Lithium ion batteries (LIBs) have gained increasing popularity due to their high potential, low self-discharge, zero priming and minimal memory effect. However, the emergence of electrical vehicles and hybrid electrical vehicles in the automobile industry, where LIBs are predominantly in use, instilled a need to improve LIB batteries by experimenting with new materials. Graphite, the commonly used anode material for LIBs suffers from low theoretical capacity (372 mA h g-1) and torpid rate performance. Germanium (Ge) seems to be a promising substitute of carbon due to its high theoretical capacity, high Li+ diffusivity and electrical conductivity. However, Ge undergoes large volumetric change (±370%). This causes deboning of the thin film Ge electrode from the substrate current collector, causing a rapid decrease in the electrolytic performance. The process of ion beam mixing claims to have overcome this problem. In our current study, the adhesion strength of Ge thin film over Nickel (Ni) substrate (with and without ion beam mixing) is being measured using nanoindentation and the superlayer indentation test. Nanoindentation is one of the popular techniques to measure the mechanical properties and adhesion of thin film coatings. In this technique, a very small indenter of a desired geometry indents the film/substrate pair and the work of adhesion is calculated by knowing the plastic depth of indentation and the radius of indentation. Superlayer indentation is analogous to normal indentation but with a highly stressed superlayer on top to restrict the out-of-plane displacements, it reduces the plastic pile up around the indenter tip. The results from our study strongly suggest the possibility of dramatically increasing the adhesion strength by ion bombardment, which can be achieved by atomic level intermixing of the film/substrate pair. These, in turn, suggest that Ge could be an effective successor to graphite in the near future.

Le mode de rupture des alliages CFC est généralement de type ductile par des mécanismes de germination, croissance et coalescence de cavités internes micrométriques et peut être modifié par l’irradiation. L’irradiation neutronique de ces alliages conduit à la création de défauts cristallins qui induisent un durcissement, une perte de la capacité d’écrouissage, une chute très importante de la ténacité et un mode de déformation localisé à l’échelle intragranulaire. La compréhension des mécanismes physiques élémentaires de la rupture ductile est indispensable au développement de modèles quantitatifs pour prédire la ténacité des matériaux CFC irradiés. Pour cela, trois différents points ont été étudiés dans cette thèse : (1) L’influence de la localisation de la déformation induite par l’irradiation sur la croissance et la coalescence de cavités : des expériences modèles in-situ MEB de croissance et coalescence de cavités micrométriques dans des matériaux irradiés aux protons ont été réalisées. Les résultats montrent un effet limité de la localisation pour des cavités de la taille des grains et une diminution de l’influence de la localisation avec l’augmentation du niveau de déformation pour des cavités intragranulaires. Par conséquent, les modèles homogénéisés de matériaux poreux développés pour les matériaux non irradiés pourraientt être utilisés en première approximation pour modéliser la rupture ductile des matériaux irradiés. (2) Le comportement sous chargement mécanique de nano-porosités d’irradiation et leur contribution éventuelle à la rupture : l’étude expérimentale et numérique de la déformation de cavités dans un matériau nanoporeux a permis de mettre en évidence la très forte hétérogénéité de la déformation à cette échelle et l’absence d’effet de taille significatif sur la déformation des cavités de diamètre supérieur à 10 nm en traction simple. (3) Le développement de modèles homogénéisés de matériaux poreux valides aux fortes porosités : deux nouveaux critères de coalescence obtenus par analyse limite sont proposés et validés par comparaison à des simulations d’analyses limites numériques, dans le cas de cavités de type fissures et de cavités ellipsoïdales. / The failure mode of FCC alloys is generally ductile through nucleation, growth and coalescence of micrometric voids, and can be modified by irradiation. Neutron irradiation of these alloys leads to the creation of crystalline defects that induce hardening, loss of work hardening capability, a very large drop in fracture toughness and a heterogeneous deformation mode at the grain scale. Understanding the elementary physical mechanisms of ductile fracture is essential for the development of quantitative models to predict fracture toughness of irradiated FCC materials. Thus, in this thesis, three different subjects have been studied. (1) Influence of the localization of deformation induced by irradiation on void growth and coalescence: Model experiments of growth and coalescence of micrometric voids in proton-irradiated materials have been performed based on SEM in-situ tests. Results show a limited effect of localization for grain-size voids and a decreasing influence of localization with increasing level of deformation for intragranular voids, so that homogenized models of porous materials developed for unirradiated materials may be used as a first approximation to model the ductile fracture of irradiated materials. (2) The behavior under mechanical loading of nanovoids generated under irradiation and their possible contribution to fracture: The experimental and numerical study of void deformation in a nanoporous material highlights the very strong heterogeneity of the deformation at this scale and the absence of significant size effect for voids of diameter greater than 10 nm under tensile loading. (3) Development of homogenized models for porous materials valid at high porosities: Two new coalescence criteria obtained by limit analysis are proposed and validated by comparison with numerical limit analysis simulations, in the case of penny-shaped cracks and ellipsoidal voids.

FeCrAl alloys are being investigated as candidate materials for replacing zirconium based alloys as nuclear reactor fuel cladding because of their superior high temperature oxidation resistance in steam environments. Unirradiated FeCrAl as well as Fe²⁺ ion irradiated FeCrAl to a peak dose of 20DPA were mechanically tested and compared against each other. Nanohardness tests were performed on both the unirradiated and irradiated conditions and it was found that the irradiated alloy was about 1GPa harder than its unirradiated counterpart. TEM <i>in situ</i> tensile tests were performed using the Bruker push to pull device alongside a PI95 Picoindenter on single crystals with grain orientations 001, 011 and 111. The unirradiated 001 grains tended to fail without yielding in a brittle manner while the irradiated 001 grain yielded and reached an ultimate tensile strength before failure. The unirradiated 011 grains behaved in a mixed manner, where one failed without yielding and one slipped many times before failing. The irradiated 011 grain yielded and failed quickly thereafter. The unirradiated 111 grain yielded, slipped and twinned before failing and both irradiated 111 grains slipped. Two general trends were observed. One, each unirradiated single grain was stronger than its irradiated counterpart. This trend is indicative of the ion irradiated microstructure facilitating bulklike mechanical behavior in the irradiated samples whereas the unirradiated samples exhibited mechanical size effects due to either the total lack of preexisting defects or the ability for existing defects to escape easily to the surface of the sample resulting in a pristine, defect free sample. Two, regardless of irradiation condition, the 001 grain orientation was brittle, the 011 grain orientation deformed in a mixed brittle/ductile manner and the 111 grain orientation was ductile through all tests. These results are indicative of the geometry of the BCC crystal structure and the slip system involving these orientations.