Abstract: Diffusion bonding of titanium and its alloys typically requires high temperatures, which may lead to structural deformation, grain coarsening, and deterioration in the properties of the workpiece. In this study, we achieved the high-quality diffusion bonding of Ti at ultra-low temperatures by nanocrystallizing the surface of the Ti substrate and introducing pure Nb foil as an interlayer. The mating surfaces of the Nb foil were subsequently nanocrystallized to further enhance the Ti/Nb/Ti diffusion bonding at ultra-low temperatures. The effective interdiffusion coefficient, diffusion activation energy, and intrinsic diffusion coefficient of Nb in the surface-processed Ti were estimated by the Sauer-Freise and Kirkendall marker plane methods, and compared with their polycrystalline coarse-grained counterparts. It is demonstrated that surface nanocrystallization (SNC) extraordinarily improves the kinetics of Nb diffusion in nanostructured Ti and alters the diffusion behaviors of Ti and Nb. The increased atomic diffusion rate caused by SNC results in a bonding temperature of ∼150-200 °C lower than that in the conventional diffusion bonding approach. This may originate from the ultrafast atomic diffusion paths provided by the high volume fraction of nonequilibrium grain boundaries and abundant triple junctions in the nanocrystalline surface layer, and extremely high-density dislocations associated with the grain refinement process. In addition, fracture analysis shows that the SNC treatment changes the fracture path and fracture mode of the joints, leading to a significant increase in shear strength.

Key words: Diffusion bonding, Titanium, Niobium interlayer, Surface nanocrystallization, Diffusion coefficient