Structure and properties of diffusion bonded transition joints between commercially pure titanium and type 304 stainless steel using a nickel interlayer

The solid-state diffusion bonding was carried out between commercially pure titanium and Type 304 stainless steel using nickel as an interlayer in the temperature range of 800-900 degrees C for 9 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in the optical and scanning electron microscopes. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis. TiNi3, TiNi and Ti2Ni are formed at the nickel-titanium (Ni-Ti) interface; however, the stainless steel-nickel (SS-Ni) diffusion interface is free from intermetallic compounds up to 850 degrees C temperature. At 900 degrees C, the Ni-Ti interface exhibits the presence of alpha-beta Ti discrete islands in the matrix of Ti2Ni and lambda + chi + alpha-Fe, lambda + FeTi and lambda + FeTi + beta-Ti phase mixtures occur at the SS-Ni interface. The occurrence of different intermetallics are confirmed by the x-ray diffraction technique. The maximum tensile strength of -276 MPa and shear strength of similar to 209 MPa along with 7.3% elongation were obtained for the diffusion couple processed at 850 degrees C. At the 900 degrees C joining temperature, the formation of Fe-Ti base intermetallics reduces the bond strength. Evaluation of the fracture surfaces using scanning electron microscopy and energy dispersive spectroscopy demonstrates that failure takes place through Ni-Ti interface up to 850 degrees C and through the SS-Ni interface of the joint when processed at 900 degrees C.