to existing literature. The convolution of time dependent plasticity in case of nanocrystalline materials in such tests will also be discussed. It is hoped that this study will pave way for routine fatigue tests of metals at the micron scale.

Predicting the fatigue response of structural metals is a long standing challenge. Addressing this challenge requires new experimental approaches, new simulation approaches and research that integrates both. This paper describes the use of a novel in-situ ultrasonic fatigue in scanning electron microscope technique (UFSEM) for characterizing the growth of small fatigue cracks and how they interact with grain scale microstructural features. A companion paper (Panwar and Sundararaghavan) describes a new approach for simulating the influence of grain orientation on crack growth retardation. In this study, the short fatigue crack growth behavior of a hot-rolled rare-earth magnesium alloy WE43 heat treated to produce large grains is investigated using UFSEM. Crystallographic crack propagation paths were selected by initiating and propagating cracks from FiB produced micronotches oriented parallel to basal planes and using the in situ instrumentation to provide high resolution details of crack growth behavior near grain boundaries. The degree of grain boundary misorientation significantly influenced the magnitude of crack growth retardation at grain boundaries and could be quantified using this new approach. This detailed quantitative information has been used to parameterize a new simulation approach. The integration of these two research activities is also an important element of the UM Center for PRedictive Integrated Structural Materials Science (PRISMS).

Attachable and wearable devices are growing interests due to their variety of applications, and evolution of flexible electronics is in present progressive. For the realization of flexible electronics, understanding both mechanical behavior and electrical properties of devices are required. In this talk fatigue behavior of bendable and stretchable devices will be discussed, and improved works will be proposed in reliability perspective.
We first investigated fatigue reliability of metal thin films including the dependence of thickness, microstructure, adhesion layer, and several parametric analysis of deformation. In-situ resistance measurement was accompanied by cyclic deformation to figure out the stage of damage growth in thin films. The reliability of fatigue damage can be improved by introducing nanoscale defects in the film, which prevents crack nucleation and growth. Aspects of fatigue damage growth varied with the deformation mode, which is closely related with methodology.
As for stretchable electronics, pairing the mechanical properties of materials is an important issue. Wide variance of physical properties in integrated materials could induce surface instabilities like wrinkling, buckling, or delamination. By tuning the Poisson’s ratio of substrate, we improved the stretching capability and reversibility of conductors regardless of the kinds of materials. We further developed the mechanically soft conductor for large-scale deformation.

Vertically aligned carbon nanotubes (CNT “forests”), present a scalable architecture for engineering the mechanics and multifunctional properties of lightweight solids. Specifically, the low relative density of CNT forests (~0.1-1% volume fraction) and the outstanding stiffness, strength, and resilience of individual CNTs enables their use as a three-dimensional scaffolding for conformal deposition coatings by atomic layer deposition (ALD). We have studied how this conformal coating of the CNTs enables wide-range tuning of stiffness, strength, and toughness; and specifically find that thin coatings enable extreme recovery (~90%) and energy dissipation, whereas thick coatings impart extreme toughness upon fracture. To leverage the scalable nature of CNT synthesis as a platform for surface engineering, we have invented a strain-engineered technique for shaping CNTs into freeform three-dimensional microstructures. The geometry of these structures can therefore be controlled independently of their mechanical properties. These combined capabilities have enabled us to engineer CNT microstructures as stamps for high-resolution flexoprinting, and as surfaces with unique biomimetic wetting behavior.

Capillary rise infiltration (CaRI), can be used to synthesize dense polymer nanocomposite films with extremely high volume fractions (>50 %) of hard fillers. These films have high hardness and toughness, making them attractive as coatings to improve the surface durability of polymers. In the CaRI process, a bilayer structure consisting of a porous nanoparticle layer on top of a polymer layer is initially fabricated; the composite is then formed by annealing above the T_g of the polymer to infiltrate polymer into the voids of the nanoparticle layer via capillary action. Here, we synthesize polystyrene-titania (particles with diameter of 30 nm) composite films with thicknesses of 2 μm and different degrees of polymer infiltration and characterize the mechanical properties, including reduced modulus, hardness, fracture strength, and fracture toughness. Modulus and hardness are characterized via standard nanoindentation techniques. The fracture strength in bending and fracture toughness measurements were measured via nanoindentation on micro-cantilever beams and micropillars, respectively, that were patterned in the films by focused ion beam milling. The fracture toughness measurements were performed by load-controlled indentation tests into micromachined pillars, an approach similar to recently reported pillar splitting methods. A critical load is measured in the pillar splitting tests and a fracture toughness is calculated based on a finite element analysis. The pillar splitting approach will be discussed. Our results show that the reduced modulus can be tuned between 12.2-22.7 GPa, the hardness between 250-530 MPa and fracture toughness between 0.032-0.25 MPa●m^1/2 through tuning of the processing conditions. In addition to experimental results, the underlying micromechanics will be discussed.

Health care and medical problems are growing interests in modern life, which makes the electronic devices to be attached to the human skin and even implanted to the human body. Electronics should overcome the curvature of human body and endure the repeated movements, requiring materials to be flexible. In this trends, mechanical reliability of devices are becoming more important as well as electrical and thermal reliability, especially focused on repeated deformation or fatigue. Metal interconnects can affect directly to the device performance due to their conducting performance. Previous researches of fatigue behavior of metal thin film on flexible substrate mainly focused on bending and tensile test which are uniform and uniaxial deformation mode. Actually, the deformation of flexible devices is more complicated; multi-axial and non-uniform strain could be developed in real cases. For high-reliable design of flexible device, understanding the fundamentals of twisting fatigue is essential due to their non-uniform strain distribution. As the initial step for the complex deformation, twisting fatigue behavior of thin Cu interconnects were investigated for the first time.
To estimate the dependence of position in the width direction, several Cu interconnects are deposited on the same PI substrate simultaneously. Fatigue cracks were generated and propagated during cyclic twisting, increasing the resistance of interconnects. As the distance of Cu interconnect became far from the rotation axis, fatigue lifetime of Cu interconnects decreased. Cu interconnects on rotation axis did not fail easily, while the interconnects out of the rotation axis failed rapidly as distance from the rotation axis increases . At the rotation axis, only shear stress was dominant, generating vertical and horizontal cracks at the center of the interconnects. However, crack-free area existed at the both edge of the interconnect, maintaining the conduction path. As the interconnects located far from the rotating axis, horizontal cracks were generated more dense. From FEM results, shear stress were same regardless of the position of width direction, while the tensile stress increases linearly from center to edge.
Strain-failure cycles curve is well matched to Coffin-Manson relationship. We confirmed the width-direction position dependence of Cu interconnects on twisting fatigue behavior and analyzed the individual stress component contribution through experiment and finite element method (FEM). This study can provide the design guideline for flexible devices under twisting deformation.

