Phase Transitions in Eutectic Combinations

In summary: At temperatures between the eutectic point and the melting points of A and B, the free energy is higher for one than the other. That is why at this point, the mixture of A and B is stable and the two metals phase transform at a constant temperature.The eutectic point is a trade-off. It is a point where the free energy of the mixture is the same as the free energy of the pure substances. At temperatures between the eutectic point and the melting points of the two substances, the mixture is stable and the two substances phase transform.
  • #1
venkypf123
1
0
for a given combination of metals phase transformations occurs at a variable temperature which is in between the phase change temperatures of constituting metals except at eutectic combination. why at this combination phase transformation is taking place at a constant temperature? what is the reason?
 
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  • #2
Not quite sure what you mean by 'variable temperature' but this might help you understand.

Consider a simple two metal ( or other substance) system with metals A and B.

There are only three points on the melting temperature v composition diagram that where a single substance occurs.

These are
Pure A
Pure B
The Eutectic point.

At all other compositions there is more than one substance present. Hence the result is a mixture.

Either Eutectic plus excess A or Eutectic plus excess B.

Elementary physics tells us that

A single pure substance has a well defined single melting point.
A mixture of substances melts over a range of temperature, determined by the components of the mixture.
 
  • #3
For simplicity, let's stick to a binary system A-B and qualitatively construct free energy versus composition plots for the system.

A "classic" eutectic occurs when A and B are completely miscible in the liquid state and completely immiscible in the solid state.

Qualitatively, if you plot free energy versus composition, you get a U-shaped curve for the liquid because the lowest free energy occurs when A and B are mixed.

The corresponding curve for the solid state is an inverted U-shape because the lowest free energy occurs in the unmixed state. Forget the curved part for now; the minimum free energy plot for the solid state is a straight line joining the ends of the inverted U. The value of free energy is a "law of mixtures" weighted average of the free energies of pure solid A and pure solid B.

The relative positions of the U-curve and straight line depend on the temperature. At higher temperatures, the liquid state is the more stable, the free energy is lower, so the the U-curve is below the straight line. Conversely, at lower temperatures where the system is solid, the straight line is below the U-curve.

It is now easy to see that there is only one temperature (and one composition) at which the straight line is an exact tangent to the U-curve. At that unique point, the free energy of solid and liquid are the same, so A, B and liquid are at equilibrium. That is your eutectic point: one temperature, one composition.
 

What is a phase transition?

A phase transition is the physical process in which a substance changes from one phase to another, such as from a solid to a liquid or from a liquid to a gas. This can occur due to changes in temperature, pressure, or composition.

What is a eutectic combination?

A eutectic combination is a mixture of two or more substances that has the lowest possible melting point. This means that when the substances are combined in the correct proportions, they form a new phase with a melting point lower than that of any of the individual components.

What are the different types of eutectic combinations?

There are two main types of eutectic combinations: binary eutectic systems, which consist of two substances, and ternary eutectic systems, which consist of three substances. Within these categories, there can be different ratios and compositions that result in different eutectic points.

What is the significance of eutectic combinations in materials science?

Eutectic combinations have important applications in materials science, particularly in the development of new alloys and composite materials. They can also be used to improve the properties of existing materials, such as increasing their strength, reducing their melting point, or improving their corrosion resistance.

How are phase transitions in eutectic combinations studied and characterized?

Phase transitions in eutectic combinations can be studied and characterized through various techniques, including differential scanning calorimetry, X-ray diffraction, and microscopy. These methods allow scientists to observe and measure changes in the physical properties of the substances as they undergo phase transitions.

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