Transmission electron microscopy observation of a deformation twin in TWIP steel by an ex situ tensile test

The formation of deformation twins in twinning-induced plasticity steels was observed in transmission electron microscope by an ex situ tensile test. The twinning process initially includes formation of extended dislocations at primary slip plane, then cross-slip to a conjugate slip plane with dissociation of the leading partial into a stair-rod dislocation and an emitted partial, and finally un-faulting of the original faults and formation of Frank partials. Repetition of the operation of the process on successive conjugate planes results in the formation of deformation twins. The formation mechanism of deformation twins can thus be explained by improving the stair-rod cross-slip model.     

电子显微学进展及其在材料科学中的应用

文章简要介绍了高分辨电子显微学方法和电子能量损失谱的进展.文中特别指出,随着电子显微技术的发展,原子分辨电子显微图像对结构问题的深入研究有重要作用.装备有能量单色器的新一代电子显微镜,可以直接给出高能量分辨率的电子能量损失谱(优于 0.1eV).这些先进技术方法的应用,推动了晶体结构学、材料科学、物理学、纳米科学及生命科学的发展,也为解决很多重要结构问题奠定了基础.文章重点讨论了几个典型功能材料体系的结构问题：利用大角度会聚束电子衍射技术,分析了应变硅器件中的应变分布;利用原位电子显微技术,研究了新型电子铁电体LuFe2O4电荷序和物理性能的关系;深入探讨了强关联体系中电子关联效应对电子能量损失谱和电子结构的影响.     

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.     

TEM study on relationship between stacking faults and non-basal dislocations in Mg

Recent interest in the study of stacking faults and non-basal slip in Mg alloys is partly based on the argument that these phenomena positively influence mechanical behaviour. Inspection of the published literature, however, reveals that there is a lack of fundamental information on the mechanisms that govern the formation of stacking faults, especially I1-type stacking faults (I1 faults). Moreover, controversial and sometimes contradictory mechanisms have been proposed concerning the interactions between stacking faults and dislocations. Therefore, we describe a fundamental transmission electron microscope investigation on Mg 2.5 at. % Y (Mg–2.5Y) processed via hot isostatic pressing (HIP) and extrusion at 623 K. In the as-HIPed Mg–2.5Y, many 〈c〉 and 〈a〉 dislocations, together with some 〈c + a〉 dislocations were documented, but no stacking faults were observed. In contrast, in the as-extruded Mg–2.5Y, a relatively high density of stacking faults and some non-basal dislocations were documented. Specifically, there were three different cases for the configurations of observed stacking faults. Case (I): pure I2 faults; Case (II): mixture of I1 faults and non-basal dislocations having 〈c〉 component, together with basal 〈a〉 dislocations; Case (III): mixture of predominant I2 faults and rare I1 faults, together with jog-like dislocation configuration. By comparing the differences in extended defect configurations, we propose three distinct stacking fault formation mechanisms for each case in the context of slip activity and point defect generation during extrusion. Furthermore, we discuss the role of stacking faults on deformation mechanisms in the context of dynamic interactions between stacking faults and non-basal slip.     

石墨烯的透射电子显微学研究

石墨烯（Graphene）是近几年迅速发展起来的研究热点材料之一.利用透射电子显微镜（TEM）研究Graphene的结构特征和原子动态过程,是 Graphene研究的重要进展.文章评述了利用透射电子衍射方法对Graphene的层数、堆垛方式、取向和表面形貌等结构特征进行的研究工作,介绍了利用高分辨透射电子显微术在Graphene的表面缺陷、边缘结构及吸附原子等研究领域取得的最新结果.     

直拉硅单晶中原生氧沉淀的透射电镜研究

利用透射电镜对掺氮(NCZ) 和普通 (CZ) 直拉硅单晶中的原生氧沉淀进行研究. 研究表明，在NCZ样品中，有高密度的粒径为5nm的氧沉淀生成，而在CZ样品中，没有观察到这种氧沉淀. 初步认为，这种细小的氧沉淀是以650℃低温下形成的N-O复合体为核心在随后的冷却过程中形成. 关键词： 直拉硅 透射电镜 氧沉淀     

原位透射电子显微学进展及应用

文章简要介绍了近年来原位透射电子显微学的进展,并指出,原位透射电子显微技术的发展使得在纳米、原子层次观察样品在力、热、电、磁作用下以及化学反应过程中的微结构演化成为可能。通过研究物质在外界环境作用下的微结构演化规律,揭示其原子结构与物理化学性质的相关性,指导其设计合成和微结构调控,促进新物质的探索和深层次物质结构研究,为解决凝聚态物理学中的具体问题提供了直接、准确和详细的方法。     

原位透射电子显微学进展及应用

文章简要介绍了近年来原位透射电子显微学的进展,并指出,原位透射电子显微技术的发展使得在纳米、原子层次观察样品在力、热、电、磁作用下以及化学反应过程中的微结构演化成为可能。通过研究物质在外界环境作用下的微结构演化规律,揭示其原子结构与物理化学性质的相关性,指导其设计合成和微结构调控,促进新物质的探索和深层次物质结构研究,为解决凝聚态物理学中的具体问题提供了直接、准确和详细的方法。     

Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

Pyramidal slip in electron beam heated deformed Titanium

The incidence of perfect glide dislocations, moving on parallel pyramidal slip bands on a particular grain boundary of deformed Titanium is studied by means of Transmission Electron Microscopy. Static experiments, performed by using the electron beam as a heating source, proved that slip propagation across the interface is possible when the angle of intersections between the activated slip planes of the incoming and the outgoing dislocations with the boundary plane is minimised. Additionally, the Burgers vector of the residual dislocations left in the boundary after slip transmission occurred should also be minimised. Due to their very small Burgers vector, residual dislocations are visualised with satisfactory results by an image simulation method.     

