Growth mechanism of Cu nanopowders prepared by anodic arc plasma

1 Introduction In recent years, the research and development for metal nanopowders has attracted significant interest and it is still the subject of intense investigation owing to their intriguing properties and extensive prospects [1-5]. Metal nanopowder…     

冷轧医用b钛合金显微组织和力学性能演变规律研究

基于微观相场动力学模型,对应变诱发镍基合金粗化过程共格沉淀相的形貌不稳定性及其发展规律进行了研究.结果表明,应变能作用下,粗化过程中沉淀颗粒界面曲率发生变化,通过沿[10]方向尺寸减小与[11]方向尺寸增大,转变为方形.低应变能作用下,沉淀颗粒形貌由圆形转变为周边圆滑,边长向外凸的方形颗粒;中等应变能作用下,沉淀颗粒形貌由圆形变为边长向内凹的方形;高应变能作用下,圆形沉淀颗粒先转变为方形,再从方形颗粒中心“分裂”为2个长条状颗粒,进一步转变为双透镜形状.     

基于形态学的焊锡粉颗粒定量分析

采用形态学的方法对超声雾化法生产的焊锡粉颗粒形态进行描述与统计分析.由于焊锡粉颗粒棱角度高、形状不规则,是影响配制焊膏质量的关键因素.建立焊锡粉颗粒的分形维度与长宽比来作为两个重要的焊锡粉颗粒形态特征参数.通过对焊粉颗粒的几何投影图像进行计算,连续运用形态学中的腐蚀和膨胀两种方法刻画焊粉颗粒轮廓,并对颗粒数目和半径等形态信息统计计算,建立分形维度的拟合方程.结果表明,文中所提出的形态学方法适用于分割粘连焊粉颗粒图像,并且对于焊锡粉颗粒的长宽比与分形维度统计能够准确地反映焊锡粉颗粒真实形态.     

DSC STUDY OF MARTENSITIC TRANSFORMATION KINETICS IN A Cu－Zn－Al－Mn－Ni SHAPE MEMORY ALLOY

ＤＳＣＳＴＵＤＹＯＦＭＡＲＴＥＮＳＩＴＩＣＴＲＡＮＳＦＯＲＭＡＴＩＯＮＫＩＮＥＴＩＣＳＩＮＡＣｕ－Ｚｎ－Ａｌ－Ｍｎ－ＮｉＳＨＡＰＥＭＥＭＯＲＹＡＬＬＯＹＧＥＮＧＧｕｉｌｉ;ＢＡＩＹｕｊｕｎ;ＰＥＮＧＱｉｆｅｎｇ（ＩｎｓｔｉｔｕｔｅｆｏｒＭａｔｅｒｉａ...     

A simple model for the oxidation of carbon-containing composites

The isothermal oxidation mechanism of the carbon-containing composites has been investigated based on the experimental data reported in the literature. The results showed that the oxidation kinetics was affected not only by temperature and time but also by carbon content and the sample shape. For the oxidation kinetics, a series of quantitative kinetic models have been developed based on the controlling step. In this model, the effects of carbon content, sample size and temperature on the reaction fraction have been especially discussed. Incorporation of the experimental data into the new model indicates that a good agreement has been reached.     

球磨时间对机械合金化制备NbMoTaW高熵合金粉末的影响

采用机械合金化技术制备了NbMoTaW高熵合金粉末,研究了球磨时间对粉末相结构、微观形貌、杂质含量、颗粒粒径的影响。利用X射线衍射仪、扫描电镜和透射电镜对高熵合金粉末进行了物相和形貌分析,利用能谱仪定量分析粉末中元素分布和杂质含量,利用激光粒度仪测试了粉末粒径分布。结果表明：随球磨时间增加,混合粉末经历了扁平化、冷焊断裂、球形化三个阶段,球磨45h后,形成了具有单一BCC结构的高熵合金粉末。粉末形貌从初始的不规则状转变成片状,而后转变成椭球状,且球形度不断优化。杂质主要来源于球磨罐和磨球,不同球磨时间段,杂质含量增速不同。颗粒粒径随球磨时间变化,呈现出先增大后减小的趋势,且颗粒粒径分布更为均匀。     

