Microstructures and properties of Zr-4 alloy diffusion bonding joint with Nb interlayer

To improve the strength of Zr-4 alloy diffusion bonding joints and reduce the effective bonding temperature, different thicknesses of Nb interlayer were used for the Zr-4 alloy diffusion bonding at 760 °C/30 min/7 MPa. The effect of the Nb interlayer and its thickness variation on the microstructures and properties of the joints was investigated. During the diffusion bonding process, the diffusion layer was formed by the mutual diffusion of Zr and Nb, which was composed of (Zr, Nb) solid solution, and its thickness was constant with the increase of the Nb thickness. Secondary phases of Zr(Cr, Fe)₂ and Zr(Fe, Nb)₂ were observed in the diffusion layer near the Nb interlayer. The tensile strength of the joints was only 75 MPa at 760 ℃ and many unbonded areas existed. The tensile strength and elongation of the joints decreased slightly with the increase of Nb thickness, and reached the maximum of 450 MPa and 13.1%, respectively, with the 20 μm Nb interlayer. The fracture position was changed from the Zr-4 matrix (20 μm) to the diffusion layer (50 μm, 80 μm). After superheated steam corrosion of 400 ℃ and 10.3 MPa, obvious corrosion occurred at the diffusion layer of the Zr-4/Nb/Zr-4 joint. The maximum corrosion depth was 108.39 μm, and the tensile strength and elongation of the joints decreased to 415 MPa and 5.1%. The Zr(Fe, Nb)₂ phases were found on the fracture surface.