• Transactions of the Korean Society of Automotive Engineers
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• v.8 no.4
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• pp.141-148
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• 2000

As the steel plates used for automobile safety the TRIP-assisted multiphase steels are being introduced to automobile industry with respect to their remarkable mechanical properties for the combination of high strength and large elongation. This multiphase structure is generated by two stage heat treatment (intercritical annealing & isothermal treatment) The metastable retained austenite can be transformed to martensite when plastically deformed which results in TRIP effect. Actually the microstructure of TRIP-assisted steels consist of a fine dispersite. There present discussion deals with bainite reaction kinetics of austenite in the process o f two stage heat treatment. In relation to bainite transformation the characteristics of bainite reaction is found to be influenced by the bainite tempering temperature and also by the relative rate in which carbides precipitate within residual austenite.

• Journal of the Korean Society for Heat Treatment
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• v.7 no.4
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• pp.233-243
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• 1994

Constitutive relation of thermoelasto-plastic material undergoing phase transformation during quenching process were developed on the basic of continuum thermodynamics. The metallic structure, temperature and residual stresses distributions were numerically calculated by the finite element technique. The metallic structure were defined by transformation from austenite to pearlite and characterized as a fuction of thermal history and mixture rule of phase. On the distribution of thermal stress along the radial direction, axial and tangential stresses are compressive in the surface, and tential in the inner part. Radial stress is tensile in the whole body. The reversion of residual stress takes plase at 11.5~15.5mm from the center.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.332-335
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• 2005

A model is developed to quantitatively analyze the dilatometry curve of carbon steel for the evaluation of phase fraction during transformation. The effect of anisotropic volume change due to transformation mismatch plasticity as well as carbon enrichment in austenite during the transformation is considered in the developed model. The developed model is applied for the analysis of dilatometry curves of carbon steels. The result shows that considering the anisotropic dilatation is very essential to quantitatively evaluate the phase fraction from the dilatation curve.

• Journal of the Korean Society for Heat Treatment
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• v.8 no.1
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• pp.12-23
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• 1995

In this a set of constitutive equation relevant to the analysis of thermo-elasto-plastic materials with phase transformation during quenching process was presented on the basis of continuum thermo-dynamic. In calculating the transient thermal stresses, temperature between coolant and specimen(SM45C) surface was determined from the heat transfer coefficient. A calculation was made for specimen with 40mm in diameter quenched in coolant from $820^{\circ}C$ and the results are as follow. Stresses at starting point of transformation always show the maximum tensile value. Reverse of stresses takes place after completion of transformation of inner part at specimen.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.241.2-241.2
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• 2011

다공 소재는 큰 비표면적과 규칙적으로 정렬된 구조의 특성으로 인해 자성메모리 소자용 재료, 나노 와이어 제작용 템플릿, 마이크로 반응기, 메타물질용 소재 등으로 각광을 받고 있다. 자기조립 수직배열 다공구조 재료를 제작하는 방법으로 흔히 알루미늄의 양극산화 방법과 이원공정계의 상분리 방법이 등이 있다. 본 연구에서는 상변태를 비롯한 패턴형성과 계면 운동을 가장 정확하게 다루는 이론적 모델로 알려진 상장모델(Phase Field model)을 이용하여 이원공정계의 박막성장과정 동안의 자발적 상분리에 의한 수직배열 자기조립 다공구조 형성을 시뮬레이션 한다. 상장모델을 기초로 하여 상분리 메커니즘에 의해 발현된 미세조직을 해석하고 다양한 공정변수가 미세조직 발현에 미치는 영향에 대해 연구한다. 또한 상장모델을 통해 얻은 결과는 기존에 발표된 연구들의 결과와 비교를 통해 유효성을 입증한다.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05a
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• pp.784-789
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• 1995

핵융합로 제1벽재로서 주목받고 있는 저방사화 Fe-Cr-Mn-W계 오스테나이트 스테인리스강에 소량의 V을 첨가하고 그 기본 조성에 시그마상의 생성억제와 합금의 고온강도 향상에 효과가 있는 질소함량을 변화시켜 전보에서 보고한 소둔열처리의 영향에 이어 미세 조직 특성 및 부식 저항성에 미치는 시효열처리의 영향을 살펴보았다. 부식저항성 평가를 위해서는 양극분극시험, 침지시험 수소취성 시험을 행하였으며, 석출상의 생성양상을 살펴보기 위해서는 광학현미경 관찰, 자성측정, EPR시험 및 경도시험을 행하였다. 시험결과, 질소의 함량이 증가할수록 시그마상의 석출이 억제되는 것을 알 수 있었으며, 시효시 실험합금의 부식저항성은 주로 $600^{\circ}C$부근에서 오스테나이트상의 입계에 석출하는 크롬탄화물과 50$0^{\circ}C$부터 비교적 넓은 온도 구간에 걸쳐 페라이트상으로부터 석출 변태되는 시그마상에 의해 지배되고 있음을 알 수 있었다.

