CHARACTERIZATION OF THE STRESS-INDUCED EPSILON-MARTENSITE IN A FE-MN-SI-CR-NI SHAPE-MEMORY ALLOY - MICROSTRUCTURAL OBSERVATION AT DIFFERENTSCALES, 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 microsco py, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory prope rties. The martensitic microstructure and the fine structure of the st acking faults are both studied to clarify the nucleation and growth me chanisms of martensite. The band structure of the martensite is pointe d out; these bands correspond to a mixture of thin martensite plates a nd extremely thin austenitic zones. Inside a grain, the monopartial na ture of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mech anism appears to be the main mechanism of martensite nucleation. (C) 1 997 Elsevier Science S.A.