Thursday, November 7, 2024 1pm to 3pm
About this Event
1400 N. Bishop, Rolla, MO 65409-0330
Steven Smith, a doctoral candidate in ceramic engineering, will defend their dissertation titled “Processing and Properties of Multi-Phase High-Entropy Ceramics.” Their advisor, Dr. William Fahrenholtz, is a curators distinguished professor of ceramic engineering. The dissertation abstract is provided below.
This work focuses on the processing and densification of multiphase, high-entropy ceramics to promote their use in friction stir welding technologies. Several high-entropy systems including high-entropy borides, carbides, and their composites were produced by spark plasma sintering and pressureless sintering. Boro/carbothermal and carbothermal synthesis were used to produce high-entropy powders from oxide powder mixtures. Composites of high-entropy borides and high-entropy carbides were produced using a sequential approach to produce both phases. The carbide was first produced using carbothermal reduction, then ZrH2 and B4C were added to the carbide powder to produce the high-entropy boride phase during a second synthesis step. The resulting powder was densified by spark plasma sintering and pressureless sintering, although higher relative densities and smaller grain sizes were obtained for the spark plasma sintered ceramics. Thermodynamic calculations were used to understand the segregation of metals to certain high-entropy phases. To investigate the densification behavior of high-entropy borides and carbides during densification by spark plasma sintering, the intermediate stage densification kinetics were examined for the boride and carbide of the Hf-Nb-Ta-Ti-Zr system. Tests in the temperature range of 1700°C to 1850°C indicated that the high-entropy carbide densified by lattice diffusion with an activation energy of 646 kJ/mol, and the high-entropy boride densified by grain boundary diffusion with an activation energy of 575 kJ/mol. Densification of both systems was limited by the diffusion of Nb into the high-entropy phase. Boride-SiC-B4C ceramics were produced with a high-entropy boride phase to determine if the addition of a high-entropy phase affected the resulting hardness and microstructure. With the addition of the high-entropy boride, it was concluded that the hardness of boride-SiC-B4C ceramics has a dependence on the hardness of the boride phase over the rule-of-mixtures.
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