Vol. 66, No. 17 (2017)
2017-09-05
SPECIAL ISSUE—Progress in research of amorphous physics
2017, 66 (17): 176101.
doi: 10.7498/aps.66.176101
Abstract +
Water absorption and wetting at metal surface have received considerable attention due to the important role in many relevant areas including catalysis and corrosion. The glassy surface has unique physical and chemical properties, displaying promising applications in surface science and technology. However, the water wetting of metallic glass surface is less studied than that of crystal metal surface. In this paper, the wetting kinetics of water droplets at the surface of Cu50Zr50 glass is studied by using molecular dynamics simulations. The water droplets show a complete wetting behavior at the glassy surface as in the cases of the CuZr (110) and (110) crystal surfaces. However, the spreading rate of water droplets on the glassy surface is remarkably fast. Despite different spreading rates, the time dependence of the spreading radius for crystal and glass surfaces consistently follows a power law, Rn t with the same exponent n = 7, which conforms with the universal law of the water spreading at non-reactive solid surfaces. An advancing adsorption monolayer of water is formed at the glassy surface, whereas the front of spreading water droplets displays a foot-like morphology at each of the (110) and (110) surfaces. The spreading of water droplets can be described as the process that water molecules diffuse from the droplet surface to the front of the adsorption layer. To reveal the microscopic mechanism of the fast spreading at the glassy surface, the interactions between surface and water are analyzed. We find that the water molecules in the adsorption layer at the glassy surface display a disordered arrangement in contrast to those of the ordered and double-layer structure. The structure of adsorption layer is closely related to the orientations of water molecules in it. The water molecules in the adsorption layer at the glassy surface are mostly parallel to the surface, and those at the crystal surface tend to point to the interiors of droplets. The molecular orientation is proved to determine the relatively weak hydrogen-bond interactions between the adsorption layer and the droplet interior at the Cu50Zr50 glassy surface, thus facilitating the diffusion of water molecules from the droplet surface to the front of the adsorption layer and improving the spreading. On the contrary, the strong interactions associated with the crystal surfaces hinder the droplet from spreading by slowing down the molecular diffusion. The present work provides an insight into the microscopic mechanism of water spreading at metallic glassy surfaces and conduces to in depth understanding the physical and chemical processes associated with metallic-glass/water interfaces.
2017, 66 (17): 176106.
doi: 10.7498/aps.66.176106
Abstract +
Metallic glasses, which exhibit outstanding mechanical, physical, and chemical properties and rich phenomena, are important technologically and fundamentally. The progress in the field of metallic glasses has largely relied on the development of new glass forming alloys. However, due to the multi-component nature of metallic glass, discovery of new alloy is slow. The fabrication combined with high-throughput characterization under the umbrella of materials genome initiative has been demonstrated to be helpful for accelerating the material discovery. In addition, the big data generated during high-throughput characterization can conduce to understanding the science behind the behaviors of various materials. In the paper, we summarize the techniques that can be used for the combinatorial fabrication of metallic glasses, and relevant approaches to realize the high-throughput characterization.
2017, 66 (17): 176107.
doi: 10.7498/aps.66.176107
Abstract +
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.
2017, 66 (17): 176402.
doi: 10.7498/aps.66.176402
Abstract +
Understanding the structure-property relationship of metallic glasses (MGs) at an atomic- or electronic level is a challenging topic in condensed matter physics. MGs usually exhibit low macroscopic plasticity, owing to the localized plastic flow in nano- and micro-meter scale shear bands upon deformation, which impedes their wide application as new structural materials. Thus, a detailed description of internal structure and establishing the structure-property relationship would underpin our knowledge of the mechanisms for the ductility/brittleness of MGs and further improve their plasticity. Due to the lack of structural defects such as dislocations and grain boundaries, the short- or middle-ranged ordered clusters are the typical deformation units in MGs, where the bonding strength and direction between atoms are the key factors that affect the cooperative displacements inside deformation unit. However, the bonding nature of MGs and their structure-property relationship are little studied systematically, which hinders our comprehensive understanding the basic problems about mechanical behaviors of MGs, such as fracture and plasticity deformation mechanism.In this paper, the potential correlation between the flexibility of bonding and ductility of MGs is discussed in detail. The first section gives a simple introduction of this topic. In the second section, the latest research progress of the electronic structural study of MGs is presented. Here, the corresponding studies of electronic structures of crystal alloys and their relationship with the mechanical properties are also presented for comparison. In the third section, the traditional and new experimental techniques employed for electronic structure measurements are presented, such as X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy and auger electron spectroscopy and the parameters such as nuclear magnetic resonance knight shift, susceptibility (χ) and specific heat (C) are also given in order to introduce electronic structure analysis methods of MGs and further reveal the bonding character of MGs and recent experimental findings of the relationship between the electronic structure and the mechanical properties of MGs.Numerous studies show that in the typical transition metal (TM)—metalloid metallic glass systems, the bond flexibility or mobility of atoms at the tip of crack that depends on the degree of bonding hybridization, determines the intrinsic plasticity versus brittleness. For instance, in these transition metal (TM)-based MGs, when metalloid element M with sp-element shells is alloyed in the TM matrix, the s-density of states (DOS) at M sites is scattered far below the Fermi level due to the pd hybridization between the p orbitals of M element and the d orbitals of TM. This causes the reduction of s-DOS at the Fermi energy (gs(EF)) at the solute M sites and exhibits a strong character. Thus, it is proposed that the gs(EF) can be employed as an effective order parameter to characterize the nature of bonding, especially in the aspect of evaluating bond flexibilities in amorphous alloys. This shows that the plastic flow and fracture process of MGs on an atomic scale can be well described using a simple bonding model where the deformation process is accompanied with the broken-down and reforming of atomic bonding inside short- or middleranged ordered clusters, since the defects are absent in MGs. We hope that this introduction can provide a much clearer picture of the bonding character of MGs, and further guide us in understanding the mechanism for ductile-to-brittle transition in MGs and exploring the novel MGs with intrinsic plasticity.directional boning
2017, 66 (17): 176403.
