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Ni-Cr-Mo-V steel alloys for high-speed railway brake discs were prepared to investigate the effect of alloying element contents on the microstructure and mechanical properties. The Cr, Mo, Mn alloying elements were incorporated into the steel alloys, which contained low carbon content in the range 0.16 wt.% ~ 0.21 wt.% to provide sufficient hardenability. The steel alloys were austenitized at 940℃ for 1 hour and quenched, and tempered at 610℃. Microstructural study showed a tempered martensitic microstructure with different sized austenite grains and packets. C-Mo alloy with high Mo content and the smallest prior austenite grain size showed the highest hardness and tensile strength. But, the alloy exhibited lower impact toughness than low Mo content alloys. The lowest tensile strength of the low Mo content Mn-Cr alloy, at room temperature and elevated temperature of 600℃, was 1053.4 MPa and 667.2 MPa, respectively. The grain refinement in the C-Mo alloy was considered to be due to the solute drag effect of the Mo element. The absorbed impact energy increased with tempering temperatures, but the impact energy of the three alloys had lower values than the generally guaranteed impact energy of the currently used disk. The low impact toughness of the Mo containing alloys was attributed to the higher Si content and higher tempered hardness of the alloys. A higher thermal conductivity and lower thermal expansion coefficient were obtained in the high Mo content C-Mo alloy, which had a higher Ac₃ transformation temperature. (Received 30 September, 2021; Accepted 10 January, 2022)

keyword : Ni-Cr-Mo-V steel alloy, heat treatment, microstructures, tensile strength, thermal analysis

The surface characteristics of a dental implant doped with Si, Mg, Ca, and P ions via plasma electrolytic oxidation (PEO) were investigated using various experimental instruments. Plasma electrolytic oxidation of the alloy was performed in a solution containing Ca, P, Si, and Mg ions at 280 V for 3 min. The potentiodynamic polarization test of the porous surface was conducted in a 0.9% NaCl solution. The Si and Mg ion-doped HA films exhibited small and large micro-pores with uniform distributions after the PEO treatment. The micro-pores formed on the 5~20Mg/5Si coated samples were smaller than those on the Si and Mg ion-free CaP coated surfaces, as the Mg ion concentration increased. The XRD peaks of Si, Mg, Ca, and P appeared and the Si, Mg, Ca, and P were well distributed on the PEO-treated surface, especially on the top, valley and crest of the screw. The Ca/P ratio decreased as Mg content increased in the electrolyte. The diffraction peak of anatase TiO₂ was observed on the Si and Mg ion HA film formed on the Ti-6Al-4V alloy. The adhesion force of the Si and Mg doped HA films increased with increasing Mg and Si ion concentrations. A passive region due to passive film formation was observed. The HA film surface with Si and Mg ions showed lower E_corr and higher I_corr compared to the HA film surface without Si and Mg ions. And the I_pp and I₃₀₀ for 5~20Mg/5Si formed on the Ti-6Al-4V alloy were lower than those of the surface coated with HA without Si and Mg ions. (Received 11 August 2021; Accepted 27 December 2021)

keyword : plasma electrolytic oxidation, dental implant, Si and Mg doped hydroxyapatite, potentiodynamic polarization test, adhesion force

In this study, the liquation cracking susceptibility in the heat-affected zone of CM247LC superalloy gas turbine blades during repair welding was quantitatively evaluated using a newly developed pre-weld Varestraint test method. The repair welding geometry was replicated through Varestraint tests for the preweld bead. The liquation cracking susceptibility, that is, the liquation cracking temperature range (LCTR), could be evaluated through temperature visualization at the time of crack formation during the Varestraint test. The LCTR of CM247LC alloy repair welds (heat-affected zone of the second layer weld) was 280 K. Compared to the LCTR of as-cast (620 K) and aged (65 K) CM247LC, metallurgical mechanisms for controlling the LCTR of repair welds were examined based on the microstructural characterization and Scheil’s solidification calculations for the pre-weld. The LCTR of the CM247LC alloy repair weld was influenced by the MC carbide fraction and the segregated concentration of trace and impurity elements, such as B and S in pre-weld solidification path. A process design capable of reducing the fraction of MC carbides and solidification segregation of trace impurity elements is required. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to analyze the solidification cracking behavior effectively in manufacturing high-soundness CM247LC superalloy welds. (Received September 28 2021; Accepted November 8, 2021)

keyword : CM247LC superalloy, repair welding, heat-affected zone, liquation cracking susceptibility, modifiedvarestraint test

