Characterization of the stress-induced ε martensite in a Fe-Mn-Si-Cr-Ni shape memory alloy:: microstructural observation at different scales, mechanism of formation and growth

The martensitic transformation induced by traction at room temperature in a Fe-16Mn-9Cr-5Si-4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation. (C) 1997 Elsevier Science S.A.