<p> The room-temperature fracture behaviour of calcia partially stabilized zirconia (PSZ) was investigated. Fracture energy measurements were made using the standard stress intensity calibration and work to fracture techniques. The detailed nature of the PSZ microstructure was studied using scanning electron microscopy, qualitative X-ray analysis and T.E.M, surface replication. The grain structure was detenninod to be bimodal with small grains of pure zirconia dispersed along the boundaries of large grains. These large grains consist of a binary pure-zirconia/stabilized zirconia mixture. An attempt was made to relate the fracture properties to the nature of the inherent flaws present in the material. </p> <p> The strength of calcia partially stabilised zirconia was observed to depend on the size and distribution of the grain boundary precipitate of pure zirconia. It is postulated that this grain boundary precipitate causes decohesion and weakening of some of the grain boundaries due to the large internal stresses associated with its martensitic phase transformation. This phenomena of grain boundary decohesion leads to elastic nonlinearity and hysteresis. Crack propagation was always observed to proceed in a slow controlled fashion in this material. A model is proposed to explain theses observations based on the formation of a microcrack zone at the tip of a propagation crack. The occurrence of continued stable crack propagation is believed associated with increasing microcrack zone size with increasing crack length. Evidence supporting this model is presented. </p> / Thesis / Master of Science (MSc)

Nous étudions la fragilisation par l'hydrogène de l'alliage d'aluminium 7108. Une technique expérimentale spécifique a été développée : Un pré-chargement en hydrogène des échantillons, à travers un dépôt de nickel de quelques dizaines de microns, qui empêche la dissolution du substrat d'aluminium, est utilisé. Il permet la comparaison de la résistance à la fragilisation de différentes microstructures modèles. Nous étudions l'effet du traitement thermique et de la précipitation sur la sensibilité à l'hydrogène pour des vitesses de déformation macroscopiques imposées variables. Différents modes de rupture sont observés ainsi que des transitions entre eux. Au moyen de simulations numériques à l'échelle mésoscopique, l'effet de taille des précipités intergranulaires pré-fragilisés sur la ténacité des joints de grains est estimé, en utilisant un modèle de zone cohésive. Nous analysons la compétition entre la diffusion de l'hydrogène vers la pointe de la fissure et la vitesse de fissuration par un couplage mécanique - diffusion basé sur la diffusion de l'hydrogène assistée par la contrainte hydrostatique. Une vitesse critique au-delà de laquelle l'hydrogène ne peut plus suivre la fissure, est mise en évidence. L'influence de la microstructure du joint de grains sur cette vitesse est analysée. La valeur est comparée à une estimation des vitesses de propagation expérimentales obtenues pour différentes vitesses de déformation macroscopiques. Nous analysons l'effet du piégeage de l'hydrogène par les précipités intergranulaires et la désorption sur la répartition de l'hydrogène le long du joint de grains en imposant un flux au niveau de l'interface précipités - matrice. / We study the hydrogen embrittlement of the 7108 aluminum alloy. A specific experimental technique was developed : A hydrogen pre-charging, through few tens of microns of deposit of nickel, which prevents the dissolution of the aluminum substrate is used. It allows a comparison of the resistance to embrittlement of different model microstructures. We study the effect of heat treatment and intergranular precipitation on the susceptibility to hydrogen embrittlement for several macroscopic strain rates. Different failure modes and transitions between them are observed. Through numerical simulations, at the mesoscopic scale, the effect of the size of pre-weakened intergranular precipitates on the grain boundary toughness is estimated using a cohesive zone model. We further analyze the competition between the hydrogen diffusion toward the crack tip and crack velocity. For this purpose, a mechanical – diffusion coupling based on the hydrogen diffusion assisted by hydrostatic stress is elaborated. A critical crack velocity, beyond which hydrogen can no longer follow the crack, is highlighted. The influence of the grain boundary microstructure on this critical crack velocity is evaluated and its value is compared with an estimate of velocities obtained for different experimental macroscopic strain rates. We analyze the effect of hydrogen trapping by intergranular precipitates and hydrogen desorption by imposing a flux at the precipitates – matrix interfaces.