红外测温法研究Al-Mg合金中的Portevin-Le Chatelier效应

试件塑性变形过程伴随着机械能向热能的转化.利用红外测温法,通过分析红外热像仪采集的温度场图像,系统研究了Al-Mg合金中的Portevin-Le Chatelier （PLC）效应.在不同应变率下,实验得到了三类锯齿形应力-应变曲线,分析了相应情况下试件温度变化曲线的异同及其原因,探讨了三种类型PLC变形带的空间传播特性.研究发现,试件表面的温升随着应变率的增加而增加;PLC带的倾角转向发生在试件的两端或者带外的温度最高处. 关键词： Portevin-Le Chatelier效应 红外测温 Al-Mg合金     

Al-Cu多晶锯齿形屈服现象中的尺度效应

对Al-Cu多晶合金锯齿形屈服现象中各特征物理量(应力跌幅、跌落时间和再加载时间)随应变演化的规律进行了系统研究，讨论了不同厚度试件演化结果的区别.同时，这种尺度效应还表现为相同加载条件下1，2mm厚度试件与3mm厚度试件的Portevin-Le Chrtelier (PLC)类型的差异.利用非线性时间序列分析的方法对前者在相空间中吸引子的几何特性进行了描述，并通过主分量分析的方法初步判定其中存在着混沌行为.后者演化过程中应力跌落事件按跌落幅度统计分布符合幂律规律，具有自组织临界性特征.最后，借助自组织 关键词： 锯齿形屈服 尺度效应 Al-Cu合金     

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     

数字散斑法在局域剪切带三维变形研究中的应用

在适当的温度、应变率和预变形下,合金材料的拉伸试验中,将会出现伴随应力锯齿形跌落的雪崩式剪切变形带,即波特文-勒夏特利埃(Portevin-Le Chatelier,PLC)效应。利用高速数字摄像系统(分辨力1000 frames/s)并结合数字散斑干涉法(Digital speckle pattern interferometry,DSPI)和数字散斑相关法设计了一套光学变形测量系统,实现了拉伸试验中对试件表面三维变形的实时、精确测量。利用该光学系统对铝铜合金试件在拉伸试验中产生的跳跃传播的局域剪切带瞬态成核过程进行捕捉。通过结合数字散斑相关法得到的面内变形定量结果和数字散斑干涉法得到的表现离面变形的条纹图,再现了剪切变形带成核和传播瞬间的三维变形过程。     

球形孔开孔泡沫铝的力学特性及准静态压缩变形机制

对胞孔形态和尺寸较为一致的球形孔泡沫铝开展静-动态压缩实验,利用数字图像相关法研究了泡沫铝在准静态压缩过程中的宏观和介观变形机理。结果表明:球形孔泡沫具有明显的应变率效应,随着应变率的增加,平台应力及屈服强度增加,吸能效率也有所提高。由于胞元壁厚不均匀和孔壁缺陷的随机分布,泡沫铝在压缩过程中会出现多条局部变形带,单个胞孔表面在孔壁缺陷处也会出现应变集中带。胞元孔的变形模式主要有3种,轴向压缩、剪切、扭转加剪切复合变形,且整体变形带处的孔壁破坏模式以剪切变形为主,孔壁的变形模式又与孔壁自身厚度以及加载方向有关。     

重掺B对应变SiGe材料能带结构的影响

硅锗异质结双极晶体管(SiGe HBT)一般以重掺硼(B)的应变SiGe层作为基区.精确表征SiGe材料能带结构对SiGe HBT的设计具有重要的意义.在应变SiGe材料中，B的重掺杂一方面会因为重掺杂效应使带隙收缩，另一方面，B的引入还会部分补偿Ge引起的应变，从而改变应变引起的带隙变化.在重掺B的应变SiGe能带结构研究中，采用半经验方法，考虑了B的应变补偿作用对能带的影响，对Jain-Roulston模型进行修正，并分析了重掺杂引起的带隙收缩在导带和价带的分布.     

固溶处理对Al-Cu合金中Portevin-Le Chatelier效应空域行为的影响

研究了在定加载速率拉伸条件下，固溶处理对Al-Cu合金材料中PLC(Portevin-Le Chatelier)效应空域行为的影响.在较低拉伸速率时，变形初期PLC变形带在试件宽度方向中央附近某点处“成核”.随着变形的继续，出现多带共存的现象.且带宽较小，带与拉伸轴向的夹角较大.在较大拉伸速率条件下，PLC变形带先连续传播，再随机出现. 关键词： PLC效应 动态应变时效 固溶处理     

晶体相场法模拟微裂纹的扩展行为

用晶体相场模型模拟预变形条件下的单轴拉伸的裂纹扩展行为.展示出裂纹扩展过程的演化图。从裂纹演化图可以清楚的观察到,随着应变量的增加,裂纹长度增加,裂纹呈钝化-扩展-钝化的扩展模式,裂纹形状为锯齿形:随着预变形量的增加,解理方式的裂纹扩展减弱,转变成锯齿形状的裂纹扩展方式。     

