• 대한기계학회논문집A
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• 제25권12호
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• pp.1933-1943
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• 2001

A micromechanical model is presented for superplasticity in which heterogeneous microstructures are coupled with deformation behavior. The effects of initial distributions of grain size, and their evolutions on the mechanical properties can be predicted by the model. Alternative stress rate models such as Jaumann rate and rotation incremental rate have been employed to analyze uniaxial loading and simple shear problems and the appropriate modeling was studied on the basis of hypoelasticity and elasto-viscoplasticity. The model has been implemented into finite element software so that full process simulation can be carried out. Tests have been conducted on Ti-6Al-4V alloy and the microstructural features such as grain size, distributions of grain size, and volume fraction of each phase were examined for the materials that were tested at different strain rates. The experimentally observed stress-strain behavior on a range of initial grain size distributions has been shown to be correctly predicted. In addition, the effect of volume fraction of the phases and concurrent grain growth were analyzed. The dependence of failure strain on strain rate has been explained in terms of the change in mechanism of grain growth that occurs with changing strain rate.

• 대한화학회지
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• 제23권4호
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• pp.217-236
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• 1979

소성변형에 대한 著者들의 이론을 초소성합금(Zn-Al eutectoid, A1-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics)에 적용하였다. 그 결과 초소성합금의 소성변형은 두 개의 grain boundary流動單位의 平行連結로 나타낼 수 있었다. 이 두 개의 流動單位는 流動式에 나타나는 parameter $X_{gj}/{\alpha}_{gj}$와 ${\beta}_{gj}$(j=1 혹온 2)로 表現할 수 있으며 이들을 實驗的으로 求할 수 있었다. 著者들의 流動式은 實驗과 잘 一致하였다. Strain rate sensitivity 對 -In(strain rate) 곡선을 이론으로 구한 결과 유동단위수만큼의 peak가 ${\beta}_{gj}$(j=1 or 2) 값에 따라 분리되어 나타났고 초소성의 조건도 ${beta}_{gj}$값에 의하여 결정됨을 알았다. ${\beta}_{gj}값의 粒子크기 依存性을 구하였고 온도변화에 따른 ${\beta}_{gj}$값 변화로부터 각 流動單位의 활성화엔탈피, ${\Delta}H_{gj}^{\neq}$도 구하였다. 그 결과 ${\Delta}H_{gj}^{\neq}$는 재료성분원소들의 grain boundary 자체확산에 의한 활성화엔탈피와 같이 나타났고 또 이들은 粒子 크기 증가에 마라 증가함을 보였다

• 소성∙가공
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• 제18권7호
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• pp.544-549
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• 2009

This study aimed to elucidate the deformation mechanism during low-temperature superplasticity of fine-grained Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy in the context of constitutive equation. For this purpose, initial coarse equiaxed microstructure was refined to $2.2{\mu}m$ via dynamic globularization. Globularized microstructure exhibited large superplastic elongations(434-826%) at temperatures of $650-750^{\circ}C$ and strain rate of $10^{-4}s^{-1}$. It was found that the main deformation mechanism of fine-grained material was grain boundary sliding accommodated by dislocation motion with both stress exponent (n) and grain size exponent (p) values of 2. When the alpha grain size, not sub-grain size, was considered to be an effective grain size, the apparent activation energy for low-temperature superplasticity of the present alloy(169kJ/mol) was closed to that of Ti-6Al-4V alloy(160kJ/mol).

• 소성∙가공
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• 제11권3호
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• pp.255-261
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• 2002

The grain size of 7010 Al alloy was refined to submicrometer level by using equal channel angular pressing (ECAP) and additional warm rolling. The mechanisms of grain refinement in ECAP process were fragmentation of coarse grain to ultra fine subgrains after a few passes and continuous recrystallization of the subgrains with the increase o( passes. Because of ultrafine grain size, essentially low temperature and high strain rate superplasticity was observed after ECAP process and warm rolling to form a sheet metal. The maximum elongation of 700% was obtained for an ECA pressed specimen after IS passes without warm rolling at $450^{\circ}C$ with strain rate of 5x$10^{-3}$/sec.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 춘계학술대회논문집
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• pp.129-130
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• 1999

The superplastic deformation characteristics is powder metallurgy(PM) Al-Li alloy has been studied within the framework of a recently proposed internal variable theory of superplasticity(SSP). The flow curves were obtained by performing a series of load relaxation tests at the temperature range from 45$0^{\circ}C$ to 52$0^{\circ}C$ It has been found that the overall flow curves were separated into the grain boundary sliding(GBS) and the accommodating dislocation glide processes/ The tensile curves were also obtained to clarify the superplastic deformation bahavior of PM Al-Li alloy. The microstructural features of PM AL-Li alloy have been examined through the transmission electron microscopy.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 추계학술대회논문집
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• pp.180-183
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• 2004

