Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium

Previous studies have revealed that dislocation structures in metals with medium-to-high stacking fault energy, depend on the grain orientation and therefore on the slip systems. In the present work, the dislocations in eight slip-plane-aligned geometrically necessary boundaries (GNBs) in three grains of near 45° ND rotated cube orientation in lightly rolled pure aluminium are characterized in great detail using transmission electron microscopy. Dislocations with all six Burgers vectors of the ½?1?1?0? type expected for fcc crystals were observed but dislocations from the four slip systems expected active dominate. The dislocations predicted inactive are primarily attributed to dislocation reactions in the boundary. Two main types of dislocation networks in the boundaries were identified: (1) a hexagonal network of the three dislocations in the slip plane with which the boundary was aligned; two of these come from the active slip systems, the third is attributed to dislocation reactions (2) a network of three dislocations from both of the active slip planes; two of these react to form Lomer locks. The results indicate a systematic boundary formation process for the GNBs. Redundant dislocations are not observed in significant densities.     

Grain boundary influence on the mechanical behaviour of oriented bicrystals

It is shown through several experiments centred on dislocation transmission through a GB that relating macroscopic mechanical properties of a bicrystalline specimen to the atomic structure of the GB or to local dislocation reactions is not straightforward. Not only the long and short range stresses and the plastic properties of the two grains must be taken into consideration, but also the kinetics of events has to be taken into account to explain the final result.     

Investigation of grain boundary activity in nanocrystalline Al under an indenter by using a multiscale method

Grain boundary activity in nanocrystalline Al under an indenter is studied by using a multiscale method.It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations.The transition processes might result in grain coarsening and refinement events.Dislocation reflection generated by a piece of stable grain boundary is also observed,because of the complex local atomic structure within the nanocrystalline Al.This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.     

Investigation of activities of grain boundaries in nanocrystalline Al under an indenter by a multiscale method

Activities of grain boundaries in nanocrystalline Al under an indenter are studied by a multiscale method. It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations. The transition processes might result in grain coarsening and refinement events. Dislocation reflection generated by a piece of stable grain boundary is also observed, because of the complex local atomic structure within the nanocrystalline Al. This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.     

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe₃C) and (Fe,Ni)₃C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1?0?0] and [0?0?1] type, [1?0?0] being the most frequent. [1?0?0] dislocations are dissociated and glide in the (0?1?0) plane. [0?0?1] dislocations glide in (1?0?0) and (0?1?0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.     

Ordinary dislocation configurations in high Nb-containing TiAl alloy deformed at high temperatures

Abstract

High Nb-containing TiAl (Nb–TiAl) alloys possess mechanical properties at elevated temperatures superior to conventional TiAl alloys. However, the strengthening mechanisms induced by Nb addition have been discussed controversial for a long time. In the present study, the dislocation structures in a polycrystalline high Nb–TiAl alloy after tensile tests at 700 and 900 °C were investigated by transmission electron microscope (TEM) observation. The results show that abundant double cross slip of ordinary dislocations is activated in the samples deformed at 700 °C. The dislocations are pinned at the jogs and numerous dipoles are observed. Debris can be commonly observed in the vicinity of screw dislocations. Trace analysis shows that the cross-slip plane is (1?1?0)γ at 700 °C but (1?1?1)γ octahedral plane at 900 °C. Three-dimensional (3D) dislocation structures, caused by cross-slip and annihilation of ordinary dislocations, were observed along the screw orientation. The dipoles and debris produced by high-temperature cross slip can be important for the strengthening of high Nb–TiAl alloys.     

Low-Energy Dislocation Structure (LEDS) character of dislocation boundaries aligned with slip planes in rolled aluminium

For the specific slip geometry of two sets of coplanar systems (a total of four systems) in fcc metals, the range of dislocation networks in boundaries aligned with one of the two active slip planes is predicted from the Frank equation for boundaries free of long-range elastic stresses. Detailed comparison with experimental data for eight dislocation boundaries in cold-rolled aluminium grains of the 45° ND rotated Cube orientation is conducted. It is concluded that the boundaries are Low-Energy Dislocation Structures, which are in good agreement with the Frank equation while also lowering the energy by dislocation reactions. Cross slip plays a role in the boundary formation process.     

