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Elucidating the evolution of long-period stacking ordered phase and its effect on deformation behavior in the as-cast Mg-6Gd-1Zn-0.6Zr alloy

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Journal of Magnesium and Alloys 2023年第8期11卷 2801-2810页

作者： Sangwon Lee Yejun Park Jongbin Go Young Mok Kim seok su sohn Jiehua Li Pyuck-Pa Choi Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology(KAIST)Daejeon34141Republic of Korea Department of Materials Science and Engineering Kyoto UniversityKyoto606-8501Japan Department of Materials Science and Engineering Korea UniversitySeoul02841Republic of Korea Institute of Casting Research Montanuniverstät LeobenLeobenA-8700Austria

Herein,the evolution of long-period stacking ordered(LPSO)phases in the as-cast Mg-6Gd-1Zn-0.6Zr(wt.%)alloy are investigated via transmission electron microscopy(TEM)and atom probe tomography(APT).The TEM results reveal that two types of LPSO phase(a bulky interdendritic phase and a plate-like matrix LPSO phase)are formed in the as-cast *** of the LPSO phases are confirmed to be of the 14H type,with a smaller proportion being of the 18R ***,the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase,and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5,thereby indicating a deficient Zn content compared to the ideal 14H phase(i.e.,1.3).In addition,the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction(EBSD)analysis to reveal twinning and slip behavior during *** results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of{1011}compressive twinning and the activation of non-basal slip systems.

关键词： Magnesium alloy Long-period stacking ordered phase Transmission electron microscopy Atom probe tomography Work hardening behavior

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Effect of solid-solution strengthening on deformation mechanisms and strain hardening in medium-entropy V_(1-x)Cr_(x)CoNi alloys

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Journal of Materials Science & Technology 2022年第13期108卷 270-280页

作者： Hyun Chung Dae Woong Kim Woo Jin Cho Heung Nam Han Yuji Ikeda Shoji Ishibashi Fritz Körmann seok su sohn Department of Materials Science and Engineering Korea UniversitySeoul 02841South Korea Center for High Entropy Alloys Pohang University of Science and TechnologyPohang 37673South Korea Department of Materials Science and Engineering and Research Institute of Advanced Materials Seoul National UniversitySeoul 08826South Korea Institute for Materials Science University of StuttgartPfaffenwaldring 55Stuttgart 70569Germany Computational Materials Design Max-Planck-Institut für Eisenforschung GmbHMax-Planck-Strabe 1Düsseldorf 40237Germany Research Center for Computational Design of Advanced Functional Materials(CD-Fmat) National Institute of Advanced Industrial Science and Technology(AIST)TsukubaIbaraki 305-8568Japan Department of Materials Science and Engineering Delft University of TechnologyMekelweg2Delft 2628 CDthe Netherlands

High-and medium-entropy alloys(HEAs and MEAs)possess high solid-solution *** investigations have been conducted on its impact on yield strength,however,there are limited reports regarding the relation between solid-solution strengthening and strain-hardening *** addition,no attempt has been made to account for the dislocation-mediated plasticity;most works focused on twinning-or transformation-induced plasticity(TWIP or TRIP).In this work we reveal the role of solidsolution strengthening on the strain-hardening rate via systematically investigating evolutions of deformation structures by controlling the Cr/V ratio in prototypical V_(1-x)Cr_(x)CoNi *** the TWIP of CrCoNi with the dislocation slip of V_(0.4)Cr_(0.6)CoNi,the hardening rate of CrCoNi was superior to slip-band refinements of V_(0.4)Cr_(0.6)CoNi due to the dynamic Hall-Petch ***,as V content increased further to V_(0.7)Cr_(0.3)CoNi and VCoNi,their rate of slip-band refinement in V_(0.7)Cr_(0.3)CoNi and VCoNi with high solid-solution strength surpassed that of *** it is generally accepted in conventional alloys that deformation twinning results in a higher strain-hardening rate than dislocation-mediated plasticity,we observed that the latter can be predominant in the former under an activated huge solid-solution strengthening *** high solid-solution strength lowered the cross-slip activation and consequently retarded the dislocation rearrangement rate,i.e.,the dynamic *** delay in the hardening rate decrease,therefore,increased the strain-hardening rate,results in an overall higher strain-hardening rate of V-rich alloys.

