Effects of brazing temperature on microstructure and mechanical performance of Al2O3/AgCuTi/Fe–Ni–Co brazed joints

Abstract Al2O3/Fe–Ni–Co joints are achieved using Ag–Cu–8Ti filler alloy, and the dependence of the joint microstructure and mechanical performance on the brazing temperature has been studied by means of SEM, EDS, XRD and tensile test. The results show that the brazing seam is composed of TiO, Ti3Al, Ag (s, s), Cu (s, s), (Cu, Ni) and Ni4Ti3 phases. A layer of Ti3Al and TiO products is observed at the Al2O3/AgCuTi interface and the fracture testing indicates that the thickness of the reaction layer plays a critical role in the joint strength. The joint strength firstly increases and then declines with the thickness of the (Ti3Al + TiO) layer increasing, and the formation of the cracks is ascribed to the existence of Ti3Al phase. The thermokinetic analysis for the interfacial reaction between Al2O3 and AgCuTi show that the Gibbs free energy equals −88.939 kJ/mol for forming Ti3Al and TiO phases, and the growth rate of the reaction layer mainly depends on the diffusion rate of Ti across the formed reaction layer. Meanwhile, the quantitative relationship among brazing temperature, holding time and reaction layer thickness has been established.