(Fe,Mn)3AlCx κ-carbide formation and characterization in pack aluminization of Fe–29Mn–9Al–0.9C lightweight steel

The Fe–Mn–Al–C lightweight steel has been widely studied due to its higher strength-to-weight ratio and potential applications for automotive vehicles, energy, transportation and mining industries. In general, the strength and ductility of the Fe–Mn–Al–C lightweight steel can be improved by the formation of nano-sized (Fe,Mn)3AlCx κ-carbide precipitates by a proper heat treatment below 800 °C. In this work, a unique micrometer scale κ-carbide phase transformation phenomenon induced by the pack aluminization in the Fe–29Mn–9Al–0.9C lightweight steel was firstly reported. Extremely large columnar-like (Fe,Mn)3AlCx κ-carbide grains with average length of 18.1 μm embedded in the Fe3Al matrix were discovered in the interdiffusion zone of aluminization layer, which were totally different from these nanoscale κ-carbides precipitated in the austenitic steel matrix. The increased Al content in the pack aluminide coating/substrate interface reduced the chemical potential of carbon and became the driving force for carbon diffusion toward the coating/substrate interface. Therefore, due to the extremely low solubility of carbon in the Fe3Al phase and the inward diffusion of carbon atoms, very long columnar-like (Fe,Mn)3AlCx κ-carbide phases were thus produced in the interdiffusion zone. The average chemical concentration of the κ-carbide was 43.9Fe–27.3Mn–20.3Al–8.5 C (in at.%). The average hardness, elastic modulus and the lattice parameter of κ-carbide were 8.8 ± 0.3 GPa, 285 ± 10 GPa and 0.3764 nm, respectively. This study provided detailed chemical compositions, hardness and elastic modulus of (Fe,Mn)3AlCx κ-carbide phase for better understanding the carbide precipitation and strengthening mechanism of Fe–Mn–Al–C lightweight steel.