Icosahedral-like Clusters Research Articles

Short and medium range order in the rapidly solidified metallic liquid Ta: Atomic packing, connection modes, and pressure effect

In this study, molecular dynamics (MD) simulations were utilized to explore the Short and Medium-Range Order (MRO) in the rapidly solidified metallic liquid tantalum (Ta). Radial distribution function (RDF) and Voronoi tessellation analysis (VTA) techniques were employed to thoroughly explore the effect of pressure on the connectivity and structural properties at the Short-Range Order (SRO) and MRO levels. Our findings indicate that, at a quenching rate of 1013 K s-1, glassy states are achieved at or below 20 GPa, while crystalline phases emerge at 25 GPa. VTA analysis indicates a significant alteration in the local structure of glassy Ta with increasing pressure. Specifically, the fraction of icosahedral-like clusters decreases while the fraction of crystal-like clusters rises notably.Furthermore, we highlight that icosahedral-like clusters strongly tend to form 3-atom connection mode, while crystal-like clusters prefer 2-atom and 4-atom connection modes. Notably, icosahedral-like clusters are identified as the primary contributors to the emergence of the left sub-peak in the second peak of the RDF. In contrast, all cluster types contribute to the appearance of the right sub-peak.

Abnormal endothermic liquid–liquid phase transition upon cooling Pd40Ni40P20 melts

Liquid–liquid phase transition (LLPT) is the transformation of a liquid from one distinctive structure to another with the same composition. However, the origin of the structural variation at LLPT is still controversial. Here, we used ab initio molecular dynamics simulation to verify and investigate a high-temperature LLPT in a Pd40Ni40P20 melt; this melt showed a reduction in the nuclei interference of the conventional low-temperature LLPT. An abnormal endothermic LLPT was confirmed with flash differential scanning calorimetry, indicating a change in the atomic short-range-order structure around the P–P bond and a decrease in the number of specific icosahedral-like clusters, such as ⟨0 2 8 0⟩ and ⟨0 2 8 1⟩. The structural change of the P-centered clusters changed the solidification path through potential energy adjustments. Our results showed the structural mechanism of the unusual endothermic phenomenon and provided a different insight into regulating the properties of metallic glasses.

Atomic and electronic structures of Ni64Zr36 metallic glass under high pressure

Amorphous metallic alloys, also known as metallic glasses (MGs), are materials with unique physical properties resulting from their disordered yet densely packed atomic structure. The packing density of MGs can be further enhanced by external pressure, forcing the decrease of interatomic distances and modifying both the atomic and electronic structure of an alloy. This work reports on classical molecular dynamics (MD) and density functional theory (DFT) studies of Ni64Zr36 MG in a hydrostatic pressure range of 0–120 GPa. The MD simulations revealed that compression leads to enhanced short-range ordering by increasing the contribution of efficiently packed icosahedral-like clusters. According to the DFT calculations, for pressure above 50 GPa, Zr atoms show a significant change in electronic configuration, with a dominant charge transfer from their s and p to d-states and charge redistribution between Ni and Zr atoms. This variation is correlated with the appearance of pairs with significantly shortened interatomic distances, as detected by the MD. We conclude that the enhanced icosahedral ordering in Ni64Zr36 MG is induced not only by the pressure-driven densification of an alloy but also by a variation of its electronic structure.

Study on the structural origins of glass-forming ability and crystallization behaviors in Ni[sbnd]Nb binary alloys

