Dislocation-mediated room-temperature mechanical behavior of multiphase entropy stabilized oxides

Entropy stabilized oxides (ESOs) exhibit a reversible entropic transformation, which can lead to the formation of secondary phases and tunable microstructures. To date, the interplay between the secondary phases and the mechanical behavior in ESOs has not been explored. In this study, the influence of these secondary phases on the mechanical behavior of the (CoCuMgNiZn)O transition metal ESO (TM-ESO) is investigated. 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 single-phase and multiphase bulk samples. Hardness values ranged from 3700 to 6400 MPa, and elastic modulus values ranged from 140 to 170 GPa. 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 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 single-phase TM-ESOs due to changes in cation-dislocation and cation-cation interaction energies. Overall, our findings indicate that the room-temperature mechanical behavior of single-phase and multiphase TM-ESOs is influenced by solid-solution and precipitation strengthening mechanisms, respectively, due to significant dislocation activity. These results demonstrate that the entropic phase transformation can be used to engineer microstructure and tailor the mechanical behavior of ESO materials.