Models and observations of fcc/bcc interfaces

Whereas models of the structure and periodicity of CSL or near CSL high angle grain boundaries are relatively well developed and have been to some extent verified by experiment, the nature of more general (e.g. fcc/bcc) interfaces is very imperfectly understood. One of the major differences between homo- and heterophase boundaries is the occurrence, in the latter case, of reproducible orientation relationships due to the crystallographic requirements of phase transformation (e.g. a glissile interface for martensitic growth, low energy interfaces for classical nucleation). A review is given of the relationships commonly observed to obtain between fcc and bcc crystals (as well as ordered phases based upon these structures). Within a given relationship, it is possible to use models of the available interfaces to predict their dislocation content; the success of the various approaches is considered. It is shown that the dislocation arrays identified on fcc/bcc boundaries are consistent with the occurrence of a primary structural relaxation, but that the latter does not appear to be complete. Possible reasons for this are discussed, together with the extent to which secondary structural models may be applicable. Attempts to rationalize the observed orientation relationships and morphologies are discussed, and it is pointed out that these all rely on searches for situations of minimum primary misfit, but that the way in which the latter is quantified determines the results of the analyses. Nevertheless, the implication is that the primary misfit does appear to determine the actual behaviour, though no structural or mechanistic conclusions can safely be drawn from this observation.     

Recent progress in the modeling of high-temperature creep and its application to alloy development

Recent progress in the understanding of high-temperature creep of alloys is discussed in the context of theoretical modeling and its application to alloy development. Emphasis is placed upon those engineering alloys specifically designed for high-temperature applications, such as precipitation and dispersion-strengthened (DS) alloys and metal-matrix composites (MMCs). Currently, these theoretical models use one of two different approaches, (a) a phenomenological approach, which is used in such models as those based on the internal stress concept, and those based on empirical creep equations; and (b) micromechanical models that are based on dislocation mechanisms and the interactions of dislocations with solute atoms, second-phase particles, and other reinforcements such as fibers. All these theoretical models have a common goal, namely, the understanding of high-temperature strengthening mechanisms and the relationship between high-temperature strength and the micromechanical mechanisms during high-temperature plastic deformation of the alloys. These theoretical studies can provide information that is useful in alloy design and processing, such as the selection of alloy chemistry, and the optimization of phase microstructural features (e.g., reinforcement amount, shape, size, and distribution; matrix grain size; and matrix and reinforcement interfaces) by optimization of processing methods. L. Shi, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22901, USA     

A kinetics formulation for low-temperature plasticity

The mechanism of low-temperature plastic deformation is controlled by thermally activated dislocation movements. An evolutionary constitutive law based on the principles of deformation kinetics is described in this article. The constitutive law is expressed with a sinh function designed for computational efficiency. It is derived from rigorously defined kinetics principles. The approximation involved in the sinh function is defined so that in applications an exact evaluation can be made of the validity limits. The system of the constitutive law and the external constraints lead to the operational equations. Applications are developed for constant strain-rate loading, constant stress-rate loading, stress relaxation, creep, and ratchetting processes. The analysis provides a unified treatment for low-temperature plastic deformation.     

A study of the influence of grain boundaries on short crack growth during varying load using a dislocation technique

The propagation of short cracks in the neighbourhood of grain boundaries have been investigated using a technique were the crack is modelled by distributed dislocation dipoles and the plastic deformation is represented by discrete dislocations. Discrete dislocations are emitted from the crack tip as the crack grows. Dislocations can also nucleate at the grain boundaries. The influence on crack growth characteristics of the distance between the initial crack tip and the grain boundary has been studied. It was found that crack growth rate is strongly correlated to the dislocation pile-ups at the grain boundaries.     

