Stress Field of Non-equilibrium Grain Boundaries in Nano-crystalline Metals

Introducing the stress distribution near grain boundaries to improve the dislocation pileup model for the Halt-Petch (H-P) relation, the continuous distribution of dislocations in the pileup could be solved by means of Tschebysheff polynomials for the Hilbert transformation. An analytical formula of the stress intensity factor for the dislocation pileup is obtained. The reverse H-P relation may be explained by the modified dislocation-pileup-model.

Phase-field model of isothermal solidification with multiple grain growth

This paper develops a new phase-field model for equiaxed dendrite growth of multiple grains in multicomponent alloys based on the Ginzberg-Landau theory and phase-field model of a single grain. Taking Al-Cu and Al-Cu-Mg alloys for example, it couples the concentration field and simulates the dendrite growth process of multiple grains during isothermal solidification. The result of the simulation shows dendrite competitive growth of multiple grains, and is reapplied to the process of dendrite growth in practical solidification.

Multi-phase field simulation of grain growth in multiple phase transformations of a binary alloy

This work establishes a temperature-controlled sequence function, and a new multi-phase-field model, for liquid- solid-solid multi-phase transformation by coupling the liquid-solid phase transformation model with the solid-solid phase transformation model. Taking an Fe-C alloy as an example, the continuous evolution of a multi-phase transformation is simulated by using this new model. In addition, the growth of grains affected by the grain orientation of the parent phase （generated in liquid-solid phase transformation） in the solid-solid phase transformation is studied. The results show that the morphology of ferrite grains which nucleate at the boundaries of the austenite grains is influenced by the orientation of the parent austenite grains. The growth rate of ferrite grains which nucleate at small-angle austenite grain boundaries is faster than those that nucleate at large-angle austenite grain boundaries. The difference of the growth rate of ferrites grains in different parent phase that nucleate at large-angle austenite grain boundaries, on both sides of the boundaries, is greater than that of ferrites nucleating at small-angle austenite grain boundaries.

The Drift of Dust Grains Induced by Transient Magnetic Field

Our experiment shows that the dust grains, suspended on the edge of the sheath of a radio-frequency discharge, undergo a contraction when switching a vertical magnetic field on, and an expansion when switching the magnetic field off. We call this kind of magnetic field "transient magnetic field". A primary analysis is proposed for the phenomenon.

Effects of low-frequency magnetic field on grain boundary segregation in horizontal direct chill casting of 2024 aluminum alloy

Effects of low frequency electromagnetic field on grain boundary segregation in horizontal direct chill (HDC) casting process was investigated experimentally. The grain boundary segregation and microstructures of the ingots, which manufactured by conventional HDC casting and low frequency electromagnetic HDC casting were compared. Results show that low frequency electromagnetic field significantly refines the microstructures and reduces grain boundary segregation. Decreasing electromagnetic frequency or increasing electromagnetic intensity has great effects in reducing grain boundary segregation. Meanwhile, the governing mechanisms were discussed.

Research on the grain refinement under pulsed magnetic field

In this paper,the principle of Pulsed Magnetic Field(PMF) force was analyzed through mathematical analyses.By theoretical analysis and calculation,the results show that the great electromagnetic force is made in the melt under pulsed magnetic field,as well as changing its direction in different places of melt at the same time.It enforces the crystallizing nucleus and brittle crystallite to fragment in the solidification processing.From the point of view,one of the main factors of grain refinement is that the fragmentations occur under a pulsed magnetic field by preliminary judgement. The feasibility of application in the metallurgical industry under PMF was discussed through comparing the results of grain refinement under EMS.According to the theoretic calculation,the power consumption under EMS is 5 -8 times the amount under PMF,when both of magnetic flux density B are 0.07T.That is to say,the better effect on grain refining can be obtained under PMF,compared with EMS,even in the lower power consumption.The solidification experiments of Sn-20%Pb alloy are conducted under the same experimental conditions that the magnetic intensity is 0.07T in the center of the crucible,it also shows that PMF has a better effect on grain refining than EMS. Combined with the continuous casting process,the influence of pulsed parameters and the metallurgical effects with applying PMF at different solidification stages was investigated.There are different grain refining effects under PMF in different solidification stages,and there are different grain refining effects under PMF in different PMF parameters.For the Sn-20%Pb alloy or silicon steel,it is more effective during the initial stage,in which the pulse frequency is 5Hz.For the Sn-20%Pb alloy,the average grain size ofβphase is the smallest,when applying the PMF during the temperature of melt decreasing from 201℃to 184℃.Further investigation of the specified technique parameters for industrial applications are required.

