Influence of solute cloud and precipitates on spatiotemporal characteristics of Portevin-Le Chatelier effect in A2024 aluminum alloys

In this paper, solute concentration and precipitate content in A2024 aluminum alloy are adjusted by solution treatment (ST) at different temperatures and tensile experiments on these treated specimens are carried out. It is found that the temperature of solution treatment (ST temperature) has a remarkable influence on the amplitude of the serrated flow and the propagation characteristics of shear bands. These results are due to the effects of solute atoms and precipitates on dynamic strain aging (DSA). When ST temperature is higher than 300~℃, solute concentration is relatively high and solute cloud is a key factor of DSA. When ST temperature is lower than 300~℃, precipitate content is relatively high and the mechanism of DSA is determined by precipitates.     

3004铝合金“反常”锯齿屈服现象的研究

在应变速率为5.56×10^-5s^-1—5.56×10^-3s^-1的范围内，在不同温度下(从223K至773K)，对3004铝合金进行系列拉伸试验，探索其锯齿屈服规律；通过激活能的计算、内耗研究、微观组织观察和能谱分析，探讨锯齿屈服的机理与物理本质.结果表明，3004铝合金在形变过程中会出现动态应变时效现象；发现了一种“反常”的锯齿屈服现象：在出现锯齿屈服的温区内，存在锯齿屈服临界应变量转变温度T_t， 关键词： 动态应变时效 锯齿屈服 铝合金 内耗     

Effect of boundary heat flux on solidification in a forced liquid metal flow: a phase-field simulation

A phase-field model coupling with velocity field is employed to study the effect of boundary heat flux on the microstructure formation of a Ni-40.8%Cu alloy with liquid flow during the solidification, and an anti-trapping current is introduced to suppress the solute trapping due to the larger interface width used in simulations than a real solidifying material. The effect of the flow field coupling with boundary heat extractions on the microstructure formation as well as distributions of concentration and temperature fields are analyzed and discussed. The forced liquid flow can significantly affect the heat and solute diffusions, thus influencing morphology formation, concentration and temperature distributions during the solidification. The solute segregation and concentration diffusion are changed by boundary heat extractions, and the morphology, concentration and temperature distributions are significantly influenced by increasing the heat extraction, which relatively makes the effect of liquid flow constrained. By increasing the initial velocity of liquid flow, the lopsided rate of the primary dendrite arm is enlarged and the growth manner of dendrite arms gets changed, and the transition of the microstructure from dendrite to cellular moves to the large heat extraction direction. Therefore, there exists the competition between the heat flux, temperature gradient and forced liquid flow that finally determines the microstructure formation during directional solidification.     

Al-4.5wt%Cu合金中溶质原子及析出相对锯齿形屈服时空特性的影响

借助不同温度下的固溶处理，得到一系列具有不同溶质原子浓度及析出相含量的Al-4.5wt%Cu合金材料试件.比较这些试件在拉伸实验中的锯齿形屈服现象的时空特性，分析溶质原子与析出相对位错运动的影响，从而探究动态应变时效产生的微观机理.实验结果显示，当固溶处理温度由500℃逐步降低，应力-时间曲线上的应力锯齿跌落幅值逐渐减小，并在300℃时达到最小.继续降低固溶处理温度至100℃，应力锯齿跌落幅值又逐渐增大.同时，热处理方式的改变对剪切带的传播特性也有显著影响.实验发现，固溶处理温度高于300℃时，溶质原子对DSA的影响强于析出相；反之，固溶处理温度低于300℃时，析出相对DSA的影响强于溶质原子；室温下析出相是影响Al-4.5wt%Cu合金DSA机理的主要因素. 关键词： 铝铜合金 锯齿形屈服 动态应变时效 固溶处理     

A new model of solution hardening in fcc metals based on the interaction with plural obstacles

A new model of solution hardening in fcc metals assumes that dislocation motion is controlled by a thermal activation process. In the model, the interaction of a dislocation with plural solute atoms is taken into account in a single activation event. The actual number of solute atoms which are involved in an activation event is determined by minimizing the activation energy. The model predicts a temperature dependence for the flow stress that agrees reasonably well with experimental results. Especially, it predicts the appearance of an inverse temperature dependence of the flow stress in the low-temperature region. Thus, the anomalous lowering of the flow stress at low temperatures, observed in some dilute alloys, can be explained solely by the dislocation–solute atom interaction. This is to be compared with the conventional explanation, in which another cause, the so-called inertial effect, was invoked. Another feature of the new model is that it provides a simple explanation for the occurrence of the stress equivalence phenomenon.     

