Joining of TiAl intermetallic Compound/CoCrFeMnNi high-entropy alloy dissimilar metals by vacuum brazing process: Microstructure, mechanical properties and processing parameters

TiAl intermetallic and CoCrFeMnNi high-entropy alloy were successfully joined by vacuum brazing using BNi-2 filler metal. The effects of brazing temperature and holding time on the interfacial microstructure, elemental diffusion, and mechanical properties were investigated, and the bonding mechanism was clarified. The brazed joints mainly consisted of solid-state diffusion reaction layers (Zones I, II, and IV) and a residual filler zone (Zone III). Zone I was mainly composed of α₂-Ti₃Al and TiNiAl. An amorphous TiNi₂Al phase formed in Zone II, while Ni(Al,Ti), Ti-rich phase, CrB phase, and Ni(Cr,Co,Fe,Mn) were found in Zone III, and Cr-rich compounds formed in Zone IV. With increasing brazing temperature and holding time, Ni atoms diffused rapidly toward the TiAl side and accumulated in Zones I and II, leading to gradual thickening of the diffusion layer on the TiAl side and thinning on the CoCrFeMnNi side. Increasing holding time enhanced interfacial diffusion, promoted the growth of reaction regions I and II, and facilitated the transformation of LAGBs into HAGBs, thereby contributing to improved metallurgical bonding. The maximum joint shear strength of 192.4 MPa was achieved at 1050°C for 30 min, which is attributed to dislocation strengthening and complete interfacial diffusion bonding. Excessively high brazing temperatures or prolonged holding times increased residual thermal stresses and induced coarse cracks in the joints, resulting in a significant deterioration of mechanical properties. The aggregation of brittle phases, especially in regions I and II, promoted premature failure and crack initiation in these regions.