Over the past decades, bulk metallic glasses (BMGs) have attracted extensive interests because of their unique properties such as good corrosion resistance, large elastic limit, as well as high strength and hardness. However, with the advent of micro-electro-mechanical systems (MEMS) and other microscaled devices, the fundamental properties of micrometer-sized BMGs have become increasingly more important. Thus, in this study, a methodology for performing uniaxial compression tests on BMGs having micron-sized dimensions is presented. Micropillar with diameters of 3.8, 1 and 0.7 £gm are fabricated successfully from the Mg65Cu25Gd10 and Zr63.8Ni16.2Cu15Al5 BMGs using focus ion beam, and then tested in microcompression at room temperature and strain rates from 1 x 10-4 to 1 x 10-2 s-1. Microcompression tests on the Mg- and Zr-based BMG pillar samples have shown an obvious sample size effect, with the yield strength increasing with decreasing sample diameter. The strength increase can be rationalized by the Weibull statistics for brittle materials, and the Weibull moduli of the Mg- and Zr-based BMGs are estimated to be about 35 and 60, respectively. The higher Weibull modulus of the Zr-based BMG is consistent with the more ductile nature of this system. In additions, high temperature microcompression tests are performed to investigate the deformation behavior of micron-sized Au49Ag5.5Pd2.3Cu26.9Si16.3 BMG pillar samples from room to their glass transition temperature (~400 K). For the 1 £gm Au-based BMG pillars, a transition from inhomogeneous flow to homogeneous flow is clearly observed at or near the glass transition temperature. Specifically, the flow transition temperature is about 393 K atthe strain rate of 1 x 10-2 s-1. For the 3.8 £gm Au-based BMG pillars, in order to investigate the homogeneous deformation behavior, microcompression tests are performed at 395.9-401.2 K. The strength is observed to decrease with increasing temperature and decreasing strain rate. Plastic flow behavior can be described by a shear transition zone model. The activation energy and the size of the basic flow unit are deduced and compared favorably with the theory.

Le magnésium est le plus léger des métaux, ce qui lui confère un fort potentiel pour être utilisé dans des applications où l’allégement des structures est requis. Pour autant, sa résistance mécanique est très faible, et doit donc être augmentée afin de rivaliser avec d’autres métaux légers tels que l’aluminium ou le titane. Une solution consiste à renforcer le magnésium et ses alliages en introduisant des nanoparticules d’oxydes. De par sa structure cristalline hexagonale compacte, le magnésium présente des propriétés plastiques complexes telles qu’une très forte anisotropie plastique et une prédisposition au maclage. La compréhension de ces mécanismes de déformation est essentielle pour le développement de nanocomposites plus performants en vue d’une utilisation industrielle plus répandue. Dans ce travail, nous nous sommes intéressés à l'élaboration et à la caractérisation de nanocomposites de magnésium pur renforcés par des particules d’oxydes. Différentes techniques ont été testées pour l’élaboration des nanocomposites : la solidification assistée aux ultrasons et le procédé de friction malaxage. L’homogénéité de la dispersion des particules a été vérifiée en 2D par observations en microscopie électronique et également en 3D par tomographie aux rayons X. On montre ainsi que le procédé de friction malaxage permet d'obtenir une distribution homogène des particules, tout en réduisant leur taille. Des essais de traction ont permis de mettre en évidence une augmentation de la limité d’élasticité pour une fraction volumique aussi faible que 0.3 %. Afin d’isoler le rôle des particules de celui des joints de grains sur le comportement plastique du nanocomposite, nous avons réalisé des essais de micro-compression sur des micro-piliers monocristallins usinés par canon à ions focalisés (FIB) dans des échantillons ayant préalablement subis un traitement thermique favorisant la croissance anormale des grains. Différentes orientations cristallines et tailles de micro-piliers ont été testées en vue d'étudier l’influence des particules d’une part sur la plasticité dans le plan basal par mouvement de dislocations et d’autre part sur la déformation par maclage. Contre toute attente, les essais sur monocristaux favorablement orientés pour un glissement basal ne montrent pas l’effet durcissant observé macroscopiquement. Nous attribuons cet effet à la densité initiale de dislocations mobiles, plus importante dans les nanocomposites que dans le magnésium pur, du fait des concentrations de contraintes autour des particules. Ces densités initiales de dislocations mobiles tendent également à supprimer l'effet de taille classiquement observé dans le magnésium pur. Les particules modifient également le mécanisme de déformation par maclage en favorisant l’apparition simultanée de plusieurs macles dans le micro-pilier qui interagissent entre elles au cours de la déformation alors que les micro-piliers de magnésium pur présentent généralement une macle unique (dans certains cas deux) qui envahi tout le monocristal. Ces résultats constituent une contribution originale à la compréhension du rôle des nanoparticules dans la déformation plastique des monocristaux de nanocomposites à base de magnésium. / Magnesium is the lightest of all structural metals, which gives it a huge potential to be used in applications that require lightweighting. However, its strength needs to be increased in order to compete with other light metals such as aluminum and titanium. A solution is the reinforcement of magnesium and its alloys with the addition of oxide nanoparticles. The hexagonal close packed crystalline structure is responsible for the complex plasticity of magnesium, which is characterized by a very strong plastic anisotropy as well as a complex twinning activity. Understanding these deformation mechanisms is crucial for the development of more performant nanocomposites, allowing widespread industrial application. The present work focuses on the processing and characterization of magnesium based nanocomposites reinforced with oxide particles. Two different processing techniques have been compared: friction stir processing and ultrasound assisted casting. The homogeneity of the dispersion of the reinforcement particles has been verified in 2 and 3 dimensions using electron microscopy and X-ray tomography, respectively. Friction stir processing produces nanocomposites with a more homogeneous dispersion of particles, while reducing their size. Tensile tests have shown strengthening of magnesium with the addition of a volume fraction of only 0.3 % of reinforcement. An annealing heat treatment has then been performed in order to promote abnormal grain growth and single crystalline microcolumns for microcompression testing have been machined by focused ion beam (FIB). The purpose is to isolate the role of particles. The orientation dependent mechanism of deformation and the size effects have been studied in order to understand the influence of the reinforcement particles on the plasticity for orientations favorable for basal slip or tensile twinning. Differently from the strengthening observed macroscopically, no clear strengthening effect is observed on microcolumns when dislocation glide operates. The reason is the higher density of potentially mobile dislocations that is generated due to stress concentrations around the reinforcement particles. In addition, the size effects usually observed on pure magnesium have also been suppressed with the addition of particles. The reinforcement particles seem to affect the twin nucleation stress and twin morphology: particles induce the nucleation of multiple twins inside a microcolumn, whereas in pure magnesium, only one or two twins have been observed. These results provide relevant insights on the role of nanoparticles on the onset of plastic deformation, as well as size effect, in single crystalline magnesium nanocomposites.

