定向凝固包晶合金相和微观组织选择研究进展

综述了定向凝固包晶合金相和微观组织选择的理论模型和实验研究进展,分析相和微观组织的选择规律,同时讨论了对流对凝固微观组织的影响。依据国内外对包晶合金凝固的研究现状,提出进一步研究的方向。     

铸态QT550-7球墨铸铁制备及组织研究

为了研究铸态QT550-7球墨铸铁组织,铁液凝固过程及石墨球化机理,用回炉铁、生铁、废碳钢、球化剂、孕育剂等材料,采用双联熔炼法制备了球墨铸铁.经测试,化学成分和力学性能满足QT550-7球墨铸铁要求,球墨铸铁组织由铁素体+珠光体+球状石墨组成,凝固过程为糊状凝固.     

Impact-activated solidification of dense suspensions via dynamic jamming fronts

Although liquids typically flow around intruding objects, a counterintuitive phenomenon occurs in dense suspensions of micrometre-sized particles: they become liquid-like when perturbed lightly, but harden when driven strongly. Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions or granular dilation. However, neither of these mechanisms alone can explain the ability of suspensions to generate very large, positive normal stresses under impact. To illustrate the phenomenon, such stresses can be large enough to allow a person to run across a suspension without sinking, and far exceed the upper limit observed under shear or extension. Here we show that these stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region, ultimately leading to extraordinary amounts of momentum absorption. Using high-speed videography, embedded force sensing and X-ray imaging, we capture the detailed dynamics of this process as it decelerates a metal rod hitting a suspension of cornflour (cornstarch) in water. We develop a model for the dynamic solidification and its effect on the surrounding suspension that reproduces the observed behaviour quantitatively. Our findings suggest that prior interpretations of the impact resistance as dominated by shear thickening need to be revisited.     

Microstructure evolution of a recycled Al-Fe-Si-Cu alloy processed by tube channel pressing

Although excellent recyclability is one of the advantages of Al alloys, a recycling process can reduce different properties of these alloys by adding coarse AlFeSi particles into the alloys' microstructures. One of the well-known methods for modifying the microstructure of metallic materials is the imposition of severe plastic deformation (SPD). Nevertheless, the microstructure evolutions of recycled Al alloys containing extraordinary fractions of AlFeSi particles during SPD processing have seldom been considered. The aim of the present work is to study the microstructure evolution of a recycled Al-Fe-Si-Cu alloy during SPD processing. For this purpose, tubular specimens of the mentioned alloy were subjected to different numbers of passes of a recently developed SPD process called tube channel pressing (TCP); their microstructures were then studied using different techniques. The results show that coarse AlFeSi particles are fragmented into finer particles after processing by TCP. However, decomposition and dissolution of AlFeSi particles through TCP processing are negligible. In addition, TCP processing results in an increase in hardness of the alloy, which is attributed to the refinement of grains, to an increase of the dislocation density, and to the fragmentation of AlFeSi particles.     

Signatures of granular microstructure in dense shear flows

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands--narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes. They are also relevant to lubricating fluids confined to ultrathin molecular layers. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.     

熔体热处理研究及其应用

明确提出了熔体热处理的概念,总结了有关熔体热处理的方法、研究与应用概况。指出与固体热处理相类似,从温度对熔体结构的影响出发,通过一定的热处理制度,控制熔体的预结晶状态和冷却速度,可以显著改善材料的组织、性能及质量。熔体热处理工艺的研究与发展,将为挖掘材料的性能潜力开辟一条有效的新途径,具有广泛的应用前景。     

Research progress of microstructure control for aluminium solidification process

Aluminum has been the secondly main metallic material in the world, whose mechanical properties are very important for industrial applications. The sizes and shapes of grains are important in determining the performance in structural applications. How to control the microstructure during solidification process has been a research focus. This paper gives an overview on the recent progress in microstructure control for aluminium alloys solidification process, and introduces the different methods to control the microstructure in detail. The mechanisms of microstructure control for different methods are also discussed. Finally, a brief prospect on future work is presented.     

