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 cooling, a martensitic transformation, γ (FCC) <-> ε (HCP), and a magnetic transition from paramagnetic to antiferromagnetic, at the Neel temperature, TN. is said to increase the stability of the γ phase, depressing MS to quite lower temperatures. In this work, the phase transition temperatures 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 cycling on the transformation temperatures MS, MF, AS, AF and TN. It was observed that, although TN was above and close to MS, the martensitic transformation was not suppressed on cooling. Besides that, depending on the temperature and time, the alloy remained above AF, the MS temperature was lowered, the AF was slightly increased, while TN remained constant. It was also observed that thermal cycling below MS and above AF can suppress further martensitic transformation, which can be recovered after heating to 1323 K. These observations are discussed in terms of composition and elastic strain changes generated during thermal cycling. (orig.)