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尹義博 成均館大學校 1980 論文集 Vol.27 No.-
Primary and secondary dendrite arm spacing increases with decreasing average cooling velocity. But primary arm spacing increases with Cu content, while secondary arm spacing does not change appreciably with Cu content over the composition range investigated. The relationships between these were obtained as follows; λ_1=0.87√Co/Va, λ_2=177Va^-1/3 wehre, λ_1 is the primary arm spacing (mm), Co is the Cu content (%), Va the average cooling velocity (℃/min) and λ_2 the secondary arm spacing (μ). As the solidification rate increases, the size and mean free path of the θ phase show an appreciable decrease. And mean free path between θ-partcles is similar to the order of the secondary dendrite arm spacing. With decreasing distance from the chill face (that is, increasing solidification rate), the amount of θ phase decreases over the range of solidification conditions investigated in this paper. As distance from chill becomes smaller, lattice constant decreases, and the Cu-content in α-matrix increased. From these results. The behavior of second phase in unidirectionally soildified Al-Cu alloys could be confirmed quantitatively.
尹義博 成均館大學校 1980 論文集 Vol.27 No.-
The influence of solidification conditions on microsegregation of dendritic structures was investigated with unidirectionally solidified Al-Cu alloy. In order to study the result of microsegregation, solute distribution at each distance from the chill surface was determind as a function of the fraction of solid which was given by two dimensional solidification model. Also the mechanism of solute redistribution during the growth of dendrite was discussed. The segregation index (S) increased by decreasing the solidification rate (R), and the S between primary dendrite arms was more remarkable than that between secondary dendrite arms. The relationship between S and R was generally obtained as follows; S=A log R+C where, A and C are the constants determined by concentration. Cu-isoconcentration curves around the dendrite element demonstrated Cu concentration to be minimum at the center of primary dendrite stalk. With decreasing distance from the chill surface (increasing R) the effective partition coefficient Ke (also minimum Cu concentration) increased, but Ke was little affected by concentration. The relationship between Ke and R was obtained as follows; Ke=0.21 log R+0.2 in which, R is cm/min. The influence of diffusion layer on the solute movement ahead of the solid-liquid interface near the dendrite tip which was not obtained by means of the prior linear analysis in Al-Cu alloys was clearly demonstrated by determining the fraction of solid from two dimensional solidification model, and it was found that the diffusion layer has a remarkable effect upon the solute redistribution. The degree of microsegregation was dependent upon the change of the effective partition coefficient with the change in the rate of solidification, and could be interpreted qualitatively by the partially mixed liquid model.
윤의박,조순형 대한금속재료학회(대한금속학회) 1987 대한금속·재료학회지 Vol.25 No.12
The microstructure of rapidly solidified Al-Cr alloys was investigated by scanning transmission electron microscopy (STEM), X-ray diffraction and micro-Victors hardness test. The temperature mariation during rapid solidification was studied to interpret the results. From the thermal analysis, the relationship between the cooling rate and the dendritic cell size was found to correspond to a previous theory which was obtained from an extrapolation of the measurements at slow cooling rate. The relatinship between the growth rate and the dendritic ceil size, however, was shown to be lower than those of general Al alloys, because the rate of the primary phase formation was suppressed by the increased fooling rate. By detailed composition measurements, the possible solubilily range of the complete supersaturation was found to be 3.43 wt% Cr. Above this concentration, the microstructure changed to a cellular structure by recoalescence in the last stage of solidification.
윤의박,김기철 대한금속재료학회(대한금속학회) 1985 대한금속·재료학회지 Vol.23 No.8
Dynamic fracture toughness of a structural steel has been estimated by employing instrumented impact testing method with small size specimens. One of the pendulum type impact testing machines equipped with semiconductor strain gage block in the tup, was used in this study. Signal from the dynamic transducer was multiplied by a dynamic strain amplifier and then stored in a digital memory before analyzing. Experimental results demonstrated that the toughness obtained from the instrumented impact testing agreed well with theoretical value, but several factors were appeared to play an important role when converting the test results into dynamic fracture toughness based on linear elastic theory. They were (1) the crack initiation point which would occur prior to maximum impact load, Pm, on the load-deflection curve, (2) compliance correction and (3) elastic inertial wave effect which had nothing to do with the specimen toughness.
계장화 충격시험에 의한 구상흑연 주철의 동적파괴인성의 고찰
윤의박,김기철,고광필 ( Eui Park Yoon,Ki Chol Kim,Kwang Pill Ko ) 한국주조공학회 1989 한국주조공학회지 Vol.9 No.2
N/A Microstructures and fracture toughness of the weld heat-affected zone of the ductile cast iron have been investigated. With increasing the cooling time from 800℃ to 500℃, the matrix structure of the heat-affected zone was transformed to martensite, martensitet pearlite, pearlite and pearlite including small amount of ferrite. The use of a proper preheat temperatures prevented the formation of martensite. These welding conditions corresponded to the preheat temperature of 200℃, heat input of 30KJ/㎝. And the instrumented charpy impact test has been carried out on a welded ductile cast iron. Result showed that the dynamic fracture toughness, K_(Id) was increased slightly with preheat temperature and heat input. In this work, it was shown that apparent impact energy includes contributions other than that of the true deflection of specimen. We found that a compliance energy correction was always needed to determine the true specimen energy. For this purpose, elastic compliance value of testing machine was measured dynamically by the low blow test to correct the apparent impact energy. From these results, the true specimen energy was nearly 62 percent of an apparent impact energy value. There, in instrumented impact testing, machine compliance corrections must be adapted to load-deflection curve.
2상혼합조직을 (相混合組織) 가진 구상흑연주철의 기계적 성질에 관한 연구
윤의박,이영호 ( Eui Pak Yoon,Young Ho Lee ) 한국주조공학회 1982 한국주조공학회지 Vol.2 No.2
N/A This paper is concerned with the improvement of impact and tensile Properties of spheroidal graphite cast iron of the following duplex matricess which were heat treated in the eutectic transformation temperature range (that is, (α+γ) coexisting range) ; ferrite-martensite, ferrite-bainite and ferrite-pearlite. The absorbed energy and maximum load was measured by recording the load-deflection curve with instrumented Charpy impact testing machine in the temperature range from +100℃ to -196℃. It was found the ferrite-bainite duplex matrix showed the highest toughness among the above matrices in the room temperature and the low temperature range. Comparison of this matrix to ferrite-pearlite matrix(that is, as east) showed a lowering of 27 ℃ in the nil-ductility transition temperature (NDT) and a lowering of 40 ℃ in the ductile-brittle transition temperature (TrE), Which seems to result from the finner dimple pattern observed using miorofractography.