Influence of oxygen ingress on microstructure and phase formation during two-step oxidation surface hardening of TiZrNbHfTa high entropy alloy

Aiming for an increased surface hardness and a better wear resistance of TiZrNbHfTa, we investigate a two-step process combining oxidation of rolled, ultrafine-grained, single-phase bcc TiZrNbHfTa in air at 550 degrees C with a vacuum heat treatment process at 1200 degrees C, including subsequent quenching. The first process step is associated with the formation of a surface oxide layer, a phase decomposition of the bulk, and oxygen ingress into the bulk, leading to an oxygen-enriched subsurface region containing internal oxides. The second process step is capable of restoring the single-phase nature of the bulk TiZrNbHfTa. At the same time, the presence of a compact surface oxide layer of 1-2 mu m and an oxygen-enriched subsurface region underneath is preserved. The oxygen present in the subsurface region stabilizes a two-phase regime consisting of a Hf- and Zr-rich hcp phase and an Nb- and Ta-rich bcc phase, with the oxygen being interstitially dissolved predominantly in the hcp phase.