A new method has been developed for testing high cycle and very high cycle fatigue properties of materials at the micro-scale based on micro-cantilevers. Focused ion beam was employed to cut micro-cantilevers with triangular cross-section into the surface of a selected grain in a bulk polycrystalline commercial pure Titanium. The bulk specimen was pre-examined by EBSD, so the crystal orientation of all micro-cantilevers were known. The bulk sample containing the micro-cantilevers was then attached to a high power ultrasonic generator, which can generate mechanical vibration at the frequency 20KHz. The bulk of the specimen moves with the ultrasonic generator, but the micro-cantilever lags somewhat behind. The resulting deflections generate cyclic stress in the micro-cantilevers. The high vibration frequency means it can easily test into the high cycle and very high cycle regime. The design challenge is to generate enough stress to cause fatigue in these ultra-small specimens because the stress amplitude achieved in vibration is inverse to the cantilever size. Previous finite element modelling and experiments had shown that the classic micro-cantilever with uniform cross-section can only generate stress of a few MPa, even with the acceleration as high as 10⁷m/s². Instead, we designed a new ‘hammer-shaped' micro-cantilever with a widened region at the free end to significantly increase the inertia. This design now generates sufficient stress and enables fatigue testing even in Titanium, which is a challenging material due to the high strength to weight ratio (both high stress and low density require higher acceleration).
SN curves in the testing range from 10⁵ to 10⁸ cycles have been obtained for 2 mu;m wide micro-cantilever single crystal Ti samples using a step test protocol. The stress to failure decreases systematically as the number of cycles to failure increases. However, there is strong dependence on the crystal orientation with the fatigue strength at 10⁷ cycles for test pieces cut along the direction being approximately twice that of those cut in the direction. The fatigue strength of micro-fatigue test is significantly lower than the static strength measured on micro-cantilevers of identical size using a nanoindenter. Due to the small specimen size, it is suggested that the results reflect the behavior of fatigue nucleation rather than propagation.

This talk presents an experimental investigation of the stress relaxation and cracking mechanisms in ultrathin nanocrystalline metals (Au and Al). Tensile tests that include stress relaxation segments were performed on ultrathin (<100 nm) specimens with micron-sized dogbone shapes, using a MEMS-based nanomechanical testing setup. The setup allows quantification of stress and strain (and their evolution with time) as well as in situ TEM observations of the governing mechanisms. The results (based on in situ TEM observations and activation volume calculation as well as evolution of strain rate with applied stress) show that the transient relaxation deformation in 100-nm-thick Au (average grain size: 75 nm) is highly localized and dominated by intergranular and intragranular dislocation activities. These dislocation activities slow down within a timescale of 1 hour, leading gradually to a diffusion-dominated deformation (steady-state creep). In contrast, the 30-nm-thick Au films (with a much smaller average grain size) do not show these sustained dislocation activities; instead their deformation behavior appears to be dominated by grain boundary activities. In addition to the stress relaxation properties, this talk will discuss the cracking behavior in ultrathin nanocrystalline gold and aluminum films.

The mechanical behavior of nanocrystalline metals has been the subject of extensive research, especially in terms of their high strengths and wear-resistance. However, the fatigue performance of these metals has received much less attention, despite initial reports of exceptional fatigue strengths. Particularly lacking is a comprehensive understanding of the cyclically-induced deformation mechanisms, structural evolution, and fracture behaviors in nanocrystalline metals. Recent studies using various post-fracture microscopy techniques have identified extensive regions of abnormally large grains near and along macro-scale cracks - but, the exact origins of these features and their overall effects on the fatigue performance and fracture are unclear. This is due, in part, to the general difficulties in performing non-destructive in situ characterization of through-thickness microstructures during fatigue testing. In the current study, a newly developed method of detecting abnormal grain growth (AGG) using synchrotron x-ray diffraction was employed to “watch” the microstructural evolution of various nanocrystalline Ni alloys during high-cycle fatigue. This in situ fatigue approach allowed us to verify with statistical confidence that the abnormally large grains do, in fact, grow during the fatigue process (i.e. they are not pre-existing) and that they develop well before final fracture of the material. The effects of the fatigue stress conditions on the microstructural evolution and AGG were also evaluated using a range of fatigue testing conditions. Additionally, traditional cross-sectional microscopy was performed after the onset of AGG, but before final fracture, to further explore the deformation and micro-scale crack mechanisms that lead to the macro-scale fracture during fatigue.
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.

Understanding the effects of a material&’s morphology upon the overall material performance requires a detailed understanding of its initial morphology and how it changes under external stimuli. Laboratory based X-ray computed tomography (CT) systems can image polymer foams as they are compressed, in an interrupted in situ modality. Due to the lower flux of a laboratory X-ray source, stress relaxation must be allowed to occur in the material before a tomogram can be collected. Otherwise, the residual sample motion leads to significant image blurring. This relaxation process requires ~20 minutes before the tomogram can be collected over 1⁺ hours. Important information regarding compressive performance and changes in morphology is lost during this stress relaxation period.
Synchrotron light sources, such as the Advanced Photon Source, provide a significant increase in photon flux, compared to lab-based X-ray tubes. Coupling this high flux with a high speed camera allows for X-ray radiographs to be acquired every millisecond. This generates tomographic data in ~1 second. By coupling a custom loading stage to the X-ray microscope, it is possible to study the dynamic in situ deformation of polymeric foams at a 10^-1 s^-1 strain rate.
In this study, synchrotron-based X-ray CT, at beam line 2-BM, was used to capture the morphology changes in polymeric foam materials during dynamic uniaxial compression. Twenty tomograms of each sample were acquired while simultaneously compressing the samples up to 60%, within a timeframe of 100 seconds. The materials studied included hydrogen blown silicone foam (LK3626), prilled urea silicone foam, syntactic Sylgard-184, and additively manufactured foams.
With this technique, we can measure and correlate the differences in mechanical performance of polymeric foams to their physical/morphological changes. For example, LK3626 is a soft-structured hydrogen blown silicone foam. It has a weak ligament structure which quickly buckles with almost no bending resistive force, even at low compressive strains. Also, it compresses with no Poisson effect. Syntactic Sylgard 184 is a much stiffer material, due to the reinforcement from the 50 mu;m glass microballoon pore former; therefore, it exhibits a stress/strain curve with definitive bending/buckling/compressing regions. Importantly, the true stress can be calculated from CT data by measuring the full 3D change in area for each tomographic step. Additionally, the tomographic data can be converted into a hypermesh for importing into Abaqus for explicit analysis. Direct comparison of the reconstructed slices to the modeled result adds a high level validation to the modeled uniaxial compressive performance of the material that cannot be acquired with other techniques.

Impact loading of spherical particulates occurs during materials handling, processing, and service conditions. This can lead to fracture of the particles and commutation into smaller particles. The dynamic fracture behavior of micro spherical particles of 5 brittle spherical solids, ranging from soda lime glass, to polycrystalline silicon and yttrium-stabilized zirconia (YSZ) was characterized with high speed, in situ X-ray phase-contrast imaging to examine the failure mechanisms in situ for spheroidal particles with diameters (d) from 0.600 to 2 mm. A new pulverization parameter was developed and is shown to predict the failure mechanism of the materials under fixed grips loading (i.e. overdriven in load) relating the hardness, toughness, modulus, and the size of the given particulate. The high speed behavior will be compared to quasi-static loading of particulates to demonstrate the effects of strain rate on particle fracture.

Zirconium is a material of great importance in nuclear applications, as it is commonly used for fuel cladding. One of the known factors potentially degrading the mechanical properties is hydrogen precipitation, leading to the formation of brittle zirconium hydride needles. Different hydride phases form, depending on the amount of hydrogen in these needles. The associated volume expansion of hydrides leads to an increasing applied strain on the zirconium matrix. Available observations suggest that such strains may induce the nucleation of dislocations in zirconium from the zirconium/hydrides interface, thus further altering the mechanical properties of the material. But these observations are rather limited, and little is known about the elastic limits, and the nucleation mechanisms.
To address this issue, we have performed molecular dynamics simulations of a strained Zr surface, using two different interatomic potentials (EAM and COMB). Different loading modes were tested, in order to mimic the influence of the hydride precipitate. We found that the onset of plasticity is controlled by heterogeneous nucleation of partial dislocations in the first-order pyramidal planes, immediately followed by partial dislocations in the basal plane. This surprising result can be understood from the analysis of generalized stacking fault energy surfaces [1]. The apparent blocking of the prismatic-plane slip, which unquestionably dominates in bulk zirconium, follows from the loading conditions imposed by the precipitate. The elastic limit depends on the geometric features of the surface making contact to the precipitate. In particular, it is lowered by the presence of surface steps. In specific circumstances, it is possible to favor the basal slip over the pyramidal one.
[1] C. Varvenne et al., Acta Mater. 78 (2014) 65 - 77

The objective of this work is to show some new and original methods to determine the fracture toughness in brittle metals. In this case the analysis was performed for tungsten due to its thermo-physical properties, tungsten is a suitable material for plasma facing applications in the future fusion reactors. However, its structural use is compromised due to its inherent brittleness. Therefore, to improve this critical feature is relevant to accurately measure its fracture toughness as a real property independent of geometrical parameters.
The most common methods to produce cracks such as fatigue, indentation microfracture or electro-discharge machining cannot be used here because they are difficult to apply, do not produce reproducible cracks or induce extended thermal damage in the material. For these reasons, in a first part of this work four experimental methods, that gradually approach a crack-like notch, were compared to determine the effect of the notch root radius on the measured fracture toughness of a nanostructured bulk tungsten material. Three-point bending tests were performed on these four types of specimens with notches introduced with: a classical diamond disc, a diamond wire, a razor blade and an ultra-short pulsed laser. The results showed that the best alternative is to introduce a notch with a femtosecond pulsed laser. This method, which produces crack-like notches without thermal damage, possesses good reproducibility, high accuracy and reliable fracture toughness. It was previously used on ceramics but there is no evidence of its use on metals.
Additionally, the ultra-short pulsed laser was used to introduce notches in tungsten foils of 0.1 mm thickness to perform in situ tensile tests inside a scanning electron microscope and determine the fracture toughness. Two types of geometries were also produced in the foils: single-edge and double-edge notched specimens. The introduction of the notches is a challenging task because of the reduced size of the samples and their slenderness. Single-edge notched specimens, easier to produce, were not suitable to provide accurate fracture toughness since the slenderness and the non-symmetric geometry resulted in complex stresses and momentum apart from the uniaxial tensile being applied to the sample. Double-edge notched specimens, however, produced accurate fracture toughness results, although to introduce both notches aligned at both sides of the specimens involves higher complications during the notching process.

Irradiation in a fast neutron environment is known to degrade the mechanical properties of materials, therefore, to ensure safe operation it is necessary to undertake measurements of these properties. To minimise doses to personnel during post irradiation testing it is appropriate to minimise the sample mass and select small test specimens. In addition, in materials with a complex microstructure, it is necessary to undertake these measurements at the required length-scale to provide input parameters for computer modelling. In this paper, we undertake measurements using micro-metre length scale cantilever beam specimens prepared by ion beam milling in a Helios Dualbeam workstation. These cantilever beams were loaded using a customised system developed at the University of Bristol. This allows load-displacement to be measured and the associated mechanical properties, such as the elastic modulus and flexural strength to be determined. With these techniques, it is possible to observe specimens throughout the duration of a test.
Pile Grade A (PGA) graphite was used as moderator and structural material within the core of UK gas-cooled Magnox reactors. It is an anisotropic, polygranular graphite comprising aligned filler particles in a matrix of graphitised binder with around 20 vol.% porosity caused by the manufacturing process. Irradiated and radiolytically oxidised PGA graphite samples were extracted from the fuel channels as part of an overall monitoring programme to support safe operation. Three cylindrical samples (7 mm height by 12 mm dia.) trepanned from bricks with increasing distance from the centre of the reactor were selected to provide a decreasing neutron dose. The microstructure, in particular the fine-scale porosity has been characterised using serial sectioning, high spatial resolution tomography. To extrapolate small-scale testing data to size of reactor core bricks, a microstructure-based multi-scale modelling approach has to be adopted. This requires the input parameters at a suitable length-scale to eliminate size effects introduced by the heterogeneity of the multi-scale microstructure. Measurement of load-displacement, elastic modulus and fracture strength have been made using 2 µm x 2 µm x 10 µm micro-cantilever test specimens in the dualbeam workstation to evaluate the changes in properties with neutron dose. This has revealed the effect of irradiation damage, independent of the macro-sized pores present in the PGA graphite. The data are discussed with respect to the possible mechanisms leading to changes in properties and their benefit as input parameters to multi-scale computer models.

The microcracking phenomenon in ceramics initiates above a critical grain size due to anisotropy in the coefficient of lattice thermal expansion. This microcrack network can result in an increase in light scattering, electrical and thermal resistance, and a decrease in the bulk coefficient of thermal expansion-even to the point of negative thermal expansion. The tunable material properties of microcracked ceramics provide a unique design opportunity but in order to utilize these materials, there is a need to understand the effect of mechanical loading on microcrack populations. In the current study, we probe the effects of microcracks in ceramics by varying the microstructure in beta-eucryptite samples. We mounted specimens with varying grain sizes into a microtensile apparatus and loaded until failure. The resulting stress-strain curves exhibit nonlinear behavior and irrecoverable strain accumulation. Digital image correlation (DIC) was used to measure the two-dimensional strain fields under applied stress for noncontact microstrain measurements. We develop a constitutive model that predicts the evolution of microcrack density within a linear elastic continuum and then applied the model to fit the experimental tensile curves. The constitutive model accounts for the accumulation of irrecoverable strain due to the increasing crack density and the evolving effect of linear-elastic modulus due to changes in crack density.

Stiction failures in radio frequency (RF) micro-electro-mechanical systems (MEMS) based devices can sometimes be attributed to stress relaxation in the metallic thin films that make up the architecture. Efforts to mitigate the extent of the relaxation through novel manufacturing and engineering techniques are critical to production of highly reliable MEMS devices. We have shown previously via thin film bulge mechanical testing that both viscoplastic and viscoelastic deformation behavior are present during the stress relaxation of a typical FCC metal film. Our prior results indicate that the mechanism responsible for the viscoelastic component is dislocation-based, but literature reports based on substrate curvature measurements imply that surface diffusion could be responsible for the viscoplastic component. In the present work, the mechanism responsible for relaxation in both regimes was explored using thin film bulge testing of metal films with and without surface layers. Surface modifications to gold films such as TiO₂ passivation layers, Al₂O₃ atomic layer deposition (ALD) coatings, and SiN_x support layers were each evaluated in an iso-strain relaxation test on the bulge tester. It was found that surface diffusion was not the dominant mechanism for relaxation under the small-strain isothermal conditions typically experienced by a MEMS device in service, but rather it is hypothesized that a dislocation double-kink nucleation mechanism controls both components of deformation

Adhesion and bonding of multilayer systems continue to be the topic of interest in the coatings and membrane industry. Generally, simple techniques such as peel testing have been used to evaluate film adhesion properties. The ability to perform controlled, repeatable, and rapid assessment of the adhesion properties in film systems, especially systems with more than two layers, is of utmost importance for formulation and application development.
In this work we present a new analytical approach to evaluate layer to layer adhesion strength for multilayered systems. A high-throughput mechanical testing (HTMECH) technique is used to measure the impact performance of polymeric films under a wide range of strain conditions. Delamination in multilayered systems is induced by puncturing the film at increasing impact speeds. Ongoing study on the mechanism of failure in the systems with varying levels of adhesion will be discussed.

Nanomechanical measurements, particularly nanoindentation, have transitioned from a purely academic research instrument into a tool for examination of complex industrial processes. In an academic setting proper design of experiments allows for the simple elimination of variables. This is not possible in an industrial setting where a complex engineering material such as the multilayers in a hard disc film stack are the accumulation of 1000&’s of engineering hours. For these complex cases both statistical information and data on the failure mechanisms are required. Here, both ex and in situ measurements are performed and then correlated, using a co-deposited hard drive film stack. Indentation depths range from 0.5 to 10 nm, with the analysis utilizing both the approach and retract curves to describe the crack growth/separation between the diamond indenter and the carbon film. Additionally, the elastic-plastic transition is thoroughly explored in an effort to understand the effects of a protective coating on a substrate and the loss of magnetic information during plastic deformation.

Functional coatings protect substrates from wear, add special electronic and optical properties to the substrates and change their gas permeability. Their functionality, however, is often limited by fracture. This contribution focuses on the flavor of fracture patterns as they arise from an interplay between the cleanliness of the substrate, the adhesion between the two substrates, the amount of ductility that the coating exhibits and the coating microstructure. The examples given range from brittle materials such as diamond like carbon, silica, titania, silicon, ink-jet printable materials to ductile coatings such as Cu, Au, Al and some of their alloys. Special analysis methods are based on in-situ tensile testing and include the measurement of electrical resistivity, AFM, SEM, optical microscopy, Raman microscopy as well as reflectance difference spectroscopy.

Currently the majority of commercial MEMS devices are fabricated out of silicon, but the development of metal MEMS alloys that possess enhanced mechanical and functional properties, requisite dimensional stability, and the ability to be shaped on the microscale would greatly expand the design space for MEMS in applications ranging from microturbine engines, to power generation, high frequency switching, microheaters and high temperature sensors. Advanced metallic alloys are especially attractive in MEMS applications that require high density, electrical and thermal conductivity, strength, ductility and toughness. Here we report the mechanical behavior of sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) thin film alloys. The as-deposited films go down as single-phase nanocrystalline solid solutions and possess ultra high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and the nucleation of Mo-W precipitates. Films annealed at 600^oC or 800^oC for 1hour still showed brittle behavior similar to the as-deposited film. Interestingly, films annealed at 1,000^oC for 1hour were found to exhibit perfect elastic-plastic deformation behavior with strength greater than 1.2 GPa and approximately 10% tensile ductility. The ultra high strength observed in the as-deposited films and significant ductility measured in the annealed films suggest that sputtering and subsequent heat treatment may offer an attractive option for depositing metallic MEMS materials with tailorable mechanical properties.

