Calcific aortic valve disease (CAVD) is associated with significant cardiovascular morbidity. While late-stage valve disease is well-described, there remains an unmet scientific need to elucidate early pathobiological processes. In CAVD, pathological differentiation of valvular interstitial cells (VICs) and lesion formation occur focally in the fibrosa layer. This VIC pathological differentiation has been shown to be influenced by matrix stiffness in vitro. However, little is known about the focal layer specific mechanical properties of the aortic valve in health and disease and how these changes in matrix moduli may influence VIC pathological differentiation in vivo. In this thesis, micropipette aspiration (MA) was shown to be capable of measuring the mechanical properties of a single layer in multilayered biomaterial or tissue such as the aortic valve, if the pipette inner diameter was less than the top layer thickness. With MA, the fibrosa of normal porcine aortic valves was significantly stiffer than the ventricularis; stiffer locations found only within the fibrosa were comparable to stiffnesses shown in vitro to be permissive to VIC pathological differentiation. Early CAVD was induced in a porcine model, which developed human-like early CAVD lesion onlays. Extracellular matrix remodeling occurred in the absence of lipid deposition, macrophages, osteoblasts, or myofibroblasts, but with significant proteoglycan-rich onlays and chondrogenic cell presence. These early onlays were softer than the collagen-rich normal fibrosa, and their proteoglycan content was positively correlated with Sox9 chondrogenic expression, suggesting that soft proteoglycan-rich matrix may be permissive to chondrogenic VIC differentiation. The findings from this thesis shed new light on early disease pathogenesis and improve the fundamental understanding of aortic valve mechanics in health and disease.

Mechanical surface treatments using an elastic-plastic cold working process can develop residual stresses on the surface of a workpiece. Compressive residual stresses on the surface increase resistance against surface crack propagation, so the overall mechanical performance can be improved by this technique. Compressive residual stresses can be created by different methods such as hammering, rolling, and shot peening. Shot peening is a well-established method to induce compressive residual stresses in the metallic components using cold working, and often ascribed to being beneficial to fatigue life in the aerospace and automobile industries. In this method, the surface is bombarded by high-velocity spherical balls which cause plastic deformation of the substrate, leading to a residual compressive stress after shot peening on the surface of the part. Computational modeling is an appropriate and effective way which can predict the amount of produced residual stresses and plastic deformation to obtain surface roughness after shot peening simulation. Finally, an experimental method to measure the magnitude of the residual stress using a nanoindentation technique was developed. The experimental indentation method was compared to both computational predictions (in aluminum) and with x-ray diffraction measurements of stress (in an alloy steel). The current study validates the relation between the nanoindentation method and numerical simulation for assessing the surface residual stresses resulting from single or multiple shot peening processes.

Cyclopentane hydrate interparticle adhesion force measurements were performed in pure cyclopentane liquid using a micromechanical force apparatus. Cyclopentane hydrate adhesion force measurements were compared to those of cyclic ethers, tetrahydrofuran and ethylene oxide, which were suspected to be cyclic ether-lean and thus contain a second ice phase. This additional ice phase led to an over-prediction of the hydrate interparticle forces by the capillary bridge theory. The adhesion forces obtained for cyclopentane hydrate at atmospheric pressure over a temperature range from 274-279 K were lower than those obtained for the cyclic ethers at similar subcoolings from the formation temperature of the hydrate. The measured cyclopentane interparticle adhesion forces increased linearly with increasing temperature, and are on the same order of magnitude as those predicted by the Camargo and Palermo rheology model.

Calcific aortic valve disease (CAVD) is associated with significant cardiovascular morbidity. While late-stage valve disease is well-described, there remains an unmet scientific need to elucidate early pathobiological processes. In CAVD, pathological differentiation of valvular interstitial cells (VICs) and lesion formation occur focally in the fibrosa layer. This VIC pathological differentiation has been shown to be influenced by matrix stiffness in vitro. However, little is known about the focal layer specific mechanical properties of the aortic valve in health and disease and how these changes in matrix moduli may influence VIC pathological differentiation in vivo. In this thesis, micropipette aspiration (MA) was shown to be capable of measuring the mechanical properties of a single layer in multilayered biomaterial or tissue such as the aortic valve, if the pipette inner diameter was less than the top layer thickness. With MA, the fibrosa of normal porcine aortic valves was significantly stiffer than the ventricularis; stiffer locations found only within the fibrosa were comparable to stiffnesses shown in vitro to be permissive to VIC pathological differentiation. Early CAVD was induced in a porcine model, which developed human-like early CAVD lesion onlays. Extracellular matrix remodeling occurred in the absence of lipid deposition, macrophages, osteoblasts, or myofibroblasts, but with significant proteoglycan-rich onlays and chondrogenic cell presence. These early onlays were softer than the collagen-rich normal fibrosa, and their proteoglycan content was positively correlated with Sox9 chondrogenic expression, suggesting that soft proteoglycan-rich matrix may be permissive to chondrogenic VIC differentiation. The findings from this thesis shed new light on early disease pathogenesis and improve the fundamental understanding of aortic valve mechanics in health and disease.

