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 400°C. The increase in austenite hardness, the decrease in the Ms temperature and the increase in the tetragonality of martensite after ausaging at 400°C are clarified as due to the formation of nanoscale particles of γ'-(Ni, Fe)3Si with the L12 structure during ausaging at 400°C. 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 γ' particles during ausaging improves the degree of shape recovery.