金属玻璃液体中的强脆转变现象

强脆转变是玻璃形成液体在从低温到高温升温过程中由强性液体转变为脆性液体的现象,反之从高温到低温冷却过程即为脆强转变.由于其意味着液体的结构发生了某种快速、非连续的变化,强脆转变现象成为异常动力学的典型代表.自1999年《Nature》杂志首次报道了水的强-脆转变现象之后,液体的强脆转变现象就作为凝聚态物理和材料科学领域中的前沿和热点问题被广泛关注.越来越多的研究表明,强脆转变现象在金属玻璃形成液体中普遍存在.为阐明金属玻璃强-脆转变现象对于深入理解玻璃转变本质、探讨液固遗传微观结构特征、揭示晶化过程相互竞争规律、提高玻璃形成能力、促进金属玻璃制备和处理工艺标准化等方面的重要意义,综合评述了强脆转变现象在金属玻璃形成液体中的普遍性、特殊性、定量表征、热力学表现以及结构起源等研究领域的最新进展,并指出了该领域今后的发展方向.     

金属玻璃形成液体的热力学特性

通过分析规则熔体的热力学模型，计算了典型金属玻璃的熔体混合焓ΔH^mix和混合熵ΔS^mix.结合临界冷却速率，归纳出典型金属玻璃形成液体的热力学特性,并提出基于原子尺寸、元素组成以及元素之间混合焓等参数的形成大块金属玻璃的成分判定方法.结果表明，当ΔH^mix<-15 kJ·mol^-1且ΔS^mix>0.6 J·K^-1mol^-1时，合金易于形成大块金属玻璃.金属玻璃的临界冷却速率R_c具有明显的尺寸效应，其值与熔体的ΔS^mix值呈指数关系，可以用R_c＝42.24×10⁴exp(-13.91ΔS^mix)+19.66粗略判断.运用该方法成功设计并制备出远离原有Zr基大块金属玻璃形成区域(55at%—65at%Zr)的Zr₄₀Al₁₀Ni₁₅Cu₃₅和四元Fe-B基Fe₅₃Co₅Nd₁₂B₃₀大块金属玻璃. 关键词： 混合焓 混合熵 大块金属玻璃 玻璃形成能力     

高温金属熔体黏度突变探索

高温金属熔体的黏度是衡量液态金属动力学性质的一个重要指标,是高温金属熔体的基本物理性能之一.熔体的黏度在表征脆性系数、金属玻璃形成能力的大小和液-液相变现象方面起关键性作用.本文在介绍高温金属熔体黏度测量方法的基础上,综合评述了单质、二元和多元合金黏度随温度的变化规律和黏度突变特征,分析了黏度突变研究的物理意义,并指出高温金属熔体黏度今后研究的发展方向。     

Non-monotonic temperature evolution of nonlocal structure–dynamics correlation in CuZr glass-forming liquids

The structure–dynamics correlations in a nonlocal manner were investigated in CuZr metallic glass-forming liquids via classical molecular dynamics simulations. A spatial coarse-graining approach was employed to incorporate the nonlocal structural information of given structural order parameters in the structure–dynamics relationship. It is found that the correlation between structure order parameters and dynamics increases with increasing coarse-graining length and has a characteristic length scale. Moreover, the characteristic correlation length exhibits a non-monotonic temperature evolution as temperature approaches glass transition temperature, which is not sensitive to the considered structure order parameters.Our results unveil a striking change in the structure–dynamics correlation, which involves no fitting theoretical interpretation. These findings provide new insight into the structure–dynamics correlation in glass transition.     

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.     

Dynamic singularity in multicomponent glass-forming metallic liquids

In the liquid state, glass-forming Ni59.5Nb40.5 and Ni60Nb34.8Sn5.2 alloys exhibit an extraordinarily high packing fraction. The self-correlation functions measured using quasielastic neutron scattering clearly show the slowing down of microscopic dynamics with an increase in packing fraction. The self-diffusivity in liquid Ni60Nb34.8Sn5.2 decreases by about 2 orders of magnitude within a temperature range of 360 K. For these highly fragile systems, the critical packing fraction obtained form the analysis of incoherent data is in excellent agreement with the prediction made by mode-coupling theory. Our results provide the first experimentally observed value for the critical packing fraction in glass-forming metallic liquids.     

Anelastic to plastic transition in metallic glass-forming liquids

The configurational properties associated with the transition from anelasticity to plasticity in a transiently deforming metallic glass-forming liquid are studied. The data reveal that the underlying transition kinetics for flow can be separated into reversible and irreversible configurational hopping across the liquid energy landscape, identified with beta and alpha relaxation processes, respectively. A critical stress characterizing the transition is recognized as an effective Eshelby "backstress," revealing a link between the apparent anelasticity and the "confinement stress" of the elastic matrix surrounding the plastic core of a shear transformation zone.     

