• Journal of Powder Materials
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• v.27 no.2
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• pp.139-145
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• 2020

The powder manufacturing process using the gas atomizer process is easy for mass production, has a fine powder particle size, and has excellent mechanical properties compared to the existing casting process, so it can be applied to various industries such as automobiles, electronic devices, aviation, and 3D printers. In this study, a modified A4032-xSn (x = 0, 1, 3, 5, and 10 wt.%) alloy with low melting point properties is investigated. After maintaining an argon (Ar) gas atmosphere, the main crucible is tilted; containing molten metal at 1,000℃ by melting the master alloy at a high frequency, and Ar gas is sprayed at 10 bar gas pressure after the molten metal inflow to the tundish crucible, which is maintained at 800℃. The manufactured powder is measured using a particle size analyzer, and FESEM is used to observe the shape and surface of the alloy powder. DSC is performed to investigate the change in shape, according to the melting point and temperature change. The microstructure of added tin (Sn) was observed by heat treatment at 575℃ for 10 min. As the content of Sn increased, the volume fraction increased to 1.1, 3.1, 6.4, and 10.9%.

• Transactions of Materials Processing
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• v.29 no.3
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• pp.135-143
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• 2020

Low-cost alloying elements were added to a modified Al-6.5Si alloy and its microstructure, tensile and impact toughness properties were investigated. The alloying elements added were Mg, Zn, and Cu, and two kinds of alloy A (Mg:0.5, Zn:1, Cu:1.5 wt.%) and alloy B (Mg:2, Zn:1.5, Cu:2 wt.%) were prepared. In the as-cast Al-6.5Si alloys, Si phases were distributed at the dendrite interfaces, and Al₂Cu, Mg₂Si, Al₆ (Fe,Mn) and Al₅ (Fe,Mn)Si precipitates were also observed. The size and fraction of casting defects were measured to be higher for alloy A than for alloy B. The secondary dendrite arm spacing of alloy B was finer than that of alloy A. It was confirmed by the JMatPro S/W that the cooling rate of alloy B could be more rapid than alloy A. The alloy B had higher hardness and strength compared to the values of alloy A. However, the alloy A showed better impact toughness than alloy B. Based on the above results, the deformation mechanism of Al-6.5Si alloy and the improving method for mechanical properties were also discussed.

• Journal of Powder Materials
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• v.24 no.1
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• pp.41-45
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• 2017

The effects of the heat treatment temperature and of the atmosphere on the dehydrogenation and hydrogen reduction of ball-milled $TiH_2-WO_3$ powder mixtures are investigated for the synthesis of Ti-W powders with controlled microstructure. Homogeneously mixed powders with refined $TiH_2$ particles are successfully prepared by ball milling for 24h. X-ray diffraction (XRD) analyses show that the powder mixture heat-treated in Ar atmosphere is composed of Ti, $Ti_2O$, and W phases, regardless of the heat treatment temperature. However, XRD results for the powder mixture, heat-treated at $600^{\circ}C$ in a hydrogen atmosphere, show $TiH_2$ and TiH peaks as well as reaction phase peaks of Ti oxides and W, while the powder mixture heat-treated at $900^{\circ}C$ exhibits only XRD peaks attributed to Ti oxides and W. The formation behavior of the reaction phases that are dependent on the heat treatment temperature and on the atmosphere is explained by thermodynamic considerations for the dehydrogenation reaction of $TiH_2$, the hydrogen reduction of $WO_3$ and the partial oxidation of dehydrogenated Ti.

