Microstructure evolution and mechanical properties of powder metallurgy CuCrNb/304 SS brazed joints enabled by multi-level heterostructure strengthening

High-performance powder metallurgy materials are ideal for fabricating complex aerospace components, owing to their near-net-shape forming and exceptional mechanical properties. This work first systematically investigated the vacuum brazing of powder metallurgy CuCrNb (PM-CCN) alloy and 304 stainless steel (304SS) with AgCuIn filler metal. It focused on the effect of modulating brazing temperatures from 750 to 780 °C on the microstructure and mechanical properties of the brazed joints. Joints formed at 750 to 770 °C exhibited a multi-level heterostructure, which includes grain size gradient and a soft-hard-soft structure of phase hardness and modulus, consisting of coarse-grained Ag-Cu eutectic zone, (Cr, Fe)₂Nb-strengthened reaction layers and fine-grained filamentary penetration zone, while excessive base metal dissolution and shrunken brazing seams occurred at 780 °C. The shear and tensile strengths first increased and then decreased with temperature, reaching maximum values of 227.04 MPa and 205.44 MPa at 760 °C, respectively. The superior performance was attributed to hetero-deformation induced (HDI) strengthening, as well as solid-solution and dispersion strengthening of (Cr, Fe)₂Nb. Microstructural finite element method (micro-FEM) revealed high stress gradients around heterogeneous interfaces and hard (Cr, Fe)₂Nb particles. This study supports the brazing of powder metallurgy materials and the fabrication of aerospace composite components.