Microstructure evolution of the Zr-4 alloy joints diffusion bonded with pure titanium interlayer and its influence on joint properties

Abstract

Zirconium-based alloy is widely applied as the fuel cladding material. Its weld needs high precision and small deformation, so the low-temperature bonding is a crucial issue to be addressed. In this study, we used pure titanium interlayer to join Zr-4 alloys by diffusion bonding at low temperature. A sound joint with pure titanium interlayer was bonded at 680 °C, which was 70 °C lower than that of the direct bonding. The effect of welding parameters on the microstructure and phase composition was investigated by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), and transmission electron microscope (TEM). Four solid solutions, (hexagonal close-packed) hcp-Ti0.92Zr0.08, hcp-Ti0.7Zr0.3, hcp-Ti0.5Zr0.5, and (body-centered cubic) bcc-Ti0.5Zr0.5, were identified forming at the Ti/Zr interface by EDS and TEM analysis. The nanohardness and Young’s modulus were measured by nanoindentation. The highest and lowest nanohardness values across the joint were 5.65 GPa for the hcp-Ti0.7Zr0.3 phase, and 4.09 GPa for the bcc-Ti0.5Zr0.5 phase, respectively. The fracture initiated in the bcc-Ti0.5Zr0.5 phase, prolonged at the Ti/Zr interface because of incoordination in the deformation process of two phases, and then the transverse tearing occurred in the hcp layer. The shear strength initially increased and then decreased as the increasing parameters. The maximum shear strength reached 255 MPa of the joint bonded at 750 °C for 60 min under 15 MPa. Besides, the formation mechanism of the Ti/Zr interface was revealed for the Zr-4 alloy joint with pure titanium interlayer.