Hydrogen embrittlement (HE) affects the cracking behavior of metals by influencing the crack nucleation and growth process, which leads to disastrous consequences. Considering that dislocations are the carrier of plastic deformation, which avoids catastrophic failure by brittle fracture, the crucial role of dislocations in HE is enlightened. Hence, in order to understand the fundamental process governing HE, it is essential to clarify the effect of HE on dislocation activity in the fracture process zone of the crack. In an elastic-plastic solid, the dislocation activity in the plastic zone of the crack is limited, either by the dislocation nucleation rate, or by the dislocation mobility.
Recent nanomechanical testing methods, based on nanoindentation, provide a versatile tool for studying both dislocation nucleation and motion. Especially with integration of an electrochemical setup into the nanoindenter, we investigated the hydrogen effect on the activation energy and activation volume for dislocation nucleation as well as interaction force of dislocation with each other that controls their mobility. In this paper, we will present the application of this method, the so called electrochemical nanoindentation, for studying the nanomechanical aspects of HE.

Dislocation organizations and slip band patterns developed under strengthening nickel crystals at room temperature was studied with different content of hydrogen. Complementary observations by AFM and TEM carried out at the same plastic strain yield correlations between different structural parameters of dislocation organization in strengthening nickel oriented for single slip, <135> and multiple slip, <001>. The impact of these correlations on the slip localization and the length scale (geometric necessary boundary, GNB spacing) which affects the hardening rate is demonstrated and discussed in relation with the wavelength of long range internal stresses and the mean free path of mobile dislocations. Additionally, it is shown that hydrogen content reduces the length scale because of a decrease of the stacking fault energy and cross-slip probability on the one hand and a shielding effect on internal stresses on the other. A similitude law between hardening rate and inverse of GNB spacing is evidenced and offers the opportunity to discuss the effect of solutes on hardening rate. In opposite, the impact of hydrogen on the equiaxe cell size and IDB (incidental dislocation boundary) size seems to be moderated for <001> multiple slip orientation. If our work gives a first quantitative insight on the impact of hydrogen on the characteristic dimensions of dislocation cell structures, the dynamics of these effects remain unclear and require more investigations.
[1] A. Oudriss, X. Feaugas, Length scales and scaling laws for dislocation cells developed after monotonic deformation of (001) nickel single crystal, Inter. J. of Plasticity, (2015), in revision.
[2] G. Girardin, C. Huvier, D. Delafosse, X. Feaugas, Correlation between dislocation organization and slip bands: TEM and AFM investigations in hydrogen-containing nickel and nickel-chromium, Acta Materialia, 91 (2015) 141-151.
[3] A. Oudriss, J. Creus, J. Bouhattate, F. Martin, X. Feaugas, Impact of dislocation distribution on hydrogen diffusion and trapping in tensile strengthening nickel (100) single crystal, Acta Materialia, (2015) in progress.

The corrosion resistance of metals relies on a passivating surface layer of dense and adherent oxide. However, the integrity of such a protective oxide is compromised in the presence of excess hydrogen which can cause blistering and eventual spallation of the oxide film. So far it remains unclear as to how a nanoscale gas bubble manages to reach its critical size at the metal/oxide interface before the oxide layer can deform. Using in situ environmental transmission electron microscopy, we have discovered that once the aluminum metal/oxide interface is weakened by the segregating hydrogen, rampant surface-diffusion of Al atoms sets in to form numerous gas-accumulating cavities on the metal side driven by Wulff reconstruction. The morphology and growth of these metal-side cavities are found to be highly orientation sensitive. The surface oxide layer remains unyielding until the metal-side cavities grow to a critical size above which the accumulated gas pressure become strong enough to blister the oxide layer. Our findings have broad implications for coating performance in nuclear, petroleum, and transportation industries, and can help optimize the material design strategies to alleviate a broad range of hydrogen induced interface failures. ( Xie et al, Nature Materials, 2015)

By using multi-scale simulation techniques, we probed the role of hydrogen-vacancy complexes on nucleation and growth of proto nano-voids upon dislocation plasticity in α-Fe. Our atomistic simulations reveal that, unlike a lattice vacancy, a hydrogen-vacancy complex is not absorbed by dislocations sweeping through the lattice. Additionally, this complex has lower lattice diffusivity; therefore, it has a lower probability of encountering and being absorbed by various lattice sinks. Hence, it can exist metastably for a rather long time. Our large-scale atomistic simulations show that when metals undergo plastic deformation in the presence of hydrogen at low homologous temperatures, the mechanically driven out-of-equilibrium dislocation processes can produce extremely high concentrations of hydrogen-vacancy complex (10^-5~10^-3). Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. The current work provides the possible route for the experimentally observed nano-void formation in the context of hydrogen-induced failure and also bridge the link from the atomic-scale events to the macroscopic failure.

Metastable austenitic stainless steels such as type 304 suffer from severe hydrogen embrittlement (HE), whereas the type 310S stable austenitic steel exhibits little degradation in ductility because of deformation localization in the presence of hydrogen. This difference between these steels is related to the intrinsic tendency towards deformation-induced martensitic transformation. Moreover, fractographic observation has often shown quasi-cleavage and planar fracture features in hydrogenated austenitic stainless steels. The cleavage is presumably formed near martensite laths and/or annealing twin boundaries. However, the role of martensite and twin boundaries in the HE of metastable austenitic steels is controversial. Therefore, this study using microtension testing has been employed to analyze the HE in single crystals and twinned bicrystals, with a particular focus on the effect of the deformation-induced martensite transformation.
The material used in this study was a solution-treated 304 stainless steel with an average grain size of 60 mu;m. Microtension specimens with gauge section dimensions of 20 mu;m × 20 mu;m × 50 mu;m were fabricated using a focused ion beam. Single-crystalline specimens were prepared so that the loading direction (LD) is close to the [123] and [111] directions, denoted as specimen A and B, respectively. For the bi-crystalline specimen, the twin boundary was arranged perpendicular to the LD. A set of specimens was electrochemically charged with hydrogen prior to microtension testing. A microtension test was performed at room temperature in air and at a loading rate of 0.1 mu;m s^minus;1. After failure, the deformed microstructure was studied by electron back scatter diffraction (EBSD).
In the uncharged single-crystalline specimen A, yielding occurred at a stress of 170 MPa, and an ultimate strength of 490 MPa and 130% strain-to-failure were attained through a three-step strain hardening process. Specimen B exhibited a high yield stress but a low strain-to-failure owing to significant strain hardening. For both orientations, the yield stress increased but the ductility decreased drastically when pre-charged with hydrogen. For the hydrogen-charged single- and bi-crystalline specimens, the fracture morphology exhibited quasi-cleavage and a planar facet fracture, respectively. The EBSD observation of the deformation microstructures suggests that hydrogen-induced fractures occurred along the martensite habit plane and the austenite twin boundary, confirming that these microstructural features play indeed a crucial role for the hydrogen embrittlement.

In complex loading conditions, mechanical properties, such as strain hardening and initial hardness, will dictate the long-term performance of materials systems. In this study, the strain hardening behaviors of metallic and metallic/ceramic nanoscale metallic multilayer systems were examined by performing nanoindentation tests within nanoscratch wear boxes. Both the architecture and substrate influence were examined by utilizing three different individual layer thicknesses (2, 20, and 100 nm) and two total film thicknesses (1 and 10 µm). After nano-wear deformation, metallic multilayer systems with thinner layers showed less volume loss. Strain hardening exponents for multilayers with thinner layers were less than was determined for the thicker layers. These results suggest that single-dislocation based deformation mechanisms observed for the thinner systems limit the extent of achievable strain hardening.

Understanding mechanical failure, crack propagation, and compressive behavior at the sub-micrometer scale is essential for tailoring material properties for structural performance. Typically, tension or compression loading is needed to understand these processes. Here, we demonstrate the coupling of a custom compression/tension load stage with a laboratory-based nano-scale X-ray transmission microscope. With this technique, 3D images of a material are collected, non-destructively, providing a probe into its internal structure. This provides a better understanding of both its manufactured and after-experiment morphologies. For some materials, it is possible to operate in an ‘interrupted in situ&’ modality to probe morphological changes during experimentation. The use of uniaxial loading (tension or compression) is needed to understand these processes. Here, we demonstrate the coupling of a custom compression/tension load stage with a nano-scale X-ray transmission microscope.
The solidification conditions used in the manufacturing of metal alloys determines the material&’s resultant properties. In order to relate the relationship between the processing (e.g., cooling rates) and morphological results (e.g., dendritic spacing, crystal structure) and how they affect the mechanical properties requires a thorough suite of characterization techniques. These techniques must capture the multi-scale phenomenon that occurs from the micro- to mesoscopic defects, solidification structures and properties. Using aluminum-copper alloys as a test, X-ray microscopy can not only directly image the resultant structure but also probe failure pathways with ~150 nm spatial resolution. Uniaxial tension experiments using micro-dogbones can image the 1-2 micrometer lamella while applying the loads. The results show that fracture failure occurs along the aluminum-copper interface. Not only does the fracture occur there, but it changes directions to follow this interface. Additionally, understanding dislocation formation within high explosive crystals is important in understanding hotspot formation which can affect performance. We will demonstrate the in situ imaging of 100 micrometer (or less) diameter single crystals of high explosive. Using phase contrast X-ray microscopy, we can directly image these dislocations and fracture progression with compression loading.

Cold spray is a process that produces thick, fully dense metallic coatings. More recently, it has been used to deposit metal matrix composites containing ceramic or solid lubricants phases for improved tribological properties. Optimization of the cold spray process to obtain robust tribological coatings involves both tailoring of the feedstock powder properties and process conditions such as gas temperature and pressure. Coatings typically have a non-homogenous microstructure and thus local mechanical properties vary somewhat. When the coatings are tested for their wear resistance, the material is modified mechanically and sometimes chemically, leading to ‘third bodies&’ with properties different from the as-prepared coating. In all of these settings, from powder to coatings to third bodies, nanomechanical testing is useful to determine mechanical properties, which are important for cold spray process optimization and understanding the material modifications while the coating is in service.
In this work, we report on nanoindentation measurements of feedstock powders, cold spray coatings and third bodies produced from the coatings after wear testing. For powder, special care was taken to produce reliable test conditions and analysis strategies for micron sized powders embedded in a soft matrix. For cold spray coatings, mapping of mechanical properties are presented and tied to various microstructural features observed by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). And for third bodies, mechanical property changes are correlated to the microstructural modifications induced by wear. Materials systems reported on include Al-Al₂O₃, Cu-MoS₂ and Ni-WC coatings.

Metallic alloys used as structural materials in the nuclear core of pressurized water reactors suffer from irradiation creep deformation. A proper understanding of the mechanisms which control the deformation is essential in order to predict the dimensional changes under irradiation. At the macroscopic scale, many experimental data are available. However, the microscopic mechanisms are still not yet fully understood.
In this study, a novel approach based on the testing of on-chip thin freestanding structures is evaluated. This on-chip test method, developed at Université catholique de Louvain, is for the first time used in the context of irradiation studies. An elementary test structure is composed of three main elements: (i) a thin specimen of the material of interest, (ii) an actuator layer of silicon nitride with strong internal tensile stresses to deform the attached specimen layer and (iii) a sacrificial layer of silicon dioxide to release the test structure from the underlying substrate. The small thickness of the material, below a few hundreds nanometers, allow full and homogenous irradiation by heavy ions with a kinetic energy of a few hundreds keV.
After adapting the method to in situ irradiation conditions, sets of test structures were successfully used to assess the room temperature creep behaviour of PVD copper films. The contribution of the irradiation creep to the deformation has been quantified by performing several irradiation steps up to a dose of 1 dpa and measuring after each step the deformation of the specimens.
Preliminary results show that while the creep behaviours before and after irradiation are very similar, the strain rate under irradiation is several times higher than out of flux. The activation volumes and the strain rate sensitivity were extracted from these experiments. Theses quantities were found to be nearly identical for the unirradiated samples and the irradiated ones. Hence the same mechanisms are thought to take place in both cases. The study of the mechanisms responsible for irradiation creep is currently in progress.
In parallel to these nanomechanical tests, the microstructure of the copper film before and after irradiation is characterized by Transmission Electron Microscopy. In particular, the influence of the grain size on the irradiation defects is investigated. No significant effect on the irradiation defects for grain size between 100 nm and 10 µm is found in this study.

Materials at small scale behave differently from their bulk counterparts. This deviation originates from the abundance of interfaces at small scale. Quantifying the properties and revealing the underlying mechanisms requires experiments with small samples in situ in analytical chambers. However, small size poses the challenge of sample handling, but offers the opportunity of in situ inspection of mechanism during testing in analytical chambers. In order to overcome the challenge and take advantage of the opportunity, we developed a MEMS based micro scale testing stage where the sample and the stage are co-fabricated. The stage suppresses any misalignment error in loading by five orders of magnitude. The stage allows in situ inspection of samples during testing in SEM and TEM. We employed the stage in two scenarios. (1) Exploring the effect of microstructural heterogeneity, such as grain size and orientation, on the deformation mechanisms in nano grained polycrystalline metals. Here the test specimens are free standing thin films subjected to uniaxial tension. We found that heterogeneity introduces two apparently dissimilar, but fundamentally linked, anomalous behaviors. The samples undergo plastic deformation during unloading, i.e., exhibit Bauschinger type phenomenon. Upon unloading, they recover a significant part of plastic deformation with time. The underlying mechanism, verified by in situ TEM inspection, is as follows: during loading, the relatively larger grains undergo plastic deformation and relax by employing dislocations, while the smaller grains remain elastically deformed. During unloading, the smaller grains apply reverse stress on the larger grains causing reverse plasticity resulting in a deviation from linear stress-strain response. Upon complete unloading, the residual stress of the elastically strained small grains continue to apply reverse stress on the larger grains resulting in biased jumps of dislocation in the larger grains and strain recovery. (2) Exploring the effect of size on brittle to ductile transition (BDT) temperature (540C) in single crystal silicon. Here the sample is a micro scale single crystal silicon beam subjected to bending which limits the high stress region to a small volume in the sample, and minimizes the probability of premature failure from random flaws. We found that silicon indeed deforms plastically at small scale at temperatures much lower than 540C. Smaller the size, lower is the temperature, e.g., 290C for 800nm sample. Ductility is achieved through a competition between fracture stress and the stress needed to nucleate dislocations from surface. Small size offers high flaw tolerance - lower the size of the sample, higher is the fracture stress. Nucleation stress is temperature dependent - higher is the temperature, lower is the nucleation stress. With decreasing sample size, fracture stress exceeds the nucleation stress even at lower temperatures resulting in a size dependent BDT temperature.

Testing of the local mechanical properties as hardness, Young&’s modulus, fracture toughness etc. by nanomechanical testing methods is now well established. Therefore these methods are often used in combinatorial studies for testing the properties of diffusion couples, where many different compositions are studied. Developing of new advanced materials with complex compositions requires a very good knowledge on the influence of the chemical compositions on the properties of the constituent phases.
A very simple method of generating a graded single crystalline NiAl sample consists of annealing a prior homogeneous crystal at high temperatures since Al evaporates out and an Al depleted zone is formed at the sample surface. By such an approach the graded sample remains single crystalline which is especially important for determining the fracture toughness, which is highly anisotropic. In addition microstructural influences are avoided which often complicates the analysis of polycrystalline diffusion couples. The fracture toughness is then determined by FIB-milled microcantilevers in dependence of the chemical composition. In previous work it has been shown that the fracture toughness of single crystals can be reliably tested with the microcantilever method.
This new approach has been used to study the properties of Ni-rich NiAl single crystals where it is found that the fracture toughness decreases with increasing the Ni content from the stoichiometric composition whereas the hardness increases with the number of constitutional defects. The results of the microcantilever tests are compared with nanoindentation measurements and will be discussed in terms of plastic deformation in the microcantilever experiments.

Since its introduction by Uchic et al. [1], micro-compression testing of pillar samples has risen to be one of the primary techniques for interrogating the deformation behavior of small volumes. The technique offers site specific interrogation of microstructural features and a simple uniaxial stress state. This uniaxial stress state provides several advantages over other techniques such as nanoindentation, which imposes complex triaxial stresses. However, fabrication of micropillars is usually performed using focused ion beam (FIB) machining techniques. These have been shown to impose significant ion damage into small pillar and cause changes in deformation behavior in metallic samples compared to pillars produced without FIB techniques [2]. Even the differences in ion beam dosage caused by different milling techniques has been shown to introduce significant variations in deformation behavior [3]. Polycrystalline aluminum micropillars are expected to be especially susceptible to gallium ion damage [4] due to the well-known embrittlement of aluminum by gallium caused by the segregation and concentration of gallium at the grain boundaries [5]. Therefore, polycrystalline aluminum samples are an ideal case for comparing the relative damage introduced by focused ion beam machining using xenon ions as an alternative to gallium ions for machining of metallic materials. In this project, micropillars will be manufactured by FIB using both ion species to produce pillars of a range of sizes. The grain boundary content of the pillars will be varied using by varying both the extrinsic pillar diameter and the intrinsic grain size of the polycrystalline aluminum. The strengths and rate sensitivity of the resulting deformation behavior will finally be investigated by strain rate jump microcompression testing of the pillars.
References
[1] Uchic MD, Dimiduk DM, Florando JN, Nix WD. Sample dimensions influence strength and crystal plasticity. Science. 2004;305:986-9.
[2] Bei H, Shim S, George EP, Miller MK, Herbert E, Pharr GM. Compressive strengths of molybdenum alloy micro-pillars prepared using a new technique. Scripta Materialia. 2007;57:397-400.
[3] Hütsch J, Lilleodden ET. The influence of focused-ion beam preparation technique on microcompression investigations: lathe vs. annular milling. Scripta Materialia. 2014;77:49-51.
[4] Kiener D, Motz C, Dehm G, Pippan R. Overview on established and novel FIB based miniaturized mechanical testing using in-situ SEM. International Journal of Materials Research. 2009;100:1074-87.
[5] Schmidt S, Sigle W, Gust W, Rühle M. Gallium segregation at grain boundaries in aluminium. Z Metallk. 2002;93:428-31.