doi: 10.7498/aps.66.176403
Abstract +
It has been observed that many glass-forming liquids are transformed from fragile to strong liquids in a supercooled region upon cooling. This is the so-called fragile-to-strong (F-S) transition. Since its discovery in water, the F-S transition, as a frontier problem, as well as a hot issue, in condensed matter physics and material science, has aroused the considerable interest of researchers. It has been generally accepted that the F-S transition might be a universal dynamic behavior of metallic glass-forming liquid (MGFL). Studying the F-S transition is important not only for better understanding the nature of glass transition, uncovering the microstructural inheritance during the liquid-solid transformation, clarifying the structural competition during crystallization, improving the stability of MGs, but also for promoting the standardization during the production and treatment technology of MGs. In this paper, the general and special features of the F-S transition for bulk and marginal MGFLs are studied and described in terms of a physical model. A characteristic parameter f is introduced to quantify the F-S transition. With two relaxation regimes, on the basis of Mauro-Yuanzheng-Ellison-Gupta-Allan model, we propose a generalized viscosity model for capturing the liquids with the F-S transition. Using this model, we calculate the F-S transition temperature for metallic glass. From the calculation results, the F-S transition might occur around (1.36±0.03) Tg. By using the hyperquenching annealing-calorimetric approach, we find that the anomalous crystallization behavior occurs in both LaAlNi and CuZrAl glass ribbons. This phenomenon implies the existence of a thermodynamic F-S transition, which could be used as an alternative method of detecting the F-S transition in MGFLs. To date, the origin of the F-S transition is far from understanding. We find that the F-S transition in CuZr(Al) GFLs is attributed to the competition among the MRO clusters composed of different locally ordering configurations. By comparing the parameter f with the parameter r that characterizes the competition between the α and the slow β relaxations in 19 MGFLs, we find that the slow β relaxation plays a dominant role in the F-S transition and the extent of the F-S transition is mainly determined by the degree of the comparability in structure units between the α and the slow β relaxations. The existence of the liquid-liquid phase transition might also be the root of the F-S transition. The tendency of investigation of the F-S transition is also evaluated.
2017, 66 (17): 176406.
doi: 10.7498/aps.66.176406
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The glass is in a non-equilibrium state in nature, and relaxation might occur towards the equilibrium state at a certain temperature. When heating a quenched glass, relaxation can be resolved as temperature approaches to the glass transition, and further heating leads to enthalpy recovery as the system turns into an equilibrium supercooled liquid. The released energy involving the relaxation relative to the original quenched state is, in magnitude, identical to the gained energy in enthalpy recovery, showing a memory effect. In this paper, we discuss the enthalpy behaviors involved in a cooling and reheating cycle around the glass transition in various glass forming systems such as oxides, metal alloys, and small molecular systems. The cooling and heating rates are fixed to be -/+ 20 K/min with the related cooling and heating heat capacity curves being determined. It is found that the relaxation enthalpy involved in the cooling/heating cycles is closely related to the enthalpy of fusion for the glass forming materials, and the basically linear correlation implies the similarity between the glass transition and melting behaviors with regard to the atomic rearrangements involved in the relaxation and solidification processes. The determining of the cooling and heating heat capacity curves also helps establish the enthalpy relaxation/recovery spectra of various glasses, and the symmetry of the spectrum is associated with the fragility of glass-forming material. For the material of low or medium fragilities, the symmetry of the enthalpy relaxation spectrum is observed to be somehow dependent on the fragility, while for the high fragility glass, the symmetry keeps almost constant. The dependence of fragility on the glass transition thermodynamics is also discussed, and low melting entropy and high fragility are shown to reduce effectively the liquid-crystal Gibbs free energy difference. Using the correlation between the relaxation enthalpy and kinetic fragility reported in our previous studies, the glass transition thermodynamics for the case of the most fragile glass with m= 175 is evaluated, especially compared with the second phase transition of thermodynamics. The results provide a new understanding of the thermodynamics of the relaxation in glassy material and the glass transition.
2017, 66 (17): 177101.
doi: 10.7498/aps.66.177101
Abstract +
Entropy is a state function of the real physical system, which relates to the chaos of a system. During the long-term exploring glass-forming systems, many empirical rules are put forward, including “confusion principle” and three empirical rules.Over a long period of exploring, many glass-forming alloys are developed based on those principles, while some questions have been raised in recent years based on the experimental results, because some other uncertain factors also have influence on the glass-forming ability (GFA) except a number of constituents, e.g., entropy. Greer claimed that in the “confusion principle” the higher the entropy value, the better the glass-formation ability will be, which does not accord with the recent experimental results.The effects of entropy on the glass-formation ability are summarized from the viewpoints of thermodynamics, kinetics, and complexity of atomic structures. In the aspects of thermodynamics and structure, the increase of entropy has a positive effect on glass formation, while in kinetics, the influence is negative. From the viewpoint of thermodynamics, the increase of entropy leads to the decrease of the entropy difference between solid phase and liquid phase, and therefore, the difference in Gibbs free energy between these two phases decreases. At a certain time during solidification, compared with the low-entropy alloy, the high-entropy alloy in the solid phase has an atomic arrangement close to that in the liquid, and it is more likely to form the amorphous phase.In the aspect of kinetics, the increase of entropy results in the decrease of the viscosity of the system according to the Adam-Gibbs equation. As a result, atoms diffuse easily in the system and the ordered-phase is more likely to form, which means that the glass-formation ability decreases with the increase of entropy. Furthermore, in the aspect of atomic structure, the increase of mismatch entropy relates to the big misfit degree between atoms, i. e., the large atomic size difference. Atoms in the high-entropy alloy tend to distribute disorderly in the system, and therefore the stress between atoms increases. As a result, with the increase of the entropy, the ordered-phase becomes unstable and the GFA will be enhanced.Furthermore, the high-entropy-glass is briefly reviewed and analyzed, which is a new system between high-entropy alloy and amorphous alloy. There have been many high-performance high-entropy-glass systems reported up to now. Researches about this unique system will contribute to developing some new amorphous alloys with excellent performances, and more importantly, to exploring the complex relationship between GFA and multicomponent alloys.
