"Deformation and Damage Resistance of Multicomponent Oxide" 

Chang Liu, MSE PhD Candidate 

Advisor: Professor Haiyan Wang

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Multicomponent oxides are promising ceramic materials for structural and extreme environment applications due to their compositional complexity, phase stability, and tunable defect structures. However, their intrinsic brittleness remains a major limitation. This dissertation investigates the deformation and damage resistance of multicomponent oxide ceramics, focusing on the high-entropy oxide (CoCuMgNiZn)O and the cobalt-free medium-entropy oxide (CuMgNiZn)O. The high temperature mechanical behavior of (CoCuMgNiZn)O was first studied by in situ SEM micropillar compression from room temperature to 600 °C. The results reveal a temperature-dependent transition from brittle fracture at room temperature to thermally activated plasticity at elevated temperatures, accompanied by dislocation activity, slip band formation, grain fragmentation, and Cu-rich secondary phases. To further improve deformability and eliminate cobalt, a cobalt-free (CuMgNiZn)O medium-entropy oxide was developed through controlled entropy destabilization. This strategy introduces Cu-rich secondary phases and planar defects that act as defect sources during deformation, enabling enhanced plastic strain accommodation and strain hardening at 600 °C. The nano-scratch testing revealed that defect-rich microstructures improve wear resistance by accommodating local strain, dissipating mechanical energy, and suppressing cracking. Finally, the irradiation response of (CuMgNiZn)O was investigated. After irradiation, the material retained its crystallinity without obvious amorphization. Irradiation-induced planar defects, defect nanodomains, and Cu-Ni-rich phase separation provided effective defect sinks, promoting defect redistribution and suppressing damage accumulation. Overall, this dissertation demonstrates that entropy control and defect engineering are effective strategies for enhancing plasticity, radiation tolerance, and wear resistance of multicomponent oxide ceramics.