Heterogeneity of grain refinement and texture formation during pulsed electric current sintering of conductive powder: A case study in copper powder

The heterogeneity of grain refinement and texture formation in conductive copper powder during pulsed electric current sintering have been investigated by means of microstructural analysis of the particle-contacting zone of the samples sintered with micron-sized powders. The particle-contacting region was indicated to experience a higher level of grain refinement than the particle center due to the inhomogeneous temperature distribution among the particle when pulsed electric current passed through. Meanwhile, two main textures of Cube (1?0?0)[0?0?1] texture and Goss (1?1?0)[0?0?1] texture were directly detected in the sintered Cu samples by a combination of electron backscatter diffraction and X-ray diffraction. Based on the experimental results, a feasible mechanism for the formation of texture during the pulsed electric current sintering has been proposed by considering the applied uniaxial pressure and pulsed electric current as well as the rearrangement of grains. In addition, the effects of heterogeneous grain refinement and texture formation on the tensile properties of the sintered Cu samples were also discussed based on the tensile properties and fractographic. These results would contribute to the basic understanding of the sintering mechanisms in pulsed electric current sintering process.     

Characterization of the prior particle boundaries in a powder metallurgy Ti_2AlNb alloy

Ti_2 AlNb-based alloy powder metallurgy(PM) compacts were prepared via hot isostatic pressing(HIP)under relatively low temperature(920 and 980℃) and at certain pressure(130 MPa).The microstructure,composition and orientation of B2,α_2 and O phases in the compacts were characterized and analyzed with an aim to investigate the effect of unsuitable HIPping parameters on the appearance of prior particle boundary(PPB),which seriously affects the mechanical properties of the alloy.The results show that more α_2 phase is the characteristics of the PPB in Ti_2AlNb-based alloy when HIPped at relatively low temperature.Increasing HIPping temperature to the upper part of the two-phase region can effectively inhibit the formation of PPB.Electron backscatter diffraction measurements show the specific orientation relationship between phases,which helps us understand the origin of a2 and O phase and the corresponding transformation path.The HIPping at a higher temperature can weaken the micro-texture intensity of the α_2 and O phase due to the increase of misorientation in B2 phase.The α_2 phase at cell wall keeps the Burgers orientation relationship(BOR) with the grain on one side,and does not satisfy the BOR with the other.It is found that some O phase variants inside the cell HIPped at 980℃ can only maintain α_2-O OR with α_2 owing to the α_2→O phase transformation forming the O phase,while these O variants deviate from B2-O OR with B2 phase.     

Observation of particle behavior in copper powder compact during pulsed electric discharge

The behavior of spherical copper powder particles of uniform size (average diameter: 550 μm) in a powder compact was observed under an optical microscope during a single-pulse electric discharge of 500 ms duration. The morphologies of necks formed between powder particles were observed under a scanning electron microscope, and their diameters were measured. The results obtained are summarized as follows: pressure and pulsed current density determine whether or not a spark occurs. Spark is more likely to occur at interparticle contacts under low pressure and high current density. Where a spark occurs, particles are joined together by melting. Regardless of whether or not a spark is observed, necks are formed at points of contact between particles and neck diameter increases with pulsed current density. These results suggest that microscopic sparking, melting, and vaporization occur by means of extremely high temperature attained by local heat generation at the interparticle contacts in the initial stage of compaction.     

Effects of heat treatment on prior particle boundary precipitation in a powder metallurgy nickel base superalloy

PPB precipitation in powdered Rene'95 is analyzed. The effects of PHT on PPB precipitation are studied. It is found that fracture along PPB is ductile in microscale. During PHT, the phase transformation on powder surface is similar to PPB interface. PHT cannot reduce the degree of contamination on powder surface. But the precipitates on surface are coarsened. The coarsening enlarges distances between precipitates. As the result, microfracture property is improved at PPB.     