铜镍合金的表面张力温度系数的蒙特卡罗模拟

本文采用面心立方体结构合金的嵌入原子模型和蒙特卡罗方法,通过计算平衡液态合金体系的内聚功求得铜镍合金的表面张力温度系数,并对模拟方法和结果进行了分析和探讨,给出了三种组分条件下的铜镍合金的表面张力与温度的关系。     

快速凝固Co-Cu包晶合金的电学性能

研究了Co-Cu包晶合金快速凝固过程中的相选择和组织形成特征, 探索了冷却速率、组织结构和晶体位向与合金电阻率之间的相关规律.实验发现, 快速凝固可使Co在(Cu)中的固溶度扩展至20%.Cu含量大于80%时, L+αCo→(Cu)包晶转变被抑制, (Cu)可从过冷熔体中直接形核析出.Cu含量在40%—70%范围时, Co-Cu合金的液相分离受到抑制, 凝固组织沿条带厚度方向分为两个晶区.细晶区中αCo和(Cu)相竞争形核并生长, αCo枝晶形态细密，细小的（Cu）等轴晶均匀分布于αCo的基体之中.粗晶区αCo相为领先相, 富Cu相分布于αCo枝晶的晶界处.随着冷速的增大, 合金组织显著细化, 晶界增多,对自由电子的散射作用增强, 合金电阻率显著增大.当晶界散射系数r＝0996—0999时, 可采用M-S模型综合分析快速凝固Co-Cu合金的电阻率. 关键词： 电阻率 快速凝固 相结构 晶体生长     

铝铜合金在范性形变过程中的低频内耗

在小型拉力试验机上测量了几种铝铜和铝镁合金试样在拉伸形变过程中的低频内耗,观测到在内耗-应变曲线上出现一系列的跳跃现象。研究了测量温度和一些冶金因素对于这种跳跃现象的影响。实验结果指出,出现这种现象的温度范围与铝铜合金中出现台阶状或锯齿状应力-应变曲线的温度范围相对应。这表明,这种现象与在范性形变过程中所进行的应变时效过程有关,是由于位错气团在形变过程中的更迭形成和解脱所引起。 关键词：     

旋转环形浅液池内双组分溶液耦合热-溶质毛细对流渐近解

为了了解水平温度梯度作用下旋转环形浅液池内耦合热-溶质毛细对流基本特征, 采用匹配渐近展开法对旋转环形浅液池内耦合热-溶质毛细对流过程进行了求解, 获得了中心区域的速度、温度和浓度分布,分析了旋转、Soret效应、浮力、溶质扩散 系数和液池的几何尺寸对流动结构的影响.将所得到的渐近解和文献中的已有结果进行对比,证明了所求结果的正确性;在浅液池内,耦合热-溶质毛细力对流体流动起主导作用, 旋转和浮力对流动的影响较小,溶质扩散系数和几何尺寸有较明显影响;当各种耦合的 驱动力作用方向相同时,流动增强;否则, 流动减弱. 关键词： 旋转 环形浅液池 耦合热-溶质毛细对流 渐近解     

相场法模拟多元合金过冷熔体中的枝晶生长

在二元合金相场模型研究的基础上，进行扩展获得了多元合金相场模型.以Al-Si-Mg三元合金为例，采用该相场模型实现了逼真地模拟多元合金凝固过程的等轴枝晶生长，得到了二次或更高次晶臂生长等复杂的枝晶形貌.随着第三组元Mg含量的减少，枝晶的二次枝晶越发达，枝晶中溶质的偏析越严重，枝晶尖端的生长速率和半径越大，与丁二腈-丙酮体系中枝晶尖端生长速率、半径随溶质浓度变化关系的理论计算和实验结果相符合.另外，枝晶初生晶臂中心的溶质浓度最低，在被二次晶臂包围的界面区域的溶质浓度最高；固液界面区域具有较大的浓度梯度，其中枝晶尖端前沿的梯度最大. 关键词： 相场法 多元合金 凝固过程 枝晶生长     

