Topological Short-range Order Research Articles

Medium Range Ordering realized in Zr80Pt20Amorphous Alloy

The short- and medium-range ordering structure in Zr80Pt20 amorphous alloy has been investigated by anomalous X-ray scattering coupled with reverse Monte-Carlo simulation. Pt prefers to construct a Zr-rich icosahedral complex with unique chemical and topological short-range ordering. The present analysis also revealed the medium-range ordering of Pt-Pt pairs corresponding to the pre-peak signal at 17 nm−1 in the diffraction profile. The chemical and topological features around Pt and bonding structure correspond well with those of Mn in the approximant phase of α-AlMnSi.

Constant real-space fractal dimensionality and structure evolution in Ti62Cu38 metallic glass under high pressure

The structure of binary $\mathrm{T}{\mathrm{i}}_{62}\mathrm{C}{\mathrm{u}}_{38}$ metallic glass is investigated under pressures up to 33.8 GPa using the pair distribution function analysis based on high-energy x-ray scattering and reverse Monte Carlo (RMC) simulations. At a global scale, its relative volume shows a continuously smooth curve as a function of pressure. The isothermal bulk modulus of $\mathrm{T}{\mathrm{i}}_{62}\mathrm{C}{\mathrm{u}}_{38}$ metallic glass is estimated as ${B}_{0}=132(3)\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ with ${B}_{0}^{\ensuremath{'}}=5.8(0.4)$. At a local scale, the atomic packing structure under compression conditions, which is extracted from RMC simulations, shows that the topological short-range order is dominated by the deformed icosahedron polyhedra and basically maintains stable. From the relationship between the relative volume and changing ratio of the atomic separation distances, the real-space fractal dimensionality of this metallic glass is determined as about 2.5 for all of the first four peaks. This experimental result reveals the consistent nature of the fractal feature on the degree of self-similarity in this sample within the entire experimental pressure range.

High temperature behavior of functional TiAlBSiN nanocomposite coatings

Abstract This article reports on the thorough characterization of structural-phase transformation in amorphous TiAlBSiN coating after high temperature annealing at 900 °C in ambient air. The influence of annealing on the tribo-mechanical behavior of the coating at nano and micro scale was also examined. The research included multiple experimental techniques, i.e. AFM, SEM, TEM, HR-TEM, EDS, XPS and Raman spectroscopy. Experiments showed that the amorphous phase of the TiAlBSiN coating undergoes a structural transformation, evidenced in the changes of parameters such as topological and chemical short-range order after the post-deposition annealing at 900 °C in air. The observed structural transformation, leads to a phase separation with the formation of a three dimensional nc-TiAl 3 /a-SiBN(O) nanocomposite structure. The relative increase of hardness, reduced elastic modulus, H/Er ratio and H 2 /E 3 r ratio after high temperature treatment of TiAlBSiN coatings is also reported. The complex interdependency between chemistry, morphology and relative composition of the amorphous TiAlBSiN coating phase, during the high temperature treatment, with the respective change of the tribo-mechanical characteristics, are evidence of the improvement of the coating properties in response to the environmental conditions and high temperature. This work contributes particularly to the development and understanding of flexible nanocomposite protective coatings and their changes at high temperature of operation.

Effect of the third element on the structure of liquid Mg65Cu25Y10 alloy

The liquid structures of Mg65Cu25Y10 and its three homologous binary liquid alloys are investigated via ab initio molecular dynamics in the present work. The chemical and topological environments in all four liquid alloys are analyzed using pair distribution function, coordination number, and the Voronoi polyhedron. It shows that the Cu atoms play significant role in deciding the chemical and topological short-range orders of the Mg65Cu25Y10 liquid alloy. The Voronoi polyhedra in the ternary liquid alloy illustrate less varieties and longer lifetime. Moreover, the diffusion coefficients are decreased significantly in the ternary liquid alloys according to the mean square displacements. All above offer a deeper insight into how the three species work in the Mg65Cu25Y10 liquid alloy.

First-Principles Study of Chemical and Topological Short-Range Orders in the Mg–Si Liquid Alloys

The structures of a series of Mg–Si liquid alloys were investigated by means of ab initio molecular dynamic simulation. The pair distribution function analysis manifests a tendency of aggregating for the Si–Si pairs in the Mg90Si10 liquid alloy. Chemical short-range orders are observed around Si atoms between unlike atoms, and the maximum is observed for the eutectic Mg47Si53 alloy. Furthermore, the topological environment changed abruptly near the eutectic Mg47Si53 alloy according to Voronoi polyhedra analysis. The variation of diffusion coefficients of Mg and Si suggests that the dynamical properties of Mg–Si liquid alloys are more sensitive to temperature than to compositions.

