Precipitation behavior and its effect on mechanical properties in weld heat-affected zone in age hardened FeMnAlC lightweight steels

Jeong, Seonghoon,Park, Gitae,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier 2019 Materials science & engineering. properties, micro Vol.742 No.-

<P><B>Abstract</B></P> <P>In this study, we investigated the effect of precipitation during the thermal cycles of age hardening and the welding process in hot rolled austenitic FeMnAlC lightweight steel. After solution treatment, samples were prepared under different age hardening conditions, which was conducted at a temperature of 550 °C in a furnace. Samples representing heat affected zones (HAZs) with three different peak temperature conditions were prepared via a Gleeble simulator. For microstructural analyses, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy (TEM) were carried out. The mechanical characteristics were investigated by tensile, hardness, and cryogenic Charpy impact tests. The results showed that precipitation behavior in the lightweight steels had specific influence in mechanical properties in age hardening and welding process. The age hardening effect was shown with increased tensile strength with loss of ductility. In the solution treated specimen, increased hardness appeared in HAZ compared to the base steel, while HAZ softening occurred in every age-hardened specimen. In the cryogenic Charpy impact test, all of the samples except the solution-treated base steel showed negligible energy absorption with inter-granular fracturing. The trends of the experimented HAZ mechanical properties revealed that the precipitation behavior of κ-carbide was strongly related to the overall mechanical properties. Moreover, we found that the welding process removed the hardening effect of aging as a result of drastic κ-carbide dissolution in the high-temperature HAZ regardless of age hardening conditions. The TEM analysis confirmed that all HAZ simulated specimens showed similar precipitation conditions for all cases. Based on our analysis, we confirmed that the precipitation behavior of κ-carbide was the main factor of different HAZ mechanical properties. We also suggested that discontinuous HAZ mechanical properties should be considered during the welding process because of κ-carbide dissolution in FeMnAlC lightweight steels.</P> <P><B>Highlights</B></P> <P> <UL> <LI> FeMnAlC alloy was analyzed for various age hardening and HAZ conditions. </LI> <LI> Mechanical properties had specific relationship with κ-carbide in all cases. </LI> <LI> Inter-granular κ-carbide rapidly formed in all heat treatment conditions. </LI> </UL> </P>

Carrier transport analysis of n-ZnO:Al/p-GaN:Mg heterojunction light-emitting diodes

Jeong, Seonghoon,Kim, Hyunsoo American Institute of Physics 2015 Journal of Vacuum Science & Technology. B Vol.33 No.2

Monolithic Inorganic ZnO/GaN Semiconductors Heterojunction White Light-Emitting Diodes

Jeong, Seonghoon,Oh, Seung Kyu,Ryou, Jae-Hyun,Ahn, Kwang-Soon,Song, Keun Man,Kim, Hyunsoo American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.4

<P>Monolithic light-emitting diodes (LEDs) that can generate white color at the one-chip level without the wavelength conversion through packaged phosphors or chip integration for photon recycling are of particular importance to produce compact, cost-competitive, and smart lighting sources. In this study, monolithic white LEDs were developed based on ZnO/GaN semiconductor heterojunctions. The electroluminescence (EL) wavelength of the ZnO/GaN heterojunction could be tuned by a post-thermal annealing process, causing the generation of an interfacial Ga2O3 layer. Ultraviolet, violet bluish, and greenish-yellow broad bands were observed from n-ZnO/p-GaN without an interfacial layer, whereas a strong greenish-yellow band emission was the only one observed from that with an interfacial layer. By controlled integration of ZnO/GaN heterojunctions with different postannealing conditions, monolithic white LED was demonstrated with color coordinates in the range (0.3534, 0.3710) (0.4197, 0.4080) and color temperatures of 4778-3349 K in the Commission Internationale de l'Eclairage 1931 chromaticity diagram. Furthermore, the monolithic white LED produced approximately 2.1 times higher optical output power than a conventional ZnO/GaN heterojunction due to the carrier confinement effect at the Ga2O3/n-ZnO interface.</P>

Heat-Affected Zone Characteristics with Post-Weld Heat Treatments in Austenitic Fe–Mn–Al–C Lightweight Steels

Seonghoon Jeong,Gitae Park,Bongyoon Kim,Joonoh Moon,Seong‑Jun Park,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.10

