Monte Carlo study of short-range order and displacement effects in disordered CuAu (original) (raw)
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Influence of modulated structures on ordering dynamics in CuAu
Physica A-statistical Mechanics and Its Applications, 1996
Using a microscopic model, we have studied the evolution of microstructure in a model metallic alloy. The Hamiltonian was derived from the effective medium theory of cohesion in metals (EMT): an approximation scheme for integrating out the electronic degrees of freedom and constructing an effective classical Hamiltonian. The alloy chosen for this study was CuAu which exhibits a sequence of
Physical Review B, 2001
We combine the first-principles, Korringa-Kohn-Rostoker coherent potential approximation based calculations of compositional fluctuations with a statistical mechanical ring approximation to study the temperature ͑T͒ and composition ͑c͒ dependence of the atomic short-range order ͑SRO͒ in disordered, face-centred cubic, Cu-Pd alloys. The fourfold splitting of SRO peaks around the equivalent X(0,1,0) points in reciprocal space is obtained in a wide T-c region. Such splitting is shown to be an ''energy'' effect caused by the absolute minima of the Fourier transform of the effective atomic interactions and related previously to the existence of nested sheets of the disordered alloy's Fermi surface. However, we find that the T dependence of the SRO peak position is mostly an ''entropy'' effect. Both the calculated T and c dependences of the SRO peaks position are in good correspondence with the experimental data. The real-space effective atomic interactions and SRO parameters indicate the tendency for longer-period structures with increasing Pd concentration, as observed.
Monte Carlo study of the [001] surface of Cu3Au for TÞ0 K
Using an N-body potential with parameters determined from the T0 K bulk properties lattice constant, cohesive energy, and the elastic properties of Cu, Au, and Cu 3 Au, we investigate the effect of the 001 surface of Cu 3 Au on the ordering for temperatures below and above the bulk order-disorder transition temperature , utilizing the constant pressure, temperature and chemical potential difference Monte Carlo method. The properties studied include the stoichiometry profile, the structure factor, the average position of the atoms along the 001 direction, short-range order, wetting phenomena, and mean-square displacements. Also, various instantaneous atomic configurations are plotted. Our interest is focused on the study of the segregation of Au to the surface, while the atoms are free to relax. We find that the surface affects the ordering of the layers close to it. Segregation is found to occur in agreement with experimental results. Oscillatory segregation appeared in our results only for the case where the atoms were not allowed to relax, and in a lower intensity in some off-stoichiometry cases, i.e., Cu 0.7 Au 0.3 and Cu 0.8 Au 0.2. S0163-18299910435-1
Effect of the [001] surface of Cu3Au on the order-disorder transition
Surface Science, 1996
Atomistic simulations of the two possible [001] surfaces of Cu3Au are performed for temperatures below the transition temperature. The method apply the constant temperature, pressure and chemical potential difference Monte Carlo method, while the potential is the N-body tight-binding in the second-moment approximation. At T= 0 K the two possible [001] surfaces have very different surface energies and the results agree with previous calculations. On the other hand the relaxation of the first layer for both cases is outwards and of similar magnitude. For temperatures 20% below the transition temperature we observe segregation of Au to the first surface atomic plane independent from the initial surface termination. Also, one can observe that the layers close to the surface exhibit below the transition temperature more disordering than the bulk material. The results are discussed in terms of known experimental results and theoretical results.
Atomic Displacements in Cu due to Transition Metal Impurities
Chinese Journal of Physics, 2003
Atomic displacements in Cu metal due to substitutional transition metal impurities are investigated using the discrete lattice model and the Kanzaki lattice static method. The effective ion-ion interaction potential, due to Wills and Harrison, is used to evaluate the dynamical matrix and the impurity-induced forces. The results for atomic displacements due to 3d, 4d and 5d impurities (Co, Ni, Pd, Ag, Pt and Au) in Cu are given up to 20 nearest impurity neighbors, and these are compared with the available experimental data; they are found to agree. The lattice shows expansion due to Co, Pd, Ag, Pt and Au impurities and contraction due to Ni impurities. The maximum displacement, 2.3% of 1NN distance, was found for the CuAu alloy, while the minimum displacements, 0.43% of 1NN distance was found for the CuNi alloy. The relaxation energies for the Ni and Pd impurities were found to be less than that of the other types of impurities, therefore these impurities may easily be solvable in Cu.
Effect of Short-Range Order on Electronic Structureof Cu 0.5 Pd 0.5 Disordered Alloy
Journal of the Physical Society of Japan, 1991
In this work we have combined the generalized augmented space method introduced by one of us with the recursion method of ͓Haydock and Te, Phys. Rev. B 49, 10845 ͑1994͔͒ ͑GASR͒, within the framework of the local density functional based linear muffin-tin orbitals basis ͑TB-LMTO͒. Using this we have studied the effect of short-range ordering and clustering on the density of states, optical conductivity, and reflectivity of 50-50 CuZn alloys. Our results are in good agreement with alternative techniques. We argue that the TB-LMTO-GASR is a feasible, efficient, and quantitatively accurate computational technique for the study of environmental effects in disordered binary alloys.
Computer simulation of local lattice distortion in Cu-Au solid solution
Metallurgical and Materials Transactions A, 1995
The local lattice-distortion effects in Cu-25 at pct Au solid solution were investigated using a computer simulation in which the nearest-neighbor Lennard-Jones interactions with thermal vibration effects were assumed. The investigation revealed that the average displacements of Cu-Cu and Au-Au pairs in the nearest neighbor were negative and positive directions, respectively, with respect to the rigid lattice before the relaxation, and that the diffuse scattering intensity due to local lattice distortion and local atomic configuration can be calculated by taking the short-range configuration up to the 12th nearest-neighbor pairs into account. The results reproduced the overall tendency of the experimental observations.
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 Cu 0.75 Au 0.25 and Cu 0.70 Pd 0.30 , although qualitative differences in the calculated energetics for these two alloys demonstrate that the splitting in Cu 0.70 Pd 0.30 may be accounted for by T0 K energetics while T0 K configurational entropy is necessary to account for the splitting in Cu 0.75 Au 0.25. Cu 0.75 Au 0.25 shows a significant temperature dependence of the splitting, in agreement with recent in situ measurements, while the splitting in Cu 0.70 Pd 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 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 Ni 0.4 Au 0.6 , like the measured data, shows a peak along the 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. S0163-18299803808-9
Thermoactivated Structure Rearrangements in a Binary CuAu 3 Alloy
Thermoactivated structure rearrangements in a binary CuAu 3 alloy under deviation from stoichiometric composition, 2010
Thermo-activated restructuring at the micro-, meso-and macro-levels of a binary alloy CuAu deviation from the stoichiometric composition are investigated. The laws of such rearrangements at these levels are identified.