Microstructure and shape-memory characteristics of Ni56Mn25−xCoxGa19 (x = 4, 8) high-temperature shape-memory alloys

NiMnCoGa alloys were studied as candidate of high-temperature shape-memory alloys with Mn substituted by Co. The results show that the hot workability and room-temperature ductility of NiMnGa alloys can be greatly improved by forming ductile γ phase through Co addition. When the amount of γ phase is about 19%, the tensile stress and strain are 491 MPa and 8.2%, respectively. When the amount of γ phase increased to about 43%, the tensile stress and strain are 729.3 MPa and 14.1%, respectively. The reversible strain due to shape-memory effect is 2.1% under a prestrain of 5.3% for x = 4, and the shape-memory ability almost disappears for x = 8 due to large amount of γ phase, which seriously hampers the reorientation of martensitic variants and/or detwinning.