Phase Equilibrium Diagrams

Phase equilibrium diagram is a graphic relationship between
temperature and weight ratios of elements and alloys contribute to the built of the diagram.

Where Phase is a uniform part of an alloy, having a certain chemical composition and structure, and which is separated from other alloy constituents by a phase boundary.

For example the salt – water solution have a four possible phases:
- Water vapor (steam)
- Liquid salt solution (sodium chloride in water)
- Crystals of water (ice)
- Crystals of salt (sodium chloride)

Alloying systems :

Alloy is a metal composing of a mixture of elements. Most of alloys are composed of a base metal with small amounts of additives or alloying elements.
The typical examples of alloys are steel/cast iron (iron base alloys), bronze/brass (copper base alloys), aluminum alloys, nickel base alloys, magnesium base alloys, titanium alloys.

There are many types of alloying systems which they are:

1- Binary system
It means that alloying have two metals only.
2- Ternary system
It means that alloying have three metals only.
3- Multi system
It means that alloying have three and more than that metals.


In general, binary alloys can be classified into the following types.

1- Simple eutectic type
The two components are soluble in each other in the liquid state but are completely insoluble in each other in the solid state.

2- Solid solution type
The two components are completely soluble in each other both in the liquid state and in the solid state.

3- Combination type
The two components are completely soluble in the liquid state, but are only partially soluble in each other in the solid state.
Thus this type of alloy combines some of the characteristics of both the previous types, hence the name ‘combination type’ phase equilibrium diagram.

Let s now consider these three types of binary alloy systems and their phase equilibrium diagrams in greater detail.

1) Phase equilibrium diagrams (Eutectic type):
In general case, consider for studying a two components presents which are referred to as metal A and metal B, with the phase diagram as shown in figure 1.







Figure 1. Phase equilibrium diagram (eutectic type).

This type of equilibrium diagram gets its name from the fact that at one particular composition (E), the temperature at which solidification commences is a minimum for the alloying elements present. With this composition the liquidus and the solidus coincide at the same temperature, thus the liquid changes into a solid with both A crystals and B crystals forming instantaneously at the same temperature. This point on the diagram is called the eutectic, the temperature at which it occurs is the eutectic temperature, and the composition is the eutectic composition.

In practice, few metal alloys from simple eutectic type phase diagrams.
It is identical with this type of phase diagram is produced for a salt (sodium chloride) and water solution, it is total solubility of the salt in water in the liquid state and total insolubility (crystals of ice and separate crystals of salt) in the solid state. As an example of e