The limited glass forming ability (GFA) poses a pervasive challenge in different metallic glass (MG) systems. This study systematically investigated the GFA of NiNb binary alloys, particularly those near the eutectic composition, aiming to explore the intrinsic factors contributing to the disparities in GFA among NiNb binary MGs. The relationship between liquid-like atomic structures and resistance to devitrification is thoroughly examined through the application of k-nearest neighbor machine learning methods. The results indicate that the critical diameter of Ni61Nb39, Ni61.5Nb38.5, and Ni62Nb38 MGs reaches upto 2 mm. An amorphous-amorphous transition is discerned within NiNb MGs exhibiting superior GFAs. These alloys exhibit low thermodynamic driving forces for crystallization, resulting in correspondingly elevated fragility coefficients. Furthermore, the presence of icosahedral-like clusters and the probability density of crystal-like clusters are observed to be intricately linked to GFA and the resistance to crystallization in NiNb MGs. The heightened abundance of crystal-like clusters within the liquid-like domain renders the alloy more susceptible to crystallization. Concurrently, icosahedral-like ordered clusters exert a “pinning effect” on the crystal-like regions, effectively inhibiting crystallization. These results collectively shed light on the fundamental nature of glass formation in NiNb binary MGs.

Role of cluster structure on the deformation behavior of Zr58Cu36Al6 metallic glass

Molecular dynamics simulations were used to simulate the process of tensile and compressive deformation of Zr58Cu36Al6 metallic glass (MG), and the variations of various cluster structures during the deformation were analyzed by the Voronoi tessellation method. The prepared Zr58Cu36Al6 alloy was found to be amorphous and the glass transition temperature obtained was about 753 K. In the deformation processes, the icosahedral-like clusters with the biggest quantity percentage decreased significantly and eventually, the damaged icosahedral-like clusters tended to be destroyed into "fragmented" clusters. Icosahedral-like clusters were more stable due to their lower average strain and shear stress compared to other clusters. The higher strain and shear stress of the clusters led to an earlier stress drop of Zr58Cu36Al6 MG during compression. However, the higher connectivity of the icosahedral network remained in Zr58Cu36Al6 MG making the overshoot stress and flow stress larger but lower plasticity in compression than those in tension.

Ductile Ni-based bulk metallic glasses at room temperature

In this work, a series of new Ni76-xCoxNb4P16B4 (x = 5, 10, 15, in at.%) bulk metallic glasses (BMGs) with the maximum diameter in the range of 1.2–1.6 mm were prepared by combining fluxing and J-quenching technology. The effects of Co content on glass forming ability (GFA), thermal stability, mechanical properties, serrated-flow behaviors and corrosion performance of the present Ni-based BMGs were systematically investigated. The ranges of Tg and ΔTx are 664–668 K and 21–31 K, respectively. Uniaxial compressive tests revealed that the present Ni-based BMGs have the compressive plastic strain (εp), yield strength (δy), and fracture strength (δf) that range from 16% to 27%, 2.1–2.6 GPa, and 2.2–3.6 GPa, respectively. The unprecedented mechanical properties originate from more icosahedral-like clusters (ILCs) and crystal-like clusters (CLCs) in the present Ni-based BMGs. It is found that the serrated-flow behaviors of the present Ni-based BMGs transition self-organized critical state (SOCs) into chaotic state (Cs) as plasticity decreases. The stick-slip dynamics were used to calculate the average shear-band velocity (ASBV), which shows an increasing trend as the Co content rises from 5 at.% to 15 at.%. Electrochemical measurements in 3.5 wt.% NaCl solution showed that the Ni71Co5Nb4P16B4 BMG has a self-corrosion current density of 3.30 × 10−8 A/cm2 and significantly stronger corrosion resistance than austenite 316L and 304L stainless steel. The present Ni-based BMGs have excellent comprehensive performance, which has great potential for industrial applications.

The Correlation among the Atomic Structure, Electronic Valence Band and Properties of Zr-Cu-Al-Ag Bulk Metallic Glasses

Investigating the relationship between the glass-forming ability (GFA), mechanical properties, and structure of metallic glasses is crucial to understanding the nature of the metallic glass state. In this study, the correlation among the atomic structure, electronic valence band, and properties have been studied using Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) bulk metallic glasses (BMGs). The results reveal that through the micro-addition of Ag, the GFA of Zr50Cu44.5Al5.5 BMG can be enhanced; meanwhile, the critical diameter of Zr50Cu44.5Al5.5 glass rods increases from approximately 2.5 mm to 5.0 mm with the addition of 3% Ag. Through the addition of Ag, the thermal stability of Zr50Cu44.5Al5.5 BMG is improved, and the proportion of icosahedral-like clusters increases. The plasticity of the Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) BMGs decreased from 4.6% to 0.8% with the addition of Ag. The valence band spectrum of the Zr50Cu44.5−xAl5.5Agx (x = 0, 1.5, 3 at.%) BMGs indicates that with the addition of Ag, the p-d hybridization near the Fermi level is enhanced, and the binding energy will move to a lower value.

