EFFECT OF THERMAL CYCLING ON THE MARTENSITIC-TRANSFORMATION IN AN FE-24MN-6SI SHAPE MEMORY ALLOY

The effect of gamma reversible epsilon cyclic transformation on the subsequent gamma --> epsilon 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 gamma --> epsilon 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 epsilon-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 epsilon-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 gamma reversible epsilon cyclic transformation. it is concluded that the rote 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.