This study investigates the relationship between the mechanical characteristics and microstructural transitions in heat-affectedzone (HAZ) of various austenitic Fe–Mn–Al–C lightweight steels. The analysis was conducted on the simulated HAZ sampleswith various post-weld heat treatment (PWHT) conditions. The base steel was prepared via vacuum induction melting, hotrolling, and solution treatment prior to water quenching. Gleeble simulator was used to manufacture the simulated HAZs, andthe samples were subjected to PWHT with various durations by electric furnace. To understand the effects of thermal historyon the mechanical characteristics, ultimate tensile tests, micro-hardness tests, and cryogenic Charpy impact toughness testswere conducted, and microstructural analyses were conducted by optical microscopy, scanning electron microscopy, X-raydiffraction analysis, and transmission electron microscopy. The experimental results confirmed the influence of Al contenton the austenitic Fe–Mn–Al–C lightweight steels, which promotes κ-carbide precipitation, both in the austenite matrix andgrain boundaries, thereby increasing the strength and hardness while decreasing the ductility and toughness. Each mechanicalproperty showed a linear relationship with the growth kinetics of the κ-carbide particles. However, abnormal coarsening ofthe inter-granular κ-carbide during welding and PWHT caused severe embrittlement, regardless of thermal history, by actingas inter-granular crack propagation channels. The experimental results suggest that Fe–Mn–Al–C lightweight steels shouldbe developed with controlling the alloying elements to overcome HAZ discontinuity and mechanical property degradationoccurring by intra- and inter-granular κ-carbide precipitation.

Enhanced optical output power of AlGaN/GaN ultraviolet light-emitting diodes fabricated with breakdown induced conductive channels

Seonghoon Jeong,Sung-Nam Lee,Chel-Jong Choi,Hyunsoo Kim 한양대학교 세라믹연구소 2020 Journal of Ceramic Processing Research Vol.21 No.S1

The enhanced optical output power of AlGaN/GaN deep ultraviolet light-emitting diodes (UV LEDs) were demonstrated by using the breakdown-induced conductive channels (BICCs). The BICCs could be made by electrical reverse biasing between two adjacent contact pads formed on top p-type layers with a certain distance, causing an electrical breakdown of pn junction and hence a generation of conductive channels. Accordingly, the reflective Ni/Ag/Pt electrodes could be formed simultaneously on the top p-type layer and the other p-type layer with the BICCs, acting as the p- and n-contacts, respectively. The deep UV LEDs fabricated with the BICCs produced the enhanced optical output power by 15 % as compared to the reference LEDs, which were fabricated with the conventional Ti/Al/Ti/Au layers formed on mesa-etched n-type layer. This could be due to the reduced light absorption at the n-contact pads, indicating that the use of BICCs will be very suitable for obtaining better output performance of deep UV emitters.

Development of an Analytic Software Using Pencil Beam Scanning Proton Beam

Jeong, Seonghoon,Yoon, Myonggeun,Chung, Kwangzoo,Han, Youngyih,Lim, Do Hoon,Choi, Doo Ho Korean Society of Medical Physics 2017 의학물리 Vol.28 No.1

We have developed an analytic software that can easily analyze the spot position and width of proton beam therapy nozzles in a periodic quality assurance. The developed software consists of an image processing method that conducts an analysis using center-of-spot geometry and a Gaussian fitting method that conducts an analysis through Gaussian fitting. By using the software, an analysis of 210 proton spots with energies 150, 190, and 230 MeV showed a deviation of approximately 3% from the mean. The software we developed to analyze proton spot positions and widths provides an accurate analysis and reduces the time for analysis.

Inhibition of brain tumor cell proliferation by alternating electric fields

Jeong, Hyesun,Sung, Jiwon,Oh, Seung-ick,Jeong, Seonghoon,Koh, Eui Kwan,Hong, Sunghoi,Yoon, Myonggeun American Institute of Physics 2014 APPLIED PHYSICS LETTERS Vol.105 No.20

Analysis of Deep-Trap States in GaN/InGaN Ultraviolet Light-Emitting Diodes after Electrical Stress

Seonghoon Jeong,Hyunsoo Kim,Sung-Nam Lee 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.12

We analyzed the deep-trap states of GaN/InGaN ultraviolet light-emitting diodes (UV LEDs) before and after electrical stress. After electrical stress, the light output power dropped by 5.5%, and the forward leakage current was increased. The optical degradation mechanism could be explained based on the space-charge-limited conduction (SCLC) theory. Specifically, for the reference UV LED (before stress), two sets of deep-level states which were located 0.26 and 0.52 eV below the conduction band edge were present, one with a density of 2:41 1016 and the other with a density of 3:911016 cm3. However, after maximum electrical stress, three sets of deep-level states, with respective densities of 1:821016, 2:321016 cm3, 5:311016 cm3 were found to locate at 0.21, 0.24, and 0.50 eV below the conduction band. This finding shows that the SCLC theory is useful for understanding the degradation mechanism associated with defect generation in UV LEDs.

