Role of nanostructured Ni surface layer in enhanced Hastelloy alloy diffusion bonding at temperatures far beyond recrystallization

Enhanced Hastelloy alloy diffusion bonding was investigated via introducing nanostructured Ni surface layer. Because the bonding temperature was significantly higher than the nanostructured Ni destabilization temperature, rapidly complete recrystallization and dramatic grain growth occurred. The beneficial effect of high-density short-circuit defects provided by nanocrystalline on enhanced diffusion ought to be limited. However, significantly accelerated diffusion was still observed in the joint. The unique contribution of the nanostructured layer in evolving microstructure is to promote the formation of diffusion-induced recrystallization at the interface, where solute elements could diffuse rapidly along the moving boundaries at a rate several orders of magnitude larger than static grain boundaries. The retained ultrafast atomic diffusion paths result in a bonding temperature of 250 degrees C-350 degrees C lower than the conventional diffusion bonding approach. The tensile strength of the achieved joint reached 833 MPa, which was nearly equal to that of the as-delivered substrate.