The Ti-6Al-4V alloy has been widely used for structural materials due to high specific strength property, however it is difficult to cast because of high reactivity at high temperature. In this study casting characteristics and post-casting heat treatment were studied in the cast Ti alloy. The cast alloy was prepared using high-frequency induction heating and horizontal vacuum centrifugal casting to reduce reaction with crucible and mold by minimizing overheating of the molten metal and casting fast. In the thin casting with a thickness of 2 mm, α’ martensite was observed, and in the casting with a thickness of 4~8 mm, α+β Widmanstatten was observed. As the thickness of the casting increased, the grain size, α-lath width, and α-case thickness increased. Accordingly, as the thickness of the casting increased, the hardness of the matrix decreased and the hardness of the surface increased. In addition, when the casting was heat treated below the β-phase transformation temperature, the α-lath width increased, and when quenched after heat treatment in the β-phase transformation temperature region, an α’ martensite was formed. On the other hand, when the casting was heat treated in the atmosphere, it was confirmed that the elongation was very low because hydrogen and oxygen in the atmosphere were induced. In addition it was confirmed that casting defects can be removed through HIP. (Received 8 September, 2021; Accepted 20 December, 2021)

keyword : Ti-6Al-4V alloy, Vacuum Centrifugal Casting, α-case, Heat treatment, HIP(hot isostatic pressing)

The recovery of valuable metals such as cobalt, nickel, and lithium from spent lithium-ion batteries (LIBs) has attracted much attention. In this study, a hydrometallurgical process for the recovery of high-purity lithium compounds like Li₂CO₃ and Li₃PO₄ from dust containing Li, Co, Ni, Cu, Fe, Mn, and Si produced during the smelting reduction of spent LIBs was investigated. The separation of Li over other metals from the dust was achieved by two methods. The first method was to leach metals from the dust with distilled water and then to precipitate the dissolved metal ions from the leaching solution at pH 10. The second one was to mix the dust with solid NaOH, following the dissolution of the mixture with distilled water. Optimum conditions for the second method were a 1:0.17 weight ratio of dust to NaOH, 50 g/L pulp density for 120 min at 22℃. These two methods resulted in a solution containing only Li(I) and Na(I). The precipitation of Li(I) to Li₂CO₃ from the leaching solution by Na₂CO₃ with the addition of acetone or ethanol was optimized a 1:0.5 molar ratio of Li(I) to Na₂CO₃, 6:5 volume ratio of solvent to Li(I) solution for 30 min at 22℃. Under the optimum conditions, the precipitation percentage of Li(I) was higher than 92.0% with above 99.0% purity. Additionally, using Na₃PO₄, 97.1 % Li(I) was precipitated from the leaching solution to Li₃PO₄ with 93.1% purity. An optimized process for the recovery of lithium compounds from the dust is proposed. (Received 25 October, 2021; Accepted 22 December, 2021)

keyword : lithium-ion batteries, lithium, dust, leaching, solvent precipitation

Currently, the need for high performance heaters for industrial application is increasing, and repeated wear, rapid temperature change, and uneven temperature can cause deteriorating quality and reduced productivity. The ceramic heater in this study consists of a substrate, an electrode layer, and a protective layer, and low-melting temperature glass frit was used for all layers to allow low-temperature processing. As a substrate for the ceramic heater, Si₃N₄, which is twice as strong as AlN and has excellent wear resistance, was used. As the electrode material, Ag-Pd, which provides good adhesion to the Si₃N₄ substrate and high electrical conductivity was used, and MoO₃ powder was mixed 8 : 2 to make a paste. The MoO₃ adjusts the resistance of the Ag-Pd electrode, thereby supplementing the heater performance. Si3N4 powder and glass frit were used for the protective film layer that protects the electrode, and MoO₃ was added to unify the materials between the layers. (Received 26 October, 2021; Accepted 6 January, 2022)

keyword : ceramic heater, Si_{3N4, protecting layer, electrode, MoO3}

Vacuum-processed oxide semiconductors have enabled incredible recent advances in the scientific research of metal oxide thin-film transistors (TFTs) and their introduction in commercial displays. Developing metal oxide transistors with low processing temperatures, on the other hand, remains a challenge. Metal oxide transistors are commonly produced at high processing temperatures (over 500°C) and have a high working voltage (30~50 V). Here, we introduce amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs that show enhanced electrical characteristics, environmental stability, and switching behavior, prepared using ultrasonicated preannealing. Before post-annealing, the ultrasonication treatment was given at 40 kHz for 20 minutes. The improved electrical characteristics of this ultrasonicated a-IGZO TFTs were: 10.78 ㎠/Vs; 1.2×10⁷on/off current ratio. The a-IGZO TFTs with ultrasonicated pre-annealing were also extremely stable under a variety of stresses. For an ultrasonicated a-IGZO TFT, the threshold voltage (V_th) shifted by +0.82 V in a positive bias stress test and -0.30 V in a negative bias stress test. This means that the sonication treatment improves both electrical and surface morphological qualities, while also lowering faults by eliminating contaminants from the a-IGZO channel layer's surface and preventing atomic rearrangement. Furthermore, the dynamic response characteristics were measured according to frequency. A dynamic inverter test was carried out at 1 kHz frequency, with the load resistance of the circuit set to 10 MW and the drain supply voltage set to 5 V (V_DD). (Received 12 November, 2021; Accepted 31 December, 2021)

keyword : oxide transistor, amorphous IGZO, ultrasonication treatment, pre-annealing, thin films transistor

