Icosahedral Short-range Order Research Articles

Molecular dynamics simulation of alloying in an Al-coated Ti nanoparticle

The alloying reaction of an Al-coated Ti nanoparticle having equi-atomic fractions and a diameter of about 4.8nm is studied using molecular dynamics simulation in combination with an embedded atom method potential. We demonstrate that the Al-coated Ti nanoparticle is much less reactive than the Ti-coated Al nanoparticle of similar size studied previously (E.V. Levchenko et al., Intermetallics 22 (2012) 193). The reason for this is that the pre-alloying in the vicinity of the interface at moderate temperatures is noticeably more pronounced in the Al-coated Ti nanoparticle. It comes about because of the distorted structure of a rather thin Al shell in the vicinity of the interface with a quite high level of short-range icosahedral order. The distorted structure is a consequence of the influence of the interface between the Ti-core and Al-shell. In the previously studied Ti-coated Al nanoparticle the interface influence was not so pronounced because the larger Ti atoms formed a greater strength shell to confine the Al core. Such a nanoparticle morphology resulted in avoidance of the strong distortions in both core and shell structures. The distorted shell structure of the Al-coated Ti nanoparticle enhances the diffusion mobility of atomic components in the Al shell in the vicinity of the interface at intermediate temperatures, thereby promoting the pre-alloying process. The pre-alloying eventually results in development of a well defined f.c.c. crystal structure in the Al-based shell at high temperatures which has good alignment of the close-packed orientation with the h.c.p. Ti-based core. As a result, the pre-alloyed interfacial layer and improved crystal structure of the Al-based shell serve as an effective reaction barrier in the Al-coated Ti nanoparticle. This slows down the reaction by extending the temperature range of the solid-state interdiffusion process in the Al-coated Ti nanoparticle.

Ab initio molecular dynamics study of the structure of undercooled Ni melt

The structure of undercooled Ni melt was studied by ab initio molecular dynamics simulations. “Cage effect” is obvious based on the calculated normalized velocity autocorrelation functions and will be enhanced with a decrease in temperature. It is found that the fractions of 1661-, 1551- and 1441-type bonded-pairs increase with the decrease in temperature and all the others decline. The fractions and lifetimes of the Voronoi clusters were calculated. The existence of icosahedral short-range orders is confirmed. The icosahedral cluster has the longest average lifetime, however, it is not the most abundant one. The most abundant cluster is the one with Voronoi cell index of <0,4,8,1>. The average lifetime of all clusters increases with a decrease in temperature.

Structure and dynamics of undercooled FeNi

Molecular dynamics simulation has been performed to explore the structural and dynamical properties of undercooled FeNi melt based on the embedded atom method (EAM), namely due to G. Bonny. The simulated partial pair correlation function (PPCF) and coordination number (CN) of liquid FeNi indicate that no stronger interaction of heterogenic atom pairs than those of the same atom pairs happen in melts. FeNi melt is close to an ideal mixing system without chemical short-range order (CSRO). The undercooled FeNi exhibits icosahedral short-range order (ISRO) that is reflected directly as a large number of 1551, 1541 bonded pairs as well as snapshot of icosahedron clusters. The mean squared displacement (MSD) and self diffusion coefficient of Fe and Ni is quite similar, which is consistent with the strong glass formation liquid of Ni36Zr64. Our work gives the detailed structure and dynamics information of undercooled FeNi melt.

Systematic mapping of icosahedral short-range order in a melt-spun Zr36Cu64 metallic glass.

By analyzing the angular correlations in scanning electron nanodiffraction patterns from a melt-spun Zr(36)Cu(64) glass, the dominant local order was identified as icosahedral clusters. Mapping the extent of this icosahedral short-range order demonstrates that the medium-range order in this material is consistent with a face-sharing or interpenetrating configuration. These conclusions support results from atomistic modeling and a structural basis for the glass formability of this system.

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.

Effects of quasicrystal formation on the crystallization of (Ti36.1Zr33.2Ni5.8Be24.9)100−xCux (x = 5, 7, 9, 11, 13, 15, 17) metallic glasses

The crystallization behaviors of the (Ti36.1Zr33.2Ni5.8Be24.9)100−xCux (x = 5, 7, 9, 11, 13, 15, 17; at. %) metallic glasses are investigated under both the continuous heating and isothermal conditions by the differential scanning calorimetry. Quasicrystals are found in the crystallization products under both conditions by X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. The evolution of the primary crystallization products of these alloys is discussed in detail. The effects of the increasing distortion energy of the icosahedral short-range order with the increasing Cu content in the(Ti36.1Zr33.2Ni5.8Be24.9)100−xCux (x = 5, 7, 9, 11, 13, 15, 17) alloys are found to be responsible for the evolution of the crystallization products. The isothermal crystallization kinetics deviates from the Johnson-Mehl-Avrami-Kolmogoroff (JMAK) model. A modified JMAK model with an impingement parameter is found to be suitable for the isothermal crystallization kinetics of the current alloys. The effects of quasicrystal formation on the crystallization kinetics are disscussed in detail. And it is found that the isothermal crystallization kinetics are influenced by both of the “soft impingement” effect and the formation of the quasicrystals.

