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Eutectic system


#1

Hi all,

There has recently been mention of eutectic systems in the
digest, and it seems to me that some confusion has occured about
these. So I’d like to give some explanation:
The whole thing has its origin from the fact that silver and
copper can only dissolve each other to a certain degree in
solid state
. Any given crystal with much silver does not
contain more than 8.8% copper, any crystal with much copper has
only 8% silver in it. These are maximum amounts at 779
Centigrade, at lower temperatures the ability to dissolve the
other metal decreases when allowed to cool slowly.
Well, with sterling, which contains 7.5% copper, there is no
problem at all (provided you quench), as the whole of the copper
can be integrated into the silver-rich crystals (aka
alpha-crystals). Solidus is well above 779 C, no eutectic
reaction, you only have the one kind of crystals with much
silver. This is the same with all copper-silver-alloys above 912
thousandths; with alloys below 80 thou silver, you get
copper-rich crystals (aka beta-crystals), and only those.
So what about alloys in between, say silver 800 thou? In the
liquid state, silver and copper mix readily (homogenous). When
the molten metal cools down, it will pass liquidus temperature,
and the forming of crystals starts (as explained by Peter).
These first crystals are of the silver-rich kind and so contain
much more than the 80% silver of the alloy as a whole. That
means, the still molten, liquid part goes down in silver
content, the more down the more crystals form and the larger
they get. These silver-rich crystals are rather big, as they
form at higher temperatures and have more time to grow. In the
meantime, temperature drops and reaches 779C. At this point, the
silver content of the still liquid part is 719 thousandths. The
liquid part solidifies. As solidifying temperature is the lowest
possible (far below the SPs of the pure metals), the number of
crystals now forming is very high, with little time to grow, so
these are very small. And, as the three phases, namely alpha-,
beta-crystals and liquid, are in a balance at this temperature,
both kinds of crystals form. So you get a very fine grained
structure of the eutectic alloy with both kinds of crystals
around comparably large alpha-crystals. This structure forms
with all silver-copper-alloys between 719 thou and 912 thou
silver content; between 80 thou and 719 thou silver, you have
large beta-crystals embedded in the fine grained structure of
the eutectic. This is the reason why all silver-copper alloys
between 80 thou and 912 thou have the same solidus temperature -
779 C. The 719 thou silver alloy is the eutectic alloy, has a
melting point (779 C) as opposed to other alloys with a
melting range, and solidifies always as a fine grained
structure of alpha- and beta-crystals. Gold-silver and
gold-copper systems don’t show this phenomenon, as they can
dissolve each other in any rate in solid state. In the 3-metal
system of gold-silver-copper, it plays a role only in alloys up
to about 400 thou gold content. I hope I didn’t bore you, but
gave some insight. If there are any questions, please ask, but
as I switched to digest mode (very busy now), you’d best e-mail
me directly. regards, Markus Ellermeier Am Wachth�gel Lindenstr. 5
D-56865 Walhausen