Fe–Mn–Si–Cr–Ni based shape memory alloy: Thermal and stress-induced martensite

The determining role of C- and Si-content on the shape memory effect of Fe–Mn–Si–Cr–Ni alloys, where Si was partly replaced by Co. In this regard, three alloys, Fe–15Mn–5Si–8Cr–5Ni, Fe–15Mn–5Si–8Cr–5Ni-0.1C, and Fe–15Mn–3Si–8Cr–5Ni-0.1C–2Co were designed to simultaneously study the shape memory effect and mechanical properties in relation to the microstructure. The microstructure of the alloys after cold rolling and annealing was analyzed by EBSD and X-ray diffraction techniques. The results indicated that grain size and the addition of C influenced the shape memory effect of Fe–Mn–Si–Cr–Ni-based memory alloy. With the increase of grain size, the annealing twin density was decreased, reducing the constraint on phase transformation of γ-austenite to ε-martensite. Furthermore, the austenite stability was decreased, such that the martensite-content was increased, and then reduced the shape memory effect of the experimental alloy. With the addition of C, the content of thermally-induced martensite was reduced, and the probability of stress-induced transformation of martensite to obtain α′-martensite was reduced, with consequent enhancement of the shape memory effect for the experimental steel. Additionally, the strength of parent phase was enhanced by the addition of C, and the plastic deformation by slip during stress-induced martensite was minimized, leading to low α′-martensite content and promotion of reversal of stress-induced martensite.