Microstructure and mechanical properties in the solid-state diffusion bonding joints of Ni₃Al based superalloy

Directional solidification (DS) of intermetallic compounds Ni₃Al based superalloys were joined by solid-state diffusion bonding (SSDB) with the joining interface perpendicular to the solidified direction. The joining process was conducted with a 4 μm pure Ni interlayer at 1100 °C under 15 MPa for 2–7 h. The results showed that the microstructure characteristics of the SSDB joints, microvoids, tiny carbides, a higher volume fraction of γ′ phase and high angle grain boundaries (HAGB) formed in the interlayer region, were significantly different from that of the base metal (BM). The joint of the best mechanical property, strength of 926 MPa and elongation of 7.2%, was produced by the bonding time of 7 h owing to the decrease of the microvoids and the large carbides with the bonding time increasing. However, the relative (ratio between the joint and the BM) tensile strength of the joint was merely 90% due to the existence of high angle grain boundaries (HAGB) generated after the recrystallization of the interlayer. Moreover, because microvoids were not vanished completely and the strain mismatch caused by the structure of the interlayer region, the hard center region (higher volume fraction of γ′ phase) and soft interface region (lower volume fraction of γ′ phase), so the stress concentration were produced between the joint and the BM under the tensile load. Therefore, the relative elongation of the SSDB joint was only 57%.