Microstructure evolution and mechanical properties of Ti[sbnd]22Al[sbnd]25Nb alloy joints brazed with Ti[sbnd]Ni[sbnd]Nb alloy

Abstract

Ti45Ni45Nb10 (at.%) brazing alloy, fabricated by arc melting, was successfully used to braze Ti22Al25Nb (at.%) alloy. The microstructures of Ti45Ni45Nb10 brazing alloy and Ti22Al25Nb alloy brazed joints were analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and micro-area X-ray diffraction (XRD). The effects of the brazing parameters on the interfacial microstructure and mechanical properties of the Ti22Al25Nb alloy brazed joints were investigated. The results showed that the joint was primarily comprised of two characteristic zones: diffusion zone I and central zone II, and the reaction phases formed in the brazed joint were the B2, O, τ3, and Ti2Ni phase. The crystal orientation of B2 phase in diffusion zone I was consistent with that in the Ti22Al25Nb substrate. The O phase was precipitated from the B2 phase. As the brazing temperature or holding time increased, τ3 was gradually replaced by the B2 phase, and the Ti2Ni phase decreased and ultimately disappeared. The maximum shear strength achieved at room temperature was 318 MPa when the joint was brazed at 1180 °C for 20 min, whereas it was 278 MPa at 650 °C. Crack primarily propagated in the τ3 compound, which was extremely hard and brittle, and partially traversed the B2 and O phases.