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Date of Award
Doctor of Philosophy (PhD)
Multiprincipal element alloys (MPEAs) are a novel class of engineering materials. As opposed to conventional alloys, where one or two elements are dominant with others in minor amount to provide improvements or supplements to the properties, there is a lack of clear solvent species in MPEAs. Since the discovery in 2004, MPEAs have stimulated the exploration of the vast compositional space offered by the increased number (≥ 3) of principal elements, and the studied alloy compositions cover an exceptionally broad range of microstructures and properties. Despite the unparalleled potential of MPEAs, the considerable number of choices in number, type and fraction of the elements complicate the process of identifying candidates for industrial applications. The conventional, one-at-a-time experimental approach is no longer adequate for the rapid development of materials. Therefore, the understanding in MPEAs is still limited. Combinatorial high-throughput methods, as a remedy, have arisen to accelerate both metallic alloys’ synthesis and characterization processes. In this dissertation, combinatorial strategies are developed to identify new MPEA compositions and microstructures, as well as to understand the underlying mechanisms of MPEAs with desired mechanical properties in an expedited manner.In particular, direct laser deposition (DLD) was used to fabricate various libraries in Al-CoCrFeNi and Nb-Ti-V-Zr MPEA family. For validation of the method, an AlxCoCrFeNi (x = 0.15 – 1.32) library with an array of 25 discrete patches was synthesized. The compositional homogeneity of the deposited alloys within patches was examined. And a remarkable consistency was found between the laser-processed and cast samples. Subsequently, three compositions of interest in the same MPEA system were selected to be cooled at different rates, which is realized by different combinations of laser parameters. This study aims to further investigate the processing-microstructure-property relationship to obtain a comprehensive understanding in the classic paradigm in MPEAs. Next, the optimal laser parameters for MPEAs to fabricate 3D structures including thin walls and pillars were also discussed. Regarding refractory MPEAs, a continuously-graded Nb-Ti-V-Zr quaternary library was produced and examined to determine the compositional effects on the crystal structures, microstructures, and mechanical properties. Meanwhile, the equiatomic NbVZr alloy were studied in detail for a better understanding of the stability and properties of intermetallic compounds, which can serve as a strengthening phase in MPEAs.
Rohan Mishra, Shankar Sastry, Kenneth Kelton, Srikanth Singamaneni,
Available for download on Sunday, May 15, 2022