Rietveld refinement, microstructure, mechanical properties and oxidation characteristics of Fe-28Mn-x Al-1C (x = 10 and 12 wt. %) low-density steels

The quantitative relationship between microstructure and properties of austenitic Fe-28Mn-xAl-1C (x=10 and 12 wt. %) low-density steels was evaluated using Rietveld method to refine X-ray diffraction (XRD) patterns. The results showed that a typical three-phase austenitic steel was obtained in the forged Mn28Al10 (i. e. Fe-28Mn-10Al-1C) steel, which included about 92.85 wt. % γ-Fe(Mn, Al, C) (austenite), 5.28 wt. % (Fe, Mn)₃ AlC_0.3 (κ-carbide), and 1.87 wt. % α-Fe(Al, Mn) (ferrite). For the forged Mn28Al12 (i. e. Fe-28Mn-12Al-1C) steel, nevertheless, only about 76.64 wt. % austenite, 9.63 wt. % κ-carbide, 9.14 wt. % ferrite and 4.59 wt. % Fe₃ Al (DO₃) could be obtained. Nanometer κ-carbide and DO₃ were mainly distributed in austenite grains and at the interface between austenite and ferrite, respectively. The forged Mn28Al10 steel had a better combination of strength, ductility and specific strength as compared with the forged Mn28Al12 steel. The ductility of the forged Mn28Al12 steel was far lower than that of the forged Mn28Al10 steel. The oxidation kinetics of Mn28Al10 steel oxidized at 1323 K for 5–25 h had two-stage linear rate laws, and the oxidation rate of the second stage was faster than that of the first stage. Although the oxidation kinetics of Mn28Al12 steel under this condition also had two-stage linear rate laws, the oxidation rate of the second stage was slower than that of the first stage. When the oxidation temperature increased to 1373 K, the oxidation kinetics of the two steels at 5–25 h had only one-stage linear rate law, and the oxidation rates of the two steels were far faster than those at 1323 K for 5–25 h. The oxidation resistance of Mn28Al12 steel was much better than that of Mn28Al10 steel. Ferrite layer formed between the austenite matrix and the oxidation layer of the two Fe-Mn-Al-C steels oxidized at high temperature.