在脉冲电流作用下钢中裂纹的愈合

对含有人工预置贯穿裂纹的钢进行脉冲电流处理．在光学显微镜和扫描电镜下观察了处理前后钢样品中裂纹和组织的变化．结果表明，脉冲电流处理可以使裂纹的局部在固态下愈合．愈合是在极短时间内发生的，不影响材料不含裂纹部分的原有结构．在电流通过裂纹时产生的较高温度使其有比较大的膨胀量，周围温度较低基体的约束导致向着裂纹内的压缩，从而使裂纹面上的原子重新成键接合．     

基于信息熵改进的粒子群算法在桁架结构

提出了信息熵改进的粒子群优化算法用于解决有应力约束、位移约束的桁架结构杆件截面尺寸优化设计问题.首先介绍了信息熵基本理论和基本粒子群优化算法理论,然后对粒子群优化算法作了合理的参数设置,并将信息熵引入粒子群优化算法的适应函数和停机判别准则中.最后对2个经典的优化问题进行求解并与其他算法进行了比较.数据结果表明信息熵改进后的粒子群优化算法在桁架结构优化设计中优于其他同类算法.     

Effect of powder preparation technology on the hot deformation behavior of HIPed P/M nickel-base superalloy FGH96

In this paper, the hot deformation characteristics of P/M nickel-base superalloy FGH96 prepared by different powder preparation technologies were studied in the deformation temperature range from 1000 °C to 1100 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹ using hot compression tests. The peak stress vs. deformation temperature curves and the peak stress vs. strain rate curves were established, respectively. The results show that the specimens prepared by plasma rotation electric pole (PREP) powder were more sensitive to deformation temperature and strain rate. On the basis of the dynamic material model, the processing maps for hot working were developed. The activation energies and Zener-Hollomon parameters were obtained by linear statistical regression method. For the specimens prepared by PREP powder, the peaks of power dissipation mainly located in lower temperature domain (1000-1030 °C), and the efficiencies of power dissipation (EPD) obtained in the strain range from 0.1 to 0.7 were essentially similar. This indicated that strain had a slight influence on processing maps. For the specimens prepared by argon atomization (AA) powder, the effects of strain on EPD and instability domains were significant. The lower activation energies and Z values indicated that the workability of the specimens prepared by AA powder is better than that prepared by PREP powder. Moreover, it was found that effects of the heat treatment time on activation energy and Zener-Hollomon parameter were significant. With the increase of heat treatment time, the dislocation density and the volume fraction of precipitation phase gradually decreased. Microstructural observation demonstrated that the phenomenon of recrystallized grains coarsening existed in the specimens prepared by longer heat treatment time. The heat treatment time of the specimens prepared by AA powder should be appropriately shortened in order to prevent recrystallized grains coarsening.     

An accurate and stable multiphase moving particle semi‐implicit method based on a corrective matrix for all particle interaction models

The Lagrangian moving particle semi‐implicit (MPS) method has potential to simulate free‐surface and multiphase flows. However, the chaotic distribution of particles can decrease accuracy and reliability in the conventional MPS method. In this study, a new Laplacian model is proposed by removing the errors associated with first‐order partial derivatives based on a corrected matrix. Therefore, a corrective matrix is applied to all the MPS discretization models to enhance computational accuracy. Then, the developed corrected models are coupled into our previous multiphase MPS methods. Separate stabilizing strategies are developed for internal and free‐surface particles. Specifically, particle shifting is applied to internal particles. Meanwhile, a conservative pressure gradient model and a modified optimized particle shifting scheme are applied to free‐surface particles to produce the required adjustments in surface normal and tangent directions, respectively. The simulations of a multifluid pressure oscillation flow and a bubble rising flow demonstrate the accuracy improvements of the corrective matrix. The elliptical drop deformation demonstrates the stability/accuracy improvement of the present stabilizing strategies at free surface. Finally, a turbulent multiphase flow with complicated interface fragmentation and coalescence is simulated to demonstrate the capability of the developed method.     

Quantitative evaluation of the displacement distribution and stress intensity factor of fatigue cracks healed by a controlled high‐density electric current field

Fatigue cracks were healed by controlling a high‐density electric current. The changes in the displacement distribution around the crack tip and the stress intensity factor before and after crack healing were evaluated quantitatively with a digital image collation method. According to the results, it was determined that the cracks were closed by approximately 2 to 7 µm in this study. On the other hand, the stress intensity factor decreased or increased depending on the conditions of the crack and the current applied. The physical restriction between the crack surfaces, such as bridging, is important with respect to lowering the stress intensity factor after healing.     

