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Re: alloys

Dr P Barker says:

In materials science you can get an effect of
cocrystallization where instead of getting a random
"solid solution" all the atoms of one element in the alloy
will occupy a particular lattice site (eg the centre in
body centred cubic). This changes the crystal structure
and (I think) is known as a "reaction". I have a feeling
the copper/gold alloys may do this under the right conditions.

I don't know what constitutes a "reaction", but you are right about copper
and gold. 'Cu 3 Au' and 'Au 3 Cu' both have face centered cubic structure
with one of the cubic sublatices being all one element, and the other 3
cubic sublatices being the other.


I worked at the Bureau of Standards (now NITS) right after graduating from
ugrad school. My project was to fabricate a high purity sample of an
_intermetallic compound_ of Au and Al in the form of a rod 3 or 4 cm long
and 1 cm in diameter. It was AuAl(sub 2), I believe. It is not a typical
alloy with a more or less random distribution of the atoms but a particular
structure. The material was brittle, not malleable. It fractured with a
conchoidal fracture, had a metallic lustre, and was _purple_. It had been
considered as a possible jewelery material when it was first discovered,
but I expect its brittleness ruled that out. I do not remember any
information about its conductivity, but given its metallic appearance I
suspect it would be a conductor.

I doubt that one should call the crystal structure a reaction, but taking
on a particular structural arrangement of the atoms is the result of, at
least a degree of, non-metallic bonding. I do not remember the specific
structure, but I remember sketching a structure with specific placement of
Au and Al in my notebook. This material I worked with was rather
exothermic when reacting. In fact the energy release led to a failure of
our first attempt to fabricate the material. (An alumina crucible heated
too rapidly and cracked allowing the melt to run out.)

This material was one of a series of these intermetallic compounds being
studied for their electronic structures by NBS and Bell Labs. The
competition seemed to have led to some less than perfectly honorable
behavior in print, but that's another story...

Dewey

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Dewey I. Dykstra, Jr. Phone: (208)385-3105
Professor of Physics Dept: (208)385-3775
Department of Physics/SN318 Fax: (208)385-4330
Boise State University dykstrad@varney.idbsu.edu
1910 University Drive Boise Highlanders
Boise, ID 83725-1570 novice piper
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