BNi2-barrier regulated microstructural evolution and strengthening mechanism of powder metallurgy CuCrNb/304SS brazed joints

Precise control of interfacial metallurgy is a critical challenge in brazing powder metallurgy CuCrNb (PM-CCN) for aerospace applications. An interfacial engineering strategy using AgCu filler with an amorphous BNi2 barrier layer was proposed. The effects of holding time (10–25 min) at 940 °C on microstructural evolution and mechanical properties of PM-CCN/304 stainless steel (304SS) joints were systematically investigated. Results indicated that direct brazing led to full penetration of the AgCu filler into the PM-CCN matrix, forming an uncontrolled mixed zone with a maximum depth of 313.4 μm at 25 min. Conversely, the BNi2 barrier induced an in-situ composite layer consisting of γ-Ni and CrB intermetallic compounds (IMCs), accompanied by a Ni-Cu solid-solution layer. This reaction-derived architecture effectively suppressed excessive filler diffusion and stabilized the seam width within 160.7–163.8 μm. Consequently, the BNi2-added joint achieved a peak shear strength of 233.14 MPa at 10 min, representing a 28.8% increase over the direct-brazed counterpart. Prolonging the holding time to 25 min, however, triggered barrier disintegration due to excessive Ni diffusion, shifting the fracture path from the ductile Ag-Cu seam back to the disordered penetration zone. This fracture mode transition, driven by the loss of barrier integrity, significantly degraded the mechanical properties. Overall, this study clarifies the underlying regulation mechanism and provides a theoretical foundation for high-reliability aerospace brazing.