The influence of thermal cycling on the transition temperatures of a Fe-Mn-Si shape memory alloy

Depending on their composition, Fe-Mn-Si based alloys can exhibit, upon coo ling, a martensitic transformation, gamma (FCC) <-> epsilon (HCP), and a ma gnetic transition from paramagnetic to antiferromagnetic, at the Neel tempe rature, T-N. is said to increase the stability of the gamma phase, depressi ng M-S to quite lower temperatures. In this work, the phase transition temp eratures were measured on a Fe-27 wt.% Mn-2.5 wt.% Si alloy by differential scanning calorimetry and by dilatometry after thermal cycles performed in the range 77 up to 623 K. The aim was to study the effects of thermal cycli ng on the transformation temperatures M-S, M-F, A(S), A(F) and T-N. It was observed that, although T-N was above and close to M-S, the martensitic tra nsformation was not suppressed on cooling. Besides that, depending on the t emperature and time, the alloy remained above A(F), the M-S temperature was lowered, the A(F) was slightly increased, while T-N remained constant. It was also observed that thermal cycling below M-S and above A(F) can suppres s further martensitic transformation, which can be recovered after heating to 1323 K. These observations are discussed in terms of composition and ela stic strain changes generated during thermal cycling. (C) 1999 Elsevier Sci ence S.A. All rights reserved.