Granular materials: packing grains by thermal cycling

A long-standing problem in managing the behaviour of a collection of solid grains concerns the nature of the grain packing, a property that is typically controlled by how the grains are poured or shaken. Here we show that a systematic and controllable increase in granular packing can be induced by simply raising and then lowering the temperature, without the input of mechanical energy. This thermal processing may have important practical implications for the handling and storage of granular materials.     

颗粒物质剪切流动的类固-液转化特性及相变图的建立

颗粒物质类固-液相态间的转化现象广泛存在于工程地质、自然环境和工业生产等诸多领域,其力学特性在相态转化过程中会发生很大的变化,是目前颗粒物质力学研究的热点和难点.通过对颗粒物质类固-液转化过程中基本力学行为的研究,可建立表征颗粒物质类固-液转化的相变图,揭示类固-液转化的内在机理.颗粒物质的类固-液转化过程可大体分为两种,即阻塞与流动状态的转化,以及在流动过程中类固态与类液态力学行为的转化.本文以周期边界条件下的多分散颗粒系统为研究对象,采用离散单元方法数值模拟了单剪流动过程;分析了在不同体积分数和剪切速率下颗粒系统的平均应力、配位数、净接触时间数、有效摩擦系数、惯性指数和广义Savage数等宏观参数的分布规律;讨论了类固-液转化的内在机理,建立了一个以孔隙率、无量纲剪切应力和无量纲剪切速率为基本变量的相变图.该相变图不仅能够表征出颗粒物质由阻塞向流动状态的转变,同时也能够描述颗粒物质在剪切流动中发生类固-液转化的演化规律.     

A Taylor vortex analogy in granular flows

Fluids sheared between concentric rotating cylinders undergo a series of three-dimensional instabilities. Since Taylor's archetypal 1923 study, these have proved pivotal to understanding how fluid flows become unstable and eventually undergo transitions to chaotic or turbulent states. In contrast, predicting the dynamics of granular systems--from nano-sized particles to debris flows--is far less reliable. Under shear these materials resemble fluids, but solid-like responses, non-equilibrium structures and segregation patterns develop unexpectedly. As a result, the analysis of geophysical events and the performance of largely empirical particle technologies might suffer. Here, using gas fluidization to overcome jamming, we show experimentally that granular materials develop vortices consistent with the primary Taylor instability in fluids. However, the vortices observed in our fluidized granular bed are unlike those in fluids in that they are accompanied by novel mixing-segregation transitions. The vortices seem to alleviate increased strain by spawning new vortices, directly modifying the scale of kinetic interactions. Our observations provide insights into the mechanisms of shear transmission by particles and their consequent convective mixing.     

离散颗粒多尺度分级模型与破碎模拟

在所建议传统离散颗粒模型基础上引入了多尺度分级模型,定义了不同尺度的基本颗粒和颗粒簇模拟颗粒破碎现象．在统计断裂力学Weibull分布公式基础上利用名义应力给出了多尺度分级模型意义下颗粒破碎概率;结合颗粒簇的解簇模型建议了一个可模拟颗粒破碎的多尺度分级离散颗粒模型．数值例题初步显示了所建议模型模拟颗粒破碎现象的有效性．并利用所提出的名义有效应变和名义体积应变显示了颗粒破碎过程对颗粒材料结构物变形的影响．     

Al-Si合金快速等轴凝固界面响应函数及组织选择

在快速枝晶及共晶生长理论模型基础上,使用最高界面生长温度判据,建立了共晶合金快速等轴凝固界面响应函数(IRF)模型,分析了Al-Si合金系各种相及组织的竞争生长,绘制了非平衡组织选择图.计算结果与实验结果基本吻合,说明所建立的界面响应函数模型可以较好地预测Al-Si合金等轴凝固过程中的非平衡组织选择及形态演化.     

真空触头成分及超微结构对其耐电压强度的影响

以普遍使用的Ｃｕ－Ｃｒ合金为基础,分析了影响真空触头材料耐电压强度的因素．通过分析真空触头材料击穿前后的成分与组织的变化,发现触头合金的成分和微观组织结构与其耐电压强度有着密切的关系．材料表面固溶体的形成有利于耐电压强度的提高;材料表面成分的均匀化也可以使材料具有更好的耐电压性能．     

Anomalous properties in ferroelectrics induced by atomic ordering.

