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Need help with Kovar and Cu/Au alloys

I have a metallized ceramic that is brazed to a Kovar sleeve with a 35% Au/65% Cu alloy. We recently noticed in a section that the Kovar was diffusing into the Cu/Au braze creating a brittle joint. We join these components be firing to 1886 degrees F. We know that we did not over fire because not all of the pieces from the furnace run came out this way. We also know that our furnace doesn't vary this much because we duplicated this diffusion phenomena by heating to 2036 degrees F. I was wondering if there was any other way, except over heating to get Kovar to change phases and start diffusing into the Cu/Au braze? I know the 2 key variables in phase changes is atomic mass (concentration) and temperature. Could the braze alloy or Kovar have different percentages of material in them then they are supposed to?

Tim Rohr
- New Bedford, Massachusetts
2002

Just wanted to add that there is exaggerated grain growth in the ceramic and especially the Kovar. This makes me believe that the whole ceramic to metal assembly was either held at a high temp for quite a while or fired too high. These are brazed in a vacuum by the way. I just can't figure out how these few assemblies could have saw this heat or temp of a long hold time.

Tim Rohr
- Massachusetts
2002