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 [100] point for all compositions studied, in agreement with diffuse scattering measurements. (ii) A fourfold splitting of the X-point SRO exists in both Cu0.75Au0.25 and Cu0.70Pd0.30, although qualitative differences in the calculated energetics for these two alloys demonstrate that the splitting in Cu0.70Pd0.30 may be accounted for by T=0 K energetics while T not equal 0 K configurational entropy is necessary to account for the splitting in Cu0.75Au0.25. Cu0.75Au0.25 shows a significant temperature dependence of the splitting, in agreement with recent in situ measurements, while the splitting in Cu0.70Pd0.30 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 [000] point. Streaking of the SRO peaks in the [100] and [1 1/2 0] 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 Ni0.4Au0.6, like the measured data, shows a peak along the [zeta 00] direction, away from the typical clustering-type [000] 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. [S0163-1829(98)03808-9].