金属凝固显微组织的计算机模拟

应用计算机数值模拟技术研究金属凝固显微组织的形成规律是材料科学发展的重要前沿领域之一。文章对该领域的最新研究进展作了简要评述；重点介绍了作者发展的一个改进的MCA(modified cellular automaton)模型的特点以及应用该模型在合金凝固组织 数值模拟方面的一些主要工作。     

Microstructural effects on ductile fracture in heterogeneous materials. Part II: Applications to cast aluminum microstructures

This is the second of a two part paper aimed at investigating the effects of microstructural morphology, material properties and loading on rate-dependent ductile fracture of heterogeneous materials. The locally enhanced Voronoi cell finite element method (LE-VCFEM) is used for micromechanical analyses of deformation and failure in complex microstructural volume elements. The first part of this paper sequence evaluates the sensitivity of strain to failure of computer simulated microstructures to loading rate, microstructural morphology and material properties. In this second part, LE-VCFEM simulations of actual microstructures of a cast aluminum alloy micrograph are used to validate a strain to failure model developed in the first part. A method for identification of critical regions within a heterogeneous microstructure is also developed and validated using in-situ observations of a two-point bending test. The influence of applied strain rates on ductile fracture of micrograph-based complex microstructures is also investigated.     

The effect of the skin thickness and spherulite size on the mechanical properties of injection mouldings

In this work are studied the relationships between the microstructure and the mechanical properties of an injection moulded propylene-ethylene copolymer. Distinct microstructures were obtained by processing, through a moulding programme that includes the variation of the injection and the mould temperatures and the injection flow rate. They were characterized by the skin ratio (measured by polarised light microscopy) and the spherulite size (evaluated by small angle light scattering system). Tensile tests were carried out at two different constant loading velocities: 2 mm/min (3.33 × 10^–5 m/s) and 3 m/s, in order to assess the initial modulus, the yield stress, the strain and the energy at break. The results are presented in terms of the relationships between the chosen microstructural parameters and the selected tensile properties. The skin thickness is evidenced as an important microstructural feature. The role of the core spherulite size is secondary or even negligible. The results also show that other microstructural parameters must be considered to establish more general microstructure-properties relationships.     

Application of single crystals to achieve quantitative understanding of weld microstructures

Abstract

The inter-relationship between the weld pool shape and the weld microstructure is a critical factor that determines the physical integrity and other important properties offusion welds. In the present work, large single crystals of an Fe–15Ni–15Cr alloy have been used to increase basic understanding of the factors that influence the development of weld microstructures. Oriented ternary alloy single crystals were used to make electron beam welds along various principal directions lying in different principal crystallographic planes. Using oriented single crystals it was possible to obtain crucial microstructural information that is ordinarily lost when welds are made on normal polycrystalline specimens. This quantitative information regarding the microstructural properties of electron beam welds has provided valuable new insight into the fundamentals of the relationships between weld pool shapes and weld microstructures. A new three-dimensional geometrical analytical method has been developed to interpret the microstructural information resulting from welds made using oriented single crystals. This analytical method establishes a direct correlation between the three-dimensional weld pool shape and the dendritic microstructures that are observed in two-dimensional transverse micrographs, and can be used to reconstruct the three-dimensional weld pool shape. Single crystal multipass and single pass bicrystal welds have also been examined. Overlapping multipass single crystal welds showed remarkable reproducibility from pass to pass and replicated the microstructural patterns observed in single pass welds. The microstructure of butt welds joining two single crystals with different orientations showed a one to one correspondence with that associated with each individual crystallographic orientation, and the microstructure essentially represented a composite of two single pass microstructures corresponding to the individual crystal orientations.

MST/3166     

Image analysis and modeling of spherical and channel microstructures of fuel-cell materials

The development of novel fuel-cell materials demands accurate and flexible microstructure characterization techniques. Conventional electron microscopy-based microstructural morphology analysis is carried out through the conceptual interpretation of transmission electron microscope images. With this method, only qualitative information on material morphologies can usually be obtained. This paper presents a digital image analysis system that deals with the automatic measurement and quantitative characterization of the microstructural morphologies of polymer electrolyte membrane fuel-cell materials. In this approach, two types of essential microstructural morphologies (spheral particles and interconnected graft channels) are modeled based on statistical geometry theory, and the statistical analysis schemes of the microstructural morphologies are designed and applied to the characterization of the phase-separated microstructures in fuel-cell components such as solid electrolyte ionomers, catalyst layers, and gas diffusion layers. Experimental results on real fuel-cell materials specimens demonstrate the effectiveness of the method.     

