Martensite transformation and superelasticity in polycrystalline Ni–Mn–Ga–Fe microwires prepared by melt-extraction technique

The effects of Fe doping on the microstructure, martensite transformation and superelasticity in melt-extracted Ni50Mn25Ga25−xFex (x=1–6) microwires were investigated. The unique solidification process during melt-extraction creates the micron-sized diameter wires with small grains and semicircular cross-section. At ambient temperature Ni50Mn25Ga25−xFex (x<4) microwires are austenite phases with a cubic L21 structure, while microwires with x>5 are martensitic phases with seven-layered modulated (7M) structure. The results point out that martensite transformation temperatures are strongly related to Fe content due to the change of valence electron concentration (e/a). Reversible superelastic strains of 0.92% and 0.75% are obtained in Ni50Mn25Ga21Fe4 and Ni50Mn25Ga20Fe5 microwires, respectively. It is demonstrated that the temperature dependence of stress-induced martensite (SIM) stress follows the Clausius–Clapeyron relation. The temperature dependence of SIM stress in Fe-doped Ni–Mn–Ga microwires is 10.5 MPa/K.