As one candidate of flexible transparent conducting electrodes (TCE), cyclic bending property of silver nanowire (AgNW) film has been examined depending on its crystallinity. AgNW contains many kinds of crystalline defects after the synthesis. Externally forced outer or inner bending motion provides tensile or compressive strain in AgNW body and junction parts. Silver atomic motions under the strain, such as dislocation movement and crack propagation, might be different depending on surrounding atomic defect existence. In this research, crystallinity of AgNW was improved by heat-treatment in the dispersed solution state. Based on XRD (111) FWHM analysis, the degree of crystallinity was improved over 1.25 times than the as-synthesized AgNW. The resultant AgNW film shows considerably low resistance change even after 200,000 cycles. The cyclic bending resistance change behaviors will be explained as a function of tensile strains, thickness of over-coating layer, and under nitrogen/dry air atmospheres. Future technology issues will be discussed in this presentation.

The development of thin film ultrabarrier coatings are necessary to protect sensitive flexible electronics from environmental exposure. Such coatings can be readily made using vacuum deposition methods such as PECVD and often contain inorganic materials such as nitrides or oxides. While most of the attention has been dedicated towards water vapor transport properties of these barrier films, the mechanical reliability and structural durability remain critical aspect for the coatings during flexural deformation. Using onset critical strain as the criterion for defining the limits of performance is not adequate enough as time-dependent processes are not taken into consideration that can occur during any mechanical deformation. In this work, we investigate the subcritical channel crack growth behavior of 250 nm thick PECVD silicon nitride barrier films on polyethylene (PET) substrates at room temperature and 85°C in both humid and dry air environments. Optical and laser scanning confocal microscopy were used for in-situ monitoring of crack growth rates for different applied strains and load conditions. For 250 nm thick SiN films on PET, an onset critical strain of 0.95% was measured while subcritical cracking was still observed at strains as low as 0.5% strain. By testing in N₂, dry air, and humid air environments, a strong environmental effect was observed showing that humidity played a major impact on subcritical crack growth rates. Tests in humid air at 25°C show crack growth rates increasing from ~200 nm/s to 60 um/s for an applied stress intensity factor, K, ranging from 1.0 to 1.4 MPa.m^0.5, below the measured fracture toughness Kc of 1.8 MPa.m^0.5. In contrast, tests in dry air at 25°C show no crack growth at all below the Kc value of 1.4 MPa.m^0.5. Tests in both humid air as well as elevated temperature (85°C) showed that humid and high temperature conditions resulted in an easier initiation and growth of crack than observed at room temperature and in dry air. Modeling results show that the relaxation of the polymer during mechanical testing was not sufficient to explain the crack growth rates observed in the film, suggesting the presence of stress corrosion cracking which is the first observation of this effect in ultrabarrier films. Overall, the results presented show the strong link between humidity, temperature, and subcritical crack growth rates in barrier films. Thus, the limits of deformation of flexible electronics with ultrabarrier films should consider both environmental and loading conditions in order to understand if they are susceptible to damage growth during application.

Numerous functional material applications such as flexible electronics, or microelectronic devices suffer from fracture due to external and internal stresses. Examples which will be discussed in the talk are Cu films on polymer substrates with adhesion promoting Cr layers and Cu-Sn intermetallic layers forming in solder joints. The Cu films on polymer substrates are subjected to tensile testing until fracture occurs and the evolving film stresses in straining and transverse direction are determined with synchrotron XRD. Interestingly, the Cr interlayer induces early fracture of the Cu films. Brittle-like fracture is also observed for the Cu-Sn intermetallics using micro-fracture testing with notched cantilevers. The measured fracture toughness values are as small as for Si. It will be discussed whether simple isotropic linear elastic fracture mechanics can be used to determine fracture toughness in a and c direction of the hexagonal Cu6Sn5 phase.

Over the past few years, miniaturization of devices has led to increasingly complex thin film combinations and geometries. As macroscale tests are not applicable for such small components, miniaturized tests are suggested to study the materials response of the complex arrangements in state of the art and future devices at small length scales. Especially residual stresses can play an important role in thin film systems concerning performance and lifetime, thus have to be fully accounted when determining fracture mechanical quantities.
Here, we concentrate on the influence of local residual stresses on the fracture behaviour of thin films. The materials investigated are sputter deposited Cu-W-Cu and W-Cu-W trilayer systems, with an individual layer thickness of 500 nm, on a Si substrate. Samples are fabricated via cross section polishing and focused ion beam (FIB) milling, and the residual stress depth profiles are determined by means of an improved ion beam layer removal (ILR) method. Here, the stress is calculated from the deflection of a cantilever that changes when parts of the film are removed. Subsequently, fracture experiments parallel and perpendicular to the interface are performed in-situ in the SEM to obtain comprehensive knowledge of the fracture and interface toughness. An accompanying finite element based modelling approach is introduced to determine crack-driving forces in the presence of interfaces and residual stresses.
With our findings we emphasize the importance of elastic and plastic incompatibilities and residual stresses when addressing fracture mechanical quantities of multi-layered thin film systems. Challenges and benefits of our approaches will be discussed

MRS Abstract for MB6 Cyclic Deformation and Fracture at the Nanoscale
W.W. Gerberich, University of Minnesota, E. Hintsala, S. Asif, Hysitron Inc./ U. of Mn

With 5 nm -= 10 nm semiconductor technology on the verge of being essential, what can be learned about defects at even slightly larger scales to assist the computational multi-scale modeling community in decision making. One aspect is the brittleness transition in silicon which can be accessed using in situ TEM at room temperature for bent beams. At thicknesses closer to 1 micron, 3-point bending beams have been evaluated as well from 300K to 873K demonstrating that slow crack growth at moderately elevated temperatures can occur. For B-doped silicon crystals 100nm to 1 micron thick it is shown that dislocation activity starts to occur at intermediate temperatures and some is even implied closer to room temperature. Both manufactured FIB slots with 5 nm tip radii at room temperature nucleate and arrest cracks at relatively low applied stress intensities. Further extension can e followed at elevated temperatures by examining crack arrest using ex-situ thinning for transmission electron microscopy. A minimum in applied stress intensity is identified with constrained plasticity developing a back stress (dislocation shielding). Tentatively, we propose a method for analyzing the observed brittleness transition which occurs near 700K.

