Effect of Fe addition on the martensitic transformation behavior and mechanical properties of Ni-Mn-Ga shape memory alloys

The microstructure, martensitic transformation behavior, mechanical properties and shape memory characteristics of Ni₅₆Mn_25-xFe_xGa19 (x=0-10) shape memory alloys were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, differential scanning calorimetry, and compression testing. A single phase of martensite with tetragonal structure is observed in the alloys with x ≤ 4, but dual phases with martensite and face-centered cubic γ phase are present when x ≥ 6. Compared with martensite phase, γ phase is rich in Ni and Fe, and its volume fraction increases with increasing Fe content. The martensitic transformation peak temperature decreases from 356℃ for x=0 to 170℃ for x=10, which is attributed to the comprehensive effect of the tetragonality and electron concentration of martensite. The introduction of γ phase by substituting Fe for Mn can greatly improve the strength and plasticity of the alloys. However, the shape memory strain drops from 5.0% for x=0 to 2.0% for x=6.