Correlation between microstructure and mechanical properties of active brazed Invar/SiO2–BN joints

The active brazing of Invar to SiO2–BN ceramic was performed using an Ag–Cu/Ti filler metal. The effects of brazing temperature and Ti content on the microstructure and mechanical properties of Invar/SiO2–BN brazed joints were investigated. The results indicated that the typical microstructure of the joint was Invar alloy/small Fe2Ti+Ni3Ti blocks/Ag(s.s)+Cu(s.s)/wave-like Fe2Ti+Ni3Ti structure/Ag–Cu eutectic/TiN+TiB2 reaction layer/SiO2–BN. The amount of brittle Fe2Ti+Ni3Ti compounds increased and the TiN+TiB2 reaction layer became thick with the increase of brazing temperature as well as Ti content. This microstructure evolution clearly affected the shear strength of the joint. The highest shear strength was obtained when the joint was brazed at 880 °C for 10 min, with a Ti content of 4.5 wt%. When a Cu barrier foil was inserted between the Ag–Cu and Ag–Cu–Ti brazing foils, the formation of brittle Fe2Ti+Ni3Ti compounds was apparently inhibited, and the shear strength of the joint increased correspondingly.