Microstructure evolution and strain partitioning of C_f/C-Nb brazing joints mediated by cold-sprayed Ti-Co interlayer

The partitioning of strain and stress among constituent microstructures fundamentally determines the mechanical performance of dissimilar material brazed joints. However, the pronounced thermal expansion mismatch (△α ≈ 6.3 × 10⁻⁶/K) between C_f/C and Nb typically triggers severe interfacial stress concentration. This mismatch, coupled with the formation of brittle interfacial reaction products, severely impairs the strain accommodation within the joint. Here, a cold-sprayed Ti-Co filler is employed to braze C_f/C to Nb, spontaneously forming a gradient structure comprising a continuous β-Ti(Nb, Co) band adjacent to Nb, a (β + α + Ti₂Co) multiphase network in the brazing seam, and a thin TiC reaction layer at the C_f/C interface. Continuous modulus-hardness transitions across the gradient layer eliminate interfacial stress singularities. Ti₂Co accommodates strain via geometrically necessary dislocations, while ductile β-Ti undergoes plastic deformation with efficient load transfer through the interpenetrating network. The coherent Ti₂Co/α-Ti interface, with only 1.34% lattice mismatch, enables elastic self-accommodation without microcrack initiation. This multi-scale strain partitioning strategy yielded a peak shear strength of 36.0 MPa in C_f/C-Nb joints, providing a robust paradigm for joining dissimilar materials with high CTE mismatches.