AZ91镁合金的凝固冷却速度与二次枝晶间距的定量关系研究

通过设计制作一套可获得不同凝固冷却速度的镁合金熔炼与浇注装置,采用多通道连续温度记录仪,获得了对应不同凝固冷却速度下的AZ91镁合金试样.用定量金相分析之截线法测定了各个试样的二次枝晶间距,并对不同凝固冷却速度下的组织特征做了简要分析.用数学回归的方法得到了AZ91镁合金的凝固冷却速度与二次枝晶间距的定量关系式.     

Complex procedure for the quantitative description of an Al–Si cast alloy microstructure

The paper describes a complex procedure applied in the quantitative analysis of a dendritic microstructure of an Al–Si alloy together with a statistical analysis of the results obtained. Result distributions are compared between individual specimens, and it is found that certain result populaces show significant differentiation in terms of the distribution of results, even when obtained from different parts of the same cast specimen.     

冷却速度对Sn-Ag无铅焊料微观组织和机械性能的影响

研究了不同冷却速度下无铅焊料Sn-3．5％(质量分数)Ag合金的微观形貌(冷却速度从0．08K／s到10^4K／s)。结果表明焊料合金中二次枝晶间距随冷却速度增加而逐渐减小，且符合公式：d=atf^n，其中d为二次枝晶间距，tf是冷却时间，a和n是由材料和其成分所决定的常数，通过计算得到对于Sn-3．5％(质量分数)Ag合金其a为3．7，而n为0．43。维氏硬度测试结果表明：快速冷却条件能使焊料合金晶粒细化，其中作为强化相的金属间化合物Ag3Sn分布更加细密，从而能使整个合金机械性能得到提高。     

铝合金均匀化扩散动力学研究

用扫描电子显微镜、电子探针微区分析研究了电磁半连续铸造Al-Zn-Mg-Cu合金铸态和均匀化态的成分分布,Cu、Zn和Mg元素存在晶界偏析,偏聚比分别为19.2、2.6、1.85.研究了铸锭均匀化动力学,导出了均匀化动力学方程,根据实验及均匀化动力学计算,最佳均匀化处理工艺为450.C,24h.     

Influence of a weak static magnetic field on the primary dendrite arm spacing of a directionally solidified Ni-based superalloy

Magnetic field applied in the solidification can influence the primary dendrite arm spacing (PDAS) of alloys. The effect of a weak static magnetic field on the PDAS of Ni-based superalloy DZ417G with different dimension is studied in this paper. The results show that the PDAS for directionally solidified superalloy DZ417G under the weak static magnetic field is significantly decreased. With increasing the intensity of magnetic field from 0 to 0.05 T, the PDAS is decreased from 0.3 mm to 0.23 mm and from 0.61 mm to 0.48 mm for the ingots with 4 mm and 10 mm in the diameter, respectively. When the intensity of magnetic field is larger than 0.05 T, the PDAS is slightly increased. The influence of the interactive competition between the magnetic damping and thermoelectromagnetic convection caused by magnetic field on the PDAS for superalloy DZ417G is discussed.     

凝固方向对单晶高温合金枝晶组织的影响

用籽晶法制备了沿不同晶体取向凝同的镍基单晶高温合金试样,研究了单晶中枝晶形貌和一次枝晶臂距随凝固取向的变化规律.结果表明:凝同方向偏离[001]取向小于15°时,枝晶排列比较规则,一次枝晶臂距随偏离角度增大而减小;偏离角度为25°时,部分二次枝晶臂阻断了相邻一次枝晶干的生长,导致一次枝晶臂距增大.沿[011]和[111...     

Effect of solidification cooling rate on the fatigue life of A356.2-T6 cast aluminium alloy

The effect of the cooling rate during solidification on the fatigue life of a cast aluminium alloy (A356.2-T6) is examined. The fatigue lives were determined for specimens removed from ingots with a gradient in cooling rates along their heights. Low- and high-cycle fatigue tests were conducted under both axial loading and reciprocating-bending conditions at a stress (strain) ratio ( R ) of −1.0, 0.1 and 0.2. Results show that the fatigue life decreases by a factor of three in low-cycle fatigue ( R = −1.0) and by a factor of 100 in high-cycle fatigue ( R = 0.1) as solidification cooling rate decreases from ~10 to ~0.3 K s⁻¹ , as indicated by measurements of the secondary dendrite arm spacings in the ingots. Fatigue cracks initiated from porosity in the material solidified at slower cooling rates. When pore size is below a critical size of ~80 μm, as a result of increasing the cooling rate, the fatigue cracks initiated from near-surface eutectic-microconstituent. When present at or near the surface, large oxide inclusions initiated fatigue cracks.     