As a natural biocomposite, Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness. It is known that the basic building block of conch shell contains a high density of growth twins with average thickness of several nanometres, but their effects on the mechanical properties of the shell remain mysterious. Here we reveal a toughening mechanism governed by nanoscale twins in the conch shell. A combination of in situ fracture experiments inside a transmission electron microscope, large-scale atomistic simulations and finite element modelling show that the twin boundaries can effectively block crack propagation by inducing phase transformation and delocalization of deformation around the crack tip. This mechanism leads to an increase in fracture energy of the basic building block by one order of magnitude, and contributes significantly to that of the overall structure via structural hierarchy.

Micro-scale mechanical testing has become a popular way to measure the strength of materials and to identify their deformation mechanisms. To date, most studies have been limited to monotonic loading (i.e., bulk indentation, compression, tension, or bending). Although the cyclic response of materials are of great importance, micro-scale fatigue tests to-date have been limited in the number of loading cycles that can be completed in a practical amount of time. Here, we present a novel in situ, high-cycle fatigue testing methodology using a combination of Focused Ion Beam (FIB) fabrication and nanoindentation. The cyclic loading is imposed by using high frequency actuator dynamics. The amplitude and frequency of the oscillating force can be optimized to the desired values. Utilizing this methodology, we conduct a systematic study on the effect of crystal size on the fracture and fatigue life of microcantilever nickel-based superalloys under different loading conditions and crystallographic orientations. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam’s dynamic stiffness and SEM imaging.

With micro-cantilever bend samples prepared by focused ion beam milling it is possible to measure the fracture toughness of extremely small samples. This method has proven to be quite successful especially on brittle materials as for example NiAl, W, TiAl alloys and coatings.
Extending the applicability to more ductile materials requires the use of concepts from elastic-plastic fracture mechanics. Therefore, a new method based on the J-Integral concept is shown which allows the continuous recording of crack resistance curves by measuring the stiffness of the micro-cantilevers continuously with a nanoindenter. The experimental procedure allows the determination of the fracture toughness directly at the onset of stable crack growth. This method has been evaluated on stoichiometric NiAl single crystals and NiAl crystals containing 0.14 wt% Fe in the so called hard orientation, where stable crack propagation is observed accompanied by plastic deformation. The fracture toughness was calculated to be 7.8 ± 0.2 MPa m^1/2 for the stoichiometric sample and 8.5 ± 0.1 MPa m^1/2 for the iron containing sample. These results are in quite good agreement with macroscopic fracture toughness measurements although the plastic zone size in these micro-cantilevers is quite large or even bigger than the specimen size. The reason why the fracture toughness results obtained from these tests nevertheless correspond very well with macroscopic tests is discussed. Since here always single crystals are tested not a full plastic zone surrounding the crack tip is formed. Instead only slip on individual slip systems is initiated and therefore the plastic zone is confined to these slip planes. This probably leads to a way weaker influence of the plastic zone on the crack tip and reduces shielding effects. The limits for the specimen size given in the ASTM standard on fracture toughness testing are not valid anymore. Therefore, the micro-cantilever approach is quite interesting for many material systems and can be used also on many multiphase materials to evaluate the fracture properties of individual small phases, which are typically also of single crystalline nature.

Graphene has extraordinary mechanical properties like high strength and elongation which make graphene enlarge the various applications, such as structural composites, multiphysical sensors, and flexible devices [1,2]. Several researches are performed to obtain fracture toughness of polycrystalline graphene grown by chemical vapor deposition process [3, 4]. However, fracture behavior of pristine graphene obtained from natural graphite is not fully understood until now. In addition, graphene is brittle material, and stress and strain relation of graphene shows nonlinear elastic behavior and large failure strain [2]. So, fracture behavior should be investigated under the consideration not of elastic fracture mechanics but of the nonlinear elastic fracture mechanics. In this study, pristine graphene with a center notch is tensioned in scanning electron microscope, and the results are analyzed with simulations to obtain fracture characteristics of atomically thin graphene.
Pristine graphene samples are exfoliated from graphite, and transferred onto silicon wafer with a silicon dioxide layer of 400 nm thickness using so-called scotch tape method [1]. After a graphene flake with strip shape is identified with an optical microscope, the graphene specimen is transferred on tensile testing jigs with fine alignments. To investigate the fracture behavior of graphene, a straight and narrow notch is fabricated at the center of the specimen with focused ion beam. Crystal structure and orientation of the specimen are analyzed with transmission electron microscope. Also, the number of the graphene specimen is measured by Raman spectroscopy. The graphene specimen with the center notch is tensioned with tensile apparatus in scanning electron microscope. From in-situ tensile test, applied load and displacement are measured by high precision sensors. Finite element analysis is performed to estimate Young’s modulus and stress distribution near the notch of the graphene specimen based on load-displacement curves obtained by the in-situ tensile test. Also, we investigate the crack propagation mechanism of graphene with high resolution scanning electron microscope image. Finally, we discuss about fracture behavior of pristine graphene based on experimental and numerical results.

References

[1] Novoselov, K. S. et al. Science 306, 666-669 (2004).
[2] Lee, C., Wei, X., Kysar, J. W. & Hone, J. Science 321, 385-388 (2008).
[3] Hwangbo, Y. et al. Sci. Rep. 4, 4439 (2014).
[4] Zhang, P. et al. Nat. Commun. 5, 3782 (2014)

Thin films possess usually depth gradients of microstructure, residual stress and mechanical properties. Currently, it is not trivial to characterize those gradients at the nanoscale and reveal their correlation to local as well as overall film functional properties. The aim of this contribution is to discuss experimental results from nanocrystalline thin films which cross-sectional properties were studied using X-ray nanodiffraction and electron microscopy.
Cross-sectional X-ray diffraction, using monochromatic point and pencil X-ray beams with diameter or thickness down to 30 nm, provides representative depth-resolved data on the evolution of phases, texture, crystallite sizes and the first-order stresses across thin film cross-sections. Results obtained at beamlines ID13 of ESRF (Grenoble) and P03 of PETRA III (Hamburg) will be used to demonstrate the complex nature of intrinsic gradients in nanocrystalline CrN, TiN and TiAlN. The gradients will be correlated with the film deposition conditions, providing an opportunity to optimize the synthesis. Additionally, results from strain and microstructure characterization (i) in in-situ indented TiN and (ii) in multilayered CrN-Cr thin films after wedge indentation will be shown, revealing the correlation between the present structural gradients and measured load-displacement curves.
Similar, results from in-situ bending experiments performed in scanning and transmission electron microscopes will be used to correlate load-deflection curves obtained from gradient thin films or particular thin film regions with films cross-sectional microstructure and strain gradients.
Finally, the presented cross-sectional approaches will be used to indicate the possibilities for functional optimization of thin films through cross-sectional design.
This work was supported by the EU Project iSTRESS.

