• Journal of the Korean institute of surface engineering
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• v.52 no.4
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• pp.203-210
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• 2019

In this work, anodizing behavior of 6xxx series aluminum alloys was studied under constant current density and constant voltage conditions in 20% sulfuric acid solution by V-t curves, I-t curves, thickness measurement, observations of surface appearance and cross-sectional observation of anodizing films. The film growth rate of the anodizing films on Al6063, Al6061 and Al6082 obtained at 20 V were $0.63{\mu}m/min$. $0.46{\mu}m/min$ and $0.38{\mu}m/min$, respectively. Time to the initiation of imperfections at the oxide/substrate interface under constant current condition was shortened and colors of anodizing films became darker with the amount of alloying elements in 6xxx series aluminum alloys. Based upon the experimental results obtained in this work, it is concluded that maximum anodizing film thickness without interfacial defects is reduced with increasing amount of alloying elements and brighter anodizing films can be obtained by decreasing amount of alloying elements in the aluminum alloys.

• Journal of the Korean institute of surface engineering
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• v.54 no.5
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• pp.285-293
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• 2021

High-entropy TiAlCrSiN nano-composite coating was designed to improve mold life for high temperature liquid molding. Alloy design, powder fabrication and single alloying target fabrication for the high-entropy nano-composite coating were carried out. Using the single alloying target, an arc ion plating method was applied to prepare a TiAlCrSiN nano-composite coating had a 30 nm TiAlCrSiN layers are deposited layer by layer, and form about 4 ㎛-thickness of multi-layered coating. TiAlCrSiN nano-composite coating had a high hardness of about 39.9 GPa and a low coefficient of friction of less than about 0.47 in a dry environment. In addition, there was no change in the structure of the coating after the dissolution loss test in the molten metal at a temperature of about 1100 degrees.

• Korean Journal of Materials Research
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• v.29 no.3
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• pp.189-195
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• 2019

The effect of C, Mn, and Al additions on the tensile and Charpy impact properties of austenitic high-manganese steels for cryogenic applications is investigated in terms of the deformation mechanism dependent on stacking fault energy and austenite stability. The addition of the alloying elements usually increases the stacking fault energy, which is calculated using a modified thermodynamic model. Although the yield strength of austenitic high-manganese steels is increased by the addition of the alloying elements, the tensile strength is significantly affected by the deformation mechanism associated with stacking fault energy because of grain size refinement caused by deformation twinning and mobile dislocations generated during deformation-induced martensite transformation. None of the austenitic high-manganese steels exhibit clear ductile-brittle transition behavior, but their absorbed energy gradually decreases with lowering test temperature, regardless of the alloying elements. However, the combined addition of Mn and Al to the austenitic high-manganese steels suppresses the decrease in absorbed energy with a decreasing temperature by enhancing austenite stability.

• Journal of Powder Materials
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• v.25 no.6
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• pp.507-513
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• 2018

This study investigates the viability of using a Na-ion battery with a tin(Sn) anode to mitigate the vulnerability caused by volume changes during discharge and charge cycling. In general, the volume changes of carbon material do not cause any instability during intercalation into its layer structure. Sn has a high theoretical capacity of $847mAh\;g^{-1}$. However, it expands dramatically in the discharge process by alloying Na-Sn, placing the electrode under massive internal stress, and particularly straining the binder over the elastic limit. The repeating strain results in loss of active material and its electric contact, as well as capacity decrease. This paper expands the scope of fabrication of Na-ion batteries with Sn by fabricating the binder as an auxetic structure with a unique feature: a negative Poisson ratio (NPR), which increases the resistance to internal stress in the Na-Sn alloying/de-alloying processes. Electrochemical tests and micrograph images of auxetic and common binders are used to compare dimensional and structural differences. Results show that the capacity of an auxetic-structured Sn electrode is much larger than that of a Sn electrode with a common-structured binder. Furthermore, using an auxetic structured Sn electrode, stability in discharge and charge cycling is obtained.

• Journal of Powder Materials
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• v.26 no.2
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• pp.101-106
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• 2019

Ni-based oxide dispersion strengthened (ODS) alloys have a higher usable temperature and better high-temperature mechanical properties than conventional superalloys. They are therefore being explored for applications in various fields such as those of aerospace and gas turbines. In general, ODS alloys are manufactured from alloy powders by mechanical alloying of element powders. However, our research team produces alloy powders in which the $Ni_5Y$ intermetallic phase is formed by an atomizing process. In this study, mechanical alloying was performed using a planetary mill to analyze the milling behavior of Ni-based oxide dispersions strengthened alloy powder in which the $Ni_5Y$ is the intermetallic phase. As the milling time increased, the $Ni_5Y$ intermetallic phase was refined. These results are confirmed by SEM and EPMA analysis on microstructure. In addition, it is confirmed that as the milling increased, the mechanical properties of Ni-based ODS alloy powder improve due to grain refinement by plastic deformation.

