Simulated Cu–Zr glassy alloys: the impact of composition on icosahedral order

The structural properties of the simulated Cu_αZr_1-α glassy alloys are studied in the wide range of the copper concentration to clarify the impact of the composition on the number density of the icosahedral clusters. Both bond orientational order parameters and Voronoi tessellation methods are used to identify these clusters. Our analysis shows that abundance of the icosahedral clusters and the chemical composition of these clusters are essentially nonmonotonic versus and demonstrate local extrema. That qualitatively explains the existence of pinpoint compositions of high glass-forming ability observing in Cu Zr alloys. Finally, it has been shown that Voronoi method overestimates drastically the abundance of the icosahedral clusters in comparison with the bond orientational order parameters one.     

Structural evolution of Ti50Cu50 on rapid cooling by molecular dynamics simulation

The structural evolution and atomic structure of the Ti₅₀Cu₅₀ compound have been investigated by means of molecular dynamics simulation using the generalized embedded-atom model (GEAM) potential. Gibbs free energy calculation manifests the large driving force of undercooled Ti₅₀Cu₅₀ for crystallization and thus the poor glass-forming ability. Radial distribution functions (RDFs) within the temperature range from 2000 K to 300 K are analyzed and reveal the increasing degree of short-range order and reducing periodic length between peaks on cooling. Atomic arrangement is characterized by the Voronoi tessellation method, showing that the frequency of icosahedral configurations is most sensitive to temperature and grows upon quenching while that of the others remains relatively stable. The thermal behavior of the structure factors follows the Debye model up to the supercooled liquid temperature. The structural investigation of amorphous Ti₅₀Cu₅₀ demonstrates that there exist a variety of polyhedral configurations in Ti₅₀Cu₅₀ amorphous alloy, where icosahedral and bcc clusters are the major types. Due to the existence of bcc clusters and the other distorted polyhedra other than full icosahedra, the structural analysis reconfirms the inference from the Gibbs free energy calculation.     

Cooling rate dependence and local structure in aluminum monatomic metallic glass

Abstract

The local atomic structure in aluminium monatomic metallic glass is studied using molecular dynamics simulations combined with the embedded atom method (EAM). We have used a variety of analytical methods to characterise the atomic configurations of our system: the Pair Distribution Function (PDF), the Common Neighbour Analysis (CNA) and the Voronoi Tessellation Analysis. CNA was used to investigate the order change from liquid to amorphous phases, recognising that the amount of icosahedral clusters increases with the decrease of temperature. The Voronoi analysis revealed that the icosahedral-like polyhedral are the predominant ones. It has been observed that the PDF function shows a splitting in the second peak, which cannot be attributed to the only ideal icosahedral polyhedron 〈0, 0, 12, 0〉, but also to the formation of other Voronoi polyhedra 〈0, 1, 10, 2〉 . Further, the PDFs were then integrated giving the cumulative coordination number in order to compute the fractal dimension (d_f).     

On the effect of heating and cooling rates on the melting and crystallization of metal nanoclusters

The effect of heating and cooling rates on melting (T_m) and crystallization (T_c) temperatures of metal nanoclusters is investigated in terms of the isothermal molecular dynamics. We report on the results obtained for nickel nanoclusters, although analogous results were also obtained for gold and aluminum nanoclusters. It is found that T_m increases, while T_c decreases with increasing heating and cooling rates, both T_m and T_c tending to the same value for heating and cooling rates tending to zero. The results indicate that the hysteresis of melting and crystallization of nanoparticles must be completely due to nonequilibrium conditions of heating and cooling. The transition of Ni nanoclusters to the amorphous state begins at very high cooling rates exceeding 10 TK/s.     

Size and shape-dependent melting mechanism of Pd nanoparticles

Molecular dynamics simulation was employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2–20 number of full shells. The original embedded atom method (EAM) was compared to the more recent highly optimized version as inter-atomic potential. The thermal stability of clusters were probed using potential energy and specific heat capacity as well as structure analysis by radial distribution function (G(r)) and common neighbor analysis (CNA), simultaneously, to make a comprehensive picture of the solid-state and melting transitions. The result shows ico is the only stable shape of small clusters (Pd₅₅–Pd₃₀₉ using original EAM and Pd₅₅ using optimized version) those are melting uniformly due to their small diameter. An exception is cub Pd₃₀₉ modeled via optimized EAM that transforms to ico at elevated temperatures. A similar cub to ico transition was predicted by original EAM for Pd₉₂₃–Pd₂₀₇₅ clusters, while for the larger clusters both cub and ico are stable up to the melting point. As detected by \(G(r)\) and CNA, moderate and large cub clusters were showing surface melting by nucleation of the liquid phase at (100) planes and growth of liquid phase at the surface before inward growth. While diagonal (one corner to another) melting was dominating over ico clusters owing to their partitioned structure, which retarded the growth of the liquid phase. The large ico clusters, using optimized EAM, presented a combination of surface and diagonal melting due to the simultaneous diagonal melting started from different corners. Finally, the melting temperature as well as latent heat of fusion were calculated and compared with the available models and previous studies, which showed, unlike the present result, the models failed to predict size-dependent motif cross-over.     

