Topological Short-range Order Research Articles

Chemical and icosahedral short-range orders in liquid and undercooled Al80Mn20 and Al80Ni20 alloys: A first-principles-based approach

Atomic structures of stable liquid and undercooled liquid Al(80)Mn(20) and Al(80)Ni(20) alloys have been calculated by first-principles molecular-dynamics simulations. For both alloys, the local structure as defined by the Faber-Ziman pair-correlation functions is characterized by a strong Al-transition-metal affinity, which leads to a well-pronounced chemical short-range order which is more temperature dependent for Al(80)Mn(20) than for Al(80)Ni(20). In addition, a structural analysis using three-dimensional pair analysis techniques has been performed in details. More particularly, we find that the fivefold local symmetry around Mn atoms is predominant in both stable and undercooled Al(80)Mn(20) alloys and displays no significant variation with temperatures. On the contrary, in Al(80)Ni(20), a strong variation of the topological short-range order is observed since in the undercooled state, the local environment of Ni atoms is characterized by the predominance of the fivefold symmetry over the close-packed local symmetry which is opposed to what occurs in the stable liquid phase.

Ageing behaviour of Pd40Cu30Ni10P20 bulk metallic glass during long-time isothermal annealing

In this paper, positron annihilation spectroscopy and differential scanning calorimetry were applied to study the structural relaxation of Pd40Cu30Ni10P20 bulk metallic glass during long-time annealing at 523 K. It was found that the relaxation of the alloy follows a two-stage process. In the initial stage, with increasing annealing time, the height of the glass transition peak (namely Δcp) increased, while the average positron lifetime decreased. The isothermal relaxation kinetics obeys a Kohlrausch law with exponent of β = 0.5± 0.03. However, when the annealing time is beyond 32 h, the height of the glass transition peak (Δcp) decreased, and the average positron lifetime increased. This is possibly due to the enhancement of the topological short-range ordering, which results in the generation of new defects on the interfaces between the solid-like regions and the liquid-like regions in amorphous phase. The experimental results were tentatively discussed in terms of free-volume theory and percolation theory.

Structural Relaxation of Amorphous Silicon-Germanium Alloys: Molecular-Dynamics Study

Thermally-induced structural relaxation of amorphous silicon-germanium (a-Si1-xGex) alloys has been examined using molecular-dynamics (MD) simulations with the Tersoff interatomic potential. The a-Si1-xGex networks were successfully observed in the present MD simulations. Major changes in the topological short-range order upon annealing occurred within the second coordination shell, while the chemical short-range order was not altered. It was found that the decrease in potential energy associated with the change in the standard deviation of bond angles becomes large as the Ge composition increases. This result was consistent with the heat release due to structural relaxation, which was observed experimentally. We also examined differences in the phonon densities of states between the as-quenched and annealed a-Si1-xGex alloys.

Structural changes and magnetic properties development in heat treated Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 metallic glass

Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 metallic glass was prepared by one-roll melt-spinning. Some samples were annealed for 1h at selected temperatures and then submitted to Differential Thermal Analysis to evaluate the effect of the annealing temperature (Ta) on the glass structure. Experimental results suggest that both Topological and Chemical Short Range Ordering (TSRO and CSRO) start above 300°C. TSRO actually occurs at Ta⩽340°C. At higher Ta CSRO and, in the glass transformation range, glass in glass phase separation do occur, depending on the annealing time and temperature.

Vibrational spectra of Ti–Zr–Si metallic glasses

Abstract The selection of specific Ti and Zr concentrations in Ti–Zr–Si metallic glasses in accordance with the Bhatia-Thornton theory allows one to determine in pseudo-binary approximation dynamic structure factors corresponding only to topological short-range order, chemical short-range order, and number-concentration. The appropriate samples were produced by the melt spinning technique. The neutron scattering intensity measurements have been carried out on the IN4 spectrometer (ILL, Grenoble) with λ =1.1 A at T =300 K. The obtained results are presented.