2017, 66 (17): 178101.
doi: 10.7498/aps.66.178101
Abstract +
Metallic glasses (MGs), as new disordered materials prepared by rapidly quenching melted alloys, have attracted tremendous attention in the material science community. Due to their long-ranged disorderd and short-ranged ordered structures, MGs usually exhibit uniquely physical, chemical and mechanical properties, which give rise to promising applications in many fields, and especially they are expected to be potentially structural materials used in irradiation conditions, such as in nuclear reactors and aerospace.In this paper, the effects of ion irradiation on the microstructure, mechanical properties, physical, and chemical properties of MGs are reviewed. It is found that the effects of ion irradiation on the microstructures and mechanical properties depend on the ion energy as well as the composition of MG. When high energy ions interact with a solid, the collisions take place between the incident ions and atoms of the solid, which are dominated by inelastic processes (electronic stopping) and elastic processes (nuclear stopping). The inelastic processes result in the excitation and ionization of substrate atoms. In contrast, the elastic processes lead to ballistic atomic displacements. Nuclear stopping can produce structure defects and irradiation damage in glassy phase. The collisions between the incident ions and the target atoms in MGs can cause the target atoms to deviate from their original positions, and leave a large number of vacancies and interstitial atoms behind. The separations between the vacancies and the interstitial atoms form displacement cascades. The interstitial atoms with a low kinetic energy can transfer self-energies to thermal energies, resulting in a thermal spike due to the accumulation of a large quantity of the thermal energies from interstitial atoms. Such a thermal spike will cause MGs to melt and resolidify, which therefore makes the structure of glassy phase changed. Furthermore, the ion irradiation can modify the structures of MGs by introducing excessive free volumes and promoting the mobilities of atoms, which leads to the dilatation of the glassy phase and nanocrystallization. The increase of free volumes softens the MGs, and then causes the plastic deformation mechanism to transform from a heterogeneous deformation to a homogeneous deformation, which significantly enhances the plastic deformation ability.This review paper can not only improve the understanding of the relationship between microstructure evolution and macroscopic mechanical properties, and provide an experimental and fundamental basis to understand the deformation mechanism of MGs, but also summarize the performances of MGs under high dosage of ion irradiation. Moreover, it is of fundamental and practical importance for engineering applications of such advanced materials.
2017, 66 (17): 176103.
doi: 10.7498/aps.66.176103
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Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of “flow unit” in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.
2017, 66 (17): 176104.
doi: 10.7498/aps.66.176104
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Uranium-based amorphous alloys are a unique family of amorphous materials, which have so far been less studied due to the high chemical activity and radioactivity of uranium metal. In this paper, we review the compositions, preparations and thermal stability characteristics of U-based amorphous alloys obtained in the early experimental studies, and summarizes our recent results of the preparations and material properties of stable U-based amorphous alloys. The latest progress in our study of U-based amorphous alloys is presented in the three aspects. Firstly, the preparation methods, alloy systems and compositions, formation and crystallization behaviors of the new U-based amorphous alloys, along with the preliminary mechanisms for their formation and structure stabilization are reviewed. A number of new uranium-based amorphous alloy systems have been established based on eutectic law and structural packing model. These alloys show high ability to form glass, and the reduction of glass transition temperatures of some alloys to those of conventional amorphous alloys. The formation rules of binary (U-Fe/U-Co/U-Cr), ternary (U-Co-Al/U-Fe-Sn) and multicomponent alloy system have been investigated. It was found that the ability to form glass is strongly related to some physical parameters such as the local cluster structure, the electron concentration, the enthalpy of mixing, the electronegativity of the alloy component as well as the atomic size. The fragilities of U-based amorphous alloys indicate that they belong to a class of strong glass forming system, which means that the critical dimensions of such amorphous alloys can be further enhanced, and bulk amorphous samples are expected to be prepared. The crystallization activation of this kind of amorphous alloy is higher, and the crystallization process is dominated by nucleation. Then, the microstructures especially the first high-resolution electron microscopic results of the unique amorphous materials are reviewed. Finally, the micro-mechanical and anti-corrosion properties are reported in great detail. It is found that U-based amorphous materials show excellent mechanical properties and corrosion resistance, and the strength and hardness are much higher than those of conventional crystalline uranium alloys, and the corrosion resistance is also superior to the latter, which may be caused by its disorderly amorphous structural characteristics. Amorphous alloys have been the subject of intense fundamental and application research in recent years. Stable U-based amorphous alloys appear to cover all physical phenomena displayed by amorphous alloys. The discovery of outstanding properties in these new alloys therefore would stimulate both the fundamental studies including structure, electronic, glass transition, crystallization, etc., and the application-orientated studies of the thermal stability, mechanical and corrosion properties.
