As previously stated, the phase diagram is simply a map showing the structure of phases present as the temperature and overall composition of the alloy are varied. It is a very useful tool for understanding and controlling the structures of polyphase materials. A binary phase diagram shows the phases formed in differing mixtures of two elements over a range of temperatures. When an alloy exhibits more than two phases, a different type of phase diagram must be used, such as a ternary diagram for three phase alloys. This discussion will focus on the binary phase diagram.
On the binary phase diagram, compositions run from 100% Element A on the left, through all possible mixtures, to 100% Element B on the right. The composition of an alloy is given in the form A - x%B. For example, Cu - 20%Al is 80% copper and 20% aluminum. Weight percentages are often used to specify the proportions of the alloying elements, but atomic percent are sometimes used. Weight percentages will be used throughout this text.
Alloys generally do not have a single melting point, but instead melt (or alternately solidify) over a range of temperatures. At each end of the phase diagram only one of the elements is present (100% A or 100% B) so a specific melting point does exists. Additionally, there is sometimes a mixture of the constituent elements which produces melting at a single temperature like a pure element. This is called the eutectic point.
At compositions other than at the pure A, pure B and the eutectic points, when the alloy is cooled from a high temperature it will begin to solidify at a certain temperature but will remain in a mushy (liquid plus solid) condition over a range of temperatures. If experiments are conducted over a range of compositions to determine the temperature at which the alloys start to solidify, this data can be potted on the phase diagram to produce a curve. This “start of solidification curve” will join the three single solidification points and is called the liquidus line.
Up to a few percent of composition, it is possible for one element to remain dissolve in another while both are in the solid state. This is called solid solubility and the solubility limit normally changes with temperature. The extent of the solid solubility region can be plotted onto the phase diagram. In this example, the alpha phase is the region of solid solution where some of B atoms have dissolved in a matrix of A atoms. The beta phase is the region where a small percentage of A atoms have dissolved in a matrix of B atoms. It is important to note that some elements have zero solid solubility in other elements. An example is aluminum/silicon alloys, where aluminum has zero solid solubility in silicon.
If an alloy's composition does not place it within the alpha or beta solid solution regions, the alloy will become fully solid at the eutectic temperature. The eutectic line on the phase diagram indicates where this transformation will occur over the range of compositions. At alloy compositions and temperatures between the liquidus temperature and the eutectic temperature, a mushy mix of either alpha or beta phase will exist as solid masses within a liquid mixture of A and B. These are the alpha plus liquid and the beta plus liquid areas on the phase diagram. The region below the eutectic line, and outside the solid solution region, will be a solid mixture of alpha and beta.