발간논문

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The flow functions for plastic deformation have been developed to describe the plastic behavior of sheet metals. In order to explain the plastic behavior of material in metal forming processes via finite element analyses, two basic input functions should be applied. One is the yield function that determines the yielding behavior. The other is flow function to describe the hardening property of sheet metal. To describe the hardening properties of sheet materials under quasi-static tension condition in a wide range of plastic straining, various different equations are known such as classical Swift, Voce, Holloman, combined Swift- Voce, and recently proposed Kim-Tuan equations, etc. Those hardening equations are based on metallurgical or phenomenological investigations, and however the application of each equation has some limitation. In this study, the random growth of the binary tree method is introduced to develop the reliable hardening equations of various sheet metals (i.e. DP980, Pure Ti, AA5052-O, STS304, Ti-Gr2, and Mg-AZ31B) with no knowledge of existing hardening equation types. To evaluate the proposed method, the proposed equations developed by new approach are compared with the Voce, Swift, and Kim-Tuan hardening equations for stress-strain curve and the plastic instability point. Consequently, the proposed approach was proven to be very efficient to find the reliable and accurate hardening equation for any kind of materials. (Received August 20, 2020; Accepted September 22, 2020)

keyword : random growth, binary tree, hardening function, curve fitting, maximum tensile force point

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution. (Received August 31, 2020; Accepted September 14, 2020)

keyword : ferrite-pearlite, low-carbon steel, hydrogen embrittlement, pre-strain, Nb

In this study, high-strength pure Cu bulk material was manufactured using a cold spray additive manufacturing process, and its microstructure, tensile and compressive deformation behaviors were investigated and compared. The cold spray additive manufactured Cu bulk material showed a heterogeneous grain structure consisting of fine-grains and coarse-grains, and only α - Cu single phase was identified. The cold spray Cu exhibited yield strengths of ~415 MPa in tensile- and compression tests, indicating that it had similar mechanical properties in different deformation modes. The yield strength values were similar to that of Cu manufactured by equal channel angular pressing (ECAP), a severe plastic deformation (SPD) method which enables ultra-high strength. Concerning tensile characteristics, the cold sprayed Cu exhibited partial plastic deformation that has not been reported to date. In addition, some nano-sized dimples, suggesting metallurgical bonding, were also found in the fracture surface. Regarding compression characteristics, the strain softening phenomenon, which is not a general tendency in room temperature deformation, appeared. This unique softening behavior was attributed to dynamic recovery and dynamic recrystallization during compression testing. Based on the above results, we discuss the tensile/compressive deformation behavior of the cold spray Cu bulk material, and predict compressive deformation behavior considering the constitutive equation. (Received August 26, 2020; Accepted September 9, 2020)

keyword : cold spray, pure cu, compression, tensile, high-strength, deformation behavior, constitutive model

Recent large-scale fires in buildings and logistic warehouses have become a major social issue, involving both property loss and multiple casualties. To make color coated steel sheets non-combustible and/ or have anti-fire properties, various ways of optimizing manufacturing parameters have been investigated for outer, inner, roof and ceiling panels. In the present study, the effect of inorganic filler content and size on the non-combustible and mechanical properties of color coated steel sheets has been investigated using samples prepared as pre-painted coating materials. Both salt spray corrosion and chemical resistance tests were also carried out. Filler distribution and size were measured by optical microscopy, scanning electron microscopy and glow discharge spectroscopy, and found to be critical factors affecting non-combustible performance. As the amount of added filler increased, the non-combustible property of the color coated steel sheets improved, while mechanical properties, corrosion resistance by salt spray and chemical resistances deteriorated. During 3t-bending tests, the adhesive strength at the interface between coated layer and hot dip galvanized steel sheets was rather strong, although the filler-added upper coated layer was mostly peeled off. The mechanical properties of 30% filler addition samples were compared to samples with less than 20% filler addition. The main reason for the poorer performance was clarified in terms of filler size and crack propagation in the 3t-bended color coated layer. (Received July 21, 2020; Accepted August 27, 2020)

keyword : adhesive strength, mechanical properties, Zn-Mg-Al hot-dip galvanized steel sheets, organic additive, additive materials

ZrO₂ is a promising candidate for knee and hip joint replacements due to its excellent combination of low density, corrosion resistance and biocompatibility. Nevertheless, a low fracture toughness of pure ZrO₂ at room temperature limits its wider application in the industry. One of the most obvious ways to solve the problem is to add a reinforcing phase, to produce a nanocrystalline composite material. Nanomaterials have been widely studied in recent years because they can improve hardness and fracture toughness. To produce nanocrystalline materials, the pulsed current activated sintering method has the advantage of simultaneously applying mechanical pressure and pulsed current during sintering. As a result, nanocrystalline materials can be produced within a very short time. Ta and ZrO₂ nanopowders were mechanically synthesized from Ta₂O₅ and 2.5Zr powders according to the reaction (Ta₂O₅ + 5/2Zr → 2Ta + 5/2ZrO₂). The synthesized powders were then sintered using pulsed current activated heating under 80 MPa uniaxial pressure within two minutes. Hardness and fracture toughness were measured using a Vickers hardness tester. The average hardness and fracture toughness of the nanocrystalline 2Ta-5/2ZrO₂ composite sintered at 1350 ℃ were 1008 kg/mm² and 10 MPa·m^1/2, respectively. Both the hardness and fracture toughness of the composite were higher than monolithic ZrO₂. The microstructure and phase of the composite was also investigated by FE-SEM and XRD. (Received July 1, 2020; Accepted September 11, 2020)

