Anisotropic fracture behaviour and brittle-to-ductile transition of polycrystalline tungsten

The fracture behaviour of polycrystalline sintered and rolled tungsten rods was investigated from ?150°C to 950°C by means of three-point bending tests and electron microscopy where special attention was drawn to the influence of the microstructure. This thorough investigation demonstrates the positive impact of the crystallographic and grain shape anisotropy in tungsten. Specimens extracted along the rolling direction exhibit twice as high fracture toughnesses and a significantly reduced brittle-to-ductile transition temperature than the other two investigated orientations. Furthermore, these specimens show a change in their fracture mode from transgranular to intergranular fracture with crack deflection occurring around 270°C. In an in situ SEM fracture test, the origin of this crack deflection could be clarified. Finally, a fracture mechanics model is presented which predicts correctly the transition between the two fracture modes and which gives an energy criterion suitable to interpret experimental fracture results.     

半导体量子阱材料微加工光子晶体的光学特性

利用聚焦离子束刻蚀方法和电子束制版结合干法刻蚀方法制备了二维近红外波段光子晶体，发现两种方法都可以制备出均匀的二维光子晶体，聚焦离子束方法操作简单，电子束制版结合干法刻蚀方法操作步骤复杂.光谱测试表明，利用聚焦离子束方法在有源材料上刻蚀的光子晶体不发光，而电子束制版结合干法刻蚀方法制备的小晶格常数光子晶体即使有些无序，其出光效率也提高到没有光子晶体时的两倍.对两种方法所加工的光子晶体不发光和提高出光效率的机理进行了分析. 关键词： 聚焦离子束 电子束制版 光子晶体 出光效率     

Integration of correlative Raman microscopy in a dualbeam FIB SEM

We present an integrated confocal Raman microscope in a focused ion beam scanning electron microscope (FIB SEM). The integrated system enables correlative Raman and electron microscopic analysis combined with focused ion beam sample modification on the same sample location. This provides new opportunities, for example the combination of nanometer resolution with Raman advances the analysis of sub‐diffraction‐sized particles. Further direct Raman analysis of FIB engineered samples enables in situ investigation of sample changes. The Raman microscope is an add‐on module to the electron microscope. The optical objective is brought into the sample chamber, and the laser source, and spectrometer are placed in a module attached onto and outside the chamber. We demonstrate the integrated Raman FIB SEM function with several experiments. First, correlative Raman and electron microscopy is used for the investigation of (sub‐)micrometer‐sized crystals. Different crystals are identified with Raman, and in combination with SEM the spectral information is combined with structurally visible polymorphs and particle sizes. Analysis of sample changes made with the ion beam is performed on (1) structures milled in a silicon substrate and (2) after milling with the FIB on an organic polymer. Experiments demonstrate the new capabilities of an integrated correlative Raman–FIB–SEM. Copyright © 2016 John Wiley & Sons, Ltd.     

Estimation of diffusion coefficient by photoemission electron microscopy in ion-implanted nanostructures

We have fabricated parallel stripes of nanostructures in an n-type Si substrate by implanting 30 keV Ga⁺ ions from a focused ion beam (FIB) source. Two sets of implantation were carried out. In one case, during implantation the substrate was held at room temperature and in the other case at 400 °C. Photoemission electron microscopy (PEEM) was carried out on these samples. The implanted parallel stripes, each with a nominal dimension of 4000 nm × 100 nm, appear as bright regions in the PEEM image. Line scans of the intensities from the PEEM image were recorded along and across these stripes. The intensity profile at the edges of a line scan is broader for the implantation carried out at 400 °C compared to room temperature. From the analysis of this intensity profile, the lateral diffusion coefficient of Ga in silicon was estimated assuming that the PEEM intensity is proportional to Ga concentration. The diffusion coefficient at 400 °C has been estimated to be ∼1.3 × 10⁻¹⁵ m²/s. Across the stripes an asymmetric diffusion profile has been observed, which has been related to the sequence of implantation of these stripes and the associated defect distribution due to lateral straggling of the implanted ions.     

Focused ion beam treatment of ZnO nanowires in the scanning electron microscope

We investigated vapor phase epitaxy-grown ZnO nanowires on a Si substrate by scanning electron microscopy. These investigations show that there are single nanowires and ensembles of nanowires, among which we found straight and bend, perfect and non-perfect nanowires, as well as nanowires with clean surfaces and surfaces with the dark spots and features. After focused ion beam polishing and milling we found that nanowires are homogeneous. The sizes of the nanowires were determined: the length is about 2–24 μm, and the width and height are about 200–500 nm.     

