Phase Diagrams

Part 1

The properties of a material depend on the type, number, amount, and form of the phases present, and can be changed by altering these quantities. In order to make these changes, it is essential to know the conditions under which these quantities exist and the conditions under which a change in phase will occur.

The best method to record the data related to phase changes in many alloy systems is in the form of phase diagrams, also known as equilibrium diagrams or constitutional diagrams.

In order to specify completely the state of a system in equilibrium, it is necessary to specify three independent variables. These variables, which are externally controllable, are temperature, pressure and composition. With pressure assumed to be constant at atmospheric value, the equilibrium diagram indicates the structural changes due to variation of temperature and composition. Phase diagrams show the phase relationships under equilibrium conditions, that is, under conditions in which there will be no change with time. Equilibrium conditions may be approached by extremely slow heating and cooling, so that if a phase change is to occur, sufficient time is allowed.

Phase diagrams are usually plotted with temperature as the ordinate, and the alloy composition as the abscissa as shown in Figure 1.

Figure 1. Sample Phase Diagram.

Experimental Methods:

The data for the construction of equilibrium diagrams are determined experimentally by a variety of methods, the most common methods are:

Metallographic Methods

X-ray Diffraction Technique

Thermal Analysis

Metallographic Methods:

This method is applied by heating samples of an alloy to different temperatures, waiting for equilibrium to be established, and then quickly cooling to retain their high-temperature structure. The samples then examined microscopically. This method is difficult to apply to metals at high temperatures because the rapidly cooled samples do not always retain their high-temperature structure, and considerable skill is then required to interpret the observed microstructure correctly.

X-ray Diffraction Technique:

This method is applied by measuring the lattice dimensions and indicating the appearance of a new phase either by the change in lattice dimension or by the appearance of a new crystal structure. This method is very precise and very useful in determining the changes in solid solubility with temperature.

Thermal Analysis:

This is by far the most widely used experimental method. It relies on the information obtained from the cooling diagrams. In this method, alloys mixed at different compositions are melted and then the temperature of the mixture is measured at a certain time interval while cooling back to room temperature.

A cooling diagram for each mixture is constructed and the initial and final phase change temperatures are determined. Then these temperatures are used for the construction of the phase diagrams.

Cooling Curve of a Pure Metal:

Under equilibrium conditions, all metals exhibit a definite melting or freezing point. If a cooling curve is plotted for a pure metal. It will show a horizontal line at the melting or freezing temperature.

Figure 2. Cooling curve for the solidification of a pure metal.

Cooling Curve of a Solid Solution:

A solid solution is a solution in the solid state and consists of two kinds of atoms combined in one type of space lattice. A solution is composed of two parts: a solute and a solvent. The solute is the minor part of the solution or the material which is dissolved, while the solvent constitutes the major portion of the solution. When solidification of the solution starts, the temperature may be higher or lower than the freezing point of the pure solvent. Most solid solutions solidify over a range in temperature. Figure 3 shows the cooling curve for the solidification of a solid solution.

Figure 3. Cooling curve for a solid solution.

Figure 4. Series of cooling curves for different alloys in a completely soluble system. The dotted lines indicate the form of the phase diagram.

Figure 5. Phase Diagram of alloy A+B

 

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Last Updated : September 17, 1999

By: Serdar Z. Elgun