Martensitic transformation in Fe-Mn-Si based shape memory alloys

The critical driving force for martensitic transformation fcc(gamma)-->hcp(epsilon) in ternary Fe-Mn-Si alloys increases with the content of Mn and decreases with that of Si. Thermodynamical prediction of M-s in ternary Fe-Mn-Si alloys is established. The fcc(gamma)-->hcp(epsilon) martensitic transformation in Fe-Mn-Si is a semi-thermoelastic and the nucleation process does not strongly depend on soft mode. Nucleation occurs directly through an overlapping of stacking fault rather than pole mechanism, and it is suggested that stacking fault energy (SFE) is the main factor controlling nucleation. Based on the phenomenological theory of martensite crystallography, a shuffle on (0001)(hcp) plane is required when d(111)not equal d(0002). The derived principal strain in Bain distortion is smaller, i.e., more reasonable than the values given by Christian. Alloying elements strengthening the austenite, lowering SFE of gamma phase and reducing T-N(gamma) temperature may be beneficial to shape memory effect (SME) of Fe-Mn-Si based alloys. Accordingly, Fe- Mn-Si-RE and Fe-Mn-Si-Cr-N (or Fe-Mn-Si-Ni-Cr-N) are worthy to be recommended as shape memory materials with improved SME.