It must get more complicated when primary creep and brightening are
happening at the same time, although I think that they are different
phenomena. Dislocations migrating and impurities migrating to catch up!
Another possibly confusion in use of words: The kind of "plastic
deformation" that can occur when handling wire and which leads to a
"dulling" of the tone, if it is related to breaking existing dislocations
away from impurities, is a different kind of thing from the plastic
deformation that occurs under stresses large enough to cause new
dislocations to be generated.
Also, it occurs to me that it is possible to have this second kind of work
hardening when a string is stressed beyond the elastic limit. It all
depends of whether the plastic flow results in failure, which in turn
depends on whether it goes into "plastic instability" whereby the wire
necks down, increasing the local stress and leading to a sort of feedback
between further flow and further stress localization. In some cases, the
wire gets stronger in just the right way to prevent necking, in which case
it can continue stretching while getting stronger. It would seem to me
that this might be happening in the procedures that were described for
pre-stressing string before bringing it up to final pitch. Or maybe a
combination of all of the above.
It seems to me that knowledge can be categorized as being merely
interesting, or useful. Dispelling some of the mystery can be useful in
some cases. For example, if I go out of a morning and start framing a
garage and find that suddenly I can't drive a nail for sour apples, I could
come up with several explanations: I could assume it had to do with
something I ate, drank, or did the night before; or I could assume that
the next-door neighbor had put a hex on the hammer; or, if I had read
about "upper-lower yield" in a metallurgy book, I would assume that the
nails were crap and go buy some different ones. Or I might assume this
without having read the book - depends on where one is coming from.