Ab initio study of chemical effect on structural properties of Ti–Al melts

We study chemical effect on the structural properties of Ti–Al melts, with the Al concentration systematically changed, via ab initio molecular dynamics simulations. By calculating the partial coordination numbers, we find a preferred connection between the nearest neighbors for Al–Ti pairs. This induces an excess Ti coordination in the cluster characterized by local five-fold symmetry in Voronoi tessellation. Structural entropy measured from the diversity of Voronoi polyhedrons shows an intriguing non-monotonic tendency with concentration: it first decreases to a minimum value at Ti40Al60, and then increases beyond this concentration. This implies a more ordered local structure induced by the chemical interaction at the intermediate compositions. The spatial correlation among the crystalline-like or the icosahedral-like clusters also exhibits the highest intensity for Al–Ti pairs, verifying the important role played by the chemical interaction in the local structure connectivity.

Effects of cooling rate on the glass formation process and the microstructural evolution of Silver mono-component metallic glass

In this study, using molecular dynamics simulations in combination with the embedded-atom approach, we investigate the effect of cooling rate on the microstructural evolution and the glass formation process of Silver monatomic metallic glass. In order to accomplish our investigation, we have utilised a variety of analytical techniques, including the pair distribution function, the bond angle distribution, the Voronoi tessellation analysis, the coordination number, and the five-fold symmetry. The splitting of the second peak in the pair distribution function during the cooling process confirms that glass formation occurs. Via the Went-Abraham parameter, we have found that a faster cooling rate leads to a higher glass transition temperature Tg, and a less relaxed glass has a lower density due to its greater free volume and disordered structure. In the same context, the bond angle distribution revealed that the cooling rate clearly influences the icosahedral short-range order in the quenched system, and the Voronoi tessellation analysis indicated that the percentage of mixed-like and icosahedral-like clusters grows as the cooling rate increases. Furthermore, the coordination number indicated that when the temperature drops throughout the cooling process, the local environment and topological structure of the amorphous Silver change. Lastly, we have also revealed that the five-fold symmetry controls the formation of the amorphous structure and that the highest cooling rate yields the greatest amount of icosahedral-like clusters in the vitreous phase, implying that fast cooling rates subserve the formation of glassy states with icosahedral-like features.

Molecular dynamics simulations of glass formation, structural evolution and diffusivity of the Pd-Si alloys during the rapid solidification process

The atomic structure, glass formation process and diffusion mechanism of Pd100-xSix (x = 10, 20, 30, 40 and 50) alloys during rapid solidification have been investigated by molecular dynamics simulations using embedded atom method potentials. The structure factors, total (or Pd90Si10) and partial pair distribution functions calculated for Pd80Si20 bulk metallic glass are in good agreement with the experimental/other data. Bond angle distribution function, Honeycutt-Andersen index and Voronoi tessellation analysis have revealed that the increasing Si amount caused a decrease in the number of icosahedral-like clusters and an increase in the number of crystal-like clusters. The majority of icosahedral-like clusters in the systems are mostly composed of Pd-centered clusters, suggesting that Pd has an effective role in the glass formation process in Pd-Si systems. It has been observed that the mobility of Si atoms decreases in environments with more Pd atoms. The critical temperature and T0 temperature for Pd-Si liquids have been determined from the self-diffusion coefficients using the mode-coupling theory and the Vogel-Fulcher-Tammann law, respectively. The present findings show us that the Pd70Si30 compound has a critical importance in terms of the glass formation process, which is consistent with experimental observations. We hope that the results will contribute to understanding the Si effect on the atomic local structure of Pd-Si systems and will encourage research on many properties of these systems, such as their mechanical properties.

