Nanoindentation has been widely used for measuring mechanical behavior of nanocrystalline (nc) metals that cannot be measured by tensile and compressive test. The hardness and elastic modulus are usually obtained by Oliver and Pharr method. However, this may not be true for materials showing viscoelastic characteristics. This study aims at clarifying the effect of testing parameters, especially loading rate and holding time, on the hardness and elastic modulus of a nanocrystalline Fe-51Ni coating obtained in nanoindentation tests as the material exhibits anelastic and creep characteristics. An analytical method based on the correspondence principle for linear viscoelasticity was developed. The holding displacement-time data obtained in indentation creep tests at a high loading rate of 20 mN/s were analyzed and material parameters related to the elastic, anelastic and creep characteristic were derived using a model containing one Maxwell unit and two Kelvin units. The anelastic deformation thus contains at least two relaxation processes having relaxation times of 0.37 s and 6.8 s, respectively and the creep deformation is described by a viscosity value of 4.2x104 GPa.s for the alloy in an as-deposited state. Moreover, electrodeposited (ED) Ni was analyzed by electron backscatter diffraction. Results indicated that the ED Ni exhibits a bimodal distribution of grain size. The grains having sizes larger than 2 £gm shows a strong fiber texture of <100>//ND, whereas the small grains (<2 £gm) are mainly randomly oriented.

abstract: Nanocrystalline (nc) thin films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. Furthermore, the mechanical behavior and microstructure of nc films is intimately related. Thus, precise control of the size, aspect ratio and spatial distribution of grains can enable the synthesis of thin films with exceptional mechanical properties. However, conventional bottom-up techniques for synthesizing thin films are incapable of achieving the microstructural control required to explicitly tune their properties. This dissertation focuses on developing a novel technique to synthesize metallic alloy thin films with precisely controlled microstructures and subsequently characterizing their mechanical properties using in situ transmission electron microscopy (TEM). Control over the grain size and distribution was achieved by controlling the recrystallization process of amorphous films by the use of thin crystalline seed layers. The novel technique was used to manipulate the microstructure of structural (TiAl) and functional (NiTi) thin films thereby exhibiting its capability and versatility. Following the synthesis of thin films with tailored microstructures, in situ TEM techniques were employed to probe their mechanical properties. Firstly, a novel technique was developed to measure local atomic level elastic strains in metallic glass thin films during in situ TEM straining. This technique was used to detect structural changes and anelastic deformation in metallic glass thin films. Finally, as the electron beam (e-beam) in TEMs is known to cause radiation damage to specimen, systematic experiments were carried out to quantify the effect of the e-beam on the stress-strain response of nc metals. Experiments conducted on Al and Au films revealed that the e-beam enhances dislocation activity leading to stress relaxation. / Dissertation/Thesis / Supplementary Video S1 / Supplementary Video S2 / Supplementary Video S3 / Doctoral Dissertation Materials Science and Engineering 2017

A better understanding of how the nanoscale environment affects the mechanical properties of materials, in particular metallic nanoparticles and nanocrystalline metals is vital to the development of next generation materials. Of special interest is obtaining a fundamental understanding of the inverse Hall-Petch Effect in nanocrystalline metals, and nanoindentation in individual nanoparticles. Understanding these subjects is critical to understanding how the mechanical properties of materials are fundamentally affected by nanoscale dimensions. These topics have been addressed by a combination of theoretical modeling and in-situ nanoindentation transmission electron microscopy (TEM) analysis. Specifically, the study of the inverse Hall-Petch effect in nanocrystalline metals will be investigated by a thorough review of the literature followed by a proposed novel theoretical model that better explains the experimentally observed behavior of nanocrystalline metals. On the other hand, the nanoindentation of individual nanoparticles is a very new research topic that has yet to aggregate a large body of experimental data. In this context, in-situ TEM nanoindentation experiments on silver nanoparticles will be first performed to determine the mechanisms of deformation in these nanostructures. A theoretical explanation for the observed deformation mechanisms will be then developed and its implications will be discussed. In addition to nanoparticles, this study will also provide unique and valuable insight into the deformation mechanisms of nanopillars, a growing area of research despite much controversy and speculation about their actual mechanisms of deformation. After studying the novel behavior of both nanocrystalline metals and nanoparticles, useful applications of both classes of materials will be explored. The discussion of applications will focus on utilizing the interesting behaviors explored in the dissertation. Of particular interest will be applications of nanoparticles and nanocrystalline materials to coatings, radiation resistance and super-plastic materials. / text

Nanocrystalline (NC) metals with averaged grain size smaller than 100 nm have shown promising mechanical properties such as higher hardness and toughness than conventional coarse-grained metals. Unlike conventional metals in which the deformation is controlled by dislocation activities, the microstructural evolution in NC metals is mainly dominated by grain rotation and stress-driven grain boundary motion (SDGBM) due to the high density of grain boundaries (GBs). SDGBM is thus among the most studied modes of microstructural evolution in NC materials with particular interests on their fundamental atomistic mechanisms. In the first part of this thesis, molecular dynamics simulations were used to investigate the influences of Triple Junctions (TJs) on SDGBM of symmetric tilt GBs in copper by considering a honeycomb NC model. TJs exhibited asymmetric pinning effects to the GB migration and the constraints by the TJs and neighboring grains led to remarkable non-linear GB motion in directions both parallel and normal to the applied shear. Based on these findings, a generalized model for SDGBM in NC Cu was proposed. In the second part, the interaction of SDGBM with crack, voids and precipitates was investigated. It was found that depending on the GB structure, material type and temperature, there is a competition between different atomistic mechanisms such as crack healing, recrystallization and GB decohesion. It is hoped that the findings of this work could clarify the micro-mechanisms of various experimental phenomena such as grain refinement in metals during severe plastic deformation, which can be used to design optimized route of making stabilized bulk NC metals. / February 2016