Contamination mitigation strategies for scanning transmission electron microscopy

Modern scanning transmission electron microscopy (STEM) enables imaging and microanalysis at very high magnification. In the case of aberration-corrected STEM, atomic resolution is readily achieved. However, the electron fluxes used may be up to three orders of magnitude greater than those typically employed in conventional STEM. Since specimen contamination often increases with electron flux, specimen cleanliness is a critical factor in obtaining meaningful data when carrying out high magnification STEM. A range of different specimen cleaning methods have been applied to a variety of specimen types. The contamination rate has been measured quantitatively to assess the effectiveness of cleaning. The methods studied include: baking, cooling, plasma cleaning, beam showering and UV/ozone exposure. Of the methods tested, beam showering is rapid, experimentally convenient and very effective on a wide range of specimens. Oxidative plasma cleaning is also very effective and can be applied to specimens on carbon support films, albeit with some care. For electron beam-sensitive materials, cooling may be the method of choice. In most cases, preliminary removal of the bulk of the contamination by methods such as baking or plasma cleaning, followed by beam showering, where necessary, can result in a contamination-free specimen suitable for extended atomic scale imaging and analysis.     

Transmission electron microscopy of single wall carbon nanotube/polymer nanocomposites: A first‐principles study

The physics of high resolution transmission electron microscopy (HRTEM) image formation and electron diffraction of single wall carbon nanotubes (SWCNTs) in a polymer matrix was investigated theoretically on the basis of the multislice method. The effect of the nanocomposite thickness on both image contrast and typical electron diffraction reflections of the nanofillers was explored. The implications of the results on the experimental applicability to study dispersion, chirality and diameter of nanofillers are discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transmission electron microscopy observations of CNT morphology before and after heating in vacuum

Heating of multi-walled carbon nanotubes is often required to obtain clean patterns in the field electron emission microscope (FEEM). A transmission electron micrograph study of morphological changes in the cap structure of multi-walled carbon nanotubes due to heating in vacuum is presented. The lack of significant structural change in the cap structure when specimens were heated to 1925 K for 2.5 h determines an upper bound for the diffusivity of multi-walled carbon nanotube surface atoms, of the order of 10⁻¹⁶ cm²/s at 1925 K.     

Direct observations of La ordering and domain structures in La0.61Li0.17TiO3 by high resolution electron microscopy

La_0.61Li_0.17TiO₃ microstructures have been studied by high resolution electron microscopy. A local lattice distortion occurs in the vicinity of the domain boundary region due to the twin with an angle of 89°. The average domain size of La_0.61Li_0.17TiO₃ is greater than 20 nm. The domain size and structures of La_0.61Li_0.17TiO₃ differ greatly from those of La-poor compounds, such as La_0.55Li_0.35TiO₃. At a nanoscopic level, microdomains of 20–100 nm in size construct a two-dimensional structure in La-rich compounds, while microdomains of 5–10 nm in size construct a three-dimensional structure in La-poor compounds. In addition, the Li-ion conduction mechanisms for La-rich and La-poor compounds are two- and three-dimensional, respectively.     

Transmission electron microscopy studies on structure and defects in crystalline yttria and lanthanum oxide thin films grown on single crystal sapphire by molecular beam synthesis

The Molecular beam synthesis and characterization are reported for Y₂O₃ thin films grown on Al₂O₃ (0001) substrate. The Y₂O₃ layer was highly oriented in the [111] direction with predominant orientation relations (111) Y₂O₃ ‖ (0001) Al₂O₃ and [110] Y₂O₃ ‖ [2110] Al₂O₃, corresponding to a lattice mismatch of ～20% at the interface. No significant interfacial layers were found at the Y₂O₃/Al₂O₃ interface and the large lattice misfit was accommodated by formation of stacking faults, dislocations and secondary orientation in the Y₂O₃ layer. A La₂O₃ interlayer improved the quality of the Y₂O₃ films. Full width at half maximum (FWHM) of the Y₂O₃ (222) peak decreased from 3.12° to 1.43° and the defect density in the Y₂O₃ layer was significantly reduced. These results may be relevant in the broader context of designing oxide heterolayers with controlled microstructures.     

高分辨扫描透射电子显微镜原理及其应用

扫描透射电子显微术是目前应用最广泛的电子显微表征手段之一,具有分辨率高、对化学成分敏感和图像直观易解释等特点。其中高分辨扫描电子显微镜可以直接获得原子分辨率的Z 衬度像,结合X射线能谱(EDS)和电子能量损失谱(EELS),可在亚埃尺度上对材料的原子和电子结构进行分析。文章简述了扫描透射电子显微镜的基本原理及其应用现状,重点论述了高角环形暗场(HAADF)和环形明场(ABF)像的成像原理、特征和应用。此外,文中还对原子尺度分辨率的X射线能谱及电子能量损失谱元素分析方法进行了简述。