2017, 66 (17): 176105.
doi: 10.7498/aps.66.176105
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Amorphous alloys are a group of novel mechanical and functional materials that possess remarkably improved properties, such as mechanical property, wear property, anti-corrosion property, magnetic property and catalytic property, compared with those of their crystalline counterparts. The interactions between amorphous alloys and hydrogen can lead to various interesting physical and chemical phenomena, and also important applications. Typically, some amorphous alloys can store more hydrogen with faster kinetics than their crystalline counterparts due to the disordered atomic structures, which make them promising candidates for hydrogen storage. Hydrogen induced optical transformation in amorphous alloy film with thickness on a nanoscale makes them suitable for developing optical switchable windows. Hydrogen could be used as a sensitive probe to study the atomic structures of amorphous alloys. Amorphous alloys, whose structures are similar to defects in crystalline alloys (vacancies, dislocations, boundaries, ect.), are a group of suitable objects to study the interactions between hydrogen and defects. Amorphous alloys are also promising membranes materials for industrial hydrogen gas purification. Micro-alloying by hydrogenation could enhance the plasticity and glass-forming ability of amorphous alloy.In this review, recent research progress of interactions between amorphous alloys and hydrogen are summarized from two main aspects: fundamental research and practical applications. In the aspect of fundamental research, we firstly review the recent study on hydrogen in the amorphous alloy, including the hydrogen concentration and distribution, hydrogen occupancy type and geometric size, hydrogen diffusion and thermodynamics and other relevant physical and chemical issues. Secondly, the studies on the effects of hydrogenation on thermal stability, magnetic property and internal friction of amorphous alloys, together with some discussion on the corresponding mechanisms are summarized. Thirdly, hydrogen embrittlement of amorphous alloy and the corresponding prevention techniques, together with the studies of the interactions between hydrogen and defects in crystalline materials such as vacancies, dislocations and boundaries in material, are also involved. In the aspect of practical applications, we firstly review recent advances in amorphous hydrogen storage alloys, focusing on transition metal based amorphous alloys and Mg based alloys. Secondly, amorphous alloy films for hydrogen purification, hydrogen sensors and optical switchable windows are reviewed. Thirdly, some positive influences introduced by hydrogenation on amorphous alloys are discussed, typically on enhancing plasticity and glass-forming ability. Besides the above, hydrogen induced amorphization on crystalline alloy, the use of amorphous alloy for preparing nanocrystalline hydrogen storage materials, and using hydrogenation to crack bulk amorphous alloys to produce amorphous alloys powders are also discussed. In the last section of this review, we try to give our own viewpoint of the future perspectives of relevant researches and applications of interactions between hydrogen and amorphous alloys.
2017, 66 (17): 176109.
doi: 10.7498/aps.66.176109
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Amorphous alloys are the glassy solids that are formed through the glass transition of high-temperature melts. They therefore inherit the long-ranger disorders of melts and many quenched-in defects such as free volume. This inevitably leads to structural heterogeneity on a nanoscale that is believed to be as fertile sites for initiating relaxation and flow. However, due to limitations of spatiotemporal measurements, experimental characterization of the nanoscale structural heterogeneity in amorphous alloys has faced a great challenges. In this paper, an in-situ tensile testing setup with synchrotron small angle X-ray scattering is designed for a Zr-based (Vitreloy 1) amorphous alloy. By the small angle X-ray scattering, the structural heterogeneity of the Vitreloy 1 amorphous alloy can be described by the fluctuation of electron density. The small angle scattering images are recorded with the charge coupled device (CCD) detector, and then are azimuthally integrated into the one-dimensional scattering intensity curves using the FIT2D software. We apply the Porod law, Guinier law and Debye law to the obtained scattering intensity curves, and attempt to obtain the information about structural heterogeneity in the Vitreloy 1 amorphous alloy at different stress levels.The results indicate that the scattering intensity curve of the Vitreloy 1 amorphous alloy exhibits the positive deviation of Porod law. This observation proves that the amorphous alloy belongs to the non-ideal two-phase system, corresponding to the complicated spatial distribution between soft/liquid-like and hard/solid-like phases. According to the Porod's law, it is revealed that the diffuse interface exists between the two phases, associated with the density fluctuations in either of phases. Furthermore, we demonstrate that different scatterers coexist in the amorphous alloy and their characteristic sizes measured by the radius of gyration are mainly distributed between 0.8 nm and 1.6 nm. It deserves to note that the range of radii of gyration of scatterers are close to the equivalent sizes (1.3–1.9 nm) of shear transformation zones (STZs) for plastic flow in amorphous alloys. In addition, the shape of scatterer is far from a sphere, reminiscent of STZ activation regions of flat discs. It is therefore concluded that the scatterers with larger gyration radius correspond to the soft regions for the potential STZs, while those with smaller gyration radius correspond to the hard regions with lower free volume concentration. Finally, based on the correlation function defined by Debye, we analyze the correlation of electron density fluctuation between two arbitrary scatterers. The result indicates that the nanoscale scatterers in the amorphous alloy are strongly correlated only within a range of about 1 nm, which is consistent with the short-range ordered and long-range disordered structural features of the amorphous alloy. The image of the nanoscale heterogeneous structures characterized by the small angle X-ray scattering is almost not changed in the elastic deformation stage of the amorphous alloy. The present findings increase our understanding of the nanoscale structural heterogeneity in amorphous alloys, which is an important step to describe glass flow and relaxation.