Kinetics of ultrafast migration-accelerated quenching in nanoparticles

It has been analytically shown that the kinetics of ultrafast migration-accelerated quenching of luminescence in nanoparticles for the case of the short-pulse-induced excitation of donors has a complex multistage character. This conclusion has been confirmed by Monte Carlo computer simulation. An order stage (exponential in time) has been observed at small times. However, in contrast to a bulk crystal, quenching in nanoparticles is exponential only to a depth determined by the average number of donors in nanoparticles. Then, differences in the quenching rates of excitations in individual nanoparticles lead to the onset of a disordered stage of ultrafast quenching, which satisfies a t ^3/S time law for the S multipole interaction type. The quenching rate of excitations at the disordered stage depends on the average number of donors in nanoparticles. In this property, ultrafast quenching in nanoparticles qualitatively differs from that in a bulk crystal: the ultrafast quenching rate in the bulk crystal is independent of the donor subsystem parameters as a whole and the donor concentration in particular. The time of the transition between stages has been determined.     

Molecular dynamics study of structure and glass forming ability of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy

In this study, the temperature effects on the structural evolution of theZr₇₀Pd₃₀ binary alloy in the glassy and liquid states werestudied using the molecular dynamics simulations based on the many-body type tight-bindingpotential. We considered the following properties in detail: the temperature dependence ofthe volume, the partial and total pair distribution functions and the simulated glasstransition temperature. The effects of the cooling rates on the glass transitiontemperature were examined. The Wendt-Abraham parameter was calculated to determine theglass transition temperature of Zr₇₀Pd₃₀ glassy alloy. The pair analysis technique ofHoneycutt-Andersen was applied to define local atomic arrangements produced from moleculardynamics simulations. The results show that the icosahedral ordering in glassy state hasbeen composed during quenching period, and the simulated glass transition temperature andthe total pair distribution functions are in good agreement with the experimental data.     

Short-Range Order in α-FeAl Soft Magnetic Alloy

The atomic structure of iron–aluminum alloy samples containing about 7 at % of aluminum (α region of the phase diagram) was investigated by X-ray diffraction. The samples were annealed in the paramagnetic (T > ^T_C) and ferromagnetic (T < T_C) states. In the first case, the structural state was fixed by quenching from the annealing temperature in water, and, in the second case, by slow cooling. Diffraction patterns of single-crystal samples were taken on an X-ray four-circle diffractometer. It is shown that local ordering, regardless of the prehistory of a sample, is a combination of B2-phase clusters (the CsCl type structure), which were previously found in iron–silicon alloys with Si content up to 10%, and small regions with D0₃ short-range order. The former consist of two B2-cells having a common face, while the latter consist predominantly of one unit cell of the D0₃-phase. Within the accuracy of the experiment, no significant difference in the structural states in the paramagnetic and ferromagnetic phases was observed.     

Homologous distortion of the aluminum crystal structure under laser radiation

It was shown by X-ray diffraction that the aluminum crystal structure is distorted under conditions of nonequilibrium laser heating, which appears in lowering the lattice symmetry. A method for describing the observed distortions, based on the transition to a new unit cell, was proposed. It was shown that the distorted aluminum crystal structure can be described using the transition from the face-centered cubic cell to the monoclinic body-centered cell. The parameters of the aluminum unit cell after laser irradiation were determined as a = 0.2870 nm, b = 0.2860 nm, c = 0.4060 nm, and β = 90.013° (for the axes of the monoclinic body-centered lattice).     

