Oxidation of CoCrFeMnNi High Entropy Alloys

Eight model high entropy alloys (HEAs) in the CoCrFeMnNi family (including one alloy each in the CoCrFeNi and CoFeMnNi subfamilies) were made, prepared, and exposed to laboratory air for 1100 h at 650A degrees C and 750A degrees C. Two commercial alloys, nickel-base superalloy 230 (N06230) and austenitic stainless steel 304H (S30409), were simultaneously exposed for comparison. Mass change oxidation kinetics were measured and cross-sections of exposed samples were observed. Seven of these HEAs contained much more Mn (12-24 wt.%) than is found in commercial heat-resistant stainless steels and superalloys. The oxidation resistance of CoCrFeNi was excellent and comparable to 304H at 650A degrees C and only slightly worse at 750A degrees C. The thin oxide scale on CoCrFeNi was primarily Cr oxide (presumably Cr2O3) with some Mn oxide at the outer part of the scale. The CoCrFeMnNi HEAs all experienced more rapid oxidation than CoCrFeNi and, especially at 750A degrees C, experienced oxide scale spallation. The addition of Y in the alloy to lower S improved the oxidation resistance of these HEAs. Alloy CoFeMnNi, without Cr, experienced much higher oxidation rates and scale spallation than the Cr-containing alloys. A linear regression analysis of the log of the parabolic rate constant, log(k(p)), as functions of wt.% Cr and Mn found a good correlation for the compositional dependence of the oxidation rate constant, especially at 650A degrees C. Mn was found to be more detrimental increasing log(k (p)) than Cr was helpful reducing log(k (p)). If CoCrFeMnNi HEAs are to be used in high temperature oxidizing environments, then examining lower levels of Mn, while maintaining Cr levels, should be pursued.