Role of vacancies in coprecipitation of δ′ – and S–phasesin Al–Li–Cu–Mg alloys

Abstract

Precipitation reactions in a series of Al–Li–Cu–Mg alloys, encompassing the composition ranges of the commercial Lital A and Lital B materials, have been studied to determine the role of solute and vacancy concentrations on homogeneous S-phase (Al₂CuMg) precipitation. Homogeneous precipitation of S-phase only occurs when a critical combination of free vacancy and copper and magnesium supersaturation is achieved. Lithium reduces the free-vacancy concentration by strong binding between vacancies and lithium atoms or atom clusters. Precipitation of δ′ (Al₃ Li) releases the bound vacancies and thereby promotes S-phase formation and, in extreme cases, vacancy condensation, producing dislocation loops.

MST/350