Early-stage oxidation behavior and mechanical degradation of HfNbZrTiTa refractory high-entropy alloy

The early-stage oxidation behavior of the HfNbZrTiTa refractory high-entropy alloy (RHEA) is investigated using reactive molecular dynamics simulations. The influences of surface orientations and nanoscale chemical inhomogeneity on oxidation kinetics, nanostructural evolution, and growth stress development are studied. The results indicate that oxidation proceeds via an initial fast vertical thickening stage followed by a slower lateral expansion stage. Surface oxides exhibit initial island-like to later layer-by-layer growth modes. Nanoscale chemical inhomogeneity [i.e., chemical short-range order and surface segregation] moderately promotes vertical oxide penetration but significantly suppresses lateral oxide growth, through influencing internal ionic diffusions. The early oxidation product exhibits an overall amorphous structure, yet it possesses a certain extent of nanoscale structural order similar to B1-type MO (M = Hf, Nb, Zr, Ti, and Ta) oxide. Additionally, the growth of surface oxide induces orientation-dependent internal stresses at the GPa level. Moreover, mechanical responses of oxidized RHEA nanopillars are examined via uniaxial tensile and compressive loadings. It is revealed that surface oxidation reduces the strength and yield strain of the oxide/alloy system, with plastic deformation and shear bands nucleating at the weak oxide/alloy interface. Nanoscale chemical inhomogeneity helps to mitigate the mechanical degradation caused by oxidation.