Preferred clusters in metallic glasses

In this work,we present a feasible scheme based on framework of the sophisticated Voronoi tessellation method in order to evaluate what clusters should be preferred for building blocks in any given metallic glass,by analysing the fivefold-symmetry axes as well as the degree of structural regularity in various clusters.This scheme is well proved by a group of experiments and calculations,which may have broad implications for exploration of obtaining explicit and proper structural pictures,and understanding the structural origin of the unique properties and glass forming ability in these novel amorphous alloys.     

Spatial correlation of irreversible displacement in oscillatory-sheared metallic glasses

We report computer simulations on the oscillatory of CuZr metallic glasses at zero temperature with different shear amplitudes. In small system a homogenous shear deformation is found, while in large system an inhomogeneous shear deformation is found with a shear band formed. Concomitantly, spatial correlation of irreversible displacement exhibits an isotropic and exponential decay in the case of homogeneous deformation, whereas a mixed power-law and exponential decay in the case of anisotropic and inhomogeneous deformation. By projecting the azimuthal-dependent correlation function onto the spherical harmonics, we found a strong polar symmetry that accounts for the emerged shear band, and a weaker quadrupolar symmetry that accounts for the elastic filed generated by Eshelby inclusions. By this, we conclude that the anisotropy and decaying formula of the plastic correlation are dominated by the homogeneity or inhomogeneity for the deformation in the metallic glasses.     

Thermodynamical properties of Zr-based bulk metallic glasses

The temperature dependence of Gibb's free energy difference (ΔG), entropy difference (ΔS) and enthalpy difference (ΔH) between the undercooled melt and the corresponding equilibrium solid phases of bulk metallic glass (BMG) forming melts has been proved to be very useful in the study of their thermodynamical behavior. The present study is made by calculating ΔG, ΔS and ΔH in the entire temperature range T_m (melting temperature) to T_g (glass transition temperature) for three Zr-based samples of BMGs: Zr₅₇Cu_15.4Ni_12.6Al₁₀Nb₅, Zr_41.2Ti_13.8Ni₁₀Cu_12.5Be_22.5 and Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8. The study is made on the basis of Taylor's series expansion and a comparative study is also performed between the present result and the result obtained in the framework of expansions proposed by earlier workers, and also with the experimental results. An attempt has also been made to study the glass forming ability for BMGs.     

钴和铁熔体短程序的分子动力学模拟

通过分子动力学模拟研究了金属钴和铁熔体从普通液态到过冷液态普通微观结构的性质.所计算两体分布函数与可获得的实验结果基本一致,从普通液态到过冷液态的局域结构中的原子配位数为11和12的分布几率随着温度的降低而变小,而原子的配位数大于12的分布几率反而增加,角分布函数位于55.有一个明显的峰,位于110°有一个宽展的峰,位于150°有一个肩膀,表明金属钴和铁熔体的微观局域结构要比规则的二十面体团簇的结构复杂得多.我们的模拟结果表明金属熔体中的二十面体短程序随着温度的降低而增加,并在过冷液态中而占优势.     

Design of high T g Zr-based metallic glasses using atomistic simulation and experiment

In this paper, we systematically investigate local atomic structures of Zr_100?x Al _x (0???x???72) alloys using molecular dynamics simulations. Radial distribution functions of Zr-Al configurations at 300 K indicate that Zr-Al metallic glasses form only when the Al atomic concentration is larger than 32%. Voronoi polyhedral analysis shows that Zr₄₀Al₆₀ has the highest fraction of ?0,0,12,0? icosahedra around Al atoms, which are characteristic of amorphous microstructures. Variations of thermal expansion coefficient and heat capacity of Zr_100?x Al _x (40???x???72) metallic glasses as a function of temperature from 1100 to 800?K reveal that Zr₄₀Al₆₀ has the highest transition temperature of 1008?K. To confirm the simulation results, Zr-Al metallic glasses were fabricated using co-sputtering deposition; differential scanning calorimetry testing suggests the highest crystallisation-onset temperature of above 920?K is within Zr_100?x Al _x where 43?<?x?<?61. The experimental finding is in a good agreement with the simulation predictions.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

Fluctuation-dissipation ratio in three-dimensional spin glasses

We present an analysis of the data on aging in the three-dimensional Edwards-Anderson spin-glass model with nearest-neighbor interactions, which is well suited for the comparison with a recently developed dynamical mean-field theory. We measure the parameterx(q) describing the violation of the relation among correlation and response functions implied by the fluctuation-dissipation theorem.     

