Comparative Effects of Thermal Treatments on the Shape Memory Phenomenon of Fe-Mn-Si and Fe-Mn-Cr-Ni-Si Steels

Two shape-memory steels were water quenched after one hour at 873 K to keep a stable austenitic structure at 298 K. The best values of the shape memory recovery were obtained after a 2.5% strain followed by an annealing at 873 K. In fact, immediately after water quenching from 1323K, the Fe-Mn-Si alloy did not present the forward γ→ε transformation. The transformation was detected after maintaining 48 h. at 298K and was considered as complete after 480 h., using electrical resistivity measurements. Samples of the two steels were slowly cooled from 1123 K under vacuum : the Fe-Mn-Si alloy showed completely ordered D03 + B2 structures and was ferromagnetic whereas the Fe-Mn-Cr-Ni-Si steel was a mixture of the γ phase and the D03 + B2 structures