Molecular dynamics simulation of polyhedron analysis of Cu–Ag alloy under rapid quenching conditions

In this study, the formation mechanism of polyhedron clusters in Cu₅₀Ag₅₀ binary alloy system consisting of 50 000 atoms has been investigated by using molecular dynamics simulations based on embedded atom method (EAM) during the rapid cooling processes. The cluster-type index method (CTIM) has been used to describe the evaluation properties of clusters and the structural development has been investigated by using radial distribution function (RDF). The simulation results show that the amorphous phase is formed by the main bonded pairs of 1551, 1541 and 1431 in the system, and ideal icosahedral (icos) cluster (12 0 12 0) and other basic polyhedron clusters, such as defective icos, Frank–Kasper, Bernal polyhedron, play a critical role under the rapid cooling conditions. The results of our simulations that have been disclosed show that high cooling rate favors the icos and defective icos clusters for model alloy system.     

Analysis of gas-phase condensation of nickel nanoparticles

Gas-phase condensation of 8000 nickel atoms is examined by molecular dynamics simulation with a tight-binding potential. A detailed study of the evolution of the system cooled at a constant rate from 1000 K to 77 K is presented. The results are used to identify four distinct stages of the evolution from a hot atomic gas to a few synthesized particles. An analysis of possible nanoparticle formation mechanisms suggests that cluster-cluster aggregation is the dominant one. The simulation shows that there two stages of cluster formation are of primary importance with regard to aggregation. At the first stage, spherical liquid clusters nucleate with uniform size distribution. The second stage is characterized by a distinct transition from uniform to bimodal size distribution due to aggregation of relatively large clusters. The particles obtained by gas-phase synthesis are analyzed by the CNA method [25]. It is found that most nanoparticles produced in the simulation have either icosahedral or mixed FCC/HCP structure.     

Effect of quenching temperature on pore formation during the cyclic heat treatment of aluminum

It has previously been shown [1] that pores are formed during the repeated heating and rapid cooling of commercially pure aluminum, and that as a result a deterioration in the mechanical properties of the material is observed. The fact that the intensity of pore formation increases with reduction in the diameter of the specimens being investigated — i.e., under conditions when the amount of plastic deformation decreases [2] and the rate of cooling, and consequently the concentration of quenching vacancies increases — leads to the assumption that the development of porosity observed in aluminum is due not to thermal stresses but to quenching vacancies.An approximate estimate of the number of fixed vacancies during repeated heat exchanges and comparison with the experimentally observed change in volume show that such an assumption is not without foundation.The present paper gives the results of further investigations into the behavior of aluminum under conditions of repeated sudden heat exchanges, and in particular the effect of quenching temperature, as a factor which increases the concentration of quenching vacancies, on the change in mechanical properties and microstructure of the material.     

Decomposition of the solid solution and the formation of a high-coercivity state in Fe<Subscript>2</Subscript>NiAl alloy upon cooling at the critical rate

Transmission electron microscopy and magnetic measurements are used to study the formation of the microstructure and magnetic properties of Fe₂NiAl (alni) alloy upon cooling at the critical rate (V ～ 2°/min) from the region of single-phase solid solution (1240°C). Cooling is interrupted by water quenching caused by temperatures Т_quench. The periodical modulated structure formed during sample cooling at the critical rate guarantees the strongest possible coercive force (Н_с = 670 Oe). The decomposition of the solid solution below 900°C includes a stage of primary modulated structural failure upon continuous cooling to temperature Т_quench ～ 850°C, which corresponds to the weakest coercive force on the Н_с(Т_quench) curve. The periodical modulated structure is recovered when the temperature falls further; this is accompanied by an increase in the coercive force (up to Н_с = 670 Oe) after cooling to 20°C.     

Local icosahedral order and thermodynamics of simulated amorphous Fe

Local icosahedral order and thermodynamics of amorphous Fe have been analyzed in detail for models containing 3000 atoms, which were obtained by the molecular dynamics (MD) method. Models were obtained by cooling from the melt. Local order in models has been analyzed by using the technique proposed by Honeycutt and Andersen; we found an existence of icosahedral order in the system. Moreover, structural properties of models were also studied via radial distribution function (RDF), static structure factor, mean atomic distances, coordination number and bond-angle distributions. Glass transition temperature, heat capacity and potential energy of the system were found in addition to the evolution of structure and mean-squared displacement (MSD) of atoms upon cooling from the melt toward the glassy state. We found the glass transition temperature of simulated liquid Fe via temperature dependence of potential energy and it is close to that observed previously in the literature, i.e. T_g≈1070 K. Calculations showed that structural properties of amorphous Fe models with the Pak-Doyama interatomic potential agreed well with the experimental data.     