BMGs (bulk metallic glasses) exhibit many exceptional advantages for engineering applications, such as high strength, good corrosion resistance, etc. Despite of having these excellent properties, the brittle nature of metallic glasses in the bulk and thin film forms inevitably imposes limitation and restricts the wide application of BMGs and TFMGs. Composite concept might be another idea to solve this dilemma. In order to manufacture the bulk metallic glass composites (BMGCs), the approaches are classified into two categories: the intrinsic and extrinsic methods. For the intrinsic method, the in situ process and heat treatment process are two kinds of ways in common uses. Adding reinforcements into the BMGs or TFMGs is extensively used to manufacture composites in the extrinsic method. In this study, the deformation behaviors of multilayer (amorphous/nanocrystalline) micropillars are studied by uniaxial microcompression tests at room temperature. The nanocrystalline layer to be coupled with the amorphous layer can be of either face-centered cubic (FCC), hexagonal close-packed (HCP) or body-centered cubic (BCC) in crystal structure. The current study demonstrates that brittle problem of a metallic glass coating can be alleviated by percolating with a nanocrystalline metallic underlayer. The brittle thin film metallic glass can become highly ductile and exhibit a plastic strain over 50% at room temperature. The present study has an important implication for MEMS applications, namely, the life span of a brittle amorphous layer can be significantly improved by using an appropriate metallic underlayer. The brittle problem of thin film ZrCu metallic glasses was also treated by invoking soft Cu layers with optimum film layer thickness. Such multilayered amorphous/crystalline samples exhibit superplastic-like homogeneous deformation at room temperature. It is found that the deformability of the resultant micropillars depends on the thickness of Cu layers. Microstructural observations and theoretical analysis suggest that the superplastic-like deformation mode is attributed to homogeneous co-deformation of amorphous ZrCu and nanocrystalline Cu layers because the 100 nm-thick Cu layers can provide compatible flow stress and ¡§plastic zone¡¨ size well matched with those of ZrCu amorphous layers. Besides, we also made attempts to investigate the critical sample size below which shear band localization would disappear and the sample can deform homogeneously. In situ TEM compression was conducted on amorphous ZrCu nanopillars to study shear band formation behavior. The nanopillar is 140 nm in diameter and with a taper angle of 3¢X. Experimental observations and simulations based on a free-volume model both demonstrate that the deformation was localized near the top of the tapered metallic glass pillar. Eventually, the interface nature of metallic glass amorphous/crystalline was characterized through evaluating its energy and validated by the mechanical response of micropillar with ~45o inclined interface under compression. The calculated results showed that the ZrCu/Zr interface energy resides several joules per meter square, meaning that the Zr/ZrCu interface is inherently strong. The high strong adhesion ability of ZrCu/Zr interface was further confirmed by shear fracture happening rightly within the Zr layers rather than along the interface when compressing the ZrCu/Zr micropillars with 45o inclined interface.