金属液体结构与性能研究的新进展

金属液体的结构演化与其性能的关联一直都是凝聚态物理和材料科学领域的研究热点。文章介绍了金属液体结构的研究方法,以及液—液相变、物性参数和易碎性指数与其结构之间的内在关联性等基础科学问题的最新研究进展。随着金属液体结构和性能研究的不断深入,特别是大量先进实验方法的引入,加深了人们对金属液体的认识,同时促进了新材料的开发和性能改善等。     

块体非晶合金的黏度与玻璃形成能力的关系

基于经典结晶理论讨论了非晶合金的晶化动力学因素和晶化热力学因素对玻璃形成能力（GFA）的影响.分析表明,合金的等温转变（TTT）曲线“鼻尖”温度T_n对应的黏度与晶化阻力因子成正比;重新加热时晶化开始温度T_x对应的黏度与晶化驱动力因子成反比.由此得到了新的GFA参数ω₀=（T_g-T₀）/（T_x-T₀）-（T_g-T₀）/（T_n-T₀）,其中T_g为玻璃转变温度,T₀为理想玻璃转变温度.统计结果显示,ω₀与临界冷却速率具有较高的相关性,R²高达09626.进一步分析表明：新提出的ω₀参数可以合理地解释过冷熔体的黏度、脆性、液相稳定性、热稳定性以及T_rg、ΔT_x、γ、γ_m、ΔT_rg、α、β、δ和φ等参数与GFA的关系. 关键词： 块体非晶合金 黏度 脆性 玻璃形成能力     

Effects of minor Cu addition on glass-forming ability and magnetic properties of FePCBCu alloys with high saturation magnetization

The effects of minor addition of Cu on glass-forming ability (GFA), thermal stability and magnetic properties of (Fe_0.8P_0.09C_0.09B_0.02)_{100? x} Cu _x (x?=?0–1) alloys were investigated. The introduction of Cu slightly increases the GFA and efficiently improves the primary crystallization of α-Fe nanoparticles. The alloy with 0.3% Cu addition shows the best GFA and the fully glassy rods can be produced up to 2?mm in diameter. The saturation magnetization of the glassy sample with 0.7% Cu addition can be enhanced from 1.44 to 1.60?T after proper heat treatment due to the precipitation of α-Fe nanoparticles in the glassy matrix. The combination of large GFA and high saturation magnetization as well as low cost makes the FePCBCu alloys as a kind of promising soft magnetic materials for industrial applications.     

非晶合金及合金液体的局域五次对称性

简要回顾了从20世纪30年代至今,有关非晶合金及合金液体的局域结构五次对称性的实验、理论和模拟研究.在简单液体的早期研究中,人们已经意识到五次对称性在简单液体的无序结构、过冷和晶化等起着重要作用,二十面体短程序作为五次对称性的典型代表受到了广泛关注.自从Frank提出简单液体中二十面体短程序的结构单元,大量的理论和实验研究已经明确在简单液体、合金液体和金属玻璃中存在局域五次对称性,并且建立了局域五次对称性与合金液体复杂动力学行为、玻璃转变、液体-液体相变以及非晶合金的形变等统一的定量描述和物理图像,表明了局域五次对称性作为结构参量的简单、普遍和有效性.     

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.     

Effect of spatial heterogeneity on level of rejuvenation in Ni80P20 metallic glass

Understanding the relation between spatial heterogeneity and structural rejuvenation is one of the hottest topics in the field of metallic glasses (MGs). In this work, molecular dynamics (MD) simulation is implemented to discover the effects of initial spatial heterogeneity on the level of rejuvenation in the Ni$_{80}$P$_{20 }$MGs. For this purpose, the samples are prepared with cooling rates of $10^{10}$ K/s-$10^{12}$ K/s to make glassy alloys with different atomic configurations. Firstly, it is found that the increase in the cooling rate leads the Gaussian-type shear modulus distribution to widen, indicating the aggregations in both elastically soft and hard regions. After the primary evaluations, the elastostatic loading is also used to transform structural rejuvenation into the atomic configurations. The results indicate that the sample with intermediate structural heterogeneity prepared with 10$^{11}$ K/s exhibits the maximum structural rejuvenation which is due to the fact that the atomic configuration in an intermediate structure contains more potential sites for generating the maximum atomic rearrangement and loosely packed regions under an external excitation. The features of atomic rearrangement and structural changes under the rejuvenation process are discussed in detail.     

Structural relaxation of metallic glass forming melts

The fragility of superheated melts, M, for 13 kinds of metallic alloys has been evaluated from the data of the dynamic viscosity above their liquidus temperatures. The authors find that the glass forming ability of metallic melts depends on the fragility of superheated melts rather than on the value of viscosity. In the present work the value of fragility is less than 1 for good glass-forming melts but more than 1 for the other melts. The variation rate of atomic coordination number with temperature indicat...     

Cooperative shear model for the rheology of glass-forming metallic liquids

A rheological law based on the concept of cooperatively sheared flow zones is presented, in which the effective thermodynamic state variable controlling flow is identified to be the isoconfigurational shear modulus of the liquid. The law captures Newtonian as well as non-Newtonian viscosity data for glass-forming metallic liquids over a broad range of fragility. Acoustic measurements on specimens deformed at a constant strain rate correlate well with the measured steady-state viscosities, hence verifying that viscosity has a unique functional relationship with the isoconfigurational shear modulus.     

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD), reverse Monte Carlo (RMC), ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral, FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model, characterized by imperfect ordered packing (IOP), was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore, the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also explored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity, then 2D periodicity, and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked. Supported by the National Natural Science Foundation of China (Grant Nos. 50431030 and 50471097), the National Basic Research Program of China (Grant No. 2007CB613901), and the Programme of Introducing Talents of Discipline to Universities (Grant No. B07003)     

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.     

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.     

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.     

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.