Thermal oxidation characteristics of Fex(CoCrMnNi)100-x medium and high-entropy alloys

The thermal oxidation behavior of Fe-x(CoCrMnNi)(100-x) high-entropy alloys (x = 20 and 40 at.%) and medium-entropy alloy (x = 60 at.%) at temperatures ranging from 900 degrees C to 1100 degrees C under air atmosphere was investigated. The oxidation kinetics of the alloys followed the parabolic law, indicated by the increase of oxidation rate constants with increasing Fe content and temperature. The addition of Fe content lowered the configurational entropy and activation energy of the alloys, supporting the acceleration of metal atoms diffusion. Furthermore, the oxide scales formed on the surface were strongly dependent on the alloy composition. In general, a Cr2O3 inner layer, spinels (NiCr2O4 and CoMn2O4) as intermediate layers, and the outer layers of Fe3O4 and Mn3O4 were formed after oxidation at 900 degrees C in all alloys. On the other hand, severe internal oxidation and pores were observed in the alloy containing 60 at.% of Fe after oxidation at 1100 degrees C. In addition, the pores were formed due to the Kirkendall effect, acting as diffusion path and reaction place which may facilitate the oxidation process.