The precipitation evolution and coarsening resistance of dilute Al-Zr-Er-Yb (-Sc) alloys

The precipitation and coarsening behavior of L1₂ nanophases in Al-0.15Zr-0.15Er-0.15Yb-xSc (wt. %) alloys during aging were characterized by Double-Cs-corrected STEM, TEM, microhardness testing, electrical conductivity measurement, and first-principles calculations. The results indicate that the Al₃(Yb, Er) initiates precipitation at approximately 175 °C with substantial precipitation occurring at 250 °C. The Al₃Sc and Al₃Zr precipitate at approximately 325 and 450 °C, respectively. The core/shell precipitates initially form in Al-Zr-Er-Yb-Sc alloys consisting of an Al₃(Yb, Er) core, an Al₃Sc inner shell, and an Al₃Zr outer shell. Upon prolonged aging, the core and inner shell remain as the Al₃(Yb, Er) and Al₃Sc, respectively, with the outer shell transforming into Al₃(Yb, Er, Zr), and the interface with α-Al remaining as the Al₃Zr. The precipitation evolution of the core-shell phases aligns with thermodynamic predictions based on solute segregation energies and phase interface energies. The increase of Sc content can effectively improve the aging response rate and strength of alloys. A modest Sc addition notably improves the coarsening resistance of the precipitation, while an excessive amount does not further improve this resistance.