Fabrication and Characterization of Functionally Graded Al/SiCp Composites Produced by Remelting and Sedimentation Process

A new process termed here as remelting and sedimentation (RAS) was developed to produce functionally graded Al/SiC composites with a smooth concentration gradient of SiC particles along the height of samples, as opposed to a step change. For this purpose, first settling velocities of different-sized SiC particles in aluminum A356 melt were measured, and the results exhibited a reasonably good agreement with those predicted via the modified Stokes law. Then slices of particulate Al/SiC composites with different SiC contents of 5, 10, 15, and 20 vol.% were stacked in a cast iron mold and heated at 650 °C resulting in remelting and unification of the different composite parts. Considering the preliminary settling experiments, the composite slurry was held at this temperature for three different times to investigate the optimum holding time for obtaining a smooth gradient of SiC concentration along the height of the sample. After quenching, the samples were sectioned and subjected to metallographic studies and hardness measurements. The results confirmed that holding the melt for 60 s provides sufficient settling and redistribution of SiC particles and results in successful production of a functionally graded material.     

基于DSMC-CFD方法的气固两相流冲蚀预测研究

目的 研究气固两相流中固体颗粒间碰撞对冲蚀的影响。方法 使用Eulerian-Lagrangian方法,将气相作为连续相,通过Navier-Stokes方程求解,颗粒平移运动由离散相模型（DPM）求解。颗粒间碰撞运动采用直接模拟蒙特卡罗（DSMC）方法进行模拟,用少量采样颗粒代替真实颗粒计算颗粒间碰撞,碰撞的发生条件通过修正的Nanbu方法判定,碰撞过程遵循颗粒间碰撞动力学模型,采用Grant-Tabakoff随机颗粒-壁面碰撞反弹模型,计算颗粒与壁面的碰撞运动。将颗粒运动信息导入5种不同的冲蚀模型,并将计算与未计算颗粒间碰撞的冲蚀预测模拟结果与实验数据进行对比。结果 颗粒间碰撞位置主要分布在90°弯头外拱侧的颗粒高浓度区,随着颗粒质量流量的增大,颗粒碰撞次数增加,且直管段中碰撞次数占比增大。随着入口速度的增大,颗粒碰撞次数减少。使用DSMC-CFD方法计算的最大冲蚀位置沿弯管外拱轴线向高角度方向偏移,且数值比忽略颗粒间碰撞的CFD方法约低5%~15%,总冲蚀率则两者区别不大。结论 引入DSMC方法计算颗粒间的碰撞,可以节省大量算力。弯管处发生颗粒间碰撞,DSMC-CFD冲蚀预测方法更符合实际,使用DSMC-CFD方法的Oka模型与实验测得值最贴近。     

离心加速条件下金属液中粒子的相互作用

建立了金属液中第二相粒子在离心加速条件下的运动规律理论模型,并根据该理论模型分析了金属液中第二相粒子的相互作用规律。结果表明：在离心加速场中粒子一直处于加速状态,不会达到均速运动状态;粒子直径越大、密度越大,在同样时间内,粒子移动的距离越远;在传输过程中,不但有因尺寸不引起的粒子追逐现象,而且在因密度不同引起的追逐现象,该现象是引起粒子碰撞、聚集及尺寸大（或密度大）粒子偏聚到试样外侧的主要原因,径向上初始位置不同的两个粒子,其间距随时间延长逐渐增长。如果在径向一条直线上有多个同间距的粒子,离心移动一段时间后,虽然任意两相邻粒子的间距比开始态增大,但各相邻粒子间距在新的时刻仍是相等的。     

Analysis of clustered dispersions of uncombined carbon in cemented carbide

A method for the quantitative assessment of the degree of clustering in dispersions of small particles has been developed. Clusters are defined as the areas where particle to particle distance is shorter than a pre-selected distance called interaction distance. The method gives the number of clusters per unit area (density) as a function of the interaction distance. Implementation on modern image analysis system shows that the time of analysis is relatively short. In order to increase the understanding of the resulting cluster density function a series of tests on computer generated particle dispersions has been done. Algorithms for the generation of both isotropic random dispersions, i.e. clumping and random clustered dispersions are given. The method of analysis was tested on two specimens of WC-10% Co cemented carbide alloys. The microstructure on polished specimen surface consisted of clustered particles of uncombined carbon. This clustering was quantified.     

