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 Ni 50 Mn 25 Ga 25− x Fe x ( 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 Ni 50 Mn 25 Ga 25− x Fe x ( x <4) microwires are austenite phases with a cubic L2 1 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 Ni 50 Mn 25 Ga 21 Fe 4 and Ni 50 Mn 25 Ga 20 Fe 5 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. [ABSTRACT FROM AUTHOR]