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The main goal of this work is to find a methodology of the fabrication of microcompressive specimens (pillars) from thin metallic film prepared by means of PVD. The studied film was prepared by the ON Semiconductor company, Roznov pod Radhostem. Its chemical composition was Al-1.5 wt.% Cu; such films are used for electric connections on integrated circuits. At first, a thin intermediate layer of W-10 wt.% Ti was deposited on the Si single crystalline substrate with the purpose of improving adhesion properties of the studied film. The geometry of the microcompressive specimen should be as close to the cylindrical shape as possible. The height of the cylinder is given by the film thickness, its diameter is approximately 1 m. Such specimens were prepared in Quanta 3D FEG Dual BeamTM facility using focused ion beams technology. Experiments were done at FEI Company in Brno. In total, 39 microcompressive specimens were prepared at various ion milling conditions. The required geometry was finally attained by the optimization of processing parameters, in particular the parallelism of lateral faces was improved, the bottom of the removed zone in the vicinity of the pillar was almost flat and the transition pillar – flat bottom was regular. The prepared pillars are suitable for the microcompression tests; the first of them have been already performed within the cooperation with the Institut of Physics, Academy of Sciences of the Czech Republic, Praha.

<p><a>Zirconium oxide (zirconia, ZrO₂) is one of the essential structural ceramics for industrial applications due to its superb strength and fracture toughness. ZrO₂ has three main polymorphs: cubic, tetragonal, and monoclinic phase, depending on temperature, type, and concentration of dopants. Stabilized zirconia with metastable tetragonal phase can transform into monoclinic phase with ~ 4% volume expansion under an applied external stress. The tetragonal-to-monoclinic transformation can hinder crack propagations by generating a compressive stress field near crack field, thereby enhancing fracture toughness. In addition, other deformation mechanisms such as dislocation activities, crack deflection, and ferroelastic domain switching can further enhance its deformability. Bulk ZrO₂ is typically prepared by sintering at high temperatures over a long period of time. Recently, field-assisted sintering techniques such as flash sintering and spark plasma sintering have been applied to effectively sinter ZrO₂. These techniques can significantly decrease sintering temperature and time, and more importantly introduce a large number of defects in the sintered fine grains.</a></p> <p>The miniaturization of sample dimension can alter the mechanical properties of materials by increasing the surface-to-volume ratio and decreasing the likelihood of retaining process-induced flaws. The knowledge of mechanical properties of ZrO₂ at micro and nanoscale is critical in that superelasticity and shape memory effect of ZrO₂ can be utilized for applications of actuation, energy-damping, and energy-harvesting at small scale. Here, we performed <i>in-situ</i> microcompression tests at various temperatures inside a scanning electron microscope to examine and compare the mechanical properties of ZrO₂ prepared by flash sintering, spark plasma sintering, plasma spray, and thermal spray. Detailed microstructural analyses were conducted by transmission electron microscopy. The unique microstructures in ZrO₂ prepared by field-assisted sintering largely improved their plasticity. Temperature and processing technique-dependent underlying deformation mechanisms and fracture behavior of ZrO₂ are discussed.</p>

X / A combined experimental and computational investigation of the deformation behavior of pure magnesium single crystal at the micron length scale has been carried out. Employing the recently exploited method of microcompression testing, uniaxial microcompression experiments have been performed on magnesium single crystals with [0001], [2-1-12], [10-11], [11-20] and [10-10] compression axes. The advantage of the microcompression method over conventional mechanical testing techniques is the ability to localize a single crystalline volume which is characterizable after deformation. The stress-strain relations resulting from microcompression experiments are presented and discussed in terms of orientation dependent slip activity, twinning mechanisms and an anisotropic size effect. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures, and further supported by 3D discrete dislocation dynamics simulations. The [0001], [2-1-12], and [10-11] compression axes results show dislocation plasticity. Specifically, the deformation due to [0001] compression is governed by pyramidal slip and displays significant hardening and massive unstable shear at stresses above 500MPa. In the case of the two orientations with compression along an axis 45 degrees to the basal plane, unsurpringly it is found that basal slip dominates the deformation. In contrast, compression along the [11-20] and [10-10] directions show deformation twinning in addition to dislocation plasticity. In the case of compression along [11-20], the twinning leads to easy basal slip, while the twin resultant during compression along [10-10] does not lead to easy basal slip. In all cases, a size effect in the stress-strain behavior is observed; the flow stress increases with decreasing column diameter. Furthermore, the extent of the size effect is shown to depend strongly on the number of active slip systems; compression along the [0001] axis is associated with 12 slips systems and displays a saturation of the size effect at a diameter of 10μm, while the other orientations still show a significant size effect at this diameter. The experimental evidence of an orientation-dependent deformation behavior in flow stress has been investigated by 3D discrete dislocation dynamics simulations. Here, the code TRIDIS was modified for hcp structure and c/a ratio of Mg. By matching the simulation results to experimental results, some proper constitutive material parameters such as initial dislocation density, dislocation source length, the critical resolved shear stress were suggested. For the case of [0001] and [2-1-12] orientation, dislocation feature in the pillar during the deformation was exhibited and strain burst was discussed.

