Reverse partitioning of Al from κ-carbide to the γ-matrix upon Ni addition and its strengthening effect in Fe–Mn–Al–C lightweight steel

Abstract

The effect of the addition of Ni to Fe–30Mn–10Al–0.9C–0.5Si–1.5Mo (wt.%) cast lightweight steel was investigated with a focus on the precipitation behavior of (Fe,Mn)3AlC κ-carbide. The addition of 3 wt.% Ni (3Ni steel) resulted in continuous hardening of the steel during aging heat treatment at 550 °C for 100 h whereas the addition of 1.5 wt.% Ni (1.5Ni steel) led to peak hardening at 550 °C for 20 h. The size of κ-carbides after aging heat treatment for 100 h was smaller in 3Ni steel compared with 1.5Ni steel, despite its higher hardness and progressed hardening. Atom probe tomography of the aged 3Ni steel revealed that Al atoms were preferentially enriched in the γ-matrix with clustering of Ni atoms observed, resulting in the relative depletion of Al in κ-carbide. This is the first report of reverse partitioning of Al from κ-carbide to the γ-matrix through Ni addition, indicating that the affinity of C–Al is lower than that of Ni–Al. The substitution of Mn to the Al site of the κ-carbide lattice is energetically favorable when Ni–Al bonding actively occurs in the γ-matrix, as determined by first-principles calculations. This is clearly evidenced by the Mn enrichment of κ-carbide in the 3Ni steel. The amount of Ni–Al clustering resulting from the reverse partitioning of Al to the γ-matrix during the precipitation of κ-carbide could be increased according to the growth of κ-carbide. Therefore, we found that the addition of Ni led to simultaneous cooperative precipitation- and solid solution-strengthening after aging, contributing to continuous hardening by effectively suppressing the movement of dislocations.