Semi-solid brazing via CoCrFeNiCu_x high-entropy alloy fillers: interfacial microstructure, mechanical properties, and joining mechanism of C/C-SiC composites and TZM alloy

High-entropy alloys have shown great application prospects in the field of high-temperature brazing due to their excellent properties. However, due to their excessively high melting temperature and poor metallurgical compatibility, achieving high-temperature, highly reliable connections between ceramic matrix composites and refractory alloys still faces significant challenges. This study proposes a semi-solid brazing strategy, which uses CoCrFeNiCu_x high-entropy alloy filler to connect C/C-SiC composites with TZM (titanium-zirconium-molybdenum alloy). This filler exhibits a dual endothermic melting behavior, forming an adjustable semi-solid window, which not only ensures sufficient fluidity but also suppresses excessive interfacial reactions. During the brazing process, the Cu-rich liquid phase promotes the formation of the initial interface. On the alloy side, the face-centered cubic (FCC) phase replaces the Cu liquid phase, generating a continuous µ-Co₇Mo₆ layer. On the composite material side, the elements dissolved in the Cu-rich liquid phase react with the composite material to form the M₇C₃ reaction layer, while Cu reacts with the SiC matrix to form the decomposition zone. As the temperature and holding time increase, the reaction intensity increases, and the M₇C₃ reaction layer and decomposition zone thicken. The optimized joint (1180 °C, 15 min) achieved a high shear strength of 40.2 MPa and maintained its integrity at 800 °C, demonstrating an anomalous strengthening effect due to stress relaxation. The finite element simulation results indicate that joint fracture primarily occurs at the Cu(s, s)/M₇C₃ boundary and is mainly influenced by the normal stress (S₂₂), consistent with experimental observations. This study provides a mechanistic framework for the relationship between the evolution of semi-solid HEA phases, interface design, and strength retention, and offers new ideas for the development of connection technologies between ceramic matrix composites and refractory alloys.