Relation of Structure, Composition and Glass Forming Ability in Zr-Cu Binary Amorphous Alloys

Molecular dynamics simulation was used to simulate Zr-Cu binary system, in which the relationship between structure, composition and glass forming ability was study. Atomic local structures were analyzed from view of chemical and topological short range order. Reduction fraction of full icosahedra ( B c ) was developed to establish the relation of structure-composition and glass forming ability(GFA) in Zr-Cu binary system. Obviously peaks were observed at some certain compositions which own the good GFA. As a structure factor, B c could be a indicator of GFA of Zr-Cu alloys. Our works contributed to further understanding the effect of atomic structures on glass forming.

X-ray diffraction study of atomic structure of Hf–W amorphous alloys

Using the X-ray diffraction method, the atomic structure of Hf100−xWx (x=16, 36, 48, 57, 61 and 68at.%) amorphous alloys was studied. The parameters of topological short-range order were obtained: the radii of coordination spheres, the coordination number of the first sphere, and the mean-square deviation of atomic positions in the first sphere. It was established that the basic parameters of topological short-range order do not depend on the composition of the alloy.

Temperature effects on atomic pair distribution functions of melts

Using molecular dynamics simulations, we investigate the temperature-dependent evolution of the first peak position/shape in pair distribution functions of liquids. For metallic liquids, the peak skews towards the left (shorter distance side) with increasing temperature, similar to the previously reported anomalous peak shift. Making use of constant-volume simulations in the absence of thermal expansion and change in inherent structure, we demonstrate that the apparent shift of the peak maximum can be a result of the asymmetric shape of the peak, as the asymmetry increases with temperature-induced spreading of neighboring atoms to shorter and longer distances due to the anharmonic nature of the interatomic interaction potential. These findings shed light on the first-shell expansion/contraction paradox for metallic liquids, aside from possible changes in local topological or chemical short-range ordering. The melts of covalent materials are found to exhibit an opposite trend of peak shift, which is attributed to an effect of the directionality of the interatomic bonds.

Evolution of structural and magnetic properties of amorphous CoFeB film with thermal annealing

Evolution of structural and magnetic properties of amorphous Co68Fe14B18 thin film with thermal annealing has been studied. Initially, the film exhibits a structural relaxation as evidenced by annihilation of excess free volume and an increase in topological short range order. Annealing at 473 K results in precipitation of primary phase followed by formation of boride phase at a still higher temperature of 598 K. Iron preferentially precipitates out in the primary phase, resulting in the formation of bcc Co58Fe41. This suggests an affinity of Co towards B. Such affinity between Co and B is evidenced even in the as-deposited film, using hard x-ray photoelectron spectroscopy (HAXPES) measurements. As-deposited film exhibits an in-plane uniaxial magnetic anisotropy which disappears at a temperature well beyond crystallization temperature, suggesting that the origin of anisotropy is mainly a chemical short range order in the system. Variation in the coercivity with thermal annealing can be understood in terms of random anisotropy model. Precise measurement of Fe self-diffusion using neutron reflectivity shows that diffusion length associated with annihilation of excess free volume in the film is about 0.5 nm. This agrees with the length scale of structural fluctuations in amorphous alloys. Secondary ion mass spectrometry measurements show that thermal annealing results in depletion of B in the region of the interface with the substrate, with associated faster Fe diffusion in this region. This faster diffusion of Fe may be a possible cause of preferential crystallization of the film in the interfacial region as seen in some earlier studies.

Controlled softening of Cu64Zr36 metallic glass by ion irradiation

We study the effect of irradiation with 5–20 keV recoils on the topological and chemical short-range order of Cu64Zr36 metallic glass using molecular dynamics simulations. We show that within the cascade region, the structural backbone of stiff Cu-centered icosahedral units is destroyed, leading to locally softened areas. Under mechanical load, the formation of shear transformation zones is thus promoted in the damaged area. Our results suggest that irradiation is a means to introduce nucleation sites for multiple shear bands and thus prevents catastrophic failure due to the presence of a single critical shear band.

