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The layer-by-layer deposition process of additive manufacturing (AM) offers the capability to design material microstructures on multiple length scales. For NiTi shape memory alloys, designing material microstructures using AM would allow for unparalleled tailoring of the multiscale martensitic transformation and shape memory response. However, the laser-based directed energy deposition (LDED) AM technique produces localized microstructures which are distinct from those found in conventionally processed alloys. This work characterizes the grain and precipitate microstructures on multiple length scales for LDED fabricated NiTi alloys and assess the capability for tailoring the martensitic transformation morphology shape memory response through post-deposition heat treatments. Build coupons were fabricated by LDED AM using elementally blended Ni and Ti powder feedstock. The use of elemental powders allowed for a Ti-rich and a Ni-rich powder feedstock composition to be blended; thus, both shape memory effect (Ti-rich) and superelastic (Ni-rich) behaviors were investigated. Specimens were extracted from the fabricated build coupons to investigate the localized microstructure and shape memory behaviors. A full-field deformation analysis technique was employed to correlate the AM microstructure to the deformation mechanisms.The results of this work show that the NiTi LDED AM builds are inherently spatially varying on multiple microstructure length scales. The grain structure resulting from the AM process was similar for both feedstock compositions: fine grains within the interfacial regions formed by overlapping passes/layers and larger columnar grains within bulk regions (i.e. away from these interfaces). As a result of the spatially varying microstructure, as built LDED NiTi alloys exhibit a hardening like response and localized strain concentrations. Post-deposition heat treatment of the Ni-rich alloys reduced the spatial variation in the Ni4Ti3 precipitate microstructure and increased the localized superelastic strains compared to the as built condition, with the solutionizing and precipitation aging treatment resulting in the most spatially uniform Ni4Ti3 precipitate morphology. For the LDED alloys, shape memory effect recovery strains of 4.0 % (for Ti-rich alloys) and superelastic recovery strains of -6.0 % (for solutionized and aged Ni-rich alloys) were achieved.
As a precipitation-strengthened alloy, the heat treatment is critical for IN718, since the desired mechanical properties, such as high-temperature tensile and creep strength, are only acquired by the formation of the strengthening precipitates, namely gamma prime and gamma double prime. Currently, the industrial standards for the heat treatment of IN718 are developed for cast and wrought cases and not specifically for AM builds. Thus, it is essential to evaluate the effect of the heat treatment on the formation of the strengthening precipitates in IN718 builds fabricated by L-PBF AM, which is the focus of the second objective. Particularly, a modification to the industry standard heat treatment is developed to maximize the fraction of the strengthening precipitates in the IN718 builds. The microstructural characterizations are performed for several modified heat treatment cases including a homogenization step, solution annealing step and aging step. The micro-hardness values are measured for as-built conditions and several heat-treated conditions including the modified homogenization, solution anneal and aging steps. Finally, the oxidation behavior during the heat treatment is also discussed and compared to that for a piece of actual cast. The third objective of the present study is the evaluation of the mechanical properties of heat-treated IN718 builds produced by L-PBF AM. Particularly, creep test are performed to quantify the mechanical properties of the heat-treated IN718 builds. The creep samples are heat-treated using the following condition: homogenization at 1100 °C for 2 hours followed by air cooling (AC), and aging at 760 °C for 10 hours also followed by AC. For the creep test, the samples are loaded at a constant stress (690 MPa or 100 ksi) at 649 °C (1200 °F) in accordance to Aerospace Material Standards (AMS) 5663. The creep rate of the heat-treated AM sample is compared with the literature data for wrought cases. The relationship of creep strength to the characteristic of the microstructures in the heat-treated IN718 builds is discussed. In summary, the research results provide insights into the microstructure-creep-strength relationship for IN718 fabricated by additive manufacturing. Particularly, a modified post-built heat treatment is developed to maximize the formation of strengthening precipitates and achieve large grains in IN718, resulting in a markedly higher creep strength when compared to the literature data for wrought cases. Taken as a whole, the new knowledge generated in this dissertation is essential to ensure the performance of additively manufactured parts in structural applications.