• 소성∙가공
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• 제10권4호
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• pp.347-354
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• 2001

Compression tests were carried out to investigate the hot-deformation behavior of Ti-6Al-4V alloy in the temperature range of $915^{\circ}C$ to $1015^{\circ}C$ and the strain rate range of $10^{-3}s^{-i}$ to $10s^{-1}$. Under the given test conditions, the hot-deformation of Ti-6Al-4V alloy was mainly led by dynamic recovery rather than by dynamic recrystallization. The activation energy for the plastic deformation in $\alpha+\beta$ field was about 894 kJ/mol and $\beta$ field was 332kJ/mo1. Processing map for hot working are developed on the basis of the variations of efficiency of power dissipation($\eta$=2m/m＋1) and flow instability criterion using the dynamic material model. The optimum process condition in the ($\alpha＋\beta$) field was obtained at the temperature ranges of $930^{\circ}C$ to $955^{\circ}C$$^{\circ}C$ and a strain rate of $10^{-3}s{-1}$.

• 소성∙가공
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• 제4권2호
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• pp.169-179
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• 1995

Static restoration mechanism during hot interrupted deformation of Cu-Zn alloy was studied in the temperature range from $550^{\circ}C$ to $750^{\circ}C$ and at a constant strain rate of 0.1/sec. At a given temperature, the hot interrupted deformations were performed with variation of interrupted time $t_i$ form 1 to 50 sec and of interrupted strain ${\varepsilon}_i$ from 0.15 to 0.90. From the analysis of the values of the critical strain of ${\varepsilon}_c$ for tje initiation of dynamic recrystallization and the peak strain of${\varepsilon}_p$, the relationship ${\varepsilon}_c{\fallingdotseq}0.7{\varepsilon}_p$ was obtained. It was clarified that the softening of the interrupted deformation was mainly the static recrystallization and the fractional softening(FS) which was over 30% mostly confirmed this result. The fractional softening of the interrupted time $t_i$ especially and pre-strain. The FS increased with increasing strain rate, interrupted time and pre-strain. The change of microstructures after hot deformation could be predicted by the FS. when the FS was 30~100%, static recrystallization was happened and grain growth was observed at the condition which was $750^{\circ}C$ deformation temperature, 0.45 prestrain and this condition's FS value was over 100%.

• 열처리공학회지
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• 제30권5호
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• pp.187-196
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• 2017

High temperature flow behaviors of AISI 4340 steel were investigated using isothermal compression tests under the temperature range from 850 to $1100^{\circ}C$ and a strain rate from 0.01 to $10s^{-1}$. The flow stress decreased with increasing compression temperature and decreasing strain rate. The dynamic softening related to the dynamic recrystallization was observed during hot deformation. The constitutive model based on Arrheniustyped equation with the Zener-Hollomon parameter was used to simulate the hot deformation behavior of AISI 4340 steel. The modification of the Zener-Hollomon parameter and lnA parameter resulted in the improvement of the calculation accuracy of the proposed constitutive model compared with the experimental flow curves. In addition, the process map of AISI 4340 steel was proposed. The instable process condition for hot deformation was predicted and its reliability was verified with the experimental observation.

• 한국재료학회지
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• 제29권6호
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• pp.343-348
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• 2019

Structural and mechanical effects of the dynamical precipitation in two copper-base alloys have been investigated over a wide range of deformation temperatures. Basing upon the information gained during the experiment, also some general conclusion may be formulated. A one concerns the nature of dynamic precipitation(DP). Under this term it is commonly understood decomposition of a supersaturated solid solution during plastic straining. The process may, however, proceed in two different ways. It may be a homogeneous one from the point of view of distribution and morphological aspect of particles or it may lead to substantial difference in shape, size and particles distribution. The effect is controlled by the mode of deformation. Hence it seems to be reasonable to distinguish DP during homogeneous deformation from that which takes place in heterogeneously deformed alloy. In the first case the process can be analyzed solely in terms of particle-dislocation-particle interrelation. Much more complex problem we are facing in heterogeneously deforming alloy. Deformation bands and specific arrangement of dislocations in form of pile-ups at grain boundaries generate additional driving force and additional nucleation sites for precipitation. Along with heterogeneous precipitation, there is a homogeneous precipitation in areas between bands of coarse slip which also deform but at much smaller rate. This form of decomposition is responsible for a specially high hardening rate during high temperature straining and for thermally stable product of the decomposition of alloy.

• 대한기계학회논문집A
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• 제24권12호
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• pp.3033-3039
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• 2000

Mechanical properties of the materials used for transportations and industrial machinery under high stain rate loading conditions have been required to provide appropriate safety assessment to these mechanical structures. The Split Hopkinson Pressure Bar(SHPB) technique with a special experimental apparatus can be used to obtain the material properties under high strain rate loading condition. There have been many studies on the material behavior under high strain rate compressive loading compared to those under tensile loading. In this paper, mechanical properties of the aluminum alloy, Al6061-T6, under high strain rate tensile loading were determined using SHPB technique.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2008년도 추계 학술발표회
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• pp.1181-1192
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• 2008

A laboratory investigation was carried out into effects of strain rate on undrained shear behavior of Holocene clay underneath Kobe Airport with an objective to evaluate the factor of safety of the retaining structure built on it. It was examined in a series of triaxial compression and extension tests performed using different rate of axial straining. A comparative compression test in which the strain rate was changed in steps was also carried out. Similar tests were performed in constant-volume direct shear box (DSB) test. And, the deformation characteristics of the clay were also examined in order to evaluate the variation of stiffness during undrained shearing. It was found that the undrained strength increased with not only the shear rate but also the consolidation period. ISOTACH properties seemed a key to govern the undrained shear behavior.

