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Difference in the Hot Compressive Behavior and Processing Maps between the As-cast and Homogenized Al-Zn-Mg-Cu(7075) Alloys

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Journal of Materials Science & Technology 2016年第7期32卷 660-670页

作者： S.y. Park W.J. kim Department of Materials Science and Engineering Hongik University 72-1 Sangsu-dong Mapo-gu Seoul 121-791 Republic of Korea Advanced Fusion Process R&D Group Korea Institute of Industrial Technology 994-32 Dongchun-dong Yeonsu-gu Incheon 406-130 Republic of Korea

The deformation mechanisms and processing maps of the as-cast Al-Zn-Mg-Cu （？0？5） aluminum （AI） alloy were studied and compared with those of the homogenized 7075 Al alloy in the temperature range of 573-723 K and in the strain rate range of 10 3-10 s-1 to explore a possibility for directly using the as- cast 7075 Al alloy as a billet for hot working. The as-cast and homogenized 7075 A1 alloys had similar grain sizes and showed similar high-temperature deformation behaviors. The as-cast 7075 Al alloy, however, exhibited higher power efficiencies and smaller unstable flow regions compared to the homogenized 7075 AI alloy because of the presence of secondary phases （MgZn2） segregated at the dendritic interfaces in the as-cast microstructure, which were formed during non-equilibrium solidification. These segregated phases provided nucleation sites for dynamic recovery or continuous dynamic recrystallization, and as a result a fine and more homogeneous （sub）grain structure could be obtained from the as-cast micro- structure compared to the homogenized microstructure. The current work suggested the possibility and advantage of using as-cast microstructures for hot working of aluminum alloys with high content of solutes.

关键词： Al-based alloysProcessing mapHot compressionAs-cast microstructureSegregationRecrystallization

Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-y magnesium alloys with different volume fractions of icosahedral phase

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Journal of Materials Science & Technology 2019年第1期35卷 181-191页

作者： T.y.Kwak W.J.kim Department of Materials Science and Engineering Hongik University

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized(DRXed) grains,and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller,higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in *** with the cast alloys, the onset of the power law breakdown(PLB) occurred at larger ZenerHolloman(Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.

关键词： Hot compression Magnesium alloy Processing map Grain refinement Dynamic recrystallization

Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling

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Journal of Materials Science & Technology 2017年第9期33卷 919-925页

作者： T.y.Kwak W.J.kim Rare Metal R&BD Group Korea Institute of Industrial Technology Department of Materials Science and Engineering Hongik University

An ultrafine-grained（UFG） Mg-13Zn-1.55 y alloy（ZW132） with a high volume fraction（7.4%） of icosahedral phase（I-phase, Mg;Zn;y） particles was prepared by applying high-ratio differential speed rolling（HRDSR） on the cast microstructure following homogenization. The alloy exhibited excellent superplasticity at low temperatures(tensile elongations of 455% and 1021% 473 K-10;s;and 523 K-10;s;,respectively). Compared with UFG Mg-9.25Zn-1.66 y alloy（ZW92） with a lower volume fraction of I-phase particles（4.1%）, which was prepared using the same processing routes, the UFG ZW132 alloy exhibited a higher thermal stability of grain size. Rapid grain coarsening, however, occurred at temperatures beyond523 K, leading to a loss of superplasticity. The high-temperature deformation behavior of the HRDSRprocessed ZW132 alloy could be well described assuming that the mechanisms of grain boundary sliding and dislocation climb creep competed with each other and considering that the grain-size was largely increased by accelerated grain growth at the temperatures beyond 523 K.

关键词： Magnesium alloy Superplastic deformation Ultrafine grain Icosahedral quasicrystal Grain growth

Influence of magnetic field and thermal radiation by natural convection past vertical cone subjected to variable surface heat flux

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Applied Mathematics and Mechanics(English Edition) 2012年第5期33卷 605-620页

作者： G.PALANI K.y.kim Department of Mathematics Dr.Ambedkar Govt.Arts CollegeChennai 600039Tamil NaduIndia Department of Mechanical Engineering Inha UniversityIncheon 402751Republic of Korea

A numerical study is performed to examine the heat transfer characteristics of natural convection past a vertical cone under the combined effects of magnetic field and thermal radiation. The surface of the cone is subjected to a variable surface heat flux. The fluid considered is a gray, absorbing-emitting radiation but a non-scattering medium. With approximate transformations, the boundary layer equations governing the flow are reduced to non-dimensional equations valid in the free convection regime. The dimensionless governing equations are solved by an implicit finite difference method of Crank-Nicolson type which is fast convergent, accurate, and unconditionally stable. Numerical results are obtained and presented for velocity, temperature, local and average wall shear stress, and local and average Nusselt number in air and water. The present results axe compared with the previous published work and are found to be in excellent agreement.