Grain level analysis of crack initiation and propagation in brittle materials

A study on the accuracy of cohesive models for capturing dynamic fragmentation of ceramic microstructures is presented. The investigation consists of a combined experimental/numerical approach in which microcracking and damage kinetics are examined by means of plate impact recovery experiments. The numerical analysis is based on a 2-D micromechanical stochastic finite element analysis. The model incorporates a cohesive law to capture microcrack initiation, propagation and coalescence, as well as crack interaction and branching, as a natural outcome of the calculated material response. The stochasticity of the microfracture process is modeled by introducing a Weibull distribution of interfacial strength at grain boundaries. This model accounts for randomness in grain orientation, and the existence of chemical impurities and glassy phase at grain boundaries. Representative volume elements (RVE) of ceramic microstructure with different grain size and shape distributions are considered to account for features observed in real microstructures. Normal plate impact velocity histories are used not only to identify model parameters, but also to determine under what conditions the model captures failure mechanisms experimentally observed. The analyses show that in order to capture damage kinetics a particular distribution of grain boundary strength and detailed modeling of grain morphology are required. Simulated microcrack patterns and velocity histories have been found to be in a good agreement with the experimental observations only when the right grain morphology and model parameters are chosen. It has been found that the addition of rate effects to the cohesive model results in microcrack diffusion not observed experimentally.     

镁合金枝晶生长的相场法研究

采用有限差分法求解考虑界面能各向异性的相场模型,对镁合金枝晶生长过程进行了模拟计算,研究了无量纲过冷度和各向异性强度对枝晶生长行为的影响。结果表明:镁合金枝晶在二维平面上具有6个最优生长方向,优先生长方向上枝晶生长迅速,形成枝晶主干,枝晶形貌具有六重对称性。随着过冷度和各向异性强度的增加,枝晶形貌由光滑的类六边形向具有侧向分支的六重对称形貌转变,温度扩散层由类球形向星形过渡,枝晶尖端生长速率增加,尖端曲率半径和温度值减小。     

Phase-Field Modeling of Austenite-to-Ferrite Transformation in Fe-C-Mn Ternary AlloysEI北大核心CSCD

The effect of Mn on the austenite-to-ferrite transformation has been widely studied by both physical models and experiments due to its technological importance for alloy design in steel industries. In recent years, an increasing interest of this issue is moved onto the effect of alloying element on the migrating interface during the austenite-to-ferrite transformation. For ternary Fe-C-Mn alloys, the interfacial condition is more complicated than that of binary Fe-C alloys in view of the large difference in the diffusivity between the interstitial and substitutional alloying elements. Generally speaking, there are two main concepts, i.e. the paraequilibrium model and the local-equilibrium model, which have been proposed to describe the phase transformation kinetics in ternary Fe-C-Mn alloys based on different assumptions about the diffusion of the substitutional elements. And many modeling attempts have been made to study the effect of Mn on the migration kinetics by using these theories. In this work, a multi-phase-field (MPF) model coupling with a Gibbs-energy dissipation model was developed to simulate the isothermal austenite-to-ferrite transformation in ternary Fe-C-Mn alloys. This model has considered the Mn diffusion inside the migrating interface in a physical manner and takes its effect on the transformation kinetics into account. Comparison simulations were made to analyze the difference in the transformation kinetics and ferrite morphologies with and without considering the energy dissipation at the moving interface. It shows that the incomplete transformation phenomenon does occur due to the Mn diffusion inside interface. The modified MPF model was then used to study the effect of Mn contents on the microstructures and kinetics of the phase transformations. It is found that the ferrite growth along the austenite/austenite boundaries is faster than that in the perpendicular direction. This difference is intensified with increasing the Mn concentration, which hence leads to the ferrite morphology changed from elliptical to flat alike. It also produces a slower transformation kinetics and a larger degree of the incomplete transformation when increasing the Mn concentration.     

