I remember mentioning before something about the stress-strain curve being the single most important thing you need to understand in material science. Well, add phase diagrams to the list of single most important won’t ya? (Trust me, I get that you can’t have two single most anythings)

When materials are made of two types of elements they form either a solution or a mixture. Often times the two are related as in the following phase diagram.

A Solution is when the two materials are maintained in a single phase.

There will be a breaking point where either there is too much concentration of the solute or the temperature is too low, and the solution will split into two phases.

A mixture is when materials are combined, but more than one phase is observed.

That point of transition is called the solubility limit. On the phase diagram above, the solubility limit at temperature = 20 degrees celsius is marked. We see that at 20 degrees Celsius, any ratio of more than 65% sugar by weight, will take the single phase syrup into a liquid + sugar mixture.

Just some simple terminology to get out of the way

components: the elements or compounds which are present in the mixture
phases: the physically and chemically distinct regions

Types of Phase Diagrams

This is a Binary phase diagram. There are only two components: 1 solid and 1 liquid.

This is a Binary Eutectic phase diagram. There is 1 liquid and 2 solids. The eutectic point is the point where the liquid phase transitions directly to the solid phase. ( Hint: it’s the bottom of the v shape, or if you prefer, the flying birdy)

Other types of phase diagrams are Eutectoid and Peritectic. I apologize as I do not have diagrams for those two right now.

A eutectoid phase diagram is characterized by 1 solid phase in equilibrium with 2 other solid phases. So in this case the 1 solid phase will transition immediately into 2 solid phases.

A Peritectic reaction is where (1 solid + 1 liquid) are in equilibrium with another solid. So (1 solid + 1 liquid) –> totally different solid.

The Lever Rule

The lever rule and the tie line are used to determine the phase composition of any point in the phase diagram

In the above diagram we are concentrating on point B (in the center). Say we want to figure out what the phase composition is at point B.

The first thing that you want to notice when you look at the diagram is that this is a binary phase diagram, with 1 liquid phase and 1 solid phase. Point B is in the middle biphasic region. To figure out the percent composition we draw a tie line.

A tie line is an isothermal line connecting the compositions of two phases in a two phase field. Looking at the tie line, we can see that Point B is disproportionately closer to the liquid phase than the solid phase. If you can imagine the tie line as a lever and Point B as the pivot, then the liquid phase is totally winning this game of see-saw.