Experimental Investigation on the Effect of Cr and Mn in High Temperature Oxidation of Cantor Alloy

High entropy alloys have emerged recently as promising candidates for high-temperature applications. This study explores the detailed oxidation behavior of CoCrFeMnNi alloy by using a combination of in-situ, short-duration and long-duration high-temperature exposure up to 1000 ^⁰C. The study reveals Cr and Mn played a significant role in the oxidation/passivation of the alloy. It was found that Cr enhanced the oxidation resistance, especially by limiting oxygen diffusion and was quite effective up to 600 ^⁰C. However, at higher temperatures, Mn continuously diffuses towards the surface and forms a poorly adherering oxide scale. Study on CoCrFeNi and CoFeMnNi alloys to investigate the roles of Cr and Mn individually revealed that under similar conditions, CoCrFeNi had a relatively continuous and less spalled oxide layer with better adherence to the alloy compared to cantor alloy. However, compared to Cr-containing alloys, the CoFeMnNi alloy did not have a continuous, well-adhering, non-protective oxide layer making the alloy prone to severe and faster oxidation. Same was apparent from significant internal oxidation, the appearance of massive cracks, and voids. Before and after the oxidation, CoCrFeMnNi and CoCrFeNi were single-phase fcc structures while CoFeMnNi decomposed into a two-phase alloy due to significant uptake of oxygen. Prolonged oxidation and molten-state studies revealed that the oxidation behaviour of HEAs is a thermodynamic-driven process, and CoCrFeMnNi is expected to gradually lose Mn to surface oxide followed by migration of Co, Ni and Fe, which otherwise hardly migrated or participated in the oxidation process. High-temperature heat treatment in vacuum confirmed that the migration to the surface was driven by its oxidation at the surface.