발간논문

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Magnesium alloys, which possess excellent specific strength and castability, are highly susceptible to corrosion. Although anodizing is widely used to resolve this problem, it requires toxic electrolytes and produces relatively thin and weak surface coatings. Recently, plasma electrolytic oxidation (PEO) has emerged as an alternative to anodizing. Although it is derived from conventional anodizing, it uses eco-friendly electrolytes and forms thicker, denser, and harder coatings on the surface of magnesium alloys. However, PEO is a complex process involving physical, chemical, and electrochemical reactions, and it is influenced by various factors such as the alloy substrate composition, electrolyte/additive composition, and the electrical variables including the mode of power supply, applied voltage/current density, frequency, and duty cycle. In this article, the detailed effects of these parameters on the microstructure and properties of the PEO coatings are reviewed, and methods of improving the coatings are proposed. (Received September 1, 2016; Accepted November 22, 2016)

keyword : magnesium alloys, plasma electrolytic oxidation (PEO), coatings, corrosion, electrolytes, process variables

High Mn twinning-induced plasticity (TWIP) steels with compositions of Fe-(18.0-18.17) Mn-1.51Al-(0.48-0.52)Si-(0.61-0.62)C-(0.52-0.54)Cr-(0.22-0.23)Ni-0.03Nb-(0-0.04)Sb (wt%) are oxidized at 650 ℃ and 750 ℃ for < 24 h in air in order to determine the effect of the alloying elements on the oxidation. They are oxidized parabolically to MnO₂,Mn₂O₃,FeO,Fe₂O₃, and FeMn₂O₄ oxides with relatively fast rates. The formed scales are prone to spallation and bi-layered. The outer scales that are formed by the outward diffusion of cations are rich in Fe and Mn, while the inner scales that are formed by the inward diffusion of oxygen are rich in Fe, Mn, Al, Cr, and Si. The isothermal and cyclic oxidation resistance progressively increases with increments in the amount of Mn, Si, and Sb. †(Received September 26, 2016; Accepted November 18, 2016)

keyword : metals, casting, oxidation, scanning electron microscopy (SEM), high Mn TWIP steel

In this paper, Cu-coated graphite powders for a low thermal expansion coefficient and a high thermal conductivity are fabricated using a chemical reaction process. The Cu particles adhere to the irregular graphite powders and they homogeneously disperse in the graphite matrix. Cu-coated graphite powders are coarser at approximately 3~4 ㎛ than the initial graphite powders; furthermore, their XRD patterns exhibit a low intensity in the oxide peak with low Zn powder content. For the passivation powders, the transposition solvent content has low values, and the XRD pattern of the oxide peaks is almost non-existent, but the high transposition solvent content does not exhibit a difference to the non-passivation treated powders. †(Received August 1, 2016; Accepted November 13, 2016)

keyword : Cu coating, Cu-coated graphite, chemical reaction process, microstructures

In order to improve the flexibility of the oscillation pattern for a wide range of casting speeds, a hydraulic cylinder is used to oscillate the mold that moves up and down according to the stroke and frequency given by controller. Currently, hydraulic oscillators are commonly used in continuous casting facilities, but there are mismatched definitions in the control parameters. The primary object of the mold oscillation is to minimize the friction force between the mold copper plate and the solidified shell. The casting speed is tested up to 7.5 m/min, and the effect of the oscillation is analyzed in a pilot plant. As the casting speed increases, the lubrication effect reaches its limits. Therefore, various oscillation patterns are examined. The orientation of the oscillation pattern that achieves the lubricating function is determined. However, efforts to reduce the friction force are constrained by the moving energy of the oscillating system. Despite these constraints, the optimal oscillation pattern is determined for a wide range of casting speeds. Furthermore, the size of the negative strip time (NST) is no longer important in the hydraulic oscillation, and the definition should be modified in order to reflect this. Therefore, a new definition of the negative strip ratio (NSR) is introduced. (Received August 23, 2016: Accepted October 29, 2016)

keyword : continuous caster, mold, oscillation, hydraulic, asymmetry, NST, NSR

In this study, the effect of the crystalline structure on the magnetic properties of Fe-based amorphous powder is investigated. The Fe-B-P-C-Nb-Hf amorphous powder with excellent soft magnetic properties is prepared via gas-atomization, and the atomized amorphous powder is annealed in order to generate a small amount of nanosized crystallites in the amorphous matrix. For the powder annealed at 375 ℃, the coercivity is decreased by 96.15% from 3632.68 to 139.89 A/m, and the magnetization is increased 19.04% from 0.192 to 0.229 Wbm/g. For the powder annealed at 425 ℃, the coercivity is decreased 95.14% to 176.72 A/m and the magnetization is increased 0.71% to 0.193 Wbm/g. The simultaneous enhancement of the coercivity and magnetization can be attributed to the generation of a small amount of nanosized crystallites and the release of the residual stresses. †(Received September 26, 2016; Accepted November 3, 2016)

keyword : Fe-based amorphous powder, ball-milling, crystallization, coercivity, magnetization