keyword : composite materials, synthesis, sintering, mechanical properties, ZrO₂

This study investigated the effects of laser power on the microstructure and mechanical properties of functionally gradient materials (FGM) produced by direct energy deposition. The FGM consisted of five different layers, which were a mixture of austenitic stainless steel (Type 316L) and ferritic steel (HSLA). During the direct energy deposition, two different laser power conditions (450W and 380W) were used. The ratio of Type 316L and HSAL at each deposition layers were 100:0, 65:35, 50:50, 25:75, and 0:100. After the direct energy deposition process, no cracks or delamination were seen between layers of the FGM. The effects of laser power on chemical composition and microstructure were not significant. However, as the laser power decreased, tensile strength and elongation changed with a small change in grain size. (Received July 20, 2020; Accepted August 5, 2020)

keyword : additive manufacturing, direct energy deposition, functionally graded material, stainless steel

Transparent and conductive Ti doped In₂O₃ (TIO) films were prepared on slide glass substrate using a radio frequency (RF) magnetron sputter and then subjected to Transparent and conductive Ti doped In₂O₃ (TIO) films were prepared on a glass slide substrate using radio frequency (RF) magnetron sputter. The film surface was then subjected to intense electron beam irradiation, to study the influence of incident energy on the visible transmittance and electrical resistivity of the films. All x-ray diffraction plots exhibited some diffraction peaks of the cubic bixbyite In₂O₃ (222), (400), (332), (431), (440), and (444) planes regardless of the electron irradiation energy, while the characteristic diffraction peak for crystalline TiO₂ did not appear even when irradiated at 1500 eV. In atomic force microscope analysis, the surface roughness of the as deposited TIO films was found to be 0.63 nm. As the electron irradiation energy was increased up to 1500 eV, the root mean square roughness decreased down to 0.36 nm. The films electron irradiated at 1500 eV showed higher visible transmittance of 83.2% and the lower resistivity of 6.4 × 10^-4 Ωcm compared to the other films. From the electrical properties and optical band gap observation, it is supposed that the band gap shift is related to the carrier density. The band gap enlarged from 4.013 to 4.108 eV, along with an increase in carrier density from 9.82 × 10¹⁹ to 3.22 × 10²⁰ cm^-3. (Received April 23, 2020; Accepted September 2, 2020)

keyword : TIO, magnetron sputtering, AFM, XRD, figure of merit

Facile, economic methods of preparing tungsten (W) nanopowder are critically needed to meet industrial demand. Herein, we report a method of preparing single-phase alpha-W (α-W) nanopowders using ammonium paratungstate (APT) as a starting material and the optimum synthesis conditions. The process involves two stages: i) the radio-frequency (RF) induction thermal plasma treatment of APT, followed by ii) thermochemical reduction at 600-900 ℃. The crystallographic phase and morphological evolution of all products were systematically investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM), and the effects of the annealing temperature on the phase and particle size of the obtained powders were also evaluated. When the RF induction thermal plasma treatment was conducted with and without H₂, the XRD and FESEM results showed the formation of mixed-phase α- and beta-W (β-W) nanopowder and WO₃ nanopowder, respectively. Single-phase α-W nanopowder was achieved by annealing the WO₃ nanopowder in an H₂ reductive atmosphere at 700 ℃ for 10 min, resulting in homogenous nanoparticles with a small particle size (d50) of 21.16 nm without any aggregation. (Received July 14, 2020; Accepted September 9, 2020)

keyword : alpha-tungsten, nanopowder, ammonium paratungstate, APT, RF induction thermal plasma, thermochemical reduction

Graphene oxide quantum dots (GOQDs) are nanometer-sized graphene oxide fragments that exhibit unique properties, making them interesting candidates for a range of new applications. Carbon black, one of the commercially available carbon precursors, is produced by the thermal decomposition or incomplete combustion of organic compounds. It is commonly used as a supporting material for catalysts because of its excellent electrical conductivity, high surface area, and stability. In this paper, we report the transformation of carbon black into GOQDs in 10 min using a one-step facile approach. This transformation was achieved by pulsed laser ablation (PLA) in ethanol using the earth-abundant and low-cost carbon black as precursor. Only ethanol and carbon black were used for the transformation. The carbon clusters ablated from the carbon black were completely transformed into GOQDs with a homogeneous size distribution and heights in the range of 0.3-1.7 nm. This confirmed that the transformed GOQDs consisted of only single- or few-layered graphene quantum dots. The UV-vis spectra showed absorption bands at 215, 260, and 320 nm, which were attributed to the π→π* transition of the C=C of the sp² C bond in the sp³ C matrix. A distinct blue emission peak at 450 nm was evident at an excitation wavelength of 360 nm. The broader PL emission spectra are due to the oxygen-related functional groups emitting PL between 300 and 440 nm. (Received September 8, 2020; Accepted September 23, 2020)

keyword : graphene oxide quantum dots, pulsed laser ablation, carbon black