Chemical Short-range Order Parameter Research Articles

Diffusion, atomic transport, and ordering in Al-Zr alloys: FCC and liquid phases

Additive manufacturing of materials with controlled microstructure demands knowledge of atomic scale properties near the solid-liquid transition state. Many of these properties are not affordable by experimental techniques and computer modeling is the possible solution to the problem. In this paper, we present the results of an extended atomistic study of intrinsic atomic transport due to vacancy diffusion in FCC and L12 solid phases and diffusion in the liquid phase of Al-Zr alloys. A deceleration of the overall self-diffusion was observed when Zr was added to Al. The effect was stronger in the solid and weaker in the liquid. Additionally, the effect was strongly temperature dependent in the solid phases, but not in the liquid. Atomic transport was chemically biased: transport of Zr atoms was significantly slower than that of Al atoms, and this bias effect was stronger in the solid phases. The overall diffusion and chemical ordering processes in the liquid state were five to six orders in magnitude faster than in the solid. Chemical short-range order parameters in the liquid saturated at values close to those in the ordered L12 structure of Al3Zr. Chemical and structural ordering in the solid phases was negligible over the modeled microsecond time scale. The results are discussed in view of optimizing additive manufacturing parameters for the controlled formation of metastable L12 precipitates.

Order Parameter Engineering for Random Systems

The chemical short-range order (CSRO) in crystalline materials influences the properties, and its effect is significant in the context of multicomponent materials. We propose a scheme for the CSRO parameter or ∆-parameter in terms of the number of like and unlike bonds in the multicomponent systems. The OPERA or Order Parameter Engineering for RAndom Systems scheme for semi-canonical and canonical ensembles is proposed. The proposed framework of ∆-parameter with OPERA framework can generate the single-phase supercell with desired CSRO without explicit energy calculations and provides a computationally efficient scheme for exploration of the CSRO. We demonstrate the applicability of the ∆-parameter as a scalar quantity for describing the CSRO in multicomponent alloys and oxides (FCC-CoCrNi, BCC-MoNbTaW, and (CoCuMgNiZn)O).

Study of Thermodynamic Properties of Bi-Pb Liquid Alloy

We have used quasi-lattice theory to describe the mixing behavior of Bi-Pb liquid alloys at a temperature of 700K by computing thermodynamic functions and structural functions. The thermodynamic functions includes free energy of mixing (GM), activity (a), heat of mixing (HM), entropy of mixing (SM). The structural functions includes concentration fluctuation in the long-wavelength limit (Scc(0)) and chemical short range order parameter (α1). Most of the computed values are in good agreement with the experimental data. The pair-wise interaction energies between the species of the liquid alloys play important role and are found to temperature dependent. Theoretical analysis suggests that Pb3Bi complex exists in the liquid state at 700K. And, it has hetero-coordination (i.e. ordering) nature but is of weakly interacting in nature.

Thermodynamics and surface properties of Fe–V and Fe–Ti liquid alloys

Abstract A simple statistical mechanical model, based on a quasi-lattice approximation in which one assumes the formation of complexes, has been used to study bulk properties, such as free energy of mixing, the thermodynamic activity and enthalpy of mixing, in liquid Fe –Vand Fe –Ti alloys. The energetics and its effect on the alloying behavior of the liquid alloys has been investigated with the aim of correlating bulk phenomena with surface effects. The analysis shows that, assuming the formation of intermetallic complexes of the form Fe2V and FeTi in the liquid alloys, one can explain the energetics of the bulk alloys. Our results for the bulk calculations indicate that Fe –V and Fe –Ti both exhibit a significant tendency for compound formation. From a perusal of the diffusion coefficient D, we observe the same trend towards compound formation, as demonstrated by the chemical short-range order parameter (CSRO) close to the assumed stoichiometric composition. Furthermore, using the model calculations in the bulk, we study some surface properties. Our calculations indicate that Fe segregates to the surface at all bulk compositions in Fe –Vand Fe –Ti liquid, though the segregation effect is more pronounced in the former alloy. The reason for this is that Fe –Ti is a more ordered system than Fe –V and, thus, the driving force for surface segregation in these alloys is their energetics.