A combined experimental and computational investigation of the deformation behavior of pure magnesium single crystal at the micron length scale has been carried out. Employing the recently exploited method of microcompression testing, uniaxial microcompression experiments have been performed on magnesium single crystals with [0001], [2-1-12], [10-11], [11-20] and [10-10] compression axes. The advantage of the microcompression method over conventional mechanical testing techniques is the ability to localize a single crystalline volume which is characterizable after deformation. The stress-strain relations resulting from microcompression experiments are presented and discussed in terms of orientation dependent slip activity, twinning mechanisms and an anisotropic size effect. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures, and further supported by 3D discrete dislocation dynamics simulations. The [0001], [2-1-12], and [10-11] compression axes results show dislocation plasticity. Specifically, the deformation due to [0001] compression is governed by pyramidal slip and displays significant hardening and massive unstable shear at stresses above 500MPa. In the case of the two orientations with compression along an axis 45 degrees to the basal plane, unsurpringly it is found that basal slip dominates the deformation. In contrast, compression along the [11-20] and [10-10] directions show deformation twinning in addition to dislocation plasticity. In the case of compression along [11-20], the twinning leads to easy basal slip, while the twin resultant during compression along [10-10] does not lead to easy basal slip. In all cases, a size effect in the stress-strain behavior is observed; the flow stress increases with decreasing column diameter. Furthermore, the extent of the size effect is shown to depend strongly on the number of active slip systems; compression along the [0001] axis is associated with 12 slips systems and displays a saturation of the size effect at a diameter of 10μm, while the other orientations still show a significant size effect at this diameter. The experimental evidence of an orientation-dependent deformation behavior in flow stress has been investigated by 3D discrete dislocation dynamics simulations. Here, the code TRIDIS was modified for hcp structure and c/a ratio of Mg. By matching the simulation results to experimental results, some proper constitutive material parameters such as initial dislocation density, dislocation source length, the critical resolved shear stress were suggested. For the case of [0001] and [2-1-12] orientation, dislocation feature in the pillar during the deformation was exhibited and strain burst was discussed.

<p> This dissertation describes the recent advancements in micromechanical testing that inform how deformation mechanisms in austenitic stainless steels (SS) are affected by the presence of irradiation-induced defects. Austenitic SS is one of the most widely utilized structural alloys in nuclear energy systems, but the role of irradiation on its underlying mechanisms of mechanical deformation remains poorly understood. Now, recent advancement of microscale mechanical testing in a scanning electron microscope (SEM), coupled with site-specific transmission electron microscopy (TEM), enables us to precisely determine deformation mechanisms as a function of plastic strain and grain orientation.</p> <p> </p> <p>We focus on AISI 304L SSs irradiated in EBR-II to ~1-28 displacements per atom (dpa) at ~415 °C and contains ~0.2-8 atomic parts per million (appm) He amounting to ~0.2-2.8% swelling. A portion of the specimen is laser welded in a hot cell; the laser weld heat affected zone (HAZ) is studied and considered to have undergone post-irradiation annealing (PIA). An archival, virgin specimen is also studied as a control. We conduct nanoindentation, then prepare TEM lamellae from the indent plastic zone. In the 3 appm He condition, TEM investigation reveals nucleation of deformation-induced <i>α</i>’ martensite in the irradiated specimen, and metastable <i>ε</i> martensite in the PIA specimen. Meanwhile, the unirradiated control specimen exhibits evidence only of dislocation slip and twinning; this is unsurprising given that alternative deformation mechanisms such as twinning and martensitic transformation are typically observed only near cryogenic temperatures in austenitic SS. Surface area of irradiation-produced cavities contribute enough free energy to accommodate the martensitic transformation. The lower population of cavities in the PIA material enables metastable <i>ε</i> martensite formation, while the higher cavity number density in the irradiated material causes direct <i>α</i>’ martensite formation. In the 0.2 appm He condition, SEM-based micropillar compression tests confirm nanoindentation results. A deformation transition map with corresponding criteria has been proposed for tailoring the plasticity of irradiated steels. Irradiation damage could enable fundamental, mechanistic studies of deformation mechanisms that are typically only accessible at extremely low temperatures. </p>