关键词： Friction stir welding   Aluminum alloy   Tool eccentricity   Microstructural properties   Material flow   Kinematics  

摘要： The effect of tool eccentricity on friction stir welding of AA6061-T651 aluminum alloy plates is evaluated. Microstructural characterization reveals that surface roughness of the weld bead is increased, while material flow and mixing in the stir zone is enhanced with increasing tool eccentricity. Moreover, grain size of examined regions are refined with increased tool eccentricity. Softening at the lower region of the stir zone was observed using both an aligned and 0.2 mm eccentric setup, where a high density of platelet-shaped precipitates are present. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Metakaolin   Hydraulic lime   Hydration heat   Early age   Microstructure  

摘要： Hydraulic lime is an ancient building repair material, and its good early performance is necessary to ensure a successful application. In this study, hydraulic lime (metakaolin (MK)-air lime (AL), MK-AL) were prepared by the amount of MK was used for different mixtures in AL, which provided the materials with both hydraulicity and the ability to air harden. The early-age (before 28 days) properties of natural hydraulic lime (NHL5), AL and MK-AL (MK20-AL and MK40-AL) pastes were studied through an examination of their physical and mechanical properties, hydration heat, thermal properties and microstructure. The strength growth trend of the MK-AL paste was similar to that of the NHL5 paste, and the compressive strength of the MK40-AL paste reached 21.06 MPa at 28 days. There was only a carbonation reaction in the AL paste, C-S-H formed mainly by the hydration reaction of C2S in the NHL5 paste, the pozzolanic reaction and carbonation reaction occurred in the MK-AL pastes, and the carbonation reaction in the MK-AL pastes was at a low level before 28 days. The MK-AL pastes had a longer hydration stage than the NHL5 paste, and there were two exothermic hydration peaks during the pozzolanic reaction for the MK-AL pastes. Laminar Ca(OH)\(2) and amorphous C-S-H gel comprised the skeleton in the NHL5 paste, and the large laminar Ca(OH)(2) structures that formed the skeleton were converted into an interweaved structure of the pozzolanic reaction products and carbonated products in the MK-AL pastes. There were many large pores (>200 nm) in the AL, NHL and MK20-AL pastes, while the pore size of the MK40AL paste was relatively small (<100 nm). The porosity of all lime pastes had the following trend: NHL (38.96%) < MK40-AL (42.89%) < MK20-AL (45.47%) < AL (51.22%).

关键词： Deformation and fracture   Grain refinement   Transformation induced plasticity   Third-generation steels   X-ray techniques  

摘要： We have investigated the capabilities of first and third-generation high strength steels for sustainable accommodation of microstructural damage before fracture occurs. Steels have been subjected to tensile tests followed by X-ray computed tomography to record the evolution of microstructural damage during deformation. Damage growth is correlated with the Rice-Tracey model. Results show that both grain refinement and transformation induced plasticity of retained austenite to martensite lead to enhanced damage tolerance, leading to the suppression of fracture and improved ductility. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Grouting material   Nano-clay   Chloride ion penetration   Permeability   Theoretical model   Microcosmic structure  

摘要： The main objective of this study was to investigate the influence of nano-clay (NC) on the mechanical and microcosmic properties of sulfoaluminate cement grouting material in 5% sodium chloride (NaCl) solution. All specimens were prepared with a water/cement ratio (W/C) of 1.0. The setting time, bleeding ratio, flow time, mass change rate, and uniaxial compressive strength were obtained by various tests. Further, the resistance toward chloride ion penetration and the permeability of pastes were also evaluated. Hydration products and micro-morphology were investigated through X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, and energy dispersive spectroscopy. The test results revealed that with the addition of NC, both the setting time and flow time were significantly shortened and bleeding capacity and mass loss rate decreased;nonetheless, the uniaxial compressive strength increased significantly. Moreover, addition of NC reduced the chloride ion penetration and permeability. Furthermore, the theoretical model based on permeability and immersion time was established, and its correctness was successfully verified by experimental data. As a result, the grouting material with added 2% NC exhibited the best experimental performance, whereas 3% and more content of NC resulted in inferior performance;however, it was still better than that of the control group (NC = 0%). Moreover, microcosmic test results showed that ettringite is the main product of the dominant hydration reaction. It was thus proved that NC could promote the hydration process, and thus it weakens the chloride ion erosion due to the porosity reduction effect. (C) 2020 Elsevier Ltd. All rights reserved.