• Korean Journal of Materials Research
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• v.10 no.9
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• pp.642-647
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• 2000

Nanocrystalline NiAl alloy containing 36at.%Al was synthesized by mechanical alloying (MA). Synthesized powder was sintered by a pulse electric current sintering (PECS) facility. Effecting parameters on the phase transformation were discussed in terms of cooling rate and time spent on heat treatment. The behavior of phase transformation for sintered parts was examined by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) method. Microstructure was observed by scanning electron microscopy (SEM). Martensitic lattice parameter and volume fraction was calculated by direct comparison method in X-ray diffraction analysis.

• Korean Journal of Materials Research
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• v.11 no.8
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• pp.629-635
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• 2001

To improve the ductility of $A1_3Hf$ intermetallics, which are the potential high temperature structural materials, the mechanical alloying behavior. the effect of Cu addition on the $Ll_2$ phase formation and the behavior of vacuum hot-pressed consolidation were investigated. During the mechanical alloying by SPEX mill, the $Ll_2 A1_3Hf$ intermetallics with the grain size of 7~8nm was formed after 6 hours of milling in Al-25at.%Hf system. The $Ll_2$ Phase of Al_3Hf$ intermetallics with the addition of 12.5at.%Cu, similar to that of the binary Al-25at.% Hf, was formed, but the milling time necessary for the formationof the $Ll_2$ phase was delayed form 6 hours to 10 hours. The lattice parameter of ternary $Ll_2(Al+Cu)_3Hf$ intermetallics decreased with the increase of Cu content. The onset temperature of $Ll_2$ to $D0_{23}$ phase in $Al_3Hf$ intermetallics was around 38$0^{\circ}C$, the temperature upon completion varied from 48$0^{\circ}C$ to 5$50^{\circ}C$ as the annealing time. The onset temperature of $Ll_2$ to $D0_{23}$ phase transformation in $(Al+ Cu)_3Hf$ intermetallics increased with the amount of Cu and the highest onset temperature of $700^{\circ}C$ was achieved by the Cu addition of 10at.%. The relative density increased from 89% to 90% with the Cu addition of 10at.% in $Al_3Hf$ intermetallics hot-pressed in vacuum under 750MPa at 40$0^{\circ}C$ for 3 hours. The relative density of 92.5% was achieved without the phase transformation and the grain growth as the consolidation temperature increased from 40$0^{\circ}C$ to 50$0^{\circ}C$ in $(Al+Cu)_3Hf$ intermetallics hot-pressed in vacuum under 750MPa for 3 hours.

• Korean Journal of Materials Research
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• v.11 no.1
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• pp.8-14
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• 2001

Microstructural evolution and creep failure behavior of GTD 111 have been studied. Solidification and precipitation behaviors of the alloy during casting have been analyzed by microstructural observations. It has been found that MC carbides solidify just before the $\gamma$/$\gamma$' eutectic solidification. The ηphase was found to be formed by transformation of Ti-rich $\gamma$'phase. PFZ has formed in the vicinity of the transformed $\eta$ phase. A few MC particles, which have been identified as TaC, precipitated within the PFZ. Creep failure along grainboundary was dominant at and above $871^{\circ}C$. Creep failure above$ 871^{\circ}C$ was caused by the propagation of surface cracks and internal cracks. Creep crack has initiated at the microporosities embedded on the grainboundary. The $\eta$phase and PFZ have been found to be little or no effect on creep crack initiation.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.706-709
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• 2004

A numerical method was developed that imposes constraint condition on the order parameters in martensitic phase transformation. In the method, an amplitude function having values of 1 or 0 was multiplied to transformation rates. The merit of the method is that the imposition of the constraint condition is more straightforward than a method with Lagrangian multiplier and easy to implement in the tangent modulus method. The developed method is applied to three-dimensional finite element analyses of single and poly crystalline shape memory alloys.