Coarsening behavior of γ particles in a nickel-base superalloy

The coarsening behavior of γ' particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140°C for different times,the relation of average particle size to time obeys the cube law ( a /2)3= kt,where k is 15.49 × 103,77.5 × 103 and 230.04 × 103 nm3/min,respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min....     

沉降性浆体在水平管道内输送时的水力坡度

从固体颗粒加速期间清水与固体颗粒的速度变化、动量传递、相关质量等基本问题分析入手,研究了沉降性浆体在水平管道内流动时其固体颗粒在３种流动状态下的水力坡度,提出了沉降性浆体在水平管道内流动的机理模型,进而用该模型对一些输送条件下的沉降性浆体的水力坡度进行了计算。对实际问题的理论计算结果与实测结果的对比分析表明,该模型能比较精确地预计上述几种流动状态下水平管道内沉降性浆体的水力坡度。     

Microstructure and high-temperature mechanical behavior of the NiAl–27 at.% Cr intermetallic composite

A NiAl–27 at.% Cr composite material was prepared by a powder metallurgical route, involving argon atomization and consolidation by hot isostatic pressing at 1350°C for 4 h at 400 MPa. The consolidated material exhibited a fine-grained microstructure consisting of a fine dispersion of Cr particles of about 1.7 μm in a NiAl matrix. The mechanical behavior at temperatures ranging from 650 to 1100°C was investigated by tensile-strain-rate-change tests. Analysis of the strain–stress data with both power law creep and Garofalo’s hyperbolic sine relation shows the transition to a low stress exponent creep regime with decreasing stress and/or increasing testing temperature. The measured activation energy for deformation of 300 kJ/mol is consistent with the activation energy for Ni self-diffusion in Ni–50Al. Experiments with coarse grain sizes established that the creep rate is independent of grain size which suggests that the deformation mechanisms must be associated with the motion of lattice dislocations.     

Cluster formation of ferromagnetic nano-particles in micro-capillary flow

This paper reports the experimental research on the growth and dispersion processes of cluster formation in a water-based magnetic fluid in micro-capillary flow. The magnetic fluids are suspensions of ultra fine particles coated with a molecular layer of dispersant in a liquid carrier, such as water or kerosene. The ultra fine particles are coated with single- or double-layer surfactants to attain stable dispersion. However, numerous experimental studies have indicated the existence of primary cluster of ferromagnetic nano-particles in a water-based magnetic fluid. The purpose of this research is to observe the characteristics of primary clusters in micro-capillary flow. The thermal behavior of ferromagnetic nano-particles in micro-capillary flow was observed through micro-visualization using the optical dark-field microscope system and particle tracking velocimetry (PTV) data processing system. Real-time visualization of the Brownian motion of primary clusters in a water-based magnetic fluid in micro-capillary flow was carried out. Furthermore, the effect of magnetic field on the growth process and dispersion process of cluster formation in micro-capillary flow was investigated.     

Level set simulations of dislocation-particle bypass mechanisms

Particle dispersions strengthen metals by acting as barriers to the motion of dislocations. A full analysis of the mechanisms by which dislocations bypass particles has been hindered because of complexities associated with elastic interactions between dislocation segments, their interactions with particle stress field, the flexibility of the dislocation line in three dimensions and the three-dimensional topological changes that occur. We present the results of full three-dimensional dislocation dynamics simulations, based on the level set method, that naturally accounts for all of these issues. We examine bypass mechanisms for penetrable, misfitting particles and impenetrable particles, with and without a misfit. The simulations show a wide range of bypass mechanisms, including particle cutting, dislocation loop formation and combinations of these. Loops are observed to form in front of and behind particles, on the sides of particles, in between particles, around particles and as combinations of these. Some of the bypass mechanisms are classical and others have never been reported previously. We also observe a novel dislocation multiplication mechanism associated with misfitting particles. The observations are compared with those in the literature and from previous theories and simulations.     

Cu-20%Co合金雾化液滴的凝固组织特征

对Cu-20%Co合金进行了高压气体雾化快速凝固实验,获得了富Co相以微细球形粒子形式分布于基体的Cu20%Co合金粉末.富Co相粒子的尺寸随着粉末尺寸的增加而增大.在凝固过程中,富Co相液滴受固-液界面推斥;凝固后,Co相粒子主要分布于晶界.由于富Co液相的表面能较高,液-液相变时弥散相液核通常在雾化液滴内部形成,并在温度梯度的作用下向雾化液滴中心迁移,从而导致在粉末表面形成很薄的富Cu基体相层.     

