The Equilibrium α (Al-Li Solid Solution) and Metastable δ′ (Al3Li) Phase Boundaries in Aluminum–Lithium Alloys
A.J. Ardell
Abstract
Abstract Data on the equilibrium solubilities of the α Al-Li solid solution phase and the ordered metastable δ ′ Al 3 Li (L1 2 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, X αe , in atom fraction Li, is given by the equation $$X_{\alpha e} \, = \,0.{6}00{\text{86 exp}}\left\{ {{-}{8669}.{55}/{\text{R}}T} \right\},$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>α</mml:mi> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mspace/> <mml:mo>=</mml:mo> <mml:mspace/> <mml:mn>0.600</mml:mn> <mml:mrow> <mml:mtext>86 exp</mml:mtext> </mml:mrow> <mml:mfenced> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>8669.55</mml:mn> <mml:mo>/</mml:mo> <mml:mtext>R</mml:mtext> <mml:mi>T</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>,</mml:mo> </mml:mrow> </mml:math> 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, X δ′e , is given by the equation $$X_{\delta \prime e} \, = \,0.{18}0{9}\, + \,{6}.{413}\, \times \,{1}0^{{{-}{4}}} {{T}}{-}{1}.{861}\, \times \,{1}0^{{{-}{6}}} T^{{2}} \, + \,{1}.{4684}\, \times \,{1}0^{{{-}{9}}} T^{{3}} ,$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>δ</mml:mi> <mml:mo>′</mml:mo> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mspace/> <mml:mo>=</mml:mo> <mml:mspace/> <mml:mn>0.1809</mml:mn> <mml:mspace/> <mml:mo>+</mml:mo> <mml:mspace/> <mml:mn>6.413</mml:mn> <mml:mspace/> <mml:mo>×</mml:mo> <mml:mspace/> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mi>T</mml:mi> <mml:mo>-</mml:mo> <mml:mn>1.861</mml:mn> <mml:mspace/> <mml:mo>×</mml:mo> <mml:mspace/> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mi>T</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mspace/> <mml:mo>+</mml:mo> <mml:mspace/> <mml:mn>1.4684</mml:mn> <mml:mspace/> <mml:mo>×</mml:mo> <mml:mspace/> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mi>T</mml:mi> <mml:mn>3</mml:mn> </mml:msup> <mml:mo>,</mml:mo> </mml:mrow> </mml:math> 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 X αe and X δ′e that are indubitably non-equilibrium in nature.