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

Youngsu Kim,Soo Yeol Lee,Ke An,Ho-Suk Choi,Wookjin Choi,Hahn Choo

doi:10.3390/cryst10020101

Abstract

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 ~66% to ~98%, and stacking fault energy (SFE) increased from ~0.65 to ~16.5 mJ/m2. The 0.1 C and 0.3 C steels underwent phase transformation from γ-austenite to ε-martensite or α’-martensite during tensile deformation. On the other hand, the 0.5 C steel underwent phase transformation only from γ-austenite to ε-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 γ-austenite to ε-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 ε-martensite phase transformation.

Highlights

High-Mn steel often shows characteristics of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) at room temperature because it contains various phases, including a metastable austenitic phase
Some annealing twins and mechanical twins were observed for all three alloys (Figure 3) The stacking fault energy (SFE)
The difference in the peak shift of γ{111} and γ{222} was not significant (Figures 7 and 8). This means that the SFE of the 17wt.% Mn steels was too small to be estimated from diffraction, and residual stacking faults are rare since the ε-martensite transformation is very active

Summary

Introduction

High-Mn steel often shows characteristics of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) at room temperature because it contains various phases, including a metastable austenitic phase. The alloying elements significantly influence the phase transformation behavior of the high-Mn steel because they change the stacking fault energy (SFE), which can control the activation of the secondary deformation mechanism [2]. In the case of carbon addition, the SFE increases and the austenite phase becomes more stabilized [7,8]. It is of great importance to understand how the addition of carbon and manganese elements influence the stability of the retained austenite associated with phase

Methods

Results

Discussion

Conclusion