• Journal of Powder Materials
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• v.24 no.2
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• pp.115-121
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• 2017

In this work, p-type Bi-Sb-Te alloys powders are prepared using gas atomization, a mass production powder preparation method involving rapid solidification. To study the effect of the sintering temperature on the microstructure and thermoelectric properties, gas-atomized powders are consolidated at different temperatures (623, 703, and 743 K) using spark plasma sintering. The crystal structures of the gas-atomized powders and sintered bulks are identified using an X-ray diffraction technique. Texture analysis by electron backscatter diffraction reveals that the grains are randomly oriented in the entire matrix, and no preferred orientation in any unique direction is observed. The hardness values decrease with increasing sintering temperature owing to a decrease in grain size. The conductivity increases gradually with increasing sintering temperature, whereas the Seebeck coefficient decreases owing to increases in the carrier mobility with grain size. The lowest thermal conductivity is obtained for the bulk sintered at a low temperature (603 K), mainly because of its fine-grained microstructure. A peak ZT of 1.06 is achieved for the sample sintered at 703 K owing to its moderate electrical conductivity and sustainable thermal conductivity.

• Journal of the Korean Society for Heat Treatment
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• v.13 no.4
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• pp.246-252
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• 2000

Stellite 12 alloy-powder was overlaid on 410 stainless steel valve seat using plasma transferred arc(PTA) process. Variation of characteristic of microstructure and hardness of deposit with current(90~150 A) and preheat temperature(R.T.~$400^{\circ}C$) was investigated. Important conclusion obtained are as follows; All welding conditions used produced a sound deposit layer with no defect in single pass welding. The maximum deposit had 4.0~4.8 mm in thickness and its bead width was increased with increase of current and preheat temperature. The deposit showed hypoeutectic microstruture, which was consisting of primary cobalt dendrite and networked $M_7C_3$ type eutectic carbides. The amount of eutectic carbides was decreased and its dendritic secondary arm spacing was increased with increase of current. Hardness of the deposit was decreased with increase of current. Preheat temperature up to $400^{\circ}C$, however, showed little influence on the hardness and microstructure. The hardness was also influenced by diluted Fe content near the interface in addition to microstructure and dendritic secondary arm spacing. Hot hardness at $500^{\circ}C$ showed higher than 300 HV.

• Journal of the Korean Society for Heat Treatment
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• v.17 no.1
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• pp.17-22
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• 2004

In the present study, newly developed spark plasma sintering(SPS) technique was introduced to refine the grain size of ${\gamma}$-based TiAl intermetallic compounds. Ti-46Al-1.5Mo and Ti-46Al-1.5Mo-0.2C(at%) prealloyed powders were produced by mechanical milling(MM) in high-energy attritor. The mechanically milled powders were characterized by XRD and SEM for the microstructural evolution as a function of milling time. And then, the MMed powders were sintered by both spark plasma sintering and hot pressing in vacuum (HP). After the sintering process, MM-SPSed specimens were heat-treated in a vacuum furnace (SPS-VHT) and in the SPS equipment(MM-SPS) for microstructural control. It was found from microstrutural observation that the microstructure consisting of equiaxed ${\gamma}$-TiAl with a few hundred nanometer in average size and ${\alpha}_2-Ti_3Al$ particles were formed after both sintering processes. It was also revealed from hardness test and three-point bending test that the effect of grain refinement on the hardness and bending strength is much higher than that of carbon addition. The fully lamellar microstructures, which is less than $80{\mu}m$ in average grain size was obtained by SPS-VHT process, and the fully lamellar microstructure which is less than $100{\mu}m$ in average grain size was obtained by MM-SPS for a relatively shorter heat-treatment time.

• Journal of Korea Foundry Society
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• v.22 no.6
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• pp.281-287
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• 2002

In order to investigate the variation of microstructure and property of the Electro-slag Remelted M2 steel, microstructure observation, hardness, and bending test were performed by using optical microscope. SEM/EDS, rockwell hardness tester, charpy impact tester and bending tester, respectively. It was revealed that the number of inclusions and content of gas elements(S, O, N) in M2 steel fabricated by ESR process decreased markedly compared to those of AIM. It seems to be due to refining effect of ESR process. The volume fraction of carbides in quenched and tempered specimens after austenitizing at 1150$^{\circ}C$ and 1240$^{\circ}C$ was measured. The volume fraction of grain boundary carbides were found to be similar for both specimens. However, The volume fraction of carbides in grain decreased with an increase of austenitizing temperature. When specimen was austenitized at 1150$^{\circ}C$, grain boundary carbides showed needle like morphology. But, the carbides were broken with an increase of austenitizing temperature. The specimen austenitized at 1240$^{\circ}C$ showed higher hardness and lower bending strength compared to that of 1150$^{\circ}C$. As expected, toughness increased with sub-zero quenching treatment.