关键词： Medium-entropy alloy Tensile property Solid-solution strength Strain-hardening rate Stacking fault energy

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Effects of cryogenic temperature on tensile and impact properties in a medium-entropy VCoNi alloy

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Journal of Materials Science & Technology 2021年第31期90卷 159-167页

作者： Dae Cheol Yang Yong Hee Jo Yuji Ikeda Fritz Kormann seok su sohn Department of Materials Science and Engineering Korea University02841 SeoulSouth Korea Metallic Materials Division Korea Institute of Materials Science51508 ChangwonSouth Korea Institute for Materials Science University of StuttgartPfaffenwaldring 5570569 StuttgartGermany Max-Planck-Institut für Eisenforschung Max-Planck-Straβe 140237 DüsseldorfGermany Department of Materials Science and Engineering Delft University of TechnologyMekelweg 22628 CD DelftThe Netherlands

Multi-principal element alloys usually exhibit outstanding strength and toughness at cryogenic temperatures,especially in CrMnFeCoNi and CrCoNi *** remarkable cryogenic properties are attributed to the occurrence of deformation twins,and it is envisaged that a reduced stacking fault energy(SFE)transforms the deformation mechanisms into advantageous properties at cryogenic temperatures.A recently reported high-strength VCoNi alloy is expected to exhibit further notable cryogenic ***,no attempt has been made to investigate the cryogenic properties in detail as well as the underlying deformation ***,the effects of cryogenic temperature on the tensile and impact properties are investigated,and the underlying mechanisms determining those properties are revealed in terms of the temperature dependence of the yield strength and deformation *** the strength and ductility were enhanced at 77 K compared to 298 K,while the Charpy impact toughness gradually decreased with *** planar dislocation glides remained unchanged at 77 K in contrast to the CrMnFeCoNi and CrCoNi alloys resulting in a relatively constant and slightly increasing SFE as the temperature decreased,which is confirmed via ab initio ***,the deformation localization near the grain boundaries at 298 K changed into a homogeneous distribution throughout the whole grains at 77 K,leading to a highly sustained strain hardening *** reduced impact toughness is directly related to the decreased plastic zone size,which is due to the reduced dislocation width and significant temperature dependence of the yield strength.

关键词： Medium-entropy alloy Cryogenic temperature Tensile property Charpy impact property Stacking fault energy

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Understanding the physical metallurgy of the CoCrFeMnNi high-entropy alloy:an atomistic simulation study

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npj Computational Materials 2018年第1期4卷 662-670页

作者： Won-Mi Choi Yong Hee Jo seok su sohn sunghak Lee Byeong-Joo Lee Department of Material Science and Engineering Pohang University of Science and Technology(POSTECH)Pohang 37673Republic of Korea

Although high-entropy alloys(HEAs)are attracting interest,the physical metallurgical mechanisms related to their properties have mostly not been clarified,and this limits wider industrial applications,in addition to the high alloy *** clarify the physical metallurgical reasons for the materials phenomena(sluggish diffusion and micro-twining at cryogenic temperatures)and investigate the effect of individual elements on solid solution hardening for the equiatomic CoCrFeMnNi HEA based on atomistic simulations(Monte Carlo,molecular dynamics and molecular statics).A significant number of stable vacant lattice sites with high migration energy barriers exists and is thought to cause the sluggish *** predict that the hexagonal close-packed(hcp)structure is more stable than the face-centered cubic(fcc)structure at 0 K,which we propose as the fundamental reason for the micro-twinning at cryogenic *** alloying effect on the critical resolved shear stress(CRSS)is well predicted by the atomistic simulation,used for a design of non-equiatomic fcc HEAs with improved strength,and is experimentally *** study demonstrates the applicability of the proposed atomistic approach combined with a thermodynamic calculation technique to a computational design of advanced HEAs.

关键词： alloy alloying entropy

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Enhancement of ballistic performance enabled by transformation-induced plasticity in high-strength bainitic steel

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Journal of Materials Science & Technology 2021年第25期84卷 219-229页

作者： Min Cheoljo Selim Kim Dong Woo suh Hong Kyu Kim Yongjin Kim seok su sohn sunghak Lee Center for Advanced Aerospace Materials Pohang University of Science and TechnologyPohang37673South Korea Graduate Institute of Ferrous Technology Pohang University of Science and TechnologyPohang37673South Korea Materials Directorate Agency for Defense DevelopmentDaejeon305-600South Korea Department of Materials Science and Engineering Korea UniversitySeoul02841South Korea