一次枝晶间距对DD26单晶高温合金持久各向异性的影响

研究了取向对高速冷却法(DD26-HRS)和液态金属冷却法(DD26-LMC)制备的DD26单晶高温合金760℃/670 MPa条件下持久性能的影响。结果表明,DD26-HRS合金一次枝晶间距明显大于DD26-LMC合金,而且DD26-HRS合金持久各向异性更为严重。位错形貌显示靠近[001]-[012]边界试样变形以基体内多滑移为主。由于DD26-HRS合金γ′相尺寸较大,抑制了层错带剪切γ′相的变形机制,从而使得DD26-HRS合金持久寿命高于LMC合金。然而,靠近[001]-[112]边界试样变形以单方向层错剪切γ′相为主,基体内开动的位错较少,DD26-HRS合金枝晶间粗大的碳化物相周围易于位错的堆积,从而萌生裂纹并沿碳化物界面迅速开裂,造成DD26-HRS合金持久寿命低于DD26-LMC合金。     

Topology optimization of creeping fluid flows using a Darcy–Stokes finite element

A new methodology is proposed for the topology optimization of fluid in Stokes flow. The binary design variable and no‐slip condition along the solid–fluid interface are regularized to allow for the use of continuous mathematical programming techniques. The regularization is achieved by treating the solid phase of the topology as a porous medium with flow governed by Darcy's law. Fluid flow throughout the design domain is then expressed as a single system of equations created by combining and scaling the Stokes and Darcy equations. The mixed formulation of the new Darcy–Stokes system is solved numerically using existing stabilized finite element methods for the individual flow problems. Convergence to the no‐slip condition is demonstrated by assigning a low permeability to solid phase and results suggest that auxiliary boundary conditions along the solid–fluid interface are not needed. The optimization objective considered is to minimize dissipated power and the technique is used to solve examples previously examined in literature. The advantages of the Darcy–Stokes approach include that it uses existing stabilization techniques to solve the finite element problem, it produces 0–1 (void–solid) topologies (i.e. there are no regions of artificial material), and that it can potentially be used to optimize the layout of a microscopically porous material. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis on fluid permeability of dendritic mushy zone during peritectic solidification in a temperature gradient

Compared with the growing applications of peritectic alloys,none research on the fluid permeability K of dendritic network during peritectic solidification has been reported before.The fluid permeability K of dendritic network in the mushy zone during directional solidification of Sn-Ni peritectic alloy was investigated in this study.Examination on the experimental results demonstrates that both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G-T) effects have obvious influences on the morphology of dendritic network during directional solidification.This is realized through different stages of liquid diffusion within dendritic mushy zone by these effects during directional solidification.The TGZM effect is demonstrated to play a more important role as compared with the G-T effect during directional solidification.Besides,it is shown that the evolution of dendrite network is more complex during peritectic solidification due to the involvement of the peritectic phase.Through the specific surface Sv,analytical expression based on the Carman-Kozeny model was proposed to analyze the fluid permeability of dendritic mushy zone in directionally solidified peritectic alloys.In addition,it is interesting to find a rise in permeability K after peritectic reaction in both theoretical predication and experimental results,which is different from that in other alloys.The theoretical predictions show that this rise in fluid permeability K after pedtectic reaction is caused by the remelting/resolidification process on dendritic structure by the TGZM and G-T effects during peritectic solidification.     