Atomic structure of liquid refractory Nb5Si3 intermetallic compound alloy based upon deep neural network potential

The knowledge of atomic structure for liquids, especially for liquid alloys with complex crystal structure and high liquidus temperatures, remains poorly understood. In this work, we have extended the development of deep neural network (DNN) potential for liquid Nb5Si3. The DNN potential captures the structural features of liquid alloys compared with ab initio results. The chemical short-range order parameter suggests that there exists strong affinity between Nb and Si atoms. The dynamic property was investigated, and the diffusion coefficient obeys the Arrhenius relationship. The atomic structure has been subsequently explored for normal and undercooled liquid Nb5Si3. Large amounts of fivefold symmetry Honeycutt–Andersen pairs have been identified in liquid Nb5Si3. However, due to the violent thermal motion in a high-temperature Nb5Si3 melt, icosahedral symmetry and distorted icosahedrons (ICOs) account for little proportion according to Voronoi polyhedron (VP) analysis. The effect of thermal motion on VPs has been discussed. Except from the well documented ⟨0,2,8,2⟩ and ⟨0,1,10,2⟩ distorted ICOs, six more quasi-ICOs (⟨0,1,9,3⟩, ⟨0,2,8,1⟩, ⟨0,2,8,4⟩, ⟨0,2,8,5⟩, ⟨0,1,10,3⟩, and ⟨0,1,10,4⟩) have been proven to deform from ICOs at high temperatures. The local environment motif obtained by the atomic cluster alignment method demonstrates the existence of dominant distorted ICOs. At last, the atomic structure during melting process is discussed by VP analysis. It is found that ⟨0,2,8,1⟩, ⟨0,2,8,2⟩, ⟨0,2,8,5⟩, and ⟨0,1,10,4⟩ prefer to form at the beginning of the melting but rapidly reduce when it is fully melted.

Effect of temperature on mixing behavior and stability of liquid Al-Fe alloys

A theoretical model based on the assumption of compound formation in binary liquid alloy has been used to investigate the thermodynamic properties (free energy of mixing, enthalpy of mixing and entropy of mixing), microscopic properties (concentration fluctuation in long wavelength limit and chemical short range order parameter), surface properties (surface tension and surface composition) and dynamic properties ( viscosity and diffusion coefficient). All the properties of Al2Fe binary melt have been measured using the same energy parameters configured for experimental values of free energy of mixing. The energy parameters are detected as independent of concentration, but depend on temperature. The findings are well consistent with the experimental standards.

Theoretical assessment for the mixing properties of AlMg liquid alloys at 1073K

The alloying behavior of AlMg alloy in the liquid form at 1073 K has been theoretically investigated in the framework of four-parameter model which is based on Maclaurin series. The analytical expressions for thermodynamic functions such as excess free mixing energy, free mixing energy, enthalpy of mixing and entropy of mixing and microscopic functions such as concentration fluctuations at the long wavelength limit and Warren-Cowley chemical short range order parameter have been derived. These expressions have been used to compute the excess Gibbs free energy of mixing, Gibbs free energy of mixing, activity, enthalpy(heat) of mixing, excess entropy of mixing, entropy of mixing, concentration fluctuations in long wavelength limit and Warren-Cowley short range order parameters of AlMg liquid alloys at 1073 K. The investigation shows the excellent concurrence between the experimental and theoretical measurements of the mixing properties of AlMg liquid alloys at 1073 K. Interaction parameters of energy depends on temperature.

Temperature-dependent mixing behavior of Pb–Sb alloys in liquid state

We have extended a computational model used to research the thermodynamic properties of binary liquid alloys to study chemical short-range order parameters and diffusion coefficients. Numerous simulations have also been employed for surface properties such as surface concentration and surface tension. The propensities for phase separation of the Pb–Sb alloy were also observed with a temperature rise. The structural and thermodynamic properties of the alloys were measured using the same interaction parameters that are suitable for the free energy of mixing and mixing enthalpy. The interaction parameters are discovered to be temperature-dependent but concentration-independent. Our results indicate that the liquid alloy Pb–Sb has a weakly interacting composition. According to the surface property review, the element Sb having a higher surface concentration than its bulk concentration segregates over the surface of the alloy.

