The ductility and shape-memory properties of Ni–Mn–Co–Ga high-temperature shape-memory alloys

Ni–Mn–Co–Ga alloys with Ni/Mn or Ni and Mn substituted by Co were investigated as candidates for high-temperature shape-memory alloys. Ni56−xCoxMn25Ga19 alloys with x < 8 consist of single phase martensite, whereas Ni56−xCoxMn25Ga19 (x ⩾ 8), Ni56Mn25−yCoyGa19 (y = 4, 8) and Ni56−z/2Mn25−z/2CozGa19 (z = 4, 6) alloys consist of a two-phase mixture of martensite and γ phase. The mechanical and shape-memory properties of Ni56Mn25−yCoyGa19 and Ni56−z/2Mn25−z/2CozGa19 alloys, which were hot-rolled into 0.5 mm thin plates by conventional hot rolling process, were investigated. The ductility and hot-workability of Ni–Mn–Co–Ga alloys were greatly improved by increasing the amount of ductile γ phase. Dynamic tensile tests and scanning electron microscopy observations of fracture surfaces confirm that the existence of γ phase plays a key role in improving the ductility of Ni–Mn–Co–Ga alloys.