In this screen cast I am going to briefly describe what a Eutectic is and then use a
temperature-mole fraction diagram to explain it in a little more detail. First we need
to understand this diagram. So showing an alpha phase here. What that corresponds to
is a solid that is mostly component B with some A dissolved in B, so A has limited solubility
in B. Likewise here we have a beta solid which is mostly component A, with some B dissolved
in it. So the eutectic reaction corresponds to liquid going to form the alpha solid plus
the beta solid. Let's look at exactly how that might happen. So if I picked a composition
that corresponds to this point, so this mole fraction, this is eutectic where the eutectic
forms. We will call that the eutectic point. The eutectic temperature is this temperature.
This would be the temperature where the eutectic forms. So let's start in the liquid phase
at this point. And we lower the temperature and we get to this point as we continue removing
heat the temperature stays constant and this eutectic reaction takes place and we end up
with a solid that is composed of the two phases, the alpha phase and the beta phase. But a
eutectic solid is going to look something, in an expanded view microstructure, alpha
phase, beta phase, alpha phase, beta phase, and so forth. This laminar structure certainly
form other broad structures, globular structures, but let's just look at this laminar structure.
Certainly the most common. The relative thicknesses would relate to, just from looking at a lever
rule, how much of the alpha and how much of beta is in the eutectic. For the particular
diagram I have drawn here this distance here is about the same as that distance which means
that it is about the same alpha and beta in the eutectic. And the composition of the alpha
phase is this mole fraction here. Composition of the beta phase at the eutectic is that
mole fraction. So we have eutectic, notice it is the lowest melting, important point,
it is the lowest melting temperature for this system and it also then, behaves very much
like pure material in that there is one temperature that we go from liquid to solid for component
A and one temperature we go liquid to solid for component B. There is one temperature
here where we form this eutectic salt. So if we cooled down to this point where the
x we would have a polycrystalline structure and imagine these crystals in the polycrystalline
structure. So we would have these layered structures. That is eutectic. So then you
might ask, what happens if we are not at that mole fraction corresponding to eutectic. So
let's look what happens then. So let's suppose we start with this composition in the liquid
phase and we cool down. We hit this line, we start forming the alpha phase and as we
continue to lower the temperature the alpha phase composition changes and the liquid phase
composition changes until we get again to this eutectic temperature. So at the eutectic
temperature now, we have alpha phase and we have a liquid of this composition, the eutectic
composition. So then what we have done is we have made a polycrystalline alpha structures
in a liquid. And the liquid when we get to the eutectic temperature has eutectic composition.
Now as we try to remove more energy from the system, in our overall composition, is still
right here. So we try and remove more energy we are going to stay at the constant temperature,
the eutectic temperature and we are going to make solid eutectic and in that solid eutectic
is going to be the alpha phase crystals. So let me draw that. This is a very schematic
representation of what we have. So when we get to this point, soon as we get below the
eutectic temperature we have alpha crystals and then most of the structure, in this case
are going to be eutectic solid, this laminar structure, and the various crystals in the
laminar structure in eutectic are the alpha phase. So this gives you a brief idea of what
we mean by eutectic. And we can certainly have eutectic forming if we have a liquid
and two solids are completely immiscible. So we form two pure solids and then the eutectic
would be the solids in the layered structure.