Under stress amplitude that is lower than nominal yield stress, the loading and unloading curves of materials with and without internal mobile defects will overlap with each other and form a loop, respectively given the instrument used having enough high resolution. The area enclosed by the loop can directly reflect the defects state of the tested materials. In this work, we demonstrate that the loading and unloading curves can be used to diagnose the state of the defects inside the tested samples. In addition, we demonstrate a mechanical healing phenomenon, i.e., when submicron-sized single crystal aluminum samples are subjected to low amplitude cyclic straining, the density of those pre-existing dislocations can be dramatically reduced with virtually no change of the overall sample geometry. This is at odds with traditional wisdom that when a metal is subjected to cyclic loading, defects are prone to accumulate progressively, leading to crack initiation and even failure. In situ transmission electron microscope (TEM) tensile tests reveal that dislocation lines behave very different in response to the external applied stress. In addition, samples experienced various degree of mechanical healing exhibit different mechanical behaviors in strain-to-failure test. Our findings are expected to find applications in submicron sized devices, as their property and performance can now be optimized by mechanically tuning the defect density in a controllable manner (Wang ZJ et al, PNAS, 2015).

Recent advances in local strain mapping using nanobeam electron diffraction (NBED) has demonstrated the ability to observe single defects and the strain fields around them at a resolution of single nanometers. In addition to measuring the strength of small-volumes, measuring the evolution of strain during plastic deformation is of great importance for correlating the defect structure with material properties. By observing dislocations, their strain fields, their movement under stress, as well as their interactions with each other and precipitates, we aim to provide insight into fundamental mechanisms of deformation in metals. This work will highlight our latest results from in situ strain mapping in an Al-Mg alloy and stainless steel using both contact loading methods (such as in situ nanopillar compression and nanoindentation) as well as non-contact methods such as tensile straining. Our method of local strain mapping consists of recording large multidimensional data sets of nanodiffraction patterns during the test. The resulting dataset contains diffraction data for every point of the STEM image, from which strain maps can be extrapolated on a scale not previously possible during in situ deformation.

We present direct observations on the incipient plasticity of dislocation-free single crystal Au [110] nanowires by in situ transmission electron microscopy nanomechanical testing. The diameter of the tested nanowires was in the range of ~80 nm-300 nm and the length-to-diameter ratio was larger than 5. The top end of the Au nanowires is bound by two inclined {111} faces in a wedge shape, on the other hand the side faces consist of four large {111} and two small {100} planes, resulting in a truncated rhombic cross-section. In our deformation setup where the wedge-shaped growth end of nanowire was compressed with a flat diamond punch, the strain becomes localized to the region under the contact. Under such a strong strain gradient condition, the initial compressive deformation began with the successive emission of prismatic dislocation loops from the top corner. Direct observations revealed that prismatic dislocation loops are formed by cross-slip mechanism; a single dislocation loop wrapping itself around a glide prism by multiple cross slip and finally reacting with itself. Not only closed prismatic loops, but also helical prismatic loops or half prismatic loops were generated when the both ends of glide loops missed each other at the final step of cross slip. The diameter of the loops was around 20 nm depending on contact area, and the Burgers vector was determined to be a/2[-1-10], which generates the vertical downward displacement of the inner area encompassed by the prismatic loops, so that it can be regarded as geometrically necessary dislocations. Detailed atomic-scale nucleation mechanism was studied by complementary MD simulations, showing that formation of stair-rod dislocations prevents further expansion of glide loop and thus promots cross slip. Right after the nucleation, these prismatic loops glided immediately down to reach a certain position where it remained stationary until newly generated loops force to glide downward in jerky manner. Once a certain number of loops were punched out (usually less than ten), they are coaxially aligned along the growth direction of the nanowire with preserving an equilibrium spacing between loops, which is determined by the stress fields of loops. Assuming the equi-sized loops, the lattice friction stress can be estimated from the inter-loop spacing, which is ~0.3 MPa. Since the nucleated loops continuously glided downward without direct interaction with other dislocations, it seems that prismatic dislocations, when confined in a small dislocation-free volume, do not necessarily attribute to strain hardening unlike geometrically necessary dislocations formed in bulk. Instead, these prismatic loops, approaching free surfaces in close proximity and under influence of surface image force, suddenly escaped through free surface with leaving slip steps, indicating that they assist surface nucleation and multiplication of ordinary dislocations.

In situ TEM straining is a widely used technique to investigate the deformation mechanisms of ultrafine-grained (UFG) and nanocrystalline (NC) metals. But obtaining statistically meaningful information to evaluate microstructural changes in these materials using traditional TEM bright-field/dark-field imaging or diffraction techniques is challenging and tedious.

Automated crystal orientation mapping in TEM (ACOM-TEM), in contrast, is highly suitable to perform crystallographic analyses on UFG/NC metals. In this technique, a precessing nanoprobe electron beam is scanned over the specimen to collect spot diffraction patterns. The orientation maps of the sample are extracted after indexing the diffraction patterns using a template matching process. ACOM-TEM enables direct acquisition of orientation/phase map over micron-sized areas while enhancing the ability to identify grains, microtexture and twin boundaries. This technique is particularly useful to monitor the microstructural evolution of UFG/NC metals during deformation.