Duralumin, a representative precipitated aluminium alloy, is a strong aircraft alloy that exhibits high tensile strength of well over 400MPa, and would not creep significantly at room or moderately elevated temperatures in the bulk condition. Here, we report a “larger-being-weaker” size effect of strength in micron-sized duralumin. Compression tests on duralumin micro-pillars in the ~1 to 6.5 microns size range at room temperature revealed that their proof strength drops from ~300MPa for the 1 micron pillars to ~120 MPa for the 6.5 micron pillars. Creep experiments conducted on duralumin micro-pillars of this size range also showed that larger samples exhibited more significant creep at room temperature than smaller counterparts. Such a “larger-being-weaker” size effect should not continue for larger specimens since their strength should revert towards that of the bulk as the specimen size increases in the tens-of-microns regime or beyond. Therefore, a critical size of duralumin, possibly in the tens-of-microns regime, at which the strength of duralumin reaches a minimum, is evident from these preliminary findings. Such a weakest-size behaviour of precipitated alloys is a significant discovery as precipitate strengthening is a deep-rooted concept in physical metallurgy, yet there has been no previous report suggesting that the specimen size in the tens-of-microns range can be an important factor hampering the effect of precipitate strengthening. A complete understanding of the weakest-size effect is therefore important when precipitated alloys are used as miniaturized structural components.

Two kinds of in-situ Al₂O₃/Al composites, sample A obtained from coarse Al flake powders (2.0 mu;m in thickness) and sample B obtained from fine Al flake powders (0.2 mu;m in thickness) have been fabricated by flake powder metallurgy (FPM). The tensile results show that the strength of sample B is much higher than that of sample A. Applying electron channeling contrast (ECC) imaging and electron backscatter diffraction (EBSD), structural parameters, such as grain size, boundaries distribution, volume fraction of Al₂O₃ particles, and spacing between Al₂O₃ particles as well as their distributions, have been measured and analyzed. Based on the quantitative microstructural analysis, the excellent strength of sample B is mainly attributed to the additional Orowan strengthening effect of the intragranular Al₂O₃ nanoparticles dispersed in the Al matrix with nano lamellar structure.

Composite of NiTi₂ and TiB have been fabricated using spark plasma sintering and mechanical properties have been investigated. Dense specimens of monolithic NiTi₂ have been obtained by the sintering at 950#8451;. The Vickers hardness of NiTi₂ is 650Hv and the fracture toughness is 3.3 MPa#12539;m^1/2 implying that monolithic NiTi₂ is a hard and brittle material. By the x-ray diffraction measurements it has been shown that NiTi₂ and TiB co-exist in equilibrium at 950#8451;. The fracture toughness of NiTi₂ increases to 6.6 MPa#12539;m^1/2 by the addition of 10wt%TiB. The Vickers hardness of NiTi₂-TiB composite increases as the amount of TiB increases up to 70wt%. The highest Vickers hardness is 1600Hv for NiTi₂-70wt%TiB. The bending strength of NiTi₂ increases as the amount of TiB₂ increases up to 60%. The bending strength of NiTi₂-60wt%TiB is as high as 710MPa.

Austenitic stainless steels are considered as structural materials for the next generation of nuclear reactors [1]. Due to the workplace, they are subjected to high irradiation doses, temperature changes and corrosive environment which locally changes their mechanical properties. Moreover, the design of future nuclear reactors and nuclear waste disposal systems forces joining of structural elements using welding techniques. Welding procedures introduces local changes of structure and phase composition which further reflects on their mechanical properties. This can potentially result in degradation of the performance of the device. One must remember that the place most vulnerable to destruction is welding. Therefore, experimental efforts have been made to examine mechanical properties of Heat Affected Zones (HAZ) and welding&’s [2], yet some phenomena are still prone to debate, i.e. production of defects during welding, dislocation movement etc. Due to the limited size of the specimen, only Small Punch Test (SPT) [3] and nanoindentation technique [4] can be applied to measure the hardness and strength in the limited volume of the sample. Moreover, reported mechanical parameters may be changed by several factors such as the surface finishing before joining, welding technique and crystallographic orientation of the joining grains. The present study aims to survey the microstructural and mechanical properties near the welding zone (non-irradiated material). The SPT technique and nanoindentation experiment with constant deflection rate were carried out at the room temperature using a small disk-type specimen (3 mm in diameter and 0.25 mm in thickness). A quantitative relationship between two different measurement techniques have been found. Reported results open a new paradigm for small volume studies which is especially important for nuclear applications where specimen size place the major role.
Literature:
[1]. S.J. Zinkle, G.S. Was, Acta Materialia 61 (2013) 735
[2]. T. Kato, Shin-ichi Komazaki, Y. Kohno, H. Tanigawa, A. Kohyama, Journal of Nuclear Materials 386 - 388 (2009) 520
[3]. T. Bai, K Guan, Materials and Design, 52 (2013) 849
[4]. C. Oliver, G.M. Pharr, Journal of Materials Research 7 (1992) 1564

Refractoryhigh-entropy alloys (HEAs) are a class of emerging multi-component alloys, showing superior mechanical properties at elevated temperatures and being technologically interesting. However, they are generally brittle at room temperature, fail by cracking at low compressive strains and suffer from limited formability. Here we reporta strategy for the fabrication of refractory HEA thin films and small-sized pillars that consist of strongly textured, columnar and nanometer-sized grains. Such HEA pillars exhibit extraordinarily high yield strengths of approximately 10 GPa— among the highest reported strengths in micro-/nano-pillar compression and one order of magnitude higher than that of its bulk form— and their ductility is considerably improved (compressive plastic strains over 30%). Additionally, we demonstrate that such HEA films show substantially enhanced stability for high-temperature, long-duration conditions (at 1100°C for 3 days). Small-scale HEAs combining these properties represent a new class of materials in small-dimension devices potentially for high-stress and high-temperature applications.

Hypervelocity impact testing is used to study fundamental aspects of materials behavior under high strain rates as well as in applications ranging from armor testing to the development of novel drug delivery platforms. In this work, we study hypervelocity impact of micron-size projectiles on gels. In an all optical laser-induced projectile impact test (LIPIT), a monolayer of microparticles is placed on a transparent substrate coated with a laser absorbing polymer layer. Ablation of a laser-irradiated polymer region accelerates the microparticles which are ejected away from the launching pad and into free space, reaching controllable speeds up to 3.0 km/s depending on the laser pulse energy and particle characteristics. The particles are monitored while in free space and after impact on the target surface with an ultrahigh-speed multi-frame camera that can record up to 16 images with time resolution of each frame as short as 3 ns. Individual projectile impact events can be followed in a variety of sample materials, with direct observation of the collision and the rebound or penetration of particles of various sizes and compositions. We present images and movies capturing individual particle impact and penetration in hydrogels, and we discuss the observed dynamics. The results can provide direct input for modeling of high-velocity impact responses and high strain rate deformation in gels and other materials. The study can help guide the design of gels with tailored mechanical properties.

Automotive companies in many countries are competitively promoting the enhancement of the fuel efficiency in order to correspond to the environmental regulation of global automotive markets. The most efficient manner to increase the fuel efficiency of auto-vehicles is the light weight design of an auto body using the non-steel light metals, plastic and composite materials instead of the conventional steel. Recently, various composite materials are applied to design of the components of an auto body because of the high specific strength/stiffness and light weight. However, it should be careful to design the part of an automotive using the composite material because of its brittle failure characteristics
In composite material, fiber sustains the most of load applied to material and the matrix binds fibers and transfers the applied load to fibers. Because of this characteristic, the composite has various failure mode under specific loading condition. However, most of cases, the fracture occurs through the growth of micro cracks at interfaces between fiber and resin. The accurate fracture prediction of composite should be achieved for prediction of the structural safety, fatigue reliability and crashworthiness of the transportation which contains composite material
In this paper, damage based fracture criterion is suggested by applying the damage evolution theory and damage initiation data which are obtained by measuring the interfacial fracture characteristics between fiber and resin in composite material. In order to determine the fracture characteristic, micro-droplet shear test is carried out. The test measures the interfacial failure load by pulling the fiber filament as fixing the resin(micro droplet) which is hardened on the surface of fiber filament. In this test, the carbon fiber(CF) and epoxy resin which is used in most carbon fiber reinforced thermos set plastics and the glass fiber(GF) and PA66 as thermoplastic composite material. The diameter of these fiber filaments is about 10~20um and the one of micro-droplet is about 100 um.
The interfacial failure shear strength(IFSS) is calculated through structural analysis of micro droplet shear test based on the failure load obtained in the experiments. It is assumed that IFSS of the micro droplet shows the damage initiation based on the stress which the micro crack occurs at the composite material in macroscopic level. And the macroscopic fracture of the composite is predicted by applying the damage evolution theory. The prediction results are compared with the results obtained by tensile experiment of composite material with various fiber orientations and laminating conditions and the reliability of this analysis scheme is verified.

Nanometer sized particle based printing method on flexible substrates is one of the most promising candidates for a mass production using roll-to-roll process with a high patterning resolution and low temperature. For these reasons, the printing technology has been emerged as an easily controlled fabrication method for metal interconnects. However, this printing method has to overcome the reliability issues including interconnects failure under electrical current stressing conditions because the porous structure which contains abundant of vacant area and voids, is vulnerable to the microstructural changes under the electrical current stressing. Even though electromigration (EM) of interconnects is well known phenomenon in Integrated Circuits, which is the atomic displacement due to momentum transfer from electron carriers to metal atoms, EM behavior of metal interconnects fabricated by utilizing nanometer sized particles has not been reported yet.
In this work, we used a printing method to fabricate Ag interconnects, and their electrical properties and associated changes of microstructures were measured under current stressing to investigate their EM behavior and verify the underlying mechanisms. Ag interconnects formed necks between Ag particles, and densified after annealing process subsequent to the deposition. Under accelerated current stressing condition, metal atoms migrated in the direction of the electron wind force resulting in the contributed grain growth and resistance increase simultaneously. The reason of reliability failure was considered as induced joule heat at a neck of particles due to current crowding effect. EM phenomenon was extremely accelerated around a neck and Ag interconnects discontinued accompanied with abnormal grain growth in the end. When the microstructures were controlled by different post heat treatments, it was observed that EM reliability of Ag interconnect depends on the distribution of grains; strong for uniform grains and weak for abnormal grains. Therefore, this implies that EM lifetime can be improved by controlling microstructure. As for enhancing EM reliability, the optimized post heat treatment conditions and the effect of substrate materials will be further discussed. This study could provide the guideline for the highly-reliable metal interconnects manufactured by printing methods and printing technology can access to new electronic industry in near future.

Ever since the Fukushima accident, accident tolerant fuel (ATF) has been widely studied, and a coating technology for a fuel cladding surface has been considered to decrease the high-temperature oxidation rate of zirconium-based alloy. Recently, Terrani et al. reported the oxidation resistance of Fe-based alloys for protecting zirconium alloys from the rapid oxidation in a high-temperature steam environment [1]. Kim and co-workers also reported the corrosion behavior of Cr-coated zirconium alloy using a plasma spray and laser beam scanning [2]. However, main concern of coating layers is crack behavior and adhesive properties due to the irradiation growth of zirconium during neutron irradiation. In this study, the cracking behavior of Cr as protective coating layers was investigated under uni-axial tensile strain, and there critical strain to crack was determined in terms of applied and actual strain during the tensile test. The crack initiation and propagation of the coating were observed with an optical microscope during the tensile force loading. The strain of the as-deposited coating was evaluated by the iso-inclination X-ray diffraction method, and the actual strain of the coating during the tensile test was calculated from the applied and initial strain of the as-deposited coating. The crack density was measured in terms of the applied strain of the coating during the tensile test.
[1] K. A. Terrani, C. M. Parish, D. Shin, B. A. Pint, J. Nucl. Mater. 2013, 438, 64.
[2] H. G. Kim, I. H. Kim, Y. I. Jung, D. J. Park, J. Y. Park , Y. H. Koo, in TopFuel 2013, 2013, 842.

Residual stress is one of the main factors in serious failure or fracture in materials. In thin films, residual stresses create cracks and affect mechanical, electrical and thermal properties, so it is important to evaluate their residual stress. Instrumented indentation testing is an attractive testing method for evaluating residual stress because it is simple, fast and needs no special specimens. Comparing the load-depth curves in the stressed and stress-free state lets us evaluate the residual stress using IIT. However, in thin film at nano-scale, it is difficult to obtain a stress-free state. In this research, we evaluate the residual stress by estimating the load-depth curve of the stress-free state from the indentation impression area of stressed specimen using the concept of invariant hardness. At constant load, the indentation impression area evaluated from optical measurement is not changed by the residual stress value because tensile residual stress reduces pile-up and compressive residual stress increases it. And since we can convert the indentation impression area from the elastic modulus, which is invariant with residual stress, we can estimate the load-depth curve in the stress-free state from the elastic modulus of the specimen.

The deformation behavior of thin films on substrates undergoing either single or cyclic thermo-dynamical loading has been investigated extensively in the last years due to the high relevance for industrial applications, ranging from cutting tools to microelectronic devices.
A popular method to evaluate the stresses occurring in the thin film during temperature cycling is to measure the change in the curvature of the thin film on the substrate during heating and cooling. From the change in curvature, knowing the elastic constants of the substrate and the film and substrate thickness, the stresses in the film can be calculated using Stoney&’s formula. Such wafer-curvature measurements are generally conducted at slow heating rates lower than several 10 K/s. This is mainly due to experimental constraints, but also originates in the fact that Stoney&’s equation is only valid if substrate and thin film have a homogeneous temperature, which might be violated for faster heating.
In power semiconductors, short high power pulses cause material heating with rates in the range of 10⁵ - 10⁶ K/s. It is questionable if the material response at low and high heating rates is comparable, which necessitates the development of methods to monitor the material behavior at heating rates comparable to the ones occurring during usage. Therefore, a new curvature measurement setup has been developed, where the curvature is measured from the reflection of incident parallel laser beams using a high speed camera, allowing much faster data acquisition rates (20000 fps) than with conventional cameras. Thin metallization films deposited onto polysilicon and single crystalline Si are heated by Joule heating using a pulse generator allowing to vary pulse shape, length, repetition rate and power. This can be used to vary the heating rate between 10e2 and 10e5 K/s and can be utilized to study the effect of cyclic heating with various temperature amplitudes and frequencies on the metallization behavior. Besides the description of the test setup, an overview on the stress evaluation procedure, necessitating the use of finite element modeling is presented. Changes in material response are deduced from changes in the stress-temperature behavior of the thin films after either one temperature cycle or after thermo-mechanical fatiguing. Microstructural and morphological changes in the coatings are investigated using SEM, FIB cross-sections and surface roughness measurements.

Nanoporous (NP) materials (e.g., Au and, of recent interest, Si) exhibit a random network structure of pores and ligaments. Modeling the mechanical properties of such structures is challenging in that the randomness inherent in complex NP geometries directly influences the nature and distribution of stress concentration points in NP materials. This is particularly important in predicting and mitigating the failure and deformation mechanisms dictating the bulk mechanical behavior of NP materials. Existing empirical models for NP materials properties are based on regularized, rectangular geometries and neither account for random network effects, nor give insight as to which geometrical structures in NP materials limit overall mechanical behavior. Here we report results of a new approach to generating model NP structures, which, when coupled with finite element method (FEM) calculations of the mechanical behavior of many model structures, can be used to deduce empirical rules for properties of NP materials from experimentally accessible geometrical characterizations of real NP materials—including, for example, details of ligament connectivity at nodes—combined with bulk elastic properties.

Ceramic reinforced metal matrix composites have interested many researchers and have been used as wear resistant materials commercially such as roll and mold materials. Among the various reinforcements, titanium carbide (TiC) is an attractive reinforcement for metal matrix composites because of its high melting temperature, excellent hardness, high modulus and thermodynamic stability. However, TiC reinforced metal matrix composites have poor interfacial properties between the reinforcement and the matrix due to contamination by impurities during fabrication processes. In contrast, composites fabricated by in-situ reactive processes shows the formation of finer reinforcement particles and clean interfaces between reinforcement particles and the matrix since thermodynamically stable reinforcement particles form within the metal matrix.
In the Ti-C phase diagram, TiC_x has a wide stoichiometry which ranges from x=0.47 to x=0.98. It has been reported that mechanical properties of TiC_x, such as hardness and modulus, are degraded, as the stoichiometry (x in TiC_x) become lower. In addition, the stoichiometry of TiC_x could influence the morphology of TiC_x. Also, residual carbons after forming non-stoichiometric TiC could have effects on phase and microstructure of metal matrix. Therefore, it is required to study on effects of the C/Ti ratio on microstructure and mechanical properties of in-situ TiC reinforced metal matrix composites.
In this work, in-situ TiC_x/Fe composites were prepared with varying of C/Ti ratios in order to investigate the effects of the C/Ti ratio on the microstructure and mechanical properties of in-situ TiC/Fe composites. The phase analysis showed the formation of in-situ TiC in the Fe matrix. Also, fine TiC particles with uniform size of 1 - 2 mu;m were homogeneously distributed in Fe matrix. Additionally, the mechanical properties of in-situ TiC_x/Fe composites were varied with reactant C/Ti ratio and compared to those of ex-situ TiC/Fe composites, indicating that hardness, wear resistance, strength and toughness of in-situ TiC/Fe composites were enhanced in comparison with those of ex-situ TiC/Fe composites due to improved interfacial coherency between TiC and Fe matrix in in-situ TiC/Fe composites with finer TiC particles.

