Efficient Atomic Packing Research Articles

Microstructural stability and mechanical property of AlCrFeMoTi high-entropy amorphous alloy thin films under He+ ions irradiation

High-entropy amorphous materials have recently proved exceptional radiation resistance, showing potential as a candidate materials for nuclear industry applications. This study synthesized nearly equi-atomic AlCrFeMoTi high-entropy amorphous thin films with a thickness of about 499 nm using magnetron sputtering. The prepared films were subjected to low dose, 0.5 MeV He+ ion irradiation, with varying irradiation damage ranging from 0.01 to 0.1 displacement per atom (dpa). Increasing the irradiation dose led to a decrease in the radius of the inner diffraction halo (r) and an increase in the crystal-like order, indicating an irradiation-induced structural relaxation process within the amorphous matrix. This relaxation results in more efficient atomic packing, yielding a denser amorphous structure and enhanced hardness of AlCrFeMoTi high-entropy amorphous thin films. This study demonstrates that AlCrFeMoTi high-entropy amorphous thin films exhibit remarkable radiation resistance, with stable microstructure and mechanical properties under He+ irradiation, making them promising candidates for applications in the nuclear industry.

Quantitative analysis of structure evolution of Zr-Cu amorphous alloys caused by cooling rates based on atomic bond proportion

The changes in the atomic structure of Zr–Cu amorphous alloys caused by different cooling rates were studied by X-ray diffraction synchrotron radiation and molecular dynamic calculation. Results show that the content of high-strength Zr–Cu bonds increases, whereas those of Zr–Zr and Cu–Cu bonds decrease as the cooling rate decreases. The increase of high-strength atomic bonds leads to the increase of Young's modulus of Zr-Cu amorphous alloys. The lower cooling rate facilitates the formation of a more efficient atomic packing model, suggesting that more atomic bonds form in the system without increasing the number of atoms. The effect of cooling rate on the atomic structure and Young's modulus of Zr–Cu amorphous alloys with different components is proportional to the content of Zr–Cu bonds in the system as a whole.

Ab initio study of ABiO3 ( A=Ba , Sr, Ca) under high pressure

Using $\textit{ab-initio}$ crystal structure prediction we study the high-pressure phase diagram of $\textit{A}BiO_3$ bismuthates ($A$=Ba, Sr, Ca) in a pressure range up to 100$~$GPa. All compounds show a transition from the low-pressure perovskite structure to highly distorted, low-symmetry phases at high pressures (PD transition), and remain charge disproportionated and insulating up to the highest pressure studied. The PD transition at high pressures in bismuthates can be understood as a combined effect of steric arguments and of the strong tendency of bismuth to charge-disproportionation. In fact, distorted structures permit to achieve a very efficient atomic packing, and at the same time, to have Bi-O bonds of different lengths. The shift of the PD transition to higher pressures with increasing cation size within the $\textit{A}BiO_3$ series can be explained in terms of chemical pressure.

ReaxFF Molecular Dynamics Simulation for the Graphitization of Amorphous Carbon: A Parametric Study.

A parametric study of ReaxFF for molecular dynamics simulation of graphitization of amorphous carbon was conducted. The responses to different initial amorphous carbon configurations, simulation time steps, simulated temperatures, and ReaxFF parameter sets were investigated. The results showed that a time step shorter than 0.2 fs is sufficient for the ReaxFF simulation of carbon using both Chenoweth 2008 and Srinivasan 2015 parameter sets. The amorphous carbon networks produced using both parameter sets at 300 K are similar to each other, with the first peak positions of pair distribution function curves located between the graphite sp2 bond peak position and the diamond sp3 bond peak position. In the graphitization process, the graphene fragment size increases and the orientation of graphene layers transforms to be parallel with each other with the increase of temperature and annealing time. This parallel graphene structure is close to the crystalline graphite. Associated with this graphitization is the presence of small voids and pores which arise because of the more efficient atomic packing relative to a disordered structure. For all initial densities, both potential parameter sets exhibit the expected behavior in which the sp2 fraction increases significantly over time. The sp2 fraction increases with increasing temperature. The differences of sp2 fraction at different temperatures are more obvious in lower density at 1.4 g/cm3. When density is increased, the gap caused by different temperatures becomes small. This study indicates that both Chenoweth 2008 and Srinivasan 2015 potential sets are appropriate for molecular dynamics simulations in which the growth of graphitic structures is investigated.