Ta-10W合金压缩力学性能实验研究

利用MTS材料试验机和分离式Hopkinson压杆(SHPB)实验装置对非退火状态Ta-10W合金进行了准静态和动态压缩实验,给出了材料的静态压缩屈服强度和应变率在700~3 100 s-1范围内的动态压缩应力-应变曲线,并获得了不同应变率下材料的动态屈服强度。通过对实验结果的分析可以发现,非退火状态Ta-10W合金具有较好的韧性,在所进行的实验中试件表面均未出现可见裂纹;试件材料具有较高的静、动态屈服强度,静态屈服强度达到930 MPa,动态屈服强度在1 GPa以上,在所进行的700~3 100 s-1应变率范围内,材料的动态屈服强度随应变率的增加略有提高。     

高应变率变形图像去模糊及数字图像相关测量

针对材料高应变率动态变形中的高速运动模糊图像,提出了一种基于加载分析的点扩散函数去模糊方法,通过对模糊像素进行卷积,有效地去除了试件高速变形过程中所产生的运动模糊。通过霍普金森杆加载实验验证了该方法的有效性,与原始模糊图像的对比表明,该方法可有效地恢复运动模糊图像,从而提高数字图像相关测量的准确性和稳定性。     

Deformation measurements of three types of Portevin--Le Chatelier bands

In this paper a technique based on high-speed digital photography and the digital speckle correlation (DSC) method is used for the quantitative measurement of the displacement and strain fields of various Portevin--Le Chatelier (PLC) bands (types A, B, and C). The experimental results clearly show the nucleation process of a type-B band and the propagation of a type-A band. The results also reveal that there exists an elastic shrinkage deformation outside a PLC band during a large avalanche-like deformation inside the PLC band.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Markov property of continuous dislocation band propagation

Tensile tests were carried out by deforming polycrystalline samples of substitutional Al–2.5%Mg alloy at room temperature for a range of strain rates. The Portevin–Le Chatelier (PLC) effect was observed throughout the strain rate regime. The deformation bands in this region are found to be of type A in nature. From the analysis of the experimental stress time series data we could infer that the dynamics of type A dislocation band propagation is a Markov process.     

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M₃C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350?°C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.     

Effect of cyclic stresses on the microstructures of hexagonal close packed metals

Slip band extrusions are formed in cadmium, magnesium and titanium, but not in zinc. The extrusions form preferentially in untwinned crystals. Filamentary growths occur at {10¯12} and {11¯21} twin interfaces during cyclic twinning.Possible dislocation interactions at these twin interfaces are described. The dislocation debris produced during cyclic strain in the slip bands and by cyclic twinning is shown to be similar and composed of a high density of dipole loops. It is therefore concluded that the occurrence and distribution of slip band extrusions in metals and the formation of twin boundary filamentary growths can be accounted for by a model based upon the glide of interstitial type dipole loops. Vacancy type loops will then cause crack nucleation in slip bands and deformation twin boundary regions.Twin boundary debris can also cause the observed fragmentation of twins by acting as a barrier to twin boundary movement.The author is grateful to Dr. A. G. Crocker, University of Surrey, for many discussions on the twinning mode in h.c.p. metals and to P. J. E. Forsyth for his interest and encouragement. The paper is published by permission of the Controller, H. M. Stationery Office. Crown copyright is reserved.     

Yield phenomenon in oriented polyethylene terephthalate

The object of this investigation was to determine the origin of deformation bands in one-way oriented polyethylene terephthalate (PET) and their relation to the upper yield point. The tensile yield point was measured at room temperature as a function of the angle (θ) between the original draw direction and the tensile axis. The effect of strain rate was observed. The growth and geometry of deformation bands were studied. The results showed that the formation of a deformation band in PET cannot be interpreted as the strength-limiting yield mechanism for all θ and that the viscoelastic behavior plays the primary part in determining the yield point of this semicrystalline polymer.     

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT

In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45?μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.     

Nature of nanocrystallization in shear bands created by megaplastic deformation in amorphous alloys

A theoretical study is made of the process of nanocrystallization upon the formation of shear bands created by megaplastic deformation in amorphous metallic alloys. Such nanocrystallization is shown to be caused by a considerable increase in temperature inside the shear bands, which in turn is associated with the stored energy of megaplastic deformation. The temperature increment depends on the degree of deformation, the rate of propagation of the shear band, and the physical parameters that determine the thermal characteristics of an amorphous matrix in the range of the shear band.     

基于数字图像相关法的Mg含量对Al合金Protein-Le Chatelier效应影响的实验研究

利用数字图像相关法研究了常温与恒定应变率(5.00×10~(-3)s~(-1))下Mg含量(质量分数)分别为2.30%,4.57%,6.10%及6.91%四种Al-Mg合金Portevin-Le Chatelier(PLC)效应的宏观变形行为.实验发现,Mg含量的增加导致强化效果的增强;低Mg含量(2.30%)合金中锯齿跌落幅值基本保持不变,而在高Mg含量(4.57%,6.10%,6.91%)合金中随应变增加而增加.锯齿跌落幅值随Mg含量增加而逐渐增大,在高Mg含量合金中趋于饱和.宏观局域变形带的观察结果表明PLC带宽不随Mg含量或者应变改变而变化,带内变形量随着Mg含量或者应变的增加而逐渐增大.此外,在低Mg含量合金的加载曲线后段(应变约为0.3时)观测到了特殊的周期性的衰减锯齿,相应的时域PLC带演化表明加载曲线的周期性转变大锯齿对应着空间上PLC带的转向,幅度逐渐减小的振荡对应着PLC带的传播,且在转向前后PLC带均向上连续传播.