• Title/Summary/Keyword: Metastable phase

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A Simulation for predicting the Refrigerant Flow Characteristics Including Metastable Region in Non-Adiabatic Capillary Tubes (증발 지연 구간을 포함한 비단열 모세관에서의 냉매 유동 특성 예측을 위한 시뮬레이션)

  • Son, Ki-Dong;Park, Sang-Goo;Jeong, Ji-Hwan;Kim, Lyun-Su
    • Proceedings of the SAREK Conference
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    • 2008.06a
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    • pp.263-270
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    • 2008
  • The capillary tube/suction line heat exchanger (SLHX) is widely used in small refrigeration systems. The refrigerant flowing in the SLHX experiences frictional and accelerational head losses, flashing, and heat transfer simultaneously. The simulation of refrigerant flow through SLHX is important since this will help engineers analyze and optimize the SLHX incorporated in a refrigeration system. The present SLHX model is based on conservation equations of mass, momentum and energy. Also a meta-stable model is included. All these equations are solved simultaneously. In this paper, HFC-134a refrigerant flow through a non-adiabatic capillary tube is simulated. The simulation results are discussed but not validated against experimental measurements yet.

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SYNTHESIS OF METASTABLE ALLOYS BY ION MIXING IN THE BINARY METAL SYSTEMS AND THEORETICAL MODELLING

  • Liu, B.X.;Zhang, Z.J.;Jin, O.;Pan, F.
    • Journal of the Korean Vacuum Society
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    • v.4 no.S2
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    • pp.148-155
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    • 1995
  • (1) The metastable crystalline(MX) phases formed by ion mixing are classified into 5 types, i.e. the super-saturated solid solutions and the enlarged HCP-I phases reported earlier, and the newly observed FCC-I phases in hcp-based alloys, The FCC-ll and HCP-ll phases in bcc-based alloys. The growth kinetics of the MX phases is discussed. (2) The interfacial free energy in the multilayered films was found to play an important role in ion beam mixing(IM) induced amorphization. By adding sufficient interfaces, amorphous alloys were obtained even in the systems with rather positive heat of formation. (3) Gibbs free energy diagrams of some representative systems were constructed, by calculating the free energy curves of all the competing phases. Steady-state thermal annealing was conducted and the results confirmed the relevance of the constructed diagrams, which were inturn employed to interpret the MX phase formation as well as the glass forming ability upon IM in the binary metal systems.

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Effect of Fe Content on Mechanical and Electrochemical Properties of Ti-Mo-Fe Alloys (Ti-Mo-Fe 합금의 Fe 함량에 따른 기계적 특성과 전기화학적 특성 비교·분석)

  • Ji-Won Kim;Jeong-Yeon Park;Min Gang;Ji-Hwan Park;Dong-Geun Lee
    • Journal of the Korean Society for Heat Treatment
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    • v.36 no.3
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    • pp.145-152
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    • 2023
  • β titanium alloys containing β stabilizing elements such as V, Nb, Ta, Mo and Fe are widely used etc, due to their excellent specific strength, corrosion resistance, fatigue strength and easy formability. New metastable β titanium alloys are developed containing low-cost elements (Mo and Fe) in this study. Fe element is a strong β-stabilizer which can affect the mechanical and electrochemical properties of Ti-5Mo-xFe (x = 1, 4 wt%) alloys. These properties were analyzed in connection with microstructure and phase distribution. Ti-5Mo-4Fe alloy showed higher compression yield stress and maximum stress than Ti-5Mo-1Fe alloy due to solid-solution hardening and grain refinement hardening effect. As Fe element increased, Fe oxide formation and reduction of ${\bar{Bo}}$ (bond order) value affect the decrease of corrosion resistance. Ti-5Mo-xFe alloys were more excellent than Ti-6Al-4V ELI alloy.

