ABSTRACT
The properties of metallic alloys depend on their microstructures, which evolve as a result of transformations occurring throughout their production processes, such as solidification, thermomechanical treatments and heat treatments. Equilibrium composition–temperature phase diagrams for atmospheric pressure are the starting point for understanding how microstructures develop during most processing. However, useful microstructures are often far from equilibrium and an understanding of the thermodynamic basis of phase diagrams for stable and metastable equilibrium helps to increase their usefulness. This chapter explains how binary and ternary phase diagrams can be understood in terms of Gibbs free energy curves and surfaces. It shows how, when temperature changes lead to nonequilibrium conditions, the driving force for transformation towards equilibrium or metastable equilibrium is derived and introduces the concepts of chemical potential and activity that are needed in later chapters to understand how atoms behave in an alloy subjected to the temperature changes that occur throughout the processing chain.