Implications for the Nb aggregation inherited from melt to γ phase of U-Nb alloy

By means of ab initio molecular dynamics (AIMD) simulations, Nudged Elastic Band (NEB) method and cluster expansion (CE) combined with the Monte Carlo (MC) techniques, we have investigated that the elemental interactions and local structural dynamics in the melt and γ phase of U-Nb alloy and hydrogen diffusion behavior in γ-U-Nb alloy. It has been found that the aggregation of Nb atoms inherited from melt to γ phase plays a significant role in improving mechanical property and protecting γ-U-Nb alloy from hydrogen damage. The analyses of chemical short-range order parameters attest that there exists the similarity of element interactions between the melt and γ phase of U-Nb alloy. The U-Nb pairs in both melt and γ phase exhibit the repulsive interactions, whereas the attractive U-U and Nb-Nb interactions exits. These results demonstrate that the U and Nb atoms prefer to stay in their own shells to form the localized clusters in both melt and γ phase of U-Nb alloy. Furthermore, the analyses of hydrogen diffusion behavior in γ-U-Nb alloy, our NEB calculations have further proved that the hydrogen atoms prefer to be trapped by Nb atoms and, this trend is further reinforced by the aggregation of Nb atoms. It implies that, once Nb is added into U alloys, the Nb addition plays an important role in slowing down the diffusion of hydrogen atoms in γ-U-Nb alloy. The uniform distribution of Nb clusters helps to avoid the occurrence of excessive local hydrogen concentration.

Ab initio molecular dynamics study on local structure and dynamic properties of liquid Ni62Nb38 alloy

In the temperature range between 1873 K and 1233 K, structural evolution and dynamic properties of liquid Ni62Nb38 alloy have been studied by ab initio molecular dynamics. The evolution of local structure was characterized by pair distribution functions, structure factors, bond angle distributions, coordination numbers and chemical short-range order parameters, and electronic structure is mainly reflected by the density of states. Self-diffusion coefficients were calculated out to represent dynamic properties. It is found that with the decrease of temperature, short-range order tends to increase and medium-range order can be discovered in the under-cooling system. Meanwhile, a series of complex local structure like tetrahedron and icosahedron gradually forms in the liquid. Moreover, the whole system is hetero-coordinating. During the whole process, both Ni and Nb diffusion coefficients show a decreasing trend, effectively prohibiting nucleation and crystal growth, and it is beneficial to the amorphous transformation of liquid Ni62Nb38 alloy. The formation of chemical bonds between Ni and Nb atoms is mainly because of the hybridization interaction between Ni and Nb d-states.

Localized Nb clusters in U-Nb liquid alloys: An ab initio molecular dynamics study

Abstract By means of ab initio molecular dynamics (AIMD), we have evidenced the formation of the localized Nb clusters in the U-Nb liquid alloys even with the very low Nb concentrations. These localized Nb clusters are characterized, qualitatively and quantitatively, in terms of the pair correlation function, structure factor, coordination number and chemical short-range order parameters. The existence of the first highest peaks of the partial Nb-Nb pairs in both their pair correlation functions and structure factors reveals that the Nb atoms prefer to accumulate themselves together to form various localized clusters with three typical Nb networks of Nb2 dimers, opening chains (Nb3 or Nb4 chains) and closed triangles (Nb3 triangles) in the U-Nb liquid alloys. These Nb-Nb affinitive facts are further supported by the derived negative Nb-Nb chemical short-range order parameters. In contrast, both U-U and U-Nb interactions in these liquid alloys are further revealed to have the nearly zero, or positive, chemical short-range order parameters, corresponding to a relatively weak U-U, or repulsive U-Nb, interactions in the U-Nb liquid alloys.

Temperature-Dependent Mixing behaviours of Bi -Mg Liquid Alloys

ABSTRACT Bi – Mg is a strongly interacting system (the free energy of mixing > 3 RT at equiatomic composition), the compositional dependence of the thermodynamic properties (free energy of mixing, heat of mixing and entropy of mixing) exhibits asymmetric behaviour about equiatomic composition and are in well agreement with experimental data literature. Such an interesting feature is explained here through complex formation model on the basis that a complex of the form Bi2Mg3 exists in the bulk phase. Its energetics is used to investigate the structural properties (concentration fluctuation in long wavelength limit and chemical short-range order parameter) and the transport property (ratio of mutual diffusion coefficient to intrinsic diffusion coefficient). The mixing behaviour of Bi – Mg system is further explored through the study of coefficient of viscosity, surface composition and surface tension. Our study indicates that the surface is enormously rich with Bismuth concentrations throughout the composition of the alloys.

