Simultaneous intermetallics suppression and residual-stress relaxation of heat-resistant Nb-interlayer-inserted Ti-6Al-4V/Si₃N₄ joints via one-step transient liquid phase bonding and brazing

This work has successfully proposed a solution to produce robust Nb-interlayer-inserted Ti-6Al-4V/Si₃N₄ joints optimized for a maximum operating temperature of 873 K; transient liquid phase bonding (TLPB) of Ti-6Al-4V/Nb side was carried out with Cu and Ni fillers to suppress brittle intermetallic compounds (IMCs), whereas brazing of Nb/Si₃N₄ side was performed using a highly ductile Ti-added Ag-rich filler for effective residual-stress relaxation. A sound yet simple one-step bonding process incorporating simultaneous TLPB and brazing was achieved with a relatively short holding time of 10 min at 1213 K. TLPB of Ti-6Al-4V/Nb side with Cu and Ni foils of 2-µm-thick each as a laminated filler suppressed brittle Ti-based IMCs and developed a homogenized microstructure consisting mainly of (α+β)-Ti via isothermal solidification. Meanwhile, brazing of Nb/Si₃N₄ side with 100-µm-thick SILVER-ABA filler (92.75Ag-5Cu-1Al-1.25Ti mass%) foil enhanced interfacial bonding with sufficient total Ti content and accommodated residual stress better than conventional eutectic Ag-Cu-based fillers, and it was verified by finite element analysis with consideration of materials’ temperature-dependent elasto-plastic properties. All joints with a bonding area of 10 mm×10 mm were tested via symmetrical four-point bending from room temperature (RT) to 873 K fractured from Nb/Si₃N₄ side. When re-heating the joints from RT to 673 K, fracture initiation gradually shifted from Si₃N₄ towards interfacial-compounds/Si₃N₄ interface and bending strengths maintained ∼220 MPa as weakening of SILVER-ABA filler was compensated by residual-stress relaxation in Si₃N₄. When tested at 873 K, joints fractured mainly across the Ag-rich solid solution in a ductile manner and bending strength degraded by ∼20% to 171 MPa as weakening of SILVER-ABA filler dominated.