The reason why thermal cycling decreased the martensite start (Ms) temperature of an Fe-17wt%Mn alloy was investigated quantitatively on the basis of a nucleation model for ε-martensite and a thermodynamic model for the martensitic transformation. The Ms temperature decreased by about 22K after 9 cycles between 303 and 573K, due to the increase in shear-strain energy (ΔGsh) required to advance the transformation dislocations through dislocation forests formed in the austenite during thermal cycling. The ΔGsh value increased from 19.3 to 28.8MJ/m3 due to an increase in austenite dislocation density from 1.5 x 1012 to 3.8 x 1013/m2 with the number of thermal cycles (in this case, up to 9 cycles). The austenite dislocation density increased rapidly for up to 5 thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.

Relationship between Austenite Dislocation Density Introduced during Thermal Cycling and Ms Temperature in an Fe-17wt%Mn Alloy. Y.K.Lee: Metallurgical and Materials Transactions A, 2002, 33[7], 1913-7