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<p dir="ltr">Additive manufacturing continues to show great promise for use in structural components due to the cost effectiveness and reduced complexity associated with optimized and targeted use of the method. However, before additive manufacturing can be widely accepted a more complete understanding of the material performance and microstructural features must be achieved. This thesis aims to further the understanding of cold dwell fatigue in additively manufactured Ti-6Al-4V and explore targeted microstructural control of additively manufactured Ti-6Al-4V through the use of printing parameter variations and hot isostatic pressing.</p><p dir="ltr">In the first portion of this thesis, experimental work was conducted to explore the effect of periodically applied load dwell and overloads on the stress-life relationship for additively manufactured Ti-6Al-4V. Samples printed using an optimized print parameter set, heat treated using hot isostatic pressing, machined, and longitudinally polished were tested across a variety of loading schemes including: constant amplitude, periodic dwell, periodic overload, and alternating periodic dwell and periodic overload. It was determined that, for the parameter set studied, periodic overload provided similar damage compared to constant amplitude cases, while periodic dwell provided greater damage compared to both constant amplitude and periodic overload cases. Additionally, a phenomenological failure prediction model for dwell, variable amplitude loading was created. The developed model combines the effects of plasticity and creep with an energy-based approach rooted in the fundamental behavior of the material.</p><p dir="ltr">In the second portion of this thesis a review of the literature is presented to explore the use of hot isostatic pressing in additively manufactured Ti-6Al-4V. The literature review holds the primary purpose of deepening the understanding of the relationships between hot isostatic pressing and microstructural control and how they are taken together to improve fatigue performance. The literature review explores many aspects of factors impacting fatigue life and how the additive manufacturing process impacts material microstructure. The final conclusion of the literature review is that 1 micrometer is the largest pore expected to achieve complete closure though hot isostatic pressing, that 40 micrometer is the critical pore size for fatigue failure, and the process for microstructural evolution during pore closure is dominated by creep and dynamic recrystallization. Using these facts targeted microstructural control can be explored to optimize fatigue performance through purposeful microstructural variations.</p>

Cette étude traite de l'impact du vieillissement thermomécanique sur la microstructure et sur les propriétés mécaniques du nouveau superalliage base Nickel pour disque de turbine René 65.Le vieillissement thermique conduit à trois évolutions microstructurales majeures, à savoir la croissance des précipités y' intragranulaires et à la nucléation de particules TCP aux joints de grains accompagnés d'une ségrégation de molybdène. Une méthode innovante basée sur des traitements thermiques adaptés a permis de dissocier les effets de ces deux évolutions microstructurales sur les propriétés en fluage et fatigue-temps de maintien à 700° Cdu René 65. La croissance des précipités y' intragranulaires est majoritairement responsable de l'abattement des propriétés mécaniques. Il s'avère néanmoins que la présence des particules TCP aux joints de grains ainsi que la ségrégation de molybdène affectent également le comportement viscoplastique et la durabilité de l'alliage, contribuant à un abattement supplémentaire des propriétés mécaniques. Ce phénomène est attribué à l'adoucissement localisé de la matrice au voisinage des particules TCP et des joints de grains par la perte d'éléments durcissants de la solution solide y. / This study focused on the impact of thermo-mechanical aging on the microstructure and on the mechanical properties of the new nickel-based superalloy René 65 for turbine disk applications.Thermal aging causes three main microstructural evolutions, namely the intragranular y'-growth, the nucleation of TCP particles at grain boundaries along with a segregation of molybdenurn. An innovative method based on appropriated thermal treatments enabled to dissociate these microstructural evolutions' impacts on the René 65 creep and dwell-fatigue properties at 700°C.The y'-growth is mainly responsible of the overall mechanical proprerties degradation. However, it turns out TCP particles and the molybdenum segregation at grain boundaries also affect negatively the alloy viscoplastic behavior and its durability, contributing to an additional decrease in its mechanical properties. This phenomenon is attributed to the softening of the matrix locally at grain boundaries by solid solution elements depletion in favor of TCP precipitation.

<p>There is a growing need to understand the deformation mechanisms in titanium alloys due to their widespread use in the aerospace industry (especially within gas turbine engines), variation in their properties and performance based on their microstructure, and their tendency to undergo premature failure due to dwell and high cycle fatigue well below their yield strength. Crystal plasticity finite element (CPFE) modeling is a popular computational tool used to understand deformation in these polycrystalline alloys. With the advancement in experimental techniques such as electron backscatter diffraction, digital image correlation (DIC) and high-energy x-ray diffraction, more insights into the microstructure of the material and its deformation process can be attained. This research leverages data from a number of experimental techniques to develop well-informed and calibrated CPFE models for titanium alloys at multiple length-scales and use them to further understand the deformation in these alloys.</p> <p>The first part of the research utilizes experimental data from high-energy x-ray diffraction microscopy to initialize grain-level residual stresses and capture the correct grain morphology within CPFE simulations. Further, another method to incorporate the effect of grain-level residual stresses via geometrically necessary dislocations obtained from 2D material characterization is developed and implemented within the CPFE framework. Using this approach, grain level information about residual stresses obtained spatially over the region of interest, directly from the EBSD and high-energy x-ray diffraction microscopy, is utilized as an input to the model.</p> <p>The second part of this research involves calibrating the CPFE model based upon a systematic and detailed optimization routine utilizing experimental data in the form of macroscopic stress-strain curves coupled with lattice strains on different crystallographic planes for the α and β phases, obtained from high energy X-ray diffraction experiments for multiple material pedigrees with varying β volume fractions. This fully calibrated CPFE model is then used to gain a comprehensive understanding of deformation behavior of Ti-6Al-4V, specifically the effect of the relative orientation of the α and β phases within the microstructure.</p> <p>In the final part of this work, large and highly textured regions, referred to as macrozones or microtextured regions (MTRs), with sizes up to several orders of magnitude larger than that of the individual grains, found in dual phase Titanium alloys are modeled using a reduced order simulation strategy. This is done to overcome the computational challenges associated with modeling macrozones. The reduced order model is then used to investigate the strain localization within the microstructure and the effect of varying the misorientation tolerance on the localization of plastic strain within the macrozones.</p>