Microstructure evolution and mechanical behavior of Ni-based single crystal superalloy joint brazed with mixed powder at elevated temperature

Brazing of a Ni-based single crystal superalloy has been investigated with the additive Ni-based superalloy and filler Ni-Cr-W-B alloy at 1260 degrees C, and attentions were paid to the microstructure evolution during brazing and the stress-rupture behavior at 980 degrees C of such brazed joints after homogenization. Microstructure in the brazed joint generally includes brazing alloy zone (BAZ), isothermally solidified zone (ISZ) and diffusion affected zone (DAZ). Microstructure evolution during this brazing process is discussed at the heating stage, the holding stage and the cooling stage respectively, according to the diffusion path of B atoms. Initially well-distributed gamma/gamma' microstructure in the homogenized bonded zone after heat treatment and substantial gamma' rafts enhance the post-brazed joint to obtain a stress-rupture lifetime of more than 120 h at 980 degrees C/250 MPa. On the other hand, the decreased stress-rupture behavior of post-brazed joint, compared with parenting material, is ascribed to the presence of inside brazing porosity and stray grain boundary, which not only reduces the effective loading-carrying area but also offers preferential sites for creep vacancy aggregation to further soften stray grain boundary. And finally an early fracture of these post-brazed joints through the intergranular microholes aggregation and growth mode under this testing condition was observed. (C) 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.