Microalloying for the controllable delay of precipitate formation in metal alloys
A model is presented to predict the effectiveness of dilute solutes in delaying precipitate formation, with application to natural and artificial aging in metal alloys. Control of aging is achieved via the binding, at natural aging temperatures, and release, at artificial aging temperatures, of excess quenched vacancies by the solute atoms. The binding of vacancies to the solute atoms reduces the vacancy concentration in the bulk lattice, and thus reduces the rate of transport processes that control aging. To be useful, this strategy requires sufficiently strong, but not too strong, vacancy-solute binding, and sufficiently slow vacancy solute diffusion, both quantified through application of the model. First-principles methods are used to compute the controlling materials properties (vacancy-solute binding energies and migration barrier) for a range of solutes in Al and Mg, and Pauli Electronegativity of the solute relative to the host metal is found useful for correlating properties. With the computed inputs, the model is applied to representative solutes (Ga, Sn, Pb) in Al that span weak to strong vacancy binding. Predictions of the model to Sn in Al are then shown to qualitatively agree with experiments, with quantitative agreement possible using a slightly stronger Sn-vacancy binding energy. The general model should be useful for identifying promising dilute solutes for control of precipitation/aging across many alloys in which quenched vacancies are used to control precipitation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.