Effect of Ni doping on mechanical properties and phase transformation in Co-V-Ga high-temperature shape memory alloys

In the present work, the effects of Ni doping on the microstructure and mechanical properties of Co-V-Ga high-temperature shape memory alloy have been studied. It has been found that γ phase occurs in the form of precipitation or even dendrite microstructure when Ni content continuously increases in the alloy. The composition distribution showed the elements Co and Ni segregated in the γ phase, while elements V and Ga concentrated in the martensite phase. Moreover, the phase transition temperature increased by Ni-doping in Co-V-Ga alloys due to the increase in e/a of the alloy. However, the abundant presence of γ phase hindered the shear phase transformation and created a large number of grain boundaries to reduce Ms temperature when Ni content continuously increased. In addition, electron backscattered diffraction (EBSD) results verified that the presence of γ phase could hinder the expansion of cracks, and improve the strength and plasticity of the alloy. Furthermore, it could be found the shape memory effect could maintain a relatively high recovery rate when Ni content was within a certain extent. However, the shape memory effect of the alloy significantly decreased in the presence of γ phase with dendrite microstructure. The current research results not only clarify the influence of Ni doping on the microstructure and mechanical behavior of Co-V-Ga alloys, but also provide guidance for element doping to prepare high-temperature shape memory alloys (HTSMAs) with excellent performance.