Here, we use ACOM-TEM in combination with quantitative in situ TEM straining using a custom MEMS device to track orientation changes in hundreds of grains in a freestanding non-textured, UFG aluminum film (thickness 200 nm, mean grain size 180 nm) and correlate those changes with the macroscopic stress-strain response of the film. Our results show extensive grain orientation changes during loading, with both the fraction of grains that undergo rotations and their magnitude increasing with strain. The rotations are reversible in a significant fraction of the grains during unloading, leading to notable inelastic strain recovery. More surprisingly, a small fraction of grains rotate in the same direction during both loading and unloading, even though the applied stress is substantially different. The ACOM-TEM measurements also provide evidence for reversible as well as irreversible grain/twin boundary migration in the film. These microstructural observations point to a highly inhomogeneous and constantly evolving stress distribution in the film during both loading and unloading.

Plastic deformation of crystals is carried out by the motion of dislocations in most metals and alloys over a wide range of experimental conditions (stress, temperature, …). When this motion is hindered, either due to intrinsic atomic bonding that limits the dislocation mobility (ceramics and semiconductors below a certain temperature), or because of a large number of obstacles (point defects, other dislocations, precipitates…), plasticity is limited and the material may become brittle.
In nanocrystalline metals (d≤100 nm) the large proportion of grain boundaries (GBs) both limits the mean free path of dislocations and diminishes their availability. Two alternative mechanisms have been recently revealed using in situ TEM on nanocrystalline (nc) Al and Au. In nc-Al shear-migration coupling is able to accommodate large strains between two grains, but seems heavily dependent on diffusion when several grains need to be deformed together. The shear-migration coupling mechanism occurs through the motion of step-dislocations, confined to GB and also called disconnections. These disconnections are also observed during the controlled deformation of nc Au free standing thin films. Using a MEMS-based deformation platform, repeated tractions led to the propagation of cracks across the films. In the case of thicker films, some intragranular dislocation activity was evidenced, but the main mode of deformation and crack propagation remained grain boundary sliding. In this mechanism, GB dislocations are also heavily involved. Incompatibilities at triple junctions and shear perpendicular to the film led to crack propagation and failure.
Similarities can be drawn between disconnections propagating along GBs and dislocations shearing a crystal, but the former is far less understood than the later. The reason why shear-migration coupling is favoured in one case and sliding in the other is for example unclear, even if the deformation carrier (disconnection) seems the same. What we have shown is that considering perfect GBs to assess the properties of sliding or coupling may not be relevant as GBs in nanocrystals contain a sufficient amount of disconnections. How these defects are activated, that is how one "GB glide system" is selected over another seems the key question to understand plastic deformation in nanocrystals.

In the present work, the fundamental plasticity/fatigue mechanisms operating at interfaces in micro/nano-scale Ni samples have been investigated. in-situ SEM fatigue tests have been performed on FIB prepared single and bi-crystal micropillars with well-known orientations as revealed by electron backscatter diffraction (EBSD). Careful characterizations of the nature and the distribution of deformation dislocations, the character and the local structure of the interface as well as the mechanisms controlling the interaction between these defects under cyclic loads were performed using ex-situ TEM techniques including diffraction contrast imaging, automated crystallographic orientation and nanostrain mapping in TEM (ACOM-TEM) as well as electron diffraction tomography. Furthermore, quantified in-situ TEM nanotensile tests were performed on both single and bi-crystal samples in order to directly observe the plasticity mechanisms.
Recently, an original method combining the measurement of dislocation mobility using commercial in-situ TEM nanomechanical testing and dislocation dynamic (DD) simulations has been used to revisit the plasticity of olivine single crystals at low temperature [1]. Cyclic deformation was applied in the load control mode. Load was increased to a given value, which is maintained constant for several minutes before unloading. During the plateau, dislocation motion is observed and characterized (hence, under a known and constant applied stress). Using this method, we found that the intrinsic ductility of olivine at low temperature is significantly lower than previously reported values which were obtained under strain-hardened laboratory conditions. More generally, we demonstrated the possibility of characterizing the mechanical properties of specimens which could be available in the form of sub-millimetre sized particles only.

References
[1] H. Idrissi, C. Bollinger, F. Boioli, D. Schryvers, P. Cordier, Low-temperature plasticity of olivine revisited with in situ TEM nanomechanical testing. Science Advances. 2 (2016) e1501671.

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 Material (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. Deformation twinning has been well documented in FCC nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy, we report 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 under cyclic loading as well as new insights into the deformation mechanism in BCC nanostructures.

Fatigue crack growth has long been investigated via post mortem analysis, leading to a phenomenological understanding of crack initiation at stress concentrators. However, post-mortem investigations can be very difficult for ultrafine grained materials, such as the Cu thin film in this study, and give little insight as to the dynamic changes in the material under cycling. In situ TEM studies can give a wealth of information, such as grain size, grain orientation, continuous monitoring of crack length/direction/radius, and plasticity present at the crack tip. Here, in situ fatigue is demonstrated using cyclic mechanical loading experiments at frequencies up to several hundred Hz. More than 10⁶ cycles can be reached within one hour. Moreover, the nanometer-scale spatial resolution of the TEM allows the observation of “incipient” crack growth rates of less than 10^-12 m●cycle^-1 very near to the minimum threshold stress intensity factor.

Work performed by K.H., B.L.B., and D.C.B. was fully supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Work by W.M.M. was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science under proposal #U2014A0026. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nanoscale metallic objects like nanowires, thin films, or nanoparticles are usually nearly void of dislocations and can consequently sustain large stresses. Cracks or crack nuclei in nanoscale objects are furthermore inherently small. Their propagation is therefore controlled by the stress to break the atomic bonds at the crack tip rather than by the release of elastically stored energy. At the high applied stresses required to propagate such short cracks, effects like tension-shear coupling can no longer be neglected. Crack tip plasticity in dislocation-starved nano objects furthermore becomes dislocation nucleation controlled and individual crack-microstructure interactions have a more pronounced effect on the fracture behavior than in bulk materials. Cracks in nano objects and nanoscale cracks in general are also typically characterized by small radii of curvature. All these aspects can only be studied to a certain extend with the conventional continuum mechanics approaches and many require explicit atomistic modeling.

Here we present the results of recent atomistic simulations of cracks in five different simulation setups of various sizes. Using EAM-type potentials for various bcc metals we studied the influence of crack length, crack front curvature and boundary conditions on crack tip plasticity. For small cracks, crack tip plasticity was facilitated by the presence of T-stresses and tension-shear coupling. Fully-3D simulations of penny-shaped cracks revealed an increased tendency for crack tip plasticity compared to straight cracks due to the availability of more slip systems and the resulting dislocation – crack interactions. Simulations of cracks interacting with individual pre-existing lattice dislocations showed stimulated dislocation nucleation and new crack tip blunting mechanisms. The results are discussed in the context of fracture and fatigue of nanoscale objects as well as crack nuclei in bulk metals.