Effect of the simultaneous application of a high static magnetic field and a low alternating current on grain structure and grain boundary of pure aluminum

Effect of the simultaneous application of a high static magnetic field and a low alternating electric current on the solidification structure of pure aluminum has been investigated. Results show that the refinement of the solidification structure is enhanced by the electric current under a certain magnetic field. However, when the magnetic field intensity exceeds a certain value, the refinement is impaired under a certain electric current. The observation by electron backscattered diffraction (EBSD) shows the complex fields have led to the increase of the low angle boundaries with the refinement. Moreover, the application of the static gradient magnetic field is capable of modifying the distribution of the refined grains. The above results may be attributed to the formation of the cavities during the electromagnetic vibration process and the high magnetic field.     

Full-text prior art and chemical structure searching in e-journals and on the internet – A patent information professional’s perspective

In depth analysis of non-patent literature prior art is a crucial step in checking patentability of new inventions and validity of competitor’s patents, since by patent law relevant subject matter disclosed in non-patent literature is as important as any patent document. E-journal articles, as well as any scientific and technical information published on the web are an important source of prior art that is very often insufficiently covered and indexed by commercial databases. This article reviews search and display capabilities of e-journal search sites of different publishers and hosts, as well as their value for full-text prior art analysis to enhance retrieval from commercial databases. Moreover, current developments and future prospects of chemical structure searching both in e-journals and on the internet are discussed.     

Effects of discharging vibration conditions and coating structures on improving discharge particle flowability in a smaller particle admixing system

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the effects of coating structures on the improvement of flowability are not yet fully understood. In this study, we focused on vibrating discharge particle flowability and investigated the effect of discharging vibration conditions and coating structures on improving the flowability. Main and admixed particles of 60.8 μm and 8 nm in diameter, respectively, were mixed in various mass ratios, and the discharge particle flow rates of the mixed particles were measured. Scanning electron microscopy and scanning probe microscopy images were used to analyze the coverage diameter, surface coverage ratio, and coverage height of the admixed particles on the main particle surfaces. As a result, the admixing mass ratio that gave maximum flowability was found to depend on the maximum value of the vibration acceleration. This could be explained by the relationship between the coating structures of admixed particles and the coated average surface distances due to the vibration acceleration.     

Effect of boron and sulphur on the electronic structure of grain boundaries in Ni

The first-principles discrete variational method is employed to study the effect of boron and sulphur on the electronic structure of the Ni grain boundary (GB). The calculated results show that boron does not strongly influence (only slightly decreases) the bonding between the atoms of the metal. In addition, B forms the strong bonding state with its neighbouring metal atoms. Our study also indicates that S strongly decreases the bonding between the atoms of the metal, and that the bonding tendency between S and the atoms of the metal across the GB plane is very weak. The calculations of environment-sensitive embedding energies show B has the strong site-competition ability and can successfully drive out S from the GB region. We conclude that the influence of impurities segregating on the GBs is closely associated with their effects on: (i) the decrease of the bonding between the atoms of the metal due to the presence of impurities; (ii) the bonding between the impurity atom and the atoms of the metal; and (iii) the site-competition ability of impurity atoms.     

改造对沸石基复合环境材料表面结构的影响及应用

以天然斜发沸石为研究对象,采用不同改造方法对其进行改造,研究发现LaCl3化学改造2h,500℃高温下焙烧1h后所得沸石基复合环境材料去除污水中磷的性能有了较大提高。SEM和EDS测试材料的结果表明,天然沸石改造后其表面形貌发生了改变,孔结构得到了扩展,改造前后材料主要成分均为Si、Al、O,但改造后材料中La离子含量明显增加。材料表面结构测试分析发现,改造后复合材料孔道得到拓宽,孔径分布更为均匀,孔容积、比表面积、孔径数量和孔隙率与沸石原材料相比均有所增加。在此基础上,研究了沸石基复合环境材料用量、处理时间及废水pH值对材料除磷效果的影响,结果表明,材料在用量1.2g/L、处理时间3h、pH值3～7的条件下,对废水中磷去除率可达98.46%。     

