Melting - Understanding a freeze after a eutectic point

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snoman701

Well-known member
Joined
Oct 8, 2016
Messages
2,108
Location
SE MI
I've been spending the last couple of days induction melting an unknown, heterogeneous mixture of shot. They ultimately melt together to form an alloy that runs around the following numbers:

70-75% Cu
5-10% Zn
5-10% Ag
5-10% Au
5-10% Pd
and other base metals.

Basically, the Au, Pd and Zn bounce around quite a bit, especially the zinc as it readily burns off depending on how long I'm taking to pour.

I experienced yesterday this weird phenomenon where I could have a perfectly fluid melt, all I can feel in the crucible is the little tiny explosions from the zinc flaring up. Usually pulled perfect looking pins.
Even had it once be good and fluid on final stir, then by the time I got the tube around, it was hard.
However, by the time I pull and inspect the pin, the alloy has frozen up in the crucible. It freezes in such a way that it's like a metal slush under the flux. I can still move it around, but I'd describe it as almost rheopectic.

The only way to exit this phase is actually to get the melt white hot, then it will liquify again.

Now, I did a quick series of googles and found an article put out by Johnson Matthey that describes a terniary braze alloy of Pd-Ag-Cu at various percentages of Pd. They describe a narrow melting range of around 50 deg C, and say that beyond the 50 degree range, it then "freezes". What they don't show is a phase diagram, or any description of the "freeze".

What I am lacking is an understanding of what the phase diagram looks like, of even a binary phase, that has a freeze, at a temp above it's melting temp...and what phase the actual freeze is. Is it liquidus plus a secondary phase, or is it entirely it's own phase? Basically, I'm not used to there being anything "above" the liquid phase, at least until the gaseous phase.
 
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