interpretation of phase diagram

7/29/2019 Interpretation of Phase Diagram

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Interpretation of Phase Diagram


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Phase Rule

1. Phase rule: P + F = C + 2

P: number of phase

(e.g. Polymorphism, solid solution)

F: number of degrees of freedom

(temperature, pressure and composition of phases)

C: number of components

(minimum number of constituents in

composition)

2. Condensed Phase Rule: P + F = C + 1

In many solids with high melting temperature, the vapor phase is

effectively non-existent


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One-Component Systems

According to phase rule: P + F = C + 2 = 3 (single component)

Independent variables in one-component system are temperature

andpressurebecause the composition is fixed.

Bivariant (F=2), Univariant (F=1) and Invariant (F=0); univariant curves,

triple points


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One-Component Systems_H2O

Ice less dense than liquid water at 0 C

The melting of ice I is accompanied by a decrease in volume; increase pressure

makes melting easier (direction YX)


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One-Component Systems_SiO2

-quartz -quartz -tridymite -cristobalite liquid


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Two-Component Condensed Systems

Two-component or binary systems have three independent variables:

pressure, temperatureand composition

Condensed Phase Rule: P + F = C + 1 = 3 (binary system: C=2)

Simple eutectic system

Liquid solution: single phase


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Simple Eutectic System

Liquidus curve (xyz) : highest temperature at which crystals can exist

Solidus curve (cyd) : highest temperature at which liquid can exist

Eutectic point (y) : invariant point (minimum on the liquidus curve)

Level rule: relative amounts of two phases in a mixture


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Simple Eutectic System

Saturation solubility curve

A small amount of soluble impurity A will lower the melting point

of B from T5 to T4. For example, H2O-NaCl.


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Binary System with Compounds

Congruent melting: It changes directly from solid AB to liquid of the

same composition at temperature T3

No changes would be observed at T1 and T2 on

heating pure AB


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Binary System with Compounds

Incongruent melting: AB A + liquid

Peritectic point: Invariant point at which three phases coexist, A, AB and liquid

Non-equilibrium effects:


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Binary System with Solid Solutions

Solid solution: shows complete

miscibility in both solid and liquid

states

Liquidus and solidus are smooth

curves which meet only at the

end-member composition A and B.

Fractional crystallization:

cooling takes place sufficiently slow Coring


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Binary System with Partial Solid Solutions

Simple eutectic systemBinary system

Binary system with

incongruent melting

compound


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Binary System with Solid-Solid Phase Transitions


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Binary System with Phase Transitions & Solid Solutions

Eutectoids: point b

(eutectoid reaction)

Peritectoids: point y


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Binary System with Liquid Immiscibility

MgO-SiO2:

interpretation of phase diagram

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