Molecular dynamics study of structural and dynamical properties of amorphous Si-Ge alloys

Structural and dynamical properties of amorphous silicon-germanium $(a\ensuremath{-}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x})$ alloys have been examined by molecular dynamics simulations using the Tersoff interatomic potential. Amorphous networks were generated by rapid quenching from liquid ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ alloys. Good agreement was obtained between the simulated and experimentally measured radial distribution functions and phonon densities of states, suggesting that the Tersoff potential is useful for analyzing the atomic configurations and vibrational properties of $a\ensuremath{-}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ alloys. Local atomistic structures, such as topological and chemical short-range order states, were also examined in detail, and we compared them with experimental and theoretical results reported previously. On the basis of the results obtained here, we proposed that the bond length and bond angle around Ge atoms have more distortion than those around Si atoms in $a\ensuremath{-}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ networks.

Mechanically alloyed Zr–Cu–Al–Ni–C glassy powders

Abstract In this contribution we report on the synthesis of glassy Zr55Cu30Al10Ni5 alloys with a carbon content up to 4 at.% by mechanical alloying which cannot be produced by casting techniques. X-ray analysis and differential scanning calorimetry, both in isochronal and isothermal mode, were carried out to assess the glassy nature of the powders. The viscosity was investigated by parallel plate rheometry and the crystallisation mechanism and kinetics are discussed in the framework of the Johnson–Mehl–Avrami (JMA) model. The comparison between carbon-free and carbon-containing alloys suggests a strong influence of increasing carbon content on the total free volume and viscous flow behaviour of the alloys and on the chemical and topological short-range order, affecting significantly the thermal stability and the crystallisation behaviour of the glassy phase and the supercooled liquid.

In-situ study on structural relaxation of Zr55Cu30Al1 0Ni5bulk amorphous alloy by SAXS

The structural relaxation of Zr55Cu30Al10Ni5 bulk amorphous alloy during annealing at 360 and 380℃ have been investigated by in-situ synchrotron radiation x-ray small angle scattering (SAXS). The experimental results o btained show that the fluctuations of electron density during the annealing proc ess are due to structural relaxations of bulk amorphous alloy before cyrstalliza tion. It is found that a homogenization in electron density takes place during a nnealing, which is attributed to the change from topological short-range order (TSRO) structural relaxation to chemical short-range order (CSRO) structural relaxation. The annealing time required for this change becomes shorter as the annealing temperature increases.

Topological and Chemical Short-Range Order in Undercooled and Stable Melts of Al13 (Co,Fe)4 Alloys

ABSTRACTThe short-range order of stable and deeply undercooled liquids of Al13(Co,Fe)4 alloys forming polytetrahedral phases is investigated by combining the containerless processing technique of electromagnetic levitation with elastic neutron scattering. Partial structure factors were inferred from the diffraction data which allow an analysis of both, the topological and the chemical short-range order of these alloy melts as a function of the temperature.

Structural relaxation process in a bulk Pd 40Cu 30Ni 10P 20 metallic glass with a high Kauzmann temperature

The structural relaxation in a Pd 40Cu 30Ni 10P 20 metallic glass was investigated by an electrical resistivity experiment. A chemical short range ordering (CSRO) process was observed together with the ordinary topological short range ordering (TSRO) one. It was found that the TSRO process was influenced by the B 2O 3 flux treatment that was used to improve the thermal stability. The kinetics of CSRO were examined by measuring the isothermal change in the room temperature resistivity, ρ 300, after annealing for a given time. The data obtained were successfully fitted by the relaxation function and the CSRO process was a thermally activated process with an energy spectrum.

Structural relaxation and viscosity in Co-based amorphous materials

The variation of viscosity of Co 69B 12Si 12Fe 4Mo 2Ni 1 and Co 70Si 14B 9Mn 5Fe 1Mo 1 amorphous materials with isothermal and isochronal annealing has been obtained. The results show that there is an increase of the viscosity as the temperature and the time of the thermal annealing increase. This behaviour is explained as a consequence of the structural relaxation of these materials due to their metastable condition in the as-quenched state. The annealing process produces changes in the topological short range order and in the chemical short range order.