单晶钨纳米线拉伸变形机理的分子动力学研究

利用分子动力学方法, 对本课题组率先采用金属催化的气相合成法制备出的高纯度单晶钨纳米线进行拉伸变形数值模拟, 通过分析拉伸应力-应变全曲线及其微观变形结构, 揭示出单晶钨纳米线的拉伸变形特征及微观破坏机理. 结果表明: 单晶钨纳米线的应力-应变全曲线可分为弹性阶段、损伤阶段、相变阶段、强化阶段、 破坏阶段等五个阶段, 其中相变是单晶钨纳米线材料强化的重要原因; 首次应力突降是由于局部原子产生了位错、孪生等不可逆变化所致; 第二次应力突降是发生相变的材料得到强化后, 当局部原子再次产生位错导致原子晶格结构彻底破坏而形成裂口、且裂口不断发展成颈缩区时, 材料最终失去承载能力而断裂. 计算模拟得到的单晶钨纳米线弹性模量值与实测值符合较好. 关键词： 分子动力学 应力应变曲线 微观机理 单晶钨纳米线     

Influence of impurities on the fracture behaviour of tungsten

Ten tungsten materials with different impurity concentrations and different microstructures have been investigated by Auger electron spectroscopy and scanning electron microscopy with respect to their fracture behaviour. For almost all samples, both inter- and transgranular fracture are observed, and the proportion of each type varies. Due to the difference in their impurity content and grain boundary area, a large variation in the grain boundary impurities can be expected. By analysing the fracture surfaces the effect of grain boundary impurities, especially phosphorous and oxygen, on the fracture resistance of the boundaries was determined. The results indicate that for the analysed tungsten materials, grain boundary impurities do not have a significant influence on the fracture resistance of the boundaries. Other factors such as the size and shape of the grains, the amount of deformation and therefore the density of dislocations within the grains have a greater impact on the fracture behaviour of tungsten.     

Evaluation on the interfacial fracture toughness of fiber-reinforced titanium matrix composites by push out test

The models for single-fiber push out test are developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. The models are based on fracture mechanics, taking into consideration of the free-end surface and Poisson expansion. Theoretical solutions to G_IIc are obtained, and the effects of several key factors such as the initial crack length, crack length, friction coefficient, and interfacial frictional shear stress are discussed. The predictions by the models are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834, and SCS/Timetal 21s. The results show that the models can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.     

聚变堆超导磁体用SS316LN铠甲低温断裂性能研究

对聚变堆中心螺管(CS)线圈中，铠装电缆导体(CICC)中的SS316LN 不锈钢铠甲在运行状态下的断裂性能进行了测试分析。结果显示SS316LN 疲劳裂纹扩展性能较稳定，断裂韧性在经历冷变形与时效热处理后出现了大幅度衰减。此结果为未来核聚变堆超导线圈的设计与性能分析提供了数据参考。     

Cu/Al引线键合界面金属间化合物生长过程的原位实验研究

铜引线键合由于在价格、电导率和热导率等方面的优势有望取代传统的金引线键合, 然而Cu/Al引线键合界面的金属间化合物(intermetallic compounds, IMC)的过量生长将增大接触电阻和降低键合强度, 从而影响器件的性能和可靠性. 针对以上问题, 本文基于原位高分辨透射电子显微镜技术, 研究了在50–220 ℃退火温度下, Cu/Al引线键合界面IMC的生长问题, 实时观测到了Cu/Al IMC的动态生长及结构演变过程. 实验结果表明, 退火前颗粒状的Cu/Al IMC 分布在键合界面, 主要成分为Cu₉Al₄, 少量成分为CuAl₂. 退火后Cu/Al IMC的成分是: 靠近Cu一端为Cu₉Al₄, 远离Cu的一端为CuAl₂. 同时基于原位观测Cu/Al IMC的动态生长过程, 计算得到了Cu/Al IMC 不同温度下的反应速率和激活能, 给出了基于原位实验结果的Cu/Al IMC的生长公式, 为优化Cu/Al引线键合工艺和提高Cu/Al引线键合的可靠性提供了指导.     

Nucleation and growth of bubbles in He ion-implanted V/Ag multilayers

Microstructures of He ion-implanted pure Ag, pure V and polycrystalline V/Ag multilayers with individual layer thickness ranging from 1?nm to 50?nm were investigated by transmission electron microscopy (TEM). The bubbles in the Ag layer were faceted and larger than the non-faceted bubbles in the V layer under the same implantation conditions for both pure metals and multilayers. The substantially higher single defects surviving the spike phase and lower mobility of trapped He in bcc than those in fcc could account for this difference. For multilayers, the bubbles nucleate at interfaces but grow preferentially in Ag layers due to high mobility of trapped He in fcc Ag. In addition, the He concentration above which bubbles can be detected in defocused TEM images increases with decreasing layer thickness, from 0 for pure Ag to 4–5 at. % for 1?nm V/1?nm Ag multilayers. In contrast, the bubble size decreases with decreasing layer thickness, from approximately 4?nm in diameter in pure Ag to 1?nm in the 1?nm V/1?nm Ag multilayers. Elongated bubbles confined in the Ag layer by the V–Ag interfaces were observed in 1?nm multilayers. These observations show that bubble nucleation and growth can be suppressed to high He concentrations in nanoscale composites with interfaces that have high He solubility.     

Effect of surface treatment on mode I interlaminar fracture behaviour of plain glass woven fabric composites: Part I. Report of the 2nd round-robin test results

A round-robin test programme has been carried out to characterise the mode I interlaminar fracture behaviour of E-glass woven fabric reinforced vinyl ester matrix composites. Special emphasis has been placed on the effect of silane coupling agent on the stability of interlaminar crack propagation and fracture toughness. Sixteen laboratories participated in this programme. Each laboratory was supplied with composite laminates of thicknesses of its own choice and conducted the tests according to its own procedures. The results showed that variations in interlaminar fracture toughness between laboratories were very large in spite of slight differences in the test procedures used, such as specimen dimensions, test speed and data reduction schemes. Nevertheless, the general trends were clearly identified with respect to different silane coupling agents. Other observations and the implications are discussed.