Chapter 3 - Structure and Stability of Alloys

Abstract

This chapter examines some of the factors that determine the limits of solid solubility in metallic systems and discusses some properties of the structure of extended solid solutions such as lattice spacings, defects, departure from randomness, and size effects. Solid solutions are phases of variable composition and in principle any number of components can be alloyed together to form a series of solid solutions. A solid solution is obtained when atoms of different elements are able to share together, with changing proportions, various sites of a common crystalline lattice. It is generally recognized that all metals and compounds show some solubility in the solid state. The replacement of copper atoms by nickel on the lattice of pure copper is an example of a “substitutional solid solution.” Because the two elements can be substituted at all proportions throughout the whole system, they form a continuous series of solid solutions. If the solid solubility is limited to only those portions of the phase diagram that are linked to pure elements, the resulting phases are known as “primary (or terminal) solid solutions.” Such solutions have the same structure as the elements on which they are based. All other phases are usually known as “intermediate phases;” they may be called “intermetallic compounds” or “valence compounds” if their solid solubility is unusually restricted around a stoichiometric composition. Intermediate phases typically possess structures that are different from the structure of either of the component elements.