冷加工变形对IF钢位错密度的影响

研究了冷加工变形对IF钢位错密度的影响。结果表明:IF钢经冷加工变形后,晶体内产生大量位错,随着变形量逐渐增高,位错密度增大,位错开始相互缠结在一起,当变形量达到20%以上时,晶体内出现纤维组织,并进一步转变成胞状亚晶粒。在一定变形量范围内,平均位错密度ρ与变形量A符合线性关系:ρ=2.205 17×1014+1.049 5×1016 A。     

D6A钢在轧制过程中的强韧化机理EI北大核心CSCD

通过轧制-热处理工艺能够使D6A钢的强度显著提高。为了探究其强韧化机理,本实验采用热轧及两相区温轧退火工艺,获得微米级D6A合金钢样品,微观组织为铁素体基体及粒状渗碳体。通过室温拉伸实验、SEM、X射线衍射、EBSD等手段对实验钢的显微组织和力学性能进行表征,结果表明:随着变形量的增加,晶粒尺寸由4.5μm细化为1.5μm,渗碳体的含量逐渐增加,小角度晶界的比例升高,屈服强度和抗拉强度不断增加,伸长率略有降低,说明轧制过程使亚晶粒的尺寸不断降低,晶界面积增加,位错滑移受到的阻力增大。同时,本研究对不同轧制变形量实验钢的位错密度进行计算,当轧制变形量为88%时,位错密度最高,此时加工硬化的程度最高。随着变形量的增加,第二相强化和晶粒细化引起的强度增量呈不断上升的趋势,位错强化引起的强度增量先升高后降低,D6A钢的主要强化方式为第二相强化、细晶强化及位错强化。     

41Cr4钢高温加工过程流变应力模型建立

为了获得高温加工过程中41Cr4钢的动态再结晶体积百分数、真应力、真应变以及稳态应力和稳态应变,在不同变形温度、变形量和变形速度条件下,在GLEEBLE 3500试验机上完成了物理模拟试验.实验结果表明:变形刚开始时位错密度增大较快,因此应力迅速增大,产生加工硬化过程,随着应变的累积,动态回复的发生导致应力增加速度减慢,当应变进一步增大到超过临界应变时,动态再结晶软化导致流变应力降低.在此基础上建立41Cr4钢的流变应力模型时,考虑了加工硬化、动态再结晶以及动态回复对流变应力的影响,给出了与实验值比较接近的41Cr4钢的流变应力模型.     

RE-Al-Zn-Mg 合金超塑变形中位错密度及位错组态的变化

本文用 X 射线线形分析方法系统研究了几种含稀土和不含稀土的 Al-Zn-Mg 合金超塑变形中位错密度和位错组态的变化,讨论了动态再结晶、空洞的形核与长大对位错密度和位错组态的影响,分析了含稀土的 Al-Zn-Mg 合金超塑性提高的原因。     

基于位错密度的钛合金电辅助压缩仿真与实验验证

目的 以新型高强韧Ti?6Cr?5Mo?5V?4Al(Ti6554)近β钛合金为对象,探讨脉冲电流对材料变形行为和温度变化的影响规律,揭示Ti6554钛合金在不同电流密度下的位错密度演化规律。方法 对材料进行不同电流密度、占空比、应变速率条件下的电辅助压缩实验,建立考虑位错密度的修正电塑性本构模型,基于ABAQUS进行UMAT子程序开发,建立电?热?力三场耦合有限元模型,模拟Ti6554钛合金电辅助压缩变形过程,并进行实验验证。结果 随着电流密度和占空比增大流动应力减小,随着应变速率增大流动应力也增大;电辅助压缩实验结果与模拟结果相比的平均误差为6.31%,验证了模型的有效性;通过子程序状态变量输出位错密度的变化发现,电流密度为15.92、23.88、27.87、31.88、39.81 A/mm²的位错密度分别下降了15.34%、55.63%、68.23%、83.84%、89.13%,表明位错密度随电流密度的增大而降低。结论 建立了基于位错密度的电塑性本构模型和电-热-力多场耦合的有限元模型,能够模拟Ti6554钛合金的电压缩变形行为,并且表征了位错增殖、位错湮灭及动态回复,获得了其位错密度的演化规律。     

