Correlation between microstructures and mechanical properties in Ni-rich Ni–Mn–Ga high-temperature shape-memory alloys

Microstructures and mechanical properties of polycrystalline and single-crystalline Ni55+xMn25Ga20-x (x = 1, 2, 3) alloys are investigated. The alloys exhibit a single martensite phase for x = 1, and two phases containing tetragonal martensite phase and face-centered gamma phase for x = 2 and 3. The appearance of gamma phase can apparently raise the strength and plasticity of the alloys. Moreover, single crystals show better mechanical properties compared with the polycrystals with the same composition. Furthermore, a kind of net-like gamma phase is observed in directionally solidified Ni58Mn25Ga17 single crystal. This alloy exhibits excellent plasticity due to that the crack propagation is seriously hindered by the plastic net-like gamma phase.