A novel FeMnAlC alloy, comprising 23˜34 wt. % Mn, 6˜12 wt. % Al, and 1.4˜2.2 wt. % C with the balance being Fe, is disclosed. The as-quenched alloy contains an extremely high density of nano-sized (Fe,Mn)3AlCx carbides (κ′-carbides) formed within austenite matrix by spinodal decomposition during quenching. With almost equivalent elongation, the yield strength of the present alloys after aging is about 30% higher than that of the optimally aged FeMnAlC (C≤1.3 wt. %) alloy systems disclosed in prior arts. Moreover, the as-quenched alloy is directly nitrided at 450˜550° C., the resultant surface microhardness and corrosion resistance in 3.5% NaCl solution are far superior to those obtained previously for the optimally nitrided commercial alloy steels and stainless steels, presumably due to the formation of a nitrided layer consisting predominantly of AlN.