大块Fe-3%Si单晶体裂纹尖端无位错区的观察

利用蚀坑技术和扫描电镜动态观察方法研究了大块 Fe-3%Si 单晶体裂纹尖端的形变行为。证实了裂纹尖端与塑性区之间存在无位错区(DFZ),且无位错区尺寸随变形量的增加而增加。     

冷变形对GH3535合金的组织和力学行为的影响

对GH3535合金进行不同变形量的拉伸,研究了冷变形对GH3535合金力学性能的影响,用OM、TEM对冷变形后的组织结构进行观察并结合真应力-真应变曲线分析了GH3535合金加工硬化的变形机制。结果表明,GH3535合金具有显著的加工硬化特性,冷变形使其强度和硬度提高,但是塑性降低;随着变形量的增大晶粒沿着变形方向伸长,孪晶变多且更粗大。GH3535合金的加工硬化规律遵循Ludwigson模型,随着冷变形量的增大GH3535合金的加工硬化指数降低,变形机制由单滑移和孪生逐渐变成交滑移和孪生。当变形量小于30%时加工硬化主要是位错的长程应力场和孪晶引起的,当变形量大于30%时主要是位错的短程应力场和形变孪晶引起的。     

一个特殊单滑移取向铜单晶疲劳位错结构研究

为了更细致地揭示面心立方金属单晶体的循环变形机制,利用扫描电镜电子通道衬度(SEM-ECC)技术观察研究了Schmid因子为0.5的[41841]单滑移取向铜单晶体的循环饱和位错结构.实验表明,在单滑移铜单晶体中,胞结构除了在高应变幅下的循环变形中出现外,还可能出现在循环应力-应变(CSS)曲线平台区的较低塑性应变幅下.驻留滑移带(PSBs)会随应变幅的增大而在试样表面聚集成内部含有位错胞的粗滑移带,带内的位错胞结构被认为是由于带内滑移阻力增大引起的应变集中所致形成的.此外,CSS曲线高应变幅区起始部分对应的循环饱和位错结构观察揭示出迷宫结构和胞结构是由PSBs逐渐演变而成的.     

纳米结构金属位错的研究进展

综述了国内外近年来对纳米结构金属位错的研究,包括位错的基本特征、研究方法以及定量分析.由于晶粒尺寸的减小,全位错的形成和运动变得困难甚至不可能,纳米结构金属更容易生成不全位错.在高分辨TEM图像观察实验中发现了大量孪晶或层错,也证实了不全位错的存在.着重讨论了晶界发射不全位错的形核、增殖以及在塑性变形过程中所起的作用.研究了纳米结构金属中的位错动力学,采用分子动力学模拟和高分辨透射电镜方法从不同层面上揭示了位错的形核、增殖、运动以及相互作用等过程.最后简单介绍了位错柏氏矢量以及密度的相关定量分析,其相关参数的表征对进一步弄清纳米结构金属的塑性变形机制具有重要意义.     

35CrMnSi钢温成形力学行为研究

为了合理制定35CrMnSi钢温成形工艺参数，利用热模拟实验对该材料在温成形条件下的变形抗力进行了研究，考察了变形温度、应变速率及变形程度与变形抗力之间的关系，并利用冶金学的方法对其进行了分析．结果表明：35CrMnSi钢在740—860℃变形时，具有较强的应变速率敏感性，变形抗力随应变速率的增加而上升；随应变速率的增大，温度对变形抗力的影响减弱；在较低温度成形时(740、780℃)，变形抗力较低，并有明显的软化趋势；随着成形温度的升高，材料在发生相变后变形抗力上升，且表现出加工硬化的趋势．     

Orientation dependence of dislocation structure evolution during cold rolling of aluminum

A well-organized dislocation structure forms in many polycrystalline metals during plastic deformation. This structure is described qualitatively with no explanation of the quantitative characterization. In this work, the evolution of dislocation structure in commercial purity aluminum is described by means of the excess dislocation density and by quantitative characterization of the cell structure as seen on a plane surface. The measurements were performed on a pseudo-internal surface of a split specimen deformed by channel die deformation. The results show a clear dependence of cell structure formation on orientation of the crystallite with respect to the imposed deformation gradient with the largest excess dislocation density occurring in grains of {0 1 1}[1 2 2] orientation for plane strain deformation. Neighboring grain and non-local effects are shown to be of importance in the type of dislocation structures that evolve.     

