Effect of copper addition on the microstructure and shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys

The effect of Cu addition on the microstructure and shape recovery behaviour of Fe–Mn–Si–Cr–Ni shape memory alloys has been studied. Microstructural characterization revealed that Cu is soluble in these alloys at least up to 3 wt% in the austenite (γ) phase and acts as a strong γ stabilizer. The γ–ε martensite transformation start temperature (MS) and the reverse ε–γ transformation start temperature (AS) were observed to decrease with Cu addition, although at different rates. Copper addition promotes the formation of incoherent Fe5Ni3Si2 type intermetallic π-phase precipitate during ageing and its amount increases with the amount of Cu in the alloy. Most importantly, Cu results in the deterioration of shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys, by increasing stacking fault energy (SFE), by decreasing the resistance to plastic deformation and by lowering the MS temperature of the γ phase.