THE HUME-ROTHERY RULES AND PHASE STABILITIES IN NOBLE-METAL ALLOYS

Results from martensitic transformations are used to evaluate stabilit ies of the equilibrium phases in the Hume-Rothery electron compounds b ased on Cu, Ag and Au, and to give arguments why the electron concentr ation plays such an important role in the selection of the crystal str uctures. It is shown that the vibrational entropy difference observed for the martensitic transformation from ordered bcc to the close packe d martensite and its e/a dependence can also account for the entropy d ifference Delta S-alpha/beta between the equilibrium alpha and beta at high temperatures, and can be made largely responsible for the compos ition dependence of the (alpha + beta) two phase field. The enthalpy o f mixing can be decomposed into a small term which depends on the aver age periodic lattice, which is different in alpha and beta but which i s nearly the same in all alloys studied, and a contribution which is d ue to the difference in the properties of the atoms and which can be e xpressed by pair interchange energies. This contribution depends stron gly on the specific alloy system, but is independent of structure, whi ch is compatible with a pair interchange energy depending only on pair distance but not on structure, as suggested by simple pseudopotential theory. The same pair interchange energies account also for long rang e order and the critical ordering temperature. The evaluation for seve ral alloy systems shows a surprisingly good agreement within this pict ure, and permits to understand better why the electron concentration p lays such an important role also for other structures, although the en ergy contribution of the conduction electrons is only a small part of the total enthalpy of formation of any of the equilibrium structures.