Phase-field simulation of the coupled evolutions of grain and twin boundaries in nanotwinned polycrystals

Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.

Primary Recrystallization of Grain Oriented Silicon Steel Strip Rolled by CSR and Annealed in Magnetic Field

The magnetic properties and textures of grain oriented silicon steel with different thickness rolled by cross shear rolling （CSR） of different mismatched speed ratio （MSR） and annealed in magnetic field under hydrogen were presented.Effects of the factors such as thickness and mismatched speed ratio on the magnetic properties and recrystallization texture were analyzed and the recrystallization principles in magnetic field annealing were discussed. The study would provide a new route for mass production of high quality ultra-thin grain oriented silicon steel strip.

MIXED-GRAINED CMOS FIELD PROGRAMMABLE ANALOG ARRAY FOR SMART SENSORY APPLICATIONS

The drive towards shorter design cycles for analog integrated circuits has given impetus to the development of Field Programmable Analog Arrays(FPAAs),which are the analogue counterparts of Field Programmable Gate Arrays(FPGAs).In this paper,we present a new design methodology which using FPAA as a powerful analog front-end processing platform in the smart sensory microsystem.The proposed FPAA contains 16 homogeneous mixed-grained Configurable Analog Blocks(CABs) which house a variety of processing elements especially the proposed fine-grained Core Configurable Amplifiers(CCAs).The high flexible CABs allow the FPAA operating in both continuous-time and discrete-time approaches suitable to support variety of sensors.To reduce the nonideal parasitic effects and save area,the fat-tree interconnection network is adopted in this FPAA.The functionality of this FPAA is demonstrated through embedding of voltage and capacitive sensor signal readout circuits and a configurable band pass filter.The minimal detectable voltage and capacitor achieves 38 uV and 8.3 aF respectively within 100 Hz sensor bandwidth.The power consumption comparison of CCA in three applications shows that the FPAA has high power efficiency.And the simulation results also show that the FPAA has good tolerance with wide PVT variations.

Microstructure refinement of AZ91D alloy solidified with pulsed magnetic field

The effects of pulsed magnetic field on the solidified microstructure of an AZ91D magnesium alloy were investigated. The experimental results show that the remarkable microstructural refinement is achieved when the pulsed magnetic field is applied in the solidification of AZ91D alloy. The average grain size of the as-cast microstructure of AZ91D alloy is refined to 104 μm. Besides the grain refinement, the morphology of the primary α-Mg is changed from dendritic to rosette, then to globular shape with changing the parameters of the pulsed magnetic field. The pulsed magnetic field causes melt convection during solidification, which makes the temperature of the whole melt homogenized, and produces an undercooling zone in front of the liquid/solid interface by the magnetic pressure, which makes the nucleation rate increased and big dendrites prohibited. In addition, primary α-Mg dendrites break into fine crystals, resulting in a refined solidification structure of the AZ91D alloy. The Joule heat effect induced in the melt also strengthens the grain refinement effect and spheroidization of dendrite arms.

Effects of Primary Annealing Condition on Recrystallization Texture in a Grain Oriented Silicon Steel

The recrystallization texture in grain oriented silicon steel sheets, which were annealed at different primary annealingtemperatures with and without an electric field, was investigated. An automated electron backscattered diffraction(EBSD) technique was used to analyze the recrystallization texture. It was found that recovery and application ofelectric field in primary annealing lead to an increase of {001} component and a decrease of {111} component afterannealing at 900℃. The development of recrystallization texture can be explained in terms of the effects of electricfield and primary annealing temperature on recovery.

Order Parameter of a Nanometre-Scale S-Wave Superconducting Grain in Quantum Tunnelling Process： Frequency Space Analysis

A nano-scale s-wave superconducting grain, coupled to a normal metallic contact through a tunnelling junction, is placed in an external magnetic field. We suppose that effect of this quantum tunnelling on the Fourier transform of the order parameter is in the form of a small additive correction to the BCS order parameter. At the first order approximation in terms of this correction term and by using an instanton method, the related Green functions （in frequency space） are obtained. By establishing a self-consistent configuration an analytic formula for the order parameter is also found. We also show that a departure from superconductivity can be captured by this formula. This change of state is indeed a manifestation of a quantum transition induced by quantum fluctuations. In this sense, this is an advantage of our simple method which, like other more elaborate methods, can detect a quantum transition in the state of the grain.

