The purpose of this Screencast is
to discuss recrystallization and grain growth
whenever we work or process a material
we can anticipate
that we're going to plastic deform it and when we do this we end
up storing
energy within the material. So this energy is going to be
stored in the form of additional dislocations or
increase grain boundary surface area. To give you an example
if we roll a material so this means that we go ahead and we take peace material
and we're trying to go ahead
and flatten it so the thickness is reduced
As you start to roll this material the one side you will have
your original size grains which will end up
entering the roller. These grains are going to end up
elongating as they go through the rolling process
in the grains that have elongated we can expect that we have additional
dislocations
and increase grain boundary surface area
so in essence we can anticipate
that these grains are going to end up having more energy
on this side after processing. Recrystallization is a process that
allows us to actually restore
our grains to the original like we see back here to this original state
and it ends up being a lower energy state for the grains
To explain the lower energy state we can use a balloon
as an analogy. If you sit on a balloon and make it flat
the balloon is gonna happen increase surface area however
a balloon wants to actually go to a lower energy state
so a decrease surface area. So if you were actually sitting on it
and you got off the balloon it would automatically pop back to its original shape
Grains work very much the same way in a metal
Once you have deformed them they've become elongated
like the flat balloon they want to pop back to their original round shape and then
this is what
recrystallization is all about. Our original drawing
of showing a piece of material that was being rolled
is representative of what happens to materials during a process called
cold working. So right after cold working our grains have typically become
elongated. So this picture represents right after cold
working you have elongated grains. You can anticipate during this time
a few different things. Number one you can expect that your
material strength has gone up. So right after cold working you have an
increase in
strength but you can also anticipate that you have a decrease in your ductility
If you were to go ahead add heat
at this point what you would find is you start to have
atomic diffusion that's enhanced and because of this
you end up reducing the number of dislocations
as well as dislocation configurations
The result of this equals a reduction in
internal energy. As a result what we start to see
is a nucleation of new grains so this nucleation
of new grains that's occurring is what we call recrystallization
During recrystallization we can anticipate that we're gonna see
our strength start to lower on us but we'll also see an increasing in our ductility
Something important to note is with recrystallization
is that the more you work a material so the more you cold work a material
the more internal energy it's going to store. As a result
with more internal energy it takes
less heat to go ahead and start recrystallization.
Recrystallization is actually how we define
hot working vs. cold working. Cold working
is defined as processes that are below
recrystallization temperatures and hot working
is considered any processes that are occurring
above recrystallization temperatures. So recrystallization is gonna actually
be dependent on both the material
as well as the percentage of working it has received
before you start to heat is up for recrystallization
So one final note is after a material has started to recrystallize
what happens if you continue to heat it so if you do continue to heat what you're
gonna end up finding out we're going to go back
up to our diagram at the top here. So if you go ahead
you continue to heat your material you'll find that these grains will actually start to
grow larger
and larger on you. So sometimes you'll hear recrystallization termed
recrystallization and grain growth
So this grain growth that has occurred right at this region
has different properties than our smaller finer materials that we had
right as recrystallization started
So recrystallization once it starts has these nice
fine grains at the beginning and then through grain growth
we start to get these larger grains and the difference between these two grains
end up being properties
Typically with grain growth what we are going to see is a decrease in our engineering
properties
So overall one can see how we can use recrystallization
to our advantage. So in the case of hot working
where we keep it above recrystallization temperatures
if we're doing something like rolling steel every time you roll it
at the end of the rolling process it's going to recrystallize. So every time you
reduce your thickness it will keep recrystallizing
keeping the engineering properties where we want them
and allowing us to process that material all the way through
to go from an initial thickness all the way down to our final thickness that we
would work for that steel.