Martensitic stainless steels are widely used for structural applications because of their superior mechanical properties and moderate corrosion resistance. These steels often retain an austenitic phase. Both positive and negative effects of the austenitic phase on the mechanical properties of martensitic steels have been reported, which may be attributed to the varied morphologies of the austenitic phase and the complexity of the martensite microstructure. The mechanical characteristics of the martensitic steels on the microstructural scale can be examined by microtension testing. In this study, we used microtension testing to analyze the plasticity of martensitic stainless steel containing austenite with particular focus being on the origin of the plastic anisotropy of the lath martensite structure.
The material used in this study was a commercial martensitic stainless steel JIS-SUS420J2. To obtain a coarse-grained lath martensitic microstructure, samples with dimensions of ~30 mm × 20 mm × 0.6 mm were solution-treated at a temperature of 1473 K for 3.6 ks followed by water quenching. The coarse-grained samples were re-heated to a temperature of 773 K and were subsequently maintained at this temperature for 5.4 ks followed by cooling in air in order to obtain the martensitic microstructure with the austenitic phase (hereafter denoted as M/A microstructure). The volume fraction of the austenitic phase was determined to be 6.5% using an X-ray method. For comparison, a sample with a fully martensitic microstructure, hereafter denoted as M microstructure, was prepared by a subzero treatment in liquid nitrogen. Microtension specimens with a gauge section of ~20 mu;m × 20 mu;m × 50 mu;m were fabricated using a focused ion beam. The crystallographic orientation of the gauge section was determined by electron backscatter diffraction analysis. Two types of specimens were prepared: the loading directions of one type are parallel to and those of the other type are inclined at an angle of ~45° to the habit plane of the lath martensite, (denoted as P- and I-specimens, respectively). Microtension testing was performed at a loading rate of 0.1 mu;m/s at room temperature in air.
In both the M/A and M microstructures, the P-specimen exhibited a high yield stress and tensile strength when compared to the I-specimen. This lath-orientation-dependent plastic anisotropy is commonly observed in the lath martensite structures in carbon steels. For the M/A microstructure, the critical resolved shear stress (CRSS) values of the activated slip system for the I- and P-specimens were determined to be 489 and 665 MPa, respectively. The ratio of these CRSS values indicates plastic anisotropy. The CRSS ratio for the M/A microstructure, 0.73, was lower than the value of 0.90 obtained for the M microstructure. This finding suggests that the presence of the austenitic phase may play a crucial role in the plastic anisotropy of the lath martensitic steel.

The B2-type FeCo alloy exhibits good soft magnetic properties with a very high saturation magnetization. However, its low workability is a major drawback. It is very brittle in the ordered state owing to the occurrence of intergranular fracture. On the other hand, it was reported that ordered FeCo single crystals could be deformed by the activation of slip. To improve the mechanical properties of the FeCo alloy, it is essential to understand the relationship between the deformability and the grain boundary strength. Meanwhile, we have recently used microtension-testing techniques to analyze the mechanical properties on the order of a few tens of micrometers. This method enables us to not only to characterize the crystal plasticity but also evaluate the strength of the grain boundary. In this study, we conducted microtension testing using single- and bi-crystalline specimens of an FeCo intermetallic to investigate the crystal plasticity and the intergranular fracture strength.
The material used in this study was an Fe#8210;51 mass% Co alloy prepared by arc melting. The orientations of the grains were determined by electron backscatter diffraction analysis. Microtension specimens with gauge dimensions of 50 mu;m × 20 mu;m × 20 mu;m were fabricated using a focused ion beam. Single- and bi-crystalline microtension specimens were prepared. The loading direction (LD) was nearly parallel to the [111] direction for the single-crystalline specimen, and the LDs were close to the [111] and [123] directions for the bi-crystalline specimen. A microtension test was performed at a loading rate of 0.1 mu;mmiddot;s^#8210;1, at room temperature in air. For comparison, a millimeter-sized polycrystalline specimen with gauge dimensions of 1 mm × 0.4 mm × 0.4 mm was tested at a loading rate of 0.1 mmmiddot; s^#8210;1.
An ultimate tensile strength of 294 MPa and an elongation to failure of 5% was determined by tensile testing using the millimeter-sized polycrystalline specimen. Fractographic observation showed that intergranular cracking was followed by cleavage fracture. In contrast, the [111] single-crystalline specimen exhibited a high ultimate tensile strength (695 MPa) and a high elongation to failure (31%) compared to the polycrystalline specimen. The critical resolved shear stress (CRSS) for the {110} <-111> slip system, whose Schmid factor is 0.308, was determined to be 110 MPa. In the bi-crystalline specimen, although plastic deformation commenced in the [123] crystal oriented favorably to the slip gliding, intergranular cracking occurred finally. The intergranular fracture stress was determined to be 357 MPa through microtension testing.

Due to the shift of the flexible electronics technology from basic R&D to industrial manufacturing the question of mechanical reliability becomes more and more relevant. At the same time, there are no standardized test methods which allow for fast and versatile characterization of mechanical reliability of electronic components on flexible substrates. Here a new technique (FLEX-E-TEST) for evaluation of mechanical reliability of polymer-supported metal films is presented. FLEX-E-TEST technique provides several important advantages compared to the existing bending tests. It is fully automated, allows for testing of up to 8 samples simultaneously and there is no restriction in sample form-factors. The bending radius can be changed from few millimeters to infinity with bending possible in both directions within a single test. The new bending test technique was applied to different polymer-supported thin films including ink-jet printed silver, PVD silver, copper and gold lines, ITO as well as hybrid rigid-flexible circuits. With the help of a new bending test apparatus it is shown that cyclic tensile, compressive, and mixed tensile-compressive bending strains result in different amounts of induced mechanical damage in printed silver lines. In contrast, evaporated silver lines with the same geometry show no dependence on the type of strain. A detailed comparison of the fracture mechanisms in printed and evaporated silver is given using scanning electron microscopy and focused ion beam analysis.

Here we introduce two in-situ scanning electron microscope (SEM) techniques for measuring mechanical properties of small-scale materials at elevated temperatures. The first technique is tensile testing of sub-micron thin films using a custom-built apparatus. Measurements at elevated temperatures are performed through use of two silicon-based micro-machined heaters that support the sample. Each heater consists of tungsten heating element that also serves as a temperature gauge. The mechanical behavior of Cu, Au, ZrB₂ thin films are studied at temperatures up to 740°C using tensile testing. Films tested at elevated temperatures exhibit a significant decrease in flow stress and stiffness. This behavior is attributed mainly to diffusion-facilitated grain boundary sliding.
In the second technique, the high temperature plastic response during compression test of single crystal microcrystals are achieved through in-situ SEM nanoindentation with independent heating of both the indenter and the sample. Thermocouples are located next to tip and sample, and the temperature of both is precisely controlled to reduce contact thermal drift. Compression tests of a-axis single crystal Magnesium micro-pillars using this setup will be presented. The effect of temperature and crystal size on the competition between twinning and dislocation slip will be discussed.

Self-assembled monolayers (SAMs) provide a platform for tailoring interfacial properties between a thin film and a substrate. In this work, we investigate changes in dynamic interfacial fracture energy of a transfer printed gold (Au) film on a silicon (Si) substrate with variable SAM-mediated interfaces. SAMs are selected with mixed end groups of thiol (-SH) and methyl (-CH₃), which are formed by varying the molar ratio of 11-mercapto-undecyltrimethoxysilane (MUTMS) and dodecyltriethoxysilane (DTES) solution during fabrication. The lengths of the monolayers are held constant to allow direct comparison of interfacial dynamic fracture energy across varying thiol and methyl functionalized SAM varieties. Dynamic interfacial fracture energy between the SAM-mediated interface and Au transfer printed film is measured through laser-induced dynamic delamination protocol. Measured dynamic interfacial fracture energy varied from 1.86 J/m² to 3.63 J/m² with changing concentrations of SAM end-group functionality. We find that interfacial failure properties between the silicon substrate and Au film are highly sensitive to the end-group composition and concentration of the SAM-mediated interface.

Diffusion aluminide bond coats are compositionally and microstructurally graded materials with significant variation of engineered mechanical properties across the cross-section. The thermal cycles treatment of bond coats during service leads to extensive interdiffusion of elements between the substrate and the coating which create further gradient in chemistry and microstructure and hence modifies properties. Nanoindentation, particularly in situ, can be considered as a well-suited technique for measuring the properties of such complex microstructural materials as the deformation volume can be carefully controlled to probe different precipitates and microstructural zones. In this study, an SEM nanomechanical instrument, PicoIndenter 87xR, with an integrated high-temperature stage and an active tip heating was used to conduct nanoindentation on the cross-section of aluminide bond coating at room temperature, 200^oC, 400^oC, 600^oC, and 800⁰C. With combined analysis of chemistry and microstructural changes, the indentation results are used to understand local mechanical properties variation as a function of temperature.

In this paper, molecular dynamics (MD) calculations have been used to examine the physics behind continuum models of misfit dislocation formation and to assess the limitations and consequences of approximations made within these models. These MD calculations consider arrays of dislocation dipoles, and use periodic boundary conditions to create a direct correspondence between atomistic and continuum representations of the dislocations. Using large systems and long-time averages of system properties, we directly compare our MD calculations with a rigorously derived continuum energy expression for the same dipole configuration. This comparison provides insights on the level of accuracy needed to estimate dislocation core radius and energy, and an identification of precise definitions of dislocation spacing and Burgers vector in lattice mismatched systems. We show that when these insights are incorporated into the continuum model, the resulting energy density expression of the lattice mismatched systems is essentially indistinguishable from the MD results.

Recently, experimental studies have shown that fracture surfaces in metallic glasses (MGs) generally exhibit nanoscale corrugations, which may be attributed to the nucleation and coalescence of nanovoids during crack propagation. To explain this behavior, we propose a model of a heterogeneous solid containing a distribution of weak zones to represent a brittle MG. 3D continuum finite element analysis of cavitation in such an elastic-plastic solid is performed with the weak zones idealized as periodically distributed regions having lower yield strength than the background material. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress for the onset of cavitation which is controlled uniquely by the local yield properties of these zones. Also, the presence of weak zones decreases the sensitivity of the cavitation stress to the volume fraction of a pre-existing void.

Halloysite nanotubes (HNT) have wide applications due to mechanical strength, thermal stability and natural abundance. Industry produced HNT generally possess scattering size distributions, with length range between 0.2 and 40 microns. Longer tubes are preferable due to their higher efficiency in transferring stress upon loading; however the size effects from HNT on coating materials&’ abrasion resistance have not been studied. The current research focuses on studying the influence of HNT dimensions to tailor the structural development in the nano-composite coatings, and how the microstructure affects mechanical robustness. HNT as obtained will be separated into multiple length categories, followed by mixing with polymers and coating on hard substrates. Mechanical properties will be examined for both control and composite coatings for reinforcement comparisons and to correlate their microstructural features. The study will also bring to light the potential of a facile coating method usage for producing high-performance composites of anti-abrasion applications, which is attractive for commercial products in terms of cost.

Grain boundaries are one of the major types of extended defects in crystalline metals that their interaction with hydrogen alters the mechanical behavior of metals. To investigate the role of grain boundary on hydrogen degradation, there are some difficulties. First of all, hydrogen has a volatile nature and interpretation of the ex situ testing of hydrogen charged metal due to the outgassing of the hydrogen in the course of the testing is very hard. Secondly, hydrogen interaction with a specific type of GB demands the examination in the same microstructural length scale.
In this paper, to overcome these difficulties, in situ electrochemical micro-cantilever bending test method were utilized on bicrystal cantilevers. To address this issue, the bending tests were done on hydrogen charged and hydrogen free conditions on bicrystal cantilevers with different kinds of predefined grain boundaries and the results were compared to single crystal cantilevers which were milled in the adjacent grains using focused ion beam.
Furthermore, post mortem misorientation characterization of bended cantilevers with a predefined grain boundary was done using electron backscatter diffraction method to study the interaction of grain boundary with dislocations in hydrogen charged and H-free conditions.

Nanocolloidal crystals have emerging applications in photonics and optoelectronics, but their poor mechanical robustness is a major hindrance to their widespread application. Understanding the mechanical behavior of NCCs is critical to propose effective reinforcement techniques. In this study, NCCs composed of monodisperse 254 nm and 289 nm sillica nanocolloids were synthesized and also treated by two reinforcement techniques: sintering (800~1150°C) and alumina atomic layer deposition (ALD) (1~10nm). Nanoindentation was used to characterize the mechanical properties of the samples.
The as-assembled NCCs yield low hardness and modulus values, exhibiting granular behavior with intrinsically weak interparticle bonding (IPB), implying that increasing the IPB is essential for reinforcement. In addition, as-assembled NCCs exhibit unstable deformation, manifested as pop-ins upon nanoindentation. By deepening indentation, the unstable deformation mode transitions from NC dislodging to shear band (SB) formation. Shear band formation is a mode common for disordered granular materials and metallic glasses. To our best knowledge, this is the first report on shear band formation in NCCs (ordered granular materials). The shear band formation at different strain rates and indentation depths are also investigated.
Furthermore, the reinforcement mechanisms and effects of sintering and ALD on NCCs were systematically studied. Thermal sintering is currently the most common reinforcement technique; however, this method could induce serious cracking. We observed that ALD can be used to tune the mechanical properties of NCCs by depositing atomically-controlled layers around the nanocolloids within NCCs. We find that, both sintering and ALD treatments can locally harden and stiffen the NCCs. The IPB strength increases with increasing the sintering temperature and ALD-cycle number. One major difference between the two reinforcement methods lies in the crack formation. Thermally sintered NCCs are prone to indentation-induced cracking due to large residual tensile stress, significantly impairing the toughness. Interestingly, the ALD treatment is much more effective for stiffening (up to 30 folds), hardening (up to 150 folds), and more importantly toughening (up to 250 folds) NCCs, because ALD reinforces NCCs through the second phase material infiltration, which does not induce residual tensile stress. Also, we find that ALD significantly suppresses NC dislodging and SB formations in NCCs. By increasing the atomic-layer-deposition thickness (or sintering temperature), the deformation mechanism of NCCs transitions from granular to bonded granular and, finally, to monolith-like behavior. We believe this work presents the first systematic study of such transitions based on both experimental and theoretical approaches. Our finding provides insights for reinforcing and toughening various nanoparticle-based and nanoporous materials.

Montmorillonite clays and Halloysite nanotubes (HNTs) were modified by surface adsorption of resorcinol di (phenyl phosphate) (RDP) oligomers. Nanocomposites have been prepared by melt blending Poly (lactic acid) (PLA) with either, RDP coated montmorilonite clays, RDP coated HNTs, ditallow functionalized clays (Cloisite 20A), unfunctionalized sodium clays and HNTs. The exfoliation or intercalation phenomenon of those clays in PLA has been determined using XRD and confirmed by transmission electron microscopy (TEM). TEM images also showed that both treated and untreated HNTs were well dispersed in the PLA matrix. Gas permeability testing indicated that the clays with the higher degree of exfoliation were more effective at improving the gas barrier properties of PLA, while the tubular particles, regardless of treatment, did not influence the PLA gas permeability. In all the thermal and mechanical tests, both HNTs and RDP coated HNTs samples increased the Young&’s modulus, storage modulus and the tensile strength of neat PLA, but decreased the elongation a little bit, which resulted in an insignificant influence on the impact toughness. In contrast, all the nanocomposites with sheet structure particles performed much worse results on elongation and hence, significantly decreased the impact strength, especially at higher particle loading. These results were interpreted in terms of the morphology of the particles, since the interactions with the PLA matrix were similar.

The interest in the interfacial adhesion phenomenon stems from the utmost technological importance of composite and sandwich structures as well as the fundamental science underlying the phenomenon of adhesion [1]. The problem of strain-rate dependences of interface strength and adhesion energy, and mechanisms of failure in multi-material systems has long been the focus of research alongside the interface roughness effects on adhesive properties of multi-material systems. The advances in miniaturization of materials and devices, however, have revealed the length scale-dependent nature of the roughness effects on adhesion - i.e., what is called the adhesion paradox [2]. Using atomistic simulation studies [3] of the topographical properties of interfaces, formed due to a tensile failure of bi-materials, we characterized the rough-surface profiles and dependences of the roughness exponent and correlation length on the strain rate. Also investigated is the adhesion energy dependence on the strain rate for the adhesive and cohesive modes of failure. In all the cases, scaling relations of the corresponding quantities with strain rates are derived. The analysis of our simulation results shows that there exists a correlation between the roughness exponents of the surfaces, formed in either adhesive or cohesive mode of failure with the adhesion energy behavior. We put forth empirical arguments, explaining these observations, based upon the roughness-induced increments to adhesion energies between bodies having a characteristic dimensions at micro- or nano-scale [4].
[1] A. V. Pocius, Adhesion and Adhesives Technology: An Introduction (Hanser/Gardner Publications, Cincinnati, OH, 2002).
[2] K. Kendall, Molecular Adhesion and Its Applications: The Sticky Universe (Kluwer Academic/Plenum Publishers, New York, 2001).
[3] M. A. Makeev, P. H. Geubelle, N. R. Sottos, and J. Kieffer, ACS Appl. Mater. Interfaces 5, 4702 (2013).
[4] M. A. Makeev, Sol. State Commun. 166, 12 (2013); Phys. Lett. A 377, 2806 (2013).

Polymers are ubiquitous in our daily lives, and are cheap, light, and easy to process. However, in bulk form, their Young&’s modulus and tensile strength are low. Study has shown that an increase in crystallinity and crystallite alignment increases their Young&’s modulus significantly.
In this work, we endeavor to measure the mechanical properties of polyethylene nanofibers using a high fidelity micromachined test platform in which displacement is measured with 5 nm resolution and force is determined to 25 nN accuracy. The fibers are fabricated using a two-step method from a polyethylene/decalin gel. We have successfully fabricated high quality fibers with diameters ranging from 90 nm to 2 µm. Methods to perform the test are now routine. However, precisely locating nanofibers on the test platform is challenging. Our previous method using a tungsten probe to manipulate these fibers onto the testing device was tedious, time consuming and afforded low yield. We have now developed a robust method to position the nanofibers. After pulling the fiber, we directly attach it to a micromachined silicon frame. The frame next is positioned above the test platform and brought into contact with grips, to which it is subsequently glued. The fiber is then cut short to a length of 30 µm using a tungsten micro heater.
The mechanical properties of these nanofibers are now being tested with the test platform, which incorporates a micro machined stepper motor, a spring and grips. The actuator is capable of stretching the nanofiber up to 75 µm in 50 nm steps, thereby applying up to 250% strain on the nanofibers. We plan to characterize and report on the stiffness, strength and creep properties of these highly crystalline nanofibers using a temperature controlled chamber.

