Chemistry and structure of core/double-shell nanoscale precipitates in Al-6.5Li-0.07Sc-0.02Yb (at.%)

Highlights: → An Al-Li alloy with dilute Sc and Yb-additions was double-aged for strengthening. → Due to kinetic effects, core/shell precipitates form during the first aging. → Lithium, though fast-diffusing, delays overaging of the alloy. → A second aging treatment resulted in core/double-shell precipitates. → Distinct hardening events correspond to the core, first shell, and second shell. - Abstract: An Al-6.3Li-0.07Sc-0.02Yb (at.%) alloy is subjected to a double-aging treatment to create nanoscale precipitates, which are studied by atom-probe tomography and transmission electron microscopy. After homogenization and quenching, Yb atoms form clusters exhibiting L12-like order. A first aging step at 325 deg. C leads to a doubling of microhardness as a result of the formation of coherent precipitates with an Al3Yb-rich core and an Al3Sc-rich shell. The core and shell both exhibit the L12 structure and both contain a large concentration of Li, which substitutes for up to 50% of the Sc or Yb atoms at their sublattice positions. These core/single-shell precipitates provide excellent resistance to overaging at 325 deg. C. Subsequent aging at 170 deg. C increases the microhardness by an additional 30%, through precipitation of a metastable δ'-Al3Li second shell on the core/single-shell precipitates, thereby forming a chemically and structurally complex core/double-shell structure. The metastable δ'-Al3Li phase is observed to form exclusively on pre-existing core/shell precipitates.