Phase Equilibrium and Triple Point

Phase Rule and Phase Diagrams

• Super cooled water can change into ice when in contact with a small particle of ice, and the curve merges into OB.
• The equilibrium at any point on the curve is Super-cool water vapour.
• The degree of freedom of the system is one, making it univariant.

Triple Point

• The triple point is where the curves OA, OB, OC, and OA1 meet, and all three phases (ice, water, and vapour) are in equilibrium.
• The temperature and pressure at the triple point of water are 0.0098°C and 4.58 mm, respectively.
• The degree of freedom is zero, making it an invariant system.

Areas on the Phase Diagram

• The areas AOB, AOC, and BOC contain vapour, liquid, and ice phases, respectively.
• Within these single-phase areas, the system is bivariant, as predicted by the phase rule.

Two-Component System

• In a two-component system, when P=1, F=C-P+2, which means three variables must be specified to describe the state of the phase.
• A three-dimensional solid model phase diagram is required to represent the variables of temperature, pressure, and composition.

Iron-Carbon Phase Diagram

• The transformation temperature of 910°C for γ- to α- iron changes is lowered to 723°C as the carbon content goes up to 0.8%.
• The point G represents the eutectoid point, where three phases (austenite, ferrite, and cementite) co-exist in equilibrium.
• The eutectic mixture of austenite and cementite is called ledeburite.

Features of Fe-C Phase Diagram

• Upto 0.088% C are regarded as pure iron.
• 0.088% to 2% C represents steel.
• Above 2% C represents cast iron.
• Steel can be further divided into hypo-eutectoid steel (0.08 to 0.8% C), hyper-eutectoid steel (0.8 to 2% C), and eutectoid steel (0.8% C).

Heat Treatment of Steel

• Heat treatment is a combination of heating and cooling of metal or alloy in the solid state to get desired properties.
• During heat treatment, the size and shape of the grain or the composition of the phase undergoes change with respect to the micro constituents, and internal stress is relieved.

Ag/Pb System Phase Diagram

• The phase diagram consists of two curves (OA and OB) and five areas (AOB, BOD, AOC, COE, and DOE).
• The eutectic point is represented by 'O'.
• Curve OA is the melting point curve of Ag, and curve OB is the freezing point curve of Pb.
• Both curves represent univariant systems.