Stacking fault energies and slip in nanocrystalline metals

The search for deformation mechanisms in nanocrystalline metals has profited from the use of molecular dynamics calculations. These simulations have revealed two possible mechanisms; grain boundary accommodation, and intragranular slip involving dislocation emission and absorption at grain boundaries. But the precise nature of the slip mechanism is the subject of considerable debate, and the limitations of the simulation technique need to be taken into consideration. Here we show, using molecular dynamics simulations, that the nature of slip in nanocrystalline metals cannot be described in terms of the absolute value of the stacking fault energy-a correct interpretation requires the generalized stacking fault energy curve, involving both stable and unstable stacking fault energies. The molecular dynamics technique does not at present allow for the determination of rate-limiting processes, so the use of our calculations in the interpretation of experiments has to be undertaken with care.     

Ni-Al-Re/Ru合金的层错能

使用置换原子计算层错能的热力学模型计算了Ni-Al-Re(Ru)合金的层错能,研究了合金元素和温度对层错能的影响.结果表明:随着温度的提高,Ni-Al-Re(Ru)合金的层错能增加.Ni-6?-4%Re合金的层错能随着温度的提高线性增加.Ni-6%Al-4%Ru合金的层错能,在温度低于500℃时随着温度的提高呈抛物线规律增加,高于500℃时随着温度的提高呈线性规律增加.Al原子可明显降低Ni-6%Al-4%Re(Ru)合金的层错能.随着Al含量的提高,原子偏聚自由能(ΔGγ→εs)降低,使Al原子自发偏聚,并促进γ'有序相的形成和数量的增加,是合金层错能降低的主要原因.而Ru原子降低合金的偏聚自由能,可提高γ'有序相的稳定性.随着温度的升高,原子偏聚引起的自由能(ΔGγ→εs)增加可抑制原子偏聚;当温度高于500℃时,含Ru合金有比Ni-Al-Re合金高的原子偏聚自由能(ΔGγ→εs),故元素Ru能抑制原子的偏聚和TCP相的析出.     

Stacking fault directed growth of thin ZnO nanobelt

Thin ZnO nanobelts with an average width of 7.5 nm have been synthesized using vapor-phase transport method. It was found that stacking faults directed the growth of the thin nanobelts along the <01 1¯0> direction with {2¯ 1¯10} top/bottom surfaces and {0001} side surfaces. The {0002} stacking fault with translation of 1/3<01 1¯0> extends throughout the entire length of the ZnO nanobelts. The growth steps at the {01 1¯0} growth fronts resulted from the {0002} stacking fault are believed to direct fast axial growth of the thin ZnO nanobelts. The thin ZnO nanobelts are expected to be promising candidates for highly sensitive chemical and biological sensor applications.     

Structural transformations of stacking fault tetrahedra upon the absorption of point defects

Mechanisms of the structural modification of stacking fault tetrahedra (SFTs) upon the absorption of point defects have been studied by the method of molecular dynamics. The sequential absorption of vacancies by a perfect SFT is accompanied by the following transformations: (i) the formation of a step on one of the SFT faces, (ii) a change in the step sign upon reaching the middle of the face, (iii) the formation of an SFT with truncated vertex, and (iv) the formation of the perfect SFT. Upon the absorption of interstitial atoms, the stages of SFT transformation follow the reverse order.     

Electron microscope weak-beam imaging of stacking fault tetrahedra: observations and simulations

Weak-beam diffraction-contrast electron microscope images of stacking-fault tetrahedra (SFT) have been simulated by solving numerically the Howie–Basinski equations, which are well suited for studying the dependence of image contrast on experimental parameters. These simulated images are in good qualitative agreement with experimental transmission electron micrographs. The visibility of small SFT and the relationship between measured image sizes and real SFT sizes are discussed.

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Stacking fault energy and planar defects on the basal plane in the beta-alumina system

Commercial forms of beta alumina have been examined using transmission electron microscopy. Measurements of the stacking fault energy on the basal plane have been made from the separation of partial dislocations and the equilibrium separation of partials at dislocation nodes, and estimated as 0.6 to 1.65×10^–3 Jm^–2. The structure of two- and three-block beta-alumina has been discussed and their relationship and transformation examined. It has been shown that transformation from two- to three-block beta-alumina cannot be accomplished by a simple shear. Structures generated by the passage of partial dislocations on the glide plane are discussed, and simple twining on the basal plane is examined and shown to be possible in the three-block, but not in the two-block material.     

Filling space with tetrahedra

In the context of 3D finite element meshes various options for filling an indefinite space (such as would be approached within a fine mesh) with tetrahedra are considered. This problem is not trivial as it is in 2‐D since, unlike equilateral triangles, regular tetrahedra cannot be fitted together to fill space. Various groupings, or assemblies, which can be repeated indefinitely to fill space are considered. By altering the shape of the tetrahedra in one of these to minimize a suitable function a unique shape of tetrahedron is obtained which optimizes the conditioning. The mesh thus produced is shown to be better conditioned than alternatives based on assemblies of different shaped tetrahedra. A number of conditioning measures are used to confirm this. Finally, actual meshes which fit boundaries are briefly considered. Copyright © 1999 John Wiley & Sons, Ltd.     