High-strength bainitic steels have created a lot of interest in recent times because of their excellent combination of strength,ductility,toughness,and high ballistic mass *** steels have great potential in the fabrication of steel armor *** various approaches and methods have been conducted to utilize the retained austenite(RA)in the bainitic matrix to control mechanical properties,very few attempts have been conducted to improve ballistic performance utilizing transformationinduced plasticity(TRIP)*** this study,high-strength bainitic steels were designed by controlling the time of austempering process to have various volume fractions and stability of RA while maintaining high *** dynamic compressive and ballistic impact tests were conducted,and the relation between the effects of TRIP on ballistic performance and the adiabatic shear band(ASB)formation was *** results show for the first time that an active TRIP mechanism achieved from a large quantity of metastable RA can significantly enhance the ballistic performance of high-strength bainitic steels because of the improved resistance to ASB ***,the ballistic performance can be effectively improved by a very short austempering time,which suggests that the utilization of active TRIP behavior via tuning RA acts as a primary mechanism for significantly enhancing the ballistic performance of high-strength bainitic steels.

关键词： High-strength bainitic steel Ballistic performance Split Hopkinson pressure bar(SHPB) Adiabatic shear band(ASB) Retained austenite(RA) Transformation-induced plasticity(TRIP)

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Omnidirectional color wavelength tuning of stretchable chiral liquid crystal elastomers

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Light(Science & Applications) 2024年第6期13卷 1062-1074页

作者： Seungmin Nam Wontae Jung Jun Hyuk Shin su seok Choi Department of Electrical Engineering Pohang University of Science and Technology(POSTECH)PohangKorea

Wavelength-tunable structural colors using stimuli-responsive materials,such as chiral liquid crystals(CLCs),have attracted increasing attention owing to their high functionality in various tunable photonic ***,on-demand omnidirectional wavelength control is highly desirable from the perspective of wavelength-tuning ***,despite numerous previous research efforts on tunable CLC structural colors,only mono-directional wavelength tuning toward shorter wavelengths has been employed in most studies to *** this study,we report the ideally desired omnidirectional wavelength control toward longer and shorter wavelengths with significantly improved tunability over a broadband wavelength *** using areal expanding and contractive strain control of dielectric elastomer actuators(DEAs)with chiral liquid crystal elastomers(CLCEs),simultaneous and omnidirectional structural color-tuning control was *** breakthrough in omnidirectional wavelength control enhances the achievable tuning freedom and versatility,making it applicable to a broad range of high-functional photonic applications.

关键词： tuning tunable directional

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Effects of Craddition on Charpy impact energy in austenitic 0.45C-24Mn-(0,3,6)Cr steels

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Journal of Materials Science & Technology 2020年第15期47卷 21-30页

作者： seok Gyu Lee Bohee Kim Min Cheol Jo Kyeong-Min Kim Junghoon Lee Jinho Bae Byeong-Joo Lee seok su sohn sunghak Lee Center for Advanced Aerospace Materials Pohang University of Science and TechnologyPohang37673South Korea Steel Products Design Team Hyundai SteelDangjin31779South Korea Steel Products Research Group 1 Technical Research LaboratoriesPOSCOGwangyang57807South Korea Department of Materials Science and Engineering Korea UniversitySeoul02841South Korea

Effects of Cr addition(0,3,and 6 wt%) on Charpy impact properties of Fe-C-Mn-Cr-based steels were studied by conducting dynamic compression tests at room and cryogenic *** room temperature,deformation mechanisms of Charpy impacted specimens were observed as twinning induced plasticity(TWIP) without any transfo rmation induced plasticity(TRIP) in all the *** cryogenic temperature,many twins were populated in the Cr-added steels,but,interestingly,fine ε-martensite was found in the OCr steel,satisfying the Shoji-Nishiyama(S-N) orientation relationship,{111}γ//{0002}ε and γ//ε.Even though the cryogenic-temperature staking fault energies(SFEs) of the three steel were situated in the TWIP regime,the martensitic transformation was induced by Mn-and Cr-segregated *** the OCr steel,SFEs of low-(Mn,Cr) bands lay between the TWIP and TRIP regimes which were sensitively affected by a small change of *** dynamic compressive test results well showed the relation between segregation bands and the *** of Cr were known as not only increasing the SFE but also promoting the carbide *** order to identify the possibility of carbide formation,a precipitation kinetics simulation was conducted,and the predicted fractions of precipitated M23C6 were negligible,0.4-1.1×10-5,even at the low cooling rate of 10℃/s.

