Microstructural evolution and mechanical properties of Zr-4 alloy joints diffusion bonded with Nb interlayer

To obtain a robust diffusion -bonded joint at a relatively low temperature, Nb foil was applied as an interlayer to the diffusion bonding of Zr-4 alloy, and a wide process parameter involving the bonding temperature of 720-820 degrees C and holding time of 30-120 min was investigated to reveal the evolution of interfacial structure and mechanical properties of the joints. The beta-(Zr, Nb) and Widmansta center dot dter microstructure formed at the interface induced by the eutectoid transformation of the diffused Zr and Nb at the bonding temperatures over 740 degrees C. The phase structure and the thickness of beta-(Zr, Nb) layer were not considerably influenced by the increase in diffusion parameters while the Widmansta center dot dter zone thickened significantly. Moreover, the tensile strength and elongation of the resultant joints could be stabilized above 433 MPa and 7.8%, respectively, when the bonding temperature exceeded 760 degrees C with a holding time of 30 min. By extending the holding time to 60 min at 760 degrees C, the tensile strength and elongation could reach 454 MPa and 12.6%, respectively, comparable to those obtained at an elevated temperature of 820 degrees C for 30 min. Additionally, the samples fractured at the diffusion layer were characterized by the in -situ tensile test under the scanning electron microscope, and it was discovered that the crack initiation and propagation process occurred mainly between the beta-(Zr, Nb) and Widmansta center dot dter diffusion zone.