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The solubility and stability of three possible ordered Al3Li structures in Al-Li alloys are studied using first-principles calculations: δ′-Al3Li(L12), δ-Al3Li(DO22), and β-Al3Li(DO3). We find that δ′-Al3Li(L12) is the most stable phase and β-Al3Li(DO3) is energetically unfavorable. The vibrational formation entropy makes a significant contribution to the solubility for all three ordered Al3Li structures and yields a 1.6-fold increase in the calculated solubility of δ′-Al3Li(L12), a 1.8-fold increase for δ-Al3Li(DO22), and a 2.5-fold increase for β-Al3Li(DO3). The solubility of δ′-Al3Li(L12) is greater than those of δ-Al3Li(DO22) and β-Al3Li(DO3), and the δ′-Al3Li(L12) solvus curve is in good agreement with the experimental one.