Growth process and microstructure of epsilon martensite in an Fe-Mn-Si-Cr-Ni shape memory alloy

Growth process and microstructure of epsilon martensite formed by cooling in thermomechanically-treated Fe-Mn-Si-Cr-Ni shape memory alloys have been investigated in order to clarify whether or not very fine martensite plates such as observed in the case of stress-induced transformation are also formed in the case of simple cooling. Either such fine martensite plates or the lamella structure which consists of h.c.p. and f.c.c, phase is not observed in the case of the simple cooling, both in thermomec:hanically and non-thermomechanically-treated specimens, but there is a tendency that thinner martensite plates are formed in the former specimens. In the present study, additional diffraction spots, besides h.c.p. spots, are formed, which may have resulted from double hexagonal structure or some kinds of long period stacking structures. Stacking fault density in the h.c.p, martensite was examined by electron diffraction and it was found that the values of fault parameter alpha are almost the same in both the streses-induced and thermally-induced martensites.