无机材料显微结构化学

本文综述了为说明无机材料显微结构形成有关的最重要的物理化学因素。根据显微结构图象判读的进一步要求,可首先在样品中对晶相和它们的形态外貌作鉴定,然后对特征区分别用一系列相变方程式来描述。结果,这些论述即构成了显微结构化学的基本内容,从而使显微结构分析的科学意义取得了大的进展。可以予期,显微结构化学将成为材料科学的重要分支。     

Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications

Microstructure plays an important role in strengthening of metallic materials. Various microstructures can be developed in titanium aluminide (TiAl) alloys, which can enable different combinations of properties for various extreme environments in advanced nuclear systems. In the present paper the mechanisms for microstructural strengthening and the effects of various microstructural features on thermal creep and radiation damage resistance of TiAl alloys are reviewed and compared. On the basis of the results, the evidence-based optimum microstructure for the best combination of thermal creep and radiation damage resistance of TiAl alloys is proposed. The heat treatment processes for manufacturing the optimal microstructure are also discussed.     

Traditional Metallurgy,Nanotechnologies, and Structural Materials: A Sorby Award Lecture

Traditional metallurgical processes are among the many “old fashion” practices that use nanoparticles to control the behavior of materials. Many of these practices were developed long before microscopy could resolve nanoscale features, yet the practitioners learned to manipulate and control microstructural elements that they could neither see nor identify. Furthermore, these early practitioners used that control to modify microstructures and develop desired material properties. Centuries old colored glass, ancient high-strength steels and medieval organ pipes derived many of their desirable features through control of nanoparticles in their microstructures. Henry Sorby was among the first to recognize that the properties of rocks, minerals, metals, and organic materials were controlled by microstructure. However, Mr. Sorby was accused of the folly of trying to study mountains with a fsmicroscope. Although he could not resolve nanoscale microstructural features, Mr. Sorby’s observations revolutionized the study of materials. The importance of nanoscale microstructural elements should be emphasized, however, because the present foundation for structural materials was built by manipulating those features. That foundation currently supports several multibillion dollar industries but is not generally considered when the nanomaterials revolution is discussed. This lecture demonstrates that using nanotechnologies to control the behavior of metallic materials is almost as old as the practice of metallurgy and that many of the emergent nanomaterials technologists are walking along pathways previously paved by traditional metallurgists. This article was presented at "Microscopy & Microanalysis 2007" in Fort Lauderdale, Fla., on Auguest 6, 2007.     

Effects of Nickel Distribution on the Strengthening and Toughening of Alumina Ceramics

Three types of composite materials were designed and fabricated by hot pressing powder blends of alumina with 20 vol. pct nickel particles. The composites differ in the shape, size and distribution of the nickel particles. Composite microstructures are described and measurements of density, hardness, flexure strength, and fracture toughness are reported. The results showed that the fracture strength of the composite with dispersed nickel particles is higher than the other two composites （network microstructure and mixed microstructure） and the alumina matrix. For all the composites studied, tougher materials than the monolithic alumina were produced. The fracture toughness of the composite with a network microstructure is much higher than that of the other composites. The toughening mechanisms were described based on the observation of the fracture surfaces and the crack-particle interactions. Moreover, the parameters for microstructural tailoring of these materials have been deduced. The toughening of the produced composites was explained in light of the interracial bond strength.     

A method for modelling microstructural material discontinuities in a finite element analysis

A new method is proposed for studying the effects of various microstructural material discontinuities, within a body, in a finite element analysis. The material discontinuities are accounted for by introducing a transformation strain in those regions. This formulation leads to two matrix equations; the first corresponds to the finite element analysis of the body without material discontinuities, and the second accounts for the microstructure. An important feature of the new method is that the first equation is solved only once, then the second equation can be solved repetitively for different microstructures. Thus, it is possible to study the effect of different microstructures within the body without reanalysing the entire body. It is expected that this method will be particularly useful in materials research to study the mechanisms that occur in materials at the microstructural level. The transformation strain formulation is reviewed, and the matrix equations for the new method are derived. Several numerical examples are presented to illustrate the versatility of the new method.     

Diffuse ultrasonic backscatter in a two-dimensional domain

The scattering of elastic waves in polycrystalline materials is relevant for ultrasonic materials characterization and nondestructive evaluation (NDE). Diffuse ultrasonic backscatter measurements are used widely to extract the microstructural parameters such as grain size and also to detect flaws in materials. Accurate interpretation of experimental data requires robust scattering models. Line transducers are often used for ultrasonic experiments such that an appropriate model for these two-dimensional problems is needed. Here, a theoretical expression for the temporal diffuse backscatter is derived for such domains under a single-scattering assumption. The result is given in terms of transducer and microstructural parameters. In addition, the problem is examined in terms of numerical simulations using Voronoi polycrystals that are discretized using finite elements in a plane-strain formulation. The material properties of the individual Voronoi cells are chosen according to appropriate material distributions. Such numerical models also allow scattering theories, including the one discussed here, to be examined for well-controlled microstructures. Example numerical results for materials with varying degrees of scattering that are of common interest are presented. The numerical results are compared with the theory developed with good agreement. These results are anticipated to impact ultrasonic NDE of polycrystalline media.     

