(Fe,Mn)₃AlC_xκ-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.