Microstructure and mechanical properties of the C/C composite and Nb joints brazed with an active Ag-based filler metal

An active Ag-based filler metal, containing trace alloy elements of Al, Cu and Ti, was successfully applied to braze C/C composite and Nb. The microstructure and formation mechanism of the C/C composite and Nb brazed joint were investigated in this study. Moreover, the influence of brazing parameters on microstructural evolution and mechanical properties of brazed joints was evaluated. The typical interfacial microstructure of the joint obtained at 950 °C for 600 s was C/C/TiC + AlCu₂Ti/Ag(s, s) + AlCu₂Ti + particle Cu/AlCu₂Ti + AlCuTi + (Ti, Nb)₃Al + Nb(Ti)/Nb. The dispersive AlCu₂Ti phase was uniformly distributed in the Ag matrix, which was a beneficial structure for the brazed joint. The shear strength of the brazed joint was sensitive to the brazing temperature and holding time, which was closely related to the TiC layer bordering the C/C composite. The thickness of the TiC layer first increased as temperature increased to 950 °C, and then decreased when temperature reached 970 °C. The carbon fiber eroded by the filler at 970 °C entered to the brazing seam and reacted with Ti, resulting the reduction of the thickness of TiC, thus damaging the strength of the joint. With extension of holding time from 300 s to 1200 s, the interface reaction became more sufficient. Therefore, the thickness of the TiC layer increased. However, at 1200 s, the over-thick TiC broke the C/C substrate because of the mismatch of coefficient of thermal expansions. The maximum shear strength of the joint (950 °C/600 s) reached 55 MPa at room temperature and 35 MPa tested at 550 °C.