I had understood that much increased hardness from overheating or
slow cooling came from copper oxides being formed (in the alloy),
migrating to the grain boundaries, increasing grain size, all
contributing to an increase in hardness?
The copper does not need to oxidize to do this. If the alloying
metal is one in which copper is not completely soluable at room
temperature, then a percentage of it will come out of solution,
migrating to the grain boundries. That decreases the flexibility of
those same grain boundaries, thus increasing the hardness. That’s the
basis of precipitation hardening in sterling silver, where copper
with still a small amount of silver dissolved in it, concentrates at
grain boundaries of the mostly silver crystals. Oxides don’t need to
be part of this, though of course they may also do something the same
in some alloys.
Oxides don't need to be part of this, though of course they may
also do something the same in some alloys.
There are some speciality alloys that use an oxide hardening process
but to achieve this the items are held at elevated temperatures for
an extended period of time like a few hours to allow for sufficient
diffusion of oxygen into the metal matrix for this hardening to
occur. Also this type of hardening requires a metal like aluminum,
titanium, molybdenum etc that forms a highly stable oxide to be
alloyed with the parent metal. The reason this type of alloy is not
common is that the hardening change is irreversible once these metals
oxidize there is no way to reduce them back to the non oxidized state
in the alloy. It requires refining to separate out the oxides and
return the main component metal to its soft state. There was at least
one commercial sterling alloy that was designed to harden this way
but not enough people wanted to use it due to the costs of needing to
refining all scrap before reuse.
Oxides don't need to be part of this, though of course they may
also do something the same in some alloys.
The other reason copper alloys don’t harden this way is that copper
is so reactive with oxygen that the oxygen atoms will not travel far
into the matrix. The first atom of copper the oxygen contacts will
bind with it. The oxygen will react and bond with the surface copper
first then the layer of atoms behind that and so forth. This stable
oxide will not allow the oxygen atom to break away to travel further.
Also the oxide layer tends to act as a barrier to slow further oxygen
penetration. With silver or gold alloys the oxygen will not combine
with the silver or gold to form stable oxides so the oxygen atoms
are much more free to move around in the matrix. This is the cause of
fire stain in sterling. The oxygen penetrates into the silver copper
matrix and bonds with the copper atoms forming a stable copper oxide
in the matrix. If you leave a piece of unprotected sterling at a high
enough temperature for long enough all the copper will be converted
to copper oxide. If you do the same with a copper alloy you just
loose the outer layer of the item when you pickle it as the oxide is
dissolved by the pickle.