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.