7xxx series Al-Zn-Mg-Cu-base wrought Al-alloy products are widely used for aerospace structural applications where monotonic and cyclic mechanical properties are of prime concern. Microstructure of these commercial alloys usually contains brittle coarse constituent particles or inclusions of Fe-rich intermetallic compounds and Mg2Si, typically in the size range of 1 to 50 micron. Plastic deformation and fracture of 7xxx series alloys (as well as of numerous other wrought Al-alloys) is associated with gradual microstructural damage accumulation that involves cracking of the coarse constituent particles, growth of voids around the cracked particles, and the void coalescence. To understand and model the microstructural damage evolution processes such particle cracking, quantitative microstructural data associated with the damage nucleation are required under monotonic as well as cyclic loading conditions. In the past quantitative characterization of particle cracking damage in these alloys has been problematic. However, with recent advances in digital image analysis and stereology based techniques, it is now possible to quantitatively characterize the damage nucleation in hot-rolled 7075(T6) Al-alloy (a typical alloy of 7xxx series) due to cracking of the Fe-rich coarse constituent particles. The objectives of this work are: * Quantitative characterization of the cracking of Fe-rich constituent particles as a function of strain under quasi-static loading. This involves measurements of number density of cracked particles, volume fraction of the cracked particles, their size, shape, and orientation distribution, as well as nearest neighbor distribution and two-point correlation functions to quantify spatial dispersion of the cracked particles in a series of interrupted uniaxial tensile test specimens at different strain levels. * Quantitative characterization of the cracking of Fe-rich constituent particles under cyclic loading to study the differences between the particle cracking damage due to monotonic and cyclic loading.

<p>This work is an experimental approach to understand the effect of the microstructure on large strain deformation behavior of A356 alloy (with various additions of titanium and strontium) under uniaxial tensile, compressive and V-bend loading conditions. The studies were carried out on unmodified and Sr-modified A356 Al alloy, where Sr was added to refine the morphology of the eutectic Si phase particles. The two variants were then used to study the effect of size and shape distributions of Si eutectic phase particles within the material on the deformation behavior under uniaxial tension, compression and V-bending conditions. The alloy with a modified and refined eutectic Si phase particles showed significant improvement in ductility and bendability, whereas the differences in compression were not appreciable. In addition to Sr addition, Ti was also added as a grain refiner to the alloys at three different levels to obtain microstructures with different grain sizes. The results in the form of full field strain maps show that the larger grained alloy exhibits extensive strain inhomogeneity, whereas the grain refined alloy shows a more homogenous loading pattern.</p> / Master of Applied Science (MASc)

State-of-the-art Li-ion cathode materials are based on LiMO2 (M=Ni, Mn, Co) layered transition metal oxides (denoted NMC) with Ni-rich composition because of their high specific capacity. Yet, these materials suffer from poor capacity retention due to crack formation during de-/lithiation cycling. Particle cracking leads to exposure of new electrode surface which leads to Li-inventory loss, increased side reactions, and electric disconnection. Quantification of the extent of cracking is therefore desirable, especially during in situ whilst cycling of the Li-ion cell. Herein, we evaluate and improve an analytical methodology based on electrochemical impedance spectroscopy (EIS) in order to estimate the changes in electrochemically active surface area of both poly- and single-crystalline Ni0.8Mn0.1Co0.1(NMC811) active materials. A transmission-line model (TLM) applied to both non-blocking and blocking electrode condition was utilized in order to deconvolute and interpret the acquired experimental data. Fits of the complex TLM equivalent-circuits to the impedance spectra was facilitated by developing a global stochastic iterative function based on local multivariate optimization. Impedance analysis during short- term cycling showed that the single-crystalline NMC811 suffered from less particle cracking and side reactions compared to polycrystalline NMC811, which was also confirmed from post-mortem gas adsorption analysis. A novel approach to estimate the extent of particle cracking in commercial Li-ion cells by utilizing an empirically strong positive correlation between the charge-transfer capacitance and resistance was proposed. The work presented herein demonstrates the unique prospects of the EIS methodology in the development and research of future rechargeable batteries

