Rietveld refinement, microstructure and high-temperature oxidation characteristics of low-density high manganese steels

Three forged low-density high manganese steels Mn28Al10,Mn28Al8 and Mn20Al10 were used as experimental materials in this study.The forged microstructure and external oxidation characteristics at 1323 K and 1373 K for 5–25 h in air were investigated by microstructural observation and X-ray diffraction（XRD）technique.The phase compositions and abundance in the forged and oxidized samples were quantitatively obtained by Rietveld method on the basis of XRD pattern analysis.The results showed that an austenitic microstructure formed in steels Mn28Al10 and Mn28Al8,and 18.02 wt%ferrite could be found in Mn20Al10.The relative amount of;.28 wt%-carbide(Fe;AlC;)in Mn28Al10 was far greater than that in Mn28Al8 and Mn20Al10.The oxidation kinetics of forged steels oxidized at 1323 K for 5–25 h had two-stage parabolic rate laws;and the oxidation rate of the first stage was lower than that of the second stage.When they were oxidized at 1373 K for 5–25 h,the oxidation kinetics followed only a parabolic law and the oxidation rates were respectively greater than those at 1323 K for 5–25 h.When they were oxidized at 1323 K for 25 h,detached external scales contained Fe;MnO;and-Fe;O;oxides.-Al;O;and（Fe,Mn）;O;oxides could only be indexed in steels Mn28Al8 and Mn28Al10,respectively.When they were oxidized at 1373 K for 25 h,Fe;MnO;,Fe;O;,-Fe;O;and-Al;O;oxides could all be indexed in the external detached scales.The main phase of detached external scales was Fe;MnO;;and the relative amount of-Al;O;in steel Mn28Al8 was higher than that in steels Mn28Al10 and Mn20Al.The external oxidation layers of these three forged steels oxidized at 1323 K and 1373 K for 25 h were essentially followed the sequence of-Al;O;,Fe;MnO;,Fe;O;,FeMnO;,and Fe;O;from the substrate to the outside surface.The forged Mn28Al10 steel with austenitic microstructure and a certain amount of-carbide（;.28 wt%in the present work）possessed a better combination of strength,ductility,specific strength,and oxidation rate when compared to that of the forged Mn28Al8 and Mn20Al10 steels.