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Eutectic Alloys


#1

Was: Casting with bezels in place

Ok the misuse of the word eutectic is one of my pet peeves. It seems
to get inserted into discussions of melting regularly to signify
some mystical property. So here is the dictionary definition

eutectic |adjective

relating to or denoting a mixture of substances (in fixed
proportions) that melts and solidifies at a single temperature
that is lower than the melting points of the separate
constituents or of any other mixture of them.

So now on to the metallurgy. Alloys normally have a melting range as
opposed to pure metals that have a distinct melting point. The
exception to this is when an alloy system has a eutectic point, not
all of them do. A eutectic alloy transitions from solid to liquid at
a precise single point just like a pure metal. The one that we
typically deal with as metalsmiths and jewelers is the silver-copper
(72Ag-28Cu) eutectic that melts at 1435 F. Gold-Silver and
Gold-Copper do not have eutectic alloys. The gold-silver-copper
system does not have a eutectic alloy either but it is influenced by
the silver copper eutectic to some degree in lower gold
concentration (10k) alloys.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#2
Ok the misuse of the word eutectic is one of my pet peeves. It
seems to get inserted into discussions of melting regularly to
signify some mystical property. 

Ok, James, I apologize and stand corrected, being the one who did
the dastardly deed of inserting the word “eutectic” into the
conversation on melting bezels. Coming from an enamelist’s
perspective, and knowing how cloisonne wires suddenly melt in a
flash if they sink deep enough to reach the copper (proper use of
the word “eutectic” effect), or knowing how a light plating of
copper helps the fusing in silver granulation, it seemed logical to
me that the copper in the 14K gold could possibly have caused the
same effect. It was an erroneous leap of logic. I did speculate that
it “could” be the cause, and others wrote also to say it was not,
and explained why, given the melting temperatures of 14K and silver.
Yet others picked up and ran with the word “eutectic”. This is
what’s so wonderful about this forum, that these things can be
discussed, and those with greater knowledge on a particular subject
can weigh in. Makes for a good learning experience all around.

Linda Gebert
http://homepage.mac.com/lgebertsilverjewelry


#3
Gold-Silver and Gold-Copper do not have eutectic alloys. The
gold-silver-copper system does not have a eutectic alloy either but
it is influenced by the silver copper eutectic to some degree in
lower gold concentration (10k) alloys. 

Thank you for bringing up the definition of eutectic soldering.
According to Erhard Brepohl (The Theory & Practice of Goldsmithing,
p.320), fine gold-copper is a eutectic alloy. Are you saying that
something happens to connect the metal parts but the interaction
does not meet the strict definition of eutectic? That is, with
gold-copper are you just soldering with copper? Oppi Untract (Jewelry
Concepts and Technology, p. 348) calls this interaction “fusion weld
bonding” where metals are joined at a molten state at their
contacting surfaces and where some interpenetration of atoms occur
(i.e., bi-metal). Is that more accurate?

Please elaborate. Thanks.

Nancy
www.psi-design.com


#4
where some interpenetration of atoms occur 

Probably not relevant, but I used to produce short-lived prototype
circuit boards with an electroless plating of gold on the contact
fingers. This (very thin) coating of gold would disappear over a few
months, letting the copper corrode. Our metallurgical lab told me
that the gold actually dissolved in the copper. Permanent contacts
needed a layer of nickel between the copper and gold to prevent this.

Al Balmer
Sun City, AZ


#5
Thank you for bringing up the definition of eutectic soldering.
According to Erhard Brepohl (The Theory & Practice of
Goldsmithing, p.320), fine gold-copper is a eutectic alloy. Are you
saying that something happens to connect the metal parts but the
interaction does not meet the strict definition of eutectic? That
is, with gold-copper are you just soldering with copper? Oppi
Untract (Jewelry Concepts and Technology, p. 348) calls this
interaction "fusion weld bonding" where metals are joined at a
molten state at their contacting surfaces and where some
interpenetration of atoms occur (i.e., bi-metal). Is that more
accurate?

With all due respect to both Brephol and Untract the process is
"transient liquid phase bonding". By adding copper to the joint you
are setting up for a localized reduction in melting point of the
gold alloy. The high copper concentration depresses the melting point
but once it melts the copper rapidly diffuses into the bulk gold
alloy and the melting point rises back up to near the same
temperature as the original bulk alloy hence the transient part of
the name. Gold and copper atoms are virtually the same size and
easily substitute for one another in the crystal matrix so at the
very high temperature present when granulating you will see the
copper move rapidly away from the joint and the metal will then
solidify. The process does not depend on a eutectic reaction, only
the depression of the melting point by the high local concentration
of copper. In transient liquid phase bonding depending on the
elements present in the alloy you could have eutectic, or peritectic
reaction or neither one. You would need to examine the grain
structure at the joint to be certain as eutectic alloys solidify with
a lamellar grain structure rather than dendritic ones. But given that
you have a high karat gold alloy with some small amount of copper and
silver present in the alloy you will not be seeing a eutectic
reaction. There are three or four metal eutectic alloys but not these
three. Now if you were to granulate fine silver you would most
certainly have some eutectic alloy present briefly but again the
copper would diffuse rapidly to such a concentration where you would
no longer have the Ag-Cu eutectic present. So the best name for this
process is still transient liquid phase bonding even though it is a
more cumbersome name than eutectic bonding :slight_smile:

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550