Ni–Mn–Ga high-temperature shape memory alloys

The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni50−xMn25Ga25−x (at.%) alloys by optical and transmission election microscopy, X-ray diffraction and differential scanning calorimeter measurements. The martensitic transformation temperatures almost linearly increase with increasing Ni content up to 670 K showing their potentials as high-temperature shape memory alloys. The results show that the tetragonality ratio (c/a) increases with the Ni substitution for Ga, but the unit-cell volume reduces simultaneously. The martensitic transformation characteristics of Ni54Mn25Ga21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni54Mn25Ga21 alloy exhibited a shape-memory effect of 4.2% and compressive strain larger than 10% by using grain refining technique. Addition of Fe as a fourth element is an effective method to improve the mechanical properties, especially plasticity of polycrystalline Ni–Mn–Ga alloy due to the precipitation of γ-phase. A compressive strain of 30% and a shape-memory effect of 2.8% are obtained for Ni56Mn17Fe8Ga19 alloy.