冰水两相流管道冲蚀磨损影响与防护

目的研究低温条件下冰晶颗粒随水流进入弯管并对弯管造成的冲蚀磨损,确定弯管关键防护区域。方法通过欧拉-拉格朗日双向耦合法,研究了冰晶颗粒的斯托克斯数、流速、粒径、质量流率以及管道弯径比对磨损特性的影响。结果冰晶颗粒的斯托克斯数会显著影响最大磨损率区域变化,当斯托克斯数由2.8增大至5.84时,最大磨损率区域由弯头内侧拱壁向弯头外拱壁与出口管道连接处转移,斯托克斯数高出或低于该范围时,最大磨损率位置不再发生变化,斯托克斯数的增加在一定范围内对最大磨损率没有绝对性影响。流速、粒径和质量流率的增大会使得最大磨损率不断升高,粒径和流速的变化会改变最大冲蚀磨损区域,而质量流率的改变对最大冲蚀磨损区域没有明显影响。弯径比的增大也会使得最大冲蚀磨损区域由弯头内拱壁向外拱壁与直管连接处转移,并降低最大磨损率。结论冰水两相流弯管的最大冲蚀磨损区域主要集中在弯头内拱壁、弯头外拱壁与出口直管连接处、靠近弯头侧壁三处,且大弯径比的管道可实现减磨防护。     

A fast control law for nano-positioning

To achieve fast nano-positioning control in a large traveling distance with a single motion stage and without a waiting time, this paper presents a micro-dynamic model together with a simple switching rule and two controllers, one for macro-dynamics and the other for micro-dynamics, such that the moving stage can smoothly travel from macro-dynamic range into micro-dynamic range. First, a micro-dynamic model is proposed to capture the complicated hysteresis behavior of the nonlinear micro-dynamics. Next, two controllers of simple structures are designed for the micro-dynamic traveling range and the macro-dynamic traveling range, respectively, based on the previous model. Finally, a simple switching law is proposed to complete a design. Experiments show that the resultant control system has a fast response speed without a sluggish response in the conjunction of the macro-dynamic range and the micro-dynamic range. Furthermore, it can sustain large disturbances while holding the motion stage at the set point.     

High temperature deformation behavior and mechanism of spray deposited A1-Fe-V-Si alloy

Al-8.5Fe-1.3V-1.7Si alloy was prepared by spray deposition and hot extrusion. The high temperature plastic deformation behavior of the spray deposited A1-8.SFe-1.3V-1.7Si alloy was investigated in the strain rate range of 2.77× 10^4-2.77×10^-2 s^-1 and temperature range of 350-550℃ by Gleebe-1500 thermomechanical simulator. The mechanism of the high temperature plastic deformation of the alloys was studied by TEM associated with the analysis of Rosler-Artz physical constitutive relationship based on the model of dislocation detaching from dispersion particles. The results show that A1-Fe-V-Si alloy has low strain hardening coefficient, and even exhibits work softening. Stress exponent n and activation energy Ω were calculated based on Zener-Hollomon relation and Rosler-Artz physical model respectively. The Rosler-Artz physical model can give a good prediction for the abnormal behavior of high temperature deformation of spray deposited A1-Fe-V-Si alloy, that is, n larger than 8 and Ω higher than 142 kJ/mol. However, because of the highly refined microstructure, the high temperature deformation behavior of spray deposited Al-Fe-V-Si alloy deviates more or less from the law predicted by using Rosler-Artz physical     

高速运动平台的快速精密定位控制

针对高速运动平台到位阶段的惯性振动造成的运动平台调节时间长、定位精度差等问题,提出了一种结合速度-加速度前馈控制、增益调度PID控制及扰动观测器(DOB)的复合控制方法.其中速度-加速度前馈用于实现平台的高加速运动特性;增益调度PID控制可以有效改善平台定位阶段由于非线性摩擦力对平台定位性能的影响,使平台的跟踪误差快速...     

On particle size effects: An internal length mean field approach using field dislocation mechanics

A mean field approach including an internal length scale is developed in order to capture the particle size effects on the overall mechanical behavior of particle-reinforced alloys. A generalized self-consistent scheme (with coated particles) is employed, with a new “phase” representing the “layers” where orderly dislocations between the matrix and the particles are present. The thickness of these “layers” is the internal length scale introduced in the model. It is determined dynamically by using a one-dimensional field dislocation mechanics model, where localized geometrically necessary dislocation densities accommodate the lattice incompatibility between matrix and inclusions. The beneficial influence of this scheme, as compared to classical mean field approaches, is shown from comparisons with experimental data on the particle size effects in Al/SiC composites.     

