Unveiling the atomistic mechanism of oxide scale spalling in heat-resistant alloys

Abstract Metallic materials often undergo oxidation and corrosion at high temperatures. Only by forming an intact and dense oxide scale adhering well to the matrix can metallic materials be safely utilized. Due to the large difference of thermal expansion properties, the formed oxide scale often spalls from alloy matrix, leading to premature failure of the material. Few mechanisms have been proposed for understanding this phenomenon, nevertheless, consensus has not yet been reached. In this study, we revealed that trace sulfur impurities contaminated in high-purity raw materials prominently segregates to the interface between the oxide scale and alloy matrix, forming a thin intermediate amorphous-like layer during the oxidation process of a model alloy at 900 oC. Preferential cracking between the sulfur-rich layer and the alumina scale easily occurred due to the weak chemical bonding between sulfur and alumina atoms, as confirmed by our atomistic simulation. Based on these findings, we successfully eliminated S segregation by microalloying and significantly improved the oxide scale adhesion. Our work clearly verifies the atomistic spalling mechanism of the oxide scale (i.e., the sulfur effect), which is useful for improving oxide scale adhesion and enhancing heat-resistant properties of high-temperature alloys.