A study of NiMnGa high temperature shape memory alloys

NiMnGa shape memory alloys (SMAs) have recently drawn more attention as promising high temperature SMAs (HTSMA) due to large shape memory effect (SME), pseudoelasticity (PE), high thermal stability and interesting aging effect. A systematic substitution of Ni for Ga has been performed in the non-stoichiometric NiMnGa alloys. The relationship among the composition, structure and martensitic transformation temperatures was studied in detail. The martensitic transformation temperatures increase with increasing Ni content from lower than 0^oC up to 400^oC until the appearance of the second phase. A high-temperature shape-memory alloy, Ni₅₄Mn₂₅Ga₂₁, was subsequently developed with a shape-memory effect of 6.1% and a martensitic transformation temperature higher than 250 °C for single crystals. The measured compressive strength and strain were 845 MPa and 20.5%. It was also found that the martensitic structure, the transformation temperatures and the enthalpy of this alloy almost keep invariable even after 1000 thermal cycles. In general, polycrystalline NiMnGa alloys have an extremely brittle feature. The results show that the second phases in Ni₅₈Mn₂₅Ga₁₇ and Ni₅₆Mn₁₇Fe₈Ga₁₉ alloys can raise the strength and plasticity. The shape memory effect of the former is 5.1%, which is better than the latter due to the favorable γ phase distribution. Moreover, a plate with the thickness of 1.5mm has been made by Pack-Hot-Rolling method for Ni₅₆Mn₁₇Fe₈Ga₁₉ alloy.