This is the liquidus projection for
a ternay system showing complete
solid solubility.

This space model shows complete
solid solubility.

It has no eutectic or peritectic
reactions. The liquidus is a single surface.

This is the liquidus projection for
a ternay system with one
eutectic valley.

This ternary system is made up of two binaries with eutectic reactions and one showing complete solid solution.

A eutectic valley runs between the eutectic points. The liquidus is formed from two surfaces, one either side of the eutectic valley.

This is the liquidus projection for a system
with a ternary eutectic point. At the
eutectic point (where the three eutectic
valleys meet) a Class I(ternary eutectic)
reaction occurs: L → α + β + γ.
Class I reactions can be identified on
liquidus projections easily as all of the down
temperature arrows point in, towards
the eutectic point.

This ternary system has a ternary eutectic
point. Each of its constituent binaries has
a binary eutectic point. There are three
eutectic valley, one leading from each binary eutectic point to the ternary
eutectic point.
The liquidus is formed of three surfaces, one between each pair of eutectic valleys.

This is the liquidus projection for a system with
a class II invariant point. At the invariant point
(where the three valleys meet) a Class II(sometimes
termed Ubergang) reaction occurs:
L + β → α + γ.

Class II reactions can be identified on
liquidus projections as two of the three
down temperature arrows point in,
towards the invariant point.

This ternary system has a class II invariant point. Two of its constituent binaries have a eutectic point while the third has a peritectic point. There are two eutectic valleys and one peritectic valley leading from the binary invariant points to the ternary invariant point. The liquidus is formed of three surfaces, one between each pair of valleys.

This is the liquidus projection for a system with
a class III invariant point. At the invariant point
(where the three valleys meet) a Class III (ternary
peritectic) reaction occurs:
L + α + β → γ.

Class III reactions can be identified on
liquidus projections as only one of the three
down temperature arrows point in,
towards the invariant point.

This ternary system has a class III
invariant point. Two of its constituent binaries have a peritectic point while the
third has a eutectic point. There is one
eutectic valley leading from the eutectic
point to the ternary invariant point.
There are two peritectic valleys leading
from the invariant point to the
peritectic points. The liquidus is formed
of three surfaces, one between each pair
of valleys.

Liquidus projections are a useful way to summarise
the reactions involving liquids in a ternary
system. Most systems are more complex than
the idealised models shown in this animation;
they have multiple reactions and can become
quite complex.

On this diagram, the traces of the eutectic
and peritectic valleys (bivariant equilibria)
are plotted, the down temperature direction
is shown for each. Each invariant point
(where three valleys meet) is labelled
with its reaction class.The solid in
equilibrium with the liquid is
indicated for each region of the diagram.