The first alloy we're going to consider is the aluminum copper alloy, again.
In this particular case we have two phases that form behind the advancing liquid.
So we have three phases in equilibrium which is what we should have
at the eutectic temperature.
When the material comes all the way down in temperature for
an alloy which is off you tactic.
What we see in our microstructure
is a composite structure that is made up of the black areas.
Our solid solution alpha.
So that's aluminum with a certain amount of copper dissolved in it.
The background is made up of
the alpha phase plus the aluminum two copper, intermetallic compound.
Now, we're looking at this in a particular technique in
the SCM where the elements, like copper,
which are heavier than aluminum, they wind up in dark field being bright.
And the aluminum, which has a lower atomic number in comparison, is dark.
So, we see that microstructure of the Eutectic in the background
between the primary alpha phase that forms.
So this is exactly what we calculated in the previous lesson.
Now we're just going to look at the microstructure at a slightly higher
magnification.
So now what we've got is the dark regions are the alpha solid solution.
And in between those primary alpha phases we have
the material that is the liquid of
the composition that transformed into alpha and theta.
Now, if you go back to the picture that helped us come
up with the understanding of the way these structures develop,
one of the things that's very important for us to understand is that in order for
the lamellar structure that we've been describing to develop,
what we need to have is a cooperative diffusion of process.
Where A moves from in front of the beta phase to the alpha phase and
component B moves in the other direction.
And what happens then is at that solid/liquid interface,
what we're able to do is to produce the lamellar eutectic.
Now, the key that
must be maintained is that in order to have this cooperative diffusion.
We essentially have to have a plainer interface or a relatively flat interface
where the A and B can move in opposite directions.
So we don't want one of the phases, either the alpha phase or
the beta phase to advance.
Because as a result of one phase going faster
than the other phase that's forming.
This very pivotal co diffusion process can't occur.
Now as it turns out Not all of the microstructures
have this regular lamellar eutectic.
