Microstructure evolution and mechanical behavior of Ni-rich Ni-Mn-Ga alloys under compressive and tensile stresses

Abstract The microstructure evolution and mechanical behavior in directionally solidified Ni-rich Ni-Mn-Ga alloys with nominal compositions of Ni58Mn25Ga17 and Ni60Mn25Ga15 under compressive and tensile stresses have been investigated. The composition distribution shows the element Ni segregates in gamma phase, while elements Mn and Ga segregate in martensite phase. Furthermore, the microstructure orientation examined by electron backscatter diffraction (EBSD) indicates that beta phase has a preferred growth orientation of (001)A in Ni58Mn25Ga17 alloys, while gamma phase has a preferred growth orientation of (001)γ in Ni60Mn25Ga15 alloys. The fracture morphology suggests that the existence of ductile γ phase can reduce the crack propagation and promote fracture strain, particularly in the Ni60Mn25Ga15 alloys. Finally, Schmid factor and deformation gradient tensor were calculated to well explain the crystallographic evolution during the detwinning under compressive and tensile stresses. The present findings not only elucidate the mechanism of γ phase on the mechanical behavior of Ni-rich Ni-Mn-Ga alloys, but also shed light on the composition design of high temperature Ni-Mn-Ga shape memory alloys.