Dispersion, solid solution, and covalent bond coupled to strengthen K4169/TiAl composite brazed joints: first-principles and experimental perspective

Ni/TiAl composite brazed joints could significantly reduce the aircraft's weight. However, low interfacial adhesion, coarse and brittle-hard intermetallic compounds (IMCs) seriously limited the application of Ni/TiAl composite joints in the next generation of aerospace applications. So enhanced K4169/TiAl composite joints were investigated by vacuum brazed with (Ni_53.33Cr₂₀B_16.67Si₁₀/Zr₂₅Ti_18.75Ta_12.5Ni₂₅Cu_18.75) composite filler metal (CFM) designed based on cluster-plus-glue-atom model. The shear strength of the joint reached 485 MPa, comparable to the 491 MPa of TiAl substrate. The flat and brittle-hard diffusion reaction layer between Zones I and II was eliminated, simultaneously generating CrB₄ dispersion strengthening due to the CFM developed with the interfacial solid-liquid space-time hysteresis effect. In Zones II and III, IMCs all transformed into Ni_ss(Cr,Fe)_[0-88], Ni_ss(Ti, Al)_[004], and Ni_ss(Zr,Si)_[11-2] of circular and oval shapes through isothermal solidification. Meanwhile, the residual stresses and hardness were distributed in reticulated cladding characteristics. Thereby, lattice distortion led to solid solution strengthening and increased plastic toughness through crack termination and bridging mechanisms, which inhibited dislocations from plugging and crack propagation. Various interfaces in Zone Ⅳ were regulated into semi- and coherent interfaces. Ni₃(Ti,Al)/(Ni,Ti,Al) and (Ni,Ti,Al)/AlNi₂Ti were composed of higher interfacial bonding energy (2.771 J/m², 2.547 J/m²) and Ni-Ni covalent bonds. Interfacial covalent bonding and large interfacial bonding energy coupling strengthened Zone IV. Consequently, cracks initiated at the (Ni,Ti,Al)_[013]/Ti₃Al_[010] and expanded rapidly into TiAl substrate. Therefore, applying this method to design CFMs and regulate the phase, grain morphology, and interface's fine structure could provide new pathways for dissimilar hard-to-join metals.