Topological Disorder and Reactivity of Borosilicate Glasses: Quantum Chemical Calculations and 17O and 11B NMR Study

We present 17O and 11B NMR and quantum chemical calculations based on Hartree−Fock (HF) and density functional theory to investigate the site connectivity, the degree of chemical short-range order (CSRO), and the topological short-range order in binary borosilicate glasses, which can serve as a simple model system for more complex borosilicates and other covalent oxide glasses and melts. We also calculate the enthalpy of hydrolysis for each oxygen cluster in such glasses. The stability of [4]Si−O−[3]B was evaluated from the relative energy differences among the bridging oxygen clusters ([4]Si−O−[4]Si, [4]Si−O−[3]B, and [3]B−O−[3]B) calculated at the B3LYP/6-311+G(2d,p)//HF/3-21G and B3LYP/6-311+G(2d,p) levels of theory as well as data from 17O 3QMAS NMR. The fraction of [4]Si−O−[3]B at a boron mole fraction of 0.57 is about 37% (±1), suggesting a significant inter-dispersion of [3]B and [4]Si, where a random distribution of [4]Si and [3]B and complete phase separation lead to 50 and 0% of [4]Si−O−[3]B, respectively. 11B MAS NMR shows that the fraction of boroxol rings increases with increasing boron content. The bond angle distribution function G(α) for each cluster was obtained from the lattice energy variation with angle. The calculation for [4]Si−O−[4]Si shows close similarity to that obtained by recent high energy photon diffraction experiments. We also predicted the effects of composition, fictive temperature and degree of disorder among framework cations on G(α). The calculated enthalpies of hydrolysis of [4]Si−O−[4]Si, [4]Si−O−[3]B, and [3]B−O−[3]B are −16.98, −3.03, and 16.61 kJ/mol, respectively, implying differential reactivity with water and that [4]Si−O−[3]B clusters should have intermediate kinetic stability. Quantum chemical simulations also suggest a possible mechanism for the formation of the oxygen tricluster, related to the presence of five-membered rings.

A Study of the Temperature Dependence of Topological Short-Range-Order and Surface Tension of Liquid Elements

This work presents a new approach to study the variation of the surface tension with temperature through the temperature dependence of the topological-short-range-order (TSRO) existing in the liquid phase. A relation has been deduced for the temperature dependence of the topological-short-range-order. Both the characteristic features of the liquid phase viz. the strong correlation among the atoms over smaller distances and the high temporal disorder have been incorporated in the development of the theory. A relation has also been obtained for the temperature dependence of the coefficient of surface tension σ. Present study indicates that both, the decrease in TSRO as well as the increase in the bulk volume, govern the variation of surface tension with temperature. Calculated values of the surface tension, for inert and alkali elements, are in good agreement with the available experimental data, over a wide range of temperatures, the maximum deviation is 6.7%.

Growth kinetics of quasicrystalline and polytetrahedral phases of Al-Pd-Mn, Al-Co, and Al-Fe from the undercooled melt

Melts of the alloys Al-Pd-Mn, Al-Co, and Al-Fe are containerlessly undercooled and solidified using the technique of electromagnetic levitation. The (quasi)crystal growth velocity is measured as a function of undercooling for the various phases by a high-speed infrared photosensing device. Additionally, a video image technique is applied to measure growth velocities especially at low undercoolings where sluggish growth prevails. Scanning and transmission electron microscopy as well as x-ray diffraction is utilized to identify the phases primarily solidified from distinct levels of undercooling. The experimental results are analyzed within current theories of dendritic growth. The investigations give insight into topological and chemical short-range order effects in the growth kinetics of phases with polytetrahedral ordering in the solid state.

