Cu-modified oxide-scale heterogeneity and subsurface evolution in medium-Mn steel

The effect of 1 wt.% Cu on oxide-scale heterogeneity and subsurface microstructural evolution was studied in Fe–0.2C–8Mn–3Al–3.5Ni medium-Mn steel exposed to static air at 700–800 °C for 2 h. Both Cu-free and Cu-bearing steels formed stratified scales. The scales consisted of Fe- and Mn-rich external oxides and an Al-enriched inner oxidation region. Thus, Cu did not change the basic scale architecture. Its main effect was on the inner oxidation front. In the Cu-bearing steel, localized Cu–Ni-rich particles and island-like regions appeared near the inner oxide and the substrate. Similar regions were also found within the inner oxidation region. Their discontinuous distribution differed from that of a continuous protective layer. Their spatial association with Al–O-rich regions is consistent with rejection of relatively noble Cu and Ni from the growing oxides. It also suggests a local link between Cu–Ni enrichment and Al-rich internal oxidation. Root-like oxidation fronts and the more continuous distribution of Al–O-rich oxides along grain boundaries indicate grain-boundary-associated internal oxidation. EBSD results showed lower local misorientation and a higher high-angle grain-boundary fraction in the air-exposed peripheral region of the Cu-bearing steel than in the Ar-treated reference. These EBSD characteristics indicate recovery-dominated orientation evolution near the oxidized surface. The results identify Cu-associated chemical heterogeneity at the oxidation front and an atmosphere-dependent subsurface orientation response.