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Journal of Materials Science & Technology 2020年第24期59卷 37-43页

作者： Timothy Alexander Listyawan Hyunjong Lee nokeun park School of Materials Science and Engineering Yeungnam University280 Daehak-roGyeongsangbuk-do38541Republic of Korea Korea Institute of Industrial Technology Incheon21999Republic of Korea Institute of Materials Technology Yeungnam University280 Daehak-roGyeongsanGyeongsangbuk-do38541Republic of Korea

Ultrasonic nanocrystal surface modification (UNSM) treatment on non-equiatomic medium-and highentropy alloy (HEA) of Fex(CoCrMnNi)100-xis firstly introduced and its impact on microstructure and mechanical properties are *** UNSM,severe plastic deformation-induced dislocation and deformation twins (DTs) arise at the topmost ***,Fe60(CoCrMnNi)40(Fe60),which is classified as a medium-entropy alloy (MEA),exhibits ε-martensitic *** the room temperature tensile test,a high strength of ~600 MPa and ductility of ~65%elongation (strain to failure) is accomplished in *** formed DTs and ε-martensitic transformation by UNSM treatment plays a key role in retardation of necking point via both twinning-induced plasticity and transformation-induced ***,Fe20(CoCrMnNi)80(Fe20) comparatively shows low strength of ~550 MPa and ~40% elongation,owing to the low accommodation of DTs than *** research will provide new guidelines for enhancing the mechanical properties of MEA and HEA.

关键词： High-entropy alloys Ultrasonic nanocrystal surface modification Deformation twin Microstructures Mechanical properties

Microstructure and mechanical properties of CoCrFeMnNi high entropy alloy with ultrasonic nanocrystal surface modification process

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Journal of Materials Science & Technology 2020年第22期54卷 123-130页

作者： Timothy Alexander Listyawan Hyunjong Lee nokeun park Unhae Lee School of Materials Science and Engineering Yeungnam University280 Daehak-roCyeongsanGyeongsangbuk-do 38541Republic of Korea Advanced Process and Materials R&D Group Korea Institute of Industrial TechnologyIncheon 21999Republic of Korea Institute of Materials Technology Yeungnam University280 Daehak-roGyeongsanGyeongsangbuk-do 38541Republic of Korea POSCO Technical Research Laboratories Gwangyang 57807Republic of Korea

In this study, mechanical properties improvement of equiatomic CoCrFeMnNi treated with an ultrasonic nanocrystal surface modification(UNSM) was studied. The applied UNSM treatment with static loads of 10 N, 20 N, and 60 N provided a severe plastic deformation, which produced a gradient structure. The nearsurface area exhibited a high number of dislocation densities and deformation twin interaction, leading to a surface strengthening and hardness improvement of up to 112% than the deformation-free interior region. Increment of dislocation densities and deformation twin formation on the surface also enhanced the yield and ultimate tensile strength of the UNSM-treated specimens. Furthermore, the combination of hard nanocrystallites layer on the surface and ductile coarse grain in the specimen interior as a result of the UNSM treatment successfully maintained the strength–ductility balance of the CoCrFeMnNi.

关键词： High-entropy alloy Ultrasonic nanocrystal surface modification Severe plastic deformation Microstructures Mechanical properties

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Journal of Materials Science & Technology 2021年第26期85卷 95-105页

作者： Nana Kwabena Adomako Giseung Shin nokeun park Kyoungtae park Jeoung Han Kim Department of Materials Science and Engineering Hanbat National UniversityYuseong-guDaejeon34158Republic of Korea Department of Materials Science and Engineering Pusan National UniversityGeumjeong-guBusan46241Republic of Korea School of Materials Science and Engineering Yeungnam University280 Daehak-roGyeongbuk38541Republic of Korea Rare Metal R&D Group Korea Institute of Industrial TechnologyIncheon21999Republic of Korea

