Icosahedral Short-range Order Research Articles

Pressure effects on icosahedral short range order in undercooled copper

There is not a wide consensus on the role played by the icosahedral short range order on the stability of undercooled simple metals. The scenario is even less clear for undercooled metals under external pressure. Classical molecular dynamics simulations are performed to explain experimental results recently obtained on liquid and undercooled liquid copper under pressure. The atomic configurations are characterized by a common neighbor analysis to reveal the icosahedral short range order and its relation with external pressure. External pressure increases the probability to find atomic bonds with icosahedral symmetry both in the liquid and in the undercooled copper.

Surface-Sandwich Segregation Phenomena in Bimetallic Ag-Ni and Pd-Ni Nanoparticles: A Molecular Dynamics Study

By molecular dynamics simulation it is shown that interdiffusion in the initial f.c.c. Ag-core ( 28 at. %) – Ni-shell ( 72 at. %) and Ni-core ( 34 at. %) – Pd-shell ( 66 at. %) nanoparticles can lead to surface–sandwich segregation. It is observed that there is a separation of the initial Ag-Ni core-shell structure into Ag-core – Ni-intermediate shell – Ag-disperse surface monolayer. The initial crystal Ni-Pd core-shell structure transforms to the core of a non-crystalline Pd-rich solid solution with quite strongly developed icosahedral short-range order, which is covered by a surface–sandwich shell, where Ni atoms are located in the centres of interpenetrating icosahedra of a subsurface Kagomé net layer while the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from inside and outside. We demonstrate that under certain conditions a surface–sandwich segregation phenomenon at the nanoscale can be observed in systems with completely different phase diagrams in the bulk states: in systems displaying the extremely low mutual solubility as in the Ag-Ni system, or in systems exhibiting a continuous mutual solid solubility like the Pd-Ni system.

Al-centered icosahedral ordering in Cu46Zr46Al8 bulk metallic glass

Icosahedral short-range order, of which Al atoms are caged in the center of icosahedra with Cu and Zr atoms being the vertices, has been evidenced in the Cu46Zr46Al8 glassy structure by ab initio molecular dynamics simulation. These Al-centered clusters distribute irregularly in the three-dimensional space and form a “backbone” structure of the Cu46Zr46Al8 glass alloy. It is suggested that this kind of local structural feature is attributed to the requirement of efficient dense packing and the chemical affinity between Zr–Zr, Zr–Al, and Cu–Zr atoms. Our calculated results are found to be in good agreement with the experimental data.

Structure and dynamics of liquidNi36Zr64studied by neutron scattering

We report on investigations on the atomic dynamics and the static structure factors of binary ${\text{Ni}}_{36}{\text{Zr}}_{64}$ alloy melts. In order to undercool the melts deeply below the melting temperature and to avoid reactions with crucible materials, the liquids are containerlessly processed by the application of the electromagnetic levitation technique. This technique is combined with quasielastic neutron scattering at the time-of-flight spectrometer TOFTOF of the Munich research reactor (FRM II) and neutron diffraction at the diffractometer D20 of the Institut Laue-Langevin. Partial static structure factors of liquid ${\text{Ni}}_{36}{\text{Zr}}_{64}$ have been derived via isotopic substitution. The quasielastic neutron-scattering experiments indicate a large activation energy for Ni self-diffusion in liquid ${\text{Ni}}_{36}{\text{Zr}}_{64}$ of 0.64 eV. This may result from a peculiar short-range order of the Ni-Zr melts that differs from the icosahedral short-range order that was previously found to prevail in most melts of pure metals and of metallic alloys with a small difference of the atomic radii of the components and that is characterized by a high nearest-neighbor coordination number of $⟨Z⟩\ensuremath{\approx}13.9$, as found by elastic neutron scattering.

Structural evolution during crystal nucleation in supercooled liquids with icosahedral short-range order

The structural evolution during crystal nucleation in supercooled Lennard–Jones liquids at a supercooling of 0.3T m (T m is the melting temperature) has been studied by molecular dynamic simulations. The icosahedral clusters are observed to compete with crystalline clusters in space, and rearrange before crystal nucleation. Both the stable face-centered-cubic and hexagonal-close-packed (hcp) structures and metastable body-centered-cubic (bcc) structures nucleate simultaneously, resulting in the formation of an incomplete bcc meso-layer in the nuclei. The nuclei form twinned crystal with five-fold axes through a successive twinning process bounded by planes with hcp atoms.