• Korean Journal of Materials Research
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• v.29 no.4
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• pp.252-257
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• 2019

Fabrication of soft magnetic composite powders for the $Fe_2O_3-Ca$ system by mechanical alloying(MA) has been investigated at room temperature. It is found that soft magnetic composite powders in which CaO is dispersed in ${\alpha}-Fe$ matrix are obtained by MA of $Fe_2O_3$ with Ca for 5 hours. Changes in magnetization and coercivity also reflect the details of the solidstate reduction process of hematite by pure metal of Ca during MA. The saturation magnetization of MA powders increases with increasing MA time and reaches a maximum value of 65 emu/g after 7 hours of MA. The average grain size of ${\alpha}-Fe$ in MA powders, estimated by diffraction line-width, gradually decreases with increasing MA time and reaches 52 nm after 5 hours of MA. It can also be seen that the coercivity of the 5-hour MA sample is fairly high at 190 Oe, suggesting that the grain refinement of already-produced ${\alpha}-Fe$ tends to clearly occur during MA.

• Journal of Powder Materials
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• v.30 no.6
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• pp.478-483
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• 2023

High-entropy alloys (HEAs) are characterized by having five or more main elements and forming simple solids without forming intermetallic compounds, owing to the high entropy effect. HEAs with these characteristics are being researched as structural materials for extreme environments. Conventional refractory alloys have excellent high-temperature strength and stability; however, problems occur when they are used extensively in a high-temperature environment, leading to reduced fatigue properties due to oxidation or a limited service life. In contrast, refractory entropy alloys, which provide refractory properties to entropy alloys, can address these issues and improve the high-temperature stability of the alloy through phase control when designed based on existing refractory alloy elements. Refractory high-entropy alloys require sufficient milling time while in the process of mechanical alloying because of the brittleness of the added elements. Consequently, the high-energy milling process must be optimized because of the possibility of contamination of the alloyed powder during prolonged milling. In this study, we investigated the high-temperature oxidation behavior of refractory high-entropy alloys while optimizing the milling time.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.23 no.6
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• pp.314-319
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• 2013

Fabrication of nanocomposite material for the $Fe_2O_3-Al$ system by mechanical alloying (MA) has been investigated at room temperature. It is found that ${\alpha}-Fe/Al_2O_3$ nanocomposite powders in which $Al_2O_3$ is dispersed in ${\alpha}-Fe$ matrix are obtained by mechanical alloying of $Fe_2O_3$ with Al for 5 hours. The change in magnetization and coercivity also reflects the details of the solid state reduction process of hematite by pure metal of Al during mechanical alloying. Densification of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies at $1000^{\circ}C$ and $1100^{\circ}C$ under 60 MPa. Shrinkage change after SPS of MA'ed sample for 5 hrs was significant above $700^{\circ}C$ and gradually increased with increasing temperature up to $1100^{\circ}C$. X-ray diffraction result shows that the average grain size of ${\alpha}-Fe$ in ${\alpha}-Fe/Al_2O_3$ nanocomposite sintered at $1100^{\circ}C$ is in the range of 180 nm. It can be also seen that the coercivity (Hc) of SPS sample sintered at $1000^{\circ}C$ is still high value of 88 Oe, suggesting that the grain growth of magnetic ${\alpha}-Fe$ phase during SPS process tend to be suppressed.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.1
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• pp.21-27
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• 2018

Synthesis of ferromagnetic composite materials for the $Cu-BaFe_{12}O_{19}$ system by mechanical alloying (MA) has been investigated at room temperature. A mixture of copper and barium ferrite with a weight ratio of $Cu:BaFe_{12}O_{19}=4:1$, 3 : 2, 2 : 3 and 1 : 4 was used. It is found that $Cu-BaFe_{12}O_{19}$ composite powders in which $BaFe_{12}O_{19}$ is dispersed in copper matrix are successfully produced by mechanical alloying of $BaFe_{12}O_{19}$ with Cu for 80 min. in all composition. The change in X-ray diffraction patterns and magnetic properties reflects the details for the formation of ferromagnetic metal matrix composite of pure Cu and $BaFe_{12}O_{19}$ during mechanical alloying. Magnetization of $Cu-BaFe_{12}O_{19}$ composite powders gradually increases with increasing the amounts of barium ferrite, whereas coercive force of MA powders gradually decreases due to the refinement of barium ferrite powders with ball milling. However, it can be seen that the coercivity of $Cu-BaFe_{12}O_{19}$ MA composite powders with a weight ratio of $Cu:BaFe_{12}O_{19}=4:1$ and 3 : 2 ball-milled for 80 min. is still high value of 1400 Oe and 1450 Oe, respectively suggesting that the refinement of barium ferrite powders during ball milling process tend to be suppressed due to the ductile copper.

• Journal of Electrochemical Science and Technology
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• v.1 no.1
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• pp.19-24
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• 2010

Alloying degree is an important structural factor of PtRu catalysts for direct methanol fuel cells (DMFC). In this work, carbon supported PtRu catalysts were synthesized by reduction method using anhydrous ethanol as a solvent and $NaBH_4$ as a reducing agent. Using anhydrous ethanol as a solvent resulted in high alloying degree and good dispersion. The morphological structure and crystallanity of synthesized catalysts were characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HR-TEM). CO stripping and methanol oxidation reaction were measured. Due to high alloying degree catalyst prepared in anhydrous ethanol, exhibited low onset potential for methanol oxidation and negative peak shift of CO oxidation than commercial sample. Consequently, samples, applying ethanol as a solvent, exhibited not only enhanced CO oxidation, but also increased methanol oxidation reaction (MOR) activity compared with commercial PtRu/C (40 wt%, E-tek) and 40 wt% PtRu/C prepared in water solution.