Structure Of Metallic Glasses Research Articles

Shear susceptibility – A universal integral parameter relating the shear softening, heat effects, anharmonicity of interatomic interaction and “defect” structure of metallic glasses

We studied shear modulus relaxation and heat effects occurring upon structural relaxation of La-, Zr- and Pd-based bulk metallic glasses. On this basis, we suggest a new method for the determination of the shear susceptibility, which appears to constitute a universal integral parameter relating different physical phenomena – shear softening, heat effects and anharmonicity of interatomic interaction – with the “defect” structure of metallic glasses.

A microscopic continuum model for defect dynamics in metallic glasses

Motivated by results of the topological theory of glasses accounting for geometric frustration, we develop the simplest possible continuum mechanical model of defect dynamics in metallic glasses that accounts for topological, energetic, and kinetic ideas. A geometrical description of ingredients of the structure of metallic glasses using the concept of local order based on Frank–Kasper phases and the notion of disclinations as topological defects in these structures is proposed. This novel kinematics is incorporated in a continuum mechanical framework capable of describing the interactions of disclinations and also of dislocations (interpreted as pairs of opposite disclinations). The model is aimed towards the development of a microscopic understanding of the plasticity of such materials. We discuss the expected predictive capabilities of the model vis-a-vis some observed physical behaviors of metallic glasses.

Effects of pressure on structure and mechanical property in monatomic metallic glass

In this work we studied the effects of pressure on the local structures and mechanical properties of pure iron metallic glasses based on molecular dynamics simulations. It is found that, at a quenching rate of 1×1013K/s, glassy states are obtained for pressure not >20GPa and crystalline phases are obtained at 25GPa. Voronoi analysis indicates that the local structure of glassy iron changes with pressure, and the degree of local fivefold symmetry in the quenched systems decreases as the pressure increases. Compression test results indicate a negative correlation between the pressure and the yield strength, with the strength of the glass obtained at 20GPa being 0.56GPa (or about 10%) lower than that at 0GPa. This study sheds light on the understanding of pressure-induced changes of local structure and mechanical property of monatomic metallic glasses.

Computational modeling sheds light on structural evolution in metallic glasses and supercooled liquids

This article presents an overview of three challenging issues that are currently being debated in the community researching on the evolution of amorphous structures in metallic glasses and their parent supercooled liquids. Our emphasis is on the valuable insights acquired in recent computational analyses that have supplemented experimental investigations. The first idea is to use the local structural order developed, and in particular its evolution during undercooling, as a signature indicator to rationalize the experimentally observed temperature-dependence of viscosity, hence suggesting a possible structural origin of liquid fragility. The second issue concerns with the claim that the average nearest-neighbor distance in metallic melts contracts rather than expands upon heating, concurrent with a reduced coordination number. This postulate is, however, based on the shift of the first peak maximum in the pair distribution function and an average bond length determined from nearest neighbors designated using a distance cutoff. These can instead be a result of increasing skewness of the broad first peak, upon thermally exacerbated asymmetric distribution of neighboring atoms activated to shorter and longer distances under the anharmonic interatomic interaction potential. The third topic deals with crystal-like peak positions in the pair distribution function of metallic glasses. These peak locations can be explained using various connection schemes of coordination polyhedra, and found to be present already in high-temperature liquids without hidden crystal order. We also present an outlook to invite more in-depth computational research to fully settle these issues in future, and to establish more robust structure-property relations in amorphous alloys.

Micropatterning of Ni-based metallic glass by pulsed wire electrochemical micro machining

The technique of wire electrochemical micro machining (WECMM) is proposed firstly for the micropatterning of Ni-based metallic glass in this paper. Metallic glass (MG) exhibits many outstanding properties such as high hardness and strength, which enable it to be used as functional and structural materials in micro electromechanical systems (MEMS). A significant limitation to the application of MGs is the challenge of shaping them on micro scale. WECMM is a non-traditional machining technique to fabricate microstructures that has some unique advantages over other methods, which will be a promising technique for micro shaping of metallic glass structures. Taking the example of a Ni-based glassy alloy, Ni72Cr19Si7B2, the polarization and fabrication characteristic in dilute hydrochloric acid electrolyte were investigated. Changes in the machined slit width in terms of several experimental parameters were investigated to find the optimal ones. Finally, the optimal machining parameters: HCl electrolyte concentration of 0.1 M, applied voltage of 4.5 V, pulse duration of 80 ns, pulse period of 3 μs and feed rate of 0.3 μm s−1 were employed for the fabrication of microstructures. Such as a micro square helix with a slit width of 14.0 μm, standard deviation of 0.2 μm and total length up to 2000 μm, along with a micro pentagram structure with side length of 90 μm and sharp corner of 36°, were machined with a high level of stability and accuracy.

