Study of Ni_50+xMn₂₅Ga_25-x (x = 2-11) as high-temperature shape-memory alloys

Ni_50+xMn₂₅Ga_25-x (x = 2-11) alloys were studied as high-temperature shape-memory alloys, with regard to their microstructure, martensitic transformation behavior and high-temperature shape-memory effect. Single phase of martensite with tetragonal structure was present for x < 7, and dual phases containing martensite and γ were observed for x ≥ 7. The martensitic transformation peak temperatures M_p increase monotonically from 39.1 °C for x = 2 to 443.8 °C for x = 7, then remain almost constant at 440 °C for x ≥ 7. The shape-memory strains of the alloys decreased gradually from 6.1% for x = 4 to 2.8% for x = 8 and 0% for x = 11 under the same pre-strain. The variations of the martensitic transformation temperatures and the shape-memory effects with Ni contents correlate with changes in size factor, electron concentration and precipitation of γ phase.