On the optimal design of radial basis function neural networks for the analysis of nonlinear stochastic systems

In this paper, an iterative selection strategy of Gaussian neurons for radial basis function neural networks (RBFNN) is proposed when the RBFNN method is applied to obtain the steady-state solution of the Fokker–Planck–Kolmogorov (FPK) equation. A performance index is introduced to rank neurons. Top rank neurons are selected, leading to a RBFNN with optimal number and locations of Gaussian neurons for the FPK equation under consideration. The statistical properties of the performance index are studied. It is found that the index assigned to the $j$th neuron is proportional to the probability of the system falling into the small neighborhood of the mean of this neuron as well as proportional to the weight of the neuron. The RBFNN method with the optimally selected neurons is then applied to several challenging examples of nonlinear stochastic systems in 2, 3 and 4 dimensional state space. The RBFNN solutions are also compared with the results of extensive Monte Carlo simulations. It is observed that the RBFNN method with optimally selected neurons by the proposed iterative algorithm is much more efficient than the RBFNN method with uniformly distributed neurons, and is very accurate in terms of the root mean squared (RMS) errors of the FPK equation or the RMS errors of the PDF solution compared with simulation results.     

Predictions of tumour morphological stability and evaluation against experimental observations

The hallmark of malignant tumours is their spread into neighbouring tissue and metastasis to distant organs, which can lead to life threatening consequences. One of the defining characteristics of aggressive tumours is an unstable morphology, including the formation of invasive fingers and protrusions observed both in vitro and in vivo. In spite of extensive biological, clinical and modelling study and research at physical scales ranging from the molecular to the tissue, the driving dynamics of tumour invasiveness are not completely understood, partly because it is challenging to observe and study cancer as a multi-scale system. Mathematical modelling has been applied to provide further insights into these complex invasive and metastatic behaviours. Modelling a solid tumour as an incompressible fluid, we consider three possible constitutive relations to describe tumour growth, namely Darcy''s law, Stokes'' law and the combined Darcy–Stokes law. We study the tumour morphological stability described by each model and evaluate the consistency between theoretical model predictions and experimental data from in vitro three-dimensional multicellular tumour spheroids. The analysis reveals that the Stokes model is the most consistent with the experimental observations, and that it predicts our experimental tumour growth is marginally stable. We further show that it is feasible to extract parameter values from a limited set of data and create a self-consistent modelling framework that can be extended to the multi-scale study of cancer.     

Evolution of the most common English words and phrases over the centuries

By determining the most common English words and phrases since the beginning of the sixteenth century, we obtain a unique large-scale view of the evolution of written text. We find that the most common words and phrases in any given year had a much shorter popularity lifespan in the sixteenth century than they had in the twentieth century. By measuring how their usage propagated across the years, we show that for the past two centuries, the process has been governed by linear preferential attachment. Along with the steady growth of the English lexicon, this provides an empirical explanation for the ubiquity of Zipf''s law in language statistics and confirms that writing, although undoubtedly an expression of art and skill, is not immune to the same influences of self-organization that are known to regulate processes as diverse as the making of new friends and World Wide Web growth.     

Microstructure,phase evolution and corrosion behaviour of the Zn–Al–Mg–Sb alloy coating on steel

ABSTRACT

In the present work, the influence of antimony (Sb) addition in Zn–Al–Mg alloy on the microstructure, phase characteristic, solidification behaviour and corrosion resistance of hot dipped Zn–0.5Al–0.5Mg–xSb (x?=?0, 0.1, 0.3 and 0.5?wt-%) coated steel wires were evaluated. Thermal analysis revealed that cooling rate of the liquid metal using the steel mould (5.3°C?s^–1) was higher than using ceramic mould (0.3°C?s^–1). Based on the phase analysis and verified by thermodynamic calculations, it was revealed that Zn₁₁Mg₂ and Zn₂Mg phases appeared for Zn–Al–Mg alloy at slow and fast cooling rates while, the Mg₃Sb₂ phase was observed after addition of Sb at both cooling rates. Corrosion behaviour of the alloys determined through electrochemical measurements shows that Zn–Al–Mg alloy with 0.3?wt-%Sb has the lowest corrosion rate indicating an excellent corrosion resistance.     

On the fracture pressure of liquid metals

Fracture pressure of six liquid metals, Pb, Al, Cu, Ni, Ti and Fe have been calculated at their melting points in three different ways; (i) by extrapolating molecular dynamic (MD) data reported in the literature for various temperatures to their melting points, (ii) by using the fracture pressure equation developed by Fisher and (iii) by using the work of nucleation suggested by Fletcher. Results have shown that the Fisher equation and MDs estimates agree closely, whereas the ones based on the work of nucleation are systematically lower than the other two. In all cases, calculated fracture pressures are several orders of magnitude different from those assumed in the literature, emphasizing the extent of weakening by extrinsic factors in liquid metals.     