Atomic structure of Mg-based metallic glasses from molecular dynamics and neutron diffraction.

We use a combination of classical molecular dynamics simulation and neutron diffraction to identify the atomic structure of five different Mg-Zn-Ca bulk metallic glasses, covering a range of compositions with substantially different behaviour when implanted in vitro. There is very good agreement between the structures obtained from computer simulation and those found experimentally. Bond lengths and the total correlation function do not change significantly with composition. The zinc and calcium bonding shows differences between composition: the distribution of Zn-Ca bond lengths becomes narrower with increasing Zn content, and the preference for Zn and Ca to avoid bonding to themselves or each other becomes less strong, and, for Zn-Ca, transforms into a positive preference to bond to each other. This transition occurs at about the same Zn content at which the behaviour on implantation changes, hinting at a possible structural connection. A very broad distribution of Voronoi polyhedra are also found, and this distribution broadens with increasing Zn content. The efficient cluster packing model, which is often used to describe the structure of bulk metallic glasses, was found not to describe these systems well.

Five-fold local symmetries in metallic liquids and glasses

In this article, we review the experimental, theoretical and simulation studies on five-fold local symmetries in metallic liquids and glasses. In the early study on simple liquid structure, it has been realized that five-fold local symmetry plays a key role in irregular structures, supercooling and crystallization of simple liquids. In particular, icosahedral short-range order, representative of five-fold local symmetry, has attracted much attention. In addition, researches proposed a dense random packing model for simple liquid structure in 1959, and found a wide variety of polyhedra and absolute predominance of pentagonal faces in simple liquids, and also pointed out that pentagonal arrangements can only occur in very complex structures such as some of the alloy structures. Based on the Frank's hypothesis of icosahedral short-range order as blocking unit in a simple liquid, a lot of theoretical and experimental efforts have been made to confirm its existence in simple liquids, metallic liquids and glasses. So far, several theoretical methods have been developed for characterizing local atomic structures in simple liquids, such as bond-orientational order parameter, Honeycutt-Andersen index, and Voronoi tessellation. Although the local atomic symmetries in atomic structures in metallic liquids and glasses can be characterized by these methods and the geometries of the atomic structures in liquids and glasses have received much more attention, an atomic cluster model has been developed for establishing the structure-property relationship in metallic liquid and glass. Due to the diversity of the atomic clusters in both type and population of different metallic liquids and glasses, the atomic cluster model could not present a simple description of structure-property relationship. Based on the fundamental characteristics of metallic liquids and glasses, five-fold local symmetry, the structure-property relationship in metallic liquids and glasses, such as dynamic crossover, glass transition, liquid-liquid phase transition, and deformation can be well described in simple, quantitative and unified ways, and therefore a clear physical picture can be provided. All these studies indicate that five-fold local symmetry as a structural parameter is simple, general and effective.

Comparatively studying the local atomic structures of metallic glasses upon cyclic-loading by computer simulations

Through a variety of local structural analysis methods, it is revealed that the NixZr100–xMGs exhibit a combination of the icosahedral-, fcc- and hcp-like configurations, while the icosahedra or distorted icosahedra cover a dominant fraction.

High-throughput drawing and testing of metallic glass nanostructures.

Thermoplastic embossing of metallic glasses promises direct imprinting of metal nanostructures using templates. However, embossing high-aspect-ratio nanostructures faces unworkable flow resistance due to friction and non-wetting conditions at the template interface. Herein, we show that these inherent challenges of embossing can be reversed by thermoplastic drawing using templates. The flow resistance not only remains independent of wetting but also decreases with increasing feature aspect-ratio. Arrays of assembled nanotips, nanowires, and nanotubes with aspect-ratios exceeding 1000 can be produced through controlled elongation and fracture of metallic glass structures. In contrast to embossing, the drawing approach generates two sets of nanostructures upon final fracture; one set remains anchored to the metallic glass substrate while the second set is assembled on the template. This method can be readily adapted for high-throughput fabrication and testing of nanoscale tensile specimens, enabling rapid screening of size-effects in mechanical behavior.

