The study of the possible ways in which various materials can exist by themselves or in contact with others, as a function of temperature, pressure, and time, is a very important part of Materials Science. The reason is that we have to be able to tell what the stability of materials will be in all sorts of environments, since the mechanical performance of these materials depends very much on this. An obvious example is that we cannot use structural materials above their melting point. Other less obvious changes can, however, happen inside materials that may be just as important from the point of view of strength. These change in phase, called phase transformations, can be recorded in diagrams called PHASE DIAGRAMS. The phase diagram is a type of map that allows us to predict what will happen when we change the temperature or the overall composition of the material.