Metastable surface alloys produced by ion implantation,laser and electron beam treatment

Ion Implantation, Laser and Electron-beam Treatment (LET) of metals have been employed extensively to produce metastable surface alloys. Recent published work on implanted alloys is reviewed first. The dilute implanted alloys (solute concentration <10 at. %) are shown to lead to crystalline metastable solid solutions. At higher solute concentrations, an amorphous phase has been observed for several binary systems and recently for a ternary system. The physical mechanisms at play, are discussed in detail. A review of the surface alloys produced by LET of metals is then presented—with an emphasis on the mechanisms involved. In particular, general criteria governing formation of metastable solid solutions under LET are proposed and shown to have excellent agreement with available data on metals and Si.     

相场法模拟NiCu合金非等温凝固枝晶生长

利用相场模型与溶质场、温度场进行耦合计算,以Ni-40.83%Cu合金为例模拟了二元合金枝晶生长过程.系统研究相场模型中相场和温度场耦合强度对枝晶形貌和浓度分布的影响.模拟结果表明:随着耦合强度的增加,相场受温度场的影响加大,界面前沿变得不稳,扰动被放大,主枝上出现了二次枝晶.同时,枝晶尖端的生长速率增大,而枝晶尖端的曲率半径减小,枝晶前沿的溶质富集现象也更严重;另外,计算结果与Ivantsov理论符合较好. 关键词： 相场法 NiCu合金 枝晶生长 Ivantsov理论     

电阻率与强度性能的关联及铜合金性能分区

铜合金以低电阻率为特征,由于电阻率与强度存在着共同的微观结构机理,两者往往协同变化,而导致难以对合金进行性能的全面评估和选材.本文以Cu-Ni-Mo合金作为研究对象,以团簇结构[Mo_1-Ni_(12)]构建固溶体的近程序结构模型,解析了电阻率和强度依赖于成分的定量变化规律,并定义了拉伸强度/电阻率的值为代表合金本质特性的"强阻比",得到了完全固溶态Cu-Ni-Mo合金的强阻比为7×10~8MPa/?·m,完全析出态的强阻比为(310—490)×10~8MPa/?·m.进而应用强阻比对常用铜合金进行了性能分区,给出铜合金材料选材的依据,得出了基于Cu-(Cr, Zr, Mg, Ag, Cd)等二元基础体系的铜合金适用于高强高导应用,而基于Cu-(Be, Ni, Sn, Fe, Zn, Ti, Al)等为基础二元体系的铜合金不能实现高强高导.该强阻比为310的特征性能分界线的发现为合金性能的全面评估提供了量化依据,可指导高强高导铜合金的选材和研发.     

Effect of rotating magnetic field on microstructure formation of directionally solidified Sn–1.6Cd peritectic alloy

In order to investigate the effect of rotating magnetic field on the microstructure formation of peritectic alloys, directional solidification experiments of Sn–1.6Cd peritectic alloy have been conducted under different rotating magnetic field conditions. The directional solidification microstructure of Sn–1.6Cd peritectic alloy changes from banded structure to axisymmetric isolated banded structure to axisymmetric oscillatory tree-like banded structure and to single primary phase structure when the magnetic Taylor number of forced-melt flow generated by a rotating magnetic field increases from 0 to 91 to 364 and to 1456. The second and third banded structures are observed in a peritectic alloy for the first time. The results indicate that it is possible to control solidification microstructure and prepare a single primary phase structure by using a rotating magnetic field during directional solidification of peritectic alloys. The experiments show that the effect of forced-melt flow on solute distribution transforms from solute buildup to homogenization with an increase in the magnetic Taylor number. The formation mechanisms of those structures are also discussed.     