The influence of periodic shear deformation on nonaffine atomic displacements in an amorphous solid is examined via molecular dynamics simulations. We study the three-dimensional Kob-Andersen binary mixture model at a finite temperature. It is found that when the material is periodically strained, most of the atoms undergo repetitive nonaffine displacements with amplitudes that are broadly distributed. We show that particles with large amplitudes of nonaffine displacements exhibit a collective behavior; namely, they are organized into compact clusters that become comparable with the system size near the yield strain. With increasing strain amplitude, spatial correlations of nonaffine displacements become increasingly long-ranged, although they remain present even in a quiescent system due to thermal fluctuations.

Microstructural features, such as grain orientations and grain boundaries, play an important role in determining the characteristics of microstructurally short cracks. A new simulation method has been developed that takes into account the effects of the grains' misorientations and the grain boundaries on the crack growth. The effect of grains' misorientations is taken into account by modifying the Bilby, Cottrell, and Swinden's (1963) model. This modification is done by embedding a damage law into the plastic zone in front of the crack tip. The properties of the grain boundary affect the magnitude of the crack retardation, and this effect is taken into account.
The damage law would only approximate the mechanism occurring in front of the crack tip. Thus, to accurately predict the crack tip mechanics, the damage law parameters are experimentally calibrated from micro beachmarkings on fracture facets. The calibration procedure is explained in detail, and the fatigue crack growth rates are predicted using a crystal plasticity finite element code and compared with experimentally determined growth rates. Jacob Adams, J. Wayne Jones, and John Allison present the experiments in a companion paper. In their paper, they have described a novel in-situ approach for characterizing the growth of short fatigue cracks and their interaction with microstructural features, and they have provided high resolution details of crack growth behavior near the grain boundaries.

Current understanding of fracture at individual microstructural features—such as grain boundaries (GBs)—is incomplete and often not predictive. We present a statistical method for inferring the conditions for failure at individual GBs based on data collected from high-throughput experiments. This method minimizes the Kullback-Leibler divergence of fracture probabilities constructed based on hypothesized fracture mechanisms from the experimentally determined fracture probability. We apply this approach to hydrogen-assisted crack initiation at coherent twin boundaries in Ni-base alloy 725 and find that the conditions for fracture along these GBs involve simultaneous mode-I loading and slip along the GB plane. Our findings enable improved lifetime predictions for Ni-base alloys that have been embrittled by H as well as for other materials that undergo intergranular fracture. The relation of these findings to intergranular fatigue failure will be discussed.

Mechanical testing was supported by the BP-MIT Materials and Corrosion Center and the development of implementation of inference methods was supported by the Department of Energy Office of Science, Office of Basic Energy Sciences, under award number DE-SC0008926.

For several decades, considerable attention has been paid to the strain localization of fcc metals. Intragranular crack initiation in these metals is assumed to be mainly a consequence of the slip band emergence in the form of extrusion and/or intrusion on the material surface [1,4]. Recently, a large amount of data have been compiled which emphasize the need for a deep understanding of the mechanical conditions for dislocation pattern formation with respect to the slip localization in order to understand the physical foundation of crack initiation [4,5]. Various forms of the plastic deformation have been observed on f.c.c. alloys. Particular attention is paid to the heterogeneity of deformation observed at two distinct scales: the slip bands and the dislocation organizations. A unified view of the various dislocation structures can be proposed in the form of a modified Pedersen’s map in the case of tensile loading and different kinds of cyclic loading: uni-axial and multi-axial tests under stress or strain amplitude control [5]. In present work, the specificities of each domain defined in the map are discussed in terms of slip localization and slip bands emerging at the materials surface. A correlation between dislocation organizations and slip bands emerging at the surface is found which allow defining three kinds of strain localization. These ones depend on loading path in deformation map and need to be distinguished in damage approach. The combination of techniques (AFM, EBSD) enables one to partially explain the emergence of slip planes in fatigue by taking crystallographic and TEM microstructural data into account. In f.c.c. alloys, cyclic slip irreversibility at local scale has been identified as a major parameter in damage initiation. It is proved here that reaching a critical height value is not a sufficient condition for an extrusion to induce the apparition of a transgranular crack [3,4]. This critical value can be understood in terms of an interaction between vacancy mobility and oxygen and/or hydrogen diffusion.
Finally, the opportunity to define a microscopic law of fatigue crack initiation using Manson-Coffin law formulated in terms of cyclic slip irreversibility is discussed for a polycrystalline Ni-base superalloy. The results show that the modified Manson-Coffin law that relates cyclic slip irreversibility parameter to fatigue crack initiation life is sustained through a two-parameter power law.

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[3] M. Risbet, X. Feaugas, Eng. Fract. Mech. 75 (2008) 3511.
[4] Ho, H.S., Risbet, M., Feaugas, X., Acta Materialia, 85 (2015) 155.
[5] X. Feaugas, P. Pilvin, Advanced Engineering Materials, 11 (2009) 703.

Fatigue of materials and components is one of the main reasons that limit their lifetime and impacts sustainability. The ongoing miniaturization in many areas of modern technology, e.g. microelectronics, medical devices etc., requires the knowledge of mechanical properties in small dimensions to guarantee their reliability. The size of typical fatigue dislocation structures, which control the damage evolution, is in the order of micrometers. Hence, reducing the size of parts and components down into this regime raises the question, whether such microstructures can occurs or not and how this affects the damage evolution. For this reason, the development of fatigue microstructures and the damage evolution will be studied by in-situ fatigue tests in the scanning electron microscope on single and bi-crystalline micro samples depending on specimen size (0.5 to 15 μm), initial dislocation density and crystal orientation. Modern methods will be utilized to measure not only the microstructure and damage evolution, but also local stresses and strains in the samples. This allows the correlation between microstructure and damage and the local loading of the sample. In the case of grain boundaries incompatibilities in local stresses and strains are of special interest as these are correlated with the damage evolution at the grain boundary by different models in the literature. The main advantage of using micron-sized specimen is the knowledge of the local stresses and strains, which allows to associate changes in the stress vs. strain curves with microstructural events. For example, it is possible to correlate the (local) Bauschinger-effect with the back stress of dislocation pile-ups at grain boundaries. The aim of the presentation is to understand the development of fatigue microstructures in dependence of the specimen size in order to predict the lifetime of miniaturized parts and components. Additionally, it will lead to a better understanding of the role of grain boundaries in fatigue damage evolution and can help to improve current models. In principle, the achieved knowledge can also help to better understand fatigue phenomena at the macro scale.

