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The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg_2Si in situ composites

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International Journal of Minerals,Metallurgy and Materials 2018年第9期25卷 1080-1089页

作者： Dong-tao Wang Hai-tao Zhang Lei Li Hai-lin Wu Ke Qin Jian-zhong Cui Key Laboratory of Electromagnetic Processing of Materials Ministry of Education Northeastern University

The effect of high-speed direct-chill(DC) casting on the microstructure and mechanical properties of Al–Mg_2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg_2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg_2Si particles(from 28 to 12 μm), the spacing of eutectic Mg_2Si(from 3 to 0.5 μm), and the grains of AA6061 alloy(from 102 to 22 μm). The morphology of the eutectic Mg_2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg_2Si phase and the grains in the Al–Mg_2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg_2Si composites and AA6061 alloy billets.

关键词： Al–Mg2Si in situ composite casting speed grain size primary Mg2Si mechanical property

Multi-revolution low-thrust trajectory optimization using symplectic methods

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Science China(Technological Sciences) 2020年第3期63卷 506-519页

作者： E ZhiBo GUZZETTI Davide School of Aerospace Engineering Tsinghua UniversityBeijing 100084China Department of Aerospace Engineering Auburn UniversityAuburn AL 36849USA

Optimization of low-thrust trajectories that involve a larger number of orbit revolutions is considered as a challenging *** paper describes a high-precision symplectic method and optimization techniques to solve the minimum-energy low-thrust multi-revolution orbit transfer problem. First, the optimal orbit transfer problem is posed as a constrained nonlinear optimal control problem. Then, the constrained nonlinear optimal control problem is converted into an equivalent linear quadratic form near a reference solution. The reference solution is updated iteratively by solving a sequence of linear-quadratic optimal control sub-problems, until convergence. Each sub-problem is solved via a symplectic method in discrete form. To facilitate the convergence of the algorithm, the spacecraft dynamics are expressed via modified equinoctial elements. Interpolating the non-singular equinoctial orbital elements and the spacecraft mass between the initial point and end point is proven beneficial to accelerate the convergence process. Numerical examples reveal that the proposed method displays high accuracy and efficiency.

关键词： low-thrust trajectory optimization symplectic method multi-revolution transfers

Genome editing in insects: current status and challenges

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National Science Review 2019年第3期6卷 399-401页

作者： Jun Xu Xia Xu Shuai Zhan Yongping Huang CAS Key Laboratory of Insect Developmental and Evolutionary Biology CAS Center for Excellence in Molecular Plant Sciences Shanghai Institute of Plant Physiology and Ecology Chinese Academy of Sciences

Insects constitute the largest group in the animal kingdom, and include many economically important species as well as pests. This abundance provides largescale genetic resources, and research on insects is beneficial from the perspectives of human health, agricultural production and basic science (Fig.1).The availability of many genetic tools has made Drosophila melanogaster a crucially important model organism

关键词： Genome editing in insects current status and challenges

A new nonlinear force model to replace the Hertzian contact model in a rigid-rotor ball bearing system

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Applied Mathematics and Mechanics(English Edition) 2018年第3期39卷 365-378页

作者： Yulin JIN Zhenyong LU Rui YANG Lei HOU Yushu CHEN School of Astronautics Harbin Institute of Technology School of Energy Science and Engineering Harbin Institute of Technology

A new nonlinear force model based on experimental data is proposed to replace the classical Hertzian contact model to solve the fractional index nonlinearity in a ball bearing system. Firstly, the radial force and the radial deformation are measured by statics experiments, and the data are fitted respectively by using the Hertzian contact model and the cubic polynomial model. Then~ the two models are compared with the approximation formula appearing in Aeroengine Design Manual. In consequence, the two models are equivalent in an allowable deformation range. After that, the relationship of contact force and contact deformation for single rolling element between the races is cal- culated based on statics equilibrium to obtain the two kinds of nonlinear dynamic models in a rigid-rotor ball bearing system. Finally~ the displacement response and frequency spectrum for the two system models are compared quantitatively at different rotational speeds, and then the structures of frequency-amplitude curves over a wide speed range are compared qualitatively under different levels of radial clearance, amplitude of excitation, and mass of supporting rotor. The results demonstrate that the cubic polynomial model can take place of the Hertzian contact model in a range of deformation.

关键词： rolling element bearing, Hertzian contact, fractional index cubic polyno-miali rotor ball bearing system

The transformation of agriculture in China: Looking back and looking forward

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Journal of Integrative Agriculture 2018年第4期17卷 755-764页

作者： JIAO Xiao-qiang Nyamdavaa Mongol ZHANG Fu-suo Center for Resources Environment and Food Security Department of Plant Nutrition/Key Laboratory of Plant-Soil InteractionsMinistry of Education China Agricultural University Lancaster Environment Centre Lancaster University

China’s grain yield increased from 1 t hain 1961 to 6 t hain 2015, while successfully feeding not only its large population but also supplying agricultural products all over the world. These achievements were greatly supported by modern technology and distinct governmental policy. However, China’s grain production has been causing a number of problems mainly related to declining natural resources and a lack of environmental protection. Due to the growing population and changing dietary requirements, increasing food production must be achieved by increasing resource use efficiency while minimizing environmental costs. We propose two novel development pathways that can potentially sustain agricultural crop production in the next few decades:(i) enhancing nutrient use efficiency with zero increase in chemical fertilizer input until 2020 and(ii) concurrently increasing grain yield and nutrient use efficiency for sustainable intensification with integrated nutrient management after 2020. This paper provides a perspective on further agricultural developments and challenges, and useful knowledge of our valuable experiences for other developing countries.