6082铝合金板带热连轧规程优化

根据应力随应变、变形温度和应变速率变化的数据,利用MATLAB软件拟合6082铝合金的变形抗力模型;利用测试函数对基本粒子群算法(BPSO)和改进粒子群算法(IPSO)的性能进行对比,结果表明,IPSO在收敛速度和精度上均优于BPSO。利用IPSO,在设备、工艺约束下,以能耗、板形为目标函数,对6082铝合金的4机架热连轧规程进行优化,并将优化结果与以等压下率分配的规程进行对比,结果表明,优化后的规程,其能耗和板形均优于等压下率制定的规程,对实际生产有一定的指导意义。     

Internal Mechanism Analysis of Modeling on Particles Size Distribution Characteristics of Impact Attrition Beryllium Powders

基于铍(Be)冲击制粉颗粒的形貌特征,建立了一个描述Be粉颗粒尺寸分布的物理模型,该模型仅含一个物理意义明确的参数。模型中考虑了2种重要的内在影响机制:具有不同初始动能的颗粒在完成冲击后的存在状态应该遵从Maxwell-Boltzmann分布,即大颗粒具有高的能量,状态不稳定,在冲击过程中容易碎成小颗粒,这一影响可以用颗粒尺寸的负指数函数来描述;获得低表面能的倾向又使得颗粒尽可能具有大的体积,这一影响可以用颗粒尺寸的立方函数来描述。实际的颗粒尺寸分布是这2种影响竞争的结果。计算结果与实验数据符合得很好。     

Effect of pH on Crystal Size and Photoluminescence Property of ZnO Nanoparticles Prepared by Chemical Precipitation Method

The effects of pH value on crystal size and optical property of zinc oxide nanoparticles prepared by chemical precipitation method were investigated.Prepared samples have been characterized by means of X-ray diffraction,scanning electron microscopy,ultraviolet-visible spectrometer and photoluminescence spectrometer.From X-ray diffraction profile,it is found that the particle size of sample increases from 13.8 to 33 nm when the pH value of the solution was increased from 6 to 13.Microstructural study also shows that the particle size increases with pH value.Hexagonal shape of the zinc oxide nanoparticle has been confirmed by the scanning electron microscopy image.The recorded ultraviolet-visible spectrum shows excitonic absorption peaks around 381 nm.The energy gap of the prepared samples has been determined from the ultraviolet-visible absorption spectrum,effective mass model equation and Tauc's relation.It was found that the energy gap of the prepared samples decreases with increase in pH value.The recorded blue shift confirmed the quantum confinement effect in the prepared zinc oxide samples.Photoluminescence spectrum infers that the increase in pH value shifts the ultraviolet-visible emission but not the violet and green emissions.     

A Multi-phase Field Model for Static Recrystallization of Hot Deformed Austenite in a C–Mn Steel

A multi-phase-field model has been developed to simulate the microstructure evolution and kinetics of the austenite static recrystallization(SRX) in a C–Mn steel. In this model, the bulk free energy that coupling the deformation stored energy with a special interpolation function is incorporated. Both the deformed grain topology and the deformation stored energy have been included in order to investigate the influence of pre-deformation on the subsequent austenite SRX at different hot deformation levels. Diverse scenarios of microstructure evolution show different deformation-dependent recrystallized grain sizes. The transformation kinetics is then discussed by analyzing the overall SRX fraction and the average interface velocity on the recrystallization front.     

Model of oxidation of SiC microparticles at high temperature

The oxidation kinetics of SiC microparticles has been studied from both experimental data and theoretical aspects based on recent research work. The effects of temperature, especially the particle size on the oxidation kinetics are explained not only qualitatively but also quantitatively based on our theory and model. The results show that our new model and the calculated results can reach a good agreement. It may also be seen that the important role of particle size in oxidation of SiC powder, especially in the nano era for SiC material.     

Morphological development and oxidation of elongated β-SiAlON material

Elongated β-SiAlON whiskers and ribbons have been synthesized at 1700 K under flowing nitrogen atmosphere. A series of experiments have been performed using thermogravimetry (TG) and thermal field emission scanning electron microscope (FE-SEM) to investigate its oxidation resistance. The results showed that the synthesized β-SiAlON material possessed good oxidation resistance and the shape can be well kept up to 1373 K. Chou’s model has been used to investigate the oxidation kinetics, from which it was concluded that β-SiAlON in shape of ribbons has better oxidation resistance than that in shape of whisker from the view point of a new index of the characteristic oxidation time.     

