Effects of Li additions on precipitation-strengthened Al–Sc and Al–Sc–Yb alloys

Abstract

Two Al–Sc-based alloys (Al–0.12Sc and Al–0.042Sc–0.009Yb, at.%) and their counterparts with Li additions (Al–2.9Li–0.11Sc and Al–5.53Li–0.048Sc–0.009Yb, at.%) are aged at 325°C. For both base alloys, the addition of Li results in greater peak hardness from incorporation of Li in the L12-structured α′-Al3(Sc,Li) and α′-Al3(Sc,Li,Yb) precipitates, and a concomitant increase in number density and volume fraction of the precipitates and a reduction in their mean radius. These changes result from a combination of: (i) an increase in the driving force for precipitate nucleation due to Li; (ii) a decrease in the elastic energy of the coherent misfitting precipitates from a decrease in their lattice parameter mismatch due to their Li content; and (iii) a decrease in the interfacial free energy, as determined from measurements of the relative Gibbsian interfacial excess of Li. In Al–2.9Li–0.11Sc (at.%), the Li concentration of the precipitates decreases from 9.1at.% in the peak-aged state (8h) to 5.7at.% in the over-aged state (1536h). As a result, the precipitate volume fraction decreases from 0.56% at peak aging time to 0.45% at 1536h. In Al–5.53Li–0.048Sc–0.009Yb (at.%), the relatively limited Li concentration produces only a small increase in Vickers microhardness from precipitation of metastable δ′-Al3Li upon a second aging at 170°C following the primary aging at 325°C.