In this study, we investigated the phase formation behavior of the FeSe₂-FeTe₂ system and the evolution of their thermoelectric transport properties. A series of Fe(Se_1-xTe_x)₂ (x = 0, 0.2, 0.6, 0.8 and 1) compounds were synthesized by conventional solid-state reaction. Single phase orthorhombic structures were formed, except for FeSe_0.4Te_1.6 (x = 0.2). For FeSe_0.4Te_1.6 (x = 0.2), in which a mixture of FeSe₂ and FeTe₂-based phases was observed. The electrical conductivity of FeSe2 was as low as 31 S/cm at room temperature, and greatly increased to 1295 S/cm in FeSe_0.8Te_1.2. Then it decreased to 335 S/cm for FeTe₂. It was shown that the bipolar conduction of electrons and holes pronouncedly exists in the sample. For FeSe₂ and FeTe₂, the Seebeck coefficient was positive at room temperature, becoming negative at high temperatures. For the other samples, the Seebeck coefficient was negative at room temperature and the magnitude of Seebeck coefficient increased with temperature. As results, the maximum power factor of 0.19 mW/mK² was observed for FeSe_0.8Te_1.2 (x = 0.6) at room temperature mostly due to its high electrical conductivity. The maximum power factor observed at 600 K was 0.34 mW/mK² for FeSe₂ (x = 0) with the largest Seebeck coefficient of -101 μV/K. The Hall carrier concentration was measured and compared, and effective masses were calculated to further investigate electrical transport in the samples. (Received 25 January, 2022; Accepted 22 Fabruary, 2022)

keyword : thermoelectric, FeSe2, FeTe2, solid solution, bipolar conduction

3D printing technology has advanced rapidly over the last decade. However, for ceramic materials, drying and sintering steps are required after printing, and excessive shrinkage that occurs during these steps is a major factor that has hindered the development of the ceramic 3D printing technology. In this study, a non-sintering ceramic 3D printing method was developed using a hydraulic material to overcome the size deformation issue encountered during the post-processing of a scaffold-type printed green body. The deformation characteristics occurring during the curing process were confirmed. Tricalcium silicate (C3S) and tricalcium aluminate (C3A), which are well-known hydraulic materials, were selected. They were prepared into a printable paste by mixing with a viscous hydrophilic oil such as polyethylene glycol and polypropylene glycol, which helped the printout survive without collapse while it was cured. The scaffold was printed by Fused Deposition Modelling (FDM), which is the simplest and most economical printing method, and was cured by immersion in a water bath. The hydrated scaffold of the C3S/C3A mixture exhibited a smaller strain than the scaffold of the single materials, and the deformation amount depended on the printing direction. Remarkably, a scaffold with the smallest deformation, of less than 1%, and the highest compressive strength was obtained with a C3S/C3A mixing ratio of 65/35. (Received November 29 2021; Accepted December 20, 2021)

keyword : 3D printing, additive manufacturing, sintering-free, cement scaffold, FDM process, size deformation

The effect of Cu content on the microstructure and mechanical properties of Al-8.0Zn-2.5Mg-xCu (x: 0, 1, 2, 3) aluminum alloys manufactured by the twin-roll casting process was investigated. The Al-8.0Zn-2.5Mg-xCu alloy showed an increase in surface defects with increasing Cu content. This is because the amount of residual liquid in the final solidification region increased from 9.6 wt.% to 18.3 wt.% as the Cu content increased from 0Cu to 3 Cu alloy. For the 3Cu alloy, as the amount of residual liquid in the final solidification region exceeded the critical point, a large number of surface defects and internal shrinkage defect were observed. The main secondary phases of the four alloys were the T(Mg₃₂(Al, Zn)₄₉) and η(MgZn₂) phases, and their fraction increased with Cu content. These secondary phases mainly existed in the center segregation band, and a fine η(MgZn₂) phase was additionally observed. In terms of mechanical properties, as the Cu content increased, the hardness of the center matrix, secondary phase, and overall hardness increased respectively. Although the yield strength increased, the tensile strength and elongation decreased because the center segregation band was widened from 684 μm to 790 μm with increasing Cu content. (Received 18 October, 2021; Accepted 4 January, 2022)

keyword : Al-Zn-Mg-Cu, twin roll casting, Cu addition, microstructure, mechanical properties