Y. Li, F. Liu, H. Xu, Beijing University of Aeronautics and Astronautics, Beijing, China
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 0oC up to 400oC 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.
Summary: 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 0oC up to 400oC until the appearance of the second phase. Ni54Mn25Ga21 single crystals exhibits a shape-memory effect, compressive strength and strain of 6.1%, 845 MPa and 20.5%, respectively. The martensitic structure, the transformation temperatures and the enthalpy of this alloy almost keep invariable even after 1000 thermal cycles. 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 ã phase distribution.
