Theory

Binary phase diagrams represent the relationships between temperature, composition and quanties of phases. Pressure is constant for binary phase diagrams. These diagrams aid in the prediction of phase transformations and the resulting microstructures.

A general binary phase diagram:

In the liquid and solid (L +α) portion of the diagram , the alloy exists as a mixture of liquid and solid.

At temperatures above the liquidus line, the alloy exists as a liquid.

At temperatures below the solidus line the alloy exists as a solid.

A video, summarising binary phase diagrams and the information represented by them:

Binary phase diagrams are very useful. For example, a binary system of known temeperature and composition can be used to work out:

1. The phases present

2. The composition of these phases

3. The percentage/fractions of these phases

Using binary phase diagrams to find out the phase compositions:

Using binary phase diagrams to find out the phase quantities:

Using binary phase diagrams to work out the composition of phases in the eutectic composition:

Summary of interpretation of binary phase diagrams:

• The compositions of phases are expressed by weight percentage (wt%) of the elements.

• For an alloy consisting of a single phase:

1. Composition of that phase is equal to the composition of the total alloy.

2. To determine fractional phase quantity (i.e. the mass fraction of liquid phase), the alloy is completely that phase.

• For an alloy consisting of a two phases:

1. Construct a tie line, and the extremeties determine the compositions of the two repsective phases.

2. To determine fractional phase quantity, use the lever rule and the following equations, where a ratio of the tie line segment lengths is determined.

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