Experimental investigation and thermodynamic analysis of glass forming ability of the Al–Co–Sm ternary system

Based on an effective atomic radius instead of a nominal atomic radius in the efficient atomic packing cluster method, the best glass forming composition in the new Al–Co–Sm ternary system was forecasted. Thermodynamic description of the Al-rich corner of Al–Co–Sm system was performed using the CALPHAD approach. Results show that the effective atomic radii of Sm and Al are approximately equal to the nominal radii, but the effective atomic radius of Co atom is obviously smaller than its nominal radius. From thermodynamic calculation, the composition dependencies of glass forming ability (GFA) in the Al-rich corner of the system were forecasted by finding the relatively low driving forces for crystalline phases, and the forecast of GFA agrees well with the experimental data.

Electron-band theory inspired design of magnesium\u2013precious metal bulk metallic glasses with high thermal stability and extended ductility

Magnesium-based bulk metallic glasses (BMGs) exhibit high specific strengths and excellent glass-forming ability compared to other metallic systems, making them suitable candidates for next-generation materials. However, current Mg-based BMGs tend to exhibit low thermal stability and are prone to structural relaxation and brittle failure. This study presents a range of new magnesium–precious metal-based BMGs from the ternary Mg–Ag–Ca, Mg–Ag–Yb, Mg–Pd–Ca and Mg–Pd–Yb alloy systems with Mg content greater than 67 at.%. These alloys were designed for high ductility by utilising atomic bond-band theory and a topological efficient atomic packing model. BMGs from the Mg–Pd–Ca alloy system exhibit high glass-forming ability with critical casting sizes of up to 3 mm in diameter, the highest glass transition temperatures (>200 °C) of any reported Mg-based BMG to date, and sustained compressive ductility. Alloys from the Mg–Pd–Yb family exhibit critical casting sizes of up to 4 mm in diameter, and the highest compressive plastic (1.59%) and total (3.78%) strain to failure of any so far reported Mg-based glass. The methods and theoretical approaches presented here demonstrate a significant step forward in the ongoing development of this extraordinary class of materials.

Amorphous phase stability and the interplay between electronic structure and topology

It is well-known that the stability of Pd-Si amorphous alloys can be improved substantially by minor additions of alloying elements such as Cu and Ag. Such improvement in stability is explained herein, whereby microscopic models based on efficient atomic packing and electronic structure were applied to the results of first principles simulations of Pd82Si18, Pd77.5Si16.5Cu8 and Pd75Si15Cu7Ag3 metallic glasses. It was revealed that while the atomic packing model fails to unequivocally explain the stabilizing effect of the binary Pd-Si alloy due to minor additions of Cu and/or Ag, the contribution of electronic states with lower energies to the stability of the amorphous structure is increased markedly. Further, the observed enhancement in the compressive ductility as a result of the addition of Ag to the Pd77.5Si16.5Cu8 alloy can be correlated to the combined effects of an increased heterogeneity in the local topology, weakened covalency and, hence, reduced directionality of Pd-Pd bonds as well as enhanced metallicity in the Pd75Si15Cu7Ag3 amorphous alloy. The analysis was further expanded to amorphous alloys where their characteristic binary prototypes are synthesised from late transition metals and non-metals as well as those comprising early transition metals and late transition metals. The findings shed light on the effects of minor alloying on the cooperative and competitive relationship between the topological and electronic structure of amorphous alloys.

Efficient atomic packing-chemistry coupled model and glass formation in ternary Al-based metallic glasses

An efficient atomic packing-chemistry coupled model has been described for ternary Al-based metallic glasses based upon efficient atomic packing and electrochemical potential equalization principle. For Al-TM (transition metal)-RE (rare earth) system, RE-centered clusters are arranged in a mode of spherical periodic order, and TM atoms locate at the interstitial sites between RE-centered clusters to form an efficient atomic packing. The equalization of electrochemical potential between the RE-centered clusters and the TM-centered clusters accounts for the stability of amorphous structure. Further, complementary inverse structure is utilized to select efficient clusters and solute elements for the best glass-forming ability. The validity of this model was testified in the Al–Ni-RE (Y, Ce, La, Gd) systems. A new Al–Ni–Ho glass with a wedge thickness of about 718 μm has been discovered at the predicted composition.