In-situ high cycle fatigue tests were carried out in a forged Inconel 718 alloy with an average grain size of approximately 100 mm at the ID11 beam line of the ESRF using a beam energy of 60 keV. Specimens with a cross-section of approximately 1 mm² were cut by electro-discharge machining (EDM) and electropolished to remove surface oxides and the machining damage layer. Half of the specimens were fatigued without pre-notches to study crack nucleation processes, while the rest of the specimens were notched using a focused ion beam to study fatigue crack propagation. Before testing, diffraction contrast tomography was performed to reveal the 3D grain structure of each specimen. The specimens were then fatigued in-situ at room temperature under a sinusoidal waveform at 20 Hz and R=0.1, with a maximum stress of 700 MPa. Phase contrast tomographs were acquired at regular intervals using white beam radiation to study crack nucleation and propagation as a function of the number of fatigue cycles. The experimental results were analyzed to the light of crystal plasticity simulations of the material.

The accurate measurement of deformation in response to thermo-mechanical loads is a fundamental requirement in the characterization of materials and structures. Of particular interest is the connection between the macroscopic and microscopic length scales, where strain localization at the grain or constituent level can play critical roles in overall material deformation and ultimate failure of the material. The identification of specific microstructural characteristics that lead to local damage accumulation and accelerated failure, and their mitigation, is key for the informed development and optimization of materials.

This talk will present our recent work on exploring these connections using a combination of distortion-corrected digital image correlation and scanning electron microscopy to measure deformation fields at small length scales, including a creation of functionalized nanoparticles for sub-grain deformation tracking. These approaches enable us to glean critical insights into material behavior, including the impact of constituent architecture on damage accumulation in aerospace composites and the relationship between processing and performance in metallic alloys. Recent studies examining microscale deformation mechanisms in forged and annealed polycrystalline alpha-Ti alloys will be discussed as an illustrative example of these emerging experimental approaches and associated analysis. The talk will also include a discussion of on-going work to develop unsupervised learning approaches that statistically link microscale deformation behavior and microstructural attributes.

In the recent past, the mechanical properties of low-dimensional materials attracted enormous attention showing increasing yield strengths reaching the ultimate limit of the respective material for defect free nanostructures [1, 2]. Despite numerous experimental and theoretical works the mechanical behavior and, in particular, the onset of plasticity on the nanoscale is still not fully understood.
To shed additional light on this topic, in situ experimental setups are being designed for monitoring the evolution of the structures during the mechanical deformation. So far, in situ mechanical tests coupled with X-ray diffraction techniques concentrated on micrometric samples [3, 4]. For in situ nano-mechanical tests, a scanning force microscope was developed which can be installed at different 3^rd generation synchrotron beamlines and, thus be combined with different nano-focused X-ray diffraction techniques [5, 6, 7]. Here, we will present the coupling of this new tool with Bragg coherent X-ray diffraction imaging (BCDI) and Laue microdiffraction for in situ nano-indentation on Au nano-crystals and in situ three-points bending tests on self-suspended Au nanowires, respectively [7, 8]. These in situ experiments enabled us for the first time to image by BCDI a prismatic loop in a Au crystal which had been induced by nano-indentation and trapped inside the crystal. While the sample must not be moved during mechanical loading to avoid any vibrations which may potentially damage the nanostructure, a singular position within a bent nanowire is probed during the experiment. We developed a X-ray beam scanning technique which facilitates the mapping of the whole nanowire by Laue microdiffraction during mechanical deformation giving access to its complete profile and, thus to its boundary conditions as well as to its elastic and plastic properties [8, 9].
This work was funded by the French National Research Agency through project ANR-11-BS10-01401 MecaniX.

[1] B. Wu et al., Nature Materials 4 (2005) 525
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[3] C. Kirchlechner et al., Acta Materialia 60 (2012) 1252
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[5] Z. Ren et al., J. Synchrotron Radiat. 21 (2014) 1128
[6] C. Leclere et al., J. Appl. Cryst. 48, 291 (2015)
[7] M. Dupraz, PhD thesis, Université Grenoble Alpes, Grenoble, France (2015)
[8] C. Leclere et al., submitted to J. Synchrotron Radiat. (under review)
[9] Z. Ren, PhD thesis, Aix-Marseille Université, France (2015

Metals catastrophically fail under a repeated load that would not be sufficient to cause failure when applied once. The origin lies in the dislocation organization at the mesoscale that is responsible for long-range internal stresses and lattice rotation. Newly developed computational schemes that can predict dislocation patterning such as for instance the crystal plasticity finite element model [1] require experimental validation.

Using x-ray Laue diffraction scans with sub-micron spatial resolution in 2D, the developing microstructure during cyclic shear was followed in a single crystal Cu sample [2]. The experiment was carried out with a miniaturized shear device installed at the MicroXAS beamline of the Swiss Light Source. The Cu single crystal oriented for single slip and shaped in Miyauchi’s geometry using picosecond laser ablation [3] was cyclic deformed with different strain amplitudes. Spatial resolved Laue diffraction patterns were recorded in transmission mode at different intermediate cycles. The evolving dislocation microstructure was analyzed in terms of lattice rotation, lattice curvature, apparent geometrically necessary dislocation densities [2]. The evolution of the GND traces showed a clear redistribution of the pre-existing GNDs and the appearance of regions surrounded by GND walls with no GNDs inside. Such regions were recognized as developing vein structures. Using the newly developed crystal plasticity model [1] a geometrical interpretation is obtained for the developing vein structure by comparing the experimental and simulated lattice curvatures.