Enthalpy recovery on thermal cycling within the non-equilibrium state of a glass

The so-called reversible structural relaxation of a Ni-based glassy metal-alloy, two organic polymers and As 2Se 3 glass has been studied by differential scanning calorimetry (DSC) on their thermal cycling below T g. The similarity of the relaxation in these glasses creates a difficulty in reconciling the earlier conclusion that reversible relaxation in glassy metal-alloys involves changes in both chemical (i.e., a preference for unlike neighbors) and topological short range orders. Here, its origin is seen in the time- and temperature-dependent regain of the equilibrium state. Groups of atoms diffuse fast enough to allow local loss of configurational enthalpy and entropy on annealing. On heating, they absorb energy and reach their new equilibrium configurations of higher enthalpy and entropy while still below T g. Hence each mode of atomic diffusion in the structure has its own `mini glass-softening endotherm', and reversible relaxation is a reflection of a broad distribution of diffusion times arising from temporal and spatial variations in the atom’s environment. The relaxation is modeled in terms of the stretched-exponential, non-linear character of glass relaxation. An experimentally testable consequence is that reversible relaxation will be minimum in a glass with a narrow distribution of diffusion times and low contribution from the sub- T g relaxation processes.

The reversible enthalpy relaxation in glassy metal alloys and polymers

Abstract Spontaneous changes in the enthalpy and entropy of six isothermally annealed Ni-based glassy metal alloys and one network structure organic polymer have been studied by differential scanning calorimetry. Both time and temperature effects of annealing have been investigated. The two classes of materials show a remarkably similar structural relaxation, thus creating a difficulty in reconciling the earlier conclusions that the so-called reversible relaxation observed for glassy metal alloys is unique, involves both chemical and topological short-range orders and needs to be modelled within the concepts of independent two-level systems. This is resolved by showing that the reversible relaxation is a reflection of the time- and temperature-dependent regain of the equilibrium state at temperatures when the enthalpy of the annealed state is less than that of the equilibrium state. Those local groups of atoms, whose diffusion is fast enough to allow loss of enthalpy and entropy on their approach to an equilibrium configuration state of low energy on annealing, absorb heat to reach their new equilibrium Configuration state of higher energy at a higher temperature on heating at a certain rate. Thus each mode of atomic diffusion in the structure has its own ‘mini glass transition temperature’, when the gain of its configurational entropy conies into evidence on heating at a certain rate. The origin of the reversible relaxation is in the broad distribution of activation energy for diffusion times arising from the temporal and spatial variations in the environments of atoms, similar or dissimilar, in the structure of a glassy metal alloy. The reversible relaxation can be modelled in terms of the non-exponential nonlinear character of structural relaxation in glasses, and its strength is expected to be less for glasses with a narrow distribution of diffusion times and lacking a significant contribution from other relaxations.

Hydrostatic pressure effect on the electrical resistivity of Ni 80P 20 amorphous alloys

An influence of hydrostatic pressure on electronic transport in amorphous alloys Ni 80P 20 has been studied. The nonmonotonic behaviour of the pressure resistance coefficient is explained in terms of the Ziman–Faber theory by differences in resistance change velocities at the expansion of the Fermi surface and a topological short-range order.

Structure of Vitreous Germania

A full partial structure factor analysis of vitreous germania has been obtained by a combination of neutron diffraction, x-ray diffraction, and anomalous x-ray scattering. The results are consistent with the identification of the first three peaks in the average neutron and x-ray structure factors with intermediate-range order, chemical short-range order, and topological short-range order, respectively. The values for the shortest distances for the three element pairs are in agreement with previous work and suggest that possible complications with this technique, which might arise for example from shifts of the centers of the electron distributions from the nuclei, are not serious in the case of oxide glasses. {copyright} {ital 1998} {ital The American Physical Society}

Structural study of liquid Te–I mixtures

The atomic structure of liquid Te 1− x I x mixtures with x=0.2, 0.4, 0.5, 0.6, 0.7 and 0.8 has been investigated in the temperature range 300–600°C. Using the high intensity total scattering spectrometer (HIT) (National Laboratory for High Energy Physics at Tsukuba) the neutron diffraction measurements were carried out. The structural changes in Te–I mixtures are mainly due to the variation in the number–number correlation function. A change in the structure factor has been observed in the concentration range from 0.2⩽ x⩽0.4. The metal–nonmetal transition in this system is related to a change in topological short-range order.

