Diffusion bonding mechanisms of pure Zr with a Ti interlayer: Microstructural characterization and polycrystalline molecular dynamics simulations

In this study, pure Zr was diffusion-bonded with a pure Ti interlayer via spark plasma sintering at temperatures ranging from 560 to 710 degrees C at a pressure of 25 MPa. The effects of the bonding temperature on the mechanical properties and microstructure of diffusion-bonded joints were evaluated using microstructural characterization and molecular dynamics simulations. The mechanical properties of the joints first increased and then decreased at increasing bonding temperatures, and then reached a maximum tensile strength of 311 MPa at 660 degrees C, with a joint efficiency of 91.5 % compared with the base material. Fracture analysis of the tensile specimens indicated that the diffusion-bonded joints exhibited good mechanical properties, with the weakest region located at the interface between the base material and diffusion layer. Excessive bonding temperatures led to grain growth in the base material, whereas excessively low temperatures resulted in the formation of unbonded voids. When the bonding temperature exceeded the phase transition temperature, the strain energy induced by the phase transformation provided the driving force for recrystallization nucleation at the joint, achieving a strong metallurgical bond. Molecular dynamics simulations of polycrystalline Zr/Ti joints revealed that the beta-phase preferentially nucleated at grain boundaries with higher free energy. Grain boundaries serve as rapid pathways for atomic diffusion, the diffusion coefficient is positively correlated with temperature and Zr exhibited higher diffusion coefficients than Ti. This study elucidates the interface bonding mechanisms of Zr diffusion-bonded joints, thus providing theoretical guidelines for controlling the interplay between the microstructure and performance of the material.