Notch fatigue of Cu_(50)Zr_(50) metallic glasses under cyclic loading: molecular dynamics simulations

Molecular dynamics simulation is performed to simulate the tension–compression fatigue of notched metallic glasses(MGs), and the notch effect of MGs is explored. The notches will accelerate the accumulation of shear transition zones, leading to faster shear banding around the notches' root causing it to undergo severe plastic deformation. Furthermore, a qualitative investigation of the notched MGs demonstrates that fatigue life gradually becomes shorter with the increase in sharpness until it reaches a critical scale. The fatigue performance of blunt notches is stronger than that of sharp notches. Making the notches blunter can improve the fatigue life of MGs.     

纳米结构非晶合金材料研究进展

迄今为止,人类社会新技术的发展主要是基于各种晶体材料(如金属、半导体等)的应用.晶体材料的性能可以通过改变它们的微观缺陷结构和/或微观化学结构来调控,但这对于当前的非晶材料而言却是难以实现的.新型的纳米结构非晶材料可以通过引入大量的非晶/非晶界面来改变非晶材料的微观缺陷结构和/或微观化学结构,从而实现对其性能的调控.本文主要讨论了目前纳米结构非晶合金材料的研究进展,包括其制备方法、结构特征和新性能.通过利用这些新特性,有可能会开启一个基于非晶材料的新技术时代.     

金属玻璃的键态特征与塑性起源

由于缺乏位错、晶界等典型的晶格缺陷,金属玻璃体系中承载力的形变单元为短程序或中程序原子团簇,键的强度及成键方向是影响原子间协调变形能力主要因素.本文通过与晶态合金对比,指出金属玻璃中原子键合方式与宏观力学性能的潜在关系,综述了金属材料电子结构与力学性能内在关系的最新研究进展,并系统介绍了金属玻璃电子结构特征、表征参量和主要测试手段,使读者对金属玻璃体系中原子间的键态特征有较清晰的认识,对进一步探索本征塑性较好的金属玻璃体系具有一定指导意义.     

Cu-Zr-Ti系Cu基块体非晶合金的形成和成分优化

利用与团簇相关的变电子浓度判据研究了过渡金属Cu-Zr-Ti系中Cu基块体非晶合金的形成区域和成分特征.据此判据在Cu-Zr-Ti系相图中确定出三条特殊的成分线，(Cu_9／13Zr_4／13)_100-xTi_x，(Cu_0.618Zr_0.382)_100-xTi_x和(Cu_0.56Zr_0.44)关键词： 块体非晶合金 Cu-Zr-Ti合金 原子团簇 电子浓度     

非晶合金的离子辐照效应

非晶合金作为一种快速凝固形成的新型合金材料,引起了材料研究者的极大兴趣.微观结构上长程无序、短程有序的特征使其具有独特的物理、化学和力学性能,在许多领域展现出良好的应用前景,尤其是有望成为核反应堆、航空航天等强辐照环境下的备选结构材料.本文深入探讨非晶合金的辐照效应,主要讨论离子辐照对非晶合金微观结构、宏观力学性能以及其他物理化学性能的影响,可为进一步理解非晶合金的微观结构和宏观力学性能之间的关系提供有效的实验和理论基础,也可为非晶合金在强辐照环境下的服役性能预测提供实验依据,对推进非晶合金这一先进材料的工程化应用具有重要的理论与实际意义.     

LaGa-based bulk metallic glasses

We report the formation of La Ga-based bulk metallic glasses. Ternary La–Ga–Cu glassy rods of 2–3 mm in diameter can be easily formed in a wide composition range by the conventional copper mold casting method. With minor addition of extra elements such as Co, Ni, Fe, Nb, Y, and Zr, the critical diameter of the full glassy rods of the La–Ga–Cu matrix can be markedly enhanced to at least 5 mm. The characteristics and properties of these new La Ga-based bulk metallic glasses with excellent glass formation ability and low glass transition temperature are model systems for fundamental issues investigation and could have some potential applications in micromachining field.     

金属玻璃的热塑性成型

金属玻璃在其过冷液相区内表现出随着温度升高黏度逐渐降低的特性,因此可以对其进行热塑性加工.该性质颠覆了传统金属的加工成型方式,使得其在远低于传统金属材料加工的温度和应力作用下可以按照人们的要求进行成型.因此,一些具有低玻璃转变温度的金属玻璃又被称作金属塑料.另外,由于金属玻璃是一种无序结构材料,不存在位错、晶界等晶体缺陷,且热膨胀系数小,在热塑性成型中具有优异的尺寸精度,因此被认为是理想的微成型材料,有广阔的应用前景.本文系统介绍了金属玻璃的热塑性成型性质及其应用,从热塑性成型的基本概念出发,阐述了金属玻璃热塑性成型能力的评估指标、热塑性成型技术、热塑性微成型及其理论、热塑性微成型的应用等,对认识金属玻璃的热塑性及扩展其应用有重要的意义.     

