Al-4%Cu多晶合金中锯齿形屈服现象的初步时序分析

对Al-4％Cu合金试样拉伸实验的锯齿形应力曲线进行了系统的统计分析,研究了锯齿应力跌幅、跌落时间以及再加载时间等特征物理量随应变、试样厚度和加载应变率的时序演化规律．结果表明,跌落时间对应变不敏感;1和2mm厚试样的平均应力跌幅和平均再加载时间均随应变线性增加,3mm厚试样的统计结果比较复杂．展示了PLC效应中自组织临界性存在的证据,并通过与经典沙堆模型的对比,结合动态应变时效原理和位错理论,解释了PLC变形带形成的非线性机制．     

Al-Cu合金中PLC效应时域和空域行为的实验研究

在不同拉伸应变速率下,得到了Al-Cu合金中PLC(Portevin-Le Chatelier)效应在时域上的三类“锯齿形”应力一时间曲线,比较并研究了三种曲线的异同及其产生的定性解释．与之相对应,观察并分析了PLC效应空域上三种类型PLC变形带的空间传播特性．针对3mm厚的试件,探讨了在不同拉伸应变率下,类型A和B的PLC变形带的传播速率随变形过程的演化;得到了在相同应变下,变形带的传播速率和加载应变速率之间的幂律关系．同时,还将时域上的“锯齿形”应力一时间曲线与空域上的带的传播位置时间曲线相对应,得到了PLC效应发生时宏观上时域和空域之间的关系、另外,还讨论了试件厚度对PLC变形带的空间相关性的影响．     

自然时效对AA7075板材锯齿形屈服行为的影响

基于高强铝合金低温形变热处理工艺,研究了自然时效对AA7075铝合金力学性能的影响并使用透射电镜(TEM)对热处理后的组织形貌进行了观察.将板材试样单向拉伸与三维数字图像相关(3D-DIC)技术相结合,研究了自然时效对板材拉伸变形锯齿形屈服行为(Portevin-Le Chatelier,PLC效应)的影响,对B型、C...     

自然时效对6061铝合金锯齿形屈服的影响

对固溶处理淬火后自然时效时间不同的6061铝合金在室温下进行恒定应变率的拉伸试验,研究了自然时效对锯齿形屈服效应(Portevin-Le Chatelier,PLC)的影响。实验发现,材料强度、锯齿周期和临界应变三者在自然时效0~1 h期间均无明显变化,在自然时效超过1 h以后随自然时效时间延长而增大;锯齿幅值在自然时效0~2 h期间随自然时效时间延长而减小,在自然时效超过2 h后趋于稳定值。自然时效过程中,强化效果明显的β'相在自然时效时间1~2h之间开始形成并随自然时效时间线性增加。结合动态应变时效模型的分析结果与实验相符合,揭示了有强化效果的析出相对PLC效应的作用机制:具有强化效果的析出相含量增多将缩短可动位错的等待时间,使得可动位错更难被钉扎,从而导致锯齿周期和临界应变增大。分析结果还表明,该合金中溶质原子主要以体扩散方式向可动位错聚集从而实现对可动位错钉扎。     

Al-4%Cu合金中锯齿形屈服剪切带的变形测量

通过高速CCD（1000 fps）连续记录Al—4％Cu合金材料在拉伸实验中出现的B型锯齿形屈服（PLC）剪切带发生前后一系列非相干光散斑图,结合数字散斑相关法图像分析技术,定量地给出了PLC剪切带及周围区域的位移和应变的空间分布,再现了PLC剪切带随时间演化发展的过程．实验结果清楚地证实了B型PLC剪切带的一种演化机制,即先在试样一侧成核,然后与拉伸轴成一定角度横向贯穿整个试样,最终形成非均匀塑性变形带．实验结果表明,在PLC剪切带形成瞬间变形带外存在弹性收缩变形．     

热处理2024-O铝合金在自然时效下的拉伸性能及断口形貌演变

本文研究了时效时间对热处理后的2024-O铝合金试件的拉伸性能及断口影响。采用室温拉伸方法,研究了2024-O态铝合金经过热处理后不同时效时间试件的应力-应变曲线、屈服强度、抗拉强度,建立了能够预测应力的本构方程,并且通过观察断口,分析了不同时效时间对材料组织的影响。试验结果表明：随着时效时间的延长,试件逐渐强化,到某一时刻达到峰值,屈服强度、抗拉强度下降;应用SEM扫面电镜观察到断口处有大量韧窝,为韧性断裂,而且随着自然时效时间增长,试件断口的韧窝越来越少,塑性越来越差;建立了2024铝合金在自然时效下强化的本构方程,该模型能够较好地预测材料在自然时效下的应力。     

