C. Wolverton et al., FIRST-PRINCIPLES THEORY OF SHORT-RANGE ORDER IN SIZE-MISMATCHED METAL-ALLOYS - CU-AU, CU-AG, AND NI-AU, Physical review. B, Condensed matter, 57(8), 1998, pp. 4332-4348
We describe a first-principles technique for calculating the short-ran
ge order (SRO) in disordered alloys, even in the presence of large anh
armonic atomic relaxations. The technique is applied to several alloys
possessing large size mismatch: Cu-Au, Cu-Ag, Ni-Au, and Cu-Pd. We fi
nd 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-po
int SRO exists in both Cu0.75Au0.25 and Cu0.70Pd0.30, although qualita
tive differences in the calculated energetics for these two alloys dem
onstrate 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 necess
ary to account for the splitting in Cu0.75Au0.25. Cu0.75Au0.25 shows a
significant temperature dependence of the splitting, in agreement wit
h recent in situ measurements, while the splitting in Cu0.70Pd0.30 is
predicted to have a much smaller temperature dependence. (iii) Althoug
h 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 p
eaks in the [100] and [1 1/2 0] directions for Ag-and Cu-rich composit
ions, 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 meas
ured data, shows a peak along the [zeta 00] direction, away from the t
ypical clustering-type [000] point. (v) The explicit effect of atomic
relaxation on SRO is investigated and it is found that atomic relaxati
on can produce significant qualitative changes in the SRO pattern, cha
nging the pattern from ordering to clustering type, as in the case of
Cu-Ag. [S0163-1829(98)03808-9].