Structure and Optical Properties of Chalcogenide Glassy As–Ge–Te Semiconductor

The structure and optical properties of a chalcogenide glassy As–Ge–Te semiconductor film are studied by X-ray diffraction analysis, Raman spectroscopy, and optical transmission and density measurement. The main structural elements and chemical bonds forming an amorphous matrix as well as the optical width of the band gap are determined. The results are explained taking into account the main principles of chemical ordering and short-range order parameters in the atomic arrangement.

Theoretical and experimental study of 1-butyl-3-methylimidazolium nitrate with 1-pentanol, 1-hexanol and 1-heptanol: COSMO-RS and structure factor

In this paper, some thermodynamic and structural properties of the binary mixtures containing [Bmim][NO3] and 1-alkanols (1-pentanol, 1-hexanol, and 1-heptanol) was studied using theoretical and experimental methods. Density and viscosity for the mixtures above were measured at temperatures (293.15 K to 323.15 K), and values of excess molar volumes, VmE and viscosity deviations Δη were calculated. For binary mixtures of [Bmim][NO3] with 1-octanol, 1-nonanol, and 1-decanol a miscibility gap was observed in the mole fraction regions between 0.1 and 0.9. Findings indicate that strong interactions occur in the binary solutions containing short-range 1-alkanol, and increasing the length of the alcohol causes the weakening of the intermolecular interactions. To obtain more information about the molecular structure of aforesaid mixtures, the conductor-like screening model for real solutions (COSMO-RS) method was applied, and the D parameter, the concentration-concentration structure factor, Scc(0), and the chemical short-range order parameter α′, were calculated for the binary mixtures.

Molecular Interactions in [Hmim][NO3] Ionic Liquid and 2-Alkanol Mixtures: Kirkwood–Buff Integrals and Structure Factor

In this paper, the investigation on thermophysical properties, structure, and dominant interactions in the binary systems containing 1-hexyl-3-methylimidazolium nitrate ([Hmim][NO3]) and 2-alkanol (2-propanol, 2-butanol, and 2-pentanol) were carried out through the Kirkwood–Buff (KB) integrals, structural factor, and excess molar volumes. Binary i and j parameters are positive and indicate that unlike molecules tend to form the strong interactions via forming hydrogen bonds and stay alongside while an increase in the carbon atom of 2-alkanols reduces the strength of bonds and tendency of the unlike molecules to be together. Also, the structure of mixtures was studied using the concentration–concentration structure factor, $${{S}_{{CC}}}(0)$$. Results from the application of this parameter show that in all binary mixtures, heterocoordination is predominant, ordering occurs in solutions and fluctuations are less than random orientation. Moreover the chemical short-range order parameter α′, which is an important function to understand the complex formation and phase segregation in the liquid mixtures, was calculated and discussed for mentioned mixtures. For current binary mixtures, experimental data and theoretical investigations are novel and reported for the first time.

Theoretical and experimental study of valeric acid and 1-alkanol: COSMO-RS method and structure factors

Abstract This paper attempts to describe the types and nature of interactions present in binary mixtures containing valeric acid and 1-alkanols (1-pentanol up to 1-decanol) using theoretical and experimental methods. The density and viscosity of the mixtures above were measured at temperatures of 293.15 K to 323.15 K. Values of excess molar volumes (VmE) and viscosity deviations(Δη) were calculated. The findings indicate that strong interactions occur in binary solutions containing short-range 1-alkanol, and increasing the length of the alcohol chain causes a weakening of intermolecular interactions. To confirm the experimental results, the conductor-like screening model for real solutions (COSMO-RS) method was used to calculate the activity coefficients of the compounds in the liquid mixtures and the contact probability (CP) of alcohols with the valeric acid. Then, the D parameter, the concentration-concentration structure factor (SCC(0)), and the chemical short-range order parameter (α') were obtained and used to describe the existing interactions for binary mixtures. For binary systems, viscosity and density data are novel and have not previously been reported.

