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맹선재,신동혁,김기석 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.4
A thermomechanical process was developed which gives a fine grain size of 8㎛ in 7475 Al alloy. Tensile and superplastic properties of this material were evaluated in the temperature range of 673-818 K over the strain-rate range of 2.8×10^(-4)-2.8×10^(-2) s^(-1). The maximum ductility exhibited by the alloy was approximately 1300%, and optimum su-perplastic properties were achieved at a strain rate which is higher by an order of mag-nitude than other 7475 Al alloys. This result is attributed to the presence of fine diaper-soils which maintain the fine grain size at high homologous temperatures. The superplastic 7475 Al alloy has strain rate sensitivity of about 0.67, and an activation energy which is similar to the one for lattice diffusion of aluminum. The superplastic deformation can be explained by a diffusion accomodated grain boundary sliding model.
7075 Al 합금의 초소성 변형시 공동형성에 관한 연구
맹선재,신동혁,최능열,주연준 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.9
Cavitation during superplastic flow was investigated for 7075 Al alloy strained under uni-axial tension at 516℃. The 7075 Al alloy was thermomechanically processed to develop fine grain sizes of Bum and 12㎛, respectively. Cavitation and superplastic elongations between specimens with 8㎛ and 12㎛ were observed to be different. In the 8㎛ microstructure, the cavity stringers were formed along the tensile direction, whereas in the 12㎛ microstructure, the stringers were always oriented along the rolling direction, regardless of the tensile direction. The difference in the formation of cavity stringers seems to be mainly due to the distribution of the second phase particles in the matrix.
상이한 초기조직에서 얻은 이상조직강의 열처리에 따른 인장특성
맹선재,신동혁,김일영 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.6
The effect of tempering temperature on the tensile properties of dual phase steels, obtained from different microstrctures, were studied. The starting microstructures of the studied dual phase steels were three kinds ; ferrite+coarse pearlite(F+P₁), ferrite+fine pearlite(F+P₂) and martensite(M). By increasing the tempering temperature, tensile strength was decreased, independently of the starting microstructures. Yield strength of the F+P starting structures was increased at the tempering temperature around 200 C and then decreased with the increased tempering temperature. However, the DP steel of M-starting structure has shown progressive decrease in yield strength with increased tempering temperature. The tempering effect on the elongation was similar, independently of the starting structures. At the early stage of tempering, tensile properties were influenced predominantly by the segregation of carbon at dislocation sites. At the later stage of the tempering, softening effect by the decomposition of martensite is more effective.
상이한 초기조직에서 얻은 이상조직강의 열처리에 따른 인장변형 및 파괴거동
맹선재,신동혁,김일영 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.6
The different starting structures of the present study were ferrite + coarse pearlite (F+ P₁), ferrit + fine pearlite(F+P₂) and martensite(M). True stress vs. true strain curves were analysed before and after tempering. When compared the work hardening rate before tempering, the DP steel with the M-starting structure has shown the highest value. Uniform true strain was decreased with increasing tempering temperatures up to 400℃, and then increased above 400℃. Void formation behavior beneath the fracture surfaces of the tensile specimens has been studied. Preferred sites of void formation are the ferrite-martensite interfaces, the elongated martensite ends and the precipitate particles. Voids are also formed by splitting of martensite phases in the case of tempering.
7475 알루미늄합금의 중간가공 열처리에 따른 파괴인성
맹선재,신동혁,최해창,류성인 대한금속재료학회(대한금속학회) 1990 대한금속·재료학회지 Vol.28 No.1
ISML-ITMT(Institute Sperimentleder Metalli Leggeri-Intermediate Themomechanical Treatment) increases greatly fracture toughness of 7475 aluminum alloy. The ISML-ITMT samples have shown fine microstructure with small volume fraction of coarse particles, hence small number and large spacings of particles. This distribution of coarse secondary particles contributed to increasing the fracture toughness. The larger the spacing of intermediate particles in ITMT samples is, the higher the toughness is. The dimple spacing of the fractured surfaces was almost the same as the coarse particle spacing, suggesting that the coarse particles act as nucleation sites of micro-voids. Grain refinement does not seem to contribute to increasing the toughness. The most important parameter for increasing the fracture toughness is the distribution of coarse particles rather than the grain size.
