Microstructure and mechanical properties of Al2O3/TiAl joints brazed with B powders reinforced Ag-Cu-Ti based composite fillers

In this manuscript, Al2O3 ceramic and TiAl alloy were brazed with Ag-Cu-Ti based composite fillers containing B powder additive. The effects of B content, Ti content, and the brazing temperature on the microstructure and mechanical properties of the brazed joints were investigated. The typical interfacial microstructure of the brazed joint was: Al2O3/Ti3(Cu, Al)3O/Ag(s.s)+TiB+Ti(Cu, Al)+fine AlCu2Ti/blocky AlCu2Ti+AlCuTi/TiAl. TiB whiskers synthesized in situ by the reaction of the B additive with Ti, served as nucleation sites for the formation of fine Ti(Cu, Al) and AlCu2Ti grains and improved the bonding properties. With increasing B content, more fine-grains were formed in the brazing seam and the thickness of the Ti3(Cu, Al)3O reaction layer adjacent to the Al2O3 ceramic decreased. Brazing parameters also influenced the joint microstructure. Growth of Ti3(Cu, Al)3O layer and AlCu2Ti formed with increase in brazing temperature. Moreover, the formation of dispersed fine-grains in the brazing seam depended largely on the initial Ti content in the composite filler. With increase in initial Ti content, the TiB-reinforced area was broadened while the blocky AlCu2Ti content decreased gradually. The maximum shear strength of Al2O3/TiAl joint brazed with B-reinforced composite filler was 148 MPa, which was 246% higher than that brazed without B addition. The joint shear strength was determined by the combined influences of the thermal expansion mismatch between the joined materials, the thickness of the Ti3(Cu, Al)3O layer, and the presence of defects in the joints.