Solidification and melting are transformations between crystallographic and non-crystallographic states of a metal or alloy. These transformations are of course basic to such technological applications as ingot casting, foundry casting, continuous casting, single-crystal growth for semiconductors, directionally solidified composite alloys, and more recently rapidly solidified
alloys and glasses. Another important and complex solidification and melting process, often neglected in textbooks on solidification. Concerns the process of fusion welding. An nderstanding of the mechanism of solidification and how it is affected by such parameters as temperature distribution, cooling rate and alloying. is important in the control of mechanical properties of cast metals and fusion welds. It is the objective of this chapter to develop some of the basic concepts of solidification. and apply these to some of the more important practical processes such as ingot
casting, continuous casting and fusion welding. We then consider a few practical examples illustrating the casting or welding of engineering alloys in the light of the theoretical introduction.
This chapter 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.