High-order cubature rules for tetrahedra

In this paper, we construct new high-order numerical integration schemes for tetrahedra, with positive weights and integration points that are in the interior of the domain. The construction of cubature rules is a challenging problem, which requires the solution of strongly nonlinear algebraic (moment) equations with side conditions given by affine inequality constraints. We present a robust algorithm based on a sequence of three modified Newton procedures to solve the constrained minimization problem. In the literature, numerical integration rules for the tetrahedron are available up to order p=15. We obtain integration rules for the tetrahedron from p=2 to p=20, which are computed using multiprecision arithmetic. For p≤15, our approach provides integration rules that have the same or fewer number of integration points than existing rules; for p=16 to p=20, our rules are new. Numerical tests are presented that verify the polynomial-precision of the cubature rules. Convergence studies are performed for the integration of exponential, rational, weakly singular and trigonometric test functions over tetrahedra with flat and curved faces. In all tests, improvements in accuracy is realized as p is increased, though in some cases nonmonotonic convergence is observed.     

电化铝烫印的若干故障现象分析与处理

电化铝烫印是包装印刷常用的生产工艺.但是,如果烫印质量出现问题,那么,产品质量和工艺效果将大打折扣.所以,认真把好烫印工艺操作和技术关,是生产中质量控制的重要一环.     

Universal finite element matrices for tetrahedra

Methods are described for forming element matrices for a wide variety of operators on tetrahedral finite elements, in a manner similar to that previously employed for line segments and triangles. This technique models the differentiation and product-embedding operators as rectangular matrices, and produces finite element matrices by replacing all required analytic operations by their finite matrix analogues. The method is illustrated by deriving the conventional matrix representation for Laplace's equation. Computer programs are available, which generate universal finite element matrices for use in various applications.     

Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition

Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m⁻² at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations.     

Triangles and tetrahedra in explicit dynamic codes for solids

Explicit dynamic codes which are used currently for the study of plastic deformations in impact, or with some modification for metal forming, suffer two serious limitations. First, only quadrilateral or hexahedral linear elements can be used thus limiting the possibilities of adaptive refinement and adaptive meshing. Second, even with the use of such elements, special devices such as reduced integration must be introduced to avoid locking and reduce costs. These necessitate complex hour glass control, mending-type procedures. The main difficulties are those due to the need of treatment of (almost) incompressible deformation modes. Recently, similar difficulties have been overcome in the context of fluid dynamics soil dynamics and we show here how the processes introduced there can be adopted effectively to the present problem, thus allowing an almost unrestricted choice of element interpolations. © 1998 John Wiley & Sons, Ltd.     

工程图学在堆垛方式与堆垛强度研究中的应用

指出了堆垛强度的计算方法，阐述了堆垛方式对堆垛强度的影响，得出了齐平式堆压强度最高，而交错式堆垛稳定性最好的结论，计算机辅助设计使计算更准确、合理。     

堆垛方式对堆垛强度的影响

对瓦楞纸箱包装的各种堆垛方式进行受力分析，阐述堆垛方式对堆垛强度的影响，得出纵横交错式堆垛的耐压强度最低，而齐平式堆垛其耐压强度最高的结论。     

High-order symmetric cubature rules for tetrahedra and pyramids

In this article, we present an algorithm to construct high-order fully symmetric cubature rules for tetrahedral and pyramidal elements, with positive weights and integration points that are in the interior of the domain. Cubature rules are fully symmetric if they are invariant to affine transformations of the domain. We divide the integration points into symmetry orbits where each orbit contains all the points generated by the permutation stars. These relations are represented by equality constraints. The construction of symmetric cubature rules require the solution of nonlinear polynomial equations with both inequality and equality constraints. For higher orders, we use an algorithm that consists of five sequential phases to produce the cubature rules. In the literature, symmetric numerical integration rules are available for the tetrahedron for orders p = 1 –10, 14 , and for the pyramid up to p = 10 . We have obtained fully symmetric cubature rules for both of these elements up to order p = 20 . Numerical tests are presented that verify the polynomial-precision of the cubature rules. Convergence studies are performed for the integration of exponential, weakly singular, and trigonometric test functions over both elements with flat and curved faces. With increase in p, improvements in accuracy is realized, though nonmonotonic convergence is observed.     

镁合金中的长周期有序堆垛结构

本文详细阐述了镁合金中长周期有序堆剁相的最新研究进展，对镁合金中长周期有序堆剁相的结构进行了系统的论述，展望了镁合金中的长周期有序堆剁结构的研究趋势。     

直拉硅中氧化诱生层错研究进展

硅中的氧化诱生层错（OSF）是一种重要的工艺诱生缺陷,会对微电子器件产生重大的影响,为提高硅片质量,要求对OSF有充分了解。综述了氧化诱生层错的研究进展,着重讨论了OSF的形成动力学、影响因素和检测方法,并指出：OSF是一种由点缺陷行为决定的工艺诱生缺陷。