Multiscale structures and phase transitions in metallic glasses: A scattering perspective**Project supported by Shenzhen Science and Technology Innovation Commission of China (Grant No. R-IND8701) and the Croucher Foundation (Project No. CityU 9500020). Si Lan is partially supported by the Fundamental Research Funds for the Central Universities of China (Grant No. 30915015103) and the National Natural Science Foundation of China (Grant

Amorphous materials are ubiquitous and widely used in human society, yet their structures are far from being fully understood. Metallic glasses, a new class of amorphous materials, have attracted a great deal of interests due to their exceptional properties. In recent years, our understanding of metallic glasses increases dramatically, thanks to the development of advanced instrumentation, such as in situ x-ray and neutron scattering. In this article, we provide a brief review of recent progress in study of the structure of metallic glasses. In particular, we will emphasize, from the scattering perspective, the multiscale structures of metallic glasses, i.e., short-to-medium range atomic packing, and phase transitions in the supercooled liquid region, e.g., crystallization and liquid-to-liquid phase transition. We will also discuss, based on the understanding of their structures and phase stability, the mechanical and magnetic properties of metallic glasses.

Correlation between initial structure and athermal quasi-static compressive deformation in a metallic glass

Athermal quasi-static compressive deformation for a model metallic glass is investigated by molecular dynamics simulations. A new structural parameter, quasi-nearest atom (QNA), is used to characterize local atomic packing of initial structure. We find that regions with higher nonaffine displacement have more atoms with more QNA. The coarse grained number of QNAs exhibits significant spatial heterogeneities and these structural fluctuations show close correlation with the nonaffine displacement. The correlation between the coarse grained number of QNA and nonaffine displacement does not decrease as the strain increases but show the maximum at a certain strain, indicating that strain localized regions are activated individually. Therefore, our results provide a direct evidence of the correlation between atomic structure and deformation in metallic glass.

3D printing of large, complex metallic glass structures

Metallic glasses (MGs) or amorphous alloys although have superior mechanical properties their products are limited to simple geometries such as foils/plates or rods with thin section-thickness due to the requirement of high cooling rates. In this study, 3D, large dimensions of amorphous structures with complex geometry are manufactured by our newly developed Laser Foil Printing (LFP) technology. Zr-based (LM105, provided by Liquidmetal Technologies, Inc.) amorphous foils of 100μm thickness are used as feedstock, and they are laser welded, layer-by-layer, to become 3D amorphous structures. Test results by X-Ray diffraction (XRD), differential scanning calorimetry (DSC), and micro-hardness confirm that the printed structures at selected process parameters achieve the same or better degree of amorphization as the raw foils. A mathematical model was developed to calculate the heating and cooling rates during structure manufacturing which helps the selection of process parameters. This study expands MG products to 3D arbitrary geometries with large dimensions due to the inherited advantages of the LFP technology which would open many potential applications.

Micro injection of metallic glasses parts under ultrasonic vibration

This work investigates the evolution of structure and mechanical performance of metallic glasses (MGs) under a proposed rapid forming approach. Through the unique ultrasonic-assisted micro injection method, micro MGs parts with fine dimensional accuracy were successfully fabricated. The temperature during the micro injection is higher than the glass transition temperature and lower than the crystallization temperature. Differential scanning calorimeter curve and X-ray diffraction pattern show that the MGs micro parts keep the amorphous nature after the ultrasonic-assisted micro injection. Our results propose a novel route for the fast forming of MGs and have promising applications in the rapid fabrication of micro scale products and devices.

Plastic deformation induced anisotropy in metallic glasses: A molecular dynamics study

The atomic structure of a Cu13Ni34Pd53 metallic glass was studied by molecular dynamics simulation at different temperatures along a shear deformation cycle. A simulation box of 1 million atoms was deformed in the x axis and then the original orthogonal shape was recovered. Directional pair distribution functions were computed in the coordinate planes and some significant directions along the shear deformation cycle. No anisotropy was found in the initial state, while post-deformation anisotropy was revealed by significant differences of the partial pair distribution functions. The analysis of atomic environments concluded that the remnant anisotropy remains constrained into the shear plane. Low temperature samples showed remnant anisotropy after the full shear sample, while close to the glass transition the anisotropy induced by the initial shear process is removed by the subsequent recovery process.

Effect of Al on the atomic structure in Zr50Cu40Al10 metallic glass

The icosahedral short-range order and medium-range order of Zr50Cu50 and Zr50Cu40Al10 metallic glasses have been investigated by molecular dynamics simulations. Our analysis shows that, although the replacement of a few percent of Cu by Al enlarges the population of full icosahedra (FIs), the degree of interpenetrating aggregation for FIs is reduced which might be caused by the strong avoidance of Al-centered FIs in the ternary system. However, this fact does not mean the weakness of icosahedral medium range order in Zr50Cu40Al10 metallic glass. If the interpenetrating and non-interpenetrating connections are both considered, the largest super-cluster formed by the connected FIs in Zr50Cu40Al10 is rather larger than that in Zr50Cu50, which might play an important role in slowing down the kinetics and thus enlarge the glass-forming ability.

