60Si2CrVA弹簧钢循环细晶热处理

在盐浴加热条件下对弹簧钢60Si2CrVA 进行了多种循环热处理试验。结果表明:常规热处理后钢的奥氏体晶粒直径约为20μm(8级),采用循环热处理则可晶粒细化至5μm(12级)。在所研究的几种循环热处理工艺中,循环淬火显著地提高强度,但对塑性和韧性影响不大;提出的一种改进的循环热处理使各项机械性能指标得到提高,尤其是显著提高屈服强度和断裂韧性。对热处理后的显微组织进行了分析。指出改进的循环热处理对性能的改善归因于奥氏体品粒的细化和挛晶马氏体量的减少。     

Forming and resolving wedges in the spatial twist continuum

The spatial twist continuum (STC) is a powerful extension of the dual of a hexahedral mesh Murdochet al.,Int. J. Numer. Math. Engng (submitted). The STC captures the global connectivity constraints inherent in hexahedral meshing. Whisker wwaving is an advancing-front type of algorithm based on the STC[Tautges et al., Int. J. Numer. Math. Engng (submitted]. During the whisker weaving algorithm, certain types of degenerate elements calledwedges [Biacker; Myers (1993)Engng with Computers 9, 83–93 arise. This paper describedwedges and how they are formed, and presentscollapsing anddriving, two strategies for removing these degeneracies.     

Ultrafine grain refinement in a low alloy steel

The phenomenon of grain refinement was studied in a steel containing 0.15% C, 0.32% Si, 1.4% Mn, and 0.43% V. Initial austenite grain size was found to be 50 μm, determined by quenching the specimen in an iced brine solution from 1150 °C. Transformational grain refinement (TGR) was applied to give a reasonable refinement in the grain size. A rolling reduction of about 67% was given to specimens at 900 °C, which was followed by air cooling. Cold rolling and recrystallization of these specimens gave refinement of grains down to 1 μm size was obtained. The electron backscattered diffraction (EBSD) technique was used to determine low- and high-angle grain boundaries that are effectively used to determine the substructure contribution at various stages of recrystallization.     

Problem representation for refinement

In this paper we attempt to develop a problem representation technique which enables the decomposition of a problem into subproblems such that their solution in sequence constitutes a strategy for solving the problem. An important issue here is that the subproblems generated should be easier than the main problem. We propose to represent a set of problem states by a statement which is true for all the members of the set. A statement itself is just a set of atomic statements which are binary predicates on state variables. Then, the statement representing the set of goal states can be partitioned into its subsets each of which becomes a subgoal of the resulting strategy. The techniques involved in partitioning a goal into its subgoals are presented with examples.     

Achievements of New Generation Steel Project in China

Major achievements of the national project, Fundamental Research on New Generation of Iron and Steel Materials in China (NG Steel), are reviewed in the paper. Ultrafine grained steel technology, based on deformation induced ferrite transformation (DIFT)and successive microstructure changes, is illustrated for grain refinement in both plain low carbon steel and microalloyed steel. Delayed fracture resistance of alloy structure steel can be improved through prior austenite grain refinement. It is shown by results that nano scale precipitates play an important role to grain refinement in thin slab casting and rolling (TSCR)process. Progresses on super cleanliness, high homogeneity, welding and metallurgical process simulation are also briefly introduced.     

纳米钛酸钡陶瓷的晶体结构

致密的平均尺寸约为30nm钛酸钡陶瓷由压力辅助烧结得到.在变温Raman光谱的基础上,用Rietveld精修方法成功地确定了纳米钛酸钡陶瓷的晶体结构.在室温下,在30nm钛酸钡陶瓷中观测到四方相和正交相的多相共存.这种现象可以用相变产生的内应力来解释.     

Adaptive multiresolution refinement with distance fields

This paper describes a multiresolution approach to field modeling that can be used with any meshfree or mesh‐based method for adaptive solution refinement. The refined solution is represented as a superposition of a coarse (unrefined) solution and a sequence of refinements that provide additional degrees of freedom with higher spatial or functional resolution. Each refinement is treated as a solution to a boundary‐value problem within a specified refinement window. The proposed approach is based on the meshfree method with distance fields (Comput. Mech. 2000; 25 :305–316, Eng. Comput. 2002; 18 (4):295–311) and guarantees C^m continuity of the refined solutions with matching or non‐matching grids. The method does not restrict the shape of the refinement window and does not place any constraints on the type of basis functions, or relative position and resolution of the refinement grids. Combining the proposed approach with hierarchical space decompositions and a posteriori error estimators results in an effective tool for automatic solution refinement. Carefully chosen numerical examples illustrate the power and advantages of the proposed approach. Copyright © 2007 John Wiley & Sons, Ltd.     

Three-dimensional simulation studies on electrostatic predictions for carbon nanotube field effect transistors

Analysis of the electrostatic characteristics and the gate capacitance of typical nanostructured carbon nanotube field effect transistors (CNTFETs) were performed numerically. A previously developed parallelized electrostatic Poisson's equation solver (PPES) is employed, coupled with a parallel adaptive mesh refinement (PAMR) to improve the numerical accuracy near the region where variation of potentials are significant. CNTFETs with four typical configurations of the gate electrode, the bottom gate (BG), the double gate (DG), the top gate (TG), and the surrounding gate (SG) were simulated. Effects of the nanotube arrangement and the gate length on the gate capacitance are presented and discussed. The simulation results show that SG-CNTFET possesses the largest gate capacitance among various structures. However, TG-CNTFET is recommended for practical applications by taking into account both the device performance and the difficulty of fabrication. According to the simulated gate capacitance, estimation of the on-state current of CNTFETs is possible.     

Adaptive Geometrically Balanced Clustering of <Emphasis Type="Italic">H</Emphasis>-Matrices

In [8], a class of (data-sparse) hierarchical (-) matrices is introduced that can be used to efficiently assemble and store stiffness matrices arising in boundary element applications. In this paper, we develop and analyse modifications in the construction of an -matrix that will allow an efficient application to problems involving adaptive mesh refinement. In particular, we present a new clustering algorithm such that, when an -matrix has to be updated due to some adaptive grid refinement, the majority of the previously assembled matrix entries can be kept whereas only a few new entries resulting from the refinement have to be computed. We provide an efficient implementation of the necessary updates and prove for the resulting -matrix that the storage requirements as well as the complexity of the matrix-vector multiplication are almost linear, i.e., AMS Subject Classifications: 65F05, 65F30, 65N38, 65N50.