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이성학,김낙준,오진근,이의휘 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.10
The effects of microstructural factors on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of a cast Ti-6Al-4V alloy were investigated in this study. High-cycle fatigue test and fatigue crack propagation test were conducted for the welded regions, which were processed by two different welding methods, i.e., tungsten inert gas (TIG) welding and electron beam (EB) welding, and then the test data were analyzed in relation to microstructures, tensile properties, and fatigue fracture mode. The base metal was composed of Widmansta¨tten structure, while the EB and TIG welds consisted of basket-weave type structure having thin α platelets. The high-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of coarse pores formed during welding, although it had highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of very thin α platelets had the faster crack propagation rate than the base metal or the TIG weld. The effective microstructural factor determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed plastic zone size calculated at near-threshold ΔK.
SA 508 강 용접 열영향부의 미세조직과 충격인성의 상관관계
김상호,홍준화,김주학,권순주,이성학,강석영,오세진 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.4
In this study, microstructures of the heat affected zone (HAZ) of SA 508 steel were identified by Mo¨ssbauer spectroscopy in conjunction with microscopic observations, and were correlated with impact toughness. Specimens with the peak temperature risen up to 1350℃ showed mostly martensite, but the microstructures were not changed, much from the base metal because of the minor tempering effect when it was risen to 650℃ or 700℃. With the peak temperature risen to 900℃ over the A₃ temperature, the martensite fraction was reduced, while bainite or martensite island were formed because of the slow cooling from the lower austenite region. As the martensite fraction present inside the HAZ increased, hardness and strengths tended to increase, whereas impact toughness decreased. However, impact toughness of the subcritical HAZ with the peak temperature risen to 650℃-700℃ was seriously reduced after post-weld heat-treatment (PWHT) since carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting impact toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.
Ampicillin과 Chloramphenicol 내성 Salmonella typhimurium 분리의 증가
한상순,권오헌,정윤섭,이삼열,정태화 대한미생물학회 1987 大韓微生物學會誌 Vol.22 No.1
Salmonella typhi infection, which was the most frequent enteric infection in Korea, has been decreasing, while the infectian of other serogroups of Salmonella has been increasing since the later part of 1970s. In 1986, the number of serogroup R isolated by us increased to 46, which corresponds 21.1% of all Enteropathogenic bacteria isolated from stool specimens. Salmonella isolates resistant to antimicrobial agents were extremly rare in Korea, in the 1970s. However, 7 of 13 serogroup R isolates showed resistance to ampicillin or to chloramphenicol in 1984. Among the serogroup B isolates in 1986, 71.7% and 69.6% were resistant to ampicillin and to chloramphenicol respectively. The minimum in- hibitory concentrations of ampicillin and chloramphenicol against these isolates were $gt;128 pg/ml apd 128 pg/ml respectively.
Ti-(43-54)Al-xMn-yNb(x=0,3,4, y=0,4) 합금의 미세조직 및 변형거동 (1) : 미세조직 및 기계적 성질
표성규,김낙준,오진근 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.11
The main objective of the present paper is to study the effect of heat treatment and temperature on mechanical properties of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) alloys. The compressive yield strength as well as fracture strain was correlated with composition, microstructure, tetragonality, and strain rate. The controlled microstructures obtained by heat treatments were introduced in order to correlate the microstructural parameters with mechanical properties. The variation behavior of compressive yield strength of Ti_(49)Al_(51) base alloys showed the positive temperature dependence and the highest compressive yield strength among the measured six alloys was obtained in (Ti_(49)Al_(51))_(92)Mn₄Nb₄. The c/a ratio of (Ti_(53)Al_(47))_(97)Mn₃ alloy was measured using hot stage in-situ XRD technique at different temperatures. The c/a ratio was decreased significantly at 1273K. The c/a ratio of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) base alloys also varied with the compositions. The compressive fracture strain of Ti_(53)Al_(47) base alloys was increased with decrease of the c/a ratio, while Ti_(49)Al_(51) base alloys had no correlation with the c/a ratio.
