In Situ Neutron Diffraction Study of Phase Transformation of High Mn Steel with Different Carbon Content

In situ neutron diffraction was employed to examine the phase transformation behavior of high-Mn steels with different carbon contents (0.1, 0.3, and 0.5 wt.%C). With increasing carbon contents from 0.1 C to 0.5 C, the austenite phase fraction among the constituent phases increased from similar to 66% to similar to 98%, and stacking fault energy (SFE) increased from similar to 0.65 to similar to 16.5 mJ/m(2). The 0.1 C and 0.3 C steels underwent phase transformation from gamma-austenite to epsilon-martensite or alpha'-martensite during tensile deformation. On the other hand, the 0.5 C steel underwent phase transformation only from gamma-austenite to epsilon-martensite. The 0.3 C steel exhibited a low yield strength, a high strain hardening rate, and the smallest elongation. The high strain hardening of the 0.3 C alloy was due to a rapid phase transformation rate from gamma-austenite to epsilon-martensite. The austenite of 0.5 C steel was strengthened by mechanical twinning during loading process, and the twinning-induced plasticity (TWIP) effect resulted in a large ductility. The 0.5 wt.% carbon addition stabilized the austenite phase by delaying the onset of the epsilon-martensite phase transformation.