An assessment of binary metallic glasses: correlations between structure, glass forming ability and stability

<title/>This manuscript explores the influence of atomic structure on glass forming ability and thermal stability in binary metallic glasses. A critical assessment gives literature data for 628 alloys from 175 binary glass systems. The atomic structure is quantified for each alloy using the efficient cluster packing model. Comparison of atomic structure with amorphous thickness and thermal stability gives the following major results. Binary glasses show a strong preference for discrete solute to solvent atomic radius ratios R*, which give efficient local atomic packing. Of 15 possible R* values, only five are common and only four represent the most stable glasses. The most stable binary glasses are also typically solute rich, with enough solute atoms to fill all the solute sites and roughly one-third of the solvent sites. This suggests that antisite defects, where solutes occupy solvent atom sites, are important in the glass forming ability of the most stable glasses. This stabilising effect results from an increase in the number of more stable solute-solvent bonds in solute rich glasses. Solute rich glasses also enable efficient global atomic packing. Together, these structural constraints represent only a narrow range of topologies and thus give a useful predictive tool for the exploration and discovery of new binary bulk metallic glasses (BMGs).

Efficient local atomic packing in metallic glasses and its correlation with glass-forming ability

We have probed local atomic structure of Zr-Cu metallic glasses using time-of-flight neutron and synchrotron x-ray diffraction techniques with high resolution. Our results provide evidence for a scheme of efficient local atomic packing where atomic clusters encompass multiple types of atoms in the first coordination shell. We also demonstrate experimental evidence of a strong correlation between the number of unlike atom bonds and the glass-forming ability. Our findings may provide insights into a broad range of scientific problems where efficient space filling by packing spheres is essential.

Efficient atomic packing clusters and glass formation in ternary Al-based metallic glasses

In this article, we propose an efficient atomic packing cluster-based composition protocol to help design Al-based metallic glasses. Its validity is verified by some typical experimental data from the literatures. Furthermore, with this understanding, the Al–Ni–Y alloy system is re-evaluated. As a result, the best glass former Al86Ni9Y5 in this system, with the critical thickness of about 500 µm, is successfully fabricated by wedge casting.

Structural characterization of Mg 65Cu 25Y 10 metallic glass from ab initio molecular dynamics

Ab initio molecular dynamics simulation (AIMD) was performed on the structural evolution of Mg 65Cu 25Y 10 alloy from 2000 K to 300 K. Pair correlation functions (PCFs), coordination numbers (CNs) and structure factors (SFs) of this glassy alloy were characterized. With the temperature decreased, the first peaks of both PCF and SF become narrower and higher, and shift to higher displacement or higher scattering vector. At the room temperature, the second peaks of both PCF and SF curves occur splitting, and the height of first peak of partial PCF for heterogenic atomic pairs is larger than that for homogenic atomic pairs. The generalized coordination numbers in this alloy are in good agreement with the results calculated by using efficient atomic packing model, which approves the atomic configuration of this alloy is in terms of dense packing. It was also found that the shape of both PCF and SF, and the CNs occur distinct change around 750 K.

Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview

Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses.

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.

The efficient cluster packing model – An atomic structural model for metallic glasses

A structural model is described for metallic glasses based on a new sphere packing scheme – the efficient filling of space by solute-centered clusters. This model combines random positioning of solvent atoms with atomic order of solutes. It shows that metallic glasses contain ⩽4 topologically distinct species and that solutes possess specific sizes relative to solvent atoms to produce efficient atomic packing. Validation is achieved by quantitative predictions of nearest-neighbor partial coordination numbers, medium-range solute ordering, density and metallic glass topologies. Good agreement is achieved in each of these areas. This model is able to reproduce compositions for a broad range of metallic glasses, provides specific guidance for the exploration of new bulk metallic glasses and may give new insights into other metallic glass studies. The new scheme introduced here for the efficient filling of space in extended systems of unequal spheres may have relevance to other fields.

