DETERMINATION OF THE SHORT-RANGE ORDER STRUCTURE OF AU-25 AT PCT FE USING WIDE-ANGLE DIFFUSE SYNCHROTRON X-RAY-SCATTERING

The short-range order (SRO) structure of a Au-25 at. pct Fe single cry stal was studied using wide-angle diffuse synchrotron X-radiation scat tering at room temperature. Two heat treatments were investigated: a 4 00-degrees-C aging treatment for 2 days and a 440-degrees-C treatment for 5 days, both preceded by solution treatment in the single-phase fi eld and water-quenched to room temperature. Evolution of the SRO struc ture with aging was determined by fitting the two sets of Cowley-Warre n (C-W) SRO parameters to a pair of 140,608-atom models. The microstru ctures, although quite disordered, showed a trend with aging for an in creasing volume fraction of an Fe-enriched and an Fe-depleted environm ent. The Fe-enriched environment displayed a preference for Fe segrega tion to the {110} and {100} face-centered cubic (fcc) matrix planes. A major portion of the Fe-depleted environment was found to contain ele ments (and variations of these elements) of the D1a ordered superstruc ture. The SRO contained in the Fe-depleted environment may be best des cribed in terms of the standing concentration wave packet (SCWP) model . This study provides, for the first time, a quantitative real-space v iew of the atomic arrangements, which make up both environments in the SRO structure of Au-25 at. pct Fe.