• Journal of Korea Foundry Society
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• v.13 no.4
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• pp.369-381
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• 1993

Aluminum alloy(AC8A) matrix composites reinforced with SiC particles(10% in vol.) were fabricated by Centrifugal Spray Deposition(CSD) process. The microstructures were investigated in order to evaluate both the mixing mode between aluminum matrix and SiC particles, and the effect of SiC particles on the cooling behaviours of droplets during flight and preforms deposited. A non-continuum mathematical calculation was performed to explain and to quantify the evolution of microstructures in the droplets and preforms deposited. Conclusions obtained are as follows; 1. The powders produced by CSD process showed, in general, ligament type, and more than 60% of the powders produced were about 300 to 850 um in size. 2. AC8A droplets solidified during flight showed fine dendritic structure, but AC8A droplets mixed with SiC particles showed fine equiaxed grain structure, and eutectic silicon were formed to crystallize granularly between fine aluminum grains. 3. SiC particles seem to act as a nucleation sites for pro-eutectic silicon during solidification of AC8A alloy. 4. The microstructure of composite powders formed by CSD process showed particle embedded type, and resulted in dispersed type microstructure in preforms deposited. 5. The pro-eutectic silicon crystallized granularly between fine aluminum grains seem to prohibit grains from growth during spray deposition process. 6. The interfacial reactions between aluminum matrix and SiC particles were not observed from the deposit performs and the solidified droplets. 7. The continuum model seem to be useful in connecting the processing parameters with the resultant microstructures. From these results, it was concluded that the fabrication of aluminum matrix composites reinforced homogeneously with SiC particles was possible.

• Korean Journal of Materials Research
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• v.30 no.1
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• pp.31-37
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• 2020

In this study, the effects of Sm addition (0, 0.05, 0.2, 0.5 wt%) on the microstructure, hardness, and electrical and thermal conductivity of Al-11Si-1.5Cu aluminum alloy were investigated. As a result of Sm addition, increment in the amount of α-Al and refinement of primary Si from 70 to 10 ㎛ were observed due to eutectic temperature depression. On the other hand, Sm was less effective at refining eutectic Si because of insufficient addition. The phase analysis results indicated that Sm-rich intermetallic phases such as Al-Fe-Mg-Si and Al-Si-Cu formed and led to decrements in the amount of primary Si and eutectic Si. These microstructure changes affected not only the hardness but also the electrical and thermal conductivity. When 0.5 wt% Sm was added to the alloy, hardness increased from 84.4 to 91.3 Hv, and electric conductivity increased from 15.14 to 16.97 MS/m. Thermal conductivity greatly increased from 133 to 157 W/m·K.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.4
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• pp.330-336
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• 2000

Effects of microstructure and indentation stress on fracture behavior of glass-infiltrated alumina composite for dental restorative application were investigated by the Hertzian and Vickers indentation method. Indentation stress-strain curve of glass-infiltrated alumina has showed the quasi-plastic behavior - deviation from linearity at high stress and the classical Hertzian cone crack, which could be confirmed the subsurface damage micrographs using bonded-interface specimen technique. The indentation stress-strain curves for the starting preforms are strongly dependent on porosity and microstructure of the preforms. On the other hand, the curves for the infiltrated composites are relatively insensitive to these factors. The failure of composite is originated at quasi-plastic deformation region. Damage and fracture behavior due to Hertzian stress field is theoretically examined, so that the indentation stress field plays a great role in material degradation. After Hertzian indentation annealing processing changes fracture behavior of alumina composite, so that stress field in material is healed through annealing.