Titanium scavenging in Ag-Cu-Ti active braze joints

Poor hermeticity was observed for Al2O3-Al2O3 braze joints using a Fe-29Ni-17Co alloy spacer and Ag-Cu-Ti active filler metal. Titanium was scavenged from the filler metal by formation of a (Fe, Ni, Co)-Ti lacework phase. The scavenging mechanism was further studied using braze joints made with elemental Fe, Ni, or Co spacers. The Fe spacer caused development of FexTiy phases at its interface but allowed a significant TixOy layer to form at the Ag-Cu-Ti/Al2O3 interface, resulting in 100% hermetic joints. The Co spacer caused a lacework phase and only intermittent TixOy reaction layer at the Ag-Cu-Ti/Al2O3 interface; 75% of the buttons were hermetic. The Ni spacer caused extensive scavenging of Ti, resulting in an absence of a TixOy layer at the Ag-Cu-Ti/Al2O3 interface and no hermetic joints. Braze joint mechanical strength correlated primarily with the presence or absence of the TixOy layer. The Fe, Ni, and Co spacer experiments suggested that lacework phase formation in Fe-29Ni-17Co/Ag-Cu-Ti joints was primarily a result of scavenging by the Ni and Co components of the spacer material. Mechanical strength behavior and poor hermeticity were commensurate with the absence of a TixOy reaction layer at the Ag-Cu-Ti/Al2O3 interface. An Fe or inactive metal barrier layer would provide a means to curtail the scavenging reaction.