冷轧态AI-Li-Cu-Mg-Zr合金在脉冲电流作用下超塑形变中的位错形态

冷轧态 ＡＩ－Ｌｉ－Ｃｕ－Ｍｇ－Ｚｒ（２０９１ＡＩ－Ｌｉ）合金在脉冲电流作用下进行超塑形变的过程中不断 产生动态再结晶与此同时,晶内位错由稀少、平直的形态随着形变量的增加而逐渐增殖、弯曲,最后转变 成为蜷线位错．脉冲电流加强空位活动,首先影响位错的攀移,其次影响晶界粘滞滑动．     

Interaction of a transonic dislocation with subsonic dislocation and point defect clusters

The moving speeds of all observed dislocations in crystals are subsonic. There has been a view in the literature that the speed of subsonic dislocations can not be accelerated above the speed of sound because the energy required would be infinitely large. Recent molecular dynamics (MD) simulation had shown that it is possible to generate dislocations with an initial moving speed higher than the velocity of sound in solids. This raises a question: what will happen when a supersonic dislocation meets other defects along its moving path? This work reports the results of MD simulation on the interaction of a transonic dislocation with other subsonic dislocations as well as with point defect clusters. The results show that a vacancy cluster such as a void has an insignificant slow-down effect on the transonic dislocation, while a subsonic dislocation slows down the transonic dislocation to subsonic one. In some cases, the subsonic dislocation (or a subsonic part of a transonic dislocation) can overcome the traditional sound barrier.     

Defect hardening modeled in 2D discrete dislocation dynamics

Two-dimensional discrete dislocation dynamics simulations are used to model the plastic deformation of an fcc metallic material containing large densities of defects. An obstacle model is proposed, based on the line tension concept. Increasing yield strength and hardening are obtained when the obstacle density is increased and destroyable junctions are included. A high dislocation source density is used to obtain a good dissemination of dislocations. Over 30% of the total density is stored as junctions. Slip is shown to be localized within a few intense slip bands, whatever the obstacle density. This localization is quantified as a function of the density of obstacles.     

The combined influence of grain size distribution and dislocation density on hardness of interstitial free steel

Understanding the relationship between microstructure features and mechanical properties is of great significance for the improvement and specific adjustment of steel properties. The relationship between mean grain size and yield strength is established by the well-known Hall-Petch equation. But due to the complexity of the grain configuration within materials, considering only the mean value is unlikely to give a complete representation of the mechanical behavior. The classical Taylor equation is often used to account for the effect of dislocation density, but not thoroughly tested in combination with grain size influence. In the present study, systematic heat treatment routes and cold rolling followed by annealing are designed for interstitial free(IF) steel to achieve ferritic microstructures that not only vary in mean grain size, but also in grain size distribution and in dislocation density, a combination that is rarely studied in the literature. Optical microscopy is applied to determine the grain size distribution. The dislocation density is determined through XRD measurements. The hardness is analyzed on its relation with the mean grain size, as well as with the grain size distribution and the dislocation density. With the help of the variable selection tool LASSO, it is shown that dislocation density, mean grain size and kurtosis of grain size distribution are the three features which most strongly affect hardness of IF steel.     

Microcrack initiation and growth in heat-resistant 15Kh2MFA steel under cyclic deformation

This paper presents the results of investigation of a nuclear reactor pressure vessel steel 15Kh2MFA of two strength levels under cyclic loading. The mechanism of microcrack formation on the surface and in the bulk of 15Kh2MFA steel under cyclic deformation was investigated. Analysis of the specimen surfaces has shown that microcracks are caused by cyclic sliding in grains most favourably oriented with respect to the direction of the maximum shear stresses. Transmission electron microscope investigations show that microcracks in the material inside the grains are formed mainly along the band‐type dislocation structure parallel to the dislocation subboundary. During cyclic deformation, the dislocation density on the subboundaries increases, in the local areas the dislocation density becomes limiting and it reaches the plasticity limit and causes microcrack formation. The interrelation of the average length of microcracks and their surface density with the energy density of inelastic deformation has been found.     