不同弹性畸变程度下Ni75Al14Mo11合金沉淀过程的多尺度模拟

基于微观相场模型和第一性原理方法模拟了Ni75Al14Mo11合金在不同弹性畸变能常数下的相变过程。当弹性畸变能从无到有时,沉淀相形貌由椭圆状变为块状,分布由不规则随机排列变为沿弹性"软"方向规则排列,具有一定方向性;弹性畸变能对短程DO22有序相的析出有抑制作用,并且对沉淀粗化过程的影响要大于对析出过程的影响;弹性畸变能对沉淀相的有序化及原子簇聚的影响并不是随着其增加而简单的起抑制或促进作用,而是与其取值范围有关。在相同的外界条件下,Ni、Al元素形成Ni3Al要比Ni、Mo元素形成Ni3Mo容易,导致Ni75Al14Mo11合金在1073 K时效下较易析出Ni3Al。     

Hf含量对镍基粉末高温合金中γ′相形态的影响

研究了含Hf镍基粉末高温合金在长期时效处理过程中γ′相形态的演化过程.结果表明:合金在高温长期处理过程中立方状γ′相发生分裂、呈现出二重平行状和八重小立方体组态.八重小立方体组态作为择优形态不再发生分裂,处于低能稳定状态.不同Hf含量合金的错配度发生明显变化,γ′相的长大或粗化过程可以粗略地分为"界面控制"和"应变控制"2个阶段,γ′相间弹性相互作用能对合金中析出相的形态变化起着重要作用.     

The influence of partial cycling on the martensitic transformation kinetics in shape memory alloys

The martensitic transformation kinetics during partial cycling and the so-called “hammer” effect has been carefully characterized in Cu–Al–Ni shape memory alloys. These temperature memory effects were measured by adiabatic calorimetry and analyzed within the frame of a new thermodynamic model. A straightforward study of the nucleation processes explains on quantitative grounds the shift of the reverse transformation to higher temperatures and the presence of a secondary C_p peak associated to these phenomena. The optical observations support the calorimetric results. Finally, the release of the elastic energy in the martensitic state due to the different thermal cycles has been determined.     

THE EFFECT OF SiC PARTICLE AND WHISKER ON THE STRENGTH OF ALUMINUM ALLOY MATRIX COMPOSITES

1.IntrodurtionDiscontinuousfiberreinforcedMMCscanbemanufacturedandformedbyc0nventionalmetallurgicalmethodl1],s0theywillbefirstlyusedinindustryonamassproductionscalebecause0ftheirhighqualityandlowc0st.Sincetherearemanytypesofparticleswhichcouldbeusedt0reinforcedifferelitlightmetalalfoys,manyexperimentsneedt0bedone.Severalresearchersdevelopedsomethe0reticaJpredictionmethodstodescribethestrengthofwhiskerorparticlereinf0rcedMMCs-Theshearlagm.del[2]wasdevelopedtopredicttheStrengthofcomposit6s,b…     

Determination of the interface energies of spherical,cuboidal and octahedral face-centered cubic precipitates in Cu–Co,Cu–Co–Fe and Cu–Fe alloys

The coarsening theory of a spherical particle in a ternary alloy developed by Kuehmann and Voorhees (KV) has been generalized to any centro-symmetric particle. A classical thermodynamic analysis reveals that the generalized KV theory enables us to estimate the interface energy of a particle with a fixed shape, even if the shape of the particle is not controlled by minimization of the interface energy. Data on the coarsening of spherical, {0 0 1}-faceted cuboidal and {1 1 1}-faceted octahedral precipitates in a Cu–Co alloy, a Cu–Fe alloy, and three Cu–Co–Fe alloys with different Co and Fe contents during aging at 873–973 K have been collected by transmission electron microscopy and electrical resistivity. By applying the generalized KV theory to the experimental data, the energies of sphere, {0 0 1} and {1 1 1} interfaces have been determined. Their energies increase with increasing the Fe composition in the alloys.