Double-Shelled L12 Nano-structures in Quaternary Al–Er–Sc–Zr Alloys: Origin and Critical Significance

The formation of highly coherent double-shelled L1 2 nano-precipitates in dilute Al–Er–Sc–Zr alloys was investigated with the combined use of Cs-corrected transmission electron microscopy characterization and first-principles energetics calculations. The double-shelled nano-precipitates are primarily featured with an Er-rich core surrounded by a Sc-rich inner shell and a Zr-rich outer shell. First-principles energetics analyses based on the classic homogenous nucleation theory suggested that once forms, this double-shell structure can be thermally stable. The predominant formation of this double-shell structure has thus both profound kinetic and thermodynamic origins. Its formation and stability preference to all other possible L1 2 nano-structures would become more pronounced as its size increases, no matter what the solute ratio and aging temperature of interest.