Reverse Transformation Characteristics of Thermally Induced ε(hcp) Martensite in an Fe–Mn–Si Based Alloy

The microstructural change of thermally induced ε(hcp) martensite during reverse transformation in an Fe–30Mn–6Si alloy was investigated with TEM dynamic observation. The result shows that the main characteristic of reverse transformation is the backward movement of Shockley partials. The density of stacking fault or ε martensite shows a positive dependence on the cooling rate, indicating that the quenched-in vacancy is an effective resource for the formation of stacking faults. As and Ms were also found to be affected by the cooling rate. Since the lateral interface between γ(fcc) and ε(hcp) can move reversibly upon heating, martensitic transformation in Fe–Mn–Si based alloys with large thermal hysteresis is considered to be semi-thermoelastic rather than non-thermoelastic. Reverse transformations from thermally and stress-induced ε martensites in Fe–Mn–Si based alloys were compared and discussed. The mobility of dislocation in Fe–Mn–Si based alloys and in cobalt was also discussed in term of their differences in hysteresis.