<p>  </p> <p>Aluminum (Al) possesses a plenitude of remarkable properties, such as strong corrosion resistance, high thermal and electrical conductivity, and high specific strength. However, Al and its alloys are still remarkably weaker than most high strength steels and susceptible to drastic softening at high temperatures, preventing many applications where its low density would be beneficial. Severe plastic deformation can yield ultra-fine grained Al alloys with similar strengths as steels, although they are highly unstable even at room temperature. Nanotwinned (NT) metals have demonstrated concomitant strength and ductility, enabled by twin boundaries which simultaneously act to inhibit dislocation motion and generate partial dislocations that aid in plasticity. In spite of having a high stacking fault energy, nanotwins have been introduced into Al alloys using transition metal solutes during magnetron sputtering. This thesis aims to explore the impact Zr has on the microstructure, deformation, and thermal stability of nanotwins in NT Al.</p> <p>Our studies identify how Zirconium (Zr) aids in the formation of a significant volume fraction of 9R phase and an abundance of finely spaced incoherent twin boundaries, leading to a maximum hardness of 4.2GPa. They further uncover through <em>in-situ</em> micropillar compression that NT Al-Zr alloys are highly deformable and reach a flow stress of ~1.1GPa. Constant strain rate nanoindentation tests demonstrate the enhanced strain rate sensitivity in NT Al-Zr alloys. Zr is also identified to be a remarkable thermal stabilizer when incorporated into NT Al-Co alloys, with no apparent softening up to 450 °C (0.78 T­m). The influence of substrate texture on nanotwinned Al-Zr alloys microstructure was also thoroughly explored.</p>

Primary water stress corrosion cracking (SCC) of metals in pressurized water reactors (PWRs) is known to be one of the most challenging and cost intensive modes of failure in the nuclear industry. Even though it is known that cracking in Ni-base alloys proceeds mainly intergranular (IG), the initiation and propagation of cracks in ductile metals are not yet understood and a much-desired accurate prediction of SCC related failure seems unobtainable. In this thesis, a combination of microcantilever fracture experiments, scanning electron- (SEM) and transmission electron microscopy (TEM) techniques was employed to study and compare the failure of oxidized grain boundaries of Ni-base Alloy 600 with high and low intergranular carbide coverage and different sample history. A new technique for lifting-out whole cantilevers after testing and for performing 3D focussed ion beam sequencing (3D FIB-SEM) while preserving a thin central region of the cantilever for further TEM sample preparation was developed and is presented. In lieu with recent efforts of the main project sponsor Électricité de France (EDF) to build a predictive model for IGSCC based on localized/microscopic information, one of the main objectives was the extraction of the stress at failure of individual oxidized GBs. Supported by finite element simulations, microcantilever fracture tests revealed that surface oxides on top of individual GBs have the capability to alter the mechanical response by delaying/suppressing the onset of failure. An overestimation of the failure stress (&GT; 230 MPa) was observed, proving that the presence of the surface oxide on top of the test structures cannot be neglected. The failure stress on both samples, tested without influence of the surface oxide, was found to cover a range of 300 - 600 MPa, which agreed well with finite element simulations of the tests and further demonstrates the reliability of the obtained data. The second objective was to gain a better understanding of the observed fracture behaviour and the role of local microstructure. Using the gathered microscopy data, it was found that the crack clearly favours a progression along the IG oxide-metal interface in the presence of carbide precipitates. Electron energy loss spectroscopy (EELS) revealed that the observed crack path can be linked to compositional and density variations of the IG oxide. In the presence of carbides the oxide was layered. An oxide close to the stoichiometry of chromia was located at the original GB and next to the carbides. Next to this Cr-rich oxide, Fe-rich mixed spinel oxides of varying composition and density were found. An explanation for density variations based on the possible formation of defective spinel oxides of the type A²⁺B³⁺₂O₄, due to an unavailability of certain cation species is presented. No clear interface preference was observed in the absence of precipitation, where the IG oxide was found to be thin and often incomplete with Cr-richer oxides preferentially located at the original GB. While these observations were consistent on both samples (high and low carbide coverage), bigger void-like defects were located at the Fe-richer oxide-metal interface of the cold worked sample with high IG carbide precipitation only. These weak spots seemed to be the preferred path for crack propagation on this sample. The sample with low intergranular carbide coverage showed no obvious porosities at this interface but a Cr- depleted region was seen. Introducing a multi-faceted investigation strategy, supported by finite element simulations, the presented thesis provides the most accurate determination of the failure stress of oxidized GBs on Alloy 600 to date and and adds new valuable insights to our understanding of IGSCC and the future prediction of SCC related failures.

