What's a Phase?

In the previous section, we observed some of the physical consequences of phase behavior. We reduced the temperature of the iron piano wire, which caused it to change phase and, effectively, volume. We mixed together indium and gallium and observed a change of phase (indeed, a change of state of matter from liquid to solid). The change in phase was related to some change in properties.

Indeed, in materials science, we consider a phase a homogeneous region of matter than has is chemically uniform and physically distinguishable. We can define properties to each phase, and those properties are uniform throughout the phase. This fits well into our paradigm - clearly, we will have some region in space that can be defined as a crystal, an amorphous solid, a liquid a gas, and so on - and we can connect that region to all of the properties of interest: mechanical, chemical, optical, etc. The most familiar example to you might be that of $\ce{H2O}$. We know that at low temperatures (and standard pressures), we have solid water ice. At higher temperatures - like those of a nice summer day in Evanston, we have liquid water. And if we put liquid water on a stove to make tea, we can get a gas phase of $\ce{H2O}$ upon boiling. These are, indeed, all different phases of water, each with their own "structures" and properties.

Read on or, if you like, watch the complimentary video in Section 10.3.4.

Other systems have multiple phases as well, of course. Let's look at a cartoon metal alloy, shown in Figure 10.3.2. There, by inspection, you can probably observe that there's two distinct regions with different structures. In one, we have copper and silver balls randomly distributed in a close-packing arrangement. We've seen this before - it is a solid solution. We also see a region in which the atoms seem more ordered, with alternating positions of copper and silver balls. This region looks perhaps more like the structures we discussed in the ionic materials chapter. These are two different phases within the material and are distinguished by their individual structure.

So - we know that different phases can exist in different conditions (temperature, pressure, etc.) We know that these different phases are important with regards to understanding and predicting properties. It might, then, be a good idea if we could create some sort of map of which phases are present in different conditions! Over the next few sections, we'll learn how to explore these maps, called phase diagrams and extract valuable information. By the end of these sections, you'll be able to, when provided with 1. certain thermodynamic information (i.e., temperature, pressure, composition) and 2. the phase diagram, determine the following information:

1. The phases present in the system.
2. The chemical compositions of the phases, typically in weight percent or atomic percent.
3. The phase fractions - or how much of each phase in in the system.
4. A schematic microstructure for the system.

Before we continue, a caveat: we will show equilibrium phase diagrams in this chapter. This means that we assume no macroscopic change is continuing. The system will not tend towards any other structures, and so the observed structures are stable.

Before we show you how to interpret these phase diagrams, complete Exercise 10.3.1.