Efficient local packing in metallic glasses

A simple topological model in an earlier manuscript has provided additional support for the concept that efficient atomic packing is a fundamental principle in the formation of metallic glasses. In that work, an approach for defining and quantifying the local packing efficiency, P, was developed for solute-centered clusters that contained only solvent atoms in the first coordination shell. In the present work, this methodology is extended to allow quantification of P when more than one atomic species is present in the first coordination shell. This analysis is applied to several metallic glasses using published experimental data of partial coordination numbers. It is shown that packing in the first coordination shell is generally very efficient, even though the systems studied have significant differences in atomic species, compositions and relative atomic sizes. It is shown that packing is generally efficient around both solute and solvent atom species. Local packing efficiencies much less than unity are expected to be uncommon, since the global average packing efficiency is near unity and local packing efficiencies greater than unity are physically improbable. Deviations from efficiently packed configurations are discussed with respect to the local packing efficiencies in competing crystalline structures and with poorer glass forming ability. The values of P obtained for metallic glasses are essentially identical to the values obtained from a similar analysis of the competing crystalline structures. These results are consistent with frequent earlier reports of topological short range ordering in metallic glasses and with developments that have established the relationship between dense atomic packing and glass formation.

Structure-forming principles for amorphous metals

The purpose of this research is to establish structure-forming principles that govern the atomic structures of metallic glasses. A structural model based on efficient atomic packing will be discussed and applied to the topological systems that represent most metallic glass alloys. The concept of efficient atomic packing has direct and specific implications regarding the local structure and composition of metallic glasses. Specific solute-to-solvent atomic radius ratios and specific solute concentrations related to these ratios are shown to be preferred in this model, and analysis of a wide range of metallic glass systems shows a very strong correlation with these predicted values. Relationships between atomic size and concentration are discussed, and new insights are proposed based on the current structural model. Possible local atomic configurations (i.e., atomic clusters) are defined based on topological constraints that are derived from the requirement of efficient atomic packing. Experimental observations drawn from the literature that provide support for this model are presented.

The influence of efficient atomic packing on the constitution of metallic glasses

Efficient atomic packing is shown to be a fundamental consideration in the formation of metallic glasses. A simple concept of packing efficiency, based on atom packing in the first coordination shell of solute-centred clusters, is proposed and developed. This model leads to the prediction that specific radius ratios, defined as the radius of the solute atom divided by the radius of the solvent atom, are preferred in the constitution of metallic glasses. Analysis of a large number of binary and complex metallic glasses shows that these specific critical radius ratios R* are indeed preferred in known metallic glasses. The predictions of this model extend previous proposals to describe the influence of topology on the formation of metallic glasses. Although this model represents a simple idealization, the strong agreement with published metallic glasses suggests that efficient atomic packing, enabled by solute-centred clusters, forms a fundamental consideration in the constitution of metallic glasses.

A geometric model for atomic configurations in amorphous Al alloys

A representative model for the atomic structure of amorphous Al alloys is proposed based on the fundamental structure-forming principle of high packing efficiency, topological concepts, and available partial and total radial distribution functions from diffraction studies. Selection of rare earth (RE)-centered atomic clusters as representative structural elements in this model is supported by the large coordination number (∼17±2), efficient atomic packing (100±5%), and small mean intersolute spacing (∼2 atom diameters center-to-center) associated with RE solutes in amorphous Al alloys. Using Al–Y and Al–Y–Ni alloys as a base, five idealized Y-centered clusters and two Ni-centered clusters are described with specific atomic configurations that are consistent with the observed coordination numbers and high density relative to crystalline alloys of the same composition. Significant configurational complexity, required for an amorphous structure, is offered by this structural model. A distribution in Y–Y intersolute spacing is provided by the model that is consistent with the expectation of a random distribution of Y atoms. Topological similarities with other amorphous metal alloy systems suggest that the structure described here for amorphous Al may also be relevant for many other amorphous metals with marginal glass-forming ability (critical cooling rate⩾1000 K/s), including alloys based on Mg, Fe, Ni and Co.