Microstructure, residual stress and mechanical properties of Al2O3/Nb joints vacuum-brazed with two Ag-based active fillers

Vacuum brazing was applied to join Al2O3 ceramic and pure Nb with two active Ag-based fillers, Ag-Cu-Ti and Ag-Cu-Ti + Mo. The influences of the brazing parameter and the content of Mo particles on the microstructure and mechanical properties of the brazed joint were studied. The typical interfacial structure of Al2O3/Nb joint brazed with Ag-Cu-Ti filler metal at 900 °C for 10 min was Al2O3/Ti3(Cu,Al)3O/TiCu+Ti2Cu3+TiCu4+Ag(s,s)+Cu(s,s)/Nb. In the joints brazed with Mo particle-modified composite filler, the large blocky Ti-Cu compounds disappeared, and a large number of fine phases formed in the brazing seam. The dissolved Mo played a better role in refining the microstructure compared with residual Mo particles. In addition, the finite element method (FEM) calculated results showed that the residual stress peak value of Al2O3/Nb brazed joint decreased from 296 MPa to 253 MPa when 8 wt. % Mo particles were added in the composite filler. The shear strength of Al2O3/Nb brazed joints could reach up to 203 MPa at 900 °C for 10 min. The joining properties of the Al2O3/Nb brazed joint were primarily dependent on the dispersion degree of fine Ti-Cu compounds and the thickness of Ti3(Cu,Al)3O reaction layer.