The Effect of Electrolyte Types on the Electrochemical Polishing Induced Martensitic Transformation of Metastable Austenite Stainless Steel (전해액 종류에 따른 준안정 오스테나이트계 스테인리스강의 전해연마 유기 마르텐사이트 상변태에 미치는 영향)

  • J. Chae;C. Jeong;H. J. Cho;H. Lee;S. J. Kim;H. N. Han
    • Transactions of Materials Processing
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    • v.32 no.4
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    • pp.191-198
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    • 2023
  • We examined the martensitic transformation kinetics for metastable stainless steel during electrochemical polishing (EP) using different types of electrolytes. Martensite fraction measured with EBSD showed that the electrolyte with high relative permittivity exhibited comparably higher levels of martensitic transformation. The amount of charge build-up on the specimen surface during EP with different types of electrolytes was calculated using COMSOL multiphysics simulations to understand these phase transformation characteristics. The effect of charge build-up-induced stress was analyzed using previously published first-principles calculations. We discovered that the electrolyte with high relative permittivity accumulated a greater amount of charge build-up, resulting in a stronger driving force for stress-induced martensitic transformation.

Powder Preparation by Hydroxide Coprecipitation and Phase Development of Pb0.97La0.02(Zr0.64Sn0.25Ti0.11)O3 Ceramics

  • Lee, Joon-Hyung;Chiang, Yet-Ming
    • The Korean Journal of Ceramics
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    • v.4 no.3
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    • pp.260-267
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    • 1998
  • A homogeneous and stoichimetric fine powder of the ferroelectric $Pb_[0.97}La_{0.02}(Zr_{0.64}Sn_{0.25}Ti_{0.11})O_3$ (PLZST) has been prepared by the hydroxide coprecipitation method. Studies on the crystallization behavior of precursor as a function of temperature by X-ray powder diffraction and transmission electron microscopy technique were consistent with the formation of the pyrochlore phase from amorphous, initially at low temperatures around 500~$550^{\circ}C$. Further heat treatment up to $750^{\circ}C$ resulted in development of the perovskite phase with no significant pyrochlore crystallite growth. At intermediate temperatures the precursor yields a fine mixture of pyrochlore and perovskite phases. When the pyrochlore phase was heat teated in air, slight weight increase was observed in the temperature range of 300~$700^{\circ}C$, which is thought to be caused from oxygen absorption. In argon atmosphere, weight increase was not observed. On the other hand, weight loss began to occur near $700^{\circ}C$, with giving off mostly CO2 gas. This implies that the pyrochlore phase seems to be crystallorgraphycally and thermodynamically metastable. An apparent activation energy of 53.9 ㎉/mol was estimated for the pyrochlore-perovskite phase transformation.

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Tunable Metal-Insulator Phase Transition in $VO_2$ Nanowires

  • Seong, Won-Kyung;Lee, Ji-Yeong;Moon, Myoung-Woon;Lee, Kwang-Ryeol
    • Proceedings of the Korean Vacuum Society Conference
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    • 2012.08a
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    • pp.385-385
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    • 2012
  • Understanding the thermodynamics and structural transformation during the Metal-Insulator Transition (MIT) is critical to better understand the underlying physical origin of phase transition in the vanadiumdioxide ($VO_2$). Here, through the temperature-dependent in-situ high resolutiontransmission electron microscopy (HR-TEM), and systematic electrical transport study, we have shown that the tunable MIT transition of $VO_2$ nanowires is strongly affected by interplay between strain and domain nucleation by ion beam irradiation. Surprsingly, we have also observed that the $VO_2$ rutile (R) metallic phase could form directly in a strain-induced metastable monoclinic (M2) phase. These insights open the door toward more systematic approaches to synthesis for $VO_2$ nanostructures in desired phase and to use for applications including ultrafast optical switching, smart window, metamaterial, resistance RAM and synapse devices.

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The Formation Behavior and Thermal Stability of $E_6$ Eutectic Phase in Unidirectionally Solidified Al-Fe Alloys (일방향 응고시킨 Al-Fe계 합금에서 $E_6$ 공정상의 생성특성과 열적안정성)

