Microstructure, characterization of interfacial phases and mechanical properties of high Nb-TiAl/Al2O3 joints brazed by novel Nb particle-reinforced Ag-Cu filler alloy

Abstract High niobium content TiAl alloys and Al2O3 ceramics were successfully joined using novel Nb particle-reinforced Ag-Cu composite filler. The changes in microstructure brazing at different temperatures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The corresponding mechanical properties were studied by shear tests. The results showed that the brazed joints prepared at 900°C can be divided into three zones. The typical microstructure of the brazed joints is high niobium content TiAl/AlCuTi+AlCu2Ti/Ag(s,s)+Cu(s,s)+Nb/Ti3(Cu,Al)3O/Al2O3. The brazing temperature was shown to have a significant influence on the microstructure and reaction layer thickness. After brazing at 810°C, macrodefects could be observed in the brazing seam. At higher brazing temperatures, brazed joints without any defects were obtained. The addition of Nb particles and the suitable thickness of the Ti3(Cu,Al)3O reaction layer are the main reasons for the high shear strength. When brazing at temperatures higher than 900°C, the volume fraction of the brittle AlCu(Ti, Nb) phase was increased and a thick Ti3(Cu,Al)3O reaction layer was formed, which deteriorated the shear strength of the joint. The maximal shear strength joint reached 228 MPa at 870°C for 10 min, which is a substantial increase compared with conventional brazing processes.