关键词： apex magnetohydrodynamics radiation finite difference skin friction,vertical cone

Extrusion limit diagram of AZ91–0.9Ca–0.6y–0.5MM alloy and effects of extrusion parameters on its microstructure and mechanical properties

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Journal of Magnesium and Alloys 2022年第12期10卷 3447-3458页

作者： Dong HeeLee ye Jin kim Sang-Hoon kim Byoung Gi Moon Sung Hyuk Park School of Materials Science and Engineering Kyungpook National UniversityDaegu 41566Republic of Korea Agency for Defense Development Daejeon 34186Republic of Korea Advanced Metals Division Korea Institute of Materials ScienceChangwon 51508Republic of Korea

An AZ91–0.9Ca–0.6y–0.5MM(AZXWMM91100) alloy, which has higher corrosion resistance, ignition resistance, and extrudability than a commercial AZ91 alloy, has been developed recently. In this study, the AZXWMM91100 alloy is extruded at various temperatures(300–400 ℃) and ram speeds(1–14.5 mm/s), and the cracking behaviors, microstructure, and tensile properties of the extruded materials are systematically analyzed. On the basis of the pressure limit and surface and internal cracking limit, the extrusion limit diagram providing a safe extrusion processing zone is established. All of the materials extruded at temperatures and speeds within the safe extrusion processing zone have high surface quality and moderate tensile ductility with an elongation higher than 10%. Moreover, they have a fully recrystallized grain structure and contain undissolved particle stringers arranged parallel to the extrusion direction. The grain size of the extruded material does not show any relationship with the Zener–Hollomon parameter(Z). However, the yield strength(yS) of the extruded material is inversely proportional to the logarithm of the Z value, and their relationship is expressed as yS =-31.2·log(Z) + 536. These findings may broaden the understanding of the AZXWMM91100 alloy with excellent chemical and physical properties and provide valuable information for the development of high-performance extruded Mg products using this alloy.

关键词： Magnesium Extrudability Extrusion limit diagram Microstructure Tensile properties

Separation of closely spaced modes by combining complex envelope displacement analysis with method of generating intrinsic mode functions through filtering algorithm based on wavelet packet decomposition

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Applied Mathematics and Mechanics(English Edition) 2013年第7期34卷 801-810页

作者： y.S.kim 陈立群 Shanghai Institute of Applied Mathematics and Mechanics Department of Mechanics Kim Ⅱ Sung University Department of Mechanics College of Sciences Shanghai University Shanghai Key Laboratory of Mechanics in Energy Engineering Shanghai University

One of the important issues in the system identification and the spectrum analysis is the frequency resolution, i.e., the capability of distinguishing between two or more closely spaced frequency components. In the modal identification by the empirical mode decomposition （EMD） method, because of the separating capability of the method, it is still a challenge to consistently and reliably identify the parameters of structures of which modes are not well separated. A new method is introduced to generate the intrin- sic mode functions （IMFs） through the filtering algorithm based on the wavelet packet decomposition （GIFWPD）. In this paper, it is demonstrated that the CIFWPD method alone has a good capability of separating close modes, even under the severe condition beyond the critical frequency ratio limit which makes it impossible to separate two closely spaced harmonics by the EMD method. However, the GIFWPD-only based method is impelled to use a very fine sampling frequency with consequent prohibitive computational costs. Therefore, in order to decrease the computational load by reducing the amount of samples and improve the effectiveness of separation by increasing the frequency ratio, the present paper uses a combination of the complex envelope displacement analysis （CEDA） and the GIFWPD method. For the validation, two examples from the previous works are taken to show the results obtained by the GIFWPD-only based method and by combining the CEDA with the GIFWPD method.

关键词： empirical mode decomposition (EMD) wavelet packet decomposition com- plex envelope displacement analysis (CEDA) closely spaced modes modal identification

Atomic Dispersion of Rh on Interconnected Mo_(2)C Nanosheet Network Intimately Embedded in 3D Ni_(x)MoO_(y) Nanorod Arrays for pH-Universal Hydrogen Evolution

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Electrochemically stable lithium-ion and electron insulators(LEIs)for solid-state batteries

