Effect of Si doping on microstructure and martensite transformation in Ni-Mn-Sb ferromagnetic shape memory alloys

We have recently reported that the quaternary Ni_49.0Mn_38.4Sb_11.7Si_0.9 alloy showed enhanced refrigeration capacity due to the wide working temperature interval and narrow magnetic hysteresis loss via Si-doping. Here we systematically investigate the microstructure, martensite transformation and magnetic transition behaviors of Ni₅₀Mn₃₈Sb_12-xSi_x (x = 0, 1, 2, 3) alloys. Ni₅₀Mn₃₈Sb_12-xSi_x (x = 0, 1, 2) alloys exhibited a full solid-solution state with a four-layer modulated orthorhombic (4O) martensite structure. However, three phases formed in Ni₅₀Mn₃₈Sb₉Si₃ alloy, and lath-like martensite twins existed only in α phase at room temperature. The martensite transformation temperature decreased with the increasing Si content when the Si atomic content increased from 0% to 2%, but significantly increased when Si content reached to 3% because of the increase of the valence electron concentration (e/a) of matrix phase (α phase). The martensitic transformation also occurred in the β phase below room temperature in Ni₅₀Mn₃₈Sb₉Si₃ alloy. Furthermore, the martensite transformation temperature range enlarged and the thermal hysteresis narrowed with the addition of Si atomic content from 0% to 2%, of which Ni₅₀Mn₃₈Sb₁₀Si₂ alloy had the widest martensite transformation temperature range and narrowest thermal hysteresis, which was supposed to have great magnetocaloric effect (MCE). However, when x = 3, the thermal hysteresis increased significantly and the transition character changed to a second-order phase transition above room temperature; a magneto-structural coupling of the first-order martensitic transformation and second-order magnetic transition also existed below room temperature, which all would result in low MCE.