초 고순도 TiAl 금속화합물의 미세조직 및 변형거동에 미치는 Al 조성의 영향
표성규,김낙준,오진근,Yamaguchi, M . 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.5
The present study is concerned with the effect of Al composition on the deformation behavior of ultra high pure (UHP) Ti-(48,50,52) at.%Al alloys using ultra high pure Ti with 30 wt. ppm oxygen. It is suggested that the deformation mechanism of UHP binary TiAl is strongly dependent on Al compositions. The deformation substructure of UHP Ti-48 at.%Al is shown to be dominated by ordinary dislocation with b = 1/2$lt;11 ̄0] as well as deformation twinning and small portion of 1/2$lt;112 ̄] superdislocation. Deformation mechanism in UHP Ti-50 at.%Al alloy consists of 1/2$lt;11 ̄0] type ordinary dislocation and 1/2$lt;112 ̄] superdislocation and a small portion of $lt;01 ̄1] superdislocation, but deformation twinning is not observed. Most of dislocation structure in UHP Ti-52 at.%Al alloy consisted of the faulted dipoles. Major deformation mode of UHP Ti-48 at.%Al and UHP Ti-50 at.%Al alloys was ordinary dislocation in deformation orientation, which take an advantage of ordinary dislocation slip. However, the major deformation of UHP Ti-52 at.%Al alloys was superdislocation slip in this orientation. It seems that there is little difference between major deformation mode of nominal purity TiAl and that of TiAl contained ultra low oxygen.
고에너지 전자빔투사방법을 사용한 TiB2/Ti 표면합금화 재료의 제조 연구
구양모,이성학,어광준,김낙준,오준철 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.12
The present study is concerned with the fabrication and the microstructural analysis of a TiB₂/Ti surfacealloyed material fabricated by high-energy electron beam irradiation. The mixture of TiB₂ powders and flux was deposited on a Ti-10V-2Fe-3Al alloy substrate, and then electron beam was irradiated on these mixture using an electron beam accelerator. The melted region of about 1.5 ㎜ thickness was homogeneously formed without pores or cracks, and was composed of primary and eutectic TiB borides in the β phase matrix. The formation of these TiB borides greatly improved hardness, especially high-temperature hardness up to 450℃. These findings suggested that the surface-alloying method using high-energy electron beam irradiation was economical and useful to the development of new advanced materials with improved high-temperature properties.
박경원,이설아,오인환,남석우,하홍용,임태훈,홍성안,성영은 한국공업화학회 1999 응용화학 Vol.3 No.2
Direct methanol fuel cell (DMFC) needs better anode electrocatalysts than Pt, which suffers from self CO poisoning phenomena. The improved oxidation kinetics enabled by Pt promoters such as Ru, Sn, Mo, W or Os has been attributed to a bifunctional and/or an electronic effect. The alloy electrode materials were prepared by reduction and colloidal methods. It was generally proved that binary system such as those based on Pt, these components are much more active in promoting DMFC reactions than pure platinum catalysts. The oxidation onset on pure Pt was 0.45 V and the potentials decreased with alloying. Pt-Ru showed the lowest onset potential. However, chronoamperometry provided high stability at Pt-Mo on CO poisoning, which shows a promising candidate as DMFC anode material. The strategy of ternary or quaternary electrocatalysts, the effects of synthesis and particle sizes will be discussed.
A STUDY ON THE EXPLOSION SAFETY ASSESSMENT OF HYDROCARBON REFRIGERANT REFRIGERATOR
Oh,Kyu-Hyung,Kim,Min-Kyu,Chu,Euy-Sung,Lim,Byung-Han,Kim,Man-Hoe,Park,Yoon-Ser 한국화재소방학회 1997 한국화재소방학회 학술대회 논문집 Vol.1997 No.-
This paper discribes an experimental explosion risk assessment study on refrigerators containing flammable hydrocarbon refrigerant. A refrigerator used in this study is a larder fridge type, 215 liter in volume. The hydrocarbon refrigerant used in the refrigerator is iso-butane(C<TEX>$_4$</TEX>H<TEX>$_{10}$</TEX>). For the explosion safety assessment of the refrigerator, temperature of compressor, cooling air circulation fan motor, defrost heater and inner lamp were measured during the operation. And to confirm the ignitablity of flammable gas by the electric spark of the switches of the refrigerator, ON-OFF test of all switches were conducted with compulsorily near the stoichiometric concentration atmosphere of iso-butane-air mixture. As the result of experiment above mentioned and another experiment for the explosion safety assessment, we can conclude that explosion hazard in connection with the use of hydrocarbon refrigerant was few.w.