Role of nanostructured surface layers in enhancing pure titanium diffusion bonding above their destabilization temperatures

The benefits of the nanostructured materials are believed to be probably absent if they are used at high temperatures. In this study, enhanced diffusion bonding of pure Ti was investigated by introducing a nanostructured surface layer through the surface mechanical attrition treatment (SMAT) technique. Complete recrystallization and remarkable grain growth have taken place in the original nanostructured surface layer due to the diffusion bonding conditions far beyond the nanostructured Ti destabilization temperatures. However, they still played a significant role in enhancing the Ti/Ti interfacial bonding. The consistency analysis of relative grain boundary energy, relaxation time, and microstrain reveals that although the original nanocrystalline has disappeared, the newly formed grain boundaries (GBs) during recrystallization probably inherit part of non-equilibrium state of the corresponding GBs in the as-SMATed surface layer, where the atomic diffusivity may increase greatly compared to the general relaxed GBs in their coarse-grained polycrystalline counterparts. The presence of these high-energy non-equilibrium GBs retains ultrafast atomic diffusion paths, leading to a diffusion bonding temperature of at least 100 °C lower than the traditional approach. The joint achieved at a low temperature of 750 °C exhibited a shear strength approximately 2 times higher than that prepared using raw substrates.