Liquid-liquid transition and inherited signatures in Zr-Cu-Ni-Al metallic glasses

Up to now, the research of liquid-liquid transition (LLT) has significantly improved the understanding of its liquid structure and evolution, which is crucial to addressing the longstanding problem with glass formation. However, it remains unclear how LLT affects the vitrification and structures of glasses, due to the challenge in experimentally exploring LLT in high-temperature melt well above the liquidus temperature. In the present study, an experiment is conducted to observe LLT in three Zr-Cu-Ni-Al melts at around 1450 K from both dynamic and thermodynamic perspectives. It is discovered that superheated fragility is closely associated with glass-forming ability. By controlling the melt temperature, two types of metallic glasses are obtained, showing difference in crystallization behaviors, which is attributed to the competition between crystal-like clusters and icosahedral-like clusters. Prior to the LLT, the formation of crystal-like clusters is dominant. Afterwards, the LLT promoted formation of icosahedral-like clusters becomes dominant. Due to the inherited signatures in glass, LLT is manifested by the changes in both medium-range order structure and short-range order structure. This study further reveals the pattern of microstructural evolution accompanied by LLT in high-temperature multi-component melts, which is conducive to the control on structure and properties of glasses from liquids.

Atomic structure transition for various solidification paths of Pd40Ni40P20 at different cooling rates

Demonstrating the cooling rate effect on the solidification path is a massive challenge. In this paper, the atomic structure of the Pd40Ni40P20 melt was statistically “snapped” at different temperatures with two different cooling rates. The structure evolution was analyzed with in situ high-energy X-ray diffraction method and Reverse Monte Carlo simulation. It indicates that the presence of Liquid-Liquid Phase Transition (LLPT) makes the melt more inclined to form crystal but the higher cooling rate leads to an insufficient transition retaining more icosahedral-like clusters. This work provides dynamic guidance for understanding the competition between crystallization and vitrification.

Composition dependence in glass-forming ability of Cu–Ag binary alloys

Alloys around eutectic points are thought to be good glass formers. However, the glass-forming ability (GFA) as a function of composition from pure metals to the eutectic alloy has not been quantified and the underlying microstructural and dynamical origins are unclear. In this work, the GFA of Cu–Ag alloys at various compositions was thoroughly investigated by using molecular dynamics simulations. According to the results, the critical cooling rate for glass formation Rc (K/s) varies with solute content x′ (at%), following Rc ≈ Aexp(kx′) where A = 4.61 × 1012 and k = -0.25 on the Cu side and A = 4.43 × 1012 and k = -0.19 on the Ag side. The Rc of the eutectic alloy is about 5 orders of magnitude lower than that of the pure metals. Structurally, the number of densely packed icosahedral-like clusters in Cu–Ag metallic glasses increases with increasing solute content, resulting in the most densely packed structure at the eutectic composition. The dynamical properties of Cu–Ag melts are basically independent on composition at high temperatures, but obviously change with composition at low temperature. Besides the largest reduced glass transition temperature, the highest five-fold symmetry (FFS) local structure and more importantly the retarded dynamics of liquid atoms below the liquidus temperature around the eutectic composition impart the eutectic alloy with the best glass-forming ability.