Increased Eciency of Range Verification in Routine QA for Pencil-Beam Scanning Proton Therapy

Seonghoon Jeong,Myonggeun Yoon,Jaeman Son,Kwangzoo Chung 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.7

Quality assurance (QA) is required when performing pencil-beam scanning proton therapy, but the eciency of QA is degraded in proportion to the energy of the protons. We developed a method to assess the preferred energy range and distal fall-off by combining multiple Bragg peaks to increase the eciency of QA. Beams of 70, 110, 150, 190, and 230 MeV for exposure were planned using a treatment planning system. The Bragg curves for therapeutic proton beams were modeled using three different fitting function models, allowing the feasibility of a simple modeling of the Bragg curve to be investigated. The planned beams were exposed and measured using a multi-layered ionization chamber. Software developed using a Python tool could detect five Bragg peaks from the integrated curves that were fitted based on polynomial, cubic spline and Landau distributions. This software could calculate the range and distal fall-off of the five fitted peaks. For the verification of the accuracy of this method, the calculated results were compared with the range and distal fall- off obtained by exposing and analyzing five single-energy beams individually. Comparisons of the Bragg peaks for the five energies exposed individually with the results obtained by exposing them all at once showed that the ranges of the energy beams when using the polynomial fitting and the cubic spline modes were 0.16 mm and 0.10 mm longer, respectively, while the distal fall-offs were 0.14 mm and 0.06 mm shorter, respectively. When using the Landau distribution fitting, the range was 0.06 mm longer and the distal fall-off was 0.04 mm shorter. Analyses of the ranges and distal fall-offs of the five energy beams exposed at once with single-beam loading by using the method developed in this study showed no significant differences from the results obtained by exposing the energy beams individually. Thus, range verification QA by using the proposed method is not only suitable for single-proton beams with multiple energies but also reduces the measurement time.

Influence of κ-carbide precipitation on the microstructure and mechanical properties in the weld heat-affected zone in various FeMnAlC alloys

Jeong, Seonghoon,Kim, Bongyoon,Moon, Joonoh,Park, Seong-Jun,Lee, Changhee Elsevier Sequoia 2018 Materials science & engineering Structural materia Vol.726 No.-

<P><B>Abstract</B></P> <P>This study aims to investigate the effects of κ-carbide precipitation behavior in the heat affected zone (HAZ) in FeMnAlC lightweight steels. Three alloys with different amounts of Al were prepared by vacuum induction melting and hot rolling. After solution treatment, the HAZ samples were simulated by a Gleeble simulator with two heat inputs of 10 and 30 kJ/cm. Microstructural analysis with XRD and TEM were carried out while sub-sized tensile test, hardness test, and V-notched Charpy impact test were performed for investigating the mechanical properties of the base steels and HAZ. The results showed that the mechanical properties and precipitation of κ-carbide within the HAZ were strongly related to the Al content and heat input; the tensile strength and hardness of the HAZ increased as the Al content and heat input increased while elongation decreased. On the other hand, in the Charpy impact test, fracture mode transitions in the HAZ (ranging from ductile fracture to brittle inter-granular fracture) occurred in accordance with the Al content and heat input. The different fracture behavior was explained by TEM results, which showed precipitation behavior of κ-carbide in HAZ. Coherent intra-granular κ-carbide was found to cause a transition from ductile fracture to trans-granular cleavage, and we observed that a severe drop of the impact toughness occurred when partially coherent inter-granular κ-carbide appeared. Therefore, our results illustrate that the HAZ of lightweight steels with the proper Al content can be strengthened with minimal loss of impact toughness due to κ-carbide precipitation during the welding process.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The effect of κ-carbide in the HAZ for FeMnAlC alloys was characterized. </LI> <LI> Relationship between mechanical properties and κ-carbide in HAZ was confirmed. </LI> <LI> Formation of intra- and inter-granular κ-carbide affected impact toughness. </LI> </UL> </P>