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.
Jim
James Binnion
James Binnion Metal Arts