Thermodynamic properties of the Cu-Al system: correlation with bonding mechanisms

Abstract

A wide-ranging study of the thermodynamic properties of the Cu–Al system at 773 K has been carried out. An e.m.f. technique, using solid CaF₂ as electrolyte was used to obtain the partial free energies of Al in a series of selected alloys. Enthalpies of formation (ΔH_f) were measured using a liquid-tin solution calorimeter and combined with integral free energies (ΔG_f) to evaluate the excess entropies (ΔS^xs_f). Volume changes on alloying (ΔV_f) were calculated from existing room temperature lattice parameters. Plots of ΔH_f, ΔS^xs_f and ΔV_f against composition show parallel relationships, particularly between ΔS^xs_f and ΔV_f. Due to lack of heat capacity data for the alloys, it is not possible to separate vibrational and configurational components of entropy but reasons are given for assuming that both may be related to volume changes.

Assuming volume contraction is associated with orbital overlap it is concluded that covalency is present in all the intermediate phases to varying extents. There is electronic spectroscopic evidence to support the view that there is an ionic contribution to bonding in the α-phase alloys which diminishes with increasing Al-content beyond this range. From other electron spectroscopic data, it is deduced that the covalent bonding in the γ₂-phase is largely due to orbital overlap while that of the ζ₂- and θ-phases arises mainly from charge localisation.