热处理2024-O铝合金在自然时效下的拉伸性能及断口形貌演变（英文）

研究了时效时间对热处理后的2024-O铝合金试件的拉伸性能及断口的影响。采用室温拉伸法,研究了2024-O态铝合金经过热处理后不同时效时间试件的应力-应变曲线、屈服强度、抗拉强度,建立了能够预测应力的本构方程,并且通过观察断口,分析了时效时间对材料组织的影响。结果表明:随着时效时间的延长,试件逐渐强化,到某一时刻达到峰值,屈服强度、抗拉强度下降;采用SEM观察到断口处有大量韧窝,为韧性断裂,而且随着自然时效时间增长,试件断口的韧窝越来越少,塑性越来越差;建立了2024铝合金在自然时效下强化的本构方程,该模型能够较好地预测材料在自然时效下的应力。     

微型铜圆柱体镦粗实验研究

为了找出微小尺度下材料流动及流动应力等的新特点及为微成形工艺设计、模具设计提供相关数据,在常温下以不同变形速度和变形量进行了微小尺度下圆柱体镦粗实验。结果表明,材料流动呈现出各向异性,且随压下量增大而明显;屈服应力降低,出现较大波动;流动应力出现明显波动;变形速度对材料流动、屈服应力、流动应力没有明显影响。本研究对于微成形技术的实用化具有一定意义。     

时效制度对大冷变形2024铝合金力学性能的影响

采用力学性能检测、透射电镜（TEM）观察等手段，研究了时效制度对大冷变形2024铝合金力学性能的影响。结果表明，大冷变形后合金的时效响应速度提高，时效20min时就接近峰值强度，比传统处理工艺（T62）缩短6h；合金的时效强化曲线呈双峰状，时效40min左右出现第一个峰，此时合金的强度最高，抗拉强度σb=580MPa，伸长率δ5=9．2％，时效120min左右出现第二个峰，但两个峰值点的屈强比（σ0.2／σb）差别较大，第二个峰值点的屈强比明显地大于第一个峰值点的屈强比；合金的伸长率δ5值呈阶梯形变化，时效时间≤40min时，δ5≥8％；时效时间≥60min时，δ5≤5％，且随着时效时间的延长，δ5值变化不大，说明大冷变形2024铝合金存在一个临界时效时间。     

6061铝合金板材锯齿形屈服行为的研究

对6061铝合金板材锯齿形屈服行为中应力跌幅、跌落时间等多个特征物理量随应变率的变化规律进行了系统的研究,探讨了锯齿形行为对材料力学性能的影响。结果表明,应力跌幅个数,跌落时间及其个数,再加载时间及其个数均随着应变率的增大而逐渐减少;当应变率为1．4×10^-3s^-1时,6061铝合金板材的锯齿形屈服行为最弱,板材的力学性能最好。     

LA41镁合金的PLC效应及其解释

通过单向拉伸实验在LA41镁合金中观察到了PLC(Portevin—Le Chatelier)效应,在整个实验的应变速率范围(3．33×10~(-4)—6．66×10~(-3)s~(-1))内这种塑性不稳定性始终存在,合金的抗拉强度和屈服强度随应变速率的增加而减小,其应变速率敏感系数始终为负值,且该不稳定性总是在一个临界应变之后出现,基于以上特征,运用动态应变时效(dynamic strain aging,DSA)机制,即形变过程中溶质原子与可动位错的交互作用,解释了所观察到的力学现象。     

Investigation into Characteristics of Portevin-Le Chatelier Effect of an Al-Mg Alloy

In this study, the plastic instabilities associated with the Portevin-Le Chatelier (PLC) and their effects on the mechanical properties and the fracture surfaces have been investigated for AA 5083. Tensile tests were performed at various temperatures and strain rates in order to do so. Then, serrated and smooth yielding domains were determined in Ln  $ \dot{\upvarepsilon } $ -1/T diagram. The stress-strain curves related to the serrated domain show the values of flow stress decreases by increasing the strain rate at a constant temperature. In addition, the plot of critical strain versus imposed strain rate indicates an inverse manner at very low strain rates. It is confirmed that the type of PLC bands alters the values of ductility. Also, scanning electron microscopy (SEM) was utilized to study the fracture surfaces of tensile test specimens. SEM images show that PLC effect or dynamic strain aging changes the fracture type and the feature of failure.     

The Effect of Electric Current and Strain Rate on Serrated Flow of Sheet Aluminum Alloy 5754

Electrically assisted tensile tests are carried out on sheet aluminum alloy AA5754 at electric current densities ranging from 0 to 30.4 A/mm² and strain rates ranging from 10^?3 to 10^?1 s^?1. The strain rate sensitivity and the serrated flow behavior are investigated in accordance with dynamic strain aging mechanism. The strain rate sensitivity changes from negative to positive and keeps increasing with current density. The tendency toward serrated flow is characterized by the onset of Portevin-Le Chatelier (PLC) instabilities, which are influenced by strain rate, temperature, and electric current. The evolutions of three types of serrated flow are observed and analyzed with respect to strain rate and current density. The magnitude of serration varies with strain rate and current density. The serrated flow can be suppressed by a high strain rate, a high temperature, or a strong electric current. The threshold values of these parameters are determined and discussed. Conventional oven-heated tensile tests are conducted to distinguish the electroplasticity. The flow stress reduces more in electrically assisted tension compared to oven-heated tension at the same temperature level. The electric current helps suppress the serrated flow at the similar temperature level of oven-heating.     

