Interface microstructure evolution and bonding mechanism of low-temperature diffusion bonding of hydrogenated Zr-4 alloy

Abstract

Zr-4 alloy is widely used as fuel cladding in the nuclear industry. And welding is one of the most significant procedures during the manufacturing of fuel assembly. Bonding at high temperature risks safe operation of nuclear reactors because high temperature deteriorates the properties of Zr-4 alloy. Herein, Zr-4 alloy was hydrogenated to a hydrogen content of 0.2 wt.% before diffusion bonding. Direct diffusion bonding of hydrogenated Zr-4 alloy was performed at various bonding temperatures. The microstructural evolution of the bonding interface and the interface bonding mechanism were systematically investigated. The bonding temperature of the hydrogenated Zr-4 alloy was reduced by 150 °C compared with the corresponding nonhydrogenated alloy. Furthermore, the shear strength of the joints was as high as 257 MPa (4.6 × higher than corresponding joints comprised of the nonhydrogenated alloy) at a bonding temperature of 700 °C. Transmission electron microscopy and electron backscatter diffraction indicate that plastic deformation of the joints increased because of a phase transition of the hydrides (to the β-Zr phase) during bonding, leading to high-density dislocations that accumulated at the bonding interface. The migration of the interface grain boundary was facilitated by high-density dislocations of sufficiently high energy. The bonding interface was no longer evident, enabled by dynamic recrystallization induced by migration of the interface grain boundary. Furthermore, the fracture mode of the joints was initially brittle but became ductile/brittle mixed mode with increasing temperature.

Keywords

• Zirconium alloy
• Hydrogenation
• Diffusion bonding
• Microstructure
• Mechanical property