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.     

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.     

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.     

判定金属玻璃微观结构中的二十面体类团簇

基于Voronoi几何分形法, 分析了理想二十面体团簇和ZrCu二元金属玻璃中各种团簇的结构特点, 提出了一种判定金属玻璃原子结构中二十面体类团簇的方法. 并选取三个ZrCu 非晶成分作为研究对象, 基于Voronoi团簇, 利用该方法提取了各种构型团簇, 证实其中四种构型团簇的基本几何结构与理想二十面体相似, 并具有同样近似于理想二十面体的高致密度、高规则度和高五次对称性, 因此可称之为二十面体类团簇. 此类二十面体类团簇可作为金属玻璃的主要结构单元, 普遍存在于非晶结构中; 二十面体类团簇及其连接能包含几乎所有的原子, 从而形成非晶结构. 研究结果提供了一种新的团簇判定方法, 有助于从微观结构层面分析合金中的非晶形成机理.     

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.     

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.     

基于频域纹理消除的结构性纹理缺陷检测方法

针对金属加工表面等结构纹理表面图像缺陷检测问题,结构纹理的存在会对缺陷（比如划痕）检测带来干扰,该文开展在频率域中消除背景纹理的方法来进行缺陷检测的研究。首先基于傅里叶变换的图像复原技术,空间域图像中的结构性纹理对应傅里叶域中高能频率分量,使用最小二乘法直线拟合操作去除,并将这些能量设置为零,经傅里叶逆变换为空间域图像。在复原的图像中,原始图像中的结构纹理区域将变为近似的均匀灰度级,但其中缺陷部分将被保留下来。再使用统计过程控制来设置阈值的方法就能从复原图像中分离出缺陷。最后在一系列的结构性纹理图像上的实验证实了所提方法可行且有效。     

La65X35(X=Ni,Al)非晶合金原子结构的第一性原理研究

运用基于密度泛函理论的第一性原理分子动力学和静态电子结构计算,研究了La₆₅X₃₅（X=Ni,Al）非晶合金体系原子结构随温度演化的规律及其相关电子结构特性.使用径向分布函数、Voronoi团簇以及键对分析等给出了从高温液体快速冷却到玻璃态过程中原子结构的演化规律.研究发现,该类合金体系的原子排布符合局域密堆模型,两体系中占比最大的特征多面体类型由溶质与溶剂原子半径比调控;两体系中高五次对称性局域结构随温度的下降而增加验证了其在抑制晶化方面的重要作用;利用投影态密度分析两体系电子结构之间的差异,指出La-5d与Ni-3d电子间强烈的杂化是La–Ni 间键长缩短的电子结构起源,为理解成分相关的结构和物性提供了重要线索.     

Molecular dynamics simulation of plastic effects upon spalling

The molecular dynamics method is applied to simulate spalling during the plane shock interaction between plates. The effect of lattice defects in a material on the propagation of a shock wave and the process of spalling is studied. The plastic effects are described using a model of imperfect particle packing with defects (vacancies). The model proposed can describe the separation of the shock-wave front into an elastic precursor and a plastic front and give velocity profiles for the free target surface close to the experimental profiles.     

Local structural transformations in the fcc lattice in various contact interaction. Molecular dynamics study

The work is a molecular dynamics study of the peculiarities of local structural transformations in a copper crystallite at the atomic level in contact interaction of various types: shear loading of perfectly conjugate surfaces, local shear loading and nanoindentation. Interatomic interaction is described in the framework of the embedded atom method. It is shown that initial accommodation of the loaded crystallite proceeds through local structural transformations giving rise to higher-rank defects such as dislocations, stacking faults, interfaces, etc. In further plastic deformation, the structural defects propagate from the contact zone to the crystallite bulk. The egress of structural defects to a free surface causes deformation of the model crystallite. The deformation pattern can evolve, depending on the loading conditions, with a change in crystallographic orientation of the crystallite near the contact zone, generation of misoriented nano-sized regions, and eventually formation of a stable nanostructural state. The obtained results allow conceptually new understanding of the nature of defect generation in a crystalline structure during the nucleation and development of plastic deformation in loaded materials.     

