The primary difference between the eutectoid point and the eutectic point lies in the phases involved in the reaction that occurs at that specific temperature and composition.
Understanding Eutectic and Eutectoid Reactions
These points represent specific invariant reactions that happen at a constant temperature in a material system (like an alloy) at a particular composition. The core distinction, as highlighted by the reference, is the state of matter involved before and after the reaction.
Eutectic Point
The eutectic point corresponds to the temperature and composition where a eutectic reaction occurs.
- Eutectic Reaction: In this type of reaction, a liquid state having a fixed composition gets solidified into two solids at a constant temperature.
Think of it like a specific melting point where, instead of turning into one solid, the liquid transforms directly into a mixture of two distinct solid phases simultaneously upon cooling. A classic example is the lead-tin solder alloy system. At the eutectic composition (around 61.9% Sn, 38.1% Pb) and temperature (183°C), liquid solder solidifies into a mixture of solid alpha (lead-rich solid solution) and beta (tin-rich solid solution) phases.
Eutectoid Point
The eutectoid point, on the other hand, represents the temperature and composition where a eutectoid reaction takes place.
- Eutectoid Reaction: In this type of reaction, two different solids gets transformed into a third solid at a constant temperature.
This reaction occurs entirely within the solid state. One solid phase transforms into a mixture of two other solid phases. A prime example is the iron-carbon system (steel). At the eutectoid composition (0.76% Carbon) and temperature (727°C), solid austenite (γ-Fe) transforms into a mixture of solid ferrite (α-Fe) and solid cementite (Fe₃C), known as pearlite.
Key Differences Summarized
Here's a table to clearly outline the distinctions:
| Feature | Eutectic Point/Reaction | Eutectoid Point/Reaction |
|---|---|---|
| Reactant Phase(s) | One Liquid Phase | One Solid Phase |
| Product Phase(s) | Two Solid Phases | Two Different Solid Phases (from the reactant) |
| Temperature | Constant | Constant |
| State Transition | Liquid → Solid + Solid | Solid → Solid + Solid |
| Example System | Lead-Tin (Solder), Aluminum-Silicon | Iron-Carbon (Steel) |
In essence, the eutectic point involves a transformation from liquid to solid, while the eutectoid point involves a transformation entirely within the solid state. Both are crucial points on phase diagrams indicating invariant reactions at specific compositions and temperatures.