Structural evolution in Ni–Nb and Ni–Nb–Ta liquids and glasses — A measure of liquid fragility?

The structures of Ni59.5Nb40.5, Ni62Nb38, and Ni60Nb30Ta10 liquids and glasses were studied using synchrotron high-energy X-ray diffraction. To avoid reactions between the liquids and their containers and to deeply supercool them below their equilibrium liquidus temperatures, the liquids were processed without a container using the beamline electrostatic levitation (BESL) technique. The total static structure factor, S(q), and the total pair-correlation function, g(r), were obtained for all liquid compositions over a temperature range of approximately 250°C; S(q) and g(r) were measured for the corresponding glasses at room temperature. All of the S(q)s have a shoulder on the high-q side of the second peak; this becomes more pronounced as the liquid is supercooled, and is most prominent in the glass. Based on a Honeycutt–Andersen analysis of the atomic configurations obtained from Reverse Monte Carlo fits to the total structure factors obtained from the scattering data, icosahedral short-range order (ISRO) is dominant in all liquids and becomes particularly pronounced in the glasses. No correlation is noted, however, between the amount of ISRO and easy glass formability. Structural features show evidence for an acceleration of ordering in the supercooled liquid above the glass transition temperature, consistent with the behavior expected for fragile liquids. This suggests that scattering data can provide a new method to assess liquid fragility, which is typically obtained from the temperature behavior of the viscosity near the glass transition temperature.

Does an icosahedral short-range order prevail in glass-forming Zr-Cu melts?

We report on investigations of the static structure factors of glass-forming Zr-Cu alloy melts by combination of the containerless processing technique of electrostatic levitation with diffraction of neutron and synchrotron radiation. The partial Bhatia-Thornton structure factors SNN and SNC were determined from the two total structure factors. While it is widely assumed in literature that the good glass-forming ability of Zr-Cu is related to an icosahedral short-range order prevailing in the melt, our partial structure factors demonstrate that the liquid Zr-Cu is not characterized by a dominant icosahedral short-range order.

Local order and dynamic properties of liquid AuxSi1−x alloys by molecular dynamics simulations

Molecular dynamics simulations are performed to examine structural and dynamic properties of liquid Au-Si alloys around the eutectic composition, with interactions described via a modified embedded-atom model suitable for the liquid properties. The local structure as defined by the partial pair-correlation functions, coordination numbers, and partial structure factors is found to display a strong evolution with composition. In addition, a structural study using a three-dimensional pair-analysis technique evidences a strong evolution of the icosahedral short-range order over the range of concentrations, 0 < x(Si) < 0.5. In examining the dynamic properties of these alloys, we show a strong interplay between the structural changes and the evolution of the self-diffusion coefficients as a function of composition.

Local order evolution of liquid Cu during glass transition under different pressures: A molecular dynamics study

Based on the second-moment approximation of tight-binding scheme, constant-pressure molecular dynamics simulations are performed for liquid Cu during the glass transition under different pressures. By means of pair analysis technique and bond orientational order analysis we find that the dominant bond pairs are those related to fcc and hcp crystalline order not those representing icosahedral short-range order (ISRO) when the systems enter into glass transition region. Although these two kinds of bond pairs compete with each other, the system tends towards a mixture of crystalline bond pairs during glass formation. The effect on various bond pairs brought about by higher pressure is much less for liquids than for glasses. The experimental observation of a shoulder on the second peak of the structure factor for supercooled liquids might not merely attribute to ISRO, since supercooled liquid Cu exhibits such a shoulder, but does not display an enhanced icosahedral symmetry.

Local Topology vs. Atomic-Level Stresses as a Measure of Disorder: Correlating Structural Indicators for Metallic Glasses

This brief overview presents a critical assessment of the choice of structural parameters that may be used to describe the degree of local order/disorder in metallic glasses (MGs) and to establish the relationship between the amorphous structure and the properties of an MG. By summarizing our own quantitative analysis of representative MG models, we compare, and relate to one another, several measures of structural disorder in amorphous alloys, including the topology of quasi-equivalent clusters, the concept of free volume, and the atomic-level stresses. The model systems used for demonstrating the correlations are characterized by two different and prototypical types of short-range order (SRO): a Cu64Zr36 MG exhibiting pronounced icosahedral SRO, and a Pd82Si18 MG representative of a metal–metalloid system, with solute-centered tri-capped trigonal prisms as the dominant local motif. Also outlined are some ideas in terms of the relationship between the glass structures and properties, including the challenges in correlating the local structures with the propensity for relaxation and shear transformations.