Short-range order of undercooled melts of PdZr2 intermetallic compound studied by X-ray and neutron scattering experiments

The short-range order in undercooled melts of the intermetallic Zr2Pd glass-forming alloy is investigated by combining electrostatic levitation (ESL) with high-energy X-ray diffraction and neutron diffraction. Experimentally determined structure factors are measured and analyzed with respect to various structures of short-range order. The comparative X-ray and neutron scattering experiments allow for investigations of topological and chemical short-range order. Based on these studies, no preference of a specific short-range order is found for the liquid Zr2Pd glass-forming alloy, even in the metastable state of the deeply undercooled melt. This is in agreement with an earlier report from X-ray diffraction and molecular-dynamics studies of a Zr75.5Pd24.5 liquid, which showed a broad distribution of cluster types. The results for the Zr2Pd liquid are discussed with respect to the glass-forming ability of this melt.

Chemical and topological short-range orders in the ternary Ni–Zr–Al metallic glasses studied by Monte Carlo simulations

Based on the recently constructed Ni–Zr–Al n-body potential, Monte Carlo simulations are performed to study the glass formation and associated structural evolutions in the system. The micro-chemical inhomogeneity (MCI) parameter and Honeycutt and Anderson (HA) pair analysis are employed to investigate both the chemical short-range orders and topological short-range orders for the ternary Ni–Zr–Al metallic glasses. Results reveal that remarkable chemical short-range orders (CSROs) exist in the ternary Ni–Zr–Al metallic glasses and are strongly influenced by the chemical interactions among the constituent elements. Moreover, topological short-range orders are clearly formed in the ternary Ni–Zr–Al metallic glasses, with the most remarkable characteristic being the icosahedral local packing. Similarly to CSRO, the extent of icosahedral short-range orders formed in the Ni–Zr–Al system varies distinctly with the chemical composition. In addition, simulation results reveal that chemical short-range orders and topological short-range orders turn out to be influenced by different factors. Unlike CSRO, both chemical interactions and geometrical constraints play important roles in forming the topological short-range orders.

Study of Amorphous-Crystalline Phase Transformations by DSC and Dilatometer in the Case of Al-Based Amorphous Alloys

The amorphous-crystalline transformation is studied by DSC and dilatometer. From the DSC signal the characteristic temperatures and the heat evolved during each transformation step can be determined. The dilatometer depicts the thermal contractions accompanying the changes in topological short range order. Although the characteristics temperatures determined by DSC and dilatometer, two methods are slightly different their dependences on the heating rate are very similar, where from the activation energies of each transformation steps can be determined using the Kissinger plots. As an example, two aluminum-based amorphous ribbon samples (Al88Mm5Ni5Co2 and Al85Y8Ni5Co2) will be measured and compared.

Atomic packing and short to medium range order in a U-Fe metallic glass

The atomic structure of the U85.7Fe14.3 metallic glasses is studied in terms of topological short range order, chemical short range order and medium range order using reverse Monte Carlo simulations. The existence of solute-centered clusters and Fe-enriched clusters clearly demonstrates a strong chemical short range order in both glasses. Two major types of medium range order are detected, one is the Fe-enriched super-clusters and the other is the close packing of the solute-centered clusters. Nevertheless, no existing model can be employed to fully describe the medium range order in U-based metallic glasses, indicating the structural complexity of these metallic glasses.

Crystallization behavior of Ce70−xAl10Cu20Cox (x = 0, 1, 3, 5 at.%) amorphous alloys

The effect of Co addition (substituting for Ce) on crystallization behavior of Ce70Al10Cu20 amorphous alloys has been investigated using X-ray diffraction (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The Co addition has an obvious effect on topological short-range ordering of Ce–Al–Cu–(Co) amorphous alloys. Moreover, the Co addition can slightly improve the thermal stability of Ce–Al–Cu based amorphous alloys. The 1 and 3 at.% Co additions do not obviously change the crystallization behavior of the Ce–Al–Cu–(Co) amorphous alloys, and the final crystallization products are FCC–CeAlCu(Co)O. However, the 5 at.% Co addition can alter the crystallization behavior of the Ce70Al10Cu20 amorphous alloys. Proper content of Co can effectively suppress the formation of oxide phases during annealing of the Ce–Al–Cu–(Co) amorphous alloys.