Collective properties of defects,multiscale plasticity,and shock induced phenomena in solids

A statistically based approach is developed for the construction of constitutive equations that provides linkages between defect-induced mechanisms of structural relaxation, thermally activated plastic flow, and material response to extreme loading conditions. The collective properties of defects have been studied to establish the interaction of multiscale defect dynamics and plastic flow, and to explain the mechanisms leading to the universal self-similar structure of shock wave fronts. Pn explanation for structural universality of the steady-state plastic shock front (the four power law) and the self-similarity of shock wave profiles under reloading (unloading) is proposed. Structural characterization under transition from thermally activated dislocation glide to nonlinear dislocation drag effects is developed in terms of scaling invariants (effective temperatures) related to mesodefect induced morphology formed during the different stages of plastic deformation.     

Activation energy of shear transformation zones: a key for understanding rheology of glasses and liquids

Key manifestations of the glassy and liquid states, such as viscous flow and structural relaxation, occur spatial and temporal heterogeneously, within highly localized rare events, termed shear transformation zones. Characterization of these basic entities with respect to thermal activation and mechanical response is vital for understanding the rheology of glasses across length scales. This is achieved in classical molecular dynamics computer simulations on the model glass, CuTi, by determining the activation energy barrier and plastic yield strain of individual shear transformation zones as a function of size and external stress loading. Sizes of approximately equal to 140 atoms are identified to be especially energetically favorable with an activation energy barrier of approximately equal to 0.35 eV. Using these parameters, a rheology model is proposed to quantitatively explain viscosity.     

支持向量机方法在热波检测图像分割中的应用

作为热波无损检测技术中的关键环节,热波图像分割对结构损伤的有效识别与准确评估具有重要影响。为克服红外热波图像背景噪声大,对比度低等因素对损伤识别的影响,提出了一种基于支持向量机的热波图像分割方法。该方法首先采用Wiener滤波对热波图像进行预处理,然后随机选取目标区域和背景区域内多个像素点的像素值组成目标向量与背景向量,对基于多项式核函数的支持向量机进行训练,最后将训练好的分类器应用于不同的热波图像,实现热波图像的分割。试验结果表明:该方法可有效克服热波图像背景噪声大的问题,较好地保留了缺陷区域分割的完整性;与基于硬阈值的图像分割方法相比,该方法能更好地抑制背景区域的噪声干扰,更有利于损伤的识别与评估。     

On the nature of low-temperature anomalies of the anelastic properties of metallic glasses

In Memory of A. M. Roshchupkin The low-temperature (30<T<300 K) internal friction and elastic modulus of the metallic glass (MG) Ni₆₀Nb₄₀ subjected to preliminary cold working by rolling, high-temperature uniform straining, or electrolytic hydrogenation is investigated. It is established that cold rolling, which induces localized plastic flow, or hydrogenation radically alters the temperature dependences of the internal friction and elastic modulus: hysteresis appears in the background damping and intense relaxational peaks arise in the internal friction, accompanied by a defect of the elastic modulus. A uniform strain does not affect the low-temperature anelastic behavior of MGs. Microplastic deformation is observed to accompany the hydrogenation of weakly loaded samples. It is asserted that localized microplastic deformation also occurs on hydrogenation with no load. Plastic flow accompanying both rolling and hydrogenation occurs by the formation and motion of dislocationlike defects, which in the presence of an external load of alternating sign give rise to the observed anelastic anomalies. It is concluded that the low-temperature internal-friction peaks, described in the literature, in the “as-quenched,” cold-deformed, or hydrogenated MGs are all of a dislocation nature. Zh. Tekh. Fiz. 67, 35–46 (October 1997) In Memory of A. M. Roshchupkin     

Study on inspection method of glass defect based on phase image processing

To solve the problem in projecting grating method, the paper presents an inspection method based on phase image processing for glass defects. In the proposed method, the wrapped phase difference is achieved according to the images of defect-free and defect-containing. The unwrapping phase algorithm, based on jump corrected is used to eliminate jump error. The segmentation of defect region is implemented by integrating grayscale mathematical morphology with high-low threshold segmentation, and the boundary coordinate of connected region is used to calculate the size and location of defect. The results demonstrate that the proposed method provides reliable identification of defects.     

