From what I understand is that fine silver does not age or
precipitate harden because the lack of copper. Is this process the
two metals working together. In other words does near pure copper
age harden?
Age hardening occurs because copper, while completely soluble in the
molten gold or silver, is only partially (and not that much) soluble
in room temperature solid gold and silver. Alloys of the two metals
form crystals in which the two are mixed, but it’s not a fully stable
solid solution. Age hardening is heating the alloys enough to allow
ions to become mobile, at which point, though the metal remains
solid, some recyrstalization is starting to occur (almost at the
annealing temp) and in this temp range, the copper, being
“uncomfortable” in solution in the gold or silver alloy migrates to
the crystal boundaries and forms new crystals of mostly copper.
Because metals ductility depends not only on the deformability of the
crystals, but also on the flexibility and ability of the crystal
boundaries to move and stretch along with the crystals they bound,
and the fact that the copper rich crystals now concentrated at the
crystal boundaries reduce the flexibility and stretchability of the
crystal boundaries, the overall hardness of the metal ends up
increasing. Pure or near pure copper generally does not then harden
this way, because it lacks the other metals that would dissociate
from the copper and mess up the crystal boundaries. One big
exception to this would be Beryllium copper, which is exactly such a
situation. Small amounts of beryllium will allow heat treatment to
harden the copper, in pretty much the same way as described. This
also explains why an alloy of just gold and silver, without copper,
does not age harden, since gold and silver and completely
intersoluble at room temperature, and heat treating does not cause
any separation to take place.
Also could someone explain the problems associated with over
annealing silver or copper.
Annealing silver or copper (or most of the other metals we work with
too) at too high a temperature or for too long, simply allows more
time for the metal crystals to combine and grow in size. Normally,
annealing is used to allow deformation of the crystals to “relax”, as
the deformed crystals break apart and reform into new, smaller,
undeformed crystals. If you continue to heat or heat more than needed
to do this, the new smaller crystals grow together and combine,
resulting in fewer, larger crystals. These larger grain sizes mean
that grain boundaries have less surface area. Grain boundaries are
less strong than the crystals themselves, so metal with very large
crystal size can be more prone to fracturing along the grain
boundaries once those boundaries become more and more reduced in
overall area and more and more consisting of large flat planes within
the metal. Also, grain boundaries deform differently from the grains
themselves, so if you’ve got large crystals, bending the metal will
appear to stretch the boundaries slightly more than the grains
themselves at the metal surface, resulting in a web of slightly
depressed lines, the grain boundaries. The result is what we call and
orange peel surface. If the metal is properly annealed, and grain
size remains small, while the same thing is happening when the metal
is bent or worked, it happens on a much smaller scale, and instead of
a visible orange peel, all you see is a slight reduction in the
polish or smoothness of the metal, a matting down that is much easier
to deal with than a coarse orange peel surface.
Peter