Papers by Keyword: Short Range Order

Abstract: Pairwise effective potentials in first seventeen shells of the Ni-22.5at.%Fe alloy are calculated using model potential method with account of the linear size effect. Using obtained values of pairwise effective potentials, the short range order parameters on the first seventeen shells of alloy are calculated by Krivoglaz-Clapp-Moss method. The calculated values ​​of the short-range order parameters were fitted to the experimental values ​​by varying the parameters of static atomic displacements. Reliable value of critical temperature of order-disorder phase transition in Ni-22.5at.%Fe alloy was calculated using obtained meanings of pairwise effective potentials.

Abstract: The aim of the study is to construct a mathematical model to describe the effect of a magnetic field on the melt crystallization, in particular cast iron. The authors’ research is based on the hypothesis of the certain energy function existence in the short-range order region, which describes the equilibrium state of the "fluid - short-range order" thermodynamic system. Using the hypothesis, we simulated the effect of a magnetic field on the melt crystallization process, which is based on the fundamental laws of statistical physics and thermodynamics and includes four components: a model of the stationary state of the short-range order region, a model for determining the energy function of the short-range order region, a model of the effect of a proportional magnetic field, a model the effects of a commensurate magnetic field on the crystallization of molten iron. Being based on the simulation results, test calculations were performed, the results of which are confirmed by previously known studies. The simulation results showed that the influence of a magnetic field on the crystallization of melts is insignificant in comparison with thermal motion. The authors of the work believe that the magnetic field acts on the region of short-range order at the formation time at a fluctuation temperature that is much lower than the equilibrium one.

Abstract: The aim of this investigation is to provide theoretical justification of the crystallizing nucleus formation in liquid iron. To solve this problem, the authors put forward the fluctuation hypothesis: short range order region formation in liquid metal takes place due to temperature fluctuations, and the temperature of the short range order is the temperature fluctuation around Kelvin scale zero in the course of crystallization time. The investigations were carried out for liquid state of the substance, in particular, for iron in paramagnetic state at the melting temperature. Analytical derivation of the temperature fluctuation value was given. Analytical expressions were obtained to define the average number of atoms in the short range order region for crystallizing iron. The investigation results show that short range order contains enough atoms to provide interaction with pulsed magnetic field resulting in the increase in the number of crystallization nuclei and in the decrease of their size. The obtained results can be used in the decision-making process concerning some manufacturing operations aimed at providing homogeneous structure of metals, particularly, in ferrous and non-ferrous metallurgy.

Abstract: The liquid lead-bismuth eutectic (LBE) alloy is of great interest for applications in future nuclear reactors. The structure and clustering of alloying elements have been investigated on an extended range of temperature (125-720 °C) by high temperature X-ray diffraction (HT-XRD), XPS and scanning photoemission microscopy (SPEM) at the ELETTRA synchrotron in Trieste. After melting the short-range order in liquid metal corresponds to a cuboctahedral arrangement of atoms that progressively evolves towards an icosahedral one as temperature increases. Such process, which is completed around 720 °C, is accompanied by a micro-chemical re-arrangement of atoms with changes of cluster size and composition. At high temperature the atom distribution results to be more homogeneous and the average size of clusters noticeably smaller.

Abstract: The new technique of atomic-scale X-ray Photon Correlation Spectroscopy (aXPCS) makesuse of a coherent X-ray beam to study the dynamics of various processes in condensed matter systems.Particularly atomistic migration mechanisms are still far from being understood in most of intermetallicalloys and in amorphous systems. Special emphasis must be given to the opportunity to measureatomistic diffusion at relatively low temperatures where such measurements were far out of reach withpreviously established methods. The importance of short-range order is demonstrated on the basis ofMonte Carlo simulations.

Abstract: Short-range order formation in dilute Fe-Si and Fe-Al alloys has been investigated by statistical Monte Carlo simulations with effective interactions deduced from first principles calculations for different magnetic structures of bcc Fe. We find that the variation of the magnetic order from ferromagnetic to paramagnetic leads to significant changes in effective cluster interactions and, as follow, in short-range order parameters of alloys. It is shown in agreement with experiment the B2 type short-range order is formed above the Curie temperature, T_C, while the D0₃ type short-range order is preferred below T_C.

Abstract: Single crystals of Ni-25.6 at.% Pt and Ni-87.8 at.% Pt were investigated by diffuse x-ray scattering for states of thermal equilibrium (923 K and 603 K, respectively). The separated short-range order scattering showed local maxima at 100 positions. Effective pair interaction parameters, as determined by the inverse Monte Carlo method, show a strong composition dependence of the nearest-neighbor interaction parameter. First-principles calculations are consistent with this finding and reveal a large contribution due to lattice strain. Based on values of the ordering energy, NiPt₃ with L1₂ structure was considered as a plausible new intermetallic phase, with Monte Carlo simulations giving an order-disorder transition temperature of 650 K. A single crystal of Ni-75.2 at.% Pt, quenched from 1073 K and aged at 613 K, showed L1₂-type ordering, reaching a long-range order parameter of 0.50(4) after 800 h.

Abstract: We describe a comprehensive ab initio investigation of phase stability and mechanical properties of W-Ta and W-V alloys, which are candidate materials for fusion power plant applications. The ab initio density functional calculations compare enthalpies of mixing for alternative ordered atomic structures of the alloys, corresponding to the same chemical composition. Combining the ab initio data with large-scale lattice Monte-Carlo simulations, we predict several low-energy intermetallic compounds that are expected to dominate alloy microstructures, and hence the low-temperature phase diagrams, for both alloys. Using the predicted ground-state atomic alloy configurations, we investigate the short-range order, point defect (vacancy and self-interstitial atoms) energies, and thermodynamic and mechanical properties of W alloys as functions of their chemical composition. In particular, we evaluate the anisotropic Young modulus for W-Ta and W-V alloys from ab initio elastic constant calculations, with the objective of comparing the predicted values with experimental micro-cantilever measurements. Also, using the calculated Poisson ratios for binary W alloys, which combine tungsten with more than 40 different alloying elements, we investigate if alloying improves the ductility of tungsten-based materials.

Abstract: Molecular dynamics simulation was used for investigating hydrogen migration in Pd-Si alloy at a temperature Т = 300 K. The strong affect of hydrogen dynamics and its defects creation to structure of palladium matrix is stated. The partial radial distribution function calculation for silicon specifies a preferable arrangement of silicon atoms relative to each other in the second coordination sphere. Model calculations have shown that not only silicon atoms can affect hydrogen mobility. Hydrogen itself also can significantly change the diffusion of the other components in the alloy.

Abstract: Blueshifts of photoluminescence (PL) peak wavelength from GaInNAs/GaAs quantum well (QW) at various annealing temperatures have been studied. Our results indicate that as-grown GaInNAs/GaAs QW sample has N-Ga3In1 phase, which changes to a mixture of N-Ga3In1 and NGa2In2 after annealing. The activation energy characterized for short range order is 2.38 eV, which is smaller than that for the diffusion process (3.196 eV). This indicates that the short range order is the dominant mechanism for PL blueshift at relatively low annealing temperature and for short time annealing.