So here is our solid liquid interface, and the motion of A and

B in front of their required components wind up increasing

the local composition so that the alpha and the beta phase can advance.

In a previous lesson, I presented a eutectic diagram

where a number of the compositions along the phase boundaries had been indicated.

What I'd like to do here is to use that same data, and

what I'm looking at is, in particular the eutectic alloy composition.

If I look at a temperature above the eutectic,

I have essentially 100% liquid.

When I reach the eutectic temperature,

I begin to form a very small amount of alpha and beta.

And I'm assuming that the amount of solid alpha and

beta I have is very, very, very small and

the compositions can be given by the compositions on the eutectic tie line.

As soon as I drop below T6, what happens is the liquid phase disappears and

what I have left are the two phases alpha and beta.

And from their compositions and the composition of the alloy,

which is the eutectic composition, I'm able to calculate what

the fraction of solid alpha and the fraction of solid beta are.

And at the next temperature, at T8, I'm able to do the same thing.

So what I can do is I can actually plot the data in the form of a phase

fraction chart.

What I see is above T6 I have 100% liquid and

as soon as I reach T6 I am in equilibrium with three phases,

just a little bit of solid alpha and beta and essentially 100% liquid.

As soon as the temperature begins to drop the amount of liquid I have goes

to zero and I'm beginning to develop my alpha phase and

my beta phase, according to the compositions

that are given along the phase boundaries at those two respective temperatures.

Now I can also show how the microstructure changes as a consequence of

cooling above the eutectic temperature down to below the eutectic temperature.

So when I'm above the eutectic temperature I see I have 100% liquid and

I'm observing that in the microscope.

As soon as I drop below the eutectic what I begin to form is that alternating

alpha and beta mixture that comes about as a result of eutectic composition.

We'll be continuing this description and looking at

eutectic systems that are off the eutectic composition in the next lesson.

Thank you.