Five-fold local symmetry in metallic liquids and glasses**Project supported by the National Natural Science Foundation of China (Grant Nos. 51271195 and 51271197), the National Basic Research Program of China (Grant No. 2015CB856800), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China (Grnat No. 16XNLQ01).

The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as periodicity for crystals, for establishing the structure–property relationship in amorphous materials. In this paper, we briefly review the recent developments of the five-fold local symmetry in metallic liquids and glasses and the understanding of the structure–property relationship based on this parameter. Experimental evidence demonstrates that five-fold local symmetry is found to be general in metallic liquids and glasses. Comprehensive molecular dynamics simulations show that the temperature evolution of five-fold local symmetry reflects the structural evolution in glass transition in cooling process, and the structure–property relationship such as relaxation dynamics, dynamic crossover phenomena, glass transition, and mechanical deformation in metallic liquids and glasses can be well understood base on the simple and general structure parameter of five-fold local symmetry.

Universal structural parameter to quantitatively predict metallic glass properties.

Quantitatively correlating the amorphous structure in metallic glasses (MGs) with their physical properties has been a long-sought goal. Here we introduce ‘flexibility volume' as a universal indicator, to bridge the structural state the MG is in with its properties, on both atomic and macroscopic levels. The flexibility volume combines static atomic volume with dynamics information via atomic vibrations that probe local configurational space and interaction between neighbouring atoms. We demonstrate that flexibility volume is a physically appropriate parameter that can quantitatively predict the shear modulus, which is at the heart of many key properties of MGs. Moreover, the new parameter correlates strongly with atomic packing topology, and also with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. These correlations are expected to be robust across a very wide range of MG compositions, processing conditions and length scales.

Characterization of nano-quasicrystal-formation in correlation to the local structure in Zr-based metallic glasses containing Pd

Abstract The effect of a noble metal (Pd) on the local atomic structure of the glassy state and the transformation behavior of quasicrystal (QC) precipitation in Zr 70 (Cu and Ni) 30-x Pd x (x = 0, 5, 10, 20) alloys were investigated. A QC phase precipitates in Zr-Cu glassy alloys at Pd concentrations of 5–20 at.% and in Zr-Ni glassy alloys at Pd concentrations of 10–20 at.%. The radial distribution function (RDF) indicates a change in the local atomic structure upon the addition of Pd. The QC phase is abruptly formed from the glassy structure above a certain temperature. The activation energy for nucleation is much larger than that for the precipitation of conventional intermetallic compounds. The QC growth is suddenly suppressed when their diameter reaches approximately 20 nm. The cooperative motion of icosahedral clusters as the precipitation mechanism was discussed. It was assumed that QC nucleus has a Zr-centered icosahedral medium-range order (MRO) as its core, and it grows by the aggregation of surrounding small icosahedral clusters. Noble metals might play a role of stabilizing the individual Zr-centered icosahedral MROs.

Correlation Between Local Structure and Boson Peak in Metallic Glasses

We made a systematic study of the boson peak for six different Zr-based metallic glasses and found a universal correlation between average local atomic structure and boson peak. It is found that the boson peak can be decomposed into six characteristic vibratory modes, i.e., Debye’s vibratory mode and five Einstein’s vibratory modes. By using the Ioffe–Regel condition over all studied Zr-based metallic glasses, we reveal that atomic pair correlation function exactly maps on the low-temperature dynamics and the origin of the boson peak, which is the sum of vibrations of local density fluctuation domains in the glasses. In addition, it is found that the Debye’s type oscillators are the major contributors to the low-temperature specific heat capacities. This study opens a new way of understanding the relationship of the physical properties with the atomic arrangements in glasses.

Local atomic structures of single-component metallic glasses

In this study we examine the structural properties of single-component metallic glasses of aluminum. We use a molecular dynamics simulation based on semi-empirical many-body potential, derived from the embedded atom method (EAM). The radial distribution function (RDF), common neighbors analysis method (CNA), coordination number analysis (CN) and Voronoi tessellation are used to characterize the metal’s local structure during the heating and cooling (quenching). The simulation results reveal that the melting temperature depends on the heating rate. In addition, atomic visualization shows that the structure of aluminum after fast quenching is in a glassy state, confirmed quantitatively by the splitting of the second peak of the radial distribution function, and by the appearance of icosahedral clusters observed via CNA technique. On the other hand, the Wendt-Abraham parameters are calculated to determine the glass transition temperature (T g ), which depends strongly on the cooling rate; it increases while the cooling rate increases. On the basis of CN analysis and Voronoi tessellation, we demonstrate that the transition from the Al liquid to glassy state is mainly due to the formation of distorted and perfect icosahedral clusters.