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A C B Temperature R
Solidification of Alloy R

   

A C B Temperature R
Alloy R 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.

   

A C B Temperature R
Alloy R cools through the two phase
region. Solid β 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.

   

A C B Temperature R
Alloy R 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 β) 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 III reaction begins.

   

A B L C L + α β L + β β + α L + α+ β L R α
A B L C L + α β L + β β + α L + α+ β L R α
A B L C L + α β L + β β + α L R α α L β γ
A B C β L + α L + β β + α L R α γ α + β + γ L + γ L + β + γ L + α + γ α + γ β + γ
A B L C α + γ β α β + γ β + α L α + β + γ L + γ γ R
A B L C α + β β γ α + γ β + γ α + β + γ α R
The effect of the invariant reaction
on alloy R is best explained with
a series of isothermal sections.

Just below the AB binary eutectic
temperature the alloy lies in
the L + β phase field.

   

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

Class III reactions are represented
by one tie triangles at high temperatures
which persists to the invariant temperature
at which point it splits into three different
tie triangles. These tie triangles then
sweep out across composition space,
often with two terminating at binary
reactions.

   

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

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