Thermal vibrations of atoms in equilibrium Al-Ag solid solutions

Abstract

An expression is derived for the effective mean square amplitude of thermal vibrations of atoms in a crystal lattice for the case of a binary, substitutional, metallic solid solution. All the possible configurations in the first coordination shell are respected. The dependence of$\overline {u^2 }$ on the Ag content is measured at 525°C for an equilibrium solid solution of Ag in Al from X-ray data. The results show that the values of$\overline {u^2 }$ increase for both the Al and Ag atoms as unequal neighbours are added to the first coordination shell. The relation between the force constant and$\overline {u^2 }$ is also derived. By using this relation for our measurements it was shown that the force constants decrease in both Al and Ag atoms as the number of nearest neighbours of different kinds increases; it thus follows that the bonding forces between different kinds of atoms are smaller than between atoms of the same kind. We therefore suppose that this describes the tendency of the equilibrium solid solution to form local regions with increased concentration of identical atoms.