W-In体系溶质晶界偏聚行为的第一性原理计算

基于第一性原理构建了钨基合金体系的溶质偏聚模型,以W-In体系为例研究了不同浓度下溶质的晶界偏聚行为和成键特征,从电子结构层面揭示了W-In体系的键合作用,预测了W-In体系界面稳定性随溶质浓度的变化规律.结合键布居、电荷密度、差分电荷密度和态密度等电子结构分析,发现了W-In体系中溶质原子在偏聚过程中的键性转变特征,阐明了W-In键由晶粒内部的离子键过渡为晶界区域强共价键的微观机理.模型计算首次得到了W-In体系中溶质本征偏聚能随In浓度的非单调变化规律,结合键合作用和能量分析揭示了溶质浓度对本征偏聚能的影响机制.计算预测了W-In体系达到高热稳定性所需的最佳溶质浓度范围和应避开的溶质浓度范围.本研究为具有高温稳定性的钨基合金材料的设计与制备提供了理论基础和定量化指导.     

析出相在铝镁合金Portevin-Le Chatelier效应中的作用研究

本文研究了在相同退火热处理条件下,不同析出相含量的两种铝镁合金5456和5052在不同加载应变率下的P-LC效应. 结果表明析出相对两种材料的P-LC效应有明显影响,并且析出相含量的不同所产生的影响不同. 此外,析出相含量较少的5052合金的溶质原子的扩散方式是管扩散,而析出相含量较多的5456合金不再局限于管扩散的方式,而是更为复杂. 关键词： Portevin-Le Chatelier effect 动态应变时效 铝镁合金     

Experimental determination of interfacial energies for Ag2Al solid solution in the CuAl2--Ag2Al system

The equilibrated grain boundary groove shapes of solid solution Ag2Al in equilibrium with an Al-Cu-Ag liquid were observed from a quenched sample with a radial heat flow apparatus.The Gibbs-Thomson coefficient,solid-liquid interfacial energy and grain boundary energy of the solid solution Ag2Al have been determined from the observed grain boundary groove shapes.The thermal conductivity of the solid phase and the thermal conductivity ratio of the liquid phase to solid phase for Ag2Al-28.3 at the %CuAl2 alloy at the melting temperature have also been measured with a radial heat flow apparatus and Bridgman type growth apparatus,separately.     

Oxygen generation in anodized Ta–Cu alloys

Comparison is made of the influences of copper in the development of amorphous anodic films on Ta-Cu alloys, containing up to 57 at.%Cu, in ammonium pentaborate electrolyte at 293 K. The various films are based on tantala, with low concentrations of copper, relative to those of the alloys, since copper species migrate through the films more rapidly than Ta⁵⁺ ions and are lost to the electrolyte on reaching the film surface. Of particular interest, film growth on Ta-1.5at.% Cu alloy at 1, 0.1 and 0.01 mA cm^?2 proceeds with generation of oxygen, which is contained in nanoscale bubbles within the oxide. Bubbles are more numerous as the current density decreases. In the case of Ta-12at.% Cu and Ta-57at.%Cu alloys, the films are flawed extensively by more numerous oxygen bubbles, which are present mainly in the outer parts of the anodic films; oxygen production and film damage due to the bursting of bubbles limit the anodizing voltage to relatively low values. It is suggested that copper species modify the electron levels, and possibly structure, of the tantala-based films, thereby facilitating oxygen generation within the bulk film by oxidation of O^2- ions of the oxide.     

Effect of geometrical stress concentrators on the band formation and the serrated deformation in aluminum–magnesium alloys

The effect of holes on the band formation and the serrated deformation in planar specimens of aluminum–magnesium alloys AlMg5 and AlMg6 is studied by high-speed video filming of moving deformation bands. It is found that the concentration of an elastic field near a hole causes early nucleation of macrolocalized deformation bands and decreases the critical deformation of the first stress drop. Differences between the spatial–temporal patterns of deformation bands near holes under various deformation conditions are revealed.     

Selective appearance of ϵ-martensitic transformation and dynamic strain aging in Fe–Mn–C austenitic steels

The influence of stress-induced ε-martensitic transformation on the serrated flow behavior associated with dynamic strain aging was investigated. The ε-martensitic transformation was controlled by changing the deformation temperature and adding Si to Fe–17Mn–xSi–0.3?C alloys. The addition of Si promoted the ε-martensitic transformation, and suppressed the slip deformation due to solution hardening. The initiation of serrations around room temperature was delayed by the promotion of ε-martensitic transformation which initiated plastic deformation. The critical stress for the occurrence of serrations and the critical stress for the occurrence of slip deformation were found to have a linear relationship.