Flow Behavior and Mechanical Properties of a High Silicon Steel Associated with Dynamic Strain Aging

Flow behavior of two grades of steel including a high silicon (HS) steel and a plain low carbon steel as the reference were considered in this work. Tensile testing at temperatures varying between 25 and 550?°C and different strain rates in the range of 4?×?10^?5 to 0.1?s^?1 were conducted and the mechanical properties, such as elongation at fracture point and strain rate sensitivity were then determined. It is observed that for both steels, dynamic strain aging occurs in the employed deformation conditions, however, the region of serrated flow and the type of the serration were somehow different. For the case of the HS steel, the serrated flow region is shifted to the higher temperatures and also, the activation energy for appearance of dynamic strain aging increases as well.     

Serrated flow and stick–slip deformation dynamics in the presence of shear-band interactions for a Zr-based metallic glass

In this paper, shear-band interactions (SBIs) were introduced by a simple method and their effect on the dynamics of shear bands and serrated flow was studied for a Zr-based metallic glass. Statistical analysis on serrations shows that the stick–slip dynamics of interacting shear bands is a complex, scale-free process, in which shear bands are highly correlated. Both the stress drop magnitude and the incubation time for serrations follow a power-law distribution, presenting a sharp contrast to the randomly generated, uncorrelated serrated flow events in the absence of SBIs. Observations on the fracture morphologies provide further evidence and insights into the deformation dynamics dominated by SBIs. A stick–slip model for multiple shear bands with interactions is also proposed and numerically calculated. The results, in good agreement with the experimental results, quantitatively show how multiple shear bands operate and correlate, especially for those with large serrated flow events. Our studies suggest that one serration in the stress–strain curve may correspond to collective stick–slip motions of multiple shear bands for those ductile bulk metallic glasses where a large number of shear bands are observed during deformation.     

The influence of serrated flow on necking in tensile specimens

Three-dimensional finite element simulations are used to investigate the role of serrated flow on the strain at the onset of necking in a cylindrical uniaxial tension specimen. The material is idealized using a modified form of the McCormick constitutive equation, which has an additional material parameter that allows the rate of transient aging to be varied without affecting its steady-state response. Stability calculations and direct simulations show that, if the transient response is sufficiently slow, serrated flow can be suppressed, even though the material has negative steady-state strain rate sensitivity. This result is then used to determine the effect of suppressing serrated flow on the strain to localization. We find that negative steady-state sensitivity significantly reduces the strain required to initiate necking failure in a tensile specimen. However, the strain to failure is largely unaffected by the transient response of the material, and suppressing the serrated flow in particular has a negligible effect on the localization strain. We conclude that, while both serrated flow and reduced ductility are observed in materials with negative rate sensitivity, the reduction in ductility is not a direct consequence of serrated flow.     

Flow behavior and fracture of Al−Mg−Si alloy at cryogenic temperatures

The tensile and fracture behaviors of AA6061 alloy were investigated in order to provide quantitative data about this alloy at cryogenic temperatures. Specimens of AA6061 alloy were solution heat treated before tensile tests at 298, 173 and 77 K and tested at strain rates in the range from 0.1 to 0.0001 s^?1. The results indicate the suppression of the Portevin?Le Chatelier (PLC) effect and dynamic strain aging (DSA) at 77 K. In contrast, at 298 K, a remarkable serrated flow, characteristic of the PLC effect, is observed. Furthermore, the tensile behavior at 77 K, compared with that observed at 173 and 298 K, shows a simultaneous increase in strength, uniform elongation, modulus of toughness, strain-hardening exponent and strain rate sensitivity, which is related to a decrease in the dynamic recovery rate at low temperature. These responses are reflected on the fracture morphology, since the dimple size decreases at 77 K, while the area covered by dimples increases. Comparisons of the Johnson?Cook model show that a good agreement can be obtained for tests at 173 and 77 K, in which DSA is suppressed.     

INFLUENCE OF AGING ON SERRATED FLOW OF ALUMINUM ALLOY LC9

The serratedflow of alloy LC9 with different regimes of heat treatment has been investigatedat constant rate of stretching.The critieal strain ε_c at the starting of serrated flow increaseswith the extension of aging.The activation energy of the process is 0.63 e V which is consistentwith the diffusion activation energy of vacancies in A1.The ε_c decreases while the strain rate εincreases,i.e.,with negative strain rate relation.This relation is contrary to that of the an-nealed samples.It shows that the serrated 17ow of annealed sample is controlled bydeformation vaeancies,and that of aged sample is controlled by quenching vacancies.The den-sity and shape of the serration are changed by aging.The mechanism of the above mentionedresults is discussed.