Alloy Z lies in the liquid phase
field at high temperatures.
It cools through the liquid region
until it reaches the liquidus
and crosses into the two phase region.

Alloy Z cools through the two phase
region. As the alloy cools, the
composition of the solid and liquid
change according to the ends of the
tie lines spanning the region.

Eventually, the alloy reaches the edge
of the two phase region and passes
into the three phase region.

The three phase region is composed of
a stack of isothermal tie triangles.
As alloy Z cools through the three
phase region, it passes through a
series of isothermal tie triangles.

It is important to remember that the
position of the tie triangle relative
to the bulk composition determines
the proportions of
each phase present.

If the tie triangle is such that
the bulk composition lies on
one edge of the triangle, only the
two phases at each end of this
side are present.

The cooling processes of alloy Z is best
explained with a series of isothermal
sections.

Initially, the alloy lies in
the single phase liquid field.

The proportion of the phases present change
as the eutectic reaction proceeds. This
requires relative motion of the tie triangle
and bulk composition. As the bulk composition
is fixed, this motion comes from the tie
triangle moving across the composition
triangle.

The tie triangle moves.
The composition does not move.