Icosahedral-like Clusters Research Articles

Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy.

The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

Structural differences of amorphous Cu65Zr35 between rapidly quenched and topologically destabilized crystalline Cu and Zr metals by molecular dynamics simulations

In this work we demonstrate, by means of molecular dynamic simulations, that the topological instability caused by successive substitutions of Zr in the Cu lattice, or Cu in the Zr lattice towards a final alloy composition of Cu65Zr35, transform the initial systems to the same amorphous structure at 300K. Such a structure, however, is composed of less ordered clusters, i.e. less icosahedral-like clusters in comparison with the same amorphous alloy produced by rapid quenching from the melt. After a short time annealing at 800K the less ordered structure evolves towards the structure of the rapidly solidified sample. From these results it comes out that no structural differences are to be expected in metallic glasses produced by mechanical alloying and by rapid quenching; any differences found in metallic glasses produced by these two different routes should be attributed to insufficient alloying treatment during the mechanical alloying procedure.

Composition-dependent structural and electronic properties of Mg(95-x)Zn(x)Ca5 metallic glasses: an ab initio molecular dynamics study.

Recent progress in the synthesis of Mg-based metallic glasses (MGs) has allowed them to be considered as potential candidates for biodegradable and bioabsorbable implant materials. In this work, we use the Mg-Zn-Ca system as a representative to investigate the effect of composition on the atomic-level structure and local chemical environment in Mg-based MGs from ab initio molecular dynamics simulations. The results suggest that the short-range order of Mg(95-x)Zn(x)Ca5 (x = 21, 25, 29, and 33) MGs is characterized by Zn-centered icosahedral and icosahedral-like clusters, which show an increasing number and a rising tendency to interpenetrate each other with the enrichment of Zn constituents. A considerable degree of charge transfer between Zn and the surrounding Mg/Ca atoms is observed through electronic structure and bonding character analysis. At Zn-rich compositions, a percolated Zn-Zn network extended throughout the entire sample is formed, upon which the accumulated charges around Zn atoms are associated into a continuous conductivity path. Such results may shed light on the improved corrosion resistance of the Zn-rich Mg-Zn-Ca MGs.

Evolution of atomic structure in Al75Cu25 liquid from experimental and ab initio molecular dynamics simulation studies

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al75Cu25 alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al75Cu25 alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt–Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al75Cu25 alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid–liquid transition appears in the bulk liquid Al75Cu25 alloy in the studied temperature range.

Structural origin of the different devitrification behaviors between ZrCu and ZrPd metallic glasses

The microstructures of Zr70Pd30 and Zr70Cu30 metallic glasses (MGs) were investigated via the synchrotron radiation techniques combined with the reverse Monte Carlo simulations. It is revealed that, compared with Zr70Cu30, Zr70Pd30 has more fruitful icosahedral-like clusters, which have a relatively high geometrical regularity closer to that of the ideal icosahedron. Therefore, clusters in Zr70Pd30 are prone to change into the structural units of icosahedral quasicrystal phase. This should be the structural origin of the amorphous-to-quasicrystal transformation found in Zr70Pd30 while such phase change never appears when heating Zr70Cu30 MG. This work provides an understanding of the devitrification mechanism from the atomic level in this class of glassy materials.

Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses

We studied atomic and electronic structures of binary Cu-Zr metallic glasses (MGs) using combined experimental and computational methods including X-ray absorption fine structure spectroscopy, electrical resistivity, thermoelectric power (TEP) measurements, molecular dynamics (MD) simulations, and ab-initio calculations. The results of MD simulations and extended X-ray absorption fine structure analysis indicate that atomic order of Cu-Zr MGs and can be described in terms of interpenetrating icosahedral-like clusters involving five-fold symmetry. MD configurations were used as an input for calculations of theoretical electronic density of states (DOS) functions which exhibits good agreement with the experimental X-ray absorption near-edge spectra. We found no indication of minimum of DOS at Fermi energy predicted by Mott's nearly free electron (NFE) model for glass-forming alloys. The theoretical DOS was subsequently used to test Mott's model describing the temperature variation of electrical resistivity and thermoelectric power of transition metal-based MGs. We demonstrate that the measured temperature variations of electrical resistivity and TEP remain in a contradiction with this model. On the other hand, the experimental temperature dependence of electrical resistivity can be explained by incipient localization of conduction electrons. It is shown that weak localization model works up to relatively high temperatures when localization is destroyed by phonons. Our results indicate that electron transport properties of Cu-Zr MGs are dominated by localization effects rather than by electronic structure. We suggest that NFE model fails to explain a relatively high glass-forming ability of binary Cu-Zr alloys.

Evolution of local atomic structure during solidification of Al2Au liquid: An ab initio study

The local atomic structure evolution in Al2Au alloy during solidification from 2000K to 400K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt–Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of 〈0,4,4,0〉, 〈0,3,6,0〉 and 〈0,4,4,2〉 with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy.

On the role of Icosahedral-like clusters in the solidification and the mechanical response of Cu–Zr metallic glasses by Molecular Dynamics simulations and Density Functional Theory computations

Cu–Zr Metallic glasses (MG) are considered to be composed of Icosahedral-like (ICO) clusters that may be distorted, truncated and/or interconnected. Aiming in gaining insight on the role of these units in the solidification-glass formation processes, as well as under mechanical solicitation, we performed detailed analysis of the electronic density of states, based on the Density Functional Theory, of the basic ICO-like clusters that were extracted from Cu65Zr35 equilibrium Molecular Dynamics (MD) configurations at various temperatures within the range from 2000 K to 300 K. In line with previous studies, we found that the d valence electrons of the Zr shell atoms occupy states close to the Fermi level, while s–s and d–s electronic hybridizations of the Cu-core/Cu-shell with the Cu shell atoms, respectively, occur at lower energy states and account for the deformation of the clusters, which is exclusively accomplished by these Cu–Cu distortions. Moreover, we found that upon quenching the mean atomic distances of the ICOs are decreasing due to shortening of the Cu-core/shell – Cu shell bonds, resulting in shifts of the corresponding states towards lower energies. Interestingly, it came out that the temperature dependence of these energies is linear, exhibiting slope changes at the melting and glass transition points. We applied the same analysis on ICOs extracted from Cu50Zr50 MD configurations under tensile solicitation from where we deduced the alterations that are caused in the electronic densities of states. These results could be of use for the understanding of the bonding characteristics of Cu–Zr model MGs and the mechanisms for the accommodation of the mechanical deformation.

The formation of FeZn13 phase from atomic cluster during hot dip galvanizing

The liquid structure of Zn95Fe5 alloy was studied by ab initio molecular dynamic simulations. The important variation tendency of pair correlation function has been characterized. It is found that the interaction of unlike atoms is stronger than the like atoms. Voronoi polyhedron analysis reveals that icosahedral-like clusters are frequently found around Fe and the dominated voronoi polyhedrons around Zn atoms are icosahedral-like and deformed-bcc clusters. Chemical short range order analysis suggests that there are 12 Zn atoms around Fe, which is similar to the structure of FeZn13 phase. More importantly, this simulated result is proved indirectly by the experiment that a mass of FeZn13 phase was confirmed in zinc slag by X-ray diffraction, which indicates that the liquid structure is partially kept during solidification process.

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1 nm.

