Internal oxidation behaviour of Fe-Mn-Al-C duplex light-weight steels with good combination of strength and ductility

•Increased carbon content degraded oxidation resistance of experimental duplex steel.•High carbon hindered the formation of induced ferrite phase transformation layer.•Oxidation of carbon and manganese offered more pathways for Al diffusion.•Aluminium firstly enriched in the boundaries of austenite internal oxidation.•Al2O3 layer healed the porous defects by preventing the oxidation of carbon. The oxidation behaviour of Fe-20Mn-8Al-xC (x=0.25-0.35wt. %) duplex light-weight steels at 1000°C in dry air has been investigated. The results show that the increased carbon content enlarges austenite phase fractions and degrades oxidation resistance to high temperature. The preferential oxidation of manganese at high temperature promotes phase transformation of austenite→ferrite, and offers pathways for aluminium diffusion. Aluminium enriches at the boundary of the austenite internal oxidation layer. A continuous and compact pure Al2O3 layer has been observed between iron and manganese oxides and oxidation-induced ferrite layers. The formed Al2O3 layer can further prevent oxidation of Fe-20Mn-8Al-xC duplex steels.