Abstract
The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni 50− x Mn 25Ga 25− 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 Ni 54Mn 25Ga 21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni 54Mn 25Ga 21 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 Ni 56Mn 17Fe 8Ga 19 alloy.
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