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IN738LC 초내열합금 정밀 주조의 주조 및 열처리 미세조직에 구성되는 성분 편석 현상
최병학,한성희,김대현,안종기,이재현,Choe, Byung Hak,Han, Sung Hee,Kim, Dae Hyun,Ahn, Jong Kee,Lee, Jae Hyun 한국재료학회 2021 한국재료학회지 Vol.31 No.7
The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ' and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ' and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ', and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ' with Ni<sub>3</sub>(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ' forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.
최병학,심종헌,김승언,현용택,박찬희,강주희,이용태,김영욱,Choe, Byung Hak,Shim, Jong Heon,Kim, Seung Eon,Hyun, Yong Taek,Park, Chan Hee,Kang, Joo-Hee,Lee, Yong Tai,Kim, Young Ouk 한국재료학회 2015 한국재료학회지 Vol.25 No.8
Metastable phase characteristics of beta Ti alloys were investigated to consider the relationship of the microstructure and diffraction pattern in TEM. TEM analysis showed that the microstructure was mottled as a modulated structure, and the diffraction pattern was composed of spot streaks between the main spots of a stable beta phase with a specific lattice relationship. The modulated structure may be induced by short distance slip or atom movement during a very short interval of solution treated and quenched (STQ) materials. The athermal ${\omega}$ phase, which could be precipitated at low temperature aging, is also analysed by the metastable phase. The metastable phases including athermal ${\omega}$ phase had a common characteristic of hardened and brittle behavior because the dislocation slip was restricted by a super lattice effect due to short distance atom movement at the metastable state.
최병학,이범규,장현수,전우일,박용성,임재균,이진희,박찬성,김진표,Choe, Byung Hak,Lee, Bum Gyu,Jang, Hyeon Su,Jeon, Woo Il,Park, Yong Sung,Lim, Jae Kyun,Lee, Jin Hee,Park, Chan Sung,Kim, Jin Pyo 한국재료학회 2017 한국재료학회지 Vol.27 No.3
The aim of this study is to consider the effect hydrogen on dezincification behavior of Cu-Zn alloys. The investigations include microstructural observations with scanning electron microscope and chemical composition analysis with energy dispersive spectrometer. The dezincification layer was found to occur in high pressure hydrogen atmosphere, not in air atmosphere. In addition, the layers penetrated into the inner side along the grain boundaries in the case of hydrogen condition. The shape of the dezincification layers was porous because of Zn dissolution from the ${\alpha}$ or ${\beta}$ phase. In the case of stress corrosion cracks formed in the Cu-Zn microstructure, the dezincification phenomenon with porous voids was also accompanied by grain boundary cracking.
최병학,이범규,장현수,박찬성,김진표,박남규,김청인,김보미,Choe, Byung Hak,Lee, Bum Gyu,Jang, Hyeon Su,Park, Chan Sung,Kim, Jin Pyo,Park, Nam Gyu,Kim, Cheong In,Kim, Bo Mi 한국재료학회 2017 한국재료학회지 Vol.27 No.8
This study investigated the surface pit corrosion of SS420J2 stainless steel accompanied by intergranular crack. To reveal the causes of surface pits and cracks, OM, SEM, and TEM analyses of the microstructures of the utilized SS420J2 were performed, as was simulated heat treatment. The intergranular cracks were found to have been induced by a grain boundary carbide of $(Cr,Fe)_{23}C_6$, which was identified by SEM/EDS and TEM diffraction analyses. The mechanism of grain boundary sensitization occurred at the position of the carbide, followed by its occurrence at the Cr depleted zone. The grain boundary carbide of $(Cr,Fe)_{23}C_6$ type precipitated during air cooling condition after a $1038^{\circ}C$ solid solution treatment. The carbide precipitate formation also accelerated at the band structure formed by cold working. Therefore, using manufacturing processes of cooling and cold working, it is difficult to protect SS420J2 stainless steel against surface pit corrosion. Several counter plans to fight pit corrosion by sensitization were suggested, involving alloying and manufacturing processes.
최병학,이범규,심종헌,고형순,조남철,이재성,박경균,김유찬,Choe, Byung Hak,Lee, Bum Gyu,Shim, Jong Hun,Go, Hyung Soon,Jo, Nam Chul,Lee, Jae Sung,Park, Kyung Gyun,Kim, Yu Chan 한국재료학회 2016 한국재료학회지 Vol.26 No.10
The aim of this paper is to consider the effect of the manufacturing processes on corrosion and centerline cracking of ancient bronze spoons. The ancient bronze spoons in question were made by several steps of forging, in reheated condition with cast ingots. The manufacturing method is similar to that of the modern spoons. The investigations include observations from light and scanning electron microscopes of the microstructure in terms of the crack propagation. Cracks in the centerline are caused by solute segregation in the center-line region; this solute is solidified in the final stage of bronze spoon manufacture. Centerline cracking is also caused by ${\alpha}$ phase segregation, accompanied by forged overlapping along the longitudinal direction of the spoons. A vertical stripe with cracks along the centerline of the spoon's width is formed by folding in the wrought process. The overlapping area causes crack propagation with severe corrosion on the spoon surfaces over a period of a thousand years. The failure mechanisms of ancient bronze spoons may be similar to that of modern spoons, and the estimation of the failure mechanisms of ancient spoons can be appropriate to determine failure causes for such modern spoons.