Nanosize icosahedral quasicrystal in Mg90Ca10 glass: An ab initio molecular dynamics study

Rapid solidification of Mg(90)Ca(10) from its liquid state is studied by means of an ab initio molecular dynamics technique, and its local structure is investigated by various analyzing methods. The liquid and amorphous states are found to have slightly different short range order even though the perfect and defective icosahedral bonding environments are major bonding elements of both liquid and amorphous states. Perfect icosahedrons with a small frequency exist in the liquid state, more develop during the cooling process and they become the leading building units in the glass state, indicating an icosahedral short range order in Mg(90)Ca(10) glass. Also the linked icosahedrons lead to an icosahedral medium range order. Furthermore, an ordered arrangement of some icosahedrons in the hexagonal symmetry is observed in the glass model, representing a nanoscale icosahedral quasicrystalline phase in Mg(90)Ca(10) glass.

Atomic structure of Ca40+XMg25Cu35−X metallic glasses

The atomic structures of four Ca40+XMg25Cu35−X (X = 0, 5, 10, and 20 at. %) ternary metallic glasses have been determined using a synergistic combination of neutron diffraction, ab initio molecular dynamics (MD) simulation, and constrained reverse Monte Carlo modeling. It is described as close-packing of efficiently packed Cu-centered clusters that have Ca, Mg, and Cu atoms in the first coordination shell. The close-packed arrangement of the clusters provides a characteristic medium range order in these alloys. An average coordination number (CN) of 10 (with about 5–7 Ca, 2–3 Mg, and 1–2 Cu atoms) is most common for the Cu-centered clusters. The average coordination numbers around Mg and Ca are 12–13 (∼6–8 Ca, 3 Mg, and 1–4 Cu) and 13–15 (7–9 Ca, 3–4 Mg, and 2–5 Cu), respectively, and they are composition dependent. Strong interaction of Cu with Mg and Ca results in pair bond shortening. Icosahedral short range order does not dominate in these amorphous alloys, although polytetrahedral packing and five-fold bond configurations resulting in pentagonal bi-pyramids have been found to be the most common nearest atom configurations.

Crystallization and compressive behaviors of Ti40Zr25Ni8Cu9Be18 BMG cast from different liquid states

Abstract The initial microstructure of Ti 40 Zr 25 Ni 8 Cu 9 Be 18 metallic glass is sensitive to preparation condition. The Ti 40 Zr 25 Ni 8 Cu 9 Be 18 metallic glasses, prepared at different liquid temperature by copper mold casting, would possess different first crystallization product because of difference in initial microstructure. During first crystallization, Crystallization product is mainly metastable icosahedral phase for metallic glass prepared at relative low liquid temperature. However, the mixture of α-Ti + icosahedral + unknown phase precipitates from metallic glass prepared at relative high liquid temperature. Moreover, the difference in initial microstructure also has a significant influence on compressive properties. Ti 40 Zr 25 Ni 8 Cu 9 Be 18 metallic glasses prepared at relative low temperature exhibit better plasticity. One of important reasons is that dominant icosahedral short range order (SRO) increases shear viscosity of shear band, and the possibility of catastrophic failure caused by fast and continuous slide of shear band is depressed.

Icosahedral short-range order in amorphous Cu80Si20 by ab initio molecular dynamics simulation study

Short-range order in liquid and amorphous structures of Cu80Si20 is studied by ab initio molecular dynamics simulations. We performed the simulations at 1140 and 300K respectively to investigate the local structure change from liquid to amorphous. The result of structure factor in comparison with experimental data indicates that our simulation of amorphous Cu80Si20 is reliable. By using the bond-angle distribution function, Honeycutt–Andersen index, Voronoi tessellation method, and the atomistic cluster alignment method, the icosahedral short-range order in the system is revealed. Strong Cu–Si interaction was also observed.

Dominant shear bands observed in amorphous ZrCuAl nanowires under simulated compression

We observed the formation of dominant shear bands in model ZrCuAl metallic glass (MG) nanowires (18-nm-long) in molecular dynamics simulations, which implies size-independent incipient plasticity in MG materials. The MG nanowires were prepared using the simulated casting technique to ensure proper relaxation of sample surfaces. Under uniaxial compression, shear bands initiate at the surfaces and lead to reduced icosahedral short-range order. The shear band formation is sensitive to sample thermal-history, which calls for careful consideration of sample preparation effects in both experimental and numerical studies of size-effect in MG samples.

