Unveiling different oxide scales in a compositionally complex polycrystalline CoNi-base superalloy

Abstract

The formation of multiple types of oxide scales on the specimen surface as well as along the grain boundaries was investigated in detail in a compositionally complex polycrystalline CoNi-base superalloy after oxidation at 900 ℃. The complex multilayered oxide scale forms under steady-state condition (oxidation time till 24 h). After the breakdown of the steady-state oxide scale (oxidation for 48 h), different types of oxide scales occur. Additionally, it is found that oxidation can gradually extend into the specimen interior along the grain boundaries. The oxide scale formed along the grain boundaries evolves when the boundaries extend from specimen surface into interior, which is caused by the selective oxidation of elements by the differences in equilibrium oxygen partial pressures. It is proposed that factors, i.e., local microstructure and oxygen concentration, greatly influence the formation and configuration of the oxide scales of the investigated alloy.