Effect of Mn and C on Age Hardening of Fe–Mn–Al–C Lightweight Steels

The effects of Mn and C content on the age hardening of Fe–Mn–Al–C lightweight steels, which have austenitic or duplex (austenite and ferrite) microstructures, were investigated. An increase in Mn content induced a delay of the age hardening that is caused by the formation of intra-granular κ-carbides. In order to interpret the effect of Mn content, first-principles calculations were conducted using the supercells of Fe₂₄Al₈C₈, Fe₂₄Al₈C₇, Fe₂₄(Al₇Mn)C₈, and Fe₂₄(Al₇Mn)C₇. The calculations showed that an increase in Mn content could be the source of the delay of the intra-granular κ-carbide formation by suppressing C atom’ occupation of the vacancy at the body-centered site of L1₂. An increase in C content accelerated the formation of intra-granular κ-carbides, which induced the intense age hardening, and coarse inter-granular κ-carbides, which resulted in significant decrease in impact absorbed energy due to inter-granular fracture.