Anode-supported yttria-doped barium cerate (BCY) electrolyte cells are fabricated and durability tests are conducted as a function of the current density (cell voltage sign), air flow rate, and BCY electrolyte thickness. As the air flow rate increases and the current density decreases (from negative to positive cell voltage), the cell voltage drop rate is reduced during the constant current test. The thick (18 μm) electrolyte cell exhibits relatively stable performance in comparison with the thin (10 μm) electrolyte cell. Electrolyte/electrode delamination is not observed on the cells, even after the negative voltage operation, and this is attributed to sufficient electronic conduction inside the BCY electrolyte. Thus, the present work indicates that the durability of BCY-based cells can be improved with optimization of the air utilization and electrolyte thickness. †(Received September 20, 2016; Accepted November 18, 2016)

keyword : solid oxide fuel cell, proton conductor, barium cerate, degradation mechanism

Feasibility studies are performed to determine the suitability of a novel simple synthesis technique for fabricating a new Ti alloy with improved strength and ductility, while exhibiting lower cell toxicity. Through consolidating pure Ti powders under a C atmosphere at elevated temperatures, a bulk form of the Ti alloy, in which a quantifiable amount of C is dissolved, is synthesized. While the alloy is free from toxic elements such as Al and V, the strength and ductility of the developed alloy are comparable to, or better than, those of its commercial Ti-6Al-4V alloy counterpart. In this study, the method to design the alloy, its synthesis, and the resultant properties are reported. (Received September 7, 2016; Accepted October 24, 2016)

keyword : diffusion, solid solution, powder metallurgy, titanium alloy, mechanical properties

The effects of the temperature and flow velocity on the cathodic protection and underwater electromagnetic signature of ships are investigated using electrochemical tests and boundary element simulations. The corrosion resistance of the materials for the hull and propeller increase at lower temperatures, and the corrosion resistance is significantly decreased under the flow conditions because the oxygen concentration on the surface accelerates the cathodic reaction. Due to the corrosion resistance reduction, the current requirement for cathodic protection increases under the flow conditions, while it decreases at lower temperatures. Furthermore, the underwater electromagnetic field is investigated at different axis directions. The electromagnetic field increases under the flow conditions and at higher temperatures due to the increase in the applied current for cathodic protection. (Received October 6, 2016; Accepted November 24, 2016)

keyword : cathodic protection, ocean, corrosion, electromagnetic signature, hull, propeller

The phase transformation behavior of the oxides formed in mechanically alloyed Fe-5Y₂O₃ powder is investigated. Non-stoichiometric Y-rich and Fe-rich oxides with sizes of less than 300 nm are observed in the mechanically alloyed powder. The diffusion and redistribution reactions of the elements in these oxides during heating of the powder above 800 ℃ were observed, and these reactions result in the formation of a Y₃Fe₅O₁₂ phase after heating at 1050 ℃. Thus, it is considered that the Y₂O₃ powder and some Fe powder are formed from the non-stoichiometric Y-rich and Fe-rich oxides after the mechanical alloying process, and a considerable energy accumulated during the mechanical alloying process leads to a phase transformation of the Y-rich and Fe-rich oxides to Y_αFe_βO_γ-type phase during heating. (Received September 1, 2016; Received October 19, 2016)

keyword : ODS(Oxide Dispersion Strengthened), mechanical alloying, nano-sized oxide particle, phase transformation, Y_αFe_βO_γ-type phase

The effects of the annealing temperature on the microstructure and tensile properties of cold-rolled light-weight steels are investigated using two Fe-30Mn-xAl-0.9C alloys that contain different Al content. The initial alloy microstructure is composed of a single austenite or a mixture of austenite and ferrite depending on the nominal aluminum content. For the alloy with 9 wt% Al content, the recrystallization and grain growth of austenite occurrs depending on the annealing temperature. However, for the alloy with 11 wt% Al content, the β-Mn phase is observed after annealing for 10 min at 550~800 ℃. The β-Mn transformation kinetics is the fastest at 700 ℃. The formation of the β-Mn phase has a detrimental effect on the ductility, and this leads to significant decreases in the total elongation. The same alloy also forms κ-carbide and DO₃ ordering at 550~900 ℃. The investigated alloys exhibit a fully recrystallized microstructure after annealing at 900 ℃ for 10 min, which results in a high total elongation of 25~55% with a high tensile strength of 900~1170 MPa. (Received October 21, 2016; Accepted November 21, 2016)

keyword : low-density steel, annealing, microstructure, beta-manganese, tensile properties, cold-rolling