La décrépitation (fracturation) des matériaux actifs de batteries associée à leur variation volumique lors des cycles de charge/décharge a pour effet d'accélérer leur corrosion par l'électrolyte et/ou d'induire une perte de connectivité électronique au sein de l'électrode ce qui réduit notablement leur durée de vie. C’est particulièrement le cas des hydrures métalliques utilisés dans les batteries Ni-MH. À ce jour, l'évaluation de leur fracturation se limite généralement à une observation post mortem des électrodes par microscopie ce qui ne permet pas une analyse détaillée du processus de décrépitation. À ce titre, un de nos principaux objectifs dans le cadre de ce travail de recherche a été de développer une méthode d'analyse novatrice et performante basée sur l'émission acoustique (EA) afin d'étudier in situ la fracturation d'électrodes négatives pour batteries Ni-MH. Dans une première étape, nous avons analysé en détail les signaux acoustiques produits lors de la charge (hydruration) d'un alliage commerciale à base de LaNi5 et d'un alliage MgNi obtenu par broyage mécanique. Nous avons ainsi pu séparer les signaux générés par la fracturation des particules d’hydrures métalliques de ceux associés à la formation de bulles de H2, ce qui a permis d’établir les mécanismes qui régissent leur fracturation. Par la suite, un montage expérimental, constitué d’une cellule électrochimique connectée à un capteur de force en compression et d’un équipement d’EA, a été mis point pour suivre in-situ la fracturation et la force générée par l’expansion/contraction lors du cyclage des électrodes MgNi et LaNi5. Nous avons ainsi pu confirmer que l’expansion/contraction volumique de l’alliage MgNi est plus progressif que pour l’alliage à base de LaNi5. Par la suite, l’étude comparée par EA des alliages MgNi, Mg0.9Ti0.1Ni et Mg0.9Ti0.1NiAl0.05 a permis de mettre en évidence l'influence de leur composition sur leur résistance à la pulvérisation. Finalement, nous avons étudié en détail l’influence de l’addition de palladium dans l’alliage Mg0.9Ti0.1NiAl0.05 sur son comportement électrochimique et sa résistance à la fracturation. / The pulverization (cracking) of active materials in batteries, induced by their volume change during charge/discharge cycles, accentuates their corrosion by the electrolyte and/or leads to a loss of electronic connectivity within the electrode, which notably reduces their cycle life. This particularly occurs for metallic hydrides used in Ni-MH batteries. To date, the evaluation of their cracking is generally limited to post mortem observations of the electrodes by microscopy, which does not allow for a detailed analysis of the decrepitation process. In this respect, one of our main research objectives was to develop an innovative and efficient analysis method based on acoustic emission (AE) for in situ monitoring of the cracking of negative electrodes for Ni-MH batteries. As a first step, a detailed analysis of the acoustic signals generated during the charge (hydriding) of a commercial LaNi5-based alloy and a MgNi alloy obtained by mechanical alloying was performed. This allowed separating the signals generated by the cracking of the metallic hydride particles from those induced by the formation of H2 bubbles. We have shown that the mechanism which governs the pulverization of the MgNi alloy remarkably differs from that of the LaNi5-based alloy. In a second step, an experimental set-up made of an electrochemical cell linked to a compression force cell and an AE equipment was elaborated, in order to monitor concomitantly the cracking and the force generated by the expansion/contraction of the MgNi and LaNi5 during cycling. We have thereby been able to confirm that the volume expansion/contraction of the MgNi alloy is more progressive than that of the LaNi5 alloy. The AE-based comparative study of MgNi, Mg0.9Ti0.1NiAl5 and Mg0.9Ti0.1NiAl0.05 alloys then allowed demonstrating the positive effect of the partial Mg substitution by Ti and adding of Al on the alloy decrepitation resistance. As a final step, we have studied the impact of palladium addition in the Mg0.9Ti0.1NiAl0.05 alloy on its electrochemical behaviour and cracking resistance.