Chemical and topological order in shear bands of Cu64Zr36 and Cu36Zr64 glasses

Shear bands in binary Cu64Zr36 and Cu36Zr64 metallic glasses are studied by molecular dynamics simulations with respect to their chemical and topological short range order. In both glasses, shear band formation goes along with an increase in excess volume inside the shear band. Only in the Cu-rich alloy, where Cu-centered icosahedra represent the most abundant topological unit, the dilatation within the shear band is related to a decrease in the number of icosahedral units, while the degree of cross-linking and the cluster size decrease when a shear band forms. In the Cu36Zr64 glass, in contrast, no topological feature changes when the material starts yielding and a shear band is formed. The chemical short range order, however, is affected in both materials within the shear band, which is revealed by an increase in the number of Cu–Cu and Zr–Zr bonds. Since all structural modifications are rather the result of plastic deformation than a pre-requisite, we conclude that the presence of distinct topological features, such as icosahedral units, is not a pre-requisite for shear band formation.

A New Model for the Configurational Entropy of Mixing in Liquid Alloys Based on Short-Range Order

A new model has been developed to calculate the configurational entropy of mixing in liquid alloys involving consideration of chemical and topological short-range order. The entropy of mixing for an equiatomic random mixture was naturally reached by this model. The application of this model to both hypothetic and real binary liquid alloys demonstrated that the chemical short-range order always decreased the configurational entropy of mixing, whereas complicated behavior was found with the atomic size effect. The configurational entropy of mixing increased when the larger atoms entered into the matrix of the smaller atoms, whereas it decreased when the smaller atoms were mixed into the matrix of the larger atoms. The maximum of the configurational entropy of mixing was not located at the eutectic composition in these alloys.

Thermal annealing of shear bands in deformed metallic glasses: Recovery mechanisms in Cu 64Zr 36 studied by molecular dynamics simulations

Molecular dynamics simulations on the recovery of shear bands in deformed binary Cu 64Zr 36 glasses are presented. While the shear band (SB) formation induced by plastic deformation leads to an increase in excess volume, the material surrounding the SB experiences a compressive strain due to the dilatation of the SB. This is accompanied by changes in topological and chemical short-range order within the shear band. Isothermal annealing of the sample leads to redistribution and annihilation of excess volume, which is coupled to the recovery of local order and depends on the temperature. At lower temperatures (500 K) diffusion by a chain-like process is the dominating mechanism of structural recovery, similar to processes occurring in supercooled liquids. At higher temperatures (800 K) individual atomic displacements also start to contribute to the recovery process. At temperatures close to the glass transition, the recovery occurs on timescales of about 20 ns. If the annealing temperature is higher than T g , thermal activation is sufficient to rejuvenate the glass structure.

Structure and dynamics of liquid Ni36Zr64byab initiomolecular dynamics

First-principles molecular dynamics simulations are performed to investigate the interplay between the local structure and atomic dynamics of liquid Ni${}_{36}$Zr${}_{64}$ eutectic alloy in the equilibrium and undercooled states. The topological and chemical short-range orders (SROs) as characterized by the partial pair-correlation functions show good agreement with recent neutron-scattering experiments. Three-dimensional structural analysis shows that the local environment around both Ni and Zr atoms is more complex than the icosahedral SRO due to the frustration of icosahedra driven by the strong directional interaction and large size difference between Ni and Zr. The atomic dynamics analysis shows that the Ni-centered $\ensuremath{\langle}0,2,8,1,0\ensuremath{\rangle}$ and Zr-centered $\ensuremath{\langle}0,1,10,4,0\ensuremath{\rangle}$ Voronoi polyhedra with relatively longer lifetime play a key role in slowing down the dynamics. The fraction of these clusters increases significantly as the liquid is cooled down and thus facilitates glass formation in the rapidly quenched alloys.

Chemical and topological short-range order evolution of Mg65Cu25Gd10 alloy in the process of rapid solidification

The structural evolution of Mg65Cu25Gd10 alloy from liquid to glass state was investigated by ab initio molecular dynamics simulations. From 500 K, the splitting of the second peaks of the three partial pair correlation functions related to Mg has been perceived which implicates that short-range order is strengthened owing to the atomic rearrangement during the quenching process; medium-range order up to 7.59 Å has been observed from gcc(r) curves. The Gd-centered Voronoi polyhedron with index of 〈 0, 1, 10, 5 〉 was observed to increase drastically to an extraordinary level as the melt transformed to glass at 500 K, suggesting that it plays important role to keep the amorphous system stable. The variation of diffusion coefficients indicates that the solidification occurred at the temperature between 750 and 500 K.