Insights into Early Stages of Oxidation Behavior of an Equiatomic TiNbCr Multi-Principal Element Alloy: Microstructural Aspects and Anisothermal Oxidation Testing in Air

This study provides new insights into the microstructure and early stages of surface oxidation behavior of an equiatomic TiNbCr multi-principal element (MPE) alloy in both as-cast and hot isostatic pressing (HIP) processed conditions. The alloy comprises two phases: a Nb-rich body-centered cubic (BCC) matrix and Cr-rich Laves phase (C15) precipitates, with a reduction in Laves phase fraction observed at increasing temperatures. Dilatometry tests revealed the precipitation of a Ti-rich hexagonal close-packed (HCP) phase, particularly at a cooling rate of 10°C/min. Anisothermal oxidation tests in air were conducted to compare the early oxidation behavior of the MPE alloy with alloy 188. Notably, the as-cast condition exhibited a lower mass gain/area than the HIPed condition, attributed to the thinner Laves precipitates in the former. Furthermore, the TiNbCr alloy demonstrates a prolonged duration to achieve supersaturation and initiate oxide scale formation compared to alloy 188, with the surface oxidation process driven by the cooperative interaction between the Nb-rich BCC matrix and Cr-rich Laves phases.