Elastic core effect and clustering tendency by EXAFS in as-quenched Al-Zn alloys

Random distributions of solute atoms are observed for as-quenched solid solutions for concentrations of zinc up to 2.6 at.%. For Al-4.4 at.% Zn or Al-6.8 at.% Zn, evidence of clustering effects is observed: the first peak of the Fourier transform of the spectrum is reduced to 73% and 47% respectively by the mixing of Al and Zn backscatterers in the nearest-neighbour shell. For the as-quenched Al-4.4 at.% Zn, 1.5 (+or-0.25) zinc atoms are present among the twelve atoms of the first shell and 3.25 (+or-0.25) zinc atoms for Al-6.8 at.% Zn. As for Al-Cu, it has been possible to determine the magnitude of the 'elastic core effect' (strain on the first shell) in dilute solutions. As expected, this effect is weak, db approximately=-0.02+or-0.01 AA, where 'b' is the radius of the first shell in aluminium. Nevertheless, it is about three times higher than the calculated value derived from the slope of Vegard's law (da/ac=-0.020) throughout an elastic theory of a continuous medium.