Ag–Al based air braze for high temperature electrochemical devices

Silver–aluminum based air brazing was attempted using an in situ alloying and brazing process. In this process, layers of foils of aluminum and silver were laid up between alumina plates in alternating fashion to achieve three target compositions representing Ag, Ag3Al, and Ag2Al phases. Each alloy composition revealed different microstructure, mechanical properties and fracture mechanisms. Joints brazed with foils containing 9.8 at% Al formed a long continuous layer parallel to the direction of the original aluminum foil. The fracture occurred at low bend strength (6–12 MPa) mainly through the interface between this newly formed long alumina layer and the braze filler. Joints containing 26.5 at% Al in the braze filler metal experienced the series of phase transformations, leading to cracks in as-brazed specimens. The fracture initiated through these pre-existing cracks, thus the joint strength observed in these specimens was extremely low. The joints prepared using foils with 35.1 at% Al exhibited a good interface even though interfacial alumina particles formed during air brazing. Crack propagation occurred along the interface between the alumina substrate and in situ formed interfacial alumina particles or directly through these particles. Due to the good interface, the best bend strength (46–52 MPa) was achieved for the braze filler containing 35 at% Al.