Martensitic transformation and shape memory effect in ausaged Fe-Ni-Si alloys

Martensitic transformations in the Fe-(24-30)Ni-(5-8)Si (mass%) alloys have been investigated by means of optical and transmission electron microscopy, differential scanning calorimetry and hardness-testing. The Ms temperature is decreased by Si addition and the morphology of martensite is mainly lenticular in the unaged specimens. However, a pronounced decrease in the Ms temperature and a change in the martensite morphology from the lenticular to the thin plate type are observed on ausaging at 400degreesC. The increase in austenite hardness, the decrease in the Ms temperature and the increase in the tetragonality of martensite after ausaging at 400degreesC are clarified as due to the formation of nanoscale particles of gamma'-(Ni,Fe)(3)Si with the L1(2) structure during ausaging at 400degreesC. The Fe-Ni-Si alloys that form thin plate martensite show the shape memory effect, which arises from the reverse transformation of stress-induced martensite to austenite, Precipitation hardening of the austenite phase by fine gamma' particles during ausaging improves the degree of shape recovery.