Relation of the Coercivity with the Grain Alignment Degree in Nd－Fe－B Sintermagnets

The experimental results show that the coercivity of Nd-Fe-B sintermagnet decreases with increasing of the grain alignment degree of the magnets (i.e. decreasing of the orientation distribution coefficient σ).The coercivities of the magnets with different orientation degree are calculated based on the pinning,nucleation and starting field theory,respectively.The comparison of the calculated values with the experimental results shows that the theoretical results are deviating from the experiments seriously for both pinning mechanism and nucleation mechanism. In contrast with that,the calculated values based on the starting field theory are in good agreement with the experimental results.

渗流作用下粗粒土冻结壁交圈规律及预测模型探索

由于渗流作用下的冻结壁发育规律不明等原因,部分冻结法施工工程依靠人工经验,缺乏科学的预测模型,与精准化设计和控制还有很长的距离。为探索渗流作用下冻结法施工中粗粒土层冻结壁的演化规律,本文首先建立冻结法施工数值模型,并通过室内模型试验反演验证所建立模型的有效性和适用性。之后,考虑渗流速度、冻结管间距、环境温度、含水率和导热系数等五个因素,开展冻结壁形成正交数值试验,确定冻结壁交圈时间和厚度的主控因素。最后,通过数值模拟研究主控因素作用下冻结壁的发育和演化规律。基于结果建立临界流速、冻结壁厚度、冻结壁交圈时间的预测模型,给出冻结法施工参数的选择方案。结果表明,建立的冻土水热相变耦合模型可有效模拟渗流条件下土体冻结过程,能够满足冻结壁发育过程计算分析的需要。基于正交数值试验结果发现,渗流速度对冻结壁厚度和交圈时间的贡献百分比分别为78%和52%,是影响冻结壁能否交圈和最终厚度的主控因素。当渗流速度超过一定阈值时,冻结壁无法交圈。本文将这一阈值定义为冻结法施工中的临界流速。当冻结壁能够交圈,其最终形成的冻结壁厚度也主要由渗流速度决定,二者呈负指数函数关系。冻结管间距对冻结壁最终厚度影响不大,但是对冻结壁交圈时间影响较大,它们之间存在负指数函数关系。本文给出了0~6 m·d^(-1)条件下的临界流速、冻结壁厚度、冻结壁交圈时间计算模型,它们可为粗粒土地层的冻结法施工参数选择和施工工艺设计提供理论指导。

晶界效应下的焊接热裂倾向判据适用性研究

材料焊接热裂倾向评判在产品制造中具有重要作用,以往热裂倾向判据并未考虑晶界效应,故其适用性一直是讨论的热点问题。文中以Al-Cu合金为例,通过多晶凝固定量相场模型获取熔池凝固过程枝晶形貌,提取凝固路径。基于凝固路径开展了目前常用热裂倾向判据的适用性研究,包含RDG判据、Clyne-Davies判据和Sindo Kou判据。研究发现,RDG判据同实际焊接热裂倾向一致,且裂纹产生主要受孤立晶界段枝晶间压力降主导;Clyne-Davies判据及Sindo Kou判据评判由于计算过程忽略部分孤立晶界区间段,导致评判结果出现失真。通过对三个判据的研究,为材料热裂倾向评判提供了参考。

粗颗粒硫酸盐渍土水盐迁移规律及变形特性研究

针对封闭系统下粗颗粒硫酸盐渍土在冻结过程中的水盐迁移和变形特性开展了理论和试验研究。基于非饱和多孔介质热弹性理论,考虑孔隙水盐相变,建立了适用于粗颗粒盐渍土水-热-盐-力多场耦合模型,并对单向冻结条件下粗颗粒盐渍土的温度场、水分场、盐分场和位移场分布进行了数值模拟。通过配制含硫酸盐的细砂作为土样开展了单向冻结条件下的室内试验,测定了冻结过程中的温度、水分、盐分以及变形的分布,并与数值结果进行了比较,验证了理论模型的有效性。结果表明:砂土结构孔隙更大,水分和盐分更容易渗透和迁移,在单向冻结试验中,水盐迁移速度更快;细砂的轴向位移呈现先下降后上升的变化趋势,且收缩变形持续时间较黏土更长;由于暖端水分向冷端迁移致使暖端土体孔隙减小,下部土体变得更加密实,土柱下部侧壁压力大于上部。