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Austenitic stainless steels are commonly used in nuclear reactors and have been considered as potential structural materials in fusion reactors due to their excellent corrosion resistance, good creep and fatigue resistance at elevated temperatures, but their relatively low yield strength and poor radiation tolerance hinder their applications in high dose radiation environments. High angle grain boundaries have long been postulated as sinks for radiation-induced defects, such as bubbles, voids, and dislocation loops. Here we provide experimental evidence that high angle grain boundaries can effectively remove radiation-induced defects. The equal channel angular pressing (ECAP) technique was used to produce ultrafine grained Fe-Cr-Ni alloy. Mechanical properties of the alloy were studied at elevated temperature by tensile tests and in situ neutron scattering measurements. Enhanced dynamic recovery process at elevated temperature due to dislocation climb lowers the strain hardening rate and ductility of ultrafine grained Fe-Cr-Ni alloy. Thermal stability of the ultrafine grained Fe-Cr-Ni alloy was examined by ex situ annealing and in situ heating within a transmission electron microscope. Abnormal grain growth at 827 K (600°C) is attributed to deformation-induced martensite, located at the triple junctions of grains. Helium ion irradiation studies on Fe-Cr-Ni alloy show that the density of He bubbles, dislocation loops, as well as irradiation hardening are reduced by grain refinement. In addition, we provide direct evidence, via in situ Kr ion irradiation within a transmission electron microscope, that high angle grain boundaries in nanocrystalline Ni can effectively absorb irradiation-induced dislocation loops and segments. The density and size of dislocation loops in irradiated nanocrystalline Ni were merely half of those in irradiated coarse grained Ni. The results imply that irradiation tolerance in bulk metals can be effectively enhanced by microstructure refinement.

L’ablation de cibles d’Al nanocristallines (taille moyenne des cristallites d = 3,1 et 6,2 nm) par impulsions laser ultrabrèves (200 fs) a été étudiée par l’entremise de si- mulations combinant la dynamique moléculaire et le modèle à deux températures (two- temperature model, TTM) pour des fluences absorbées allant de 100 à 1300 J/m2. Nos simulations emploient un potentiel d’interaction de type EAM et les propriétés électro- niques des cibles en lien avec le TTM sont représentées par un modèle réaliste possédant une forme distincte dans le solide monocristallin, le solide nanocristallin et le liquide. Nous avons considéré l’effet de la taille moyenne des cristallites de même que celui de la porosité et nous avons procédé à une comparaison directe avec des cibles mono- cristallines. Nous avons pu montrer que le seuil d’ablation des métaux nanocristallins est significativement plus bas, se situant à 400 J/m2 plutôt qu’à 600 J/m2 dans le cas des cibles monocristallines, l’écart étant principalement dû à l’onde mécanique plus im- portante présente lors de l’ablation. Leur seuil de spallation de la face arrière est aussi significativement plus bas de par la résistance à la tension plus faible (5,40 GPa contre 7,24 GPa) des cibles nanocristallines. Il est aussi apparu que les contraintes résiduelles accompagnant généralement l’ablation laser sont absentes lors de l’ablation de cibles d’aluminium nanocristallines puisque la croissance cristalline leur permet d’abaisser leur volume spécifique. Nos résultats indiquent aussi que le seuil de fusion des cibles nano- cristallines est réduit de façon marquée dans ces cibles ce qui s’explique par la plus faible énergie de cohésion inhérente à ces matériaux. Nos simulations permettent de montrer que les propriétés structurelles et électroniques propres aux métaux nanocristallins ont toutes deux un impact important sur l’ablation. / The ablation of nanocrystalline (mean crystallite size d = 3.1 and 6.2 nm) Al tar- gets by ultrashort (200 fs) laser pulses was studied using hybrid simulations combining molecular-dynamics and the two-temperature model (TTM) for a range of absorbed flu- ence of 100 to 1300 J/m2. Our simulations employ an EAM interatomic potential and the TTM-related electronic properties are modelled using three distinct functions to rep- resent the monocrystalline solid, the nanocrystalline solid, and the liquid in an accurate way. Comparison between targets displaying two mean grain sizes, porous targets, and monocrystalline targets are reported. This study showed a significantly reduced abla- tion threshold of 400 J/m2 instead of the 600 J/m2 obtained for the single crystals, the discrepancy being mainly accounted for by an increase in the magnitude of the pressure wave generated during ablation. The spallation threshold of the back side of the target is also reduced owing to a lower tensile strength (5.40 GPa against 7.24 GPa). This work also allowed to discover that residual stress generally associated with laser ablation is totally absent in nanocrystalline samples as crystal growth provides a mechanism for volume reduction near the melting temperature. Furthermore, our results demonstrate that the melting threshold shows an important decrease and the melting depth an im- portant increase in the nanocrystalline samples which can be explained by their lower cohesion energy. Our simulations shed light on the fact that a realistic modelling of both structural and electronic properties of the nanocrystalline target is important to produce a reliable representation of laser ablation.

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