Behaviors in Oxidation at 1100°C of an Equimolar CoNiFeMnCr High-Entropy Alloy and of its Versions Moderately Added with HfC or TaC Carbides

Three cast alloys based on a HEA composition were exposed to air at 1100°C for 50 h, a simple equimolar CoNiFeMnCr alloy, an equimolar CoNiFeMnCr alloy added with 3.7 wt.%Hf and 0.25 wt.%C, and an equimolar CoNiFeMnCr alloy added with 3.7 wt.%Ta and 0.25 wt.%C. The CoNiFeMnCr alloy is single-phase, and the two others are two-phase with a HEA matrix and either HfC or TaC interdendritic carbides. The three alloys resisted isothermal oxidation, with the formation of a M2O3 scale comprised of Cr and Mn. In this scale, the relative proportions of Cr and Mn varied from the alloy/scale interface (much more Cr than Mn) to the scale/atmosphere interface (much more Mn than Cr). Internal oxidation took place too, with locally noticeable deep oxidation penetrations in the simple equimolar alloy. The carbides-containing alloys were also affected by internal oxidation (M2O3, HfO2 and CrTaO4). Cr and Mn obviously diffused outward. Quantification of Cr and Mn lost by the alloys allowed for estimating the total oxide masses formed. The Cr and Mn losses were rather great, and equivalent values of parabolic constants were estimated to allow comparison with a model chromia-forming Ni-based binary alloy. The calculated oxidation kinetics were deduced to be faster than in the case of a pure chromia-forming behavior.