Effects of Ti and Gd for Ga substitution on microstructure, magnetic and mechanical properties of polycrystalline Ni-Mn-Ga magnetic shape memory alloy

The present study is focused on the influence of Ti and Gd doping at the Ga site on the microstructure, magnetic and mechanical properties of the polycrystalline Ni₅₀Mn₂₅Ga_20-xZ_x (x = 0 or 5, Z = Gd, Ti) magnetic shape memory alloys. Microstructure investigations show that reference Ni₅₀Mn₂₅Ga₂₅ and Ti-doped alloys, both in the as-cast and annealed state, are single phase materials, whereas Gd-doped sample reveals dendritic dual-phase structure with substantial distinction between Gd-rich and Gd-poor regions. Thermomagnetic measurements expose reversible martensitic transition in the Ni₅₀Mn₂₅Ga₂₅ and Ni₅₀Mn₂₅Ga₂₀Ti₅ alloys, where Ti addition to NiMnGa composition leads to the decrease of phase transformation temperature from T_M = 193 K for reference sample to T_M = 172.5 K for Ti-doped material. Furthermore, the Ni₅₀Mn₂₅Ga₂₀Gd₅ alloy does not experience fully martensitic transition. Temperatures of magnetic transformation also varies with chemical composition and equals to 379 K, 318 K and 370 K for the annealed Ni₅₀Mn₂₅Ga₂₅, Ni₅₀Mn₂₅Ga₂₀Ti₅ and Ni₅₀Mn₂₅Ga₂₀Gd₅ alloy, respectively. Mechanical properties investigation based on the nanoindentation measurements shows beneficial influence of doping elements on material hardness. In addition, planar distributions of hardness allow to deconvolute mechanical properties of each individual phase of the Ni₅₀Mn₂₅Ga₂₀Gd₅ alloy.