TYPES OF PHASE DIAGRAMS

Two metals are completely soluble in the solid state, the type od solid phase formed will be substitution solid solution.

The diagram show the series of cooling curves for different alloys in a completely soluble system. The dotted lines the form of phase diagram.

The diagram plotting temperature vs. composition.

TYPE-1: Two metals are completely soluble in liquid state and solid states

The cooling curves pure metals A and B show only a horizontal line because the beginning and end of solidification take place at constant temperature.

These cooling curves show two changes in slope.

The first break is at temperature T1, which indicates beginning od solidification and lower break at T2 indicates end of solidification

The two points TA and TB represent the freezing point at two pure metals

Liquids line: is upper line showing the beginning of solidification

Solidus line: is the lower line showing the end of solidification.

Between the liquids and solidus lines there exists a two phases region.

Two phase region does not give any information regarding mixture

In order to determine this information, It is necessary to apply two rules.

Rule-1: Chemical composition of phases

To determine the actual chemical composition of the phases of an alloy in equilibrium at any specified temperature in two phases region, draw horizontal temperature line called tie line

Draw the tie line mo to the boundaries of the field

Point m the intersection of the tie line with the solidus line, when dropped to base line, give the composition od phase that exits boundary.

Similarly, point O, when dropped to base line, will give the composition of the other phase constituting the mixture, in this liquid solution of composition.

Rule-2: Relative amount of each phases

To determine the relative amounts of the two phases in equilibrium at specified temperature in a two phase region

A vertical line represting the alloy and horizontal temperature line to the boundaries of the field.

The vertical line will divide the horizontal line into two parts whose length are inversely proportional to amount of phases present. This is also known as the lever rule.

The lever rule may be expressed mathematically as

TYPE-2: Two metals completely soluble in the liquid state and completely insoluble in the

solid state

This type of phase diagram is produce, when the two alloy components A and B are completely soluble in the liquid state but completely insoluble in solid state.

The points M and N indicate the melting points of two pure metals.

T1 show the beginning of solidification and TE end of solidification.

MEN is liquidus line and show beginning of solidification.

The point at which the liquidus lines intersect the minimum point E, is known as the eutectic point. TE is called eutectic temperature.

This phase diagram consists of four areas.

The area above the liquids line is a single-phase homogenous liquid solution.

The remaining three areas are two phase areas

The alloys to left of the eutectic composition as hypoeutectic alloys and those to right as hypereutectic alloys.

Alloy 1: is eutectic composition

It is cooled from temperature To

it remains a uniform liquid solution until the point E, the eutectic temperature line is reached

The eutectic reaction may be written as

The amount of percentage A and L determine by lever rule

TYPE-3: Two metals completely soluble in liquid state but only partly soluble in

solid state

This type of phase diagram show the partial solid solubility.

TA and TB indicate the melting points of two pure metals.

TAETB is liquids line and TAFEGTB show solidus line

Above the liquids line, the is only single phase liquid solution.

The lines FH and GH show that with the deceasing the temperature the maximum amount of solute can be dissolve deceases.

In alloy 1: liquids line is cross T1 it will begin to solidify by forming crystals of α solid solution extremely rich in A.

The process continuous with liquid getting richer in B and gradually moving down along the liquids line.

The α solid solution also getting richer in B, moving down along the solidus line.

When the solidus line crossed at T4 and with diffusion keeping pace with crystal growth the entire solid will be homogenous α solid solution.

Alloy 2: is the eutectic composition and remains liquid until the eutectic temperature is reached at point E.

Since this is also the solidus line, the liquid now undergoes the eutectic reaction, at constant temperature

The eutectic reaction may be written as

The lever rule is a tool used to determine weight percentages of each phase of equilibrium phase diagram.

The relative amounts of α and β in the eutectic mixture may be determined by lever rule