2017, 66 (17): 176404.
doi: 10.7498/aps.66.176404
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The viscosities of metallic glasses gradually drop with temperature rising in their supercooled liquid region (SLR) which enables them to be thermoplastically formed and totally overturns the processing method of traditional metallic materials: their forming can be realized under temperature and stress far below those of traditional metallic materials. Based on this property, metallic glasses are considered as the ideal miniature fabrication materials due to their unique amorphous structures and no crystalline defects such as dislocation and grain boundary.The thermoplastic micro forming of metallic glasses in their SLR is studied in the present paper. A universal equation which describes the filling kinetics of viscous metallic glasses in the non-circular channel is proposed with the help of fluidic mechanics, and the results may be theoretically useful for the micro application of metallic glasses.In addition, some applications in the micro thermoplastic forming of metallic glasses are introduced. A metallic glass mold insert for hot embossing of polymers is fabricated by the micro thermoplastic forming of metallic glass, and it is found to have many advantages in mechanical property, fabrication efficiency, surface quality, etc. compared with the traditional material and method. A similar approach is used to fabricate gratings, which may provide a new material and technology to produce gratings. The superhydrophobic metallic glass surface with excellent abrasion and corrosion resistance is also fabricated by constructing micro-nano hierarchical structures on metallic glass surface. The bulk metallic glass micro fuel cell is also finished and found to have good performance.
2017, 66 (17): 176405.
doi: 10.7498/aps.66.176405
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The inherent atomic packing mode of glassy solid is still one of the most interesting and fundamental problems in condensed-matter physics and material science. Although significant progress has been made and provided insights into the atomic-level structure and short-to-medium-range order in glass, the way of leading to the medium-range order is still unclear. Does a universal rule exist in nature to construct a glass structure as what has been discovered for crystals? Is there any connection between glassy and crystalline structures? If so, what does the connection look like and how is the connection related to the properties of the glassy solids? A glassy state is usually obtained through supercooling a liquid fast enough to avoid crystallization. The amorphous nature of glassy solid is experimentally ascertained by X-ray diffraction (XRD), transmission electron microscopy or selected area electron diffraction (SAED). Almost all kinds of glassy solids exhibit similar maze-like SAED patterns without any local lattice fringes and broad diffraction maximum characteristics in XRD data. However, the glassy solids are inherently different in atomic-level structure, demonstrated by their different response behaviors under certain conditions, for example, the diverse annealing-precipitated crystallinephases, the distinct mechanical strengths and ductilities, and the different thermal stabilities against crystallization. Unfortunately, such a difference in inherent structure among glassy solids cannot be easily differentiated from a trivial analysis of the experimental diffraction data. However, the diffraction data such as structure factors or pair correlation functions (PCFs) are not as trivial as they look like. On the contrary, some studies have demonstrated that plenty of structural information is hidden behind the data of structure factors or PCFs, for example, global packing containing both spherical-periodic order and local translational symmetry has been revealed by analyzing PCFs of many metallic glasses. A fractal nature of medium-range order in metallic glassis also found by examining the relationships between the first peak positions in structure factors and atomic molar volumes in many metallic glasses. In fact, the oscillation in the structure factor or PCF is an indication that a certain order does exist in amorphous solid. Therefore, a more careful scrutiny of the diffraction data is desired to gain a more in-depth insight into the glassy structure features and find a clue to unveil the natures of the inherent structures in different glasses. In this paper, we briefly review the recent molecular dynamics simulation results that the distinct hidden orders of atomic packing formula in medium range in these pure glassy solids are unveiled to be inherited from bcc order in glassy Fe and fcc order in glassy Ni, respectively, reflecting nontrivial structural homology between glassy and crystalline solids. By analyzing the partial PCFs of three two-component metallic glasses of CuZr, NiAl, and NiCu which are similar but have distinct glass-forming ability via MD simulations, very different hidden orders are observed in each individual system, indicating that the hidden orders are more complex in multicomponent metallic glasses. The different hidden orders in a multicomponent metallic glass may be entangled topologically. More different hidden orders lead to more complex topological entanglement. Further analysis indicates that the formation of the hidden orders during cooling and their topological entanglement produces the geometrical frustration against crystallization and is closely correlated with the glass-forming ability of metallic alloys. A “genetic map” of hidden orders in metallic glass is finally constructed, which provides new insights into the structural properties and structure-property relationships in metallic glass-forming liquids and glasses.
2017, 66 (17): 178102.
doi: 10.7498/aps.66.178102
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Mechanical properties of micro- and nanoscale fibers are superior to their bulk counterparts, and their mechanical behaviors are different from each other. Homogeneous amorphous fibers with smooth surfaces and controllable sizes can be continuously drawn from supercooled liquid. Compared with the preparing of crystalline fibers, the manufacturing of amorphous fibers saves much energy and time. Furthermore, amorphous materials have excellent mechanical properties due to their short-ranged ordered and long-ranged disordered structures. Therefore, amorphous fibers have wide engineering applications and research interest. In this paper we review the fabrication and mechanical behaviors of amorphous fibers with excellent mechanical properties including oxide glass fibers and amorphous alloy fibers.There are continuous and discontinuous oxide glass micro-fibers. Discontinuous oxide glass micro-fibers can be fabricated by techniques in which a thin thread of melt flowing from the bottom of a container is broken into segments. Continuous oxide micro-fibers can be fabricated by techniques in which a filament of supercooled liquid is drawn from melt. However, oxide glass nano-fibers can be fabricated by chemical vapor deposition, laser ablation, sol-gel, and thermal evaporation methods. Fabrication techniques of amorphous alloy fibers are very different from those of oxide glass fibers. These techniques adopt in-rotating-water spinning method, melt-extraction method, Taylor method, nanomoulding method, fast drawing method, melt drawing method, and gas atomization method.Microscale oxide glass fiber has a facture strength as high as 6 GPa. The fracture strength of nanoscale oxide glass fiber can reach 26 GPa which is close to the theoretical strength of 30 GPa. On the other hand, the plasticity of microscale amorphous alloy fibers is mediated by shear banding. The shear band spacing decreases with reducing sample size in bending. However, there is no tensile plasticity in microscale amorphous alloy fibers. When the sample size is smaller than the size of shear band core (500 nm), inhomogeneous plastic deformation transforms into homogeneous plastic deformation. The tensile plasticity of amorphous alloy is significantly improved. The homogeneous plastic deformation is mediated by catalyzed shear transformation. The catalyzed shear transformation may be the origin of hardening behaviors of nanoscale amorphous alloy fibers.Fianlly, we summary the unsolved problems in the fabrications and mechanical behaviors of amorphous fibers, and discuss the prospect of amorphous fibers.
