At the eutectic point in a binary phase diagram, there are three phases in equilibrium.
Understanding the Eutectic Point
The eutectic point is a specific point on a phase diagram that represents the lowest melting temperature of a mixture of components. It's an invariant point, meaning that at this precise temperature and composition, the phases are in equilibrium and do not change as heat is added or removed (until the transformation is complete).
As described in the reference: "In a binary phase diagram when a liquid phase (L) changes completely to two solid phases (α and β) on cooling and vice versa on heating, the state of the system is called the eutectic point."
This transformation involves:
- On Cooling: A single liquid phase transforms into a mixture of two distinct solid phases (α and β).
- On Heating: A mixture of the two solid phases (α and β) transforms into a single liquid phase.
Phases Coexisting at the Eutectic Point
For this transformation to occur reversibly at a single temperature and composition (the eutectic point), all the phases involved in the reaction must be in equilibrium simultaneously at that point.
Therefore, at the eutectic point itself, the phases present are:
- Liquid (L): The single liquid phase.
- Solid Phase α: One of the solid solutions or pure components.
- Solid Phase β: The other solid solution or pure component.
These three phases coexist in equilibrium at the specific eutectic temperature and composition.
Phase Diagram Illustration
A binary phase diagram visually represents the phases present at different temperatures and compositions. The eutectic point is located at the intersection of the liquidus lines and the solidus line where the invariant reaction occurs.
| Phase | Symbol | Description |
|---|---|---|
| Liquid | L | The molten state of the alloy. |
| Alpha Solid | α | One of the distinct solid phases (e.g., a solid solution). |
| Beta Solid | β | The other distinct solid phase (e.g., another solid solution). |
This coexistence of three phases at a single point is a defining characteristic of an invariant reaction like the eutectic transformation in a binary system, governed by the Gibbs' Phase Rule.