Cobalt (Co), a magnetic material, is of significant interest to data storage industry. The mechanical properties of Co films is, however, less well understood. In this study, we will present systematic studies on microstructure and mechanical properties of Co films prepared by a physical vapor deposition technique. By tailoring deposition parameters, we were able to obtain Co with both face centered cubic (FCC) and hexagonal closely packed (HCP) structures. Furthermore, various types of growth defects were observed in these films. The influence of growth defects on mechanical and magnetic properties of Co films will be presented. This work is supported by NSF.

Cubic boron nitride (cBN) is a very hard polymorph of boron nitride, formed under extreme temperatures and pressures, similar in crystalline structure to the diamond allotrope of carbon. The synthesis methodology in use leads to a grain based microstructure which ranges from 10 to 50 microns, with unknown material properties within and between the individual grains. By utilizing various testing modalities, the composition and structure of the grains may be determined for use in material development. Microscale characterization methods were performed using a MicroMaterials Nanotest Vantage nano-indenter, capable of indentation, scratch, and impact testing. By utilizing the multiple capabilities of the machine, an enhanced view of the behavior of the cBN may be determined. Simple indentation testing, high strain rate testing (both single and repetitive impact wear testing), and single and multiple scratch (wear) testing were performed at multiple points on the sample and under multiple loads. The majority of the nanoscale characterization methods are performed using a TI-750 UBI Nano-indentation machine manufactured by Hysitron. Like the Vantage, indentation tests were performed to determine the hardness and reduced modulus at various contact depths while the finer indent resolution and lower loads afforded by this machine allows multiple indents on a single cBN grain. It was determined that in general a single grain on the cBN surface is harder in the center than near the edges and on a larger scale, the cBN specimen is harder towards the center of the entire specimen than the edges of the specimen. Additionally, a cube-corner indenter was used to perform sharper indents to induce cracking in order to determine the fracture toughness K of the cBN.

In the last few decades new carbon based materials have been discovered. Important examples of these structures are fullerenes, carbon nanotubes and graphene. Single-wall carbon nanotubes (CNT) consists of a single graphite sheet (graphene) rolled up into a cylindrical shape. Many theoretical and experimental studies investigated their electronic and mechanical properties. Theoretical studies showed that mechanical deformation produced by torsional deformations (twistings) on carbon nanotubes can significantly alter their electronic properties [1]. In the present work, we have investigated through reactive (ReaxFF) molecular dynamics simulations, the dynamical aspects of the hydrogenation of twisted carbon nanotubes at several different temperatures and torsion angles. We investigated the structural changes of twisted carbon nanotubes at fixed angles (00, 1800, 3600, 4500 and 7200) when exposed to a gaseous atmosphere composed of hydrogen atoms. Our results show that the chemical reactions (hydrogenations) occur at a higher rate on highly curved regions of the deformed (twisted) nanotubes. This kind of behavior indicates that these regions have higher chemical reactivity, as expected. Our data also show that the number of hydrogen atoms incorporated into twisted CNTs increases almost linearly as a function of twisting angle values [3]. Untwisting the tubes favors the dehydrogenation processes. These results suggested that twisting/untwisting CNTs could be an effective way to storage hydrogen into CNTs or into related structures as the already experimented realized torsional carbon nanotube artificial muscles [2].
[1] L. Yang et al., Phys. Rev. B 60, 13874 (1999).
[2] J. Foroughi et al.. Science, v. 334, n. 6055, p. 494-497, 2011.
[3] J. M. de Sousa, P. A. S. Autreto, R. Paupitz, and D. S. Galvao - to be published.

Physical orientation process of polymer materials has been an essential research topic for designing new tough materials and improving existing materials. In this wok, the broadband coherent anti-Stokes Raman scattering (CARS) microscopy was performed to investigate the deformation of a homologous PE blend during stretching. Broadband CARS microscopy as a noninvasive and stain-free microscopy can acquire compositional and orientation images simultaneously. The microscopic structure and localized molecular orientation of both the amorhpous and crystalline phases in the PE blend were obtained. We demonstrate critical issues for performance of PE products during mechanical failure and the deformation mechanisms for semi-crystalline polymer materials.

Deformation of hydrided Zircaloy-4 has been examined using in situ loading of hydrided micropillars using synchrotron X-ray Laue microbeam diffraction, and in the SEM. Results suggest that both the matrix and hydride can co-deform, with storage of deformation defects observed within the hydrides, which were twinned. Hydrides placed at the plane of maximum shear stress showed deformation within the hydride packet, whilst packets in other pillars arrested the propagation of shear bands. X-ray Laue peak broadening, prior to deformation, was associated with the precipitation of hydrides, and during deformation plastic rotation and broadening of both the matrix and hydride peaks were observed. Post-mortem TEM of the deformed pillars has indicated a greater density of dislocations associated with the precipitated hydride packets, while the observed broad- ening of the hydride electron diffraction spots further suggests that plastic strain gradients were induced in the hydrides by compression.

The hydrogen assisted material degradation of structural materials has received increasing attention during the past decades. Various strong views on micromechanisms of hydrogen embrittlement has been proposed based on a large number of experimental studies, however, none of them is widely agreed so far. The main difficulty lies on the complexity of hydrogen interaction with materials and on the lack of critical experiment approaches to sense the effect.
To this end, we introduce a novel in-situ micro-cantilever bending test, which is conducted with nanoindentation device inside the environmental scanning electron microscope (ESEM), to sense the hydrogen effect both from the aspects of mechanical property and microstructure evolution. The hydrogen is introduced into the FeAl cantilevers during the bending test, the mechanical response is recorded by the Pico-Indent system and the microstructure evolution is tracked by the SEM. Moreover, various multi-scale characterization techniques (e.g. secondary electron imaging, electron backscatter diffraction, transmission electron microscopy, etc) are employed to analyse the deformation substructures after the test. The primary results reveal a decrease in flow stress when the cantilevers are bended with the presence of hydrogen, which are comparable with the results obtained from the bending tests with in-situ electrochemical hydrogen charging. This new approach introduced here gives the opportunity to exclude the extrinsic influence from outside environment and to sense merely the hydrogen effect.

Crystal plasticity in micropillars under uniaxial compression has been characterized by two major observations: Smaller is stronger (yield stress decreases with width) and plastic deformation is predominantly stochastic and abrupt with a non-Weibull event distribution, with the average size tending towards ultra small length scales. The connection and range of its validity between the two observations has remained unknown.
We present a model that combines both effects in an intrinsic manner and we perform an extensive analysis of the level of their potential association. Our model is qualitatively consistent with experimental observations on uniaxial compression data in micropillars with diameters smaller than 2 micrometers.
We find strengthening consistent with experimental data with a non-trivial exponent 0.42. The observed events in stress-drops grow as the system becomes smaller in a characteristic fashion and we characterize the constitutive behavior. The boundary of the pillars display characteristic structure that scales also with the width. However, the statistics of the events become a power-law only in the limit of very small widths -- where bulk sources are absent. Our simulation data is qualitatively consistent to experimental observations. We discuss the limitations of our modeling approach and possible extensions.

Continuing device miniaturization (e.g., MEMS/NEMS, semiconductor devices) and microstructural refinement has opened up questions on the fundamental mechanical response of a material as a function of the governing intrinsic and/or extrinsic size. Today, it is well-accepted that the strength and ductility of a material show size effects with shrinking material dimensions and therefore substantially deviate from the observed macro-behavior. While several approaches for studying the plastic deformation behavior at the micron and submicron length scale exists, downscaling existing test geometries for fracture testing is not straight-forward. Valid experiments for analyzing the fracture toughness require certain sample geometries, which are as of today defined by ASTM standards. Clearly, miniaturized samples cannot conform to the macro ASTM standards. During the last decade several testing geometries for analyzing the fracture toughness of individual grains or interfaces have been developed worldwide and will be reviewed in this talk.
Various micro-scale fracture testing geometries, which are either modified forms of bulk test geometries or entirely new techniques, will be presented and their applicability to different material systems and advantages and limitations discussed. Four such commonly used test geometries are chosen for a validation test on single crystal Si, whose fracture toughness is well established. To maintain consistency with respect to sample composition and purity, specimen preparation induced damage and artifacts as well as the same testing equipment and environment, all the four geometries are machined on one Si (100) wafer using the FIB and tested in-situ in the SEM using the same indenter for loading all of them. The results show a consistent average value of fracture toughness of 0.8 MPam^1/2 for all the four test geometries. Implications of these results to the small-scale fracture mechanics community at large will be discussed.

Magnetic materials are a limiting factor for the efficiency of many devices for energy conversion such as electrical generators and transformers. The use of Soft Magnetic Composites (SMC) offers high magnetic permeability, high resistivity, and thus small eddy current losses, as well as reduced weight and size of devices. These unique properties of the SMCs lead to new design options for 3D components compared to conventional laminated soft magnetic materials. Those new designs, however, may demand besides the magnetic properties of SMC a certain mechanical robustness of the material.
Typically, SMCs consist of large iron particles coated with fairly thin inorganic layer. This insulating layer in the SMC leads to an increase in resistivity reducing eddy current losses. On the other hand, the iron particles dominate in a favorable manner the magnetic properties. In terms of mechanical properties, however, the combination of soft iron particles with a brittle insulating layer causes a rather poor mechanical behavior of the SMC. For instance, macroscopic samples fail in tension at stresses lower than 100 MPa in a brittle fashion whereas in compression the material can be deformed plastically at flow stress of more than 300 MPa [1].
In this work, SMCs are studied which consist of pure iron particles that are coated by a thin inorganic, phosphorous layer and which were annealed after compaction in order to form a strength-enhancing oxide. The resulting structure of the boundary layers is complex consisting of several individual layers and typically pores along the interfaces. In order to better understand the role of these interfaces for the mechanical properties, micromechanical testing methods were applied to measure locally the intrinsic strength of the Fe matrix as well as of the boundary layers as a function of their specific structure. Using focused ion-beam preparation, micropillars and cantilevers, containing only one boundary, were produced and tested by a nanoindenter.
In principle, the load-displacement curves of the cantilever bending experiments are indicative of brittle fracture. Scanning electron microscopy observations confirm that failure occurs along the boundary layers. Different fracture morphologies are observed ranging from crack opening to complete fracture, depending on details of the boundary structure. In micro-compression tests, the strength was measured as a function of the position and structure of the boundary. Overall, the results of the tests suggest that the intrinsic strength of the boundary layers limits the mechanical behavior and needs to be improved for making more robust SMCs.
[1] H. G. Nguyen, G. D., A. Hartmaier (2013). Grenze der Einsetzbarkeit eines weichmagnetischen Pulververbundwerkstoffes aus Sicht der Mechanik. 19. Symposium Verbundwerkstoffe und Werkstoffverbunde. Karlsruhe.

Ever since quasicrystalline materials, or quasicrystals (QC), were discovered Shechtman, they have been found to be a host of many unusual mechanical and physical properties. However, they are intrinsically brittle: steady-state plastic deformation is only possible at high temperatures (above ~75% of their melting temperatures) or under confining hydrostatic pressure. Their engineering application is significantly impeded and the plasticity at lower temperatures is poorly understood.
Here, we report micro-compression and micro-bending of micron- and submicron-sized icosahedral Al-Pd-Mn QC-pillars at room temperature. We demonstrate that QC Al-Pd-Mn exhibits a brittle-to-ductile transition at the pillar diameter of ~400 nm. Above this critical size, the QC pillars fail by cracking along shear direction, while below this size QC pillars show extraordinarily high compressive strain over 50% and a remarkably high yield strength of ~5 GPa.
Additionally, we employed in-situ micro-compression technique to deform decagonal Al-Ni-Co quasicrystalline micro-pillars from room temperature to 500°C. The quasicrystal micro-pillars do not exhibit temperature-dependent yield strengths below 400°C, but they obviously show three different regimes (below 200°C, 200-400°C, above 400°C) in terms of serration phenomena and dislocation activities. The different behaviors in each temperature regime could be attributed to the activities of phason faults.
Deformation of small-scale QCs reveals a unique mechanism to with crystalline and amorphous metals which is peculiar to quasicrsytals rather than regular crystals and amorphous solids.

Capacity fade in commercial lithium ion batteries has been partially attributed to fracture resulting from large volumetric changes that occur in lithium-hosting ceramics upon repeated cycling. LiCoO₂ was extracted from a cycled battery cathode and was subjected to nanoindentation at loads high enough to induce fracture. Instead of radial cracking initiating from the indentation corners, cracks took on crystallographically preferred orientations. Site-specific focused ion beam extraction and subsequent TEM analysis showed that intra- and inter-granular fracture occurred on {001} planes. Computation of the charge density structure for LiCoO₂ indicated that the Li-O bonds along the {001} planes require the lowest energy for cleavage, supporting the experimental findings. The implications of these findings on LiCoO₂ electro-chemo-mechanical behavior will be discussed.

This paper presents the results of recent studies of failure mechanisms in organic light emitting devices (OLEDs), organic solar cells and stretchable/bendable electronic structures. The failure of OLEDs is shown to be associated with the formation of surface blisters that form as a result of the thermal expansion mismatch and Joule heating around interfacial impurities. The degradation of organic solar cells and OLEDs is also shown to be associated with the microstructural evolution of active layers in bulk-heterojunction solar cells. The implications of the results are then discussed for the pressure-assisted fabrication of OLEDs and organic solar cells with improved performance. Finally, the paper explores the effects of micron-scale wrinkling and buckling modes on the deformation of stretchable and bendable electronic structures with gold thin films or organic solar cells/OLEDs deposited on stretchable or bendable polymeric substrates. The implications of the results are discussed for the design of robust electronic textiles and organic solar cells/OLED structures.

The ideal strength and structural evolution of methane hydrate has been determined at the nanoscale, and dislocations' core structure and mobility has been modeled at the microscale. Methane hydrates are crystalline compounds in which hydrogen-bonded water molecules entrap methane gas within cages at high pressures and/or low temperatures. Although they exist abundantly almost all over the world and are considered an alternative energy resource, the mechanical properties of these compounds have not yet been fully investigated. All past studies on pure hydrates have been expermental and involved polycrystalline aggregates with all kinds of defects combined into a single model.
Density Functional Theory (DFT) and molecular statics (MS) have been used to model plastic deformation. Using DFT, the ideal strength has been determined under uniaxial and shear deformations in characteristic lattice directions. Under uniaxial tension, the hydrate loses stability at critical nearest neighbor oxygen-oxygen and hydrogen bond lengths which was found to be identical to that of ice Ih and empty hydrates. This signifies the minimum role of the methane molecules in the cages at the ideal tensile strength. Also, the ideal shear strength was found invariant in different slip systems which reflects the lack of a dominant slip system resulting from the radial bond arrangement in hydrates. These ab initio findings have been used to guide MS simulations in terms of choosing appropriate atomic force fields and slip systems for dislocations. Using MS, edge and screw dislocations have been modeled in terms of the pressure dependence of the Peierls stress, dislocation width, and mobility. Results are compared with the continuum Peierls-Nabarro model. As expected, the Peierls stress increased with pressure, and screw dislocations were found less mobile than edge dislocations. The lower mobility of screw dislocations may be due to the complex three-dimensional structure of hydrates which requires greater bond rearrangements for screw dislocations to move than edge dislocations. At a certain pressure, the dislocation was found to stabilize by dissociating into two partial ones.
While these results are important due to their novelty and ab initio basis, their physical implications are far-reaching. The existence of a critical hydrogen bond length for ice and hydrates upon tension allows the results to be extended to other hydrates, such as carbon dioxide hydrates. Also, the equivalence of slip systems in hydrates and the difference in mobility in different dislocations are essential for risk assessment studies on the stability of earth strata containing hydrates and for the design of hydrates as media for storing and transporting gas.

Fibrous materials such as nano- fibers, nanotubes, and their twisted yarns are great candidates to achieve multifunctionality facilitated by their lightweight and enhanced interaction surface. Current research is focused on improving the interfacial properties between individual elements in twisted yarns, given that interfaces are often the weakest points in the structure. The weak shear interaction between adjacent nanofibers or nanotubes prevents these materials from achieving their maximum theoretical performance. Interaction between neighboring elements in fibrous materials is often weak van der Waals (vdW) forces or hydrogen bonds if they are properly functionalized. There has been major effort in establishing hydrogen bonds at interfaces, including hydrogen bonds in carbon nanotube (CNT) yarns. Electrostatic (Columbic) interactions are much stronger than vdW interactions (sim;1 kT) and hydrogen bonds (sim;10 kT). Columbic interactions can be as strong as covalent bonds (100minus;300 kT). Engineering interfaces with endogenous electrostatic interactions can be beneficial in enhancing the interface strength and ultimately results in lightweight materials. Such interactions are believed to exist between collagen fibrils in bone. Collagen fibrils are piezoelectric materials and, hence, would generate surface charges in bone under deformation. This mechanism could be one of the reasons for high toughness of natural materials such as bone. One potential candidate for engineering interfaces with electrostatic (Columbic) interaction would be piezoelectric materials. Mechanical tension in these materials results in surface charges that could enhance the mechanics of the interface. Among piezoelectric polymers, the piezoelectric properties of polyvinylidene fluoride (PVDF), and its copolymer PVDF-TrFE (polyvinylidene fluoride trifluoroethylene) has attracted considerable interest. In this study, we report on highly stretchable piezoelectric structures of electrospun PVDF-TrFE nanofibers. We used twisting process to develop nanofibrous PVDF-TrFE yarns out of ribbons. Our results show that the twisting process not only increases the failure strain but also increases overall strength and toughness. Through overtwisting, we fabricated novel polymeric coils out of twisted yarns. Overtwisting here means that once the yarn samples were obtained by twisting the ribbons, we apply additional twist to fabricate coil samples from yarn samples. The coils can stretch up to sim;740% strain. This enhancement in mechanical properties is likely a result of increased interactions between nanofibers, contributed by friction and vdW interactions, as well as favorable surface charge interactions as a result of piezoelectric effect. We present a theoretical model to account for contribution of piezoelectric effect in mechanical properties. Reference: Baniasadi et al, ACS Applied Materials & Interfaces (2015).