2017, 66 (17): 176102.
doi: 10.7498/aps.66.176102
Abstract +
Glass-liquid transition phenomenon, usually known as glass transition, has been valuated as one of the most important challenges in condensed matter physics. As typical amorphous solid, metallic glass is composed of disordered-packing atoms, which is akin to a liquid. Thus, metallic glass is also known as frozen liquid. Metallic glass is an ideal model material for studying glass transition phenomenon. When heated up to glass transition temperature or stressed to yielding point, metallic glass flows. The flow behavior at elevated temperature or under stress plays an important role in the applications of metallic glass. In this paper, we briefly review the research developments and perspectives for the flow behavior and extended elastic model for flow of metallic glasses.
In elastic models for flow, i.e., free volume model, cooperative shear transformation model, it is assumed that the activation energy for flow (E) is a combination of shear modulus (G) and a characteristic volume (Vc), E=GVc. Most recently, it has been widely recognized that in amorphous materials, e. g. metallic glass, shear flow is always accompanied by dilatation effect. This suggests that besides shear modulus, bulk modulus (K) should also be taken into account for energy barrier. However, what are the contributions of K is still unknown. On the other hand, the physical meaning of characteristic volume Vc and the determination of its value are also important for quantitatively describing the flow behavior of metallic glass. Based on the statistical analyses of a large number of experimental data, i. e., elastic modulus, glass transition temperature, density and molar volume for 46 kinds of metallic glasses, the linear relationship between RTg/G and Vm is observed. This suggests that the molar volume (Vm) is the characteristic volume involved in the flow activation energy.
To determine the contribution of K as a result of shear-dilatation effect, flow activation energy density is defined as E =E/Vm. According to the harmonic analysis of the energy density landscape, we propose that both shear and bulk moduli be involved in flow activation energy density, as E = (1-)G+K, with 9%. This result is also verified by the relationship between elastic modulus and glass transition temperature: (0.91G+ 0.09K)Vm/RTg is a constant, that is, independent of property of metallic glass. This result is also consistent with the evolution of sound velocity with glass transition temperature.
In the end of this review, we address some prospects about the applications of the extended elastic model and its significance in designing new metallic glasses with advanced properties. This extended elastic model is also fundamentally helpful for understanding the nature of glass transition and kinetic properties of shear band of metallic glasses.
2017, 66 (17): 176108.
doi: 10.7498/aps.66.176108
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Glasses are solid materials that are far from their thermodynamic equilibrium states and their stabilities play a role in many applications as well the theoretical understanding of the natures of glass systems. Recently, ultrastable glasses (SGs) have been developed. The SGs have the stabilities that ordinary glasses can obtain only after being annealed for thousands to millions of years, thereby providing a great opportunity for studying the stabilities of glasses. In this paper we present a brief review about the properties of SGs and their formation mechanisms and novel insights into the glassy physics.
2017, 66 (17): 176110.
doi: 10.7498/aps.66.176110
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Today's technologies are primarily based on crystalline materials (metals, semiconductors, etc.), for their properties can be controlled by changing their chemical and/or defect microstructures. This is not possible in today's glasses. The new features of nanostructured glasses consisting of nanometer-sized glassy regions connected by interfaces are that their properties may be controlled by changing their chemical and/or defect microstructures, and that their interfaces each have a new kind of non-crystalline structure. In this paper we mainly discuss the research progress of nanostructured metallic glasses, including their preparation methods, structure characteristics and new properties. By utilizing these new features, an era of new technologies based on non-crystalline materials (a “glass age”) can be opened up.
2017, 66 (17): 176111.
doi: 10.7498/aps.66.176111
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Bulk metallic glass has aroused intensive interest due to its unique atomic structure and properties, while its structural application is restricted by the shortcomings of its mechanical properties-room temperature brittleness and strain softening. To make up for these shortcomings, various approaches have been proposed, including tailoring intrinsic parameters such as elastic modulus and structural heterogeneity, and changing stress state or defect concentration. Bulk metallic glass composites with ex-situ added or in-situ formed crystallites have been fabricated, series of bulk metallic glasses and their composites with good mechanical properties have been designed, especially TRIP (Transformation-induced plasticity)-reinforced bulk metallic glass composites with large tensile ductility and work-hardening. In this paper, we review the ductilization of bulk metallic glass and its composites, as well as the related mechanism. Particularly, fabrication, properties, structure control and the ductilization mechanism of TRIP-reinforced bulk metallic glass composite are introduced in detail. A perspective of the challenges of ductilization of bulk metallic glassy materials is also mentioned briefly.
