Effect of Transformation Cycling on the ε Martensitic Transformation in Fe-Mn Alloys

The effect of γ⇆ε cyclic transformation on the subsequent forward and reverse transformations was studied in low carbon Fe-17%Mn and Fe-25%Mn binary alloys. A remarkable decrease in Ms temperature by the cyclic transformation was observed in the Fe-25%Mn alloy. After 7 thermal cycles between 305 and 573 K, Ms decreased by over 60 K and the structure became fully austenitic at room temperature. On the other hand, As and Af temperatures increased with an increase in the number of thermal cycles. As the upper peak temperature of thermal cycling increased, the changes in Ms, As and Af became smaller. The decrease in Ms by the cyclic transformation was smaller in the Fe-17%Mn alloy than in the Fe-25%Mn alloy. A large number of dislocations were observed in the austenite of the Fe-25%Mn alloy after the thermal cycling between 273 and 573 K, although dislocations were scarcely observed before the thermal cycling. The hardness of austenite at 573 K was markedly increased by the thermal cycling between 273 and 573 K. A microstructure memory, which is a phenomenon that martensite forms in the same position where the martensite formed in the last forward transformation, was not observed.