Binary Phase Diagrams

A binary phase diagram consists of two variables, temperature and percentage composition. Although, phased diagrams will vary according to different pressures, binary phase diagrams are normally viewed at constant 1 atm pressure. A phase diagram can have more than two components as in the case of ternary phase diagrams but these are a lot more complex.
The binary phase diagrams are all drawn at an equilibrium state meaning that cooling rates during phase transformations are extremely low i.e. the decrease in temperatures which leads to the change in the phase is very slow. This almost never happens in real situations. The cooling rates which coincide with the change in phases is never really at an equilibrium temperature hence, the map or the data gathered from a phase diagram is not accurate. In practical situations cooling rates are usually too fast to be assumed as equilibrium. Take the iron carbide phase diagram, it shows that austenite phase of iron and carbon at a eutectoid composition (0.78% carbon) transforms to pearlite at 723 degrees Celsius, but under normal conditions with faster cooling rates this transformation occurs at lower temperature.

So then why do we use a binary phase diagram?

These phase diagrams provide a much simpler way to understand the phase transformations. It can also help us predict the microstructures which could be formed under equilibrium or non-equilibrium conditions. One could estimate the approximate solubility limit of alloy addition at various temperatures.