关键词： Austenitic high-Mn steels Charpy impact energy Split Hopkinson pressure bar Twinning induced plasticity(TWIP) Transformation induced plasticity(TRIP) Stacking fault energy(SFE)

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Effects of Cu addition on formability and surface delamination phenomenon in high-strength high-Mn steels

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Journal of Materials Science & Technology 2020年第8期36卷 44-51页

作者： Min Chul Jo Jisung Yoo Min Cheol Jo Alireza Zargaran seok su sohn Nack J.Kim sunghak Lee Center for Advanced Aerospace Materials Pohang University of Science and TechnologyPohang 37673South Korea Graduate Institute of Ferrous Technology Pohang University of Science and TechnologyPohang 37673South Korea Department of Materials Science and Engineering Korea UniversitySeoul 02841South Korea

The formability of austenitic high-Mn steels is a critical issue in automotive applications under nonuniformly-deformed environments caused by dynamic strain *** austenite stabilizing alloying elements in those steels,Cu has been known as an effective element to enhance tensile properties via controlling the stacking fault energy and stability of *** effects of Cu addition on formability,however,have not been sufficiently reported *** this study,the Cu addition effects on formability and surface characteristics in the austenitic high-Mn TRIP steels were analyzed in consideration of inhomogeneous microstructures containing the segregation of Mn and *** reveal determining factors,various mechanical parameters such as total elongation,post elongation,strain hardening rate,normal anisotropy,and planar anisotropy were correlated to the hole-expansion and cup-drawing test *** respect to microstructural parameters,roles of(Mn,Cu)-segregation bands and resultant Cu-rich FCC precipitates on the formability and surface delamination were also discussed.

关键词： High-Mn steel Cu effects Cu-rich FCC phase Hole-expansion test Formability Stretch-flangeablity surface delamination

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Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

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Journal of Materials Science & Technology 2021年第17期76卷 222-230页

作者： Yong Hee Jo Junha Yang Won-Mi Choi Kyung-Yeon Doh Donghwa Lee Hyoung Seop Kim Byeong-Joo Lee seok su sohn sunghak Lee Center for High Entropy Alloys Pohang University of Science and TechnologyPohang 37673South Korea Department of Materials Science and Engineering Division of Advanced Materials SciencePohang University of Science and TechnologyPohang 37673South Korea Graduate Institute of Ferrous Technology Pohang University of Science and TechnologyPohang 37673South Korea Department of Materials Science and Engineering Korea UniversitySeoul 02841South Korea

We present a new class of metastable high-entropy alloys(HEAs),triggering deformation-induced martensitic transformation(DIMT)from face-centered-cubic(FCC)to body-centered-cubic(BCC),i.e.,*** the ab-initio calculation based on 1 st order axial interaction model and combined with the Gibbs free energy calculation,the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy(ISFE)in Si_xV_((9-x))Cr_(10)Mn_5 Fe_(46)Co_(30)(x=2,4,and 7 at.%)alloy *** ISFE decreases from-30.4 to-35.5 mJ/m^(2)as the Si content increases from 2 to 7 at.%,which well corresponds to the reduced phase stability of FCC against *** BCC-DIMT occurs in all the alloys via intermediate HCP martensite,and the HCP martensite provides nucleation sites of BCC ***,the transformation rate enhances as the Si content increases in an earlier deformation ra ***,the BCC-DIMT is also affected by the phase stability of FCC against BCC,and the stability is the highest at the Si content of 7 at.%.Thus,the 7Si alloy presents the moderate transformation rate in the later deformation *** to the well-controlled transformation rate and consequent strain-ha rdening rate,the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room *** results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

关键词： High-entropy alloy(HEA) Deformation-induced martensitic transformation(DIMT) Phase stability Stacking fault energy(SFE)

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Stable, efficient inverted perovskite solar cells enabled by the multifunctional ytterbium oxide buffer

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Science China Chemistry 2024年第8期67卷 2423-2424页

作者： Manman Hu Sang Il seok Department of Energy and Chemical Engineering Ulsan National Institute of Science and TechnologyUlsan 44919Republic of Korea

Significant advancements in single-junction inverted perovskite solar cells (PSCs) have led to power conversion efficiencies (PCEs) exceeding 25% [1]. However, their operational stability under strict conditions falls short compared with commercial solar panels, primarily due to the ion migration. This migration alters the chemical composition within the perovskite layer, increasing the heterogeneity and forming degradation sites [2]. Additionally, ion accumulation at metal interfaces corrodes electrodes, while metal diffusion into perovskite layers further compromises the stability, highlighting critical areas for the improvement in long-term device performance [3,4].

关键词： perovskite exceeding stability

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