A nano-twinned structure and tensile properties of a thermally cycled Fe-17Mn alloy

In the present study, the effect of thermal cycling on the microstructure and tensile properties of an Fe-17Mn (wt%) alloy were investigated. The thermal cycling was carried out until 10 times between a lower temperature (T_L) of 25 °C and a upper temperature (T_U) of 300 °C. A nano-twinned γ structure was observed at T_U during thermal cycling through in-situ transmission electron microscopic observation. Many stacking faults, such as intrinsic I₁, intrinsic I₂, and twin-like T₂ stacking faults, were observed in ε martensite at T_L. The nano-twinned γ structure was formed by the stacking faults with a localized fcc structure in ε martensite during the ε → γ reverse transformation. Namely, half of the stacking faults with a localized fcc structure had the same crystallographic orientation as an initial γ austenitic phase, while the other half had a twin relationship with the γ austenite. The stacking faults with a twin relationship caused nano-twins during ε → γ reverse transformation. The volume fraction of nano-twins at T_U was linearly proportional to that of ε martensite at T_L, supporting the result that nano-twins in the γ phase resulted from preexisting stacking faults in ε martensite during ε → γ reverse transformation. Both the yield and ultimate tensile stresses of the γ austenite at T_U increased with an increase in the number of thermal cycles, while the elongation decreased. The significant increase in yield stress at T_U was due to the increase in dislocations and nano-twins introduced into γ austenite during thermal cycling.