2017, 66 (17): 176112.
doi: 10.7498/aps.66.176112
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Owing to the superior mechanical and physical properties, metallic glasses (MGs) have attracted tremendous attention as promising candidates for structural and functional applications. Unfortunately, the ability to form uncontrollable glasses, the poor stability and the unpredicted catastrophic failure stemming from the disordered structure, as the Achilles' heel of MGs, severely restrict their large-scale applications. A number of phenomenological models, such as free volume model, shear transformation zone (STZ) model, flow unit model, etc., have been proposed, intending to relate microstructures to properties of MGs. However, few sophisticated structure-property relationships are established due to a poor understanding of the microstructure of MGs. Recently, heterogeneity is commonly believed to be intrinsic to MGs, and it can be used to establish the structure-property relationship of MGs. In this paper, we review the recent progress of MGs from the angle of heterogeneity, including the static heterogeneities and dynamic heterogeneities. The perspectives of the scientific problems and the challenges of metallic glass researches are also discussed briefly.
2017, 66 (17): 176113.
doi: 10.7498/aps.66.176113
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Magnetic semiconductors hold a very special position in the field of spintronics because they allow the effective manipulations of both charge and spin. This feature is important for devices combining logic functionalities and information storage capabilities. The existing technology to obtain diluted magnetic semiconductors (DMSs) is to dope magnetic elements into traditional semiconductors. So far, the DMSs have attracted much attention, yet it remains a challenge to increasing their Curie temperatures above room temperature, particularly for those III-V-based DMSs. In contrast to the concept of doping magnetic elements into conventional semiconductors to make DMSs, here we propose to introduce non-magnetic elements into originally ferromagnetic metals/alloys to form new species of magnetic semiconductors. To demonstrate this concept, we introduce oxygen into a ferromagnetic amorphous alloy to form semiconducting thin films. All the thin films are deposited on different substrates like Si, SiO2 and quartz glass by magnetron sputtering. The structures of the deposited thin films are characterized by a JEOL transmission electron microscope operated at 200 kV. The optical transparencies of the samples are measured using Jasco V-650 UV-vis spectrophotometer. The photoluminescence spectra of the samples are measured using RM1000 Raman microscope. Electrical properties of the samples are measured using Physical Property Measurement System (PPMS-9, Quantum Design). Magnetic properties, i.e., magnetic moment-temperature relations, are measured using SQUID-VSM (Quantum Design). With oxygen addition increasing, the amorphous alloy gradually becomes transparent. Accompanied by the opening of bandgap, its electric conduction changes from metal-type to semiconductor-type, indicating that the inclusion of oxygen indeed mediates a metal-semiconductor transition. For different oxygen content, the resistivities of these thin films are changed by about four orders of magnitude. Notably, all of them are ferromagnetic. All the samples show anomalous Hall effect. Furthermore, their magnetoresistance changes from a very small positive value of about 0.09% to a negative value of about -6.3% under an external magnetic field of 6 T. Correspondingly, the amorphous structure of the thin film evolves from a single-phase amorphous alloy to a single-phase amorphous metal oxide. Eventually a p-type CoFeTaBO magnetic semiconductor is developed, and has a Curie temperature above 600 K. The carrier density of this material is ～1020 cm-3. The CoFeTaBO magnetic semiconductor has a direct bandgap of about 2.4 eV. The room-temperature photoluminescence spectra further verify that its optical bandgap is ～2.5 eV. The demonstrations of p-n heterojunctions and electric field control of the room-temperature ferromagnetism in this material reflect its p-type semiconducting character and the intrinsic ferromagnetism modulated by its carrier concentration. Our findings may pave a new way to realizing high Curie temperature magnetic semiconductors with unusual multi-functionalities.
2017, 66 (17): 176401.
doi: 10.7498/aps.66.176401
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Amorphous material usually exhibit a complex atomic structure including short-range order, long-range disorder and metastable state in thermodynamic, which is one of the existing states of matters. Amorphous alloy, also named metallic glass, is a new metallic material, and has a high strength, a good electromagnetic property, an excellent corrosionresistant and a high elasticity. The system of amorphous alloy can show some critical states and is a complicated system. In recent years, much atttentions have been paid to the researches of the phase transitions and critical phenomena of amorphous material. On a microscale, amorphous alloy can be regarded as a solid composed of many-particle systems. The investigation of the critical phenomena can significantly enhance the understanding of the interactions among these multi-particle systems. The structure of amorphous alloy is randomly and isotropic in macro performance, and ordered and anisotropic on a localized nanometer scale. The characteristics on different scales of amorphous alloy are not isolated. The structure of amorphous alloy determines the performance. The preparation process determines the nature of the microstructure. The microstructure is the internal cause dominating glass transition and deformation. Moreover, the effective cooling rate in preparation process of amorphous alloy affects the short-range rate of the amorphous phase. The nonperiodic short-range order plays a key role in the stability of amorphous phase. Furthermore, the glass transition and deformation of amorphous alloys are the responses to the external energy. The characteristics of the deformation process change with external condition. The external force can lead to the localized shear deformation and transformation between amorphous and liquid in the shear band. High temperature can cause a wide range of transformation from the amorphous solid to the liquid. So it is worth understanding in depth the basic principles of liquid and glass transition in order to prepare amorphous alloy in undercooled liquids. In this review article, we discuss the critical phenomena of amorphous alloys, which include the preparation process, the microstructure, the mechanical property and the electromagnetism. The correlation and the influence of microstructure on the macroscopic properties are analyzed. It will be helpful for understanding the nature of amorphous alloy, improving service reliability and exploring amorphous alloys with application values.
2017, 66 (17): 176407.
doi: 10.7498/aps.66.176407
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As soon as crystallization is suspended at constant pressure, cooling liquids turn inevitably into rigid amorphous solids, i.e. glasses. The process is a universal phenomenon in nature, termed as glass transition involving many fundamental problems in many-body interaction system and material science. Among the decades research on the glass transition, the universality of β-relaxation, its mechanism and its effects on the understanding of liquids and glasses have been studied argumentatively. In this paper we review the research progress of β-relaxation and also try to point out the tendency of β-relaxation study in future.
