On the α/β brass type of equilibrium

In alloys of copper and silver with many of the elements of the B subgroups, the first additions of the solute element result in the formation of a primary or α-solid solution in copper or silver, and, when the solubility limit of this is exceeded, a second phase is formed with a body-centred cubic structure, which may have a disordered (β), or an ordered (β´) structure. The two-phase areas of these equilibrium diagrams are characterized by the fact that the α/α + β and α + β/β phase boundaries are approximately parallel, and slope so that the solubility limit of the α phase increases with falling temperature; this kind of equilibrium will be called the α/β brass type of equilibrium, and is in contrast to the equilibrium between the α and β´ phases, in which the phase boundaries follow quite different principles. It is well known that the α/β brass type of equilibrium is controlled largely by the electron concentration, i.e. the number of valency electrons per atom; a theory based on this assumption was advanced by H. Jones (1937b). In this theory the electron concentration only was considered, but it was shown by Hume-Rothery (1936) that the equilibrium was influenced by the atomic size factors and lattice distortions in the different systems.