Atomic-scale Nb heterogeneity induced icosahedral short-range ordering in metallic glasses

• Atomic-scale structural evolution has been depicted via thermal agitation during heating. • The Nb heterogeneity plays an important role in the formation of short-range icosahedral-like clusters. • The coupling development of atomic-scale structure and composition is simultaneously responsible for promoting the formation of perfect orders or even long-range quasicrystalline. The intrinsic origins and formation of atomic-scale structure in multicomponent alloys remain largely unknown owing to limited simulations and inaccessible experiments. Herein, we report the formation of three-dimensional periodicity from a disordered atomic-scale structure to an imperfect/perfect ordered cluster and finally to long-range translational and rotational symmetry coupled with Nb heterogeneity. Significant atomic-scale structural clustering and atomic arrangements involving solvent or solute atoms simultaneously occurred during isothermal annealing. A close relationship between atomic-scale structural evolution and composition variation has important implications in depicting the chemical and topological packing during the early crystallization stage in metallic glasses. This work can provide a comprehensive understanding of how short-range orders evolve into long-range periodicity and will further shed light on the origins and nature of metallic glasses.

Atomic structure of Cu60Ti20Zr20 metallic glass under high pressures

During the rapid cooling process, the atomic structure of the Cu60Ti20Zr20 ternary alloy under external pressure was investigated by molecular dynamics simulations using the embedded atom method potential. The effect of the pressure was discussed in detail by using a variety of analysis methods. Glass transition temperature determined by the modified Wendt-Abraham parameter was found to increase with increasing pressure. It was observed that the calculated total (or partial) pair distribution function and structure factor are in good agreement with the experimental x-ray data (with the ab-initio molecular dynamics results). The bond angle distributions of Cu-centered triplets were more consistent with ideal icosahedral clusters than that of Ti- and Zr-centered triplets. The dominant clusters for all pressures were icosahedral-like clusters that were not much affected by pressure. In addition to the bonds between Cu–Zr pairs shortening more easily under high pressure than other pairs, it is found that the Cu60Ti20Zr20 glassy alloy remained stable under pressure and its topological and atomic structures did not change much with pressure.

Dynamical and structural properties of metallic liquid and glass Zr48Cu36Ag8Al8 alloy studied by molecular dynamics simulation

The structure and dynamics of the Zr48Cu36Ag8Al8 alloy were investigated by molecular dynamics simulation method using the embedded atom method (EAM) and tight-binding (TB) potentials. Total structure factors and pair distribution functions calculated from both potentials are in good agreement with the reported experimental data. The glass transition temperature was determined more easily by using the Wendt-Abraham parameter, which we modified with small changes. The fraction of 1551-bonded pairs and Zr-, Cu-, Ag- and Al-centered icosahedral-like clusters increased with decreasing temperature. High fractions of Cu-, Ag- and Al-centered perfect icosahedra and Zr-centered icosahedra-like clusters were detected in both supercooled liquids and glasses. The critical temperature was determined to be ~760.65 K from mode coupling theory. Although the results for the EAM and TB potentials were largely consistent with the experiment, aggregations were detected in the Ag and Al atoms at low temperatures for the EAM potential.

Effect on microstructure and plastic deformation behavior of a Zr-based amorphous alloy by cooling rate control

In this study, Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloys samples with the same diameter (8 mm) were prepared by using self-designed molds (viz. refractory steel, pure graphite, and copper molds) with different cooling capacities. Moreover, by eliminating the size effect, the effect of the cooling rate on the microstructure and compression deformation behavior of Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloys was investigated. Differentiation of the cooling curves revealed that the instantaneous cooling rates of the alloy melt at the glass transition temperature (Tg) are 45, 52, and 64 K·s―1 for refractory steel, pure graphite, and copper molds, respectively. X-ray diffraction, differential scanning calorimetry, and high-resolution transmission electron microscopy analysis revealed that with the decrease in the cooling rate, trace icosahedral-like atomic clusters and nanocrystals appear in local areas of the amorphous alloy and that the amount of free volume decreases with the increase in the amount of icosahedra-like atomic clusters and nanocrystals. Compression test results revealed that the elastic strain, yield strength, and compressive strength of the amorphous alloy marginally change with the decrease in the cooling rate, while the plastic strain gradually increases. By fitting, the effective size of the vein-like pattern was linearly related to the enthalpy released during structural relaxation and plastic strain, indicating that at a low cooling rate, the trace nanocrystals in the amorphous alloy could not effectively improve its plasticity and that the amount of free volume mainly affects its plasticity.