Superplastic formability depends on flow parameters such as temperature, strain rate, strain and stress, microstructures. Usually, superplastic properties of materials are characterized with using a uni-axial tension testing. However, superplastic sheet is formed under mutiaxial loading condition in most forming practices. In the present study, superplastic characteristics of A15083 alloys were determined with using both a uni-axial and biaxial bulging tests. Specially, cone-type die was used to achieve constant strain rate under constant pressure condition. Even though constant strain rate under a certain pressure was achieved only approximately, a cone-type bulging test was found to be quite beneficial to get a multiaxial formability of superplastic materials.

• 항공우주기술
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• 제2권2호
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• pp.133-141
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• 2003

초소성 공정을 이용하여 구형 압력용기를 제작하는 경우 두께와 구형의 형상은 일정하지 않고 성형공정이 진행함에 따라 변하게 된다. 본 연구에서는 공정과정 동안 발생하는 초기 블랭크의 두께변화 예측을 이론적으로 해석하였다. 두께변화 예측을 위한 이론적인 모델은 Kuglov 등이 적용한 박막이론을 사용하였으며 요구되는 최종 두께를 얻기 위한 초기 블랭크의 두께, 블랭크의 균일한 변형을 얻기 위한 시간에 따른 압력 프로파일, 각 성형단계에서 위치에 따른 두께분포 및 시간에 따른 두께분포를 결정하였다. 공정 단계 전반에 걸쳐서 적용 모델 및 해석 프로그램의 적합성을 문헌의 시험 결과와 비교하여 검증하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1145-1150
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• 2003

Zr-based bulk metallic glasses have a significant mechanical properties such as high strength and elastic strain limit, and a good processing ability due to the deformation behavior such as superplasticity under supercooled liquid region. Recently, many researches on the determination of optimum working condition in various bulk metallic glasses have been carried out. In this study, the deformation behavior and forming conditions of $Zr_{55}Cu_{30}Al_{10}Ni_{5}$ bulk metallic glass were investigated under three different strain rates and at various temperatures between 627K and 727K. The glass transition temperature, crystallization temperature and supercooled liquid region of $Zr_{55}Cu_{30}Al_{10}Ni_{5}$ bulk metallic glass are 680K, 762K and 82K, respectively.

• 소성∙가공
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• 제18권4호
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• pp.317-322
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• 2009

Superplastic deformation behavior of a Zn-0.3 wt.% Al was investigated. Grain sizes of $1{\mu}m$ and $10{\mu}m$ were obtained by a thermomechanical treatment. A series of load relaxation and tensile tests were conducted at various temperatures ranging from RT ($24^{\circ}C$) to $200^{\circ}C$. A large elongation of 1400% was obtained at room temperature in the specimens with the grain size of $1{\mu}m$. In the case of specimens with the grain size of $10{\mu}m$, relatively lower elongation at room temperature was obtained and, as the temperature increases above $100^{\circ}C$, a high elongation of about 400 % has been obtained at $200^{\circ}C$ under the strain rate of $2{\times}10^{-4}/s$. Dynamic materials model (DMM) has been employed to explain the contribution from GBS of Zn-Al alloy. Power dissipation efficiency for GBS was evaluated as above 0.4 and found to be very close to the unity as strain rate decreased and temperature increased, suggesting that GBS could be regarded as Newtonian viscous flow.

• 소성∙가공
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• 제11권5호
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• pp.447-456
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• 2002

A study has been made to investigate the boundary sliding and its accommodation mode with respect to the variation of $\alpha$$_2$/$eta$ volume fraction during superplastic deformation of two-phase Ti$_3$Al-xNb intermetallics. Step strain rate and load relaxation tests have been performed at 950, 970 and 99$0^{\circ}C$ to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that the grain matrix deformation and boundary sliding of the three intermetallics containing 21, 50 and 77% in $eta$ volume fractions are well described by the plastic deformation and viscous flow equations. Due to the equal accommodation of both $a^2$ and $\beta$ phases, the accommodation modes for fine-grained materials are in good agreement with the iso-strain rate models. The sliding resistance analyzed for the different boundaries is the lowest in the $\alpha$$_2$/$\alpha$$_2$ boundary, and increases in the order of $\alpha$$_2$/$\alpha$$_2$<< $\alpha$$_2$/$\beta$ = $\beta$/$\beta$, which plays an important role in controlling the superplasticity of the alloys with the various $\alpha$$_2$/$\beta$ phase ratio.