Alloy P 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 + β field.

Alloy P 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.

Alloy P cools through the three phase
region. Two solid phases form
(β and γ).The bulk composition of the
solid (shown in blue) can be linked to
the composition of the liquid by a tie
line through the bulk composition of
alloy P.

As γ is precipitated the solid enriches
in C. The composition of the liquid
moves down the eutectic valley until
it reaches the invariant point and the
ternary eutectic reaction begins.

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

Above the binary β + γ eutectic
temperature the alloy lies in
the L + β phase liquid field.

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

Class I reactions are represented
by three tie triangles at high
temperatures coming together at
the invariant temperature to form
a single tie triangle below the
invariant temperature.

On a liquidus projection, class I reactions
exist where three reactions valleys meet and
all of their down temperature arrows are
pointing in, toward the invariant point.
They are often denoted with an E on
liquidus projections.

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