A phase diagram is a chart that demonstrates material properties of a compound or element. In a crystalline structure, there exist multiple phases that are achievable. Normally, one only thinks in terms of solids, liquids, and gasses; however, there are more exact measures available. When trying to determine the structure of binary systems, it is easy to visualize. Whenever trying to determine how a compound will react to different pressures and temperatures at different compositions of alloy, that is another matter altogether.
The above is a phase diagram for different compositions of iron.
It specifically shows the melting point of iron and how it varies with regards to carbon content. It also demonstrates how the crystalline lattice structure will change as both the composition percentage and temperature change. The points where the material will be a liquid/solid mixture is referred to as the “liquidus” while the area where it is a 100% solid phase is called the “solidus.”
This is called a ternary phase diagram. It is a more modern development that demonstrates a “recipe” for a specific type of material. In this case, it is showing how to produce 18-8 stainless steel. It may seem complicated at first but it is actually fairly simple. To determine the iron composition, draw a line parallel to the opposite side of the iron vertex. When trying it at home, I found a credit card worked best. Where that line intersects with the dot is read on the Iron axis. In this case, the chart shows about 74%. Do the same for each side to get 8% Nickel and 18% Chromium respectively.
Phase diagrams were originally developed in an attempt to understand Iron-Cementite. This was an extremely strong alloy used by the Karnataka people of south west India. Roberts-Austen of Cambridge University and Bakhuys Roozeboom developed the first phase diagram in an attempt to understand how to develop this very string alloy of steel.