Structural transformations in liquid,crystalline, and glassy B<Subscript>2</Subscript>O<Subscript>3</Subscript> under high pressure

We present in situ (x-ray diffraction) and ex situ (quenching) structural studies of crystalline, liquid, and glassy B₂O₃ up to 9 GPa and 1700 K, drawing equilibrium and nonequilibrium phase diagrams of B₂O₃. Particularly, we have determined the melting curve, the stability regions for crystalline B₂O₃ I and B₂O₃ II modifications, the regions of transformations, such as densification or crystallization, for both the liquid and glassy states, including the region of sharp first-order-like transition in liquid B₂O₃ to a high-density phase near 7 GPa. Quenching experiments also show that the transition to the high-density liquid can occur at much lower pressures in nonstoichiometric melts with an excess of boron. B₂O₃ is the first glassformer whose transformations in the disordered state have been comparatively studied for both liquid and glassy phases.     

Heat capacity of PMN near an electric-field-induced phase transition

The heat capacity of a single crystal of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in an electric filed with E = 3 kV/cm applied along the [111] direction has been measured using adiabatic calorimetry over the temperature range 170–250 K. Anomalies in C _p have been found, which correspond to a field-induced phase transition from a relaxor to a ferroelectric state at 225 K under field cooling conditions or at 235–240 K on the subsequent field heating. The field-induced ferroelectric phase persists in a metastable state at low temperatures and is destroyed on zero-field heating at 210 K. The small entropy change ΔS = 0.028R in the field-induced phase transition suggests an insignificant change in the volume fraction of existing polar nanoregions.     

Atomic structure and diffusivity in liquid Al80Ni20 by ab initio molecular dynamics simulations

The atomic structure and diffusivity in liquid Al₈₀Ni₂₀ are studied by ab initio molecular dynamics simulations. The local structures are analyzed by the pair correlation function, structure factor, coordinate number, Honneycutt–Anderson bond pair, and Voronoi tessellation methods. It is observed that the amount of icosahedral clusters increases, and the liquid becomes more ordered as the temperature decreases. The predicted self-diffusion coefficients of Al and Ni via the mean square displacements are very close to each other and agree well with the quasi-elastic neutron scattering measurements in the literature. The observation of equal self-diffusivity of Al and Ni is attributed to the formation of local solute-centered polyhedra, coupling the migration of Al and Ni. The Manning dynamic correlation factor is evaluated and found to be close to unity. The predicted interdiffusion coefficients using the Darken equation agree well with experimental data in the literature.     

Quenched disorder effects in electron transport in Si inversion layers in the dilute regime

In order to reveal the effects of disorder in the vicinity of the apparent metal-insulator transition in 2D, we studied electron transport in the same Si device after cooling it down to 4 K at different fixed values of the gate voltage V^cool. Different V^cool did not significantly modify either the momentum relaxation rate or the strength of electron-electron interactions. However, temperature dependences of the resistance and the magnetoresistance in parallel magnetic fields in the vicinity of the 2D metal-insulator transition carry a strong imprint of the quenched disorder determined by V^cool. This demonstrates that the observed transition between the metallic and insulating regimes, besides the universal effects of electron-electron interaction, depends on the sample-specific localized states (disorder). We report on evidence for a weak exchange of electrons between the reservoirs of extended and resonant localized states that occur at low densities. The strong cool-down dependent variations of ρ(T), we believe, are evidence for a developing spatially inhomogeneous state in the critical regime.     

Specific Features of the Domain Structure of BaTiO<Subscript>3</Subscript> Crystals during Thermal Heating and Cooling

This paper presents the results of the study of the domain structure of barium titanate crystals in a wide temperature range including the Curie point (T_C) using the polarization-optical method in the reflected light and the force microscopy of the piezoelectric response. It is shown that a new a–c domain structure forms during cyclic heating of the crystal above T_C and subsequent cooling to the ferroelectric phase. The role of uncompensated charges appeared on the crystal surface during the phase transition and their influence on the formation of the domain structure during cooling are discussed.     

Glass-forming region in the Cd-Ge-As ternary system

In the Cd-Ge-As system, bulk glassy samples can be prepared by quenching of the melt for certain compositions. The glass-forming region is not very large. It includes the pure CdAs₂ compound, the strongest tendency to form a glass is at CdGe_0,3As₃. The characteristic phase transition temperatures for some glasses were determined by DTA. The studies also show that all these glasses recrystalize on heating. After melting and slow cooling they solidify as polycrystals.