So what would a three dimensional defect be?
And in fact, it's interesting, I think,
to think about what's the difference between a three
dimensional defect and a zero dimensional defect.
And so zero dimensional defect would
be something like, for example, an impurity atom.
Impurity atom.
That is a single atom that's sitting where it shouldn't be.
I mean, surrounded by other types of atoms.
But solvent atoms.
So it could be substitutional.
It could be interstitial.
What's different, then, when it's three dimensional?
Well, zero dimensional is one point
in the lattice, a single atom site.
Three dimensional has got to have some kind of volume.
So it's well above the scale of a single atom.
It's a collection of atoms.
It's massively big compared to a single atom, really.
So what kind of things would be three dimensional
within a crystal?
Well, you could certainly have a pore, which is basically
like a bubble, if you will.
It's filled with air.
So it's an empty space.
Not completely empty.
It has something in it.
But it's essentially an external surface
all wrapped around something inside the material.
Empty space.
Pore.
You could also have some other type
of material, another material.
So some of us call that a second phase.
It's a different crystal structure.
Different properties.
And those particularly actually are very interesting in-- well,
both of these, actually, in material science.
So when you're studying solids and materials, matter.
A pore, particularly in ceramics can cause big problems
to the mechanical properties of ceramics.
And second phases are used in many, many, many metals
as a strengthening mechanism.
Particularly there's a type of strengthening
called precipitation hardening.
Precipitation hardening.
And just very careful formation of these precipitates.
That's the second phase coming out of solution.
But just in the solid state.
In the solid state.
So what we want to look at, though,
without looking into all the specifics of how you
precipitation harden a material, just
look at how these second phase particles present an obstacle
to dislocation movement.
And much like we did when we discussed dislocations,
I'm not even drawing the dislocation.
I'm just trying to draw the lattice here.
And then say we have a dislocation that's
moving along through this.
And we describe this for passing across a grain boundary.
Well, what about if that now passes--
so this would be like a grain boundary--
it passes in and has to change direction.
But what about if we had, instead of it
passing a grain boundary, it passed into some other kind
of material?
Some other material.
Maybe it's a compound.
It could be a ceramic, a hard phase like a carbide,
like an iron carbide or something.
It's a different phase.
I know I haven't really described what a phase is,
but it's different material.
Let's leave it at that, actually.
A different material.
As an example in steel-- look for a piece of steel--
and you're almost certain to have carbide phases,
iron carbide phases in amongst the iron lattice.
And that's popular in marketing on tools and stuff.
They'll say, oh, it's got certain carbides in it.
Well, carbides are usually very hard.
And because they're hard, it means they're
hard to plastically deform.
So it's difficult for the dislocation
to cross that boundary and get into the second phase.
So these second phase particles present, again,
second phase particles, which is a type of volume defect
or three dimensional defect.
These second phase particles present an obstacle
to dislocation movement.
And as soon as we hear that, we know
that's going to make the metal stronger.
Because dislocations have to move for plastic
deformation to occur.
So if we've got an obstacle to dislocation movement,
that means this is a strengthening mechanism.
So by introducing second phase particles into a crystal--
and industrially, most relevant is precipitation hardening--
we can strengthen a metal.
And it's a really interesting process.
Anyway, pores will cover that specifically
in the context of ceramics.
Not so much as a strengthening mechanism.
But second phase particles can be introduced,
and they are introduced through different means
to strengthen metal.
Strengthening by second phase strengthening, it's called.
Or precipitation hardening is a specific form
of second phase strengthening.