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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.
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.
고크롬 백주철의 미세조직과 기계적 특성에 미치는 초기응고 조직의 영향
조현욱 ( Hyun Wook Jo ),도정현 ( Jeong Hyeon Do ),조원제 ( Won Je Jo ),정현득 ( Hyun Deuk Chung ),이재현 ( Je Hyun Lee ),조창용 ( Chang Yong Jo ) 한국주조공학회 2015 한국주조공학회지 Vol.35 No.6
Due to excellent abrasion resistance the high-chrome white irons are widely used in mining and mineral industries. Minor variation of carbon content in 28% chrome white iron resulted in difference in primarily solidified microstructure. Sub-eutectic (hypoeutectic) a) composition led to formation of primarily solidified dendrites. Formation of primarily solidified dendrites which were supersaturated with carbon and chrome also caused precipitation of fine secondary carbides that are different from relatively large plate type M7C3 carbides in the eutectic structure. Small portion of primarily solidified dendrite expected to contribute significantly to the improvement of abrasion resistance of the white iron because the dendrites provided mechanical support to carbides. The relative fraction of primary dendrite increased with reduction of carbon content from the eutectic composition. The increased fraction of primary dendrite increased hardness value of the white irons.
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.
액체로켓 연소기용 Inconel 718 주조 및 단조 합금의 전자빔 용접부 미세조직 및 극저온 특성
홍현욱,배상현,권순일,이재현,도정현,최백규,김인수,조창용,Hong, Hyun-Uk,Bae, Sang-Hyun,Kwon, Soon-Il,Lee, Je-Hyun,Do, Jeong-Hyeon,Choi, Baig-Gyu,Kim, In-Soo,Jo, Chang-Yong 대한용접접합학회 2013 대한용접·접합학회지 Vol.31 No.6
Characterization of microstructures and cryogenic mechanical properties of electro beam (EB) welds between cast and forged Inconel 718 superalloys has been investigated. Optimal EBW condition was found in the beam current range of 36~39 mA with the constant travel speed of 12 mm/s and arc voltage of 120 kV for 10 mm-thick specimens. Electron beam current lower than 25 mA caused to occur the liquation microfissuring in cast-side heat affected zone (HAZ) of EB welds. The HAZ liquation microfissure was found on the liquated grain boundaries with resolidified ${\gamma}/Laves$ and ${\gamma}/NbC$ eutectic constituents. EBW produced welds showing a fine dendritic structure with relatively discrete Laves phase due to fast cooling rate. After post weld aging treatment, blocky Laves phase and formation of ${\gamma}^{\prime}+{\gamma}^{{\prime}{\prime}}$ strengtheners were observed. Presence of primary strengthener and coarse Laves particles in PWHT weld may cause to reduce micro-plastic zone ahead of a crack, leading to a significant decrease in Charpy impact toughness at $-196^{\circ}C$. Fracture initiation and propagation induced by Charpy impact testing were discussed in terms of the dislocation structures ahead of crack arisen from the fractured Laves phase.
용탕단조법으로 제조된 AZ51-xSn 마그네슘 합금의 미세파괴기구
김병호 ( Byeong Ho Kim ),도정현 ( Jeong Hyeon Do ),이성학 ( Sung Hak Lee ),박익민 ( Ik Min Park ) 대한금속·재료학회 2009 대한금속·재료학회지 Vol.47 No.12
A study was made of the effects of a Sn addition on the microstructure and microfracture mechanism of squeeze cast AZ51-xSn magnesium alloys. Microstructural observation, in situ fracture testing, and fractographic observations were conducted on these alloys to clarify the microfracture process. The microstructural analyses indicated that Mg2Sn particles as well as Mg17Al12 particles precipitated mainly along the solidification cell boundaries; the volume fraction of these hard particles increased as the amount of added Sn increased, with increased the strength. From in situ fracture observations of the AZ51-7Sn alloy, coarse precipitates located on the cell boundaries worked as easy crack propagation sites and caused abrupt intercellular fracturing. On the other hand, the overall fracture properties of the AZ51-3Sn alloy improved because crack propagation proceeded into the Mg matrix rather than into the cell boundaries as twins developed actively, as confirmed by an R-curve analysis. These findings suggest that the addition of 3~5 wt.% Sn is effective in improving both the tensile and fracture properties on the basis of well-developed twins, the blocking of crack propagation, and crack blunting.
니켈기 초내열합금의 열간노출에 따른 미세조직 및 기계적 특성 변화
김인수 ( In-soo Kim ),최백규 ( Baig-gyu Choi ),정중은 ( Joong-eun Jung ),도정현 ( Jeong-hyeon Do ),정인용 ( In-yong Jung ),조창용 ( Chang-yong Jo ) 한국주조공학회 2016 한국주조공학회지 Vol.36 No.5
The microstructural evolution of a cast Ni base superalloy, IN738LC, has been investigated after long term exposure at several temperatures. Most of the fine secondary γ` particles resolved after 2000 hour exposure at 816℃. At higher temperatures of 871℃ and 927℃, secondary γ` resolved after 1000 hours of exposure, and cuboidal primary γ` grew with exposure time. During the ther-mal exposure, σ phase formed at all tested temperatures, and η phase was observed around interdendritic regions due to carbide degeneration. The influence of microstructural evolution during thermal exposure on the mechanical properties has been analyzed. The effects of γ` particle growth are more pronounced on the high temperature creep properties than on the room temperature tensile properties.
니켈기 초내열 합금 GTD 111에서 편석에 의한 합금원소 분포 및 미세조직 변화
최백규 ( Baig Gyu Choi ),김인수 ( In Soo Kim ),도정현 ( Jeong Hyeon Do ),정중은 ( Joong Eun Jung ),조창용 ( Chang Yong Jo ) 한국주조공학회 2015 한국주조공학회지 Vol.35 No.6
Segregation during solidification and homogenization during thermal exposure in GTD 111 were investigated. The microstructures of as-cast, standard heat-treated, and thermally exposed specimens were observed by SEM. A compositional analysis of each specimen was conducted by EDS. The dendrite core was enriched in W and Co, though lower levels of Ti and Ta were observed. An unexpected phase, in this case like the η phase, was observed due to segregation near the γ-γ`` eutectic in the standard heat-treated specimen. Segregation also induced microstructural evolution near the γ-γ`` eutectic during the standard heat treatment. A quantitative analysis and microstructural observations showed that the thermal exposure at a high temperature enhanced the chemical homogeneity of the alloy.