Effect of Zr addition on the microstructure, phase transformation and mechanical property of Ni50Mn25Ga17Cu8 alloy

This study investigated the microstructure, phase transformation and mechanical property of Ni50Mn25Ga17Cu8−xZrx (x=0, 4 and 8) alloys. The microstructure of alloys changed from a single martensitic phase to a dual phase consisting of martensitic/austenitic matrix and second γ phase after substituting Cu with Zr and the volume fraction of second phase increased with increasing Zr content. The martensitic transformation temperature and transformation enthalpy were reduced after adding Zr, which was related to the introduction of second phase that changed the composition and reduced volume fraction participating in martensitic transformation of the alloy. The decrease of transformation hysteresis after introducing the second phase was thought to be caused by the martenstic structure change of the matrix from tetragonal to orthorhombic that reduced the lattice deformation of austenite→martensite transformation. The mechanical strength of the Cu8 alloy was increased firstly after substituting 4 at% Cu by Zr and then decreased after completely substituting Cu by Zr. The change of mechanical performance was related to the fracture character of the alloys. The intergranular fracture of matrix and second phase dominated in the Cu8 and Zr8 alloys, respectively, but the fracture of Cu4Zr4 alloy was determined by a mixture of intergranular fracture of second phase and transgranular fracture of matrix.