Thermodynamics and Phase Diagrams-Lecture 1

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 equiiibrium. 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, The study of phase transformations, as the name suggests, is concerned with how one or more phases in an alloy (the system) change into a new phase or mixture of phases. The reason why a transformation occurs at all 1s because the initial state of the alloy is unstable relative to the final state. But how is phase stability measured? The answer to this question is provided by thermodynamics. For transformations that occur at constant temperature and pressure
the relative stability of a system is determined by its GIbbs free energy
(G).The Gibbs free energy of a system is defined by the equation

G=TS

H is the enthalpy, T the absolute temperature, and S the entropy of the system. Enthalpy is a measure of the heat content of the system and is given by:
H=E+PV

where E is the internal energy of the system. P the pressure, and V the
volume. 1.