Alloy Q lies in the liquid phase
field at high temperatures.
It cools through the liquid region
until it reaches the liquidus and
crosses into one of the two phase
regions, in this case the L + α field.

Alloy Q cools through the two phase
region. Solid α is deposited.
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.

Alloy Q cools through the three phase
region. As it cools it passes through
a series of isothermal tie triangles. The
proportions and compositions of the
phases present change. Two solid
phases form(α and γ) and the
proportion of liquid decreases due
to the monovariant eutectic reaction.
The composition of the liquid
moves down the eutectic valley until
it reaches the invariant point at which
point the class II reaction begins.

The effect of the invariant reaction
on alloy Q is best explained with
a series of isothermal sections.

Above the invariant temperature
the alloy lies in the L + α + γ
phase field.

Class II reactions can be generally
classified as reactions which go from
two phases at high temperatures to two
phases at low temperatures. It can also
be useful to think of a class II reactions
in terms of tie triangles.

Class II reactions are represented
by two tie triangles at high
temperatures coming together at
the invariant temperature to form
a trapezium which splits into two
different tie triangles below the
invariant temperature.

On a liquidus projection, class II reactions
exist where three reactions valleys meet and
two of the down temperature arrows are
pointing in, toward the invariant point
while the third point out, away from the
invariant point.They are often denoted
with an E on liquidus projections.

No matter how complex a liquidus
projection is, the class II reactions
can be found by looking for sets
of three arrows with two pointing in.