Microstructural aspects of Thermal Stress Resistance of high-strength engineering ceramics. Part I: Data of thermal stress resistance of high-strength engineering ceramics

Based on the criteria for the improvement of thermal shock resistance mainly two microstructural aspects of thermal stress resistance are discussed: First, the influence of microstructure on thermal shock resistance to fracture initiation, and second, the improvement of thermal shock resistance on the basis of microstructural considerations. In this connection, data of thermal stress resistance (thermal shock and thermal cycling) of various engineering ceramic materials are presented. Using laboratory grades with well-defined microstructures the interdependence between various microstructural variables and the mechanical and thermal properties, which control the thermal shock resistance, is discussed and the relation to thermal shock resistance is outlined by the example of the two materials, dense and porous reaction-bonded Si₃N₄. Moreover, the improvement of thermal shock resistance by microstructural optimization is demonstrated. Some examples of the improvement of thermal stress resistance by developing advanced composite materials are given. The paper is divided into three parts: Part I: Data of Thermal Stress Resistance of High-Strength Engineering Ceramics Part II: Influence of Microstructure on Thermal Shock Resistance of High-Strength Engineering Ceramics Part III: Improvement of Thermal Stress Resistance of High-Strength Engineering Ceramics.     

Multiscale dynamic fracture analysis of composite materials using adaptive microstructure modeling

In this study, a computational framework is proposed to investigate multiscale dynamic fracture phenomena in materials with microstructures. The micro- and macro-scales of a composite material are integrated by introducing an adaptive microstructure representation. Then, the far and local fields are simultaneously computed using the equation of motion, which satisfies the boundary conditions between the two fields. Cohesive surface elements are dynamically inserted where and when needed, and the Park-Paulino-Roesler cohesive model is employed to approximate nonlinear fracture processes in a local field. A topology-based data structure is utilized to efficiently handle adjacency information during mesh modification events. The efficiency and validity of the proposed computational framework are demonstrated by checking the energy balances and comparing the results of the proposed computation with direct computations. Furthermore, the effects of microstructural properties, such as interfacial bonding strength and unit cell arrangement, on the dynamic fracture behavior are investigated. The computational results demonstrate that local crack patterns depend on the combination of microstructural properties such as unit cell arrangement and interfacial bonding strength; therefore, the microstructure of a material should be carefully considered for dynamic cohesive fracture investigations.     

ZnO tetrapod materials for functional applications

In the last 15?years, more than 50,000 papers with zinc oxide (ZnO) in the title are listed within ISI database. The outstanding popularity of ZnO has many reasons; the most important one appears to be its multi-functionality, resulting in applications in physics, chemistry, electrical engineering, material science, energy, textile, rubber, additive manufacturing, cosmetics, and pharmaceutical or medicine, as well as the ease to grow all kinds of nano- and microstructures. A key structure is the tetrapod-shaped ZnO (T-ZnO), which we want to focus on in this mini-review to demonstrate the remarkable properties and multifunctionality of ZnO and motivate why even much more research and applications are likely to come in near future. As T-ZnO came into focus again mainly during the last 10?years, the big data problem in T-ZnO is not as severe as in ZnO; nevertheless, a complete overview is impossible. However, this brief T-ZnO overview attempts to cover the scopes toward advanced technologies; nanoelectronics/optoelectronics sensing devices; multifunctional composites/coatings; novel biomedical engineering materials; versatile energy harvesting candidates; and unique structures for applications in chemistry, cosmetics, pharmaceuticals, food, agriculture, engineering technologies, and many others. The 3D nanotechnology is a current mainstream in materials science/nanotechnology research, and T-ZnO contributes to this field by its simple synthesis of porous networks as sacrificial templates for any desired new cellular materials.     

Strength and Ductility of Bi‐Modal Cu

Engineering a microstructure with multiple length scales has been proposed as a strategy to enhance the plasticity of nanostructured materials which otherwise lack extensive dislocation activity, and therefore low ductility. To that effect, various research groups have implemented this concept by promoting the formation of so called bi‐modal microstructures (e.g., consisting of a mixture of ultrafine and micro‐grains) with balanced combinations of strength and ductility. Despite encouraging results, fundamental, information on important questions remained unanswered. For example, what is the relationship between the volume fraction of the coarse grained phase and the overall ductility? What is the influence of size of the coarse grained phase, and how does its distribution influence ductility? To provide insight into these, and other related questions, in this work, we prepared bimodal Cu with homogeneous distribution of different‐volume‐fraction micro‐grains via isothermal recrystallization of an ultrafine grained Cu matrix at 200 °C. Analysis of the tensile results and microstructural characterization suggest that both strength and ductility of the bi‐modal Cu follows the rule‐of‐mixtures, with interesting results related to volume fraction. Our work provides a pathway for optimizing the mechanical properties of multiscale materials.     

Imaging and microanalysis of liquid phase sintered silicon-based ceramic microstructures

This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si₃N₄ or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure.     

超细晶超高碳钢的研究现状及展望

超细晶超高碳钢是国外近年来发展起来的一种新型的、并具有重要发展前景的高性能钢铁材料。在系统总结超细晶超高碳钢的化学成分设计、制备工艺、室温组织性能及超塑性等各方面研究现状的基础上，对超细晶超高碳钢的发展提出展望。