Isothermal precipitation behavior of κ-carbide in the Fe–9Mn–6Al–0.15C lightweight steel with a multiphase microstructure

• κ-Carbide started to precipitate at the grain boundaries of δ-ferrite and interphase boundaries during isothermal aging.
• The lamellar κ-carbide formed by decomposition of γ-austenite to α-ferrite and κ-carbide after aging for 6 h.
• The precipitation of κ-carbide at the boundaries preceded the lamellar κ-carbide due to solute atoms pre-segregated at the boundaries.
• α′-Martensite was not reverted to γ-austenite, but tempered during isothermal aging.

The precipitation behavior of κ-carbide during isothermal aging was investigated in the Fe–9Mn–6Al–0.15C (wt.%) lightweight steel through thermodynamic calculations and microstructural analyses using various microscopes and an X-ray diffractometer. No precipitates of κ-carbide were observed in both hot-rolled and annealed specimens. The κ-carbide started to precipitate at the grain boundaries of δ-ferrite and at interphase boundaries between δ-ferrite and γ-austenite and between δ-ferrite and α′-martensite after isothermal aging at 550 °C for 1 h. While the precipitation and growth of κ-carbide at the boundaries continued, new κ-carbide precipitated alongside α-ferrite by the eutectoid reaction to form a lamellar structure in austenite during 6-h aging. This result confirmed that the precipitation of κ-carbide at the boundaries preceded the precipitation of the lamellar κ-carbide due to the pre-segregation of solute atoms to the boundaries. The α′-martensite was not transformed to γ-austenite and just tempered during isothermal aging at 550 °C.

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