Review of Thermodynamics-L3

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This lecture deals with some of the basic thermodynamic concepts that are required for a more fundamental appreciation of phase diagrams and phase transformations. It is assumed that the student is already acquainted with elementary thermodynamics and only a summary of the most important results as regards phase transformations will be given here. Fuller treatment can be found in the books listed in the bibliography at the end of this chapter. The main use of thermodynamics in physical metallurgy is to allow the prediction of whether an alloy is in equilibrium. In considering phase transformations we are always concerned with changes towards equilibrium. And thermodynamics is therefore a very powerful tool. It should be noted. however. that the rate at which equilibrium is reached cannot be determined by thermodynamics alone, as will become apparent in later chapters.
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The goal of this section is to predict what will happen to our element when it is subjected to changes in those variables that we can manipulate, usually temperature and pressure. For example, what happens when we heat a sample of pure sulfur? It will probably melt at some point. What happens when we subject carbon to very high pressures? We predict that diamond will form. We seek quantitative explanations of these phenomena and an ability to predict under what conditions they will occur.
Consider a single component, A. A component is a chemical constituent (element
or compound) that has a specified composition. For simplicity, we will assume that
component A is an element, but we will see in subsequent sections that it can be
anything that does not undergo a chemical change, including compounds. But for now, we have an element, A, that can exist in two phases, which we will designate ? and ?. A phase is defined as a homogeneous portion of a system that has uniform physical and chemical properties