Hi Helen,
This is how I explain it to my students. Jim, if you’re reading this,
please jump in if I get it wrong, I came up with this explanation
Step one: this is a vast oversimplification of what’s really going
on. It’s more to give you a mental model than anything passing for
serious metallurgical knowledge.
Step two: Imagine a shoebox, and a bucket of golfballs. Pour enough
golf balls into the box to cover the bottom in one layer. Notice
that they line up into a grid sort of like eggs in an egg-crate. If
you were to pour another layer in, they’d settle into the divots
between the balls in the previous layer. So you’d end up with the
same grid, but offset half a ball to one side, and most-of-a-ball
higher. Pour in a bunch of layers, and you’ll get a three dimensional
grid or lattice of golfballs.
The golfballs are actually metal atoms, and you can think of the
shoebox as a sort of fuzzy stand in for a metal crystal, or metal
domain, or a grain.
When jewelry metals start to solidify from liquid, they start out
solidifying around some sort of impurity or something else that
serves to get the grid going, and then the rest of the atoms sort of
join the crowd. Once they see how the grid’s forming, they know where
to put themselves, so they line up with the matrix that’s growing
from that original seed point. This happens all through the
solidifying object pretty much simultaneously, so there are lots of
different starting points, and lots of different grid matrices that
don’t line up with each other once everything solidifies. This is
actually good. If you had a perfectly even grid, you’d have something
as hard (and brittle) as a diamond. No way to forge diamonds. It’s
the discontinuities between the crystals (shoeboxes) that allow the
metal to bend and flow.
Imagine a semi-trailer full of those boxes of golf balls, loaded by
a bunch of guys at 4:45 on Wednesday afternoon before thanksgiving.
There are going to be a lot of boxes just tossed into the trailer,
with a lot of holes and gaps between them. Now imagine Godzilla
(that’s you) picks up the trailer and tries to tie it in a knot. The
boxes are full of perfectly packed golfballs, so they’re not going
to compress much, but as the trailer flexes, there’s plenty of room
for the boxes themselves to slide around, which lets the trailer
bend. Through the magic of tortured analogies, the boxes can change
size to fit into smaller spaces, so long as they change shape along
one of the axes of their internal matrix.
So, Godzilla trying to tie the trailer in knots is roughly
equivalent to you, taking out your aggressions on the poor hapless
metal with a hammer. Otherwise known as cold working. Every time you
bend or hammer the metal, the metal domains (or grains) slide around
like those boxes in the trailer. The incredibly minute gaps between
them let them slide around, and shuffle themselves into a new order.
Every time you do that, the holes get smaller, and smaller, and
smaller, until eventually there aren’t any left, and the metal snaps.
An interesting thing to keep in mind is that the holes are only going
to disappear in areas where the metal’s deforming. If Godzilla grabs
the ends of the trailer and twists, the middle will eventually shred,
but the two ends may stay just the same as they were to start with.
Annealing is roughly equivalent to Godzilla getting frustrated, and
breathing fire on the whole thing. (You never knew you were the
metallurgical equivalent of Godzilla, did you?) Godzilla breathes
fire, the trailer goes up in a ball of flame, and the boxes burn.
Which lets the balls (metal atoms) reorganize themselves into a
loose grid without quite melting. As the fire department shows up,
they hose down the trailer, (you quench the metal) and the
temperature crashes back down, the same sort of recrystalization
happens, and once again, the spirit of tortured analogies appears, to
recreate the boxes, all jumbled up in a brand new trailer, just
waiting for Godzilla to have his next fit.
In a less goofy version, annealing relaxes the grain boundaries. The
crystals don’t quite fully dissolve. (if they did, the metal would
melt), but it lets the outlier atoms start to drift around, and look
for greener pastures. However, metal atoms being susceptible to peer
pressure, the longer you hold the metal at heat, the longer the
surviving grains have to say “Oh! Oh! Come hang out with the cool
atoms!” which makes the surviving grains get bigger and bigger. As
you quench the metal, the wandering atoms are forced to pick a team,
quickly. So many of them glom onto the surviving grains, while others
form new grains based on impurities (or whim), so you end up with a
new discontinuous structure, with a fresh batch of microscopic holes
to let the grains shuffle themselves around. This is why it’s better
to push the metal as hard as you can, and anneal as quickly and
infrequently as possible: less time for the ‘cool kid’ grains to
extend their domains.
The reason big grains are bad is that the grains would really
rather shuffle around based on their external boundaries. They
really don’t like to deform themselves. They can, but it takes a
lot more energy to do that, and if the metal’s already been pretty
seriously worked, that may just be enough to shred it. So the bigger
the grains, the harder it is for them to find spaces into which they
can shuffle, and the more likely they are to just snap. (In getting a
grain to deform, you’re asking the atoms to shear along one of the
planes of the internal matrix. If one of the planes lines up with how
you want it to move, great. If not… ) It is actually possible to
see how the internal grain orientation effects a sheet’s formability,
I don’t remember all the details, but I know Charles Lewton-Brain has
done some work on this, relating to foldforming. I remember pictures
(somewhere) where you can actually see the metal moving differently
in different directions.
Who knew that Godzilla and high-school cliques had anything to do
with the inner life of metals?
On a serious note, I mostly teach older adults. I find they remember
better if (A) you give them examples they can relate to, and (B) if
they’re laughing. Thus Godzilla and the high-school prom.
So, Jim, I’m almost afraid to ask how far off I am… Remember: the
goal is a mental model that predicts what the metal’s going to do,
not textbook precision.
Regards,
Brian.
