Mechanism of training effect in the ferrous shape memory alloys associated with gamma->epsilon martensitic transformation.

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gamma->epsilon forward and epsilon->gamma reverse transformations and cyclic transformation in Fe-Mn-Si alloys have been investigated. The main findings are as follows.(1)The change in transformation temperature and microstructure by thermal cycling between room temperature and 573K(above Af)was observed in an Fe-24%Mn-6%Si alloy of which Ms was higher than room temperature. Unlike the case of Fe-24%Mn, Ms hardly changed by the thermal cycling in the Fe-Mn-Si alloy. Moreover, the Fe-Mn-Si alloy showed the good reproducebility of gamma martensite plates, i e., microstructure memory. Furthermore, lattice defects such as dislocations were scarcely observed in the austenite after the thermal cycling. These results strongly suggest that the movement of transformation dislocations is reversible during gamma->epsilon transformations in the Fe-Si-Mn alloy.(2)Microstructural observation of stress-induced epsilon martensite formed by tensile deformation at room temperature was performed in an Fe … More -33%Mn-6%Si alloy of which Ms was just below room temperature. It was confirmed by TEM observation that an epsilon plate formed by the operation of one kind of partial dislocations. A large amount of stacking faults were observed in the untransformed austenite. These stacking faults may act as nucleation sites in the following forward transformation after the annealing above Af. The reversion of stress-induced martensite started at a lower temperature and finished at a higher temperature compared with that of thermally-induced martensite.(3)The change in shape recovery by the addition of thermal cycling between 77K and 873K to the Fe-33%Mn-6%Si alloy was observed. The shape recovery was not improved by the thermal cycling, indicating that the gamma->epsilon cyclic transformation per se was not associated with the improvement of shape recovery by the training treatment.It was concluded from the above results that the role of the training effect is to introduce lattice defects into austenite which act as the nucleation sites in the following transformation. Less