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)₂ thin films for high‐efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep‐defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect‐free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)₂ thin films with two‐dimensional device simulations suggest that these defects are one origin of the reduced open‐circuit voltage of the photovoltaic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

纳米通道内液体流动的滑移现象

采用分子动力学模拟方法研究了液态氩在铂纳米通道内的流动，通过改变流体和壁面之间的势能作用获得了流体和通道表面之间浸润性质不同时的滑移现象. 研究发现：液体分子在亲水性通道表面附近呈类固体性质，数密度和有序性较大，而在疏水性表面附近的平均数密度降低，形成一个低密度层；液体流动在固体表面的速度滑移随着液体与表面势能作用的增强而减小，当液体和表面的浸润性不同时可以发生滑移、表观无滑移和负滑移现象；液体在固体表面的表观滑移是液体在固体表面的速度滑移、粘附和流体内部滑移的综合作用的结果. 关键词： 纳米尺度流动 速度滑移 浸润性 分子动力学模拟     

周期永磁聚焦电子注性能计算机模拟

 使用全三维电磁场粒子模拟程序MAFIA建立了行波管周期永磁聚焦系统模型，计算了磁系统中的磁场分布和静态电子注传输特性。分析了周期永磁系统聚焦电子注传输特性，并重点讨论了磁场强度、磁场端部效应和横向速度分布对电子注聚焦性能的影响。为周期永磁聚焦系统的工程设计和优化提供了有价值的模拟结果。     

周期永磁聚焦电子注性能计算机模拟

使用全三维电磁场粒子模拟程序MAFIA建立了行波管周期永磁聚焦系统模型,计算了磁系统中的磁场分布和静态电子注传输特性。分析了周期永磁系统聚焦电子注传输特性,并重点讨论了磁场强度、磁场端部效应和横向速度分布对电子注聚焦性能的影响。为周期永磁聚焦系统的工程设计和优化提供了有价值的模拟结果。     

平台间光束耦合传输与控制技术研究

介绍了平台间光路耦合传输系统的构成及光轴稳定控制的实现方法,开展了耦合校正系统和探测控制系统的设计,对校正系统进行了动态范围和模态仿真,优化设计后研制出光束耦合传输与控制系统。在对快反镜性能参数测试之后,开展了平台间光束耦合传输与控制实验,当振动台加载0 db振动谱且控制系统开环时,光轴X轴抖动10.9″@RSM,Y轴抖动102.3″@RSM,闭环时,光轴X轴抖动0.75″@RSM,Y轴抖动1.11″@RSM,通过频谱分析发现,快反镜光轴耦合系统闭环时对28 Hz以内光轴抖动具有较好地抑制作用,在系统开环残差较大的频率段2～6 Hz的抑制比为−40～−30 dB。实验结果表明,该光轴耦合控制系统对平台间光束传输过程中光束抖动具有较好地抑制和稳定效果。     

Fracture of polycrystalline graphene membranes by in situ nanoindentation in a scanning electron microscope

Failure of polycrystalline graphene grown by chemical vapor deposition was investigated by nanoindentation in a scanning electron microscope. Circular graphene membranes were subject to central point loads using a nanomanipulator combined with an atomic force microscope cantilever as a force sensor. The grain boundaries of the polycrystalline graphene were visualized by Raman spectroscopy coupled with a carbon isotope labeling technique. Graphene membranes without any grain boundary had a failure strength of 45.4 ± 10.4 GPa, compared to 16.4 ± 5.1 GPa for those with grain boundaries when a Young's modulus was assumed to be 1 TPa. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Defect reduction in sublimation grown SiC bulk crystals

For several years the major focus of material issues in SiC substrates was laid on the reduction of macroscopic defects like polytype inclusions, low angle grain boundaries and micropipes. Although significant improvements have been achieved, there are still shortcomings in material quality that have to be overcome. Since it is clear that dislocations are the main reason for degradation in power devices the prevailing attention has shifted to that field of material research. The aim of our work was to investigate the mechanisms that affect the generation of macroscopic and microscopic defects during sublimation growth. Intense studies were utilized on dislocation and stacking fault formation. For this reason we systematically varied parameters of the growth process and applied several methods for the characterization to evaluate material properties most precisely, e.g. KOH-defect-etching, X-ray-diffraction, electron microscopy and optical microscopy. The investigations were accompanied by failure analysis of devices of the Schottky type. We found out that for the improvement of substrate quality emphasis has to be laid on the reduction of thermoelastic stress in the growing crystal. From results of numerical calculations we were able to derive moderate growth conditions with reduced temperature gradients prevailing during the growth process. As a consequence we succeeded in decreasing the defect concentration. The best value so far achieved for the sum of both BPD and TED was 7×10³ cm⁻².     

Interdiffusion in two-layer Pd/Ag films III. TEM investigation of diffusion-induced grain boundary migration

Diffusion-induced grain boundary migration (DIGM) is studied by the transmission electron microscopy method in polycrystalline two-layer Pd/Ag thin films with a grain size (100–2000 nm). In addition to the typical features of DIGM known for coarse-grained bulk objects and foils, new features are found which are caused by a quite dense network of triple junctions and by misfit dislocations: fast increase of grain boundary curvature and inclination; back motion of grain boundaries owing to recrystallization forces and termination of DIGM. Homogenization resulted from diffusion-induced migration of misfit dislocations is observed in addition to DIGM.     

Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.