The purpose of this Screencast is to discuss
the concept of dislocation and and plastic deformation
So let's say that you have a bowl of
cold ice cream and a spoon. If you go ahead and take a bite out the ice cream
and you put your spoon in the ice cream and it bends but it does not go back to its
original shape
what you have just experienced or what you've just observed at the macroscopic
level
is a large number of dislocations moving
Another way that we end up describing this is plastic deformation
So let's go ahead and take a look at what a dislocation is
So if we have a metal that is solidifying we are going to expect that during
solidification
it will actually end up forming for us a lattice. So sometimes
the lattice that ends up forming has a defect in it and these defects
are what we call dislocations. So if our defect is a situation where
we end up having part of the plane not forming we end up calling this an edge
dislocation
and you can see how our lattice has a missing atom
So in yellow is where our missing atom is.
This becomes a problem if we end up having a situation
like our spoon with our ice cream.
If we go ahead and we place a force on the side
and this would be similar to if we dug that spoon deep into that
ice cream
what we'll end up finding is that our bonds along
plane 2. So right along this plane. They will start to stretch on us
and then they will eventually break and once they end up breaking
we end up having a situation where right
at this point you will find that these bonds are going to actually end up breaking
and reforming and allowing us to then have plane number 1
will end up having a full plane. The result is
that your defect in your material in your lattice has now moved. It's no longer along
plane 1
but instead it's a along plane 2.
If we keep applying our shear force what's going to end up happening is our dislocation
is going to keep moving. So our defect is going to keep moving all the way to the end
So then we're gonna have a situation where our final lattice will end up
looking like this.
In this case our edge dislocation has moved all the way to the end so now it's in
plane 4.
One of the things is really good to remember though is that what your seeing
here is
a single dislocation moving. So this is all we've had in this example
however anytime we see plastic deformation
of a metal we are seeing several
dislocations large large numbers moving all at once so
a little bit different than the example that we have here. So the definition of slip
is plastic deformation through
or produce rather by dislocation motion another concept that's associated with
dislocations
is this idea of dislocation density
So as our materials end up solidifying
we end up having different types of
dislocation densities associated with that metal. So dislocation density
is defined as total dislocation length per unit volume
What you end up finding is different types of processing
that we do actually will increase our dislocation density.
For instance processes that can increase your dislocation density
would include cold rolling. Another process that may end up increasing
your dislocation density would be cold forging. In these cases
we are assuming that our material is above its recrystallization temperature
We are assuming that our material is below its recrystallization temperature
So for cold rolling and cold forging these are examples of
processes below
recrystallization. So just remember the next time you end up permanently
deforming
a metal what you have actually done is
a large amount of dislocation motion.