Bonding mechanism of TC4 titanium alloy/T2 copper vacuum diffusion bonded joint with nickel as transition interlayer

Vacuum diffusion bonding of TC4 titanium alloy (TC4) to T2 copper (T2) using nickel foil as transition interlayer was explored. Ti₃Ni, Ti₂Ni, TiNi, AlNi₂Ti and TiNi₃ phases arose at the TC4/Ni bonded interface, and Cu-Ni solid solution appeared in the Ni/T2 interface. Thereinto, AlNi₂Ti was a kind of discontinuous nano precipitated phase, which distributed between TiNi and TiNi₃ phases. The crystallographic orientations of Ti₂Ni, TiNi, AlNi₂Ti and TiNi₃ phases were (201), (020), ($1\overline{1}1$) and (031), respectively. The interplanar spacing of (031), ($1\overline{1}1$) and (020) was correspondingly d₍₀₃₁₎ = 0.144 nm, $d_{\left(1\overline{1}1\right)}$ = 0.275 nm and d₍₀₂₀₎ = 0.137 nm. The lattice mismatch between TiNi₃ and TiNi was calculated to be 2.5 %, with low strain energy. The order of effective formation enthalpies for TiNi₃, TiNi and Ti₂Ni phases formed between titanium and nickel was $∆H{\prime }_{\mathrm{Ni}{\mathrm{Ti}}_2}$ > $∆H{\prime }_{\text{NiTi}}$ > $∆H{\prime }_{{\mathrm{Ni}}_3\mathrm{Ti}}$. The growth activation energy of TiNi₃, TiNi and Ti₂Ni phases was correspondingly 35.8 kJ/mol, 180.6 kJ/mol and 347.1 kJ/mol. When welding at 880 °C for 60 min, the highest shear strength of the joints could achieve 150 MPa. The joints fractured along the Ni/T2 interface, the fracture surface of joint was composed of elongated dimples and cellular pits, presenting a shear ductile fracture mode. FCC-Cu, FCC-Ni and (Ni, Cu)_ss phases were detected on TC4 and T2 fracture surfaces by XRD. The interdiffusion coefficient ratio of (Ni in Cu)/(Cu in Ni) and (Ni in Ti)/(Ti in Ni) decreased gradually with increasing temperature.