Grain size dependent indentation response of single-phase (CoCuMgNiZn)O high entropy oxides

Abstract

Vickers indentation and nanoindentation methods were used to explore the hardness, elastic modulus, and fracture toughness of single-phase (CoCuMgNiZn)O transition metal high entropy oxides. Bulk samples with grain sizes ranging from 0.075 µm to 1.4 µm were consolidated using spark plasma sintering. Measurements reveal relatively small differences in elastic modulus and comparable hardness to Rule-of-Mixture calculations, alluding to minimal effects of entropy stabilization on mechanical properties. Hardness values exhibit a Hall-Petch relationship until an average grain size of 0.11 µm. Below this grain size an Inverse Hall-Petch relationship is observed with values decreasing up to 70 %. The measured hardness deviates from calculations of hardness using a grain interior-grain boundary composite model, indicating that other mechanisms, such as nanocracking or grain boundary sliding, contribute to the decrease in hardness at smaller grain sizes. Variations in elastic modulus are attributed to grain boundary effects, and variations in fracture toughness are attributed to the absence of grain bridging and transgranular fracture at smaller grain sizes. This grain-size dependent mechanical behavior, which is similar to behavior in MgO, must be controlled when designing (CoCuMgNiZn)O materials for various applications.