考虑位错密度和损伤的NiCoCrFe高熵合金晶体塑性有限元分析EI北大核心CSCD

结合实验和晶体塑性有限元方法研究准静态加载NiCoCrFe高熵合金有限变形过程中的宏观和微观力学响应、损伤行为以及微观结构演化。使用电子背散射衍射技术(EBSD)对拉伸实验变形前后NiCoCrFe的微观结构进行表征。通过修改强化模型和流动准则分别在CPFEM模型中引入位错密度内部状态变量和连续介质损伤因子,并结合拉伸实验应力-应变曲线确定NiCoCrFe相关的模型参数。结果表明:考虑位错密度和损伤的CPFEM模型可以有效地描述NiCoCrFe宏观和微观力学响应。CPFEM模型合理预测NiCoCrFe颈缩区域的变形形状和尺寸,其中实验获得的颈缩区域长度比预测结果小7%,CPFEM预测的颈缩区域宽度比实验结果大23%。CPFEM模型预测NiCoCrFe拉伸变形后的织构演化同EBSD表征结果大致相同,均表现为弱的(100)∥RD以及强的(111)∥RD纤维织构。在三维微观结构损伤分析中,CPFEM模型预测的损伤在应力集中以及位错密度集中的晶界处萌生,表现为晶间损伤机制,并且随着变形的增加损伤逐渐向晶粒内部扩展。     

Discrete dislocation dynamics simulations of surface induced size effects in plasticity

Experiments have shown that the presence of free surfaces may induce harder as well as softer deformation behaviors in a crystalline solid. In order to shed some light on these apparently contradictory findings, two-dimensional discrete dislocation dynamics simulations are performed to investigate the surface induced size effects. The simulations indicate that, depending on the surface density of dislocation sources, a free surface may act either as a dislocation sink or as a net dislocation source, and can accordingly exert opposite effects on dislocation density over a boundary layer thickness of up to 500 nm into the bulk. This finding provides a possible explanation for the apparent contradictions in experimental observations.     

A dislocation theory of isotropic polycrystalline plasticity

A theory of polycrystalline plasticity is developed in which the polycrystalline solid is modeled as an isotropic continuum. The rate of plastic deformation tensor is shown to be a function of the mobile dislocation density and the dislocation velocity vector summed over all active glide planes. The dislocation velocity vector is expressed in terms of the stress tensor and the normal vector to the dislocation glide plane. The condition of plastic incompressibility yields the fact that the dislocation glide planes are the octahedral shear planes of the stress tensor. As a special case the rate of plastic deformation tensor reduces to a relation analogous to the Prandtl-Reuss flow rule. The theory has been implemented in a 2-dimensional finite element code and two example problems are presented.     

A dislocation model of the plastic deformation of single crystals of ice

A new dislocation model is constructed on the basis of various characteristics of dislocations revealed by X-ray diffraction topography, for interpreting plastic deformation behaviour of ice single crystals in basal glide. The model of a pair of screw dislocation arrays of opposite sign exhibits resistance for the movement, which depends upon both the configuration and the stress. Orowan's relation between the macroscopic strain rate and characteristic of dislocations in the crystal (density and velocity) is rewritten in a dynamical style taking into account the resisting stress and the empirically established linear relationship between the dislocation velocity and the stress. In this formulation, a new concept of fractional dislocation density is introduced.Examples of fractional density as a function of maximum stress are obtained from our stress relaxation experiments. Assuming that the initial fractional density profile for a fresh ice single crystal is similar to those obtained above, stress-strain curves are calculated numerically for various crosshead speeds of the test machine. Computed results coincide well with the characteristic stress-strain curves previously obtained experimentally.