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-y CoyGa19 (y = 4,8) and Ni56-z/2Mn25-z/2CozGa19 (z = 4,6) alloys consist of a two-phase mixture of martensite and gamma 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 y phase. Dynamic tensile tests and scanning electron microscopy observations of fracture surfaces confirm that the existence of y phase plays a key role in improving the ductility of Ni-Mn-Co-Ga alloys. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.