• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 춘계학술대회 논문집
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• pp.143-146
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• 2001

The evolution of dynamic recrystallization (DRX) was studied with torsion test for AISI 304 stainless steel in the temperature range of $900-1000^{\circ}C$ and strain rate range of 0.05-5/sec. The evolution of DRX was investigated with microstructural analysis and change of flow stress curve slope. The investigation of serrated grain boundaries using electron back scattered diffraction (EBSD) analysis indicated that the nucleated new DRX grain size was similar to the size of bulging part. Before the steady state, the dynamically recrystallizing grains do not remain a constant size and gradually grow to the size of fully DRX grain at steady state. The calculation of grain size was based on $X_{DRX}$ and the assumption, which the nucleated DRX grains are growing to the steady state, continuously. It was found that the calculated results agreed with the microstructure of the alloy.

• 한국분말재료학회지
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• 제11권1호
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• pp.43-49
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• 2004

The electrical and thermal conductivity of W-Cu composites were investigated as a function of the W-particle size and W-W contiguity. Powder mixtures were prepared by ball milling or mechanical alloying process, and then sintered at various temperatures. The electrical conductivity of sintered composite was increased with decreasing W grain size. Dependence of electrical conductivity on the W grain size was explained by the W-W contiguity concept. The thermal conductivity was increased with increasing the temperature up to $600^{\circ}C$ but decreased at the temperature above $600^{\circ}C$ Also, thermal conductivity value was influenced by the W particle size. Change of thermal conductivity in W-Cu composites was discussed based on the observed microstructural characteristics and theoretical considerations.

• 한국분말재료학회지
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• 제11권4호
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• pp.301-307
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• 2004

Successful implementation of the powder forming process requires a detailed understanding of several interacting phenomena. The aim is to better control the process variables and to optimize the design parameters. A number of studies were carried out using various constitutive models that take the density change during powder forming into account. Most of them were developed for powders and sintered porous metals, but few of them can describe powder agglomerates, whose behaviour is different from that of uniformly arranged powders. The modification is needed to account for the effect of agglomeration on densification behaviour. Incorporating powder agglomeration into a constitutive model is of considerable importance, as it provides a possibility of relating the powder densification response to microstructural characteristics of powder particles, especially in case of nano powders. In this paper, we proposed a new powder agglomerate model in order to describe the unique densification behaviour of nano powders. The proposed model was applied to the densification of powder agglomerates during cold isostatic pressing.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.499-504
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• 2002

The microstructural change associated with the hardness profile in friction stir welded, age-hardenable 6005 Al alloy had been evaluated. Frictional heat and plastic flow during friction stir welding created the fine recrystallized grain (Stir Zone, SZ), the elongated and recovered grain (Thermo-Mechanical Affected Zone, TMAZ) in the weld zone. Heat affected zone (HAZ), which could be only identified by hardness test because there is no difference in the grain structure compared with that of the base metal, was formed beside the weld zone. A softened region had been formed near the weld zone during friction stir welding process. The softened region was characterized by the dissolution and coarsening of the strengthening precipitate during the friction stir welding. The sound joints of 6005 Al alloys were successfully formed under a wide range of the friction stir welding conditions.

• Tribology and Lubricants
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• 제12권3호
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• pp.33-38
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• 1996

High temperature wear behaviors of plasma-sprayed ZrO$_{2}$-$Y_{2}O_{3}$ composite coatings were investigated for high temperature wear resistance applications. The composite powders containing 20, 50, 80 vol% of alumina for plasma spray were made by spray drying method. Wear tests with composite coated specimens were performed at temperature ranges from room temperature to 800$^{\circ}$C. Wear tests were also carried out with heat treated specimens at room temperature. The microstructural change of coatings and the worn surface were examined by SEM and XRD. Sharp increase of wear loss at high temperature wear test was observed in specimens containing 50 and 80 vol% alumina. Similar trend was observed in the heat treated coatings. The measured residual stress was increased with increased alumina contents and heat treating temperatures. Residual stress induced during heat treatment appeared to be responsible to the observed harmful effect of alumina additions on the high temperature wear.

• 대한전자공학회논문지
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• 제26권10호
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• pp.1545-1553
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• 1989

The ohmic contact behavior in HEMT structure was compared with that in MESFET one throughout the specific contact resistance and microstructural change in both structures. A Au-Ge-Ni based metallization scheme was used and the alloying temperature of the ohmic materials was changed from 330\ulcorner to 550\ulcorner. The alloying temperature to obtain the minimum specific contact resistance in HEMT structure was 60k higher than that in MESFET. The volume fraction of NiAs (Ge) in MESFET structure increases with alloying temperature and/or the alloying time, which makes the decrease of specific contact resistance at the initial stage of ohmic metallization. In contrast, the volume fraction of NiAs(Ge) in HEMT structure was not dependent upon the specific contact resistance, which implies that the ohmic contacts are dominantly formed by the Ge diffusion to 2-DEG(two dimensional electron gas) layer.

• Journal of Welding and Joining
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• 제6권1호
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• pp.46-52
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• 1988

Retardation or delay in fatigue crack growth due to overloads are important for the accurate prediction of fatigue lives of structural materials. In this study, retardation of fatigue crack growth in Al 6061-T6 weldments and heat affected zones (HAZ) after single overload cycle had been investigated. Retardation in both weldments and HAZ was observed. It was concluded that retardation in both weldment and HAZ was greater than in base metal due to microstructural change and crack branching and crack closure were major governing factor in retardation.

• 한국세라믹학회지
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• 제36권9호
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• pp.991-996
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• 1999

Effect of solution property on the weight varation and microstructural change of film was studied by electrophoretic deposition in order to obtain a homogeneous and dense zirconia film. As a result of weight kinetics of film which obtained in alcohol or aqueous solution having different polarity experimental data showed large deviation from theoretical ones calculated by Zhang's kinetic model. It had been shown that the weight affecting factors was largely dependent on properties other than dielectric constant and viscosity of solvent zeta potential appiled field and time. In initial stage a main factor of the drastic weight increase was the capillary drag of porous substrate. The cause of weight decrease with time in aqueous solution after 300 s was attributed to the defect of film by sagging and electrolytic reaction. The electrolyte film which prepared in alcohol solution with good wetting for substrate had better homogeneous and dense microstructure than one in aqueous solution with high surface tension.

• 한국세라믹학회지
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• 제42권8호
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• pp.588-592
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• 2005

The effect of SiC additive on the densification behavior and microstructural change of $TiB_2$ ceramics was investigated. The infiltration and direct mixing with polycarbosilane (PCS), which was converted to SiC by pyrolysis during sintering, were used for the addition of SiC. The addition of Fe enhanced the densification of $TiB_2$, but which resulted in exaggerated grain growth. However, the addition of PCS enhanced the densification and suppressed the exaggerated grain growth of $TiB_2$. Moreover, the addition of PCS using direct mixing was more effective for suppress of grain growth as compare to the addition of PCS using infiltration.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.210-215
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• 2001

The ferritic 2.25CrMo steel has been used for high temperature structure applications such as turbine rotors, boilers and pressure vessels in fossil plant and petroleum chemical facilities. However, this steel is known to result in aging degradation due to temper embrittlement, carbide induced brittleness and softening of matrix after long time exposure to high temperature. This research investigated the microstructural and mechanical changes after artificial degradation treatment and evaluated the degree of degradation by several nondestructive methods. The decrease of electrical resistivity and increase of magnetic Barkhausen noise(RMS voltage) with increasing aging time were observed. The change of electrical resistivity and Barkhausen noise showed a good correlation with the ductile-brittle transition temperature.