Effect of electric field on solute solubility in Al alloys measured by hardness

Abstract

The influence of dc electric field on pertinent solute solubility during the solution heat treatment of Al–Mg–Si–Cu AA 6111 Al alloy was determined by employing Vickers hardness and resistivity. Thermodynamic analysis of the results showed that the field decreased both entropy ΔS_s and enthalpy ΔH_s of the solution, but still gave a reduction in the Gibbs free energy ΔG_s = ΔH_s–TΔS_s and in turn an increase in solubility. The reduction in ΔG_s obtained from the hardness tests was similar to that from the resistivity measurements. The decrease in ΔG_s by the field suggests that there exists an electronic aspect to the Gibbs free energy of the solution in addition to the size misfit. The solid solution strengthening corresponding to the increase in solubility by the field was in accord with the Mott–Nabarro model. Considering several Al–Mg₂Si alloys, the solubility increase by an electric field was proportional to the difference in valence between the pertinent solute and the Al host.