Microstructure and stress-rupture property of TLP diffusion bonded DD3 single crystal superalloy joints

DD3 is a first generation nickel-based single crystal superalloy developed in China. In order to meet the possible need of manufacturing turbine blades and vanes, DD3 was TLP diffusion bonded with D1P powder interlayer alloy and the investigation on joint microstructure and properties was conducted. The results showed that, the bonding seam was mainly composed of γ+γ' two-phase matrix, dendritic compound was identified as (Mo, W, Cr, Ni) ₃B ₂ and strip-like compound was identified as (Cr, Mo) ₂₃(C, B) ₆. Some blocky or needle-like compounds were precipitated in the zone adjacent to the bonding seam, and the TEM analysis results indicated that these compounds were identified as (Mo, W, Cr, Ni) ₃B ₂ borides. Microstructural inhomogeneity existed in the joint along the flowing direction of molten interlayer alloy, and this inhomogeneity could not be eliminated even by the diffusion bonding for 24 h at elevated-temperature of 1250°C. There are some γ' phases in the form of large block. The boundaries between these large γ' phases and the bonding seam matrix became the weak part of the joint, which damaged the joint property. Under proper bonding condition, the stress-rupture strength at 980°C of the TLP diffusion bonded DD3 alloy joints reached 90% of that of the base metal.