- There's this really cool desk toy called Atomix
that you can't buy anymore.
So a couple of years ago, I made my own.
It's about 3000 ball-bearings
sandwiched between two layers of acrylic.
They're free to move in two dimensions
and they naturally fall into this regular lattice structure
except that there are imperfections in the lattice.
And actually, that's why I built this thing
in the first place
because I wanted to make a video about lattice imperfections
in things like diamond or metals.
And this Atomix toy is a really good model for that.
If you wanna watch that video
the link is in the card and in the description.
But someone commented on that video
saying that this Atomix toy might also be useful
for demonstrating what's going on
when you heat treat metals like annealing or tempering.
And that's what this video is about.
If I allow the balls to fall quickly into position
they form these small regions of regularity.
You get the same kind of arrangement
in the atomic lattice of metals,
these small regions of crystalline metal
that are neighboring each other.
And they can't form one single crystal
because they're oriented differently.
These regions are called grains, in a metal,
and it turns out that the size of grains in a metal
really has a profound effect
on the physical properties of that metal.
So let's see what happens to the grain size
when we heat treat a metal.
This is a model of an atomic lattice.
So we need to think about what heating means for the model.
Well, on the atomic scale, heat is just kinetic energy.
It's how much are the atoms and molecules jiggling around?
So if we want to increase the temperature in our model,
we need to jiggle the balls around.
I'm gonna jiggle these balls with a vibration generator,
which is basically a speaker with a prong.
And because I'm filming at 50 frames per second,
I'm gonna try vibrating at 50 Hertz and 100 Hertz.
By picking a multiple of the frame rate,
I don't need to worry about the stroboscopic effect.
You can see how the added kinetic energy
gives the lattice an opportunity to rearrange itself.
That extra movement means that the atoms
can slip and slide into new positions
and the grain size increases.
So heat treating a metal lattice in this way
increases the grain size.
But what effect does that have
on the physical properties of the metal?
Well, in metallurgy, you have all these different terms
for the physical properties of a metal,
hardness, toughness, strength.
They all sound really similar
but they all have different technical definitions
that I don't wanna get into in this video.
So we're just gonna gloss over all that and say
how hard or easy is it to bend a metal
permanently out of shape,
before and after heat treating in this way?
Well, my assumption would be that
if you have larger grains, larger crystal structures
then it will be stronger
because a regular crystal arrangement is really strong.
But it turns out that larger grains
make it easier to bend a metal out of shape.
To see why,
we need to look at what happens at the molecular level
when you permanently deform a metal.
It's called plastic deformation,
when you permanently change the position of atoms.
And you might theorize that
when you permanently deform a metal,
maybe the grains are sliding over each other,
or they're breaking up into smaller grains
or joining together.
It's actually none of that.
It turns out, the most important thing
when it comes to plastic deformation,
is imperfections in the crystal,
specifically imperfections called dislocations.
Dislocations come in a few different flavors
but generally speaking, they look a little bit like this.
And it turns out that dislocations are easy to move.
You can see why,
if you look at the bonds around a dislocation,
they're already quite strained,
so you don't need much energy to break one of those bonds.
So when you apply forces to a metal in different directions,
these dislocations will move around in different directions,
like this.
The important thing is it takes less energy
to move dislocations through a grain
than it does to move grains against each other.
That's why dislocations are sometimes called
the carrier of plasticity in a metal.
You might have thousands of dislocations
within a single grain in a metal.
So when you're bending it, you're causing those dislocations
to glide through the grain.
But crucially, when a dislocation gets
to the edge of a grain, it can't go any further.
It can't hop to the next grain.
That's the end of its journey.
So if your metal has small grains,
then the dislocations within those grains
won't have far to travel before their journey ends,
before they reach a grain boundary
and on a macroscopic level
that means that your metal isn't very ductile.
You can't plastically deform it very much.
And if you keep applying force
then it's just gonna fail in other ways.
Whereas if you grow the size of your grains
like we did here, then you're giving those dislocations
further to travel.
You're increasing the capacity of your metal
to plastically deform before braking.
You're increasing the ductility of your metal.
That's about the limit of what I can demonstrate
with this model.
And obviously it's an oversimplification
of the whole field of metallurgy,
because there are different ways to heat treat a metal
with different results.
What I've done is probably closest to annealing
but there's also tempering and quenching.
And there's those different variables
like, what temperature do you raise the metal to?
Is it above or below the recrystallization temperature?
How quickly or slowly do you let it cool down?
And you get all these different properties.
It's really, really interesting.
And it's not something I can demonstrate with this model.
But I found a couple of really good videos,
if you want to dive a bit deeper.
I'll leave those in the card and in the description.
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Someone told me that I should say slash
instead of forward slash because
I guess the forward bit is implied.
It's never backslash.
So in case anyone was wondering
"Why didn't you say forward slash?"
I made this longer really by explaining, haven't I?
Anyway, I hope you enjoyed this video.
If you did, don't forget to hit subscribe
and I'll see you next time.
(upbeat music)