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주조 합금 Alloy 718에서 미세조직과 인장특성에 미치는 열처리의 영향
도정현 ( Jeong-hyeon Do ),김인수 ( In-soo Kim ),최백규 ( Baig-gyu Choi ),정중은 ( Joong-eun Jung ),정인용 ( In-yong Jung ),조창용 ( Chang-yong Jo ) 한국주조공학회 2016 한국주조공학회지 Vol.36 No.5
The effect of various types of heat-treatment on the mechanical properties of cast Alloy 718 has been investigated. Cast Alloy 718 bars were subjected to ‘standard heat-treatment`_(SHT), ‘HIP (Hot Isostatic Pressing) heat-treatment`_(HHT), and ‘HIP-sim-ulated heat-treatment`_(HS). In the absence of long time high temperature heat-treatment, a small amount of Laves phase remained in the ‘SHT` specimen, and needle shaped δ precipitated in the vicinity of the Laves phase. Due to the formation of the Laves and δ phases in the ‘SHT` specimen, it exhibited lower tensile properties than those of the others_specimens. On the other hand, the Laves phase was completely dissolved into the matrix after ‘HHT` and ‘HS` treatments. It is known that isostatic pressure reduces the self-diffusion coefficient, because of the lower self-diffusivity under HIP conditions in the interdendritic region, Nb segregation and the high amount of γ” precipitation that occurs. Due to the higher fraction of coarse γ” phases, the ‘HHT` treated Alloy 718 showed excellent tensile strength.
고에너지 전자빔 투사방법으로 제조된 Zr계 비정질 합금 표면복합재료의 탄도충격 성능
도정현 ( Jeong Hyeon Do ),전창우 ( Chang Woo Jeon ),남덕현 ( Duk Hyun Nam ),김충년 ( Choongnyun Paul Kim ),송영범 ( Young Buem Song ),이성학 ( Sung Hak Lee ) 대한금속·재료학회 2010 대한금속·재료학회지 Vol.48 No.12
The objective of this study is to investigate the ballistic properties of Zr-based amorphous alloy surface composites fabricated by high-energy electron-beam irradiation. The mixture of Zr-based amorphous powders and LiF+MgF2 flux powders was deposited on a pure Ti substrate, and then an electron beam irradiated this powder mixture to fabricate a one-layer surface composite. A four-layer surface composite, in which the composite layer thickness was larger than 3 mm, was also fabricated by irradiating the deposited powder mixture by an electron beam three times on the one-layer surface composite. The microstructural analysis results indicated that a small amount of fine crystalline particles were homogeneously distributed in the amorphous matrix of the surface composite layer. According to the ballistic impact test results, the surface composite layers effectively blocked a fast traveling projectile, while many cracks were formed at the composite layers, and thus the surface composite plates were not perforated. The surface composite layer containing ductile β dendritic phases showed a better ballistic performance than the one without dendrites because dendritic phases hindered the propagation of shear bands or cracks.
다결정 니켈기 초내열 합금 CM247LC의 온도에 따른 인장특성 변화
최백규,김인수,도정현,정중은,석우영,이유화 한국주조공학회 2020 한국주조공학회지 Vol.40 No.4
Microstructural evolution during a heat treatment and high-temperature tensile properties have been investigated in conventionally cast CM247LC. In as-cast specimens, MC carbides with high amounts of Ta, Ti, Hf, and W were found to exist in the interdendritic regions, and γ' was observed in the form of cubes and octocubes prior to decomposition into cubes. In the heat-treated condition, some portion of eutectic γ-γ' remained, and uniform cubic γ' was observed in both interdendritic regions and dendrite core. Three types of carbides with different stoichiometries and compositions were found at the grain boundaries. MC carbides with high Hf contents were observed in the vicinity of eutectic γ-γ'. The highest tensile strength value was found at 750o C, whereas the greatest ductility appeared at 649o C. The effect of the temperature on the tensile properties was closely related to the dislocation structure. With increase in the test temperature, the density of dislocations inside γ' decreased, whereas that in the γ matrix increased. Stacking faults generated in γ' at 750o C had a strengthening effect, whereas thermally activated dislocation motion at a high temperature was considered to have the opposite effect.
Hastelloy X 주조재의 열간 노출에 따른 미세조직 및 인장 특성 변화
최백규,김인수,도정현,정중은,정인용,홍현욱,조창용,Choi, Baig Gyu,Kim, In Soo,Do, Jeonghyeon,Jung, Joong Eun,Jung, In Yong,Hong, Hyun Uk,Jo, Chang Yong 한국주조공학회 2017 한국주조공학회지 Vol.37 No.5
Microstructural evolution of cast Hastelloy X during thermal exposure has been investigated. OM, SEM, and TEM microscopy were carried out on the as-cast, the standard heat treated, and the thermally exposed conditions. Tensile tests were also conducted to understand the effect of microstructural evolution on the degradation of tensile properties. Coarse $M_6C$ and fine $M_{23}C_6$ carbides were found in as-cast Hastelloy X with fine carbides on sub-boundary. Some of $M_{23}C_6$ carbide dissolved into the matrix during solution heat treatment and dislocation network formed at the interface between the carbide and the matrix due to the misfit strain. There was no significant microstructural difference between the exposed specimens at $400^{\circ}C$ and the solution heat treated specimen. A large amount of $M_{23}C_6$ carbides precipitated along and near grain boundaries and sub-boundaries after exposure at $650^{\circ}C$. Exposure at $870^{\circ}C$ of the alloy caused precipitation of $M_6C$ and ${\mu}$. The strength increased and the elongation decreased by thermal exposure at $650^{\circ}C$ and $870^{\circ}C$ because carbides interfere with the movement of the dislocation. It was found that the precipitation of carbide gave significant effects on the tensile properties of Hastelloy X.
주조합금 IN 718 전자빔 용접부의 극저온 기계적 특성에 미치는 미세구조와 후열처리 영향
권순일 ( Soon Il Kwon ),도정현 ( Jeong Hyeon Do ),조창용 ( Chang Yong Jo ),홍현욱 ( Hyun Uk Hong ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.1
Effects of post-weld heat treatment (PWHT) on the microstructure evolution and the mechanical properties at -196 ℃ have been investigated with electron beam (EB) welds of cast Inconel 718. Coarse Laves particles in EB fusion zones after conventional PWHTs provided a preferential site for crack initiation, leading to a significant decrease in cryogenic Charpy impact toughness. A modified PWHT, which was highlighted as higher-temperature solution treatment followed by slow cooling to 980 ℃, was designed to ensure a sufficient micro-plastic region ahead of crack tip by a significant reduction of the Laves size and fraction. The EB weld after the modified PWHT exhibited no blocky Laves particles. Instead NbC with a size of 200~300 nm were observed. This modified PWHT led to 1.7 times higher CVN impact toughness than base metal. The cryogenic mechanical properties of EB welds are discussed in terms of dendrite arm spacing, the Laves size and fraction.