[1] N. Grilli, K.G.F. Janssens, H. Van Swygenhoven, J. Mech. Phys. Solids 84, 424 (2015)
[2] A. Irastorza-Landa, H. Van Swygenhoven, S. Van Petegem, N. Grilli, A. Bollhalder, S.Brandstetter, D. Grolimund, Acta Mater. 112, 184 (2016)
[3] A. Guitton, A. Irastorza-Landa, R. Broennimann, D. Grolimund, S. Van Petegem, H. Van Swygenhoven, Mater. Lett. 160, 589 (2015)

We have developed a methodology for testing small volumes of materials to failure in the (very) high cycle fatigue regime. At the micro-scale (300 nm to ~5 µm wide) cantilevers are cut by focused ion beam into the surface a bulk sample block, while at the meso-scale (50-200 µm wide) we have used laser micro machining to cut cantilever structures into thin metallic foils. In both cases cyclic deflections are excited in the cantilevers through vibration of the sample block/foil using a high power ultrasonic generator. The generator operates at a driving frequency of ~20 kHz while the cantilevers are designed so that their resonant frequency is significantly higher so that small deviations in sample shape are not critical in controlling the response as they would be very close to resonance. To achieve stress amplitudes in the range of several hundred MPa as required for fatigue failure, the cantilever design included a large block of material at the free end which increases the inertia and hence also the stress generated in the thinner test region near the fixed end. Results from testing campaigns on Ti alloys (commercially pure, and Ti-6Al-4V), Ni-based superalloy IN718, and stainless steel 304L will be presented.
The micro-cantilevers can be machined with individual grains aligned with the crystallography as measured with EBSD. Strong anisotropy is found for the fatigue strength at 10⁷ cycles in hcp Ti where the strength for loading along is double that for loading along . A significant size effect is seen with fatigue strength doubling as the cantilever width is reduced from 5 µm to 300 nm, though this increase is less marked than seen in static bending strength determined by bending similar cantilevers with a nanoindenter.
The meso-cantilevers were machined so as to randomly sample relatively small patches of microstructure in polycrystalline alloy foils. An optical microscope was fitted to the testing rig to allow observation of the samples during testing. The formation of localised slip and profuse surface extrusions associated with crack initiation was recorded, and locations correlated with EBSD maps of the entire sample obtained prior to testing. This and post-test SEM and EBSD examination clearly correlated crack initiation to annealing twins present in large numbers in the Ni-based superalloy material.

Ag nanowires network is a promising candidate that can replace the ITO or ZnO to ensure reliability in applications in flexible and/or wearable devices. For these applications, the mechanical robustness of the transparent electrode during repeated bending strains is a critical parameter in ensuring device reliability. In this work, we therefore studied the bending fatigue behavior of the Ag nanowire network with and without RGO protective layer by evaluating in-situ resistance change during application of bending strain ranging from 1-4% up to 500 000 cycles and measurement. First, the thermally annealed bare Ag nanowire networks showed a reduction in fractional resistance in the initial stages of bending cycles followed by a transient and steady-state increase at later stages of cycling. SEM analysis revealed that the initial reduction in resistance is caused by mechanical welding as a result of applied bending strain, and the increase in resistance at later stages of cycling was determined to be due to the failure at the thermally locked-in junctions. Based on the observations from this study, a new methodology for highly reliable Ag nanowire network was proposed: formation of Ag nanowire networks with no prior thermal annealing but localized junction formation through simple application of mechanical bending strain. The non-annealed, mechanically welded bare Ag nanowire network showed significantly enhanced cyclic reliability with essentially 0% increase in resistance due to effective formation of localized wire-to-wire contact. After analyzing the bare Ag nanowire network, the effect of using RGO protective layer coating on the fatigue response of the Ag nanowire network was also evaluated. Ag nanowire/RGO hybrid electrode is currently being researched to solve the problem associated with oxidation of the bare Ag nanowires, but this layer can affect the bending fatigue response of the Ag nanowire network. The Ag nanowire/RGO hybrid electrode and bare Ag nanowire electrodes were then exposed to ambient air at 70°C for 132 hours and tested for bending fatigue behavior. The in-situ resistance measurements indicate that the fatigue resistance is significantly enhanced for the case of Ag nanowire/RGO hybrid electrode as the RGO helps to reduce the surface oxidation of the Ag nanowires.

Ag nanowire networks are candidate materials for use in transparent conducting materials because of their high intrinsic electrical conductivity and relative ease of manufacture to form nanowires with diameter < 100 nm. The mechanical properties of these wires are important in designing flexible transparent conductors that must resist mechanical loads in service. The compression strength of metal nanowires is well known to increase as the diameter of the pillar decreases. Normally low strength metals such as gold have been observed to possess plastic flow stress > 1 GPa when the pillar diameter is < 100 nm. There have been few studies of the tension behaviour of small metal pillars or wires, especially when the diameter is significantly smaller than 1 mm because of the difficulty of specimen preparation and testing. Here we present a study of the strength of commercial Ag nanowires with diameter in the range 30 – 100 nm. All wires possess a pentaprism morphology and are believed to be manufactured by the polyol process. Nanowire tensile strengths are > 1 GPa with the tension strength showing approximately the same size dependence as found with the scaling law for the compression strength of single crystal fcc nanowires. Failure is by fracture with a small region of localised deformation close to the fracture surface.

Graphene exhibits excellent mechanical, electrical and thermal properties which make it an ideal reinforcement for the fabrication of a new generation of ceramic nanocomposites with tailored properties. In recent years, different types of ceramic matrices containing graphene sheets have shown substantial increases in fracture toughness. Relatively low graphene volume fractions lead to improvements of 20 – 140%, thus proving its effectiveness. In most cases, full densification of these composites is carried out by techniques such as Spark Plasma sintering, which apply pressure and simultaneous rapid heating to produce orthotropic materials with graphene layers that are preferentially aligned perpendicular to the direction of applied load.
Fracture toughness has generally been investigated for crack propagation normal to the plane of the graphene. This maximizes the toughness enhancement. However, near the percolation threshold, slight misalignment of the sheets or sites with agglomerated material leads to crack deflection and permits easy propagation.
The aim of the work presented here is to obtain a better understanding of the reinforcement mechanisms and the role of graphene orientation during fracture. Direct observations were made with microcantilever beams that were cut from Si₃N₄ reduced graphene oxide composites with a focused ion beam. Notches were cut in known directions relative to the graphene layer orientations. Stepped and continuous loads were then applied with a nanoindenter. This approach made it possible to monitor the crack propagation process, while obtaining direct in situ data from the load displacement curves. Results from these investigations are being used to develop new fracture models for 2D reinforced nanocomposites.