Enhanced Strength and Toughness of SiC_f/SiC joints though Zr-induced Core-Shell Microstructure evolution in high entropy brazing fillers

This study utilizes (FeCoNiCrCu)_100-X Zr_X (X = 10, 15) high-entropy brazing alloys to enable reliable joining of SiC_f/SiC composites to Zr using a vacuum brazing furnace. Microscopic analysis shows that increasing the Zr content to 15 at% significantly improves the wettability of the brazing alloy and promotes the formation of a distinctive “ZrCrFe core–Zr₂Cu shell” microstructure. Compared with the ZrCrFe phase, the Zr₂Cu phase exhibits enhanced plastic deformation capacity. After encapsulating the brittle ZrCrFe phase, it promotes the formation of a region near the SiC_f/SiC interface dominated by plastic FCC phases and Zr₂Cu phases. The Zr₂Cu phase exhibits a coherent crystallographic orientation with the FCC matrix phase, indicating a relatively low lattice mismatch. The wrapped structure contributes to joint toughening through two primary mechanisms: (1) it prevents direct contact between the brittle ZrCrFe phase and the SiC_f/SiC composite, thereby mitigating crack initiation; and (2) it alleviates residual stress concentrations at the ZrCrFe–FCC interface caused by lattice mismatch, enhancing the overall plastic deformation capacity. Finally, the shear strength of the joint containing 15 at% Zr reached 78.61 MPa, representing a 22.4 % increase compared to the specimen containing 10 at% Zr, and indicating enhanced plastic deformation capacity and overall joint strength.