Theoretical investigation of the precipitation of δ' in Al-Li

Abstract

This paper contains the results of a theoretical investigation of the equilibrium between a disordered fee solution and an Ll₂ phase in a model binary alloy and the transformation paths that may be followed when the disordered phase is quenched into the two-phase field. The results are specifically applied to binary Al-Li alloys, in which case the ordered phase is the metastable Al₃Li(δ') phase that precipitates from the disordered solid solution (α). The thermodynamic model assumes that the atoms interact in pairs with an interaction potential that is independent of the temperature and composition, and uses the “mean field approximation” for the entropy of mixing. The assumptions confine its applicability to temperatures well below the ordering temperature of the Ll₂ phase. The model is used to compute the two-phase field that separates the disordered solution and the Ll₂ phase. For the specific case of Al-Li, it provides results that fit the available experimental data and offer a simple explanation for the observed deviation from stoichiometry of the δ' phase. The model predicts that the disordered solution orders congruently on quenching, but is then unstable with respect to decom-position by a spinodal mechanism that leads ultimately to a state of ordered Ll₂ precipitates in a disordered matrix. The results provide plausible interpretations for the transformations observed in quenched Al-Li alloys.