What is phase transition or phase transformation?

In thermodynamics, chemistry and numerous different affiliated fields, phase transitions (or phase change) are the physical processes of change among the basic states of matter like the solid phase, liquid phase, and the gas phase along with biological phases, metallurgy phases, and so on.

For instance, a thermodynamic system phase and the various states of matter have no conversion in their respective properties. Throughout the phase transition of a particular medium, some properties often discontinuously change due to external factors like temperature, pressure, and so on. For instance, the change of liquid to gas upon heating till its boiling point leads to sudden volume change. The computation of these external factors at which the transformation occurs is known as the phase transition. Phase transitions generally occur in nature and are applied in many technologies.

Phase transition

Phase transitions occur when the thermodynamic free energy of the system is non-analytic for certain alternatives of thermodynamic variables. It is crucial to note that phase transitions can occur and are described for non-thermodynamic systems, without a temperature parameter. Samples are inclusive of quantum phase transitions, dynamic phase transitions, and topological phase transitions. At the phase transition point (e.g. the boiling point), the liquid and the vapor phase have similar free energies and are probable to exist equally. Below the boiling point, the most stable one is the liquid state of the two.

The change of system state can be led past the point of phase transition without encountering a phase transition and the final state is metastable. This takes place in supersaturation, superheating, supercooling, and so on.

Phase transition examples

• The transition between the solid phase, the liquid phase, and the gaseous phase of one component, due to pressure and temperature effects.
• A two-component single-phase liquid is cooled and gets turned into two solid phases is termed as a eutectic transformation and the same process, but initiating with a solid rather than a liquid is termed as a eutectoid transformation.
• A metastable to equilibrium phase transformation. A metastable polymorph that is a quick formation will change to an equilibrium phase given enough thermal input.
• A peritectic transformation, in which a two-component single solid phase is placed under high temperatures and transforms into a solid phase and a liquid phase.
• A spinodal decomposition, where a single phase is cooled and divided into two arrangements of the same phase.
• The transition between the paramagnetic phases and the ferromagnetic phases.
• The martensitic transformation occurs as phase transformations in carbon steel and stands as a model for displacive or diffusionless phase transformations. The martensite is a very hard formation of steel crystalline structure because martensite is filled with carbon and stuck dislocations.

Types of phase transition

Ehrenfest classification

Paul Ehrenfest categorized various phase transitions based on their manner towards the thermodynamic free energy. Under this, phase transitions were titled by the lowest derivative of the free energy. The first-order phase transition demonstrates a discontinuity in the 1^st derivative of the free energy with regards to some thermodynamic variables. The various solid, liquid, and gas transitions are classified as first-order transitions because they demand a discontinuous transformation in density which is the 1^st derivative of the free energy with respect to pressure. The order parameter which is the 1^st derivative of the free energy with respect to an external field is a constant beyond the transition. These include the ferromagnetic phase transition in substances like iron, where magnetization is the 1^st derivative of the free energy with respect to the magnetic field applied strength, increasing steadily from zero as the temperature is lowered below Curie temperature. The magnetic susceptibility as the 2^nd derivative of the free energy with the field transforms discontinuously. No discontinuity exists in any free energy derivative like supercritical liquid-gas boundaries.

Modern classifications

First-order phase transitions 

These transitions are the ones that include latent heat and the transformation is concluded over a finite range of temperatures, but occurrence like supercooling and superheating survive and hysteresis is seen on thermal cycling.

Second-order phase transitions 

These transitions are also called ‘continuous phase transitions’ like ferromagnetic and thermal expansion transitions showing abrupt change at the glass transition temperature. Easy phase transitions apart from isolated ones, the existence of transition and multi critical lines when changing of external conditions like magnetic field or composition. The liquid-gas transition is seen in various polymers which are able to supercool far under the melting point.

Characteristic properties

Critical points

The system comprising of liquid and gas phase, a special amalgamation of pressure and temperature persists called as the critical point, near this critical point fluid is hot and suppressed and the difference of the liquid and gas phase is almost none.

Order parameters

At the critical point, the order parameter susceptibility will typically diverge.

Phase transformations in alloys

The metallurgical phases that exist in an alloy have a vast impact on the metal component properties.  Phases are distinct materials that contain the elements in the alloy and their properties have an impact on the entire alloy. In addition to this, the size, location, shape within the alloy also has effects on the overall alloy characteristics. Amongst various common alloys, it is feasible to change the phases present with heat treatment. Phase transformations occur when the alloy is heated or cooled from a higher temperature.

Many phase transformation types occur when an alloy is cooled from a higher temperature and depend on the specific alloy. All transformation types do not take place in alloys and some alloys have no transformations apart from a solid-liquid transformation. The two most common phase transformations faced with common alloys are eutectoid and precipitation. The most common alloy in which eutectoid transformation is encountered is steel. In the process of slow to moderate cooling, the austenite changes to ferrite and cementite.  The microstructure consists of cementite plates with ferrite in between the plates which are referred to as perlite. During faster cooling of certain alloy, the ferrite forms of needles, and the cementite forms as particles which is referred to as bainite. 

Precipitation transformation in alloys

Precipitation transformation occurs when an alloy is cooled from a high temperature.  At the elevated temperature, the phase present comprises the major element in the alloy with the alloying elements in a solid solution.  When the alloy is cooled, the solid solution is not capable of holding all the atoms of the alloying elements in solution, so precipitates are created that comprise of the solute atoms and if possible, the atoms of the major element in the alloy. The process is modified by first quenching the alloy to room temperature to compress the atom movement.  Then the alloy is either allowed to change at room temperature, if room temperature transformation is feasible, or the alloy is heated again to a medieval temperature to speed up the transformation. 

Context and Applications

• Bachelors in Technology (Civil Engineering)
• Bachelors in Technology (Mechanical Engineering)
• Masters in Science (Metallurgy)

Practice Problems

1. Which type of transformation includes latent heat?

1. Ehrenfest classification
2. First-order phase transitions
3. Second-order phase transitions
4. Alloy transitions

Correct option- b

Explanation: First-order phase transitions include latent heat.

2. A two-component single solid phase is placed under high temperatures and transforms into a solid phase and a liquid phase is known as?

1. Peritectic transformation
2. Martensitic transformation
3. Spinodal decomposition
4. Eutectic transformation

Correct option- a

Explanation: A two-component single solid phase is placed under high temperatures and transforms into a solid phase and a liquid phase is known as peritectic transformation.

3. The most common phase transformation faced with any alloy is?

1. Metaphase
2. Bainite
3. Ferrite
4. Eutectoid

Correct option- d

Explanation: The most common phase transformation faced with any alloy is eutectoid.

4. An uncommon amalgamation of pressure and temperature persists called as?

1. Perlite
2. Microstructure
3. Critical point
4. Melting point

Correct option- c

Explanation: An uncommon amalgamation of pressure and temperature persists called a critical point.

5. Which transformation occurs when an alloy is cooled from a high temperature?

1. Precipitation transformation
2. Eutectic transformation
3. Martensitic transformation
4. Peritectic transformation

Correct option- a

Explanation: Precipitation transformation occurs when an alloy is cooled from a high temperature.

Related Concepts

• The lattice points of the critical lattice
• The melting point of phase transformation substances
• Metallurgy
• States of Matter