The Role of Macrostructural and Microstructural Morphologies on the Corrosion Resistance of Zn and a Zn-4% Al Alloy

Different structural morphologies may develop due to a wide range of the operational conditions that may exist during casting. It is well known that corrosion resistance and mechanical properties depend on solidification structures. The aim of this study is to investigate both the influence of columnar and equiaxed structures of zinc as-cast samples and of dendritic microstructural array of a Zn-Al alloy on the corrosion resistance. In order to obtain columnar and equiaxed structures, both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed using electrochemical impedance spectroscopy and Tafel extrapolation. Corrosion tests were conducted in a 3% (vol.) NaCl solution at room temperature. It was found that coarser macrostructures tend to provide higher corrosion resistance for both columnar and equiaxed morphologies. On the other hand, finer secondary dendrite arm spacing is conducive to a better corrosion behavior of a Zn-4% Al alloy.     

复合材料液体模塑成型工艺中预成型体渗透率张量的数值预测

结合均匀化理论和计算流体动力学技术, 实现了对复合材料液体模塑工艺中预成型体渗透率张量的预测。首先, 采用均匀化理论分析了流体在多孔介质内的流动问题, 推导出广义达西定律, 证明以施加周期性边界条件的单胞为研究对象, 可以预测预成型体的渗透率张量, 并以单向纤维织物为例, 对该方法进行了验证。对于复杂结构的预成型体, 渗透率的预测分为两步, 首先分别确定预成型体中流道和纤维束的渗透率, 然后计算其整体宏观渗透率。对于二维平面织物, 该方法与其他预测方法及实验的结果吻合较好。本文还考察了单胞的微观结构对渗透率的影响, 微观结构相似的预成型体如果孔隙率相同, 但束间流道的结构不同, 其整体宏观渗透率也存在很大差别。      

Numerical solution of the anisotropic Poisson equation for SiC semiconductors device simulation

Classically, in semiconductor simulation programs it is assumed that the semiconductor material is isotropic, i.e. dielectric constant, transport parameters such as carrier mobility have no direction dependence. In this work, we propose a numerical study for the Poisson's equation, addressing the anisotropic properties for the hexagonal silicon carbide (SiC) polytype. The implementation uses a vectorized code. Investigation has been conducted for the 4H- and 6H-SiC. Our results have been compared with TCAD-dessis which enables anisotropic simulation, results have shown good agreement.     

纸质玩具中BPA和BPS化合物的迁移

目的 研究纸质玩具中双酚A(BPA)和双酚S(BPS)经口暴露途径下的迁移规律,并考察BPA和BPS迁移的适用性.方法 采用间接酶联免疫(IC-ELISA)检测方法和Fick第二定律研究不同温度(4,37,60℃)、盐浓度(0.001,0.01,0.1 mol/L)、pH(4,7,9)条件下双酚类物质经口暴露下的迁移规律,模拟纸质玩具中BPA和BPS的迁移过程.结果 双酚类物质迁移量与温度和盐离子浓度呈正相关,且在酸性和碱性条件的模拟液中溶出增加,纸质玩具中BPA和BPS经口暴露途径下的迁移规律符合Fick第二定律.正常迁移模拟条件下(37℃、盐浓度0.01 mol/L、pH=7的唾液模拟液)测得BPA和BPS的迁移系数分别为1.3×10-17,6.6×10-18 cm2/s.结论 基于酶免疫检测方法,构建了纸质玩具中BPA和BPS的迁移模型,并成功应用于预测纸质玩具中双酚类化合物的迁移情况,根据纸质玩具中双酚类物质的迁移模拟实验,计算双酚类物质在不同条件下的扩散系数,并拟合迁移模型,预测盲样中BPA和BPS的迁移情况,与实际迁移情况对比,结果表明理论预测值与实际迁移量基本一致,此项工作对于儿童纸质玩具中双酚类物质的风险性评估具有重要意义.     

Small planar travelling waves in two-dimensional networks of coupled oscillators

The existence of several small planar travelling waves with arbitrary direction of propagation is shown for two-dimensional cubic networks of oscillators with linear nearest-neighbour coupling. The analysis is based on a spatial dynamics reformulation of the relevant advance–delay equation. Whereas important aspects of the analysis are discontinuous in the direction of travel, and continuity of the travelling wave is shown with respect to the direction.