Microstructure and martensitic transformation behavior of Ni_56−x Mn₂₅Fe _x Ga₁₉ shape memory alloys

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni_56-x Mn₂₅Fe _x Ga₁₉ (x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy (OM), X-ray diffraction analysis (XRD), differential scanning calorimeter (DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni₅₆Mn₂₅Ga₁₉ alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature (M _p) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni₅₆Mn₂₅Ga₁₉ alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.