• 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.

• 열처리공학회지
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• 제12권1호
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• pp.21-30
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• 1999

The present study were investigated changes of precipitation behaviour of laves phase in ferrite single phase and ferrite-martensite dual phase and precipitation of laves phase under stress. Hardness changes in ferrite phase appeared two hardness peaks by precipitation of initial fine precipitator and laves phase in 3Mo-0.3Si and 3Mo-0.3Si-C specimens, respectively. Hardness changes in martensite phase of 3Mo-0.3Si-C specimen was lower in the initial stage of aging by carbide precipitation and after this, increased by re-hardening due to precipitation of laves phase. In the ferrite phase, laves phase was mainly precipitated, whereas in the martensite phase, carbide was preferentially formed during the initial stage of aging and with increasing aging time, laves phase and carbide were simultaneously precipitated by precipitation of laves phase at around carbide. In the ferrite-martensite interface, laves phase was mainly precipitated and carbide was mainly formed at boundary of lath martensite than grain boundary. Adding the stress in aging, fine precipitator of inital precipitation of laves phase precipitated in (100) of perpendicular to tensile direction and has grown to only followed<010>direction and also, volume fraction of laves phase increased. Consequently, the stress added was accelerated initial precipitation of laves phase.

• 한국세라믹학회지
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• 제30권10호
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• pp.859-865
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• 1993

Morphotropic tetragonal and rhombohederal phase boundary(MPB) in PZT system was calculated and the effect of mechanical stress and electric field on the MPB was examined using phenomenological thermodynamic theory proposed by Devonshire. Mechanical stress and electric field was calculated to change free energies of phases and shift MPB. The stable composition range of the phase, of which spontaneous strain and spontaneous polarization aligned to the direction of tensile stress or electirc field, was expanded.

• 한국주조공학회지
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• 제29권6호
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• pp.270-276
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• 2009

After casting button-type small ingots of ternary Fe-Mn-S alloys which had three different Mn/S ratios (1, 5 and 70) in a vacuum arc furnace, the effect of the ratio on the sulfide formation was investigated. In case of the Mn/S ratio of 1, if alloy composition was located in an iron-rich corner on a Fe-Mn-S ternary phase diagram, only duplex MnS-FeS sulfide films were observed in the grain boundary. If the alloy composition was located in the miscibility gap area of the phase diagram, primary globular dendritic sulfides and dendritic sulfide slags were generated within the grain and tubular monotectic sulfides were also detected in the grain boundary. When the Mn/S ratio was 5, if the alloy composition was in the iron-rich corner, only bead-like sulfides were generated. On the other hand, if the composition was in the miscibility gap area, globular dendritic sulfides and dendritic sulfide slags were generated in the form of primary sulfide inclusions and rod-like eutectic sulfides were observed in the grain boundary. Especially, if the contents of Mn and S increased more in the miscibility gap area of the phase diagram, primary globular sulfides containing iron intrusions were observed. In case of Mn/S ratio of 70, if the contents of Mn and S was decreased in the Fe corner of the phase diagram, only bead-like sulfides were observed in the grain boundary. Despite the composition was outside the miscibility gap area of the phase diagram, if the contents of Mn and S increased, clusters of fine sulfide particles as well as fine spherical primary monophase sulfides were observed in the grain boundary.

• 한국재료학회지
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• 제5권6호
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• pp.699-705
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• 1995

질화규소 요업체에서 입계상의 변화가 파괴인성에 미치는 영향에 대해 살펴보았다 실험에는 Si$_3$N$_4$-Y$_2$O$_3$-SiO$_2$(YS)계와 Si$_3$N$_4$-Y$_2$O$_3$-A1$_2$O$_3$(YA) 계를 사용하였으며, 175$0^{\circ}C$에서 Can/HIP 처리한 후 1800~200$0^{\circ}C$ 온도구간에서 열처리시키면서 입계상의 변화에 따른 파괴인성의 변화를 조사하였다. 열처리 온도구간에서 입계상이 비정질상만으로 존재하였던 YA계의 경우는 열처리 온도가 증가되어 입성장됨에 따라 파괴인성 값이 증가되었으나, 190$0^{\circ}C$ 이상에서 열처리될 때 입계상이 결정상에서 비정질상으로 변화하였던 YS계의 경우는 오히려 파괴인성 값이 급격히 감소되었다. YS계에서 파괴인성의 급격한 저하는 열처리 온도 증가에 따라 입계상이 결정상과 비정질상의 공존 상태에서 비정질상만의 상태로 전이하며 파괴거동에 영향을 미쳤기 때문이라고 생각된다.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.466-467
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• 2006

The anatase particle was facetted at the free surface and a neck formation between the anatase particles prior to the phase transformation occured. This resulted in the severe lattice distortion at the region of the interface near the neck and this can act as the nucleation sites for the phase transformation. The grain growth of rutile particles after the phase transformation grew very fast by the sweeping phenomena of grain boundary. Therfore, It leaded to the microstructure without the rutile phase located in anatase particle.

