First-principles theory of short-range order in size-mismatched metal alloys: Cu-Au, Cu-Ag, and Ni-Au

We describe a first-principles technique for calculating the short-range order (SRO) in disordered alloys, even in the presence of large anharmonic atomic relaxations. The technique is applied to several alloys possessing large size mismatch: Cu-Au, Cu-Ag, Ni-Au, and Cu-Pd. We find the following: (i) The calculated SRO in Cu-Au alloys peaks at (or near) the (Formula presented) point for all compositions studied, in agreement with diffuse scattering measurements. (ii) A fourfold splitting of the (Formula presented)-point SRO exists in both (Formula presented) and (Formula presented) although qualitative differences in the calculated energetics for these two alloys demonstrate that the splitting in (Formula presented) may be accounted for by (Formula presented) K energetics while (Formula presented) K configurational entropy is necessary to account for the splitting in (Formula presented) shows a significant temperature dependence of the splitting, in agreement with recent in situ measurements, while the splitting in (Formula presented) is predicted to have a much smaller temperature dependence. (iii) Although no measurements exist, the SRO of Cu-Ag alloys is predicted to be of clustering type with peaks at the (Formula presented) point. Streaking of the SRO peaks in the (Formula presented) and (Formula presented) directions for Ag- and Cu-rich compositions, respectively, is correlated with the elastically soft directions for these compositions. (iv) Even though Ni-Au phase separates at low temperatures, the calculated SRO pattern in (Formula presented) like the measured data, shows a peak along the (Formula presented) direction, away from the typical clustering-type (Formula presented) point. (v) The explicit effect of atomic relaxation on SRO is investigated and it is found that atomic relaxation can produce significant qualitative changes in the SRO pattern, changing the pattern from ordering to clustering type, as in the case of Cu-Ag.