  • Jung, Hae-Ryong
    • Journal of Korea Foundry Society
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    • v.19 no.1
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    • pp.71-76
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    • 1999
  • Eutectic and off-eutectic Al-Fe alloys were unidirectionally solidified at the solidification rate of $1{\sim}50\;mm/min$ under the temperature gradients $75{\sim}80^{\circ}C/cm$. The investigation has been carried out for the microstructural variation, phase transition, mechanical properties by means of detailed analyses of stress-strain, micro-Vickers hardness and scanning electron micrography. The thermal stability at elevated temperature has been studied on $Al-Al_6Fe$ eutectic alloy held at $600^{\circ}C$ for $0{\sim}150$ hours. When the solidification rate was less than 10mm/min, the X-ray diffraction and EDS analysis showed the presence of $Al_3Fe$ compound. As the solidification rate more than 20 mm/min, $Al-Al_3Fe$ eutectic phase was transfered into $Al-Al_6Fe$ eutectic phase. The mechanical properties of unidirectionally solidified off-eutectic Al-Fe alloy is better than those of unidirectionally solidified eutecic Al-Fe alloy Maximum ultimate tensile strength was obtained in Al-2.25% Fe alloy which was unidirectionally solidified at the solidification rate of 20 mm/min. The metastable $Al-Al_6Fe$ phase was transferred into stable $Al-Al_3Fe$ phase at $600^{\circ}C$ held for 150 hours.

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A Study on Synthesis of Ni-Ti-B Alloy by Mechanical Alloying from Elemental Component Powder

  • Kim, Jung Geun;Park, Yong Ho
    • Journal of Powder Materials
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    • v.23 no.3
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    • pp.202-206
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    • 2016
  • A Ni-Ti-B alloy powder prepared by mechanical alloying (MA) of individual Ni, Ti, and B components is examined with the aim of elucidating the phase transitions and crystallization during heat treatment. Ti and B atoms penetrating into the Ni lattice result in a Ni (Ti, B) solid solution and an amorphous phase. Differential thermal analysis (DTA) reveals peaks related to the decomposition of the metastable Ni (Ti, B) solid solution and the separation of equilibrium $Ni_3Ti$, $TiB_2$, and ${\tau}-Ni_20Ti_3B_6$ phases. The exothermal effects in the DTA curves move to lower temperatures with increasing milling time. The formation of a $TiB_2$ phase by annealing indicates that the mechanochemical reaction of the Ni-Ti-B alloy does not comply with the alloy composition in the ternary phase diagram, and Ti-B bonds are found to be more preferable than Ni-B bonds.

A Study on the Microstructures of Rapidly Solidified Ti-($45{\sim}58at%$)Al Intermetallic Compound (급속응고된 TiAl 금속간화합물의 Al함량 변화에 따른 미세조직변화에 관한 연구)

  • Kim, Jae-Hoon;Jeoung, Tae-Ho;Nam, Tae-Woon
    • Journal of Korea Foundry Society
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    • v.18 no.6
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    • pp.550-554
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    • 1998
  • The microstructures of rapidly solidified binary Ti-Al alloys containing $45{\sim}58\;at%Al$ have been studied using C/S (carbon/sulfur), N/O (nitrogen/oxygen) analyser, X-ray fluorescence spectrometer (XRF), X-ray diffractometer (XRD), optical microscope (OM) and scanning electron microscope (SEM). The phases present in the alloys and their distribution were found to be a sensitive function of Al content. Essentially single-phase (${\gamma}$) microstructures were observed to alloys with 45 at%Al, 55 at%Al and 58 at%Al. In other content alloys, two phase (${\alpha}_2$, ${\gamma}$) microstructures were observed. The 48 at%Al, 52 at%Al alloys contain (${\gamma}+{\alpha}_2$) phase and ${\alpha}_2$ phase. These results indicate that rapid solidification affect the solidification path, then metastable phase forming during solidification.

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Bubble Formation in Liquid Helium under Negative Pressure by Quantum Tunneling near Absolute Zero Temperature (절대 0도 부근에서 양자터널링에 의한 헬리움(He)액체의 부압하에서의 기포형성)

  • Kwak, H.;Jung, J.;Hong, J.
    • Proceedings of the KSME Conference
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    • 2001.06d
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    • pp.354-359
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    • 2001
  • As the temperature of liquid under negative pressure approaches the absolute zero, the nucleation process due to thermal fluctuations hardly occurs. Instead of this mechanism, quantum fluctuations may lead the formation of nucleus for new phase in metastable state. In this study, the thermal as well as quantum nucleation bubble in liquid helium under negative pressure was investigated theoretically. The energy barrier against nucleation was estimated by molecular interaction due to the Londom dispersion force. It is shown that the phase transition from liquid to vapor in is possible due to the quantum tunneling below 0.2 K for Helium-4 and 0.1 K for Helium-3, at negative pressures close to the ideal tensile strength at which every liquid molecules become bubbles simultaneously.

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