On the structural changes and glass transition temperature relationship during the formation of Re-W metallic glasses

The search for advanced materials has been the main preoccupation of materials scientists for the past years. Binary Re-W metallic glasses (MGs) are very promising candidates for many applications requiring good mechanical proprieties. Molecular Dynamics (MD) simulations with the embedded atom method (EAM) potential have been devoted to investigate the behavior of these materials by exploring their local atomic structure by means of various techniques such as the radial distribution function (RDF), the Voronoi Tessellation Analysis (VTA) and the coordination number (CN). The results showed a splitting in the second peak of RDFs indicating the formation of Re-W metallic glasses. The VTA method showed that the <0,1,10,2> icosahedral Voronoi cluster is the major short-range unit in Re-W MGs. The icosahedral-like cluster abundance is proposed as a measure of vitrification which has been shown to be promoted by the increase of W content. The coordination number analysis showed the existence of transition from low- to high-coordinated atoms occurring at the glass transition temperature. These simulated structural properties were used mainly to study the glass transition temperature Tg using different methods, including the Wendt-Abraham method which is considered as a reference method. Essentially, we suggest the intersection between low- and high-coordinated atom populations to be a measure of Tg.

A plastic FeNi-based bulk metallic glass and its deformation behavior

The strength and plasticity of Fe39Ni39B12.82Si2.75Nb2.3P4.13 bulk metallic glass (BMG) are improved simultaneously by modulating atomic-scale structure through fluxing treatment. The compression strength increases from 3074 to 4220 MPa, and the plastic strain is enlarged from 10.7 % to more than 50 %. The increased mechanical properties of the fluxed FeNiBSiNbP BMG originate from the optimization of atomic-scale structure. More icosahedral-like clusters (ILCs) and crystal-like clusters (CLCs) are found in this FeNi-based BMG with fluxing treatment, and the ILCs are usually surrounded by CLCs. Furthermore, phase separation and a sandwich-like heterogeneous structure of SB are also observed during deformation, indicating the multiscale deformation mechanism and a stable shear-band evolution. The unique “ILC surrounded by CLCs” structure and phase separation lead to a stable plastic deformation process with strong interactions of multiple shear bands, thereby the improved plasticity and strength. This work provides useful guidelines to develop strong and plastic Fe-based BMGs from a structural aspect.

A molecular dynamic study of the effects of high pressure on the structure formation of liquid metallic Ti62Cu38 alloy during rapid solidification

In this study, the effects of high pressure ranging from 0 to 100 GPa on the structural evolution of liquid metallic Ti62Cu38 alloy during rapid cooling have been extensively investigated by using classical molecular dynamics simulation with embedded atom method at a cooling rate of 5 × 1010 K s−1. To investigate the first order phase transition during the solidification of the system and to determine the crystallization and glass transition temperatures, the temperature-dependent change in the curves of thermodynamic properties such as average volume per atom, specific heat and enthalpy are examined. Structural properties are expressed by using pair distribution functions, structure factors and atomic configuration. The microstructural atomic order in the system are characterized by using Honeycutt-Andersen pairs and Voronoi tessellation analysis methods. The results provide convincing evidence that the applied pressure during rapid cooling has a strong effect on determining whether the metallic liquid Ti62Cu38 will transform into a crystal-like structure or a glassy structure. The critical pressure for the glass formation are predicted to be approximately 10 GPa. While the simulated crystallization and glass transition temperatures increase linear with a slope of 11.11 K GPa−1 within the range of 0–9 GPa and with a slope of 12.10 K GPa−1 within the range of 10–100 GPa, respectively. While crystal-like clusters are dominant in the system up to 10 GPa, icosahedral-like clusters representing a short range order at 10 GPa become dominant in the system. The amount of dominant icosahedral-like clusters remains basically stable with pressure increase from 10 to 100 GPa. Also, as the pressure is applied, the calculated bond lengths for all bond pairs decrease.