The development of high capacity, light-weight, high efficiency, highly durable, and mechanically robust energy storage materials can have critical environmental and economic impacts. One such material - a graphene aerogel phase change material (GA-PCM) composite- is developed and investigated in this study. The mechanical properties and failure modes of graphene aerogel (GA) and GA-PCM nanomaterials are studied at the nano-, micro-, and macroscales and at different temperatures.
Phase change materials (PCMs), e.g., paraffin, have a high energy density and store thermal energy through phase change (melting). However, paraffin&’s intrinsic low thermal conductivity results in a low thermal efficiency, and the flow and leakage of liquid paraffin can lower the system reliability. Graphene is a two-dimensional material with ultrahigh in-plane thermal conductivity, and graphene aerogel (GA) forms a three-dimensional (3D) graphene network with extremely low packing density (<3%). In this study, we developed a GA-PCM composite, where the GA functions thermally as fast thermal transfer path and mechanically as a nanofiller to reinforce the paraffin matrix.
GA was synthesized from the reduction of graphene oxide (GO) hydrogel, showing a very high surface area (~ 476m²/g) and an extremely low density (16-36 mg/cm³). The GA-PCM was synthesized by infiltrating GA with paraffin, at a high paraffin concentration (>95%), guaranteeing a high energy storage capacity. The mechanical properties of GA, paraffin, and GA-PCM were studied using nanoindentation. GA&’s modulus and hardness are only 0.4MPa and 0.3kPa, respectively while the GA-PCM&’s hardness is ~20MPa, ~100% harder than pristine paraffin and much higher than the prediction of rule-of-mixture, implying chemical bonding between graphene and paraffin matrix. The temperature-dependent mechanical properties of paraffin and GA-PCM were tested using a digital durometer from which the moduli of the samples were estimated. By heating from 28°C to 37°C, the modulus of paraffin (melting temperature = 55.4°C) drastically drops from 10MPa to 0.5MPa, while the GA-PCM&’s modulus only slowly drops from 20MPa to 7MPa. Above the melting temperature, the GA-PCM&’s modulus reaches a non-zero plateau of 0.4MPa, matching the GA&’s modulus. More importantly, the GA-PCM keeps the original shape and holds the molten paraffin without any leakage, indicating superior shape stabilization.
To our best knowledge, this is the first study on the fundamental mechanical behavior of graphene aerogel (GA) and GA-paraffin composite at different temperatures, providing insights to understand the behavior of assembly of graphene sheets and graphene-reinforced nanocomposites.

This study presents a combine experimental and analytical investigation of the toughening behavior in natural fiber-reinforced earth-based composites. A specially designed single fiber pullout apparatus was used to provide a quantitative determination of interfacial properties that are relevant to toughening brittle materials through fiber reinforcement. The parameters investigated included a specially designed high strength earth-based matrix comprising of 60% laterite, 20% clay and 20% cement. The toughening behavior of whisker-reinforced earth-based matrix is analyzed in terms of a whisker bridging zone immediately behind the crack tip and interface strength. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker-matrix interface. Debonding with constant frictional stress was obtained and this formed the basis for the analytical model considered and the underlying crack-microstructure interactions associated with Resistance-curve behavior was studied using in situ/ex situ optical microscopy to account for the bridging contribution to fracture toughness. The effect of multiple toughening mechanisms (debonding and crack bridging) was elucidated and the implications of the results are considered for potential applications in the design of robust earth-based building materials for sustainable eco-friendly homes.

Direct coupling of Raman spectroscopy with nanoindentation instrument provides the capability for full mechanical characterization of the material at the nanoscale and its direct correlation to the localized chemical composition. The vibrational (phonon) states of molecules detected using Raman spectroscopy give a molecular fingerprint of the physical state of a matter. At the same time, a nanoindentation curve serves as a fingerprint of a material&’s mechanical properties. In situ Raman mapping performed before mechanical testing allows for precise positioning of the mechanical test based on chemical structure. Raman maps performed after nanomechanical tests will provide comprehensive information about the internal stress distribution within the material, resulting from plastic deformation during the test. Raman maps can be acquired in fully automated routines together with in situ SPM imaging, modulus maps, or electric resistivity maps.
An automated array of Raman and nanoindentation measurements was performed across the enamel and dentin with tests spaced at 30mu;m intervals. Raman spectra were collected at 785nm laser excitation wavelength within the range of 50-1800 wavenumber (cm^-1). An intensity of mineral peak v₁(PO₄) is associated with a volume of hydroxyapatite within the tissue. An overlay of an optical micrograph with correlated Raman and indentation maps shows a descending gradient in mineralization followed by decreasing elastic modulus from the enamel outer layer to the less mineralized dentin.
Other nice example of in situ Raman spectroscopy option is monitoring stress distribution around an indent in Si wafer introduced by post Raman mapping and monitoring a shift of silicon peak at 520cm^-1. Peak shift varied from 505cm^-1 up to 529cm^-1 in regions with peak bands <520cm^-1 and >520cm^-1 indicating tension and compression stress fields, respectively.

Wood possesses hierarchy of structure ranging from individual wood polymers to cells to growth rings. The development of new forest products is hindered by the lack of fundamental understanding of how molecular-scale modifications affect properties of bulk wood and wood composites. Nanoindentation-based mechanical spectroscopy is well suited to probe the individual cell wall layers and provide new information about how modifications of wood polymers affect bulk wood properties. Mechanical spectroscopy is the assessment of a mechanical index, such as the viscoelastic Young&’s modulus or the plastic flow stress, across a broad spectrum of time scale, deformation rate, temperature, or moisture content. In addition to providing thorough mechanical characterization, which is useful to predict material performance over a wide range of conditions, mechanical spectroscopy also provides information about the microphysical processes which are causally linked to the properties. Broadband nanoindentation creep (BNC) has been developed to measure viscoplastic properties across 4-6 decades of strain rate and dynamic nanoindentation can be used to measure viscoelastic properties across more than three decades of frequency. The relative humidity (RH) during the test can be controlled between dry air and 95%. Results will be presented from BNC and dynamic nanoindentation performed on the S2 secondary wood cell walls, which are anisotropic composites consisting of semicrystalline cellulose microfibrils embedded in a matrix of hemicelluloses and lignin. Experiments were performed in multiple orientations with respect to the cellulose microfibrils to study anisotropic effects and over a wide range of humidity conditions. Moisture softened the wood cell walls causing substantial decreases in BNC-derived plastic flow stress and storage modulus. The tan delta increased with moisture. In experiments on a longitudinal plane, which means the stiff cellulose microfibrils were oriented perpendicular to the indentation direction, a peak was observed in tan delta at humidity values above 55%. The peak moved to higher frequency with increasing humidity. The peaks were attributed to a moisture-induced glass transition in the hemicelluloses of the S2 secondary wood cell wall. The moisture-induced glass transition in hemicelluloses&’ is of particular interest because it is hypothesized that percolated networks of softened hemicelluloses create diffusion channels for ion transport through wood cell walls. Because the fungal decay of wood requires ion transport through wood cell walls, a chemical treatment that prevents the hemicelluloses&’ glass transition may also be an effective non-toxic wood treatment to protect wood products against decay. Nanoindentation-based mechanical spectroscopy can now be used as a tool in wood science research to study the effects of chemical modifications on the moisture-induced glass transition in hemicelluloses.

Biological cells form dynamic molecular contacts with extracellular materials, providing nanoscale to microscale features that enable adhesion to, mechanical force generation by, and motion of cells. The strength and failure of these adhesive contacts is increasingly understood through experiments, modeling, and simulation. For example, the force required to rupture or "break" these ligand-receptor complexes depends on the rate of force application, and also on the stiffness of the materials on either side of the complex. As the cell generates its own force transmitted through these adhesive contacts, the finite thickness of the underlying materials can affect the perceived stiffness of the surroundings, and in turn the bond lifetime. The lifetime of these adhesive complexes also depends on local chemistry, where acidity increases the likelihood that many such contacts form and effectively make the adhesion of cells to extracellular materials stronger. Here, we will discuss our recent findings and approaches to study the strength, failure, and implications of cell-material adhesion that are coupled strongly to local mechanics and chemistry.

Attachment pads and roots of climbing plants provide excellent permanent attachment, especially with respect to dynamic loads and wind drag. In plants with adhesive pads and adventitious roots, attachment is achieved by a form closure at sub-micrometer length scales and in some species in combination with the secretion of an adhesive. Frequently, a significant stiffness difference between the plant material and the climbing substrate is observed, which would be expected to promote failure of the attachment. The climbing plants, though, exhibit quite effective compensation mechanisms including load damping by coiling of the tendril main axis, composite or graded structures or sacrificial components.
In this study, the millimeter-sized attachment pads of the passionflower Passiflora discophora are investigated as a biological model for the design of technical attachment structures. A previous study has revealed a heterogeneous structure in the pad interior, while the functionality of this structure has not yet been clarified. The passionflower pads adapt easily to different substrates, but preferably to porous surfaces. After only a few weeks, the attached pads enter senescence, which induces changes in the material, i.e. lignification, holes and shrinkage, without compromising the reliability of the attachment.
Statistical Nanoindentation Technique (SNT) was applied to investigate successive cross sections of a lignified attachment pad of the Passiflora discophora. A large number of indents and a statistical analysis of the elastic modulus revealed the presence of up to four structural areas in each cross section and enabled the calculation of their associated surface fraction. The microstructural analysis revealed a variation of the cell structure over the pad volume, which can be correlated to the mechanical properties determined from SNT. A simplified 2D finite element model is used to investigate the influence of the different structural regions of the pad interior under external loads. The results shall be used as guidelines for the design of reliable technical attachment structures.

Montmorillonite clays and Halloysite nanotubes (HNTs) were modified by surface adsorption of resorcinol di (phenyl phosphate) (RDP) oligomers. Biodegradable poly (lactic acid) (PLA) and poly (butylene adipate-co-butylene terephthalate) (PBAT) polymers were blended together with either, RDP coated montmorilonite clays, RDP coated HNTs, Ditallow functionalized clays (Cloisite 20A), unfunctionalized sodium clays and HNTs. The thermo-mechanical response of the samples were directly compared in order to determine the role of the particle morphology (tube vs sheet) on the materials properties. TEM images of thin sections indicated that even though both clay nanotubes and platelets segregated to the interfacial region between the two immiscible polymers, regardless of coating, only the platelets, having the larger aspect ratio, were able to reduce the domain sizes. The ability of clay platelets to partially compatibilize the blend was further confirmed by the dynamic mechanical analysis (DMA) which showed that the glass transition temperatures of two polymers tend to shift closer. No shift was observed with either coated or uncoated HNTs samples. Izod impact testing demonstrated that the rubbery PBAT phase greatly increased the impact strength of the unfilled blend, but addition of only 5% of clay filler decrease the impact strength by nearly 50% while a small increase was observed with nanotubes at that concentration. A simple model is proposed. The clay platelets are observed to cover the interfacial area. Although they are effective at reducing the interfacial tension, they increase the overall brittleness, and facilitate micro-crack initiation and propagation. On the other hand, the HNTs are observed to lie perpendicular to the interface, which makes them less effective in reducing interfacial tension, but far more effective at blocking micro-crack propagation.

The deposition of metal thin films on Polydimethylsiloxane (PDMS) is well established as a strategy to pursue deformable electrical interconnects. Besides exploiting proper metal film design based on structural mechanics arguments (e.g., horse-shoe and S-shaped interconnects), metal/polymer adhesion is a key requirement to achieve the goal of reliable stretchability. Indeed, the presence of a compliant polymer prevents strain localization in the metal film upon stretch, thus retarding ductile failure. Any delamination event at the metal/polymer interface leads the metal film to become locally free-standing, hindering all the aforementioned benefits.
Since pristine PDMS features very poor surface energy (in the range of 10^-2 J/m²), metal deposition is usually preceded by radio frequency plasma treatment in order to promote strong adhesion. While surface activation is inherently temporary and subject to hydrophobic recovery, a side effect of plasma treatment consists in the permanent embrittlement of a sub-micron thick layer of PDMS at the surface. Depending on the specific plasma treatment settings, the mismatch between the mechanical properties of the embrittled PDMS layer and its bulk counterpart can trigger several surface phenomena, ranging from wrinkling to cracking before any stretch is applied. This evidence complicates not only the subsequent metal deposition step: even when a good quality deposition and strong adhesion are achieved, the PDMS layer adhered to the metal film is no longer compliant and may feature unexpected cracking upon stretch. This would hinder the desired strain localization even in the absence of metal/polymer delamination.
This study is focused on the fracture behavior of plasma treated PDMS upon stretch. PDMS samples were treated with oxygen plasma at different power (50-100 W) and exposure time values (15-60 s). Uniaxial stretch was applied by means of a miniaturized tensile stage. The latter was combined with a confocal laser scanning microscope (CLSM), enabling in-situ topography of the plasma treated surface at different strain levels. In-situ CLSM revealed that, already at low values of strain, the embrittled PDMS layer relieved excessive deformation energy by means of extensive cracks, which develop orthogonal to the stretch direction. The resulting pattern featured an alternation of two different bands: (i) transverse cracks, exposing the underlying compliant PDMS and (ii) embrittled regions, undergoing well ordered buckling instabilities. Upon increasing stretch, the evolution of failure mainly consisted in the nucleation of new transverse cracks. While the characteristic dimensions of the buckling patterns were found to be loosely sensitive to the plasma treatment parameters, deeper and wider transverse cracks were observed for samples subject to harsher plasma treatment.

The interactions of a biodegradable scaffold with cells or living tissues depend on the time-evolution of the nanoscale properties of the scaffold. We present an in-situ quantitative study of early-stage swelling and degradation of Poly(lactic-co-glycolic acid) (PLGA)¹. A novel metrology scheme based on force microscopy measurements on patterns of PLGA nanostructures is developed to characterize the evolution of topography, volume and nanomechanical properties. Volume and nanoscale roughness show an oscillating behaviour during the first eight days of immersion; at a later stage, we observe a continuous decrease of the volume. The effective Young modulus exhibits a monotonic decrease from an initial value of about 2.4 GPa down to 9 MPa at day 14. The oscillating behaviour of the volume before the onset of full degradation is explained by a coupled diffusion-swelling mechanism. The appearance of a second maximum in the volume evolution results from the competition between swelling and degradation.
[1] A. C. Dumitru, F. M. Espinosa, R. Garcia, G. Foschi, S. Tortorella, F. Valle, M. Dallavalle, F. Zerbetto and F. Biscarini, Nanoscale 2015, 7, 5403-5410.

Poly (L-lactic Acid) (PLLA) is a semicrystalline and biocompatible polymer that is increasingly used in “transient” biomedical implants, ranging from orthopedic screws and plates to treat bone fractures to vascular scaffolds to treat Coronary Heart Disease. Implants made of PLLA undergo hydrolysis to form lactic acid that is readily metabolized by the human body, allowing them to harmlessly disappear after their job is complete. The present research examines the dramatic changes in semicrystalline morphology that occur during the manufacture and use of PLLA in bioresorbable vascular scaffolds that are used to restore blood flow through occluded coronary arteries. The polymer undergoes biaxial elongation during tube expansion; after laser cutting into a mesh of struts that are roughly 500mu;m long, 200mu;m wide and 150mu;m thick, the PLLA then undergoes locally intense elongation and compression during crimping down onto the balloon of the surgical device. When it is in position in the diseased artery, the crimped scaffold is expanded by inflation of the balloon. The resulting semicrystalline nanostructure changes over distances of a few microns, requiring X-ray microdiffraction to shed light on the structural changes that occur in PLLA vascular scaffolds up to the time of implantation, which govern their therapeutic function.
Using a 200nm diameter beam, WAXS patterns were measured at points separated by 5mu;m across the 100mu;m x 150mu;m region of extreme deformation where bending occurred (at a “U-crest”). During crimping, the outer bend (OB) of a U-crest is subjected to elongational stresses while the inner bend (IB) is subjected to compressive stresses. The diffraction patterns indicate highly oriented PLLA crystallites where elongation was imposed (near the OB) and crystallites tilted out of plane where compression was imposed (at the IB). On the microdiffraction path taking 5mu;m steps from the OB to the IB, WAXS also reveals an unperturbed region in the middle with an orientation similar to that created during tube expansion alone.
Upon deployment, the deformation is profoundly altered due to the structure created during crimping. The tilting of crystallites at the IB during crimping allows them to separate (rather than undergoing elongational deformation) when the IB is placed under tension during deployment. Consequently, the OB experiences relatively mild compressive stress during deployment. The result is a highly uniform and oriented structure, inherited from tube expansion and OB elongation without disruption during deployment. Despite PLLA&’s reputation as a brittle plastic, the solid state deformation does not fracture the scaffold; rather, it leaves the PLLA crystallites in the deployed scaffold with a high degree of orientation, giving the scaffold the radial strength to hold the blood vessel open.

The manufacturing of metal connections has traditionally involved high temperature processes such as welding or soldering. These connections or seals have many advantages including high strength, low permeability to air, and high thermal and electrical conductivity. Metal connections can be difficult to produce, however, in locations near heat sensitive components such as in integrated circuits and organic LEDs and photovoltaics without causing damage. Here, we use low cost copper nanorods grown by physical vapor deposition with high spacing from indium seeds to produce a bond at low temperature and pressure. We compare the mechanical, thermal, and electrical properties of these low temperature bonds with more traditional methods.

