Creep properties and precipitate evolution in Al–Li alloys microalloyed with Sc and Yb

A dilute Al–Sc alloy (Al–0.12 Sc, at.%, Al–Sc), its counterpart with a Li addition (Al–2.9 Li–0.11 Sc, at.%, Al–Li–Sc), as well as a quaternary alloy (Al–5.53 Li–0.048 Sc–0.009 Yb, at.%, Al–Li–Sc–Yb) were isothermally aged at 325 °C, and in some cases isochronally aged to 450 °C. As the α′-Al3(Li,Sc) and Al3(Li,Sc,Yb) precipitates, with L12 structure, coarsen in the two Li-containing alloys, their Li and Yb concentrations decrease and their Sc concentration increases. A significant interfacial excess of Li also segregates at the α-Al matrix/α′-Al3Sc(Li,Sc,Yb) precipitate interface: 5.99 ± 0.05 atoms nm−2 in Al–Li–Sc and 13.2 ± 0.4 atoms nm−2 in Al–Li–Sc–Yb after aging isochronally to 450 °C. During compression creep at 300 °C, the aged alloys exhibit threshold stresses between 8 and 22 MPa. A recent threshold stress model based on elastic interactions between dislocations and precipitates predicts correctly that Li additions in the Al–Li–Sc alloy reduce the threshold stress, while Yb in the Al–Li–Sc–Yb alloy increases it. The model is also in agreement with the threshold stresses of all Al–Sc–X alloys published to date.