Microstructure and properties of transient liquid phase diffusion bonded joint for TiNi shape memory alloy and stainless steel

TiNi shape memory alloy and stainless steel were bonded by transient liquid phase diffusion bonding (TLP-DB) with AgCu metal foil as the interlayer. The microstructure, alloy elements profile and the patterns of joint were analyzed, while the microhardness and the shear strength of the joint were investigated. The results show that the interface zone of joint is composed of TiNi transition zone, middle zone and stainless steel transition zone, which contains Ti(Cu, Ni, Fe) phase, AgCu phase, TiFe phase respectively. The microhardness of diffusion zone in both TiNi side and stainless steel side wries in the range of 500-650 HV. However, the microhardness of middle zone is only about 120 HV. With the increase of heating temperature or the prolonging of the holding time, the shear strength of joint interface increases firstly and then decreases. The highest shear strength is about 239.4 MPa. The fracture occurs at diffusion interface between TiNi alloy and AgCu interlayer, and the joint presents a characteristic of mixed fracture ductile-brittle mode.