关键词： food security sustainable development agriculture grain production

Development of inclusions in 3104 alloy melt during heating and holding treatments

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International Journal of Minerals,Metallurgy and Materials 2016年第6期23卷 637-644页

作者： Xiao-xiong Luo Hai-tao Zhang Xing Han Shi-jie Guo Dan-dan Chen Jian-zhong Cui Hiromi Nagaumi Key Laboratory of Electromagnetic Processing of Materials Northeastern University Chinalco Rumin Co. Ltd. Suzhou Research Institute for Nonferrous Metals Chinalco Research Institute of Science and Technology

Developments in the contents of different typical inclusions in 3104 alloy melt were described during heating and holding processing. The settling process of inclusion particles was investigated by measuring the contents of inclusions in the surface, center, and bottom layers of the molten metal. In the results, main inclusions observed and determined by Prefil and PoD FA methods are MgO, Al2O3, spinel（MgAl2O4）, and TiB2 particles or thin films. It is found that some small particles of Al2O3 and MgO are transformed into spinel particles, and the formation rate increases as the temperature and the holding period of melt increase. The content of inclusions increases from 3.37 mm^2×kg^-1 to 7.54 mm^2×kg^-1 and then decreases to 3.08 mm^2×kg^-1 after holding for 90 min. This is attributed to a settling phenomenon and a significant increase in settling velocity after holding for 60 min. The content of inclusion particles decreases by means of settlement and flotation in liquid aluminum with an increase in holding time. The theoretical analysis and experiment results are in essential agreement with those from industrial production.

关键词： aluminum alloys inclusions heating holding time settling

Fabrication and characterization of nanoporous Ag-Pt alloy

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Rare Metals 2021年第5期40卷 1203-1207页

作者： Tao Yin Fan-Yun Meng Min Zhang Ying-Wei Yan Beijing Key Laboratory of Traditional Chinese Medicine Protection and Utilization Faculty of Geographical ScienceBeijing Normal UniversityBeijing 100875China Engineering Research Centre of Natural Medicine Ministry of EducationFaculty of Geographical ScienceBeijing Normal UniversityBeijing 100875China School of Mechanical Electronic and Control EngineeringMaterials Science and Engineering Research CenterBeijing Jiaotong UniversityBeijing 100044China

Nanoporous Ag-Pt bi-metallic alloy was fabricated by free dealloying of amorphous Ag-based precursor with the nominal composition of *** noble Ag and Pt were left after the less noble Cu and Si dissolved in a certain acid ***-continuous nanoporous microstructure was formed with asymmetric ligaments and pores with typically 30-200 nm in *** trace addition of Pt has refined the grains of the ligaments to the average size of less than 20 nm in the substrate and induced the formation of rods with *** morphologies of the rods were observed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM)both in the form of clusters and as scattered individuals with characteristic length of several micrometers and diameter of tens of *** grains sizes in the rods were finer than those in the *** good mechanical integrity might be due to the combination of continuous ligaments and clusters of rods.

关键词： Nanoporous Ag-Pt alloy Amorphous precursor Dealloying Nanograins

Bifurcation characteristics analysis of a class of nonlinear dynamical systems based on singularity theory

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Applied Mathematics and Mechanics(English Edition) 2017年第9期38卷 1233-1246页

作者： Kuan LU Yushu CHEN Lei HOU School of Astronautics Harbin Institute of TechnologyHarbin 150001China College of Engineering The University of IowaIowa CityIA 52242U.S.A. School of Energy Science and Engineering Harbin Institute of TechnologyHarbin 150001China

A method for seeking main bifurcation parameters of a class of nonlinear dynamical systems is proposed. The method is based on the effects of parametric varia- tion of dynamical systems on eigenvalues of the Frechet matrix. The singularity theory is used to study the engineering unfolding （EU） and the universal unfolding （UU） of an arch structure model, respectively. Unfolding parameters of EU are combination of concerned physical parameters in actual engineering, and equivalence of unfolding parameters and physical parameters is verified. Transient sets and bifurcation behaviors of EU and UU are compared to illustrate that EU can reflect main bifurcation characteristics of non- linear systems in engineering. The results improve the understanding and the scope of applicability of EU in actual engineering systems when UU is difficult to be obtained.

关键词： bifurcation singularity engineering unfolding (EU) universal unfolding(UU) nonlinear dynamics

The Antidepressant Effect of Light Therapy from Retinal Projections

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Neuroscience Bulletin 2018年第2期34卷 359-368页

作者： Xiaotao Li Xiang Li The Brain Cognition and Brain Disease Institute for Collaborative Research of SIAT at CAS and the McGovern Institute at MIT Shenzhen Institutes of Advanced TechnologyChinese Academy of Sciences McGovern Institute for Brain Research Department of Brain and Cognitive Sciences Massachusetts Institute of Technology

Observations from clinical trials have frequently demonstrated that light therapy can be an effective therapy for seasonal and non-seasonal major depression. Despite the fact that light therapy is known to have several advantages over antidepressant drugs like a low cost,minimal side-effects, and fast onset of therapeutic effect,the mechanism underlying light therapy remains *** far, it is known that light therapy modulates mood states and cognitive functions, involving circadian and noncircadian pathways from retinas into brain. In this review,we discuss the therapeutic effect of light on major depression and its relationship to direct retinal projections in the brain. We finally emphasize the function of the retino-raphe projection in modulating serotonin activity,which probably underlies the antidepressant effect of light therapy for depression.

关键词： Light therapy Depression Retinal projection Serotonin Opsin