Clustering, microalloying and mechanical properties in Cu/Zr-based glassy models by molecular dynamics simulations and ab-initio computations

We present results on the microstructure of Cu–Zr metallic glasses (MGs) at equilibrium and under tensile deformation by means of large scale molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We found that the MGs are composed by interpenetrating icosahedral-like (ICO) clusters forming superclusters (SCs). The deformation accommodation is associated with the destruction and recreation of these ICO clusters and SCs. DFT calculations on the Cu13Zr10 SCs with Cu–Cu core atoms revealed the presence of a free of bonds (FoB) plane that could be viewed as analog of a slip plane of the crystalline materials. The microalloying (MA) effect in a similar SC (one of the core atoms substituted by Al) yielded that Al is associated with the formation of a FoB plane. These results provide possible explanations of the experimental findings referring to the short range order, the MA effect and could potentially be used for the design of new MGs.

On the deposition mechanisms and the formation of glassy Cu–Zr thin films

We report on molecular dynamics (MD) simulations and physical vapor deposition experimental results concerning the development of glassy and nanocrystalline Cu–Zr thin films. MD has revealed that when Cu and Zr are deposited sequentially, a thin film overlayer is formed that consists of nanocrystalline a-Zr and t-Zr2Cu, while if Cu and Zr are simultaneously deposited, amorphous CuZr thin film emerges, due to the formation of icosahedral-like clusters that impede nucleation. Thin films grown by pulsed laser deposition and magnetron sputtering techniques were analyzed by x-ray diffraction and high-resolution transmission electron microscopy and yielded unequivocal evidence that validates our MD predictions. These findings may indicate an alternative pathway for the growth of metallic nanocomposites or glassy films.

Relaxation Dynamics of Photoexcited Calcium Deposited on Argon Clusters: Theoretical Simulation of Time-Resolved Photoelectron Spectra

Relaxation dynamics following photoexcitation of a calcium atom deposited on an icosahedral-like argon cluster Ar(n) (n approximately = 55) is investigated through theoretical simulations. Based on ab initio calculations of the CaAr molecule, a diatomics-in-molecules model is set up to efficiently describe the electronic excited states of the system. The excited state dynamics is studied using molecular dynamics with electronic transitions (Tully, J. C. J. Chem. Phys. 1990, 93, 1061). The signature of this dynamics in the time-resolved photoelectron spectra is investigated, to assess the possibility of detecting competing vibrational and electronic relaxations through pump-probe experiments. The vibrational relaxation, influenced by nonadiabatic transitions, can clearly be seen in the time-resolved photoelectron spectra. The details of the electronic relaxation, as well as the possible ejection of the chromophore, are found to be sensitive to the local environment of the calcium atom deposited on the argon cluster.

On the microstructure of the Cu 65Zr 35 and Cu 35Zr 65 metallic glasses

We report on the microstructure of Cu65Zr35 and Cu35Zr65 metallic glasses (MGs). We found that the systems consist of small touching and/or interpenetrating icosahedral-like clusters that obey a simple rule, which is dictated from the system’s composition, eventually yielding a microstructure of polyicosahedral clusters that follow a specific sequence of magic numbers. The approach permits the prediction of the basic MG microstructure and may thus be of use for the experimental data interpretation and the design of new MGs.

Correlation between local structure of melts and glass forming ability for Fe78M9B13 (M=Nb, Si, and Zr) alloys

The structures of Fe78M9B13 (M=Nb, Si, and Zr) melts have been studied by ab initio molecular dynamics simulation. The essential features of the structures are the same in these melts. Prism-type local atomic arrangements dominate around B atoms and icosahedral-like clusters are frequently found around Fe atoms. However, the structures of the three melts differ in the degree of structural distortion caused by introducing M atoms to the prisms. The structural and dynamical properties of these melts change with different M elements, suggesting that they are closely correlated with their glass formation. The degree of complexity of molten structure caused by the interactions among Fe, M, and B may play an important role in their glass forming ability.

