Evolution of the interfacial phases in Al₂O₃–Kovar^® joints brazed using a Ag–Cu–Ti-based alloy

Abstract

A systematic investigation of the brazing of Al₂O₃ to Kovar^® (Fe–29Ni–17Co wt.%) using the active braze alloy (ABA) Ag–35.25Cu–1.75Ti wt.% has been undertaken to study the chemical reactions at the interfaces of the joints. The extent to which silica-based secondary phases in the Al₂O₃ participate in the reactions at the ABA/Al₂O₃ interface has been clarified. Another aspect of this work has been to determine the influence of various brazing parameters, such as the peak temperature, T_p, and time at T_p, τ, on the resultant microstructure. As a consequence, the microstructural evolution of the joints as a function of T_p and τ is discussed in some detail. The formation of a Fe₂Ti layer on the Kovar^® and its growth, along with adjacent Ni₃Ti particles in the ABA, dominate the microstructural developments at the ABA/Kovar^® interface. The presence of Kovar^® next to the ABA does not change the intrinsic chemical reactions occurring at the ABA/Al₂O₃ interface. However, the extent of these reactions is limited if the purity of the Al₂O₃ is high, and so it is necessary to have some silica-rich secondary phase in the Al₂O₃ to facilitate the formation of a Ti₃Cu₃O layer on the Al₂O₃. Breakdown of the Ti₃Cu₃O layer, together with fracture of the Fe₂Ti layer and separation of this layer from the Kovar^®, has been avoided by brazing at temperatures close to the liquidus temperature of the ABA for short periods of time, e.g., for T_p between 820 and 830 °C and τ between 2 and 8 min.

Keywords:

• Brazing
• joining
• alumina
• Al₂O₃
• Kovar
• Cusil ABA

Acknowledgements

We are grateful for the contribution and support for this study by AWE plc.