Effect of thermal cycling on the martensitic transformation in an Fe-24Mn-6Si shape memory alloy

The effect of γ⇔ε cyclic transformation on the subsequent γ→ε martensitic transformation has been studied in an Fe-24%Mn-6%Si shape memory alloy in order to obtain fundamental information on the role of Si in the shape memory effect. The γ→ε martensitic transformation is markedly enhanced by the thermal cycling between 305 K (below M_s) and 573 K (above A_f), although the M_s temperature slightly decreases. The amount of ε martensite at 305 K is less than 30% after a solution treatment and increases to 40% after 10 thermal cycles. Microstructural observation demonstrates that the Fe-Mn-Si alloy exhibits a good reproducibility of ε martensite plate formation. Moreover, there are few dislocations in the austenite of a thermal-cycled specimen unlike the case of Fe-Mn binary alloys. These results suggest that the addition of Si plays an important role in the reversible movement of transformation dislocations (i.e., Shockley partial dislocations) during γ⇔ε cyclic transformation. It is concluded that the role of Si in the shape memory effect is to make the movement of partial dislocations reversible as well as to restrict the permanent slip in austenite during a shape change.