冷喷涂金属颗粒变形行为数值模拟研究进展

理解冷喷涂中的颗粒变形和沉积行为一直是科学工作的焦点。由于颗粒撞击基底后的瞬时变形行为难以通过实验观测,因此多数研究工作聚焦于数值模拟。总结了一些颗粒撞击基底的建模方法,在前人研究的基础上,针对每个模型的原理及优缺点,分析了每个方法的适用场景,给出了改善模型的方法。综述了颗粒特性、入射角度、气体预热温度等对颗粒变形行为的影响,其中粒径大小、颗粒形状等是影响颗粒变形行为的主导因素,因此重点探讨了颗粒特性的影响。颗粒的撞击变形是影响冷喷涂涂层残余应力分布的重要因素,对涂层残余应力的相关数值模拟研究进行了综述,分析了颗粒变形与颗粒残余应力的关系。最后就目前冷喷涂残余应力建模较单一的形势,探讨了如何建立一种新模型以分析涂层残余应力。     

SnZn系无铅钎料性能对比分析

对比研究了4种SnZn系无铅钎料的润湿性、抗蠕变性能以及力学性能.结果表明,SnZnAg/SnZnGa/SnZnAl的润湿性均高于SnZn钎料,合金元素的添加可以显著提高SnZn钎料的润湿性;SnZnAg钎料的抗蠕变性能最高,这主要是因为Ag-Zn金属间化合物颗粒钉扎位错引起的;合金元素的添加可以显著提高SnZn焊点的...     

镍-铝复合涂层的电沉积机理

在改进的Watts溶液中电沉积镍-铝复合涂层。采用Z电位分析、伏安法和电化学阻抗谱（EIS）,研究涂层的电化学行为。结果发现,铝颗粒的Z电位是-4 mV,与氧化铝的非常接近。添加导电铝颗粒到电解液中,导致极化曲线向负电位方向移动,且EIS曲线的环路尺寸增大。结果表明,镍-铝复合层的共沉积行为服从Guglielmi模型。加入的导电铝颗粒起惰性粒子的作用,证实在铝颗粒表面存在薄的氧化铝层。     

筒内高功率脉冲磁控溅射放电等离子体的空间分布及输运行为EI北大核心CSCD

增强对高功率脉冲磁控溅射(HiPIMS)过程中等离子体分布及输运行为的理解是控制涂层沉积过程并优化涂层性能的关键,尤其是对于筒内放电,等离子体分布与输运过程更加复杂,影响粒子运动行为的因素更加多样。针对筒内HiPIMS放电,并耦合电磁约束离子输出系统,以Ar/N_(2)/Cr磁控溅射体系中的主要组分及其相关反应和运动规律为研究对象,利用检验粒子蒙特卡罗(MC)方法对Ar、N_(2)和Cr三类主要演化粒子的空间演变过程进行仿真,发现离子输出束流中的各粒子呈现出不同的空间分布。其中,质量大、离子能量高的金属离子Cr^(+)的输出束流最窄,而质量小、离子能量低的气体离子N^(+)的输出束流范围最宽。利用高精度光谱仪测出Ar^(+)、N^(+)和Cr^(+)的特征光谱强度随空间的变化,与仿真得到的离子空间分布吻合。同时,利用该体系可在不同的沉积位置制备出不同Cr/N比的Cr_(x)N涂层,实现多种成分的一次性制备或同一成分的可控制备。     

Impact Behavior of Intrinsically Brittle Nanoparticles: A Molecular Dynamics Perspective

Impact behavior of intrinsically brittle materials at the nanoscale is a topic of growing interest, for instance, in aerosol deposition and cold spraying of ceramic materials. In this work, we examine the behavior of single-crystalline brittle nanoparticles upon impact on a rigid substrate, within the framework of a molecular dynamics model. The model is based on Lennard-Jones formulation, where brittleness is brought about by using a relatively small cut-off interaction distance. Simulations have been carried out for different values of particle size and velocity. The results show that despite the induced brittleness, particles start to deform without breaking into fragments, as the particle size falls below a critical value. They also indicate that the deformation of particles can be accompanied by poly-crystallization and bonding to the substrate. The necessary conditions for deformation and bonding are also predicted in view of an analytical model of impact and fracture, considering the effects of particle size and surface energy. The results are summarized into a parameter selection map, providing an overview of the conditions for successful deposition of intrinsically brittle materials, in terms of particle size and velocity. The predictions are interpreted with respect to the results of the relevant kinetic spraying studies.