Ab initio molecular dynamics simulation of binary Ni62.5Nb37.5 bulk metallic glass: validation of the cluster-plus-glue-atom model

We have performed ab initio molecular dynamics simulation of Ni62.5Nb37.5 alloy at descending temperatures (from 1800 to 300 K) and discussed the evolution of short-range order with temperature. The pair-correlation functions, coordination numbers, and chemical compositions of the most abundant local clusters have been analyzed. We found that icosahedral short-range order exists in the liquid, undercooled, and amorphous states, and it becomes dominant in the amorphous states. Moreover, we demonstrated the existence of Ni-centered Ni7Nb6 icosahedral clusters as the major local structural unit in the Ni62.5Nb37.5 amorphous alloy. This finding agrees well with our previous “cluster-plus-glue-atom” model for the Ni–Nb bulk metallic glasses. The positions of the first peaks of Ni–Nb pair correlation functions are lower than the sum of the metallic radii of Ni and Nb, suggesting enhanced chemical bonding between Ni and Nb atoms in Ni62.5Nb37.5 alloy. Analysis of electronic structures further revealed that the Nb-to-Ni charge transfer is responsible for the enhanced Ni–Nb bonding.

Molecular dynamics simulation of alloying in a Ti-coated Al nanoparticle

Using molecular dynamics simulation in combination with an embedded atom method potential we analyze the alloying reaction of a Ti-coated Al nanoparticle with equi-atomic fractions and a diameter of about 4.8 nm. The alloying reaction in the nanoparticle occurs mostly as a metastable solid-state alloying process. However, it quickly completes after the melting of the Al-based nanoparticle core. The final product of the reaction is the undercooled liquid Ti 50Al 50 alloy. The estimated adiabatic temperature, about 1352 K, of the alloying reaction in the nanoparticle is significantly less than the melting temperature, 1494 K, of the γ-TiAl phase in the model. We demonstrate that the amorphous structure of Ti 50Al 50 alloy is quite stable against crystallization and tends to develop short-range icosahedral order. The possibility of the alloying reaction synthesis of a nanostructured intermetallic TiAl via crystallization of the deeply undercooled liquid Ti 50Al 50 alloy formed during the self-heating of a highly compacted mixture of crystalline Ti and Al nanopowders is discussed in the light of some related experimental findings [H. Kimura, Philos. Mag., 1996, A 73, 723].

Structural and dynamical properties of liquidCu80Si20alloy studied experimentally and byab initiomolecular dynamics simulations

The local structures and dynamical properties of the liquid Cu${}_{80}$Si${}_{20}$ alloy have been studied by x-ray diffraction and ab initio molecular dynamics (MD) simulations. The pair-correlation functions and the structure factors derived from the three-dimensional coordinates of the MD configurations agree well with the experimental results. The local structure of the liquids is analyzed using Honeycutt-Andersen (HA) indices, Voronoi tessellation (VT), and an atomic cluster-alignment method. The HA indices analysis shows that the pentagonal bipyramid, a fragment of an icosahedron (ICO), plays a dominant role in the short-range order (SRO) of the Cu${}_{80}$Si${}_{20}$ liquid. The HA indices corresponding to the pentagonal bipyramid increase dramatically with decreasing temperature. VT analysis indicates that, while the liquid does exhibit a strong icosahedral SRO, fcc-like SRO is also measurable. The results from VT analysis are further confirmed using the recently developed atomic cluster-alignment method. Finally, self-diffusion constants, as a function of temperature for both Cu and Si species, are calculated.

Local order and dynamic properties in liquid Au-Ge eutectic alloys byab initiomolecular dynamics

We report the results of first-principles molecular dynamics simulations for liquid and undercooled eutectic Au${}_{72}$Ge${}_{28}$ alloys at various temperatures. In comparison with the parent Au${}_{81}$Si${}_{19}$ liquid, we find a much less pronounced chemical short-range order in Au${}_{72}$Ge${}_{28}$, mainly due to the increasing influence of Ge-Ge nearest neighbor packing. In addition, a structural analysis using three-dimensional pair-analysis techniques evidences an icosahedral short-range order and its evolution with temperature, closer to that of pure Au than to that of Au${}_{81}$Si${}_{19}$. We use such differences to understand the dynamic properties of both systems and to discuss disparities in other properties of Au${}_{72}$Ge${}_{28}$ and Au${}_{81}$Si${}_{19}$ alloys.