• 한국해양공학회지
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• 제37권3호
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• pp.122-128
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• 2023

Autonomous berthing is a crucial technology for autonomous ships, requiring optimal trajectory planning to prevent collisions and minimize time and control efforts. This paper presents a two-phase, two-point boundary value problem (TPBVP) strategy for creating an optimal berthing trajectory for a twin-propeller, twin-rudder ship with autonomous berthing capabilities. The process is divided into two phases: the approach and the terminal. Tunnel thruster use is limited during the approach but fully employed during the terminal phase. This strategy permits concurrent optimization of the total trajectory duration, individual phase trajectories, and phase transition time. The efficacy of the proposed method is validated through two simulations. The first explores a scenario with phase transition, and the second generates a trajectory relying solely on the approach phase. The results affirm our algorithm's effectiveness in deciding transition necessity, identifying optimal transition timing, and optimizing the trajectory accordingly. The proposed two-phase TPBVP approach holds significant implications for advancements in autonomous ship navigation, enhancing safety and efficiency in berthing operations.

• 한국해안·해양공학회논문집
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• 제33권3호
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• pp.93-100
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• 2021

고정된 격자시스템에서 임의형상의 불투과 경계를 갖는 물체와 유체와 연성해석이 가능한 IB(Immersed Boundary)법이 개발 된 이후로 다양한 CFD 모델에서 IB법의 활용이 증가하고 있다. 기존의 IB법의 대부분은 구조물의 경계면에서 산정되는 유체력으로부터 수치적으로 경계조건을 만족시키는 directing-forcing법이나 구조물 내부에 가상셀을 위치시켜 보간을 통해 경계조건을 만족시키는 ghost-cell법들로 알고리즘이 복잡하다. 본 연구에서는 고정된 격자시스템에서 가동물체형 구조물 해석이 가능함과 더불어 3차원으로의 확장도 용이한 SIB(Simplified Immersed Boundary)법을 제안하였다. 본 연구에서 제안한 SIB법은 각 상(phase)의 밀도함수가 국소질량의 중심과 함께 이동하는 것으로 가정한 단일유체모델(one-field model for immiscible two-phase fluid)을 기초로 하였다. 또한 이동하는 고체상태의 구조물을 취급하기 위해 고체의 밀도함수를 이용한 체적가중평균법을 적용하고, 수치확산을 방지하기 위해 이류계산에는 CIP법을 적용하였다. 제안된 SIB법의 해석성능을 검토하기 위해 자유수면으로 낙하하는 물체에 대한 수치모의를 수행하였다. 수치해석결과는 자유수면으로 낙하하는 물체를 양호하게 재현하였다.

• Structural Engineering and Mechanics
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• 제25권2호
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• pp.181-200
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• 2007

A decoupling technique for simulating near-field wave motions in two-phase media is introduced in this paper. First, an equivalent but direct weighted residual method is presented in this paper to solve boundary value problems more explicitly. We applied the Green's theorem for integration by parts on the equivalent integral statement of the field governing equations and then introduced the Neumann conditions directly. Using this method and considering the precision requirement in wave motion simulation, a lumped-mass FEM for two-phase media with clear physical concepts and convenient implementation is derived. Then, considering the innate attenuation character of the wave in two-phase media, an attenuation parameter is introduced into Liao's Multi-Transmitting Formula (MTF) to simulate the attenuating outgoing wave in two-phase media. At last, two numerical experiments are presented and the numerical results are compared with the analytical ones demonstrating that the lumped-mass FEM and the generalized MTF introduced in this paper have good precision.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 추계학술대회 논문집
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• pp.76-79
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• 2009

The current understanding for phase/grain boundary sliding and low-temperature/high-strain rate superplasticity of two-phase titanium alloys is summarized. The quantitative analysis on boundary sliding revealed increased sliding resistance on the order of $\alpha/\beta\;\ll\;\alpha/\alpha\;\approx\;\beta/\beta$ boundary, hence, led to the conclusion that approximately 50% alpha(or beta) volume fraction and/or grain refinement is beneficial for obtaining large superplastic elongation at low temperature and/or high strain rate. To predict the temperature for 50% alpha volume in various alpha/beta Ti, artificial neural network was applied. Finally, much enhanced superplasticity was achieved through grain refinement utilizing dynamic globularization.