Ab initio molecular dynamics and high-dimensional neural network potential study of VZrNbHfTa melt

The structural and dynamics properties of melts are directly related to their solidification processes, and consequently to the properties of as-cast solid alloys. Ab initio molecular dynamics (AIMD) is a powerful tool that can study both of these factors. However, the main disadvantage of this method is its low performance which is critical for simulation of the multicomponent liquids. At the same time the atomistic simulation of multicomponent liquids has found its application for prediction of the formation of high-entropy alloys—a novel class of materials with enhanced mechanical properties. An effective method to solve the problem of AIMD low performance may be the design of pair or many-body potentials for classical molecular dynamics. One of the promising approaches is high-dimensional neural networks—the method of constructing many-body potentials for classical molecular dynamics from ab initio data. Thus, in this work, the high-dimensional neural network potential for multicomponent liquid VZrNbHfTa melt was constructed. The structure of this melt obtained by AIMD and high-dimensional neural network potential was compared by analyzing partial radial distribution functions. Dynamics of the melt obtained by both methods was also compared analyzing velocity autocorrelation functions and mean-square displacement for each type of atom in multicomponent VZrNbHfTa melt. It was shown that structure and dynamics are reproduced well by high-dimensional neural network potential (HDNNP). Some differences between HDNNP- and AIMD-obtained structure and dynamics are explained by finite-size effect and lack of statistics in AIMD simulation along with inherent errors in energy and force estimations made by high-dimensional neural network potentials. Analysis of melt structure via partial radial distribution functions and chemical short range order parameters led to the conclusion that vanadium atoms are repulsed from all the atoms of another type in liquid VZrNbHfTa system, which lowers the probability of single phase disordered solid solution formation. Diffusivity in multicomponent melt was found to decrease with increasing mass and size of an atom.

Study of interactions in binary mixtures containing normal alkanols; KB integrals and structure factor

ABSTRACT In this paper, the investigation on the structure and molecular interactions in the binary systems containing aniline and 1-alkanol (1-pentanol up to 1-decanol) was performed in the framework of Kirkwood-Buff (KB) integrals and experimental techniques. The interpretation of obtained parameters shows that in binary mixtures with shorter chain alcohols, unlike molecules tend to form strong interactions, while increasing in the carbon atom of 1-alkanols reduces the strength of bonds and tendency of the heterogeneous molecules to stay along side. Mixtures structure was studied using the concentration-concentration structure factor, . Results from the application of this parameter show that the heterocoordination occurs in solutions involving 1-pentanol up to 1-heptanol, while for mixtures containing 1-octanol to 1-decanol, homocoordination is predominant. Also, the chemical short-range order parameter, α′which is an important function to understand the complex formation and phase segregation in the liquid mixtures was obtained and discussed for mentioned mixtures.

Kirkwood-Buff integrals and structure factor for binary mixtures of ionic liquid with 1-alkanol

In this paper, we have tried to describe the factors affecting molecular interactions and structure of binary systems including 1-Hexyl-3-methylimidazolium Nitrate ([Hmim][NO3]) and 1-alkanol (1-hexanol, up to 1-decanol) using Kirkwood-Buff (KB) integrals and structure factor. Obtained results indicate that unlike molecules tend to form favorable interactions via forming hydrogen bonds or dipolar interactions and stay alongside, while an increase in the carbon chain of 1-alkanols reduces the strength of bonds and tendency of the unlike molecules to stay together. Moreover, the structure of mixtures was studied using the concentration-concentration structure factor, SCC(0). Results from the application of this parameter show that in all binary mixtures, heterocoordination is predominant, ordering occurs in solutions and fluctuations are less than random orientation. In addition, the chemical short-range order parameter α′, which is an important function to understand the complex formation and phase segregation in the liquid mixtures, was calculated and discussed for mentioned mixtures. For current binary mixtures, experimental data and theoretical investigations are novel and reported for the first time.

An optimized random structures generator governed by chemical short-range order for multi-component solid solutions

Multi-component random solid solutions have caused much interest for research because they often exhibit excellent mechanical properties. Many computational efforts have been made through various length scales, by first principles, molecular dynamics or finite elements. Here, a quasi-simulated-annealing (Markov Chain Monte Carlo algorithm) with prior burn-in procedure using chemical short-range order parameters as input values is developed for searching the global optimal distribution of species with regard to the phase stability of face-centered cubic CoCrNi alloys. The results show that this new approach could effectively generate the stable structures with a desired concentration and moreover, the stacking fault energies of these configurations have less uncertainties statistically.