Local order and dynamic properties of liquid and undercooledCuxZr1−xalloys byab initiomolecular dynamics

We report the results of first-principles molecular-dynamics simulations for liquid and undercooled ${\text{Cu}}_{x}{\text{Zr}}_{1\ensuremath{-}x}$ alloys at four different compositions. The local structure as defined by the partial pair-correlation functions and the Bhatia-Thornton partial structure factors is found to display a strong evolution as a function of composition. In addition, a structural analysis using three-dimensional pair-analysis techniques evidences an icosahedral short-range order much more pronounced around ${x}_{\text{Cu}}=0.64$. In examining the dynamic properties of these alloys, we show a strong interplay between the structural changes and the evolution of the viscosity as a function of composition.

Formation mechanism of atomic cluster structures in Al–Mg alloy during rapid solidification processes

The rapid solidification processes of Al 50Mg 50 liquid alloy consisting of 50,000 atoms have been simulated by using molecular dynamics method based on the effective pair potential derived from the pseudopotential theory. The formation mechanisms of atomic clusters during the rapid solidification processes have been investigated adopting a new cluster description method—cluster-type index method (CTIM). The simulated partial structure factors are in good agreement with the experimental results. And Al–Mg amorphous structure characterized with Al-centered icosahedral topological short-range order (SRO) is found to form during the rapid solidification processes. The icosahedral cluster plays a key role in the microstructure transition. Besides, it is also found that the size distribution of various clusters in the system presents a magic number sequence of 13, 19, 23, 25, 29, 31, 33, 37, …. The magic clusters are more stable and mainly correspond to the incompact arrangements of linked icosahedra in the form of rings, chains or dendrites. And each magic number point stands correspondingly for one certain combining form of icosahedra. This magic number sequence is different from that generated in the solidification structure of liquid Al and those obtained by methods of gaseous deposition and ionic spray, etc.

Atomic structure of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glass alloy

Ab initio molecular dynamics (AIMD) calculations were performed on the atomic configuration of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glass. The local structures were characterized in terms of structure factors (SF), pair correlation functions (PCF), coordinate numbers, bond pairs and Voronoi polyhedra. The glass transition temperature, generalized PCF and SF predicated by AIMD are in good agreement with the experimental data. Icosahedral short-range orders (ISRO) are found to be the most dominant, in view of the presence of the majority of bond pairs with 1551, 1541 and 1431, and Voronoi polyhedra with <0,3,6,1>, <0,2,8,1>, <0,0,12,0> and <0,2,8,4>. Icosahedral medium range orders (IMROs) are formed from icosahedra via the linkage of vertex-, edge-, face- and intercross-shared atoms. The glass structure on the nanometer scale is accumulated by polyhedra through an efficient packing mode. It is suggested that the extraordinary glass-forming ability of this alloy is essentially attributable to the formation of ISRO and IMRO, and the dense packing of atoms.

Glass forming ability and short-range order in a binary bulk metallic glass by ab initio molecular dynamics

We have performed a series of ab initio molecular dynamics simulations of Cu64Zr36 alloy, in order to study the glass forming ability in connection to the local order. The results show that Cu64Zr36 is characterized by a pronounced icosahedral short-range order (ISRO) in the liquid, undercooled, and amorphous states associated with the existence of Cu7Zr6 and Zr6Cu10 clusters centered by Cu and Zr atoms, respectively. The latter increase the structural incompatibility of liquid and amorphous states with competing crystalline phases and give support to the view that good glass formers have high degree of ISRO already present in the liquid.

Structural evolution of Cu during rapid quenching by ab initio molecular dynamics

The structural transitions of Cu during two distinct quenching processes (Q1: 4.0 × 10 13 K / s , Q2: 2.0 × 10 14 K / s ) were investigated by ab initio molecular dynamics simulation. The variations with temperature of internal energy, pair correlation functions g ( r ) and bond pairs have been characterized in both quenching processes. It is shown that liquid Cu transforms to fcc phase at the temperature about 600 K under the quenching condition Q1. The investigation of atomic diffusion by mean square displacement further demonstrates this result. When quenched under Q2, however, the liquid Cu is frozen into glass state at the temperature about 800 K. This work also reveals that icosahedral and tetrahedral clusters are predominant in the liquid state, while the icosahedral, bcc and tetrahedral clusters predominate in the glass state. The icosahedral and bcc short range ordering (SRO) are largely enhanced during the liquid–glass quenching process, whereas the tetrahedral SRO is slightly decreased.

