Probing elastic isotropy in entropy stabilized transition metal oxides: Experimental estimation of single crystal elastic constants from polycrystalline materials

In classical elasticity, materials with cubic symmetry are subject to specific restrictions, and only a few exhibit elastic isotropy or near-isotropy. Recent DFT calculations have shown that the entropy-stabilized oxide [(MgNiCoCuZn)O] displays elastic isotropy, with a Zener ratio close to 1. However, no experimental evidence exists to confirm this. In this study, we present a robust micromechanical approach based on the Voigt-Reuss-Hill model to estimate Single Crystal Elastic Constants (SECs) from polycrystalline materials. This method requires only two diffraction elastic constants and isotropic elastic constants for materials with cubic symmetry. Validation using phase-pure nickel showed excellent agreement with literature values, with a maximum deviation of 8.8 % for C12. Applying the methodology to [(MgNiCoCuZn)O], the calculated SECs were: C11 = 219 GPa, C12 = 116 GPa and C44=51 GPa. A comparison with DFT-calculated literature values revealed significant discrepancies, with bulk and shear moduli obtained using the Voigt-Reuss-Hill average differing by 25 % to 59 %. To investigate this disparity, we performed additional DFT calculations and examined the underlying factors influencing these results. This study not only introduces a reliable and straightforward methodology for SEC estimation for cubic materials but also provides the first experimental SEC values for [(MgNiCoCuZn)O] entropy- stabilized oxides under ambient conditions. These findings are crucial for developing accurate interatomic potentials in future.