The Equilibrium α (Al-Li Solid Solution) and Metastable δ′ (Al3Li) Phase Boundaries in Aluminum–Lithium Alloys

Abstract

Data on the equilibrium solubilities of the α Al-Li solid solution phase and the ordered metastable δ′ Al3Li (L12 crystal structure) precipitate phase are critically reviewed, and a new binary alloy phase diagram is proposed. The δ′ solvus, describing the equilibrium solubility of Li in the α phase, Xαe, in atom fraction Li, is given by the equation $$X_{\alpha e} \, = \,0.{6}00{\text{86 exp}}\left\{ {{-}{8669}.{55}/{\text{R}}T} \right\},$$ X α e = 0.600 86 exp - 8669.55 / R T , where R is the gas constant, and the temperature T is in K. The α solvus, i.e. the equilibrium solubility of Li in the δ′ phase, Xδ′e, is given by the equation $$X_{\delta \prime e} \, = \,0.{18}0{9}\, + \,{6}.{413}\, \times \,{1}0^{{{-}{4}}} {{T}}{-}{1}.{861}\, \times \,{1}0^{{{-}{6}}} T^{{2}} \, + \,{1}.{4684}\, \times \,{1}0^{{{-}{9}}} T^{{3}} ,$$ X δ ′ e = 0.1809 + 6.413 × 1 0 - 4 T - 1.861 × 1 0 - 6 T 2 + 1.4684 × 1 0 - 9 T 3 , which represents a compromise between previously published theoretical curves that predict retrograde behavior. It is emphasized that that all the data cited and re-analyzed exclusively involve binary Al-Li alloys. The new phase diagram eliminates data that were previously mis-attributed. Most importantly, it is informed by considerable re-analysis of previously published data, supplemented by the inclusion of data that were not previously considered, and eschews data on both Xαe and Xδ′e that are indubitably non-equilibrium in nature.