Multiscale structures and phase transitions in metallic glasses:A scattering perspective

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.     

金属玻璃研究简史

金属玻璃的发明和研究已经整整50年了.半个世纪以来,金属玻璃不但成为性能独特的新材料,同时也是研究材料科学和凝聚态物理中一些重要问题的模型体系.金属玻璃的研究已经成为凝聚态物理的一个重要分支.文章简要介绍了金属玻璃的研究历史以及最新的进展,并扼要介绍了这门学科的发展前景.     

不均匀性:非晶合金的灵魂

非晶合金,即金属玻璃,是一类特殊的由基本化学元素组成的非晶态物质,由于其独特的微观组织结构,展现出了不同于传统晶态合金材料的特殊物性,而成为高性能材料应用领域的重要一员.由于非晶合金的结构无序性,且当前精确表征其微观结构的实验手段缺乏,相应的理论模型也不完善,人们对非晶合金中一些基础物理问题的认识尚且不足,无法形成基本的理论框架来精确地描述其物性产生的微观机理.因而,当前非晶合金研究的核心问题可以概括为:如何建立以微观特征或结构为基础的基本理论框架,准确地概括非晶合金物性的微观机理.在过去几十年里,针对非晶合金的大量研究表明,在非晶合金长程无序的特征中,隐藏着本征的不均匀性和有序,且这些不均匀性的特征与材料物性有着密切的关联,使得建立以不均匀性特征为基础的理论框架成为可能.本文从非晶合金微观特征的不均匀性包括静态不均匀性和动态不均匀性的视角出发,概括性地总结了非晶合金材料与物理研究中取得的丰硕成果,及当前需要关注的重要科学问题,并针对未来可能的研究模式进行了展望.     

Developing aluminum-based bulk metallic glasses

This paper details a systematic investigation of the formation of Al-based bulk metallic glasses, expanding on an earlier brief report [Scripta Mater. 61 (2009) p.423]. We discuss an approach for designing and predicting the best glass-forming composition in the Al–TM–RE systems, based on the atomic cluster packing model for the internal structure of the glass. The effects of additional elements in quaternary and quinary systems on the glass-forming ability and thermal stability of the glasses are also discussed. Three new compositions, Al₈₆Ni₆Y_4.5Co₂La_1.5, Al₈₆Ni₇Y₅Co₁La₁ and Al₈₆Ni₇Y_4.5Co₁La_1.5, are capable of forming fully glassy rods of 1 mm in diameter; their glass transition and other thermal properties are systematically characterized.     

Influence of free volume and medium-range order on the deformation response of rapidly solidified and bulk Zr-based (Zr52Ti6Al10Cu18Ni14) metallic glass

Free volume and medium-range order (MRO) present in rapidly solidified ribbons (RSRs) and bulk metallic glasses (BMGs) of Zr₅₂Ti₆Al₁₀Cu₁₈Ni₁₄ have been probed by high resolution electron microscopy, fluctuation microscopy, positron annihilation and differential scanning calorimetry. In the as-solidified condition, RSRs showed higher free volume and lower MRO in comparison to BMGs. Within BMGs, the central regions showed higher MRO and lower free volume than the peripheral regions. Uniform deformation of BMGs and RSRs modified their structures, where in, free volume increased in the former and reduced in the latter. These changes in structures led to work hardening in RSRs and work softening in BMGs. Such behaviour could be explained by invoking a concept of critical free volume in the glass phase. For samples (in as-solidified condition) having free volume higher than the critical value, free volume decreased with deformation and showed work-hardening behaviour. In contrast, the work softening behaviour was noticed in samples having free volume lower than the critical free volume.     

Superconducting state parameters of binary metallic glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSPs) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C , isotope effect exponent αand effective interaction strength N _O V of some binary metallic glasses based on the superconducting (S), conditional superconducting (S’) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The T _C obtained from the H-local field correction function are in excellent agreement with available theoretical or experimental data. In the present computation, the use of the pseudo-alloy-atom model (PAA) was proposed and found successful. Present work results are in qualitative agreement with such earlier reported experimental values which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSPs of the metallic glasses on the valence ‘Z’ is identified.