• 한국항공우주학회지
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• 제32권7호
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• pp.60-68
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• 2004

고속으로 비행하는 물체가 다른 물체와 충돌하는 경우에는 극히 짧은 시간에 커다란 변형이 일어나게 된다. 고변형률 변형 (high-strain-rate deformation) 에서는 소성변형이 일어나면서 상당한 열을 발생시키고 재료의 온도를 상승시킨다. 온도의 상승은 재료의 동적인 물성에 많은 영향을 미치게 되므로, 변형 시의 온도상승을 예측하는 것은 매우 중요하다. 변형시의 온도상승은 주로 전위(dislocation)의 움직임과 공공(vacancy)으로 인한 재료내의 저장에너지와 밀접한 관계를 갖게되므로, 저장 에너지의 양을 파악하는 것은 매우 중요하다. 고변형률 변형시 전위가 빠르게 움직이면서 평형상태에서의 경우보다 많은 파공공 (excess vacancies) 을 발생시키게 된다. 본 논문에서는 과공공을 포함하는 미시적 재료 모텔을 구성하고 분자동역학 (molecular dynamics, MD) 기법을 사용하여 면십입방격자 (fcc) 구조를 가지는 재료 (구리)에 대한 저장 에너지를 계산하였다.

• Journal of Mechanical Science and Technology
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• 제20권1호
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• pp.13-18
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• 2006

Ti-8Ta-3Nb, as a new biomaterial, was prepared by cast and swaging process. Their deformation behavior of Ti-8Ta-3Nb alloy has been characterized on the basis of its flow stress variation obtained from the true strain rate compression testing in the temperature of $700-900^{\circ}C$ and strain rate of $0.001-10\;s^{-1}$. At the strain rates lower than $0.1\;s^{-1}$ and the all temperature ranges which consist of two phase ${\alpha}+{\beta}$ as well as single ${\beta}$ phase fields, the flow curves show a small degree of flow softening behavior. In contrast, the shapes of the flow curves at other strain rates indicate unstable behavior. The shapes of the flow curves were similar in both as-cast and swaged specimen as well as in both ${\alpha}+{\beta}$ phase and ${\beta}$ phase. The flow stress data did not obey the kinetic rate equation over the entire regime of testing but a good fit has been obtained in the intermediate range of temperatures ($750-850^{\circ}C$). In this range, a stress exponent value of about 7.7 in as-cast specimens and about 6.2 in swaged specimens with an apparent activation energy of about 300 kJ/mol and about 206 kJ/mol respectively have been evaluated.

• 한국정밀공학회지
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• 제22권11호
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• pp.165-172
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• 2005

In MEMS (Micro-Electro-Mechanical System), packaging induced stress or stress induced structure deformation becomes increasing concerns since it directly affects the performance of the device. In the decoupled vibratory MEMS gyroscope, the main factor that determines the yield rate is the frequency difference between the sensing and driving modes. The gyroscope, packaged using the anodic bonding at the wafer level and EMC (epoxy molding compound) molding, has a deformation of MEMS structure caused by thermal expansion mismatch. This effect results in large distribution in the frequency difference, and thereby a lower yield rate. To improve the yield rate we propose a packaged SiOG (Silicon On Glass) process technology. It uses a silicon wafer and two glass wafers to minimize the wafer warpage. Thus the warpage of the wafer is greatly reduced and the frequency difference is more uniformly distributed. In addition. in order to increase robustness of the structure against deformation caused by EMC molding, a 'crab-leg' type spring is replaced with a semi-folded spring. The results show that the frequency shift is greatly reduced after applying the semi-folded spring. Therefore we can achieve a more robust vibratory MEMS gyroscope with a higher yield rate.

• 소성∙가공
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• 제9권1호
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• pp.72-79
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• 2000

Dynamic recrystallization (DRX), which may occur during hot deformation, is important for the microsturctural evolution of 304 stainless steel. Especially, the current interest in modelling hot rolling demands quantitative relationships among the thermomechanical process variables, such as strain, temperature, strain rate, and etc. Thus, this paper individually presents the relationships for flow stress and volume fraction of DRX as a function of processing variables using torsion tests. The hot torsion tests of 304 stainless steel were performed at the temperature range of 900~110$0^{\circ}C$ and the strain rate range of 5x10-2~5s-1 to study the high temperature softening behavior. For the exact prediction of flow stress, the equation was divided into two regions, the work hardening (WH) and dynamic recovery (DRV) region and the DRX region. Especially, The flow stress of DRX region could be expressed by using the volume fraction of DRX (XDRX). Since XDRX was consisted of the critical strain($\varepsilon$c) for initiation of dynamic recrystallization (DRX) and the strain for maximum softening rate ($\varepsilon$*), that were related with the evolution of microstructure. The calculated results predicted the flow stress and the microstructure of the alloy at any deformation conditions well.