Local structure of liquid Ti: Ab initio molecular dynamics study

Local order of liquid Ti is studied by ab initio molecular dynamics to address the unique liquid structure factor in experiments reported recently. The present study reveals that the local order of liquid Ti is in the form of fragments of the distorted icosahedral short range order, where the distortion is induced by strong bond order effects. We show that the fragments in the short-bond rich region separated from the background liquid account for the pronounced feature in structure factor of liquid Ti.

Molecular dynamics investigation of dynamical heterogeneity and local structure in the supercooled liquid and glass states of Al

Molecular dynamics simulations are performed to study the structure and dynamical heterogeneity in the liquid and glass states of Al using a frequently employed embedded atom potential. While the pair correlation function of the glass and liquid states displays only minor differences, the icosahedral short-range order (ISRO) and the dynamics of the two states are very different. The ISRO is much stronger in the glass than in the liquid. It is also found that both the most mobile and the most immobile atoms in the glass state tend to form clusters, and the clusters formed by the immobile atoms are more compact. In order to investigate the local environment of each atom in the liquid and glass states, a local density is defined to characterize the local atomic packing. There is a strong correlation between the local packing density and the mobility of the atoms. These results indicate that dynamical heterogeneity in glasses is directly correlated to the local structure. We also analyze the diffusion mechanisms of atoms in the liquid and glass states. It is found that for the mobile atoms in the glass state, initially they are confined in the cages formed by their nearest neighbors and vibrating. On the time scale of $\ensuremath{\beta}$ relaxation, the mobile atoms try to break up the cage confinement and hop into new cages. In the supercooled liquid states, however, atoms continuously diffuse. Furthermore, it is found that on the time scale of $\ensuremath{\beta}$ relaxation, some of the mobile atoms in the glass state cooperatively hop, which is facilitated by the stringlike cluster structures. On the longer time scale, it is found that a certain fraction of atoms can simultaneously hop, although they are not nearest neighbors. Further analysis shows that these hopping atoms form big and more compact clusters than the characterized most mobile atoms. The cooperative rearrangement of these big compact clusters might facilitate the simultaneous hopping of atoms in the glass states on the long time scale.

Local structure of equilibrium and supercooled Ti-Zr-Ni liquids

Recently, we reported the results of experimental in situ high-energy x-ray diffraction studies of electrostatically levitated equilibrium and supercooled metallic elements and alloy liquids, showing evidence for icosahedral short-range ordering (ISRO). In this paper, these studies are extended to binary Ti-Zr and ternary Ti-Zr-Ni alloys. From a cluster-based analysis of the x-ray structure factors, it is concluded that ISRO in the binary alloys becomes progressively more dominant, and the coherence length of the order becomes longer, with the addition of Ni, especially near the concentration of $21\text{ }\text{at}\text{.}\text{ }%$ Ni. The effect of chemical interactions among Ti/Zr-Ni and the atomic size on the stabilization of the ISRO is discussed.

Hydrogenation of Ti50Zr25Co25 amorphous ribbons and its effect on their structural and mechanical properties

Changes in the structure and mechanical properties of Ti50Zr25Co25 melt-spun ribbons upon electrolytic hydrogenation have been investigated. Structural analyses of the as-spun ribbons revealed the sporadic presence of icosahedral (i) quasicrystalline short-range order (SRO) in the amorphous structure. Upon cathodic charging with hydrogen, the i-SRO vanished for a hydrogen content of about 21 at.% (hydrogen-to-metal atom ratio, H/M: 0.26), resulting in a fully amorphous structure. Simultaneously, the fracture strength was found to increase while the ribbons preserved their bending ductility. However, a significant reduction in the fracture strength and ductility of the ribbons was observed for hydrogen concentrations larger than 26 at.%. Variations in the mechanical stability are discussed based on a structure-property correlation.