Structural damping identification based on an iterative regularization method

A time domain structural damping identification method is presented in this paper. The damping of a structure is assumed to be time-invariant Rayleigh damping, time-variant Rayleigh damping and modal damping, respectively, and an iterative regularization method is proposed to identify these three types of damping in turn. A sensitivity approach is adopted in the formulation of the ill-posed inverse problem. A new constraint is imposed on the identified iterative increment as well as on the unknown structural parameters to ensure their physical meaning in the identification process is not lost. The proposed method is verified in numerical studies with a space frame structure and laboratory measurements of accelerations with accurate results.     

金属玻璃温度依赖的拉压屈服不对称研究

通过引入静水应力对自由体积演化的影响, 研究了金属玻璃在不同温度下的拉压屈服行为. 结果表明, 在拉伸和压缩载荷下, 屈服强度均满足(T/T_g)^1/2的温度依赖关系; 同时, 在不同温度下, 材料的压力敏感系数保持为常值0.1. 随着温度的升高, 压力对自由体积的影响逐渐降低, 从而导致材料的拉压屈服不对称性逐渐趋于不显著. 在高温下, 显著的结构弛豫减缓了自由体积增长速率从而抑制材料迅速屈服. 这些结果将有助于更深入的认识金属玻璃屈服及其拉压不对称性的内在机理.     

Modeling material responses by arbitrary Lagrangian Eulerian formulation and adaptive mesh refinement method

In this paper we report an efficient numerical method combining a staggered arbitrary Lagrangian Eulerian (ALE) formulation with the adaptive mesh refinement (AMR) method for materials modeling including elastic–plastic flows, material failure, and fragmentation predictions. Unlike traditional AMR applied on fixed domains, our investigation focuses on the application to moving and deforming meshes resulting from Lagrangian motion. We give details of this numerical method with a capability to simulate elastic–plastic flows and predict material failure and fragmentation, and our main focus of this paper is to create an efficient method which combines ALE and AMR methods to simulate the dynamics of material responses with deformation and failure mechanisms. The interlevel operators and boundary conditions for these problems in AMR meshes have been investigated, and error indicators to locate material deformation and failure regions are studied. The method has been applied on several test problems, and the solutions of the problems obtained with the ALE–AMR method are reported. Parallel performance and software design for the ALE–AMR method are also discussed.     

Effect of energy density on defect evolution in 3D printed Zr-based metallic glasses by selective laser melting

In this study, the defects in 3D printed Zr-based bulk metallic glasses(BMGs) fabricated by selective laser melting(SLM) under different energy densities have been investigated via both experimental and simulation approaches. Different defects, including balling, interlayer pores, open pores and metallurgical pores, are detected in the 3D-printed Zr-based MGs depending on the energy inputs. Balling mainly occurs at a relatively low energy density(E<8.33 J/mm^3) due to the incomplete melting of particles, while interlayer pores and open pores are formed at modest energy densities(E=13.89-16.67 J/mm^3) because of incomplete welding and insufficient filling of molten liquid between layers. Fine metallurgical pores appear on the upper surface at relatively high energy densities(E=20.83-27.78 J/mm^3), which originate from gas escaping from molten pools during rapid solidification of the melt. Computational fluid dynamics(CFD) simulations are carried out to verify the experimental observations. The CFD simulations reveal that the various defects formed in the 3D-printed Zr-based BMG are related to the melt flow behaviours in the molten pools under different energy densities. The present work provides in-depth understandings of defect formation in the SLM process and provides methods for eliminating these defects in order to enhance the mechanical performance of 3D printed BMGs.     

A method for crack sizing using Laser Doppler Vibrometer measurements of Surface Acoustic Waves

The goal of non-destructive testing (NDT) is to determine the position and size of structural defects, in order to measure the quality and evaluate the safety of building materials. Most NDT techniques are rather complex, however, requiring specialized knowledge. In this article, we introduce an experimental method for crack detection that uses Surface Acoustic Waves (SAWs) and optical measurements. The method is tested on a steel beam engraved with slots of known depth. A simple model to determine the cracks size is also proposed. At the end of the article, we describe a possible application: fatigue crack sizing on a damaged slat track. This technique represents a first step toward a better understanding of the crack growth, especially in its early stages (preferably when the cracks can still be repaired) and when it is possible to assume a linear propagation of the crack front. The ultimate goal of this research program is to develop a useful method of monitoring aircraft components during fatigue testing.