The reason why thermal cycling decreased the martensite start temperature of an Fe-17wt%Mn alloy was investigated quantitatively; on the basis of the nucleation model for ε-martensite, and a thermodynamic model for martensitic transformation. The martensite start temperature decreased by about 22K after 9 cycles between 303 and 573K. This was due to an increase in the shear-strain energy that was required to advance transformation dislocations through the dislocation forests which formed in austenite during thermal cycling. The shear-strain energy increased from 19.3 to 28.8MJ/m3, due to an increase in the austenite dislocation density from 1.5 x 1012 to 3.8 x 1013/m2 with increasing number of thermal cycles (up to 9). The austenite dislocation density increased rapidly for up to 5 thermal cycles, and then increased gradually with further thermal cycles; exhibiting good agreement with the increase in austenite hardness with 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, 32[7], 1913-7