Structural study of amorphous Pd41Ni41Si18 alloy associated with structural relaxation by anomalous X-ray scattering

Since the amorphous state is not thermodynamically stable, it is known that this state easily transfers to a more stable state when annealed at temperatures sufficiently low that crystallization does not occur. This transfer process has now been interpreted in terms of structural relaxation. The structural relaxation is frequently described by two categories of a topological short-range ordering (TSRO) process and a chemical short-range ordering (CSRO) process [1]. The TSRO process involves irreversible structural change via highly collective atomic rearrangements with the annihilation of density fluctuation and stress relief in the as-quenched specimen [2, 3]. On the other hand, the anneal-induced reversible relaxation behaviour is also well documented in some amorphous alloys [4] and it is considered to be the atomic rearrangement between specific constituent elements. Such reversible relaxation behaviour in amorphous alloys is mainly attributed to the CSRO process [5]. Regarding the structural change due to lowtemperature annealing, much reliable information has been obtained using X-ray and neutron diffraction [6, 7]. These results clearly indicate that a highly disordered frozen-in structure, which is metastable in amorphous alloys prepared by rapid quenching from the melt, relaxes structurally owing to low-temperature annealing and the atoms occupy more stable positions in the disordered atomic arrangement. Thus, we now have sufficiently reliable information about the TSRO process [2, 3]. However, these structural studies have still had relatively little impact on the CSRO process due to low temperature annealing, because the available structural data are very limited for the individual chemical constituents in amorphous alloys. The use of anomalous X-ray scattering may bring about a significant breakthrough in this subject by obtaining the environmental structure around a specific element in amorphous alloys. The main purpose of this work is to present the results on the structural change of amorphous Pd41Ni41Si18 alloy due to low-temperature annealing by applying anomalous X-ray scattering (AXS) at the Ni K absorption edge, coupled with ordinary Xray diffraction data. Amorphous ribbons were prepared by the meltspinning method with a single copper roller in a purified Ar atmosphere. The ribbon shape was about 30 μm in thickness and 2 mm in width. The quality of samples was tested by X-ray diffraction and differential scanning calorimetry (DSC). The samples were annealed at 603 K for 330 s to reduce partially the excess free volume. It may be worth mentioning that the crystallization temperature was estimated to be 668 K, defined as a departure from the base line of the DSC thermogram taken with a heating rate of 0.33 K sy1 and the present annealing condition was determined from the temperature– time–transformation diagram determined from electrical resistance measurements. Ordinary X-ray scattering profiles of the 2θ range between 5 and 1308 were obtained by Mo Kα radiation with a pyrolitic grade monochromator in the diffraction beam path. AXS experiments were carried out with the synchrotron radiation source at Photon Factory of the National Laboratory for High Energy Physics in Japan. The X-ray energies of 8.032 or 8.307 keV, which are 300 and 25 eV below the Ni K absorption edge, were selected by the double Si(1 1 1) monochromator. The energy resolution of the monochromator is about 7 eV [8]. The 2θ range was varied from 10 to 1408. The experimental details including data processing have been reported previously [9, 10]. It must be stressed that special effort has been paid to reduce the statistical fluctuation in the AXS measurements by accumulating at least 1:5 3 105 counts so as to maintain a statistical error of less than 0.3%. Fig. 1 shows the interference functions, Qi(Q)as (solid curve) and Qi(Q)an (dotted curve), obtained from ordinary X-ray scattering for as-quenched and annealed samples together with the difference, Q i(Q) ˆ Qi(Q)an y Qi(Q)as. Fig. 2a represents the reduced radial density function, G(r), which is given by Fourier transformation of Qi(Q), for both asquenched and annealed samples. In the case of the