Microstructure evolution and phase transitions in metals simulated by the multiphase-field method

The phase-field method has emerged as the method of choice for the description of microstructure evolution and phase transitions in metallic materials.Following general thermodynamic laws a set of evolution equations for the structural variables of the system,the so called phase-fields,are derived.The paper reviews shortly the theoretical background of the multi-phase-field.Different examples demonstrating the applicability of the method to technical steels will be presented ranging from deformation of the dendritic strand shell during peritectic transformation,grain growth in Austenite to stress driven growth of Pearlite.

气相旋转螺旋槽管式换热器结构优化与性能试验

粮食干燥生产中换热损失大,存在烟气热量无法高效传递,导致换热效率低等问题。本文以集管壳与列管式换热于一体的气相旋转螺旋槽管式换热器为研究对象,基于场协同与热力学理论优化螺旋槽管关键部件,探究螺旋槽管参数对换热性能的影响。以螺距、槽深及内外径比为试验因素,以努塞尔数和阻力系数为评价指标,进行三因素五水平二次正交旋转组合试验,采用多目标优化方法确定最优参数组合,当螺距为24.845 mm、槽深为1.753 mm、内外径比为0.897时,Nu为164.637,f为0.348。对优化结果进行验证试验,试验结果与优化结果基本一致。螺旋槽管平均场协同角β相较于圆管降低约2°,揭示了置于壳程内的螺旋槽管管束进口、出口及中间截面内场协同角的分布特征,整体范围内的场协同效果有所增加,结论符合场协同原理。利用强化换热综合性能指数(Performance evaluation criteria,PEC)进行评价,结果表明气相旋转螺旋槽管式换热器PEC在1.031~1.267之间,验证了螺旋槽管在换热器应用的合理性。

半地下双层浅圆仓静态储粮温度场的数值分析

粮食温度是影响储粮安全的最重要因素之一,为揭示半地下双层浅圆仓的储粮温度场分布规律,本文建立了地上浅圆仓和地下仓的粮堆数学分析模型,并通过现场实验结果验证了模型的有效性。以此为基础建立了半地下双层浅圆仓数值模型,分析低温入粮后静态储藏1年期间的粮堆温度场变化规律。结果表明:半地下双层浅圆仓地上层靠近仓壁2 m内的粮温受外界气温影响变化明显,温度范围在10.62~27.37℃,地下层粮温常年处在准低温状态,平均温度不超过地下恒温区温度17℃;入冬时地上层粮堆在距仓壁1~3 m之间会短期形成一个热量聚集区,导致近仓壁处粮堆温差较大,随后外围区温度缓慢降低,形成“热心冷皮”;仓壁的保温隔热措施可有效减小气温对粮温的影响,地上层夏季最高粮温较未做保温的地上浅圆仓低3.86℃左右。研究结果可为半地下双层浅圆仓的推广应用提供技术支持。

地下仓粮堆局部发热温度场数值模拟研究

为了解地下仓粮堆局部发热对粮堆温度场变化的影响,以高水分玉米为研究对象,采用数值模拟的方法,通过在地下仓粮堆内加入内置热源模拟粮堆局部发热,研究了地下仓粮堆在内置热源数量与表面温度不同情况下的粮堆内部温度场变化。结果表明:当地下仓粮堆中心测点发热时,热源周围相同半径内上方温度明显高于下方,在粮堆上部容易产生次热源;当地下仓粮堆右侧临近仓壁和装粮线处以及装粮线中心处同时发热时,粮堆左下部容易产生次热源;当地下仓粮堆右侧临近仓壁和装粮线处、左侧紧贴仓壁和装粮线处以及装粮线中心处同时发热时,粮堆下部容易产生次热源,并且热源温度为45℃时的粮堆内部温度明显高于热源温度为35℃时的粮堆内部温度。