Effects of thermal cycling on the fcc-hcp martensitic transformation temperatures in the Fe-Mn system - II. Transmission electron microscopy study of the microstructural changes

The paper deals with the effect of thermal cycling (TC) upon the microstructure and the fcc-hcp martensitic transformation (MT) in Fe-Mn alloys. Binary alloys with 34.0 and 25.9wt% Mn are studied by transmission electron microscopy (TEM), the latter before and after the TC process. It is shown that the TC introduces dislocations mainly in the fee phase, which explains why the observed effects of TC upon the M-s and A(s) temperatures are qualitatively similar to those of plastic deformation of the austenite phase. Measurements of the M-s and A(s) as a function of the number of cycles reveal first a promotion effect and then an inhibition effect, which were explained in the first part of this study by Baruj et al. (1996, Z. Metallkde., 87, 765) by invoking the interplay between stacking faults and total dislocations. These ideas are supported by the present TEM observations, which provide a new basis for understanding the microstructural evolution upon TC.