A phase diagram is a two dimensional graph that represents the phase or phases that will appear in a material when it reaches equilibrium. The axis need not be temperature and pressure, in fact, for binary alloys they are often temperature and weight percent or volume percent of each of the two components. pHis another common coordinate for phase diagrams.
Materials Science and Engineering students are generally forced to study phase diagrams ad nauseam. This is because the phase of a material determines its properties.
What Phase means
The term "solid steel" means almost nothing. There several solid phases of steel and many other materials. In basic chemistry classes, we are often incorrectly taught that there are three phases of matter; Solid, Liquid, and Gas. This is far from true. Solid, Liquid, and Gas are simply three states of matter. There are many different ways in which the atoms in a mixture can arrange themselves. Each of these is a different phase. Steel can adopt a wide variety of different solid phases depending on its composition and temperature. Each one has a unique arrangement of atoms and unique properties.
Some Solid Phases of Steel
One of the key features of phase diagrams are the points where a phase transition occurs. The eutectic phase transition is the most frequently mentioned one. When such a transition occurs, a cooling homogenous single phase mixture to form a heterogeneous mixture with several different phases. For example, when water with air dissolved in it is frozen, it becomes a mixture of ice and air bubbles.
The rate of phase transitions in solids is limited by the rate of diffusion. Materials Science and Engineering students are often forced to study the kinetics and thermodynamics of phase trasnformations.
Spinodal decomposition is an important class of phase transitions in which the change in phase is spontaneous due to a decrease in Gibbs free energy. Decreases in Gibbs free energy occur when a material undergoes a transition from a metastable phase to a stable, equilibrium phase.
What Metastable Phases Are
Phase transistions require energy. Sometimes materials can spend a really long time in a phase that is not their equilibrium phase for their current state because they do not have enough energy to transform to their equilbrium phase. The state of a material is its temperature, the pressure it is under, etc... When a sword is heated, then quenched, the rapid cooling of the metal prevents diffusion from occurring fast enough for the metal to make the transition to its equilibrium phase. In other words, when you cool something down really fast, it may be stuck without enough energy to undergo a phase transition to its thermodynamically preferred phase. It is thus said to be stuck in a metastable phase. If heated gently, the metal will slowly undergo a spinodal decomposition to its equilibrium phase.
Polycrystalline Materials vs. Glass
A glass is a material with no long range order. A crystal is the exact opposite of this. It the atoms in a crystal are arranged in a pattern. Glasses form when something is cooled very fast, too fast to crystallize. Glasses are almost always a metastable phase. Quartz is the crystalline phase of SiO2, everyday glass is the metastable, disordered state of SiO2. Also, everyday glass has stuff added to it to lower its melting point and make disrupt the long range order of the molecules so that it will not crystallize as easily.
Metals are almost always in a polycrystalline state. Metallic glasses are a very Star Trekesque material. Almost all metals must be cooled extremely quickly to form metallic glasses.