Complex insulating perovskite alloys are of considerable technological interest because of their large dielectric and piezoelectric responses. Examples of such alloys include (Ba1-xSrx)TiO3, which has emerged as a leading candidate dielectric material for the memory-cell capacitors in dynamic random access memories; and Pb(Zr1-xTix)O3 (PZT), which is widely used in transducers and actuators. The rich variety of structural phases that these alloys can exhibit, and the challenge of relating their anomalous properties to the microscopic structure, make them attractive from a fundamental point of view. Theoretical investigations of modifications to the atomic ordering of these alloys suggest the existence of further unexpected structural properties and hold promise for the development of new functional materials with improved electromechanical properties. Here we report ab initio calculations that show that a certain class of atomic rearrangement should lead simultaneously to large electromechanical responses and to unusual structural phases in a given class of perovskite alloys. Our simulations also reveal the microscopic mechanism responsible for these anomalies.     

A high-strain-rate superplastic ceramic

High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10-2 to 10-1 s-1 and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based and magnesium-based alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10-5 to 10-4 s-1 for most oxides and nitrides with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and alpha-alumina phases exhibits superplasticity at strain rates up to 1 s-1. The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s-1. The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.     

大块准晶材料的制备

给出了一种大块准晶材料的制取方法,并且对所得材料的性能进行了研究。采用快速凝固制取几乎完全为二十面体相的准晶粉末,通过爆炸成形得到大块准晶材料。研究结果表明,在Al-Mn,Al-Cr和Al-Mn-Cr合金系列中,调整合金成分和冷却速度能够制得几乎完全为二十面体相的Al_(77.5)Mn_(22.5),Al_(82)Cr_(18)和Al_(78)Mn_(18)Cr_4准晶粉末,经爆炸成形后,所得的大块准晶材料仍然保持其二十面体相结构。     

Phase-field method and Materials Genome Initiative （MGI）

Predicting and controlling the microstructure evolution within a material are considered as the‘‘holy grail’’of materials science and engineering.Many important engineering materials are designed by controlling their phase transformations and microstructure evolution.Examples include the improvement of mechanical properties through solid-state precipitation reactions in Nibased superalloys and age-hardened Al alloys,the useful dielectric properties and electro-mechanical coupling effects by manipulating the phase transitions in ferroelectric crystals,and the memory effect of shape-memory alloys by utilizing martensitic transformations.Phase-field method has become the method of choice for modeling three-dimensional microstructure evolution for a wide variety of materials processes.This short article briefly discusses the potential roles that phase-field method can play in the Materials Genome Initiative.     

切削强烈塑性变形对材料微结构及硬度的影响

金属切削过程中,剧烈的大剪切变形可以产生具有超细晶结构的切屑从而使其获得了比本体材料更高的硬度和强度。本文对比了不同的金属和合金在各种刀具前角和切削速度条件下,切屑上产生的剪应变、切屑的微结构及其硬度的变化规律。实验结果显示随着刀具前角的减小切屑的微结构得到显著细化,其硬度随之得到极大地提高;而切削速度的减小提高了切屑的硬度但其微结构的变化不甚明显;采用负前角刀具在较低的切削速度下能加工出具有超细晶结构和高硬度的切屑材料,而切削速度的提高将使大剪切变形引起的硬度增长减弱。     

切削大变形对材料微结构及硬度的影响

在金属切削过程中,剧烈的大剪切变形可以产生具有超细晶结构的切屑,从而使其获得比本体材料更高的硬度和强度.文中比较了不同的金属和合金在各种刀具前角和切削速度下,切屑上产生的剪应变、切屑的微结构及硬度的变化规律.实验结果显示:随着刀具前角的减小,切屑的微结构显著细化,其硬度随之极大提高;切削速度的减小提高了切屑的硬度,但对其微结构的影响不甚明显;采用负前角刀具在较低的切削速度下能加工出具有超细晶结构和高硬度的切屑材料,而切削速度的提高将使大剪切变形引起的硬度增长变缓.