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

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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′-Al 3(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 9.1 at.% in the peak-aged state (8 h) to 5.7 at.% in the over-aged state (1536 h). As a result, the precipitate volume fraction decreases from 0.56% at peak aging time to 0.45% at 1536 h. 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.

Original languageEnglish (US)
Pages (from-to)1700-1715
Number of pages16
JournalActa Materialia
Volume59
Issue number4
DOIs
StatePublished - Feb 2011

Funding

This research is supported by the United States Department of Energy (Basic Energy Science) through grant DE–FG02–98ER45721. The LEAP tomography system was purchased and upgraded with funding from NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, and N00014-0910781) grants. ME Krug would like to thank Drs. C. Booth-Morrison and D. Isheim for helpful discussions.

Keywords

  • Aluminum alloys
  • Atom-probe field-ion microscopy
  • Nanostructure
  • Precipitation
  • Rare earth

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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