Electron factors in the occurrence of sigma and structurally related transition metal alloy phases

Stability of the sigma phase in transition metal systems is considered with respect to electron factors, and a pair of electronic coordinates derived from fundamental microscopic theory is used for a structural map of these alloy systems. Two electron factors are applied in the procedure. One factor, closely related to the e/a ratio, is the d-band vacancy count, which is the number of holes per atom in the alloy d-bands, calculated from theory. The second factor is the electronegativity difference, defined with attention to the sign of the magnitude. The structural map and its role in providing a bridge between practical metallurgy and fundamental microscopic theory are discussed.