• Journal of the Korean Ceramic Society
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• v.34 no.11
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• pp.1129-1138
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• 1997

Al2O3/20 vol%YAG composite containing equiaxed grains and Al2O3/20 vol%LaAl11O18 composite containing elongated grains were fabricated using Al2O3-Y2O3 composition and Al2O3-La2O3 composition, respectively, by hot-pressing. In order to investigate the influence of microstructural control of second phase on toughening effect of toughened ceramic composites, AE (acoustic emission) measurements have been coupled with fracture toughness experiments(SENB and SEPB method). A separation of the fracture toughness and analysis of toughening mechanism was possible using the AE technique. The fracture toughness of hot-pressed materials was estimated to be 3.2 MPam0.5 for monolithic alumina, 4.7 MPam0.5 for Al2O3/20 vol%YAG composite and 6.2 MPam0.5 for Al2O3/20 vol%LaAl11O18 composite. In monolithic Al2O3, toughening does not occur as a result of either microcracking or grain bridging, whereas, composites exhibit toughening effects by both microcracking in the frontal zone and gain bridging in the wake zone, resulting in an improvement of fracture toughness as compared with monolithic Al2O3. The fracture toughness of Al2O3/20 vol%LaAl11O18 composite is higher than that of Al2O3/20 vol%YAG composite. It may be attributed to the elongated microstructure of Al2O3/20 vol%LaAl11O18 composite, resulting relatively greater bridging effect.

• Journal of Powder Materials
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• v.4 no.1
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• pp.18-25
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• 1997

High speed steels with commercial compositions of 10V, Rex20, Rex25, T15, and ASP30 were gas-atomized and then consolidated by hot isostatic pressing (HIPping). The microstructures of gas-atomized powder, as-HiPped billet, and heat-treated billet have been characterized using optical microscope, scanning electron microscope and X-ray diffractometer. In the gas-atomized powders, the solidification structures of 10V and Rex25 alloys show that primary MC carbides embedded within the fine equiaxed dendrites, whereas those of Rex20, T15 and ASP30 alloys exhibited eutectic MC and/or M$_2$C carbides in the interdendritic region. The trace and dendritic morphologies of gas-atomized powder have been retained in as-HiPped billets. The microstructures of as-HiPped billets have been observed to consist of ferrite, $M_6C$ and MC carbides in other alloys with the exception of 10V alloy, which consists of ferrite and MC carbides. The hardness of heat-treated billet makes a favorable comparison with that of as-HIPped billet. This seems mainly to be due to the strengthening by the precipitation of secondary carbides and the change of matrix phase from $\alpha$-ferrite to martensite.

• Korean Journal of Materials Research
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• v.10 no.8
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• pp.532-539
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• 2000

Co-0.5C-(15~20)Cr-20Ni-8W-(2~7)Fe alloy bond in diamond-impregnated abrasive tool was synthesized by ball-milling and mechanical alloying process. When the powders were mechanical alloyed for 6h, micro-welding in most metal powders was observed irrespective of addition of stearic acid. Without stearic acid in metal powders, partial-ly coarse powders were obtained, which could be unfaverable to the densification of composite of composite powders. The hot-pressed compacts showed rupture strength of 1100MPa and hardness of about $46H_{RC}$, respectively.

• Journal of Powder Materials
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• v.27 no.5
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• pp.373-380
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• 2020

The purpose of this study is to investigate the densification behavior and the corresponding microstructural evolution of tantalum and tantalum-tungsten alloy powders for explosively formed liners. The inherent inhomogeneous microstructures of tantalum manufactured by an ingot metallurgy might degrade the capability of the warhead. Therefore, to overcome such drawbacks, powder metallurgy was incorporated into the near-net shape process in this study. Spark plasma-sintered tantalum and its alloys with finer particle sizes exhibited higher densities and lower grain sizes. However, they were contaminated from the graphite mold during sintering. Higher compaction pressures in die and isostatic compaction techniques also enhanced the sinterability of the tantalum powders; however, a full densification could not be achieved. On the other hand, the powders exhibited full densification after being subjected to hot isostatic pressing over two times. Consequently, it was found that the hot isostatic-pressed tantalum might exhibit a lower grain size and a higher density as compared to those obtained in previous studies.

• Journal of the Korea Furniture Society
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• v.12 no.2
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• pp.29-38
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• 2001

This study was carried out to develope a new process of flattening bamboo pieces(3 months old) by two steps of utilizing microwave oven and hot press. Internode bamboo pieces were impregnated with low molecular weight phenol formaldehyde resin (PF) under vacuum of 76 cmHg, heated in a household microwave oven in 1 minute, pressed on the temperature of $145^{\circ}C$ by the hot press for 10 minute, and then cooled by the cold press in their flattened form. The physical and mechanical . Properties of compressed flattened bamboo were as follows: 1) PF1(Mw:427) and PF2(Mw:246) sol. met the success of flattening of internode bamboo pieces in both of P. bambusoides and P. nigra var. PF2 showed the more plasticity to flatten the bamboo than PFI. The PF2 sol. with low molecular weight(Mw:246) gave the more weight gain than that of PF1 in the equal concentration. PF1 of 5% (NVC) and PF2 of 10% (NVC) sol. gave the best result for physical and mechanical properties and from a economical view point. 2) The PFI of 5% (NVC) sol. with low molecular weight decreased the water absorption of 62-63% and increased the bending strength (MaR) of 80-90%, compression strength of 43-54%. 3) The PF2 of 10% (NVC) sol. with low molecular weight decreased the water absorption of 56-57% and increased the bending strength (MaR) of 64-86%, compression strength of 39-63%.