Short-to-medium-range order in Mg 65Cu 25Y 10 metallic glass

The atomic configurations of liquid and glassy Mg 65Cu 25Y 10 alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.

Local structure of binary alloys Zr70Pd30

The rearrangements of the local environment of zirconium and palladium atoms in the binary alloy Zr70Pd30 upon the transformation from the crystalline to the quasicrystalline and amorphous states were studied by extended X-ray absorption fine structure (EXAFS) analysis. In the quasicrystalline phase, the icosahedral environment was found to be formed only around the zirconium ions because of shifts of both the palladium and zirconium atoms to the center of the cluster; in addition, the structure of the icosahedral clusters is distorted. The transformation from the crystalline to the quasicrystalline phase is accompanied by the atomic rearrangement in the immediate environment of palladium, thus resulting in the partial displacement of palladium by zirconium. In the amorphous phase, the icosahedral short-range order is not observed. A model for the description of the structure of the quasicrystalline phase of Zr70Pd30 is proposed.

Structural studies of a Ti–Zr–Ni quasicrystal-forming liquid

Employing the technique of electrostatic levitation coupled with high-energy x-ray diffraction,Ti39.5Zr39.5Ni21 liquids were shown previously to develop significant short-range icosahedral orderwith supercooling. However, that conclusion was based on the assumption ofa single dominant cluster type in the liquid and the observed evolution of thehigh-q shoulder on the second peak in the structure factor,S(q). Here, new diffraction data that were obtained using more rapid data acquisitionmethods are presented. These allow structural studies to be made down to andthrough recalescence to the icosahedral quasicrystal. The liquid structures obtainedfrom a Reverse Monte Carlo analysis of these data are characterized by theirbond-angle distributions, Honeycutt and Andersen indices and bond orientational orderparameters. These analyses indicate that while there are several different types of localorder, the icosahedral short-range order is dominant and increases gradually withsupercooling.

Nanoscale defect clusters in metallic glasses

The reverse Monte Carlo method was used to obtain three-dimensional discretedistributions of constitutional atoms in melt-spun CuZr and CuZrTi metallicglasses from neutron and x-ray diffraction data. It was found that the icosahedralshort-range order is less stable in the CuZr binary alloy than in the Ti-dopedternary alloy. The present investigation also provides evidence on the medium-rangeorder, characterized by some nanoscale clusters of defects, in the metallic-glassystate.

Structural Aspects of Metallic Glasses

AbstractA recent structural model reconciles apparently conflicting features of randomness, short-range order, and medium-range order that coexist in metallic glasses. In this efficient cluster packing model, short-range order can be described by efficiently packed solute-centered clusters, producing more than a dozen established atomic clusters, including icosahedra. The observed preference for icosahedral short-range order in metallic glasses is consistent with the theme of efficient atomic packing and is further favored by solvent-centered clusters. Driven by solute—solute avoidance, medium-range order results from the organization in space of overlapping, percolating (via connected pathways), quasi-equivalent clusters. Cubic-like and icosahedral-like organization of these clusters are consistent with measured medium-range order. New techniques such as fluctuation electron microscopy now provide more detailed experimental studies of medium-range order for comparison with model predictions. Microscopic free volume in the efficient cluster packing model is able to represent experimental and computational results, showing free volume complexes ranging from subatomic to atomic-level sizes. Free volume connects static structural models to dynamic processes such as diffusion and deformation. New approaches dealing with “free” and “anti-free” microscopic volume and coordinated atomic motion show promise for modeling the complex dynamics of structural relaxations such as the glass transition. Future work unifying static and dynamic structural views is suggested.

Local order in undercooled liquid metals: A tight binding molecular dynamics approach

We use constant (zero) pressure molecular dynamics based on a semi-empirical tight binding model to simulate the structures of liquid Ni, Cu, Pd and Au in the stable and undercooled states. We compare our data with the available experiments (neutron scattering for Ni, EXAFS for Pd). We analyze the structure in terms of Voronoi polyhedra and use the neighbors defined by this analysis to perform a Common Neighbors Analysis. We show that a small but significant difference in the number of bonds with five common neighbors exists between undercooled Ni and the other elements. We question the possible experimental observation of such a difference and its relevance to the issue of the existence of an icosahedral short range order in the liquid.