Scaling of energy dissipation in crushing and fragmentation: a fractal and statistical analysis based on particle size distribution

An extensive experimental investigation on concrete specimens under crushing and fragmentation over a large scale range (1:10) – exploring even very small specimen dimensions (1 cm) – was carried out to evaluate the influence of fragment size distribution on energy density dissipation and related size effect. To obtain a statistically significant fragment production as well as the total energy dissipated in a given specimen, the experimental procedure was unusually carried out up to a strain of approximately –95%, practically corresponding to the initial fragment compaction between the loading platens. The experimental fragment analysis suggests a fractal law for the distribution in particle size; this simply means that fragments derived from a given specimen appear geometrically self-similar at each observation scale. In addition, clear size effects on dissipated energy density are experimentally observed. Fractal concepts permit to quantify the correlation between fragment size distribution and size effect on dissipated energy density, the latter being governed by the total surface area of produced fragments. The experimental results agree with the proposed multi-scale interpretation satisfactorily.     

On the coupling between fluid flow and mesh motion in the modelling of fluid–structure interaction

Partitioned Newton type solution strategies for the strongly coupled system of equations arising in the computational modelling of fluid–solid interaction require the evaluation of various coupling terms. An essential part of all ALE type solution strategies is the fluid mesh motion. In this paper, we investigate the effect of the terms which couple the fluid flow with the fluid mesh motion on the convergence behaviour of the overall solution procedure. We show that the computational efficiency of the simulation of many fluid–solid interaction processes, including fluid flow through flexible pipes, can be increased significantly if some of these coupling terms are calculated exactly.     

The effect of martensite volume fraction and particle size on the tensile properties of a surface-carburized AISI 8620 steel with a dual-phase core microstructure

This study is focused on the production of a dual-phase steel structure in the core of a surface-carburized AISI 8620 cementation steel and the effect of martensite volume fraction (MVF) and martensite particle size (MPS) on tensile properties. Experimental results showed that, compared with specimens with a fully martensitic microstructure in the core, those with a dual-phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness. In specimens with a dual-phase microstructure in the core, the tensile strength increased and ductility decreased with increasing MVF. Both the tensile strength and the ductility increased with decreasing MPS at constant MVF. The best combination of tensile strength and ductility was obtained with a fine MPS at a constant MVF of 25%.     

Study on the structure and properties of core–shell Fe/Al composite powder synthesized by MOCVD in fluidized bed

Core–shell Fe/Al composite powder with different thicknesses of Fe layer has been prepared by MOCVD in a fluidized bed reactor. The products were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDX) and simultaneous thermogravimetry–differential scanning calorimetry (TG–DSC). The results show that a compact nano-Fe layer is covered on the surface of Al to form core–shell Fe/Al composite powder. Nano-Fe layer thickness can be controlled by adjusting deposition time. The Fe layer thickness is evaluated by weight increase in TG curve at the temperature range of 350–550 °C in air atmosphere. Combustion properties of Fe/Al composite powder have great improvement compared with raw Al.     

Exploring the influence of particle shape and air velocity on the flowability in the respiratory tract: a computational fluid dynamics approach

Dry powder inhalers (DPIs) are considered a main drug delivery system through pulmonary route. The main objective of this work is to study the flow of differently shaped microparticles in order to find the optimum shape of drug particles that will demonstrate the best flow to the deep lung. The flowability of particles in air or any fluid depends particularly on the drag force which is defined as the resistance of the fluid molecules to the particle flow. One of the most important parameters that affect the drag force is the particles’ shape. Computational simulations using COMSOL Multi Physics 5.2 software were performed for investigating the particles flow in the air pathways of lung, and the drag force was calculated for different particles shapes. This was accomplished by screening a set of 17 possible shapes that are expected to be synthesized easily in the micro-scale. In addition, the macro-scale behavior of the investigated shapes was also simulated so as to compare the behavior of the flowing particles in both cases. A very big difference was found between the behavior of particles’ flow in the micro and macro scales, but a similar behavior can be obtained if the flow velocity of the microparticles is very high. It was also found that the micro-triangle with aspect ratio 2:1 has the least drag force in both deep and upper lung; so, it should be the shape of choice during the process of particle synthesis for pulmonary drug delivery.