UC Irvine

As new materials and manufacturing techniques are developed to suit the needs of several key industries, creative methods of designing microstructures to tailor mechanical behavior must be explored to control deformation and prevent failure. Additionally, the underlying mechanisms that dictate such control must be well understood. To this end, this dissertation includes three distinct investigations: a study on the underling mechanisms that control the microstructure of samples fabricated with ultrasonic vibration-assisted directed energy deposition; an exploration of the role of phase state and composition on the room-temperature mechanical behavior of entropy stabilized oxides; and an analysis of the role of microstructure and phase state on the high-temperature deformation of entropy stabilized oxides. In the first study, ultrasonic vibration (UV) was applied in situ to directed energy deposition (DED) of 316L stainless steel single tracks and bulk parts. For the first time, high-speed video imaging and thermal imaging were implemented in situ to quantitatively correlate the application of UV to melt pool evolution in DED. Findings show that UV increases the melt pool peak temperature and dimensions, while improving the wettability of injected powder particles with the melt pool surface and reducing powder particle residence time. Through in situ imaging we demonstrate quantitatively that these phenomena, acting simultaneously, effectively diminish with increasing build height and size, consequently decreasing the positive effect of implementing UV assisted (UV-A) DED. Thus, this research provides valuable insight into the effects of UV on DED melt pool dynamics, the stochastic interactions between the melt pool and incoming powder particles, and the limitations of build geometry on the UV-A DED technique. In the second study, we investigate the influence of these secondary phases on the mechanical behavior of the (CoCuMgNiZn)O transition metal ESO (TM ESO). TM-ESOs of equimolar, Co deficient, and Cu deficient compositions were fabricated, heat treated to form secondary phases, and characterized. Room-temperature indentation was used to measure the hardness and elastic modulus of as-sintered single-phase and as-heat-treated multiphase bulk samples. As the atomic fraction of secondary phase increases, equimolar and Co-deficient TM ESO harden then soften, and Cu-deficient TM ESO continuously hardens. Hardness trends were analyzed by evaluating strengthening mechanisms, indicating that hardness is significantly influenced by the interactions between dislocations and secondary phases. The elastic modulus varies as a function of composition and quantity of secondary phases but falls within a range of values predicted by a composite model. Changing composition influences the hardness and elastic modulus of as sintered single-phase TM-ESOs due to changes in cation-dislocation and cation-cation interaction energies. Overall, our findings indicate that the entropic phase transformation can be manipulated to tailor the room-temperature mechanical properties of TM-ESOs. In the third study, we begin to address the high-temperature deformation behavior of TM ESOs. The microstructure and phase state of TM-ESOs were varied. Fine-grained and coarse-grained TM-ESOs were deformed at increasing loads over a range of elevated temperatures in both their single-phase and multiphase states. Stress exponent values were determined for all conditions, indicating that fine-grained and coarse-grained TM-ESO deformed superplastically. In fine grained TM-ESO samples, the secondary phases did not have a significant effect on the stress exponent values. At low deformation temperatures, coarse-grained TM-ESO samples had higher stress exponent values than fine-grained samples, and the stress exponent increased with the presence of secondary phases. At high deformation temperatures, the stress exponents for single phase and multiphase coarse-grained samples decreased. The drop in stress exponent for the coarse grained samples at higher deformation temperatures indicates a temperature-induced switch in the deformation mechanism from grain boundary sliding to solute-drag creep. Overall, this work demonstrates that microstructure and phase composition of TM-ESO can be used to tailor the high temperature deformation of TM-ESO. This dissertation highlights that unconventional methods can be used to tailor the microstructure of polycrystalline metal alloys and oxide ceramics to control their mechanical behavior. Future studies examining the mechanical properties of individual secondary phases in TM-ESOs, the kinetics of the reversible phase transformation of TM-ESOs, and the reversible phase transformation and room-temperature mechanical behavior of nanocrystalline TM-ESOs would be meaningful additions to the studies included in this dissertation.