Effects of intermetallic phases on the bond strength of diffusion-bonded joints between titanium and 304 stainless steel using nickel interlayer

Solid-state diffusion bonding was used to join commercially pure titanium (Ti) and 304 stainless steel (SS) with a pure nickel (Ni) interlayer of 300-mu m thickness in the temperature range of 800 degrees C to 950 degrees C in steps of 50 degrees C for 7.2 ks under 3 MPa load in vacuum. Interfaces were characterized using light and scanning electron microscopy. The interdiffusion of the chemical species across the diffusion interfaces was evaluated by electron probe microanalysis. Nickel interlayer completely restricted the migration of Ti to SS up to 850 degrees C bonding temperature. However, at 900 degrees C and 950 degrees C processing temperatures, Ni interlayer could not restrict the migration of Ti to SS, and phase mixtures of lambda + chi + alpha-Fe, lambda + alpha-Fe, and lambda + FeTi + beta-Ti were formed at SS-Ni interface. The occurrence of different intermetallics was confirmed by X-ray diffraction technique. The maximum tensile strength of similar to 311 MPa and shear strength of similar to 236 MPa along with 9.1 pet elongation were obtained for the diffusion couple processed at 850 degrees C due to better contact between the mating surfaces and without formation of brittle Fe-Ti base intermetallics. With an increase in the bonding temperature to 900 degrees C and above, the bond strength drops due to the formation of brittle Fe-Ti base intermetallics in the diffusion zone. Observation of the fracture surface in a scanning electron microscope (SEM) using energy dispersive spectroscopy demonstrates that failure occurred through Ni-Ti interface of the joints when processed up to 850 degrees C and through SS-Ni interface when processed at 900 degrees C and above.