First-principles theory of short-range order in size-mismatched metal alloys: Cu-Au, Cu-Ag, and Ni-Au
- National Renewable Energy Laboratory, Golden, Colorado 80401 (United States)
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 {l_angle}100{r_angle} point for all compositions studied, in agreement with diffuse scattering measurements. (ii) A fourfold splitting of the X-point SRO exists in both Cu{sub 0.75}Au{sub 0.25} and Cu{sub 0.70}Pd{sub 0.30}, although qualitative differences in the calculated energetics for these two alloys demonstrate that the splitting in Cu{sub 0.70}Pd{sub 0.30} may be accounted for by T=0 K energetics while T{ne}0 K configurational entropy is necessary to account for the splitting in Cu{sub 0.75}Au{sub 0.25}. Cu{sub 0.75}Au{sub 0.25} shows a significant temperature dependence of the splitting, in agreement with recent {ital in situ} measurements, while the splitting in Cu{sub 0.70}Pd{sub 0.30} 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 {l_angle}000{r_angle} point. Streaking of the SRO peaks in the {l_angle}100{r_angle} and {l_angle}1 (1) /(2) 0{r_angle} 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 Ni{sub 0.4}Au{sub 0.6}, like the measured data, shows a peak along the {l_angle}{zeta}00{r_angle} direction, away from the typical clustering-type {l_angle}000{r_angle} point. (v) The explicit effect of atomic relaxation on SRO is investigated and it is found that atomic relaxation can produce significant {ital qualitative} changes in the SRO pattern, changing the pattern from ordering to clustering type, as in the case of Cu-Ag. {copyright} {ital 1998} {ital The American Physical Society}
- OSTI ID:
- 591833
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 57, Issue 8; Other Information: PBD: Feb 1998
- Country of Publication:
- United States
- Language:
- English
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