Molybdenum thin films play an important role in many electronic applications ranging from back contact electrodes in solar cells, diffusion barriers in microelectronics, source/drain electrodes in thin-film transistors to data bus-lines in liquid-crystal displays. Molybdenum has emerged as the material of choice, due to its high thermal stability and chemical inertness, good adhesion onto glass substrates, as well as low electrical resistance. With the development of flexible electronics, attention has been focused on the deformation behavior of these brittle metal thin films on polymers and their utilization in flexible systems. This study reports on magnetron sputter deposited molybdenum thin films and how the structure and residual stress of the thin films can be tailored by changing the deposition parameters to control the electro-mechanical behavior. Mo thin films were synthesized on polyimide foils and silicon substrates using an industrial-scale in-line sputter deposition system, equipped with a rotary magnetron and DC power supply. The typical columnar microstructure from stationary sputter deposition processes is compared to a zigzag structure obtained by varying the direction of the incident particle flux during deposition by off-axis deposition. In-situ characterization techniques were used to examine the mechanical and electrical response of the films during deformation. Monotonic and cyclic tensile tests were performed with in-situ resistance measurements to determine the crack onset strain of the films and the crack formation during straining was investigated with optical microscopy. In general, a high compressive stress state enabled the Mo films to withstand much higher tensile strains before the loss of conductivity and severe cracking occurred. Comparing the different microstructures, the films with zigzag-architecture exhibited a higher failure strain than the columnar grown films under the same stress conditions. Even after 1000 cycles to 1% applied strain, zigzag-structured film remained fully electrically conductive.

Wafer bonding of III-V semiconductors to Si is an increasingly popular approach for the fabrication of hybrid photonic devices. In a recent study, we proposed a method to directly bond InP to Si and avoid the penalties incurred in oxide-assisted bonding. 400nm thick InP membranes have been successfully bonded to Si for surfaces of ~ 1 cm² using this method. In the present paper we discuss a new method to characterize the strength of InP/Si interface at the nanometric scale employing both in-situ cube corner and ex-situ Berkovich nanoindentation techniques.
The mean low-load pop-in value (~0.5 mN) of the membrane bonded to bare Si is shown to be close to that measured in a bulk specimen confirming the good crystallinity of the bonded membrane. Under relatively large loads (~80 mN), another discontinuity in the indentation loading curve was observed. In fact, buckling of the InP membrane is observed for loads higher than 10 mN, as the interfacial crack propagates far from the indentation center.
An in-depth analysis of plasticity and fracture events that occur during nano-indentation was obtained using both cross-sectional in-situ scanning electron microscopy and transmission electron microscopy. The crack introduced by a cube corner indenter in the Si substrate deviates to continue along the interface. On indenting the membrane with a Berkovich tip, the Si/InP interface discontinuity is shown to act as a barrier to plastic flow. The difference in plastic deformation between the membrane and the substrate induces membrane rotation as determined from TEM and drives the interfacial crack [1,2]. The surface bonding energy can be extracted for a given debonding crack length and a corresponding blister height to be 0.6 J/m². While significant reconstruction of the InP/Si interface is observed, the reconstruction is not yet perfect. Annealing of the membranes at higher temperatures is expected to improve the interfacial strength and is under progress.
[1] K. Pantzas, G. Patriarche, E. Le Bourhis, D. Troadec, A. Itawi, G. Beaudouin, I. Sagnes, A. Talneau, Appl. Phys. Lett., 103, 081901 (2013)
[2] K. Pantzas, E. Le Bourhis, G. Patriarche, A. Itawi, G. Beaudoin, I. Sagnes, A. Talneau, Eur. Phys. J. - Appl. Phys. 65, 20702 (2014)

The catastrophic optical degradation (COD) of high power laser diodes consists of the formation of networks of extended defects, preferentially at the front facet. The degradation of the active zone of the laser is triggered by the formation of dislocations, which propagate along the laser cavity driven by the optical field. The laser structure consists of a stack of epitaxial semiconductor layers with different thicknesses: the QW (#61627;10 nm thick) is the active zone of the laser, and it is surrounded by the so-called confinement layers (#61627;100 nm thick). The wave is guided in this central part of the structure, as it is further sanwidched in between materials of lower refractive index, labelled the cladding layers. The formation of dislocations occurs when the plastic limit of the QW is reached. The stresses are of thermal origin, generated by the very local temperature increase in defect rich zones during the laser operation. These thermal stresses can overcome the yield strength for the semiconductors in the laser structure, in particular for the QW. Therefore, the thermal strength of the QW is critical to the robustness of the laser. We present herein a thermomechanical analysis of the degradation of high power laser diodes, in which both the thermal conductivity and the mechanical strength of the QW play a major role for the degradation process.

The development of advanced structural materials for the next generation nuclear reactor applications requires a deep understanding of the structural-property relationship in steel alloys under neutron irradiation. However, mature techniques like electron microscopy and typical X-ray diffraction measurements focus on the nano scale phenomena and bulk-sample averaged statistics, respectively, largely missing the information at the individual grain level. In this talk we present the first-hand data from in-situ X-ray studies on neutron-irradiated steel specimens during tensile tests at the Advanced Photon Source, using a combination of wide-angle scattering, small-angle scattering and high-energy diffraction microscopy. The results reveal the microstructural responses to mechanical deformation at different length scales and clearly show the effect from irradiation. The unique capability of the experimental setup and the methodology we developed open up a new opportunity for understanding the structural-property relationship of materials.

Titanium alloys are widely used in aerospace applications, typically in blade and disc forms. In these applications they are subjected to significant dwell fatigue, with a time sensitive dwell component under load. This dwell fatigue problem is more dominant in blades and disks, compared with other low/high cycle fatigue, due to the unavoidable low temperature (i.e. less than 200°C) operations when engines start their duty cycles. The stress dwell can result in a significant reduction in fatigue life, where the dwell debit representing the number of cycles to failure for dwell vs saw tooth dwell loading, in some alloys (i.e. Ti-6Al-4V) can be a factor of 10 at room temperature. This is clearly a time sensitive deformation mode, where simple evaluation of the critical resolved shear stress for individual slip systems is insufficient and we must access rate sensitive material properties. We employ a combination of micro-pillar experiments and simulations, as well as nanoindentation of particular crystal orientations, to trigger slip on specific slip planes and explore load-relaxation and rate dependant deformation of individual slip systems. The opportunity now presented with high fidelity micro-mechanical tests combined with crystal level finite elements models offers a real opportunity to influence alloy design and component operation. We will present an overview of our novel experiments in Ti-6Al-2Sn-4Zr-XMo alloys, where it has been shown previously that the Mo content can render the material dwell sensitive (Ti6242) or insensitive (Ti6246). This work forms part of the HexMat programme grant (http://www.imperial.ac.uk/hexmat).

Multi-scale deformation models depend on detailed characterization of both microstructure and constitutive properties, as well as experimental benchmarks obtained at salient length scales. Traditional methods for validating models with bulk material properties provide a solid foundation but fail to capture the underlying microstructural dependence that is needed to develop multi-scale models. In the current study, micro-mechanical tests of Ni-base superalloys have been designed and carried out in order to complement and support parallel multi-scale modeling efforts. Micro-tensile and micro-bending samples have been excised and shaped from bulk materials and are being used to elucidate underlying microstructure-property relations for polycrystalline Rene 88. Employing these micro-scale experiments facilitates determination of both local and global properties. Scale specific micro-tensile experiments facilitate meso-scale characterization through collection of key microstructural features, such as grain size, shape and orientation for a finite number of grains that are tractable in crystal plasticity modeling. Moreover, correlation with digital image correlation (DIC) allows for local strain mapping, which can be directly compared with crystal plasticity predictions. Resonant micro-bending experiments are also being used to identify the microstructural features that govern fatigue crack nucleation and short crack growth. The importance of grain size, orientation and elastic compatibility will be addressed. In this way, connecting explicit intergranular microstructures to their attendant mechanical behavior, which can be captured in micro-scale experiments, provides a unique avenue for benchmarking modeling efforts (at scale) and opens a valuable pathway for model development and validation.
Support for this project is provided by the AFOSR and AFRL funded Center of Excellence on Integrated Materials Modeling.

The applicability of lightweight metallic materials for protective applications under extreme environments (high strain rate, temperature, pressure, etc.) relies on a fundamental understanding of the deformation and failure response of the material. Ti and Ti alloys, due to their high strength to weight ratio, are promising candidates for applications in protective structures. The design of these materials experiencing high strain rate and shock deformation relies on a fundamental understanding of the atomic scale deformation and failure mechanism for various microstructures and loading conditions. The deformation response of Ti is dominated by twinning due to the HCP structure as well as a martensitic phase transformation from HCP(α) to hexagonal (omega;) phase at high pressures. However the mechanisms responsible for failure in Ti at high strain rates and under shock loading conditions are not well understood.
In this work, large scale molecular dynamics (MD) simulations are carried out to investigate the high strain rate and shock response of nanocrystalline Ti with grain sizes of 50nm, 75nm and 100nm. The effect of microstructure on the deformation (twinning), phase transformation (α to omega;), and failure mechanisms (void nucleation, growth and coalescence) will be presented. The evolution of defects, the omega; phase on the micromechanisms of failure, and the spall strength for nanocrystalline Ti wlll be discussed.

A new inverse method has been developed to extract the stress-strain curves of different phases in a multi-phase steel from the load-depth curves measured in nanoindentation tests. A power law hardening response is assumed for each phase and an empirical hardness versus yield stress relationship is used. Adjustment was made to take into consideration indentation size effect and indenter bluntness effect. With the newly developed inverse method and statistical analysis of the hardness histogram for each phase, the average stress-strain curves of individual phase in a quench and partitioning (Q&P) steel, including austenite, tempered martensite and untempered martensite, are calculated and the results are compared with phase properties obtained via in-situ high energy X-ray diffraction (HEXRD). The stress-strain curves predicted by the two methods are quite similar for the various phases, thus demonstrating the feasibility of using instrumented nano-indentation test to determine the individual phase properties in multiphase alloys.

Advances in three-dimensional nanofabrication methods including two-photon lithography have enabled synthesis of lightweight lattice materials having record values of specific stiffness and other unique properties. These are typically made by patterning a polymer, depositing a conformal metal or ceramic coating, and then dissolving the polymer scaffold. The mechanical properties of these novel materials can, in some cases, be considered self-similar to those of macroscale lattices and foams; however, nanoscale control over the thickness, microstructure, and roughness of the constituent elements (e.g., beams or hollow tubes) enables unprecedented tuning of the mechanical behavior and engineering within new property regimes. Vertically aligned carbon nanotubes (CNT “forests”), while tortuous and wavy, present an alternative and possibly more 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 of ceramic and metallic materials by atomic layer deposition (ALD). We use CNT microstructures coated with various condition of aluminum oxide by ALD to investigate how the mechanical behavior of lightweight foams can be controlled. Specifically, thin conformal coatings of alumina on CNTs allow the resultant composite to recover from deformations >90% (compared to ~10% for uncoated CNTs), with only a ~5% increase in density. Moreover, the full recovery cycle is accompanied by 80% dissipation of the initial elastic energy. We study the dependence of the recovery and dissipation on the strain rate, and on the coating thickness and morphology, and find there an optimum coating thickness for maximum recovery. The unique combination of high recovery and dissipation in these materials may find applications in structural damping and shock absorption, especially under extreme thermal or chemical conditions.

Nanoparticle assemblies (NPAs) represent a group of materials composed of nanoparticles (NPs), such as nanocolloidal crystals (NCC), nanoparticle thin films, nanostructures composed of NPs (e.g., shells, freestanding films, membranes, and aerogels). Due to NPs&’ novel functionalities compared to their bulk counterparts as well as the NPAs&’ high surface to volume ratio, NPAs have emerging applications in photonics, optoelectronics, sensors, energy storage, and bone replacement. However, NPAs&’ poor mechanical properties largely hinder their applications. A full understanding on the mechanical behavior of NPAs is critical for proposing ways of reinforcement. Nanoindentation is demonstrated to be a versatile technique to characterize the NPAs&’ mechanical properties, such as hardness, strength, modulus, creep, fracture and tribology properties.
Tuning the interparticle interaction is shown to be essential to reinforce NPAs, and two feasible ways of tuning are (1) establishing interparticle bonding, and (2) developing local configurational confinement. Interparticle bonding may be established through sintering, functional-group bonding, and/or introducing a gluing-phase. We would compare these interparticle bonding techniques. Particularly, we focus on introducing a gluing-phase through atomic layer deposition (ALD). We find that, upon nanoindentation, as-assembled disordered NPA composed of 22nm-silica/7-nm-titania nanoparticles as well as as-assembled nanocolloidal crystal (NCCs) composed of monodisperse 289nm silica nanocolloids show rigid-granular behavior, such as rigid-granule-like NPs being dislodged, strain bursts during loading, creep and shear band formation. ALD enables us to nanoscale-precisely tune the interparticle interaction by depositing a reinforcing layer around all NPs. For example, the alumina-ALD-treated silica NCC is drastically stiffened up to 30 times and hardened up to 150 times, compared to the un-treated silica NCC. With increasing the ALD thickness, the deformation mechanism of order and disordered NPAs transits from granular, bonded granular, to particle-reinforced composite behavior, and the shear-band formation, strain-bursts, cracking, and creep are all suppressed. Thus, ALD enable precise tuning to make (both order and disordered) nanoparticle assemblies be simultaneously hardened, stiffened, and toughened.
We would also discuss the interparticle-interaction reinforcement through developing local configurational confinement. Particularly, we would demonstrate that, introducing shape-anisotropic (elliptical) NPs into NPAs greatly suppresses shear band formation and toughen NPAs without sacrificing their strength, implying that tuning constituent-shape-anisotropy presents a new strategy to enhance toughness of NPAs.

The relationship between the evolved microstructural state, crack mode and failure pathway is needed to bridge the mesoscale gap from understanding the behavior of single, isolated dislocations to the behavior of an assembly of dislocations and the macroscale properties. Here we demonstrate in Haynes 230, a Ni-Cr-W planar slip alloy, and 316 stainless steel that the microstructural state of a crack formed under cyclic loading influences, for some distance, the crack propagation mechanism under tensile loading. A fatigue crack was initiated and propagated through the sample to reach Stage II, the crack was then arrested and failed under tensile loading. Upon examination of the fracture surface, the loading change was distinct with the unexpected finding of striation-like markings existing between regions of fatigue striations and microvoids. The microstructure, examined via TEM imaging, mirrored this transition in fracture surface morphology and evolved from intense planar slip bands that were approximately normal to the fatigue fracture surface, seen in the fatigue crack growth region as well as in the transition region, to one typical of the intense shear associated with microvoid formation. A second finding was that the planar bands beneath the fatigue surface were not correlated with the striation markings and they did not extend to the fracture surface but were replaced by a zone of severe deformation that contained nanoscale features. The findings of this study have implications for the critical length scales associated with the modeling of fatigue crack propagation as well as the importance of the evolved microstructural state on the fracture path and mode.

Self-assembly can be harnessed to create thick, water-insoluble, films at a submerged water-oil interface. In our case, molecules are built at the interface by bringing hydrogen phosphate, in the aqueous phase, into contact with fatty amines, in the oil phase. SANS and cryo-SEM reveal that, on the molecular scale, the interfacial film is comprised of lamellae and that these are then organized into micron-scale needles [1]. Ultimately, these films can be used to stabilize temperature responsive capsules, but first we need a good understanding of the strength and failure characteristics.
We use interfacial shear rheology [2], interfacial rheo-imaging, and confocal fluorescence microscopy to study the structure and physical properties of these films and the water-in-oil emulsion droplets and capsules that they stabilize. These films have a very large interfacial storage modulus (G' ~ 10 N/m) which dominates the rheology and can be tuned across two orders of magnitude. The film is found to yield abruptly at a characteristic strain (~0.6% at 2.5mM of fatty amine). Notably, emulsions stabilized by these films coalesce upon heating to a critical temperature which is governed by film thickness. We find that the temperature increase causes the needles to dissolve in to the oil, thinning the film and reducing its interfacial storage modulus, whilst retaining its brittle nature.
References:
[1] J. Forth, D. J. French, A. V. Gromov, S. King, S. Titmuss, K. M. Lord, M. J. Ridout, P. J. Wilde, and P. S. Clegg, submitted.
[2] S. Vandebril, A. Franck, G. G. Fuller, P. Moldenaers, J. Vermant, Rheol. Acta 49, 131 (2009).

Nanowires (NWs) have gained prominence in the fields of nanoelectronics and nano-electromechanical systems thanks to continuous miniaturization of components and their utilization in day-to-day applications. The realization of their mechanical properties becomes extremely important to the next generation of devices using nanowires as their functional units. Even though there have been many studies following a growing demand for the knowledge of mechanical behavior at the nanoscale, a significant work still lies ahead in the mechanical domain due to challenges encountered such as inconsistent nanowire fabrication, difficulties of manipulation, alignment and attachment in a test setup. In this study, we aim to reveal fracture strength of single suspended silicon nanowires (SiNWs) based on the atomic force microscopy (AFM) for both inspection of SiNWs before and after fracture and also for the exertion of forces with a quantitative control in two directions (normal to and in-plane) on investigated nanostructures. In order to carry out an accurate mechanical test, we first aim to develop a reliable top-down fabrication method ensuring a good support structure for suspended SiNWs by carving the same silicon substrate for both supports and SiNWs. Specimens tested have a width (in-plane) of 30 nm, height (out-of-plane) of 100 nm, a length of 4.5 µm and a vertical distance of 10 µm from the surface of the substrate. A careful investigation of samples with scanning electron microscope (SEM) reveals the uniformity and high yield of resulting SiNWs across the whole Si wafer substrate. A thorough calibration routine is incorporated to detect forces accurately at the instant of fracture. After the fracture, the samples are further investigated by SEM to observe the location and the type of fracture. These SEM micrographs reveal two important information: 1) the reliability of support structures thus that of the developed fabrication process is confirmed since 14 out of 17 tests carried out, fracture is observed to be in the middle where the AFM tip is placed for the exertion of force, 2) SiNWs at the aforementioned sizes, fracture happens as a sudden failure, in a brittle fashion. Finite element (FE) simulations are carried out to estimate the fracture strength at the fracture force. In the FE calculations, SiNWs are simulated in a double-clamped beam configuration with a point force acting in the midspan of the beam. The fracture strength of SiNWs having the volume of 1.3x10⁷ nm³ is found to vary in the range of 15 GPa- 20 GPa, which is slightly higher than the fracture strength of SiNWs grown by VLS growth mechanism of an earlier study,12 GPa. This work provides a significant contribution to the monolithic fabrication of silicon nanowires where crystalline orientation, location and dimensions are solely determined by the layout design and the reliability of the process is further confirmed by fracture tests and SEM investigations.

