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Ti-6Al-4V 합금의 α상 형상에 따른 미세균열 생성거동
장영원,박찬경,이종수,배기만 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.1
The purpose of this study is to investigate the deformation characteristics and microcrack initiation behavior depending on the microstructure of Widmanstatten-α(Wα) and equiaxed-α(Eα) in a Ti-6Al-4V Alloy. In Wα structure, microcracks were observed to form along the α/β interfaces, the colony boundaries and the grain boundary α layers both in tension and fracture tests. In Eα structure, most of microvoids were found to form at the α/β interfaces. The lower tensile strength and ductility of Wα structure was mainly due to the strain localization along the prior β grain boundaries and colony boundaries, which was preceded by the easy slip transfer through α/βinterfaces within the colony. The large increase in fracture toughness of Wα structure was, however, primarily attributed to the crack path deviation depending on the colony orientation.
가공 및 열처리에 따른 Ti-10V-2Fe-3Al 합금의 미세조직과 기계적 성질의 변화
장영원,이종수,김성주,박찬향 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.3
The relationship between microstructure and mechanical properties of the near-βTi-10-V2Fe-3Al alloy has been investigated. Microstructural variations were made by either changing heat treatment or imposing the thermomechanical processing prior to heat treatment. In the β-solution treated(ST) and aged structure, brittle fracture occurred during the tensile tests, presumably due to the precipitations of fine secondary a particles in the metastable β matrix. The presence of primary a in the (α+β)-ST and aged structure represented high ductility, about 10%. The increase in ductility was, however, limited by the formation of thick grain boundary α-film. Thermomechanical processing was found to be effective in increasing yield and tensile strength of the alloy. In order to get a good combination of strength and toughness it is thus, suggested that microstructure should be modified not to contain any grain boundary α, but to contain globular primary αin small β grains.
Al-Li 8090 합금의 미세조직에 따른 피로거동에 관한 연구
장영원,이종수,박기종,박찬경 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.4
The present research was performed to investigate the effect of precipitates on the fatigue properties of 8090 Al-Li alloy. By applying different heat treatment paths, two types of microstructures were produced : one with the δ' phase dominant microstructure and the other with δ' + S' microstructure. It was observed that both the fatigue crack initiation resistance and fatigue crack propagation resistance of δ' microstructure were considerably higher than those of δ' + S' microstructure. Excellent resistance for fatigue crack initiation in δ' microstructure was mainly attributed to the extrinsic nature such as crack path tortuosity and higher level of crack closure, as well as to the intrinsic nature of greater slip reversibility in δ' microstructure. However, δ' + S' microstructure showed greater intrinsic resistance for crack propagation at intermediate ΔK region, probably due to the effect of slip dispersion by the presence of S' phase. It was also observed that the relative amount of crack closure was maximum at the near threshold regime and decreased rapidly as ΔK increased.
고탄소강 bloom 의 연속주조시 macro 편석 발생거동
장영원,오경식,이인열 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.11
The macrosegregation behavior in continuously cast(CC) high carbon steel blooms are systematically investigated by carrying out a plant test using a bloom caster at POSCO in connection with the electro-magnetic stirring(EMS) mode and stirring force. The degree of centre soundness is quantitatively measured in terms of the area fraction and size distribution of centre porosity as well as the segregating solutes by using a mapping analysis method. The thicknesswise solidification structures are examined to find out solidification behavior in the centre region of CC blooms and the calculation of specific permeability(k) is also presented to elucidate the formation mechanism of centre defects. The addition of a final stirring(FEMS) to an in-strand EMS(SEMS) is found to increase the solidification rate significantly in the final stage resulting in a more fine distribution of porosities and segregation spots in CC blooms. An optimum stirring forces of SEMS and FEMS are then determined to provide the best centre soundness for the high carbon steel tested. The most effective conditions at the final stage of solidification for the reduction of macrosegregation in CC blooms have also been suggested as a result of this study.
Ti-6Al-4V 합금의 초소성 특성에 미치는 α/β상분율의 영향
장영원,이종수,김지식 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.7
Present investigation has been made to study the superplastic deformation behavior of Ti-6Al-4V alloy with respect to the variation of α/β volume fraction. Load relaxation and superplastic deformation tests have been carried out at 600℃ and 800℃, to obtain stress-strain rate curves and tensile elongations for the microstructures of 3 and 11㎛ grain sizes. The experimental results have been analyzed by the theory of inelastic deformation which consists of two mechanisms, i.e., the grain matrix deformation and the phase/grain boundary sliding. Grain matrix deformation observed at 600℃ is consistent with an iso-stress model for the deformation of fine grain microstructures (3㎛) in which the β volume fractions are varied. However, the coarse grain microstructures (11㎛) reveal the deformation behavior of mixed type between. iso-stress mode and iso-strain rate mode. Phase boundary sliding occurring at 800℃ is the most effective in 48% β structure, resulting in the largest elongation, The sliding resistance in the different types of grain boundary is increased in the order of α/β≫α/α≒ β/β.
내부변형 변수를 이용한 β-CuZn 합금의 항복강도 이상 현상 연구
장영원,이종수,이기안 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.10
A new interpretation for the anomalous yield phenomena has been suggested from an internal variable approach. The high temperature deformation behavior of β-CuZn alloy(B2 structure) has been investigated through a series of load relaxation tests and analyzed by the inelastic deformation theory. The high temperature deformation behavior of β-CuZn alloy is well described by inelastic deformation theory, consisting of three deformation modes, internal, plastic and dislocation creep deformation modes. Among the above three modes, inelastic deformation has proved to be the most important one to affect the anomalous phenomena. It has also been proposed that the anomalous phenomena can only occur when the reference strain rate(ε˙_o) at higher temperature falls into the lower strain rate and lower stress region. The activation energy for dislocation creep is obtained as 159 kJ/mole, similar to that for the self diffusion of Cu or Zn. It is therefore thought here that the decrease of yield stress above T_p in tensile test is mainly due to dislocation climb process.