Relation between the electron to atom ratio and some properties of metallic systems

Hume-Rothery and his co-workers demonstrated the importance of the electron-to-atom (e/a) ratio in controlling the range of stability of solid solutions, intermetallic compound formation and liquidus temperatures in metals and alloys. Since then, this parameter has been shown to vary in a systematic manner with a number of other properties such as the axial ratio of hexagonal phases, formation of defect phases, stacking fault energy, electronic specific heat coefficient, flow stress, superconducting transition temperature, stress corrosion cracking, elastic constants, activation energy for diffusion etc. These relationships are reviewed here to show that the e/a ratio constitutes a useful parameter for rationalizing the effect of solute additions, in dilute concentration, on several properties of the solvent matrix. This approach can also be used in a limited way even in concentrated alloys.