Short- and medium-range order in aZr73Pt27glass: Experimental and simulation studies

The structure of a ${\text{Zr}}_{73}{\text{Pt}}_{27}$ metallic glass, which forms a ${\text{Zr}}_{5}{\text{Pt}}_{3}$ (${\text{Mn}}_{5}{\text{Si}}_{3}$-type) phase having local atomic clusters with distorted icosahedral coordination during the primary crystallization, has been investigated by means of x-ray diffraction and combining ab initio molecular-dynamics (MD) and reverse Monte Carlo (RMC) simulations. The ab initio MD simulation provides an accurate description of short-range structural and chemical ordering in the glass. A three-dimensional atomistic model of 18 000 atoms for the glass structure has been generated by the RMC method utilizing both the structure factor $S(k)$ from x-ray diffraction experiment and the partial pair-correlation functions from ab initio MD simulation. Honeycutt and Andersen index and Voronoi cell analyses, respectively, were used to characterize the short- and medium-range order in the atomistic structure models generated by ab initio MD and RMC simulations. The ab initio results show that an icosahedral type of short-range order is predominant in the glass state. Furthermore, analysis of the atomic model from the constrained RMC simulations reveals that the icosahedral-like clusters are packed in arrangements having higher-order correlations, thus establishing medium-range topological order up to two or three cluster shells.

Local ordering of polymer-tethered nanospheres and nanorods and the stabilization of the double gyroid phase

We present results of Brownian dynamics simulations of tethered nanospheres and tethered nanorods. Immiscibility between tether and nanoparticle facilitates microphase separation into the bicontinuous, double gyroid structure (first reported by Iacovella et al. [Phys. Rev. E 75, 040801(R) (2007)] and Horsch et al. [J. Chem. Phys. 125, 184903 (2006)], respectively). We demonstrate the ability of these nanoparticles to adopt distinct, minimal energy local packings, in which nanospheres form icosahedral-like clusters and nanorods form splayed hexagonal bundles. These local structures reduce packing frustration within the nodes of the double gyroid. We argue that the ability to locally order into stable structures is key to the formation of the double gyroid phase in these systems.

Atomic configuration of metal atoms in (Ti 0.676Zr 0.324)D 0.31 metallic glass studied by X-ray, neutron diffraction and reverse Monte Carlo modeling

We have obtained a three-dimensional atomic configuration in (Ti 0.676Zr 0.324)D 0.31 glass using the reverse Monte Carlo (RMC) modeling method with neutron and X-ray diffraction data. Voronoi polyhedral analysis for the RMC configuration of metal atoms revealed the presence of a lot of icosahedral-like clusters.

Local Atomic Structures of Amorphous Fe<SUB>80</SUB>B<SUB>20</SUB> and Fe<SUB>70</SUB>Nb<SUB>10</SUB>B<SUB>20</SUB> Alloys Studied by Electron Diffraction

Effect of Nb addition on the stabilization of amorphous Fe80B20 alloy was studied by electron diffraction structure analysis with the help of computer simulation. Atomic structure models with dense-random-packing 5000 atoms were constructed to reproduce interference functions obtained experimentally for the amorphous Fe80B20 and Fe70Nb10B20 alloys. In the structure models so obtained, prism-type local atomic arrangements dominated around B atoms in both the alloys, while deformed-bcc and icosahedral-like clusters were frequently found around Fe atoms. The icosahedral-like clusters and short Fe–Fe bondings were more frequently found in Fe70Nb10B20. The difference of glass forming abilities of these alloys is discussed in relation to the local structural differences.

Change in local atomic structure during formation of the icosahedral quasicrystalline phase in Zr70Pd30glassy alloy

The local atomic structures in glassy, supercooled liquid and quasicrystalline phases for a Zr70Pd30binary alloy have been examined by the x-ray diffraction method. It was found that the local atomic structure in the glassy phase can be identified as the distorted icosahedral-like structure around Zr and remains almost unchanged by the phase transformation into the supercooled liquid. In the formation process of the icosahedral phase, approximately one Zr atom substitutes with one Pd atom in this local structure. This kind of atomic rearrangement may improve the perfectibility of the as-quenched icosahedral-like cluster, leading to the phase evolution of the icosahedral phase.