This paper will chronicle the “evolution” of a practical Ag-1Cu-2Zr active braze filler metal for use in hermetic Kovar/Alumina braze joints.  The alloy possesses the following important attributes relevant to metal/ceramic brazing:  (a) it forms hermetic braze joints between 94% alumina ceramic and the Fe-29Ni-17Co controlled expansion alloy (known commercially as Kovar™),  (b) ASTM F19 tensile button strength for such braze joints are generally quite high, along with a large number of bulk ceramic fractures, and (c) high temperature mechanical properties of this braze alloy indicate excellent capacity for relaxation of mismatch strains in Kovar/alumina ceramic braze joints.
 
Because the alloy “evolved” - as opposed to being “developed” - it is useful to discuss two intermediate locations on the roadmap which led us to this alloy.  The first stop was a 70Ag-28Cu-2Zr alloy, which we named “Zrcusil”.  Although some hermetic braze joints were made with this alloy, it has a wide melt gap which limits it utility for many types of metal/ceramic brazing.
 
The second stop on this journey was a binary alloy with composition 98Ag-2Zr.  This alloy is apparently a binary eutectic alloy, with a eutectic between essentially pure Ag (with negligible Zr in solution) and the intermetallic compound AgZr.  Tensile buttons brazed with this alloy – primarily in dry hydrogen atmosphere – were found to have excellent strength, a high yield of hermetic joints, and a significant percentage of bulk ceramic fractures.
 
Finally, we arrived at the 97Ag-1Cu-2Zr composition for the following reasons:  (a) it has mechanical properties only slightly stronger than those of 98Ag-2Zr, (b) we evaluated the effect of Cu additions on the melting point of the alloy, and found that a 1% Cu addition results in a solidus/liquidus gap of about 20°C (940 and 960°C, respectively), (c) the ternary alloy appears to be free of the Cu₄AgZr intermetallic phase, which has been shown to have a deleterious influence on hermetic brazing yields.  The specific process conditions needed for successful braze joints with this alloy will be discussed.
 
 
* Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin  Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.