CSP工艺热轧低碳钢板的强化机制

采用金相显微镜、H—800透射电镜和正电子湮没方法分析了CSP热轧低碳钢板金相组织、析出物形貌、尺寸、分布及位错密度。结果表明:CSP工艺热轧低碳钢板的晶粒较为细小,约为5.3μm;当累积变形量较小、变形温度较高时,析出物主要在晶界上,数量少见比较粗大,其尺寸大多大于150nm;当累积变形量较大、变形温度较低时,析出物主要在晶内,细小、弥散且数量较多,其尺寸大多为20～100nm,析出物主要为Al_2O_3、MnS或Cu_7S_4;随着累积变形量的增加,位错密度明显增加,终轧后轧件的位错密度约为6.35×10~(14)m~(-2)。晶粒细化、析出物弥散分布及位错密度增加是CSP工艺热轧低碳钢板强度高的决定因素。     

含微量稀土元素的Al-Zn-Mg合金的超塑变形机制

用透射电镜详细观察了含微量稀土的 Al-Zn-Mg 合金在超塑变形过程中晶界位错的形态、运动及其与晶界第二相粒子的相互作用情况。发现晶界中的第二相粒子对位错沿晶界运动有明显的阻碍作用。实验还发现在含第二相粒子较多的晶界的超塑性合金中,晶界滑动是通过位错沿晶界运动和晶界的粘滞性流动两种机制同时发生作用进行的,它们相互促进并共同推动了晶界滑动的发展。首次在透射电镜下观察到超塑变形使晶界加宽的现象,在面缩率ψ=72.2%的样品中测得某晶界的宽度约为100nm,从而证实了超塑变形的晶界滑动和晶粒转动机制。     

氮化镓材料中的位错对材料物理性能的影响

氮化镓材料中的位错是制约GaN发光器件及电子器件的性能的一个关键因素。目前对于氮化镓材料中的位错的研究是一大热点。扼要综述了位错对于材料及器件的物理性能的影响：非辐射复合作用、造成器件的漏电流、缩短器件的寿命。并简要介绍了减少GaN外延层中的位错密度的几种方法。     

Al2O3/Al复合材料的界面结构特征

利用高分辩透射电子显微镜研究挤压铸造法制备的亚微米 Al 2O 3颗粒增强 Al 基复合材料的界面微观结构。结果表明 : Al基体的 (200) 和 (111) 面优先沿 Al 2O 3颗粒表面生长 , 在复合材料界面处 Al 基体与 Al 2O 3颗粒具有 Al (200) ∥Al 2O 3 (101 2) 、Al [011 ] ∥Al 2O 3 [0221 ] 的晶体学位向关系并形成半共格界面 , 且界面存在 Al (111) / / Al 2O 3 ( 1120) 的共格关系。界面干净无任何反应物。接近界面的 Al 基体中出现了柏氏矢量为 b= 1/ 3 [ 111 ] 弗兰克不全刃位错 , 该刃位错引起界面附近基体中明显的晶格应变场 , 位错周围晶格变形场的范围约为 20～30 层原子面宽度 , 而在 Al 2O 3颗粒靠近界面的区域中未观察到位错等缺陷。并从晶体学角度对界面的形成机制进行了分析。     

Simulation Charpy impact energy of functionally graded steels by modified stress-strain curve through mechanism-based strain gradient plasticity theory

In the present work, Charpy impact energy of functionally graded steels produced by electroslag remelting composed of graded ferritic or austenitic layers in both crack divider and crack arrester configurations has been modeled by finite element method. The yield stress of each layer was related to the density of the statistically stored dislocations of that layer and assuming by Holloman relation for the corresponding stress-strain curves, tensile strengths of the constituent layers were determined via numerical method. By using load-displacement curves acquired from instrumented Charpy impact tests on primary specimens, the obtained stress-strain curves from uniaxial tensile tests were modified. The data used for each layer in finite element modeling were predicted modified stress-strain curves obtained from strain gradient plasticity theory. A relatively good agreement between experimental results and those obtained from simulation was observed.