Influence of Ni excess on structure and alloys shape-memory effect of polycrystalline Ni₂MnGa alloys

Nonstoichiometric Ni_52+XMn_23-XGa₂₅ polycrystalline samples were made by arc melt method. The phase transformation temperatures of these alloys were determined by DSC and susceptibility measurements. The results show that the substitution of Mn atoms by Ni at constant Ga content results in higher transformation temperatures and lower Curie temperatures. X-ray diffraction was used to identify the crystal structure. Ni₅₂Mn₂₃Ga₂₅ and Ni₅₃Mn₂₂Ga₂₅ possess fcc structure whereas Ni₅₄Mn₂₁Ga₂₅ and Ni₅₅Mn₂₀Ga₂₅ possess tetragonal structure at room temperature. Shape memory effect was investigated on polycrystalline Ni₅₄Mn₂₁Ga₂₅ alloy. The strain of 1300 ppm was obtained under zero applied magnetic field and 2500 ppm was obtained under 1.2T applied magnetic field. The mechanism of magnetic field induced strain during phase transformation was also discussed.