One of the primary motivations for development of instrumented indentation was to measure the mechanical properties of thin films. Characterization of thin film mechanical properties as a function of temperature is of immense industrial and scientific interest. The major bottlenecks in variable temperature measurements have been thermal drift, signal stability (noise) and oxidation of/condensation on the surfaces. Thermal drift is a measurement artifact that arises due to thermal expansion/contraction of indenter tip and loading column. This gets superimposed on the mechanical behavior data precluding accurate extraction of mechanical properties of the sample at elevated/cryogenic temperatures. Vacuum is essential to prevent sample/tip oxidation at elevated temperatures and condensation at cryogenic temperatures.
In this talk, the design and development of a novel nanoindentation system that can perform reliable load-displacement measurements over a wide temperature ranges (from -150 to 700 °C) will be presented emphasizing the procedures and techniques for carrying out accurate nanomechanical measurements. This system is based on the Ultra Nanoindentation Tester (UNHT) that utilizes an active surface referencing technique comprising of two independent axes, one for surface referencing and another for indentation. The differential depth measurement technology results in negligible compliance of the system and very low thermal drift rates at high temperatures. The sample, indenter and reference tip are heated/cooled separately and the surface temperatures matched to obtain drift rates as low as 5nm/min at 700 °C. Instrumentation development, system characterization, experimental protocol, operational refinements and thermal drift characteristics over the temperature range will be presented. Extensive test results on standard calibration materials like fused silica and aluminum, used for validating the system, will be shown. In addition, case studies of variable temperature measurement on steels will be presented. Finally, the current status and future roadmap for variable temperature nanoindentation testing will be summarized.

Future generations of jet engines require increased turbine entry temperatures and higher rotational speeds in order to realise improved thermal efficiency. The need for a new generation of nickel-based superalloys which can withstand the demanding combination of mechanical loads in an aggressive environment is significant. For turbine disc applications there is a requirement for stronger, more corrosion resistant alloys. In particular these alloys must be resistant to dwell fatigue damage. To accelerate the development of these new alloys there is a need for small scale mechanical testing. In this work, an electro-thermal-mechanical testing (ETMT) system - that allows for small scale mechanical testing - is used for rapid assessment of the mechanical properties of commercial nickel-based superalloys at 800°C. Results from tensile tests on specimens with a cross-section of 1x1 mm show good agreement with more conventional methods. The findings confirm that the use of miniaturized test-pieces is sufficient for determining tensile behaviour. A novel experiment using the ETMT, designed to study susceptibility to intergranular oxidation-assisted cracking is proposed. Uniaxial test were conducted at slow strain rates 0.0001 /s at 800°C in both inert atmosphere and air. This test method provides a quick assessment of dwell fatigue resistance. Brittle and ductile failure mechanisms were observed in the oxidising and inert atmosphere respectively. Bending of microcantilevers - prepared using FIB milling techniques - at grain boundaries in virgin and thermally exposed material allows greater insight into the mechanism of grain boundary failure on the micro-scale. The use of these experimental techniques provides a new insight into the mechanism of environmentally assisted cracking in nickel-based superalloys.

A PM2000 oxide-dispersion-strengthened alloy (ODS) has been tested by nanoindentation testing in a novel xSOL 800 stage that allows testing samples under a controlled environment. This oxide dispersion strengthened steel PM2000 has a fine elongated grain size of around 1µm, and The is a candidate material for high temperature application typically found in power plants or high temperature fuel cells. Beside the temperature dependence of hardness and indentation modulus, a careful analysis of time-dependent features such as creep and stress exponent was performed. The H/E ratio demonstrates the relation of plastic and elastic deformation and is dominated by a transition from small hardness variations at low temperatures to a strong hardness decrease starting at about 400 °C. The two different deformation regimes have been attributed to modifications in dislocation pinning at temperatures up to 400 °C and thermal activation over obstacles, when fine oxide nanoparticles of diameter 5-20nm are present. Below 400°C a small hardness increase is observed that is related to the formation of partial dislocations.

Micromechanical testing at variable temperatures using nanoindentation systems has seen significant progress over the past two decades. In combination with current fabrication techniques, such as focused ion beam (FIB) machining, nanoindenters now represent the most versatile setups for mechanical testing at small length scales and high temperatures. Sample geometries range from flat surfaces and micro-compression and tension samples to more complicated shear and fracture testing geometries. In combination with in-situ observation in a scanning electron microscope, deformation and failure can be related directly to the loading condition. We use two different nanoindentation systems both featuring heating of tip and sample, which has been shown to be critical to minimize drift at high temperatures [1]. Different applications of high temperature and in-situ testing will be discussed, including strain rate and temperature variation in indentation to study the activation parameters of bcc metals and multilayer thin films as well as compression of nanocrystalline and multilayer pillars.
[1] Wheeler et al, COSSMS (2015), doi:10.1016/j.cossms.2015.02.002

Interfaces are known as a prime constituent when aiming to achieve ultra strong materials, which is a main driver for the development of nanostructured materials. To understand the role of grain boundary plasticity versus dislocation plasticity in these materials, we studied the rate dependent deformation mechanisms of ultra-fine grained materials, composites and foams in comparison to their coarse grained or single crystal counterparts at ambient and non-ambient temperatures, thereby distinctively changing the type of interfaces present in the material for the same structural size.
Severe plastic deformation techniques are established for creating bulk nanostructured materials and composites. In this work, we produced ultrafine grained Cr, Cu and Au, as well as Cu-Fe and Cu-Nb nanocomposites, by high pressure torsion deformation. Finally, by selective etching, Cu and Au foams were derived from the composites. To identify the rate dominating deformation mechanisms, mechanical properties such as hardness, strain rate sensitivity and activation energy were evaluated by using high nanoindentation testing at ambient, elevated, and cryo temperatures.
We find outstanding strength for all approaches, rendering them promising new materials. Moreover, the strain rate sensitivity of the composite materials and foams over temperature is increasing, in close agreement to that of the ufg bulk state, and the activation volumes and energies point towards dislocation based deformation mechanisms. This indicates that the domination governing deformation mechanism should be thermally activated interaction of dislocations with grain boundaries

Traditionally, time-dependent properties of nanocrystalline metals have been measured on bulk samples. With the advent of thin film deposition techniques like sputtering and electrodeposition for fabricating nanocrystalline materials, it has become necessary to adapt bulk mechanical testing for thin films. Nanoindentation has been extensively applied for this purpose, particularly on thin films where conventional testing is difficult or impossible, and has been demonstrated to successfully extract strain rate exponents [1]. However, the interpretation of the indentation results can be difficult due to the complex stress state, and the nearly instantaneous onset of large-strain plasticity. Microcompression, on the other hand, is advantageous due to the relatively simple, well understood uniaxial stress state.
In this talk, micro-compression creep, stress relaxation and strain rate sensitivity [2, 3] tests performed in-situ, in a scanning electron microscope, on nanocrystalline Ni at elevated temperatures (25-125 °C) will be introduced. Corresponding microtensile tests were performed to compare and validate the proposed micropillar tests. All micropillar tests were performed on the same sample to remove sample-to-sample variation and allow direct comparison to understand the correlation between these three transient tests. The observed stress relaxation and creep were found to be significant at stresses even below the 0.2% offset yield strength demonstrating the enhanced time dependent behavior of nanocrystalline materials. The extracted exponents and activation parameters (activation volume and activation energies) provided an initial estimate of the footprint of the rate controlling deformation mechanism(s). Based on these results, the role of dislocation plasticity and grain boundary mediated processes will be discussed. Overall, this study aims to bridge the gap between the three time-dependent tests and provides useful insights into developing similar indentation based tests, for creep and stress relaxation measurements in particular.
References:
[1] V. Maier, K. Durst, J. Mueller, B. Backes, H.W. Höppel, M. Göken, Journal of materials research, 26 (2011) 1421-1430.
[2] G. Mohanty, J.M. Wheeler, R. Raghavan, J. Wehrs, M. Hasegawa, S. Mischler, L. Philippe, J. Michler, Philosophical Magazine, (2014) 1-18.
[3] J. Wehrs, G. Mohanty, G. Guillonneau, A.A. Taylor, X. Maeder, D. Frey, L. Philippe, S. Mischler, J.M. Wheeler, J. Michler, JOM, 1-10.

Measuring the uniaxial creep response from nanoindentation has been of great interest to the small scale mechanics community. However, several experimental and modeling challenges pose obstacles to direct comparison of indentation and uniaxial results. In this talk, we present new experimental procedures to address some of these issues and thereby improve the precision and accuracy of high temperature nanoindentation tests. Indentation creep results at a number of temperatures up to 550 °C on commercial purity aluminum alloy will be presented. The activation energy for creep was found to be 140.2 KJ/mol/K, matching the value determined with high temperature tensile creep experiments extremely well. Uniaxial power-law creep behavior (stress exponent and pre-exponential term) is calculated from the indentation data for direct comparison of results to the uniaxial data. The results are in excellent agreement with the uniaxial compression/torsion tests over a wide range of strain rates and temperatures demonstrating the capabilities of the current experimental procedure to study high temperature creep. The relative contributions and interplay of indentation size effect, strain rate and temperature on the creep response will also be discussed. These results indicate that nanoindentation based techniques can be simple, quick and cost effective for high temperature mechanical characterization of small volumes of materials.

Thin metal films on compliant polymer substrates are of major interest for flexible electronic technologies. To improve the adhesion of the ductile charge carrying metal layer (Au, Cu) to the flexible substrate thin interlayers of brittle metals (Cr, Ti, Ta) are applied. The suitability of a film system for flexible applications is based on the electro-mechanical performance of the metal film/polymer substrate couple. This study demonstrates how a 10 nm Cr interlayer deteriorates the electro-mechanical performance of 50 nm Au films on polyimide substrates by inducing the formation of cracks in the ductile metal layer. Ex situ and in situ fragmentation experiments showed that through thickness cracks perpendicular to straining direction from at very low strains (ε=2-2.5%) if the interlayer is present. In contrast, thin Au films without an interlayer showed outstanding mechanical performance as they can support strains up to 15% without any visible deformation. Combined in situ measurements of the film stress in the Au layer with x-ray diffraction and electrical film resistance (4 Point Probe setup) during uniaxial straining confirmed the different behaviours. For Au films with Cr interlayer the film stress decreases rapidly as cracking initiates and reaches a plateau as saturation crack spacing is reached. Crack formation and stress drop correspond to a rapid increase in the film resistance. Without the interlayer the Au film stress reaches a maximum around 2% strain and remains constant throughout the experiment. The film resistance is unaffected by the applied elongation up to a maximum strain of 15%, giving no sign of deformation in the metal layer. The outstanding electro-mechanical performance of the gold film indicates that adhesion layers may not be necessary to improve the performance of ductile films on polymers. Rather, the deposition method and resulting film and interface structure may be the dominant parameters controlling the electro-mechanical behaviour of metal-polymer systems.

The fracture toughness of ceramics is often dominated by the structure of their grain boundaries. Our ability to improve life of ceramic components depends on our ability to investigate properties of individual grain boundaries. This requires development of new fracture testing methods allowing high spatial resolution and high control over the area to test. Further benefits of these ‘small scale&’ approaches will enable testing of specimens for which big volumes are not available (e.g. thin films, coating, or simply samples of dimensions limited by production process).
Recently, several techniques have been developed using small scaled mechanical testing, based within a nanoindenter, changing tip and sample geometries, including: micropillar compression; microcantilever bending; and double-cantilever compression. However, the majority of the published works utilises complex geometries resulting into complex analysis of force distribution and stress intensity factor and rely on load-displacement curves for the identification of crack initiation, with the added complication of friction.
Our approach builds upon the work of Lawn, who showed that a practical test geometry to obtain stable crack growth and calculate the fracture energy G is that of a double-cantilever beam (DCB) under constant wedging displacement. We replicate this configuration in our tests fabricating double-cantilever beams of micrometric dimensions by focused ion beam (FIB) milling and loading them in-situ in an SEM using a nanoindenter with a wedge-shaped tip. This has two benefits: the sample is well aligned for a controlled test; images are recorded during the test for later analysis. This allows us to use beam deflection and crack length rather than critical load to measure fracture toughness. Our tests have proved it is possible to initiate and stably grow a crack in a controlled manner in ceramic materials and our fracture energy results have been validated against prior macro-scale fracture data. This approach is being extended to multi-phase materials with unknown materials properties and extends our arsenal of small-scale characterisation techniques required to generate new processing strategies for the next generation of materials design.

Although since two decades silver has been studied as bonding material, it has not been widely used in packaging industry. This can be due to the limited knowledge of the relation between microstructure and mechanical behavior. In this work, the microstructure and mechanical behavior of thin (~25 µm) layers of porous nanocrystalline silver are investigated. The layers are produced by sintering nano-sized silver paste at reasonably low temperatures, pressures and times. The main application of this material is in the field of packaging of high power semiconductor devices as a replacement of high-lead solder materials. Pore morphology, bulk porosity, grain size, ligament size, twin and dislocation structures are characterized by electron microscopy and ultra-high resolution ptychographic tomography. We find a strong dependency of the microstructure on the sintering conditions. The mechanical behavior of the nano-silver layers is investigated by in-situ tensile testing during Xray diffraction. Load-unload cycles exhibit large hysteresis in the stress-strain curve. The broadening of the FWHM of the diffraction peaks recovers significantly during unloading. The amount of recovery depends on various microstructural parameters such as porosity, morphology and size of the ligaments.
A three-dimensional (3D) finite-element (FE) microstructure-based simulation is applied in order to distinguish between the interplay and role of filament size, pore morphology and porosity in final deformation mechanism. The high-resolution 3-D reconstructed images of porous silver, obtained from x-ray ptychography are used as input to create finite element meshes of the actual porous morphology. Based on volume fraction, size and volume-to-surface ratio of pore structure the size of a Representative Volume Element (RVE) and shape of meshing elements are determined. The main goal of performing a microstructure-based simulation is to develop a model that correlates the porous morphology of the silver layers with its mechanical behavior. The results can be used further to predict the mechanical behavior of the porous silver given its porous structure [1].
[1] M. Maleki, J. Cugnoni and J. Botsis. Microstructure-based modeling of the ageing effect on the deformation behavior of the eutectic micro-constituent in SnAgCu lead-free solder, in Acta Materialia, vol. 61, num. 1, p. 103-114, 2013

The deformation and failure of polycrystalline metals is directly related to the motion of dislocations and their interaction with grain boundaries, cracks and surfaces. The presence of grain boundaries (GBs) usually leads to material strengthening. The current understanding is that GBs that are impenetrable lead to an increase of dislocation density because of dislocation accumulation while a GB acting as a sink will reduce the dislocation density [1]. In order to develop fatigue-resistant microstructures, however, more fundamental understanding of these interactions is required. Cyclic compression experiments involving bi-crystalline micro-pillars provide ideal model systems which allow studying mechanical properties and the evolution of dislocation structures with respect to a single selected GB.
Single- and bi-crystalline Ni pillars of defined orientation were investigated. The micro-pillars were deformed repeatedly with up to 100 load-unload cycles to study the influence of the GB on the plastic deformation and microstructural evolution. The pillars were prepared by focused ion beam (FIB) at selected GBs as well as in the adjacent single crystalline regions. The crystal orientation of the grains was characterized by electron backscatter diffraction prior to pillar preparation. Both ex situ and in situ experiments in a scanning electron microscope were performed. Digital image correlation is used to observe the strain distribution and its changes over the full range of loading cycles. The deformed pillars were carefully characterized using scanning and transmission electron microscopy. Activated slip planes as well as the influence of the GB on the pillar strength and cyclic deformation behavior will be described.
[1] P.J. Imrich, C. Kichlechner, C. Motz, G. Dehm Acta Materialia, 73(2014), 240

Hierarchical composite laminates with a carbon nanotube (CNT) nanophase have been prepared via two approaches. Glass micro-scale fibers were coated with CNTs using electrophoretic deposition (EPD) prior to infusion with epoxy resin. The CNTs were functionalised using an ultrasonicated-ozone process followed by reaction with a polyethyleneimine (PEI) dendrimer to enhance CNT-fiber adhesion and interphase ductility. In the chemical vapor deposition (CVD) approach, CNTs were grown directly onto the quartz or alumina micro-fibers, prior to infusion with an epoxy resin modified with a SBM tri-block copolymer, thermoplastic nanophase.
The matrix-dominated shear properties of both laminate types were examined using in-plane shear testing to examine strength and stiffness changes resulting from the CNT modification. Similar trends in the shear stiffness and strength were observed for the EPD and CVD methods. Compared to the baseline condition, an 80% increase in shear strength resulted from CNT treatment at loadings around 15% by volume. The shear modulus also increased several-fold compared to the CNT-free laminates.
Whilst the mechanical performance of the CVD and EPD CNT laminates were similar, examination of the fracture surfaces indicated differences in failure paths. The EPD laminates showed increases in strength corresponding to the fracture moving away from the matrix-fiber interface and into the CNT-rich interphase region. In contrast, the CVD laminates showed that strength was limited to adhesion failure within the iron nanoparticles at the CNT-fiber interface. For both laminate types it was shown that a ductile toughening phase provide by the PEI or SBM enhanced the CNT performance by reducing brittle fracture.
CVD and EPD prepared laminates also exhibited significant differences in conductivity. The randomly oriented CNTs deposited by EPD had conductivities around 0.05 S/cm, whereas the CVD coated CNTS produced laminates with conductivites greater than 3 S/cm. The very high conductivity of the CVD laminates is suspected to be due to the metallic layer of iron nanoparticles deposited on the fibers during CVD growth of the CNTs.
The study provides a good comparison of the CVD and EPD methods for preparing hierarchical composites, where high-volume fractions of CNTs are used and functionalization and resin chemistry is manipulated to improve CNT-matrix adhesion and matrix toughness.The results indicate that the EPD can produce CNT-modified laminates with significant increases in strength without degrading micro-fiber properties. The CVD approach, if applied in a way that does not compromise fiber strength, would provide higher strengths if CNT-fiber adhesion could be improved.