• Journal of the Korean institute of surface engineering
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• v.44 no.4
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• pp.125-130
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• 2011

$Cr_2AlC$ compounds were synthesized by hot pressing, and oxidized between 900 and $1200^{\circ}C$ in air for up to 50 hours. They oxidized to a thin $Al_2O_3$ layer containing a small amount of $Cr_2O_3$with the liberation of carbon as CO or $CO_2$ gases. The consumption of Al to form the $Al_2O_3$ layer led to the depletion of Al and enrichment of Cr just below the $Al_2O_3$ layer, resulting in the formation of an underlying $Cr_7C_3$ layer. As the oxidation temperature and time increased, the $Cr_7C_3$ oxide layer and the underlying $Cr_7C_3$ layer thickened. The oxidation resistance of $Cr_2AlC$ was generally good due to the formation of the $Al_2O_3$ barrier layer.

• Korean Journal of Materials Research
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• v.30 no.2
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• pp.57-60
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• 2020

Titanium aluminides have attracted special interest as light-weight/high-temperature materials for structural applications. The major problem limiting practical use of these compounds is their poor ductility and formability. The powder metallurgy processing route has been an attractive alternative for such materials. A mixture of Ti and Al elemental powders was fabricated to a mechanical alloying process. The processed powder was hot pressed in a vacuum, and a fully densified compact with ultra-fine grain structure consisting of Ti₃Al intermetallic compound was obtained. During the compressive deformation of the compact at 1173 K, typical dynamic recrystallization (DR), which introduces a certain extent of grain refinement, was observed. The compact had high density and consisted of an ultra-fine equiaxial grain structure. Average grain diameter was 1.5 ㎛. Typical TEM micrographs depicting the internal structure of the specimen deformed to 0.09 true strain are provided, in which it can be seen that many small recrystallized grains having no apparent dislocation structure are generated at grain boundaries where well-developed dislocations with high density are observed in the neighboring grains. The compact showed a large m-value such as 0.44 at 1173 K. Moreover, the grain structure remained equiaxed during deformation at this temperature. Therefore, the compressive deformation of the compact was presumed to progress by superplastic flow, primarily controlled by DR.

• Journal of Powder Materials
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• v.25 no.4
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• pp.327-330
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• 2018

This study is conducted as a preliminary research to verify the feasibility of Ti-based Oxide dispersion strengthened (ODS) alloy. Pure-Ti powder is mixed with $Y_2O_3$ powder and subsequently, mechanically alloyed at $-150^{\circ}C$. The Ti-based ODS powder is hot-isostatically pressed and subsequently hot-rolled for recrystallization. The microstructure consists of elongated grains and Y excess fine particles. The oxide particle size is larger than that of the typical Fe-based ODS steel. Tensile test shows that the tensile ductility is approximately 25%, while the strength is significantly higher than that of pure Ti. The high-temperature hardness of the Ti-ODS alloy is also significantly higher than that of pure Ti at all temperatures, while being lower than that of Ti-6Al-4V. The dimple structure is well developed, and no evidence of cleavage fracture surface is observed in the fracture surface of the tensile specimen.

• Journal of the Korean Ceramic Society
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• v.46 no.5
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• pp.447-451
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• 2009

Continuous SiC fiber-reinforced SiC composites ($SiC_f$/SiC) were fabricated by electrophoretic deposition (EPD). Nine types of slurries with different powder contents, binder resin amounts and slurry pH were deposited on Tyranno$^{TM}$-SA fabrics by EPD at 135 V for ten minutes to determine the optimal conditions. Further EPD using the optimum slurry conditions was performed on fabrics with four different pyrolitic carbon (PyC) thicknesses. The density of the hot-pressed composites decreased with increasing PyC thickness due to the difficulty of infiltrating the slurry into the narrow gaps between the fibers. On the other hand, the mechanical strength increased with increasing PyC thickness despite the decrease in density, which was explained by the enhanced crack deflection with increasing PyC thickness. The $SiC_f$/SiC composites showed the highest density and flexural strength of 94% and 342 MPa, respectively, showing EPD as a feasible method for dense $SiC_f$/SiC fabrication.

• Journal of Powder Materials
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• v.25 no.6
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• pp.501-506
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• 2018

With the recent remarkable improvements in the average speeds of contemporary trains, a necessity has arisen for the development of new friction modifiers to improve adhesion characteristics at the wheel-rail interface. The friction modifier must be designed to reduce slippage or sliding of the trains' wheels on the rails under conditions of rapid acceleration or braking without excessive rolling contact wear. In this study, a novel composite material consisting of metal, ceramic, and polymer is proposed as a friction modifier to improve adhesion between wheels and rails. A blend of Al-6Cu-0.5Mg metallic powder, $Al_2O_3$ ceramic powder, and Bakelite-based polymer in various weight-fractions is hot-pressed at $150^{\circ}C$ to form a bulk composite material. Variation in the adhesion coefficient is evaluated using a high-speed wheel-rail friction tester, with and without application of the composite friction modifier, under both dry and wet conditions. The effect of varying the weighting fractions of metal and ceramic friction powders is detailed in the paper.