Structure factors and Debye temperatures of Al–Li solid-solution alloys

By combining the accurate low-angle X-ray structure factors of A1-Li solid-solution alloys (containing 5.25 and 8.06 at.% Li) determined by the critical voltage technique in high-energy electron diffraction (HEED) with higher-angle values obtained by interpolation between best pure-element form factors, a complete set of accurate X-ray structure factors for these alloys has been produced. From the measured Debye-Wailer factors for the alloys it was found to be difficult to determine a Debye temperature trend with composition for AI-Li solid-solution alloys because of the extent of the experimental errors, although the results suggest that the Debye temperatures of the alloys are higher than that of pure aluminium. This is obviously consistent with an increase in Young's modulus; i.e. the stiffness of the alloys appears to be greater than that of pure aluminium. This increase appears to arise predominantly from an increase in the force constant between nearest-neighbour (n.n.) lithium atoms in the alloy as compared with the value for pure lithium. This occurs because n.n. lithium atoms are closer together in Al-Li solid-solution alloys than they are in pure lithium. Because the lithium atoms are closer together in the alloys, the electron charge density, ρ, associated with the valence electrons in the alloys is likely to be higher than if ρ is considered unchanged by alloying. This suggested increase in the charge density of the alloy valence electrons was confirmed, as the experimental 111 low-angle structure factors of the alloys were found to be significantly higher than the equivalent values obtained by interpolation between the best pure-element form factors. Such electronic changes are to be expected for Al-Li alloys as aluminium and lithium have a valency difference of two.