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상이한 초미세립 구조를 갖는 OFHC Cu의 연성에 영향을 미치는 요소인자
박이주(L. J. Park),감형원(H. W. Kim),이종수(C. S. Lee),황시우(S. W. Hwang),박경태(K.-T.Park) 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.5
Tensile ductility of OFHC Cu with the different ultrafine grained (UFO) structures, which were fabricated by the different routes of equal channel angular pressing (ECAP), was associated in detail with the microstructural characteristics developed by ECAP. OFHC Cu having the lamellar and equiaxed UFO structures was prepared by ECAP of routes A and Bc respectively up to 8 and 16 passes. Their microstructures were closely examined by transmission electron microscopy and orientation image mapping. Tensile tests at room temperature were conduced on the ECAPed samples under the quasi- static condition of 10?³ s?¹ and 1 s?¹. Uniform elongation of the lamellar UFO samples decreased with increasing the ECAP passage while both uniform and total elongations of the equiaxed UFO samples increased. In the case of route A producing the lamellar UFO structure, the fractions of high angle grain boundaries and grains less than 0.5 ㎛ increased significantly but an analysis revealed that the dislocation free length decreased with increasing the ECAP passage. For route Be resulting in the equiaxed UFO structure, the fraction of high angle grain boundaries increased but the grain size distribution and the dislocation free length remained nearly unchanged with increasing the ECAP passage. From the present experiments and analyses, it was found that tensile ductility of lamellar UFO OFHC Cu is primarily controlled by the dislocation free length and that of the equiaxed one is mainly dependent on the fraction of high angle grain boundaries.
쌍정 진화의 변형률 효과를 포함한 cell 모델과 유한요소해석
안동현(D. H. Ahn),박이주(L. J. Park),김형섭(H. S. Kim) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.5
In this study, a dislocation density based constitutive model with two phase, dislocation cell wall and cell interior, is proposed. Equations including twin volume fraction and twin thickness are added to consider not only slip but also twinning mechanism. In addition, incompatibility stress induced by insufficient slip system of hcp (hexagonal closest packing) structure material is also included. For the application of the model simple compression test of CP-titanium (grade 2) at various strain rate was performed and microstructure (specially, the evolution of twinning) were analyzed by optical microscope (OM) and electron backscatter diffraction (EBSD). Based on the parameters obtained by the test, the finite element method (FEM) simulation of the forging process with high pressure was compared with the experiment.
초미세 결정립 구조를 갖는 OFHC Cu 대형봉재의 미세조직 및 기계적 특성
박경태(K.-T. Park),박이주(L. J. Park),김형원(H. W. Kim),황시우(S.W. Hwang),지정훈(J.H. Ji),이종수(C.S. Lee) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5
Bars of OFHC Cu with the diameter of 45mm were processed by equal channel angular pressing up to 16 passes via route Be, and homogeneity of their microstructrues and mechanical properties was examined at every four passes which develop the equiaxed ultrafine grains. Cross-sectional hardness exhibited a concentric distribution. Hardness was the highest at the center of bar and it decreased gradually from center to surface. After 16 passes, overall hardness decreased due to recovery and partial recrystallization. Regardless of the number of passage, yield strength and tensile strength were quite uniform at all positions, but elongation showed some degree of scattering. At 4 passes, coarse and ultrafine grains coexisted at all positions. After 4 passes, uniform equiaxed ultrafine grains were obtained at the center, while uniform elongated ultrafine grains were manifested at the upper half position. At the lower half position, grains were equiaxed but its size was inhomogeneous. It was found that inhomogeneity of grain morphology and grain size distribution at different positions are to be attributed to scattering in elongation but they did not affect strength.