摘要： We present the first results of laser-driven flyer plate experiments on a nanocrystalline copper-tantalum (NC-Cu-Ta) alloy. A pulsed Nd:YAG laser (1.2J/pulse, 10ns) is used to accelerate an Al foil disk (25 mu m x similar to 800 mu m) off a glass substrate at velocities of 0.8 and 2.4km/s through a small air gap and impact the NC-Cu-Ta target. The flyer velocities were determined from a high-speed video and extensive post-impact analyses were conducted using advanced electron microscopy revealing the formation of a band structure leading to a non-trivial upper bound for the breakdown of an extremely stable NC-microstructure and physical-properties.

关键词： Python语言   GUI插件   材料微结构   晶体学取向   二次开发  

摘要： 基于Voronoi图基本原理,通过编写Python语言在ABAQUS中构造出三维多晶体微结构几何模型。利用ABAQUS/CAE内嵌的GUI插件开发辅助工具RSG对话框构造器,定制合理的参数对话框,编写内核Python程序开发出材料微结构前处理建模插件Polycrystal Toolkit,利用该插件可以快速构造微结构几何模型并赋予晶体学取向。采用该方法建立的有限元模型结合大型仿真软件ABAQUS分别对2种任意取向下的圆柱多晶体进行扭转变形模拟。结果表明:通过编写Python语言生成的Voronoi多晶模型可以很好地描述实际多晶体变形过程中内部变形的不均匀性,验证了模型及插件的有效性,为以后织构演化行为的研究提供了有利的建模基础。

关键词： Electric fields  

摘要： The electric-field-induced and temperature induced dynamics of domains, defects, and phases play an important role in determining the macroscopic functional response of ferroelectric and piezoelectric materials. However, distinguishing and quantifying these phenomena remains a persistent challenge that inhibits our understanding of the fundamental structure-property relationships. In situ dark field x-ray microscopy is a new experimental technique for the real space mapping of lattice strain and orientation in bulk materials. In this paper, we describe an apparatus and methodology for conducting in situ studies of thermally and electrically induced structural dynamics and demonstrate their use on ferroelectric BaTiO3 single crystals. The stable temperature and electric field apparatus enables simultaneous control of electric fields up to approximate to 2 kV/mm at temperatures up to 200 degrees C with a stability of Delta T = +/- 0.01 K and a ramp rate of up to 0.5 K/min. This capability facilitates studies of critical phenomena, such as phase transitions, which we observe via the microstructural change occurring during the electric-field-induced cubic to tetragonal phase transition in BaTiO3 at its Curie temperature. With such systematic control, we show how the growth of the polar phase front and its associated ferroelastic domains fall along unexpected directions and, after several cycles of electric field application, result in a non-reversible lattice strain at the electrode-crystal interface. These capabilities pave the way for new insights into the temperature and electric field dependent electromechanical transitions and the critical influence of subtle defects and interfaces.

关键词： 纳米VO2(B)   微观形貌   结构缺陷   电化学性能  

摘要： 以V2O5为原料、草酸为还原剂,采用水热法制备不同草酸浓度下的系列纳米VO2(B)样品(c(H2C2O4)/c(V2O5)=x,1.00≤x≤2.50),对所制备样品的物相结构、微观形貌和电化学性能进行表征和分析.结果表明:所制备的样品均为单斜结构的纯相VO2(B);随着还原剂浓度的增大,VO2(B)样品的形貌逐渐由较小的纳米片变成较长的纳米棒;VO2(B)样品的结构缺陷主要为微孔;所得VO2(B)样品均具有较好的可逆性和循环稳定性,其中,当x=1.75时,VO2(B)样品具有较小的氧化还原峰电位差(0.111 V)和较小的电阻(0.770Ω),且当电流密度为30 mA/g时,具有最大的比电容值(105.00 F/g),表现出更好的电化学性能.

关键词： 位置精度   疲劳断口   金属材料   大科学装置   微结构损伤   破坏行为   疲劳损伤   点光源  

摘要： 长期以来,科学家通过测量表面裂纹长度和辨识疲劳断口揭示小试件的破坏行为,并依此来表征大块金属材料的疲劳损伤机制,从而建立了工程结构抗疲劳断裂设计的科学基础和评价体系。虽然断口保存了材料破坏过程的全部信息,但很难标定缺陷的空间形貌及与其他缺陷的耦合情况,无法提取到三维空间中真实临界缺陷尺寸。低通量点光源工业CT探测的缺陷形貌、尺寸及位置精度不足,尤其不能实现高时空分辨下的动态原位加载。