High entropy alloys(HEAs)have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace *** a dissimilar joint design is required for these applications,we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel(DSS)through laser beam welding;a technique capable of producing a sound joint between the two *** structure examination using SEM/EBSD/XRD analysis revealed that the weld metal(WM)exhibits an FCC phase regardless of the postweld heat treatment(PWHT)temperature(800 and 1000℃)without forming detrimental intermetallic compounds or *** heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no ***,a lower hardness was recorded by the WM compared to the base metal after *** PWHT,the hardness of the WM,CoCrFeMnNi-HEA,and DSS decreased with an increase in the PWHT ***,the decrease in the hardness of the HEA was more significant than in the WM and *** cause for this reduction in hardness was attributed to recrystallization and grain *** addition,a strength of 584 MPa with low ductility was recorded after *** obtained strength was lower than that of the BMs,but comparable to that of the welded *** application of PWHT resulted in over a 20%increment in ductility,with only a marginal reduction in *** defo rmation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was *** propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint,which will be the subject of future studies.

关键词： High entropy alloys Welding Duplex stainless steel Post-weld heat treatment

Improving high-temperature oxidation behavior by modifying Al and Co content in Al-Co-Cr-Fe-Ni high-entropy alloy

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Journal of Materials Science & Technology 2022年第34期129卷 115-126页

作者： Timothy Alexander Listyawan Maya Putri Agus tianingrum Young Sang Na Ka Ram Lim nokeun park School of Materials Science and Engineering Yeungnam University280 Daehak-roGyeongbuk 38541Republic of Korea Advanced Metals Division Korea Institute of Materials Science(KIMS)797 ChangwondaeroSeongsan-guChangwonGyeongnam 51508Republic of Korea Institute of Materials Technology Yeungnam University280 Daehak-roGyeongsanGyeongbuk 38541Republic of Korea

Composition modification was introduced to improve the oxidation resistance by varying Al and excluding Co from the Al-Co-Cr-Fe-Ni system. Since adjusting the composition shifted the valence electron concentration(VEC) of the alloys, the dual-phase structure of the alloys is expected to be more stable. At low temperatures(T < 1273 K), the alloys formed mixed oxide products. As oxidation temperature increased,only Cr_(2)O_(3)or Al_(2)O_(3)dominated the alloy’s surface. Compared to equiatomic AlCoCrFeNi(5-Equi), nonequiatomic AlCoCrFeNi(5-B 40) and four-component AlCrFeNi(4-B 2013) had better oxidation resistance due to monocrystalline-Al_(2)O_(3)formation. Besides the role of oxide formation, maintaining BCC and B2phases within the alloys is also beneficial to supporting the stable Cr_(2)O_(3)or Al_(2)O_(3).

关键词： High-entropy alloy Alloy design Oxidation Valence electron concentration Phase stability Transmission electron microscope

A feasible route to produce 1.1 GPa ferritic-based low-Mn lightweight steels with ductility of 47%

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Journal of Materials Science & Technology 2022年第22期117卷 225-237页

作者： Kwang Kyu Ko Hyo Ju Bae Eun Hye park Hyeon-Uk Jeong Hyoung Seok park Jae Seok Jeong Jung Gi Kim Hyokyung Sung nokeun park Jae Bok Seol Department of Materials Engineering and Convergence Technology Center for K-metalGyeongsang National University(GNU)Jinju 52828South Korea School of Materials Science and Engineering Yeungnam UniversityGyeongbuk 38541South Korea Materials Research Team R&D Hyundai MobisYongin 16891South Korea Materials Technology Development Team Doosan Heavy Industries&ConstructionChangwon 51711South Korea

High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ***, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facilestrategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embeddedin a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain,which act through dislocation engineering. We ascribe the simultaneous improvement in strength andtotal elongation to both the size-dependent dislocation movement in austenite grains and the controlleddeformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation andhigh dislocation density-induced hardening and by interface strengthening. Additionally, high-carbonpartitioned small austenite grains enhance the strength and ductility by planar dislocation glide (inthe low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strainregime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP maybe desirable for broad industrial applications at an economic cost.

关键词： Low-Mn lightweight steel Carbon partitioning Metastable austenite Dislocation movement