This project is in collaboration with the Hydro-Quebec Research Institute (IREQ) and the Canadian Centre for Electron Microscopy (CCEM) on the mechanical test performance of miniature-scale micro-tensile specimens. The objective of the thesis project is to create an efficient and reliable fabrication route for producing micro-tensile specimens and to validate the accuracy of a newly custom-built micro-tensile bench at IREQ. The fabrication techniques developed and outlined in this thesis use the underlying fundamental physical mechanisms of secondary electron microscopy (SEM), focused-ion beam (FIB), and the femtosecond (fs)-laser machining for producing optimal quality micro-tensile specimens. The mechanical testing of the specimens is geared towards studying the localized deformation occurring in the microstructure when the size of the specimen only limits a number of grains and grain boundaries in order to target the specific detailed measurement of the mechanical behaviour of individual grains and interfaces. The goal for creating an optimal fabrication route for micro-tensile specimens is to carry out micro-mechanical testing of the primary turbine steels of 415 martensitic stainless steel used in the manufacture of Francis turbine components at Hydro-Quebec. The mechanical testing of single phase and interphase interface 415 steel micro-tensile specimens are considered building blocks to developing digital twin models of the steel microstructure. The experimental data from the mechanical tests would be fed into the crystal plasticity finite element models (CPFEM) that are currently being developed by researchers at IREQ. With the development of digital twin models, engineers at IREQ would be able to predict crack initiation at the microstructure level (prior to crack propagation into macro-scale cracks) by observing the evolution of the grain’s crystallographic orientation and morphology, as well as deformation mechanisms such as martensite formation and twinning produced from localized induced strains in the microstructure. In addition, self-organized dislocation processes such as dislocation nucleation and dislocation escape through the free surface can also be studied using the CPFEM models for size-limited mechanical deformation behaviour of miniature-scale mechanical test specimens. The fabrication routes studied in this thesis project use the combination of the fs-laser and plasma focused ion beam (PFIB) to machine the micro-tensile specimens. (100) single crystal copper was the ideal material chosen to validate the accuracy of the micro-tensile bench and quality of the fs-laser-machined tensile specimens, due to its ductile nature and well-characterized properties studied in literature. A mechanical size effect was studied for single crystal copper specimens with different gauge thicknesses. It was observed from the micro-tension testing that the strength of the specimens increased with decreasing gauge thickness occurring in the size-limited tensile gauges. In addition, it was determined there was negligible differences in the size effect seen between the PFIB-machined copper micro-tensile specimens and the fs-laser-machined micro-tensile specimens, demonstrating that the fs-laser is a reliable machining route for the micro-tensile specimens. X-ray computed tomography was used to validate the correct geometry of the machined gauge section produced from an innovative gauge thinning method adopted from IREQ’s research collaborator, Dr. Robert Wheeler. As well, finite-element analysis (FEA) was performed to determine the deformation behaviour under both linear-elastic and non-linear elastoplastic conditions of (100) copper and 415 steel models simulated in pure tension, prior to the fabrication of the micro-tensile specimens, respectively. Furthermore, significant progress has been made towards targeting martensite grains in the 415-steel microstructure using electron backscattered diffraction (EBSD) analysis to produce single crystal and interphase interface micro-tensile specimens. A workflow towards grain targeting using EBSD analysis has been developed, as well as for the relocation of grains using reference fiducial marks for future fabrication of the single crystal and interphase interface 415 micro-tensile specimens. / Thesis / Master of Applied Science (MASc) / Hydro-Quebec is an energy utilities company that operates the design of Francis hydro-turbines to supply hydroelectric power across the province of Quebec. The hydro-turbines have an expected service life of 70 years. Unfortunately, the turbines can get replaced by new ones prior to reaching half of its service life, due to the development of severe fatigue crack growth in the primary components of the turbines. A solution proposed by the researchers at the Hydro-Quebec Research Institute (IREQ) is to determine a linkage between the turbine’s steel’s microstructure and the mechanical behaviour of the turbine steels. Deformation of the material starts at the microstructure level, where dislocations glide through the material lattice, causing both reversible (elastic) and irreversible (plastic) deformation. Therefore, a solution was proposed by the researchers at IREQ to create computational models of the steel microstructure to predict the deformation of the steel’s microstructure. Being able to predict the deformation mechanisms through the simulation models of the microstructures allows for engineers at Hydro-Quebec to schedule regular maintenance of the turbines more efficiently and provide metallurgists the knowledge on what is occurring at the microstructure level and what can be done to improve the chemical and physical composition of the steel. To develop the digital twin models, experimental data must be collected through mechanical testing of miniature mechanical test specimens of the turbine steels. The mechanical properties of the single phases and interphase interface specimens are fed into the models as building blocks to building a microstructure map of the turbine steels. Micro-tension testing of micro-tensile specimen provides direct information about the material’s mechanical properties. In this work, a reliable and efficient fabrication route for micro-tensile specimens was developed for the purpose of extracting mechanical properties of single phase and interphase interface turbine steel specimens using focused ion beam (FIB) and femtosecond laser machining.