Energy & Environmental Materials 2023年第5期6卷 236-250页

作者： Thi Luu Luyen Doan Dinh Chuong Nguyen Patrick M.Bacirhonde Ahmed S.yasin Abdelrahman I.Rezk Nelson y.Dzade Cheol Sang kim Chan Hee Park Department of Bionanotechnology and Bioconvergence Engineering Graduate SchoolJeonbuk National UniversityJeonju Jeollabuk-do 54896Korea Department of Nano Convergence Engineering Jeonbuk National UniversityJeonju Jeollabuk-doJeonbuk 54896Korea Graduate School of Energy Science and Technology Chungnam National UniversityDaejeon 34134Korea Department of Energy and Mineral Engineering The Pennsylvania State UniversityUniversity Park State CollegePA 16802USA Division of Mechanical Design Engineering School of EngineeringJeonbuk National UniversityJeonju Jeollabuk-do 54896Korea Department of Bionanosystem Engineering Graduate School Jeonbuk National UniversityJeonju Jeollabuk-do 54896Korea

Herein,a simple synthetic approach is employed for the atomic dispersion of Rh atoms(Rh SAs)over the surface of interconnected Mo_(2)C nanosheets intimately embedded in a three-dimensional Ni_(x)MoO_(y)nanorod arrays(Ni_(x)MoO_(y)NRs)framework;we found that the introduction of both isolated Rh SAs and Ni_(x)MoO_(y)NRs adjusts the electrocatalytic function of the host Mo_(2)C toward the direction of being an advanced and highly stable electrocatalyst for efficient hydrogen evolution at pH-universal *** a result,the proposed catalyst outperforms most recently reported transition metal-based catalysts,and its performance even rivals that of commercial Pt/C,as demonstrated by its ultralow overpotentials of 31.7,109.7,and 95.4 mV at a current density of 10 mA cm^(-2),along with its small Tafel slopes of 42.4,51.2,and 46.8 mV dec^(-1)in acidic,neutral,and alkaline conditions,*** addition,the catalyst shows remarkable long-term stability over all pH values with good maintenance of its catalytic activity and structural characteristics after continuous operation.

关键词： hydrogen evolution reaction Mo_(2)C nanosheets Ni_(x)MoO_(y)nanorod arrays pH-universal catalyst single Rh atoms

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Nano Research 2022年第2期15卷 1213-1220页

作者： Kai Pei So yeon kim Ju Li Frontier Institute of Science and Technology Xi'an Jiaotong UniversityXi'an 710049China Department of Materials Science and Engineering and Department of Nuclear Science and Engineering Massachusetts Institute of TechnologyCambridgeMA 02139USA

Rechargeable solid-state Li metal batteries demand ordered flows of Li-ions and electrons in and out of solid structures,with repeated waxing and waning of Ubcc phase near contact interfaces which gives rise to various electro-chemo-mechanical *** have been approaches that adopt three-dimensional(3D)nanoporous architectures consisting of mixed ion-electron conductors(MIECs)to combat these ***,there has remained an issue of LiBcc nucleation at the interfaces between different solid components(e.g.,solid electrolyte/MlEC interface),which could undermine the interfacial bonding,thereby leading to the evolution of mechanical instability and the loss of ionic/electronic *** this regard,the present work shows that the Li-ion and electron insulators(LEIs)that are thermodynamically stable against LiBcc could combat such challenges by blocking transportation of charge carriers on the interfaces,analogous to dielectric layers in *** searched the ab initio database and have identified 48 crystalline compounds to be LEI candidates(46 experimentally reported compounds and 2 hypothetical compounds predicted to be stable)with a band gap greater than 3 eV and vanishing Li *** these compounds,those with good adhesion to solid electrolyte and mixed ion-electron conductor of interest,but are lithiophobic,are expected to be the most *** also extended the search to Na or K metal compatible alkali-ion and electron insulators,and identified some crystalline compounds with a property to resist corresponding alkali-ions and electrons.

关键词： electrochemical stability window stress fracture adhesion contact lithium metal anode

Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy

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Journal of Materials Science & Technology 2011年第1期27卷 93-96页

作者： B.H.kim N.H.Kang W.T.Oh C.H.kim J.H.kim y.S.kim y.H.Park Department of Materials Science and Engineering Pusan National UniversitySan30Jangjeon-dongGeumjeong-guBusan 609-735Korea Department of Materials and Components Engineering Dong-Eui UniversityBusan 614-714Korea Advanced Joining Technology Team KITECH7-47Songdo-dongYeonsu-guIncheon 406-840Korea Division for Dongnam Area Technology Service KITECHJisa-dongGanseo-guBusan 618-230Korea

For Al 6k21-T4 overlap weld joint,the shear-tensile strength by using the weaving laser was improved as compared to the case of linear *** the specimen of low strength,the porosity was distributed continuously along the intersection between the plates and fusion ***,for the optimized welding condition,large oval-shaped porosities were located only in the advancing track of the concave ***,the continuity of cracks and porosities played a key role to determine the ***,the weaving width was also the important parameter to control the ***,the concave part had more significant hot and cold cracking in the weld and heat-affected zone（HAZ）,respectively,than the convex part.

关键词： Weaving laser Weld microstructure Void Al alloy