2017, 66 (17): 176408.
doi: 10.7498/aps.66.176408
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It is always desirable to develop bulk metal materials with extremely mechanical properties. Ultrahigh strength bulk metallic glass (BMG) is a kind of advanced metallic material with extremely high strength (above 4 GPa), high thermal stability (the glass transition temperature: normally above 800 K), and high hardness (normally above 12 GPa). A typical system of the ultrahigh strength BMG is Co-Ta-B alloy with a fracture strength of above 6 GPa, which is the highest value in the fracture strengths for all kinds of bulk metallic materials (including crystalline and amorphous ones) that we have known so far. In this paper, the compositions, thermal properties, elastic constants, and mechanical properties for all of the reported ultrahigh strength BMGs are summarized. The research progress of these BMGs is also introduced. The correlations among the characteristic temperature, elastic constants, hardness and mechanical properties are built, and the natures of chemical bonding for the ultrahigh strength and high hardness of these BMGs are revealed. The results relating to the structure and physical properties of this kind of ultrahigh strength BMG are significant for potential applications in advanced manufacture, super-durable components and machining.
2017, 66 (17): 176409.
doi: 10.7498/aps.66.176409
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Metallic glasses with functional properties, such as magnetic properties, are promising materials for potential applications and have aroused great interest. Magnetic phase transition is an important feature of metallic glass. The unique effect of the magnetic phase transition can be applied to the field of refrigeration. On the one hand, due to its magnetocaloric effect, the amorphous alloy can be used as a magnetic refrigeration material for magnetic refrigerator. On the other hand, because of its specific heat anomaly the amorphous alloy can be used as a magnetic regenerator material for cryogenic refrigerator. In recent years, the magnetocaloric effects and magnetic regenerator performances of metallic glasses have become hot topics in the field, and opened up possibilities for the functional applications of metallic glasses. In this paper, the principle of magnetocaloric effect and magnetic regenerator performance of metallic glass and its characteristics and application prospect are introduced in detail.
2017, 66 (17): 176410.
doi: 10.7498/aps.66.176410
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In most of liquids, densities increase as temperature decreases. However, the densities of water and water-like liquids, such as silicon and germanium, are anomalous, which increase as temperature increases. Such substances also show other anomalous behaviors, such as diffusivity anomalies (diffusivities increase as density increases), and thermodynamic anomalies (the fluctuations increase as temperature decreases). The chemical properties of these materials are very different from each other, but they all share similar physical properties. Further studies indicate that most of them have two distinct liquid states, i.e., a low-density liquid and a high-density liquid, and a first order liquid-liquid phase transition (LLPT) between these two liquids. We mainly discuss the anomalous properties of materials each of which has a predicted LLPT and their relations with anomalous behaviors (thermodynamic, dynamic and structural) as those of water and water-like liquids, such as hydrogen and gallium. In particular, we discuss the supercritical phenomenon of the liquid-liquid phase transition of hydrogen, as well as the liquid-liquid phase transition of gallium and its relation with the thermodynamic, dynamic, and structural anomalies. It is found that the liquid hydrogen and gallium both have the LLPT and share similar anomalous behaviors as water and water-like liquids, such as density anomaly, dynamics anomaly, thermodynamic anomaly Since the chemical properties of these materials are very different from those of others having the LLPT, the anomalous behaviors may be common features for substances predicted to have the LLPT.
2017, 66 (17): 176411.
doi: 10.7498/aps.66.176411
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Metallic plastic, named and developed by Chinese scientists, is a kind of new material. Here in this paper we explain how this material was discovered and its design philosophy and principle. The chemical compositions, micro-structures, and typical physical and chemical properties of these metallic plastic materials are summarized in this paper. The potential applications of the metallic plastic materials are also analyzed.
2017, 66 (17): 178103.
doi: 10.7498/aps.66.178103
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Amorphous solids are metastable materials formed by the rapid quenching of liquid melts. Under mechanical stress, amorphous solid displays unique and complex plastic flow behavior, which is both spatially and temporally inhomogeneous on different length scales. In some cases, the plastic flow behavior of amorphous solid can evolve into the self-organized critical state, which is similar to many complex phenomena in nature and physics such as earthquakes, snow avelanches, motions of magnetic walls, etc. In this paper, we briefly review the recent research progress of the plastic flows of amorphous solids, with an emphasis on the plastic flow of metallic glass which has been one of our research foci in past few years. The review begins with an introduction of the inhomogeneous flow behaviors on different scales, from the macroscopical-scale spatially inhomogeous shear bands, temporally intermittent serrated flow to the atomic-scale localized viscoelastic behavior in metallic glass. The microscopical deformation theories including free volume model and shear transformation zone model, and recent efforts to elucidate macrosopical flow behaviors with these theories, are also presented. Finally, recent progress of the self-organized critical (SOC) behaviors of the plastic flow of metallic glass are reviewed, with an emphasis on its experimental characterizations and the underlying physics. The emergence of SOC in the plastic flow is closely related to the interactions between plastic flow carriers, and based on this point, the relation between the SOC behavior and the plasticity of metallic glass is elucidated. The implications of plastic flow of metallic glass for understanding the occurence of earthquakes are also discussed. The review is also concluded with some perspertives and unsolved issues for the plastic flow of amorphous solids.
2017, 66 (17): 178201.
doi: 10.7498/aps.66.178201
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As a soft matter material, the unique properties of colloidal glasses make it a particularly useful platform to study fundamental physics of amorphous solids. In the article, we briefly review the connections between colloidal glasses and molecular amorphous materials by surveying applications of colloids in different aspects of glass researches. And we also give future directions of colloidal glasses researches in the end.