7075 알루미늄합금 초소성재에 대한 소성변형후의 기계적 성질과 응력부식균열특성
맹선재,신동혁,최해창 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.8
Standard 7075 aluminum alloy was thermonecharically processed via the method by Paton to obtain superplasticity. The alloy was then pre-deformed under superplastic deformation condition and heat-treated to T-6 condition prior to tensile test and stress corrosion cracking test. By increasing the degree of pre-deformation above 60 % elongation, both the strength and elongation have decreased. Stress corrosion cracking sensitivity was increased by the increased pre-deformation. Cavitations, which was formed during the predeformation processing, deteriorated the tensile properties and increased stress corrosion cracking sensitivity with the amount of pre-deformation. Microstructure at the grain boundaries also affected to deteriorate tensile properties and SCC resistivity.
7475 알루미늄합금의 중간가공열처리 공정변화에 따른 초소성
맹선재,신동혁,유재은 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.2
Effect of the RI-ITMT variations on grain refinement and the superplasticity of 7475 aluminum alloy has been studied. The increased number of warm rolling passes contributed for better grain refinement, that is the average grain diameter of 7.5㎛ was obtained. As the solution treatment time increased, the distribution and the size of secondary particles became more uniform and finer, respectively, and the superplasticity was enhanced. The maximum superplastic elongation of this study was 2000% at 516℃, and at the deformation rate of 1.7 × 10^(-3)/s. The attained superplasticity of this study is much higher than any of the previously published results on 7475 and 7075 alloys. Cold rolling process, instead of the warm rolling of the conventional RI-ITMT, has resulted a little decreased superplasticity. However the superplastic elongation was still quite high.
상이한 초기조직을 갖는 저합금고강도강의 이상역가열에 의한 오스테나이트 형성과 기계적 성질
맹선재,신동혁,장동현,조현권 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.6
Austenite formation and mechanical properties of HSLA steel of different starting structures has been studied by intercritical annealing(IA). The different starting structures are: ferrite+pearlite (FP1), martensite(M1), cold worked ferrite+pearlite(FP2) and cold worked martensite(M2). Austenite was nucleated at ferrite grain boundaries in FP1 and FP2 and also at the interior of the grains in case of FP2. By increasing the IA time globular austenite structures were distributed uniformly all over the space enclosing the ferrite phase. Austenite in M1 and M2 was nucleated at the interface between prior martensite laths and the globular strutctures were formed from the former acicular structures. By increasing the IA time M2 has shown well developed globular austenite structure than M1 and the agglomerated continuous austenite networks surrounding ferrite phases were formed. Tensile strength in all the tested samples of different starting structures was increased by increasing the IA time. Any noticeable change in elongation was not found in FP1, FP2 and M2 by the increase of IA time. Exceptionally elongation of M1 was increased by increased IA time. Chapy V-notch impact values of all the different starting sturctures were decreased by increasing the IA time.
7475 알루미늄합금의 파괴인성 및 응력부식균열특성에 미치는 무석출대의 영향
박희준,맹선재,신동혁,곽희석 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.9
Effect of Precipitation free zone(PFZ) width on the fracture toughness and stress corrosion cracking(SCC) of 7475 aluminum alloy has been studied. The different PFZ width was obtained by controlling the cooling rate following solution treatment in the thermomechanical treatment of this study. PFZ width was also varied by aging treatment of different stages. By increasing the PFZ width stress intensity factor, K_(lc) has been increased in the under-and peak-aged conditions. Over-aged state has shown lower value of K_(lc) than the under-and peak-aged state when compared in the same level of PFZ width. However, when compared in the same over-aged condition, increased PFZ width resulted increased fracture toughness. The effect of PFZ width on the SCC has been studied in the under-aged condition by the crack-growth-rate test. Increased PFZ width resulted increased K_(lscc) and lower plateau velocity, which denotes the decrease of SCC sensitivity.