발간논문

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The lamellar structure of two phases, the Zn-rich and Al-rich phases, is formed in a Zn-5 wt% Al eutectic alloy. In order to study the effect of microstructural changes on the tensile properties of eutectic Zn-Al binary alloy, Zn-Al and Zn-Al-Cu alloys were directionally solidified at a growth rate of 50 μm/s using a Bridgman-type method. The alloys were successively drawn until reaching area reductions of 50% and 80% at room temperature. The tensile strength of the Zn-Al alloy decreased with increasing drawing ratio. In the Zn-Al-Cu alloy, the tensile strength increased until the area reduction reached 50%, but decreased at an area reduction of 80%. The ductility of alloys prepared by directional solidification increased with increasing drawing ratio. The orientations of the lamellar structure and grains were aligned in the drawn direction. The inter-lamellar distance of the alloys also decreased with increasing drawing ratio. As a result, the trade-off between strength and ductility could be significantly reduced. The strength and ductility combination of 246 MPa-40% was achieved by cold drawing with 80% area reduction of the Zn-Al-Cu alloy. (Received January 10, 2017; Accepted March 27, 2017)

keyword : eutectic, Zn-Al alloy, directional solidification, high strength, high ductility

To study the effect of Al and Zr contents on the microstructural changes and tensile properties of Ni-Al-Bronze alloy, seven specimens were cast with different Al and Zr contents. The microstructure observation revealed that all specimens were composed of β` and κ phases in a α matrix. With increasing Al and Zr contents, the tensile strength of the Ni-Al-Bronze increased, due to an increase in the β` and κ phase fraction and grain refinement, but the elongation behavior of the Ni-Al-Bronze declined. Furthermore, the excessive addition of Zr caused a reduction in tensile strength and ductility. A successful combination of tensile strength and ductility, 688 MPa-23% was achieved by the controlled addition of Zr to Ni-Al-Bronze. (Received November 10, 2016; Accepted March 13, 2017)

keyword : Ni-Al-bronze, cast alloy, grain refinement, mechanical properties, microstructure

The delayed cracking behavior of deep-drawn lean duplex stainless steel (LDSS) was investigated, and compared with that of conventional metastable austenitic stainless steels (MASS). Hydrogen charging was performed by electrochemical method, and the hydrogen-charged specimens were subjected to the slow-strain-rate test. Fractures occurred along the phase boundaries in the LDSS, whereas the MASS specimens showed typical intergranular fractures after the hydrogen charging. In particular, the LDSS exhibited a superior limiting drawing ratio in spite of the large amount of internal hydrogen as well as high sensitivity to hydrogen embrittlement. The high resistance to delayed cracking originated from the relatively lower residual stress generated during the deep drawing process. This is a consequence of the suppression of martensitic transformation in the LDSS, due to Mn partitioning. (Received January 18, 2017; Accepted March 27, 2017)

keyword : delayed cracking, stainless steel, hydrogen, residual stress, martensite

This study investigated the feasibility of using HfO₂ as a dispersoid in the additive manufacturing process, compared to Y₂O₃. The effect of pre-annealing treatment was investigated too. Scanning electron microscopy (SEM) analyses revealed unusually coarse deposition layers for both the HfO₂ and Y₂O₃ dispersed oxide dispersion strengthed (ODS) steels, in both the as-milled and the pre-annealed conditions. The deposited layer of the HfO₂ dispersed ODS steel had relatively coarser grains than the deposited layer of the Y₂O₃ dispersed ODS steel in both the as-milled and the pre-annealed conditions. Moreover, the SEM results also revealed the presence of nanometer sized particles in all the deposition layers of both Y₂O₃ and HfO₂ dispersed ODS steels, and their number densities were far lower than those in conventional bulk ODS steels. However, transmission electron microscopy analyses revealed that the dispersion and retention of nanoparticles within the melt were not achieved, even with HfO₂ as a dispersoid, in contrast to the results from the SEM analyses. Furthermore, the deposition layers of both the as-milled Y₂O₃ and HfO₂ ODS steels also exhibited an unusual nano-grained structure. The microhardnesses of the HfO₂ and the Y₂O₃ dispersed ODS steels in both the as-milled and the pre-annealed conditions were higher than the substrate. Furthermore, the Y₂O₃ dispersed ODS steel had a higher microhardness than the HfO₂ dispersed ODS steel in both the as-milled and the pre-annealed conditions. (Received February 8, 2017; Accepted March 27, 2017)

keyword : oxide dispersion strengthened steel, direct laser deposition, mechanical alloying, ultra fine grain, nano-particle

Spent nuclear fuel is stored and transported in cask bodies, but they become embrittled with the passage of time resulting in lattice defects due to neutron irradiation. Cask bodies made by the quenching and tempering (QT) treatment are not safe and are hard to store because of embrittlement caused by high levels of radiation and heat over long periods. Recently, it was shown that the double austenitizing tempering (DAT) treatment resulted in a significant improvement in strength and toughness as a result of austenite grain refinement. In this study, we evaluated the effects of DAT treatment for improving the mechanical properties of low alloy steel. We also investigated the effects of cooling rate on the DAT treatment. It was found that the charpy impact properties of the alloy were improved by the DAT treatment, which used water cooling comparing, compared to the QT heat treated specimen. As a result of microstructure analysis, it was determined that the grain size and carbide morphology were affected by mechanical properties such as the tensile, hardness and impact property. (Received January 20, 2017; Accepted March 24, 2017)

keyword : low alloy steel, DBTT, quenching, tempering, cooling rate

The Paris climate change agreement made a commitment to keep global warming below 2 ℃. The Korean government confirmed that it would seek a 37% reduction in CO₂ emissions by 2030 from the Business As Usual level. CO₂ emissions from Korean iron and steel industries account for approximately 15.4% of that reduction. Therefore, breakthrough technologies are urgently required to reduce the CO₂ emitted from iron and steel making processes. A high efficiency iron making process is needed, because present methods consume huge amounts of energy. One of the key technologies for high efficiency iron making is more effective slag making. Understanding the effects of basic oxide additives such as lime and dolomite on the slag melting is required. In this study, the effects of additives (CaO and MgO) on the melting behavior the CaO-SiO₂-Al₂O₃(-MgO) slag were investigated using a high temperature in-situ observation technique. For the CaO-SiO₂-Al₂O₃ system, when the particle size of CaO was increased, slag melting was delayed. When 10 wt% of 1-2 mm diameter CaO or MgO was added, it was observed that the MgO addition delayed melting of the CaO-SiO₂-Al₂O₃-MgO system. Accordingly to facilitate slag melting, finer additives are recommended. In addition, it was determined that lime was more useful for making molten slag than dolomite. (Received December 8, 2016; Accepted March 2, 2017)

keyword : in-situ observation, ironmaking process, melting, oxides, softening

We investigated solution-processed indium zinc oxide (IZO) thin-film transistors (TFTs) which were fabricated by spin coating with simultaneous irradiation by ultraviolet light. The proposed treatment enhanced the efficient condensation and densification of the oxide semiconducting channel layer by decreasing oxygen-vacancy-related defects and increasing metal-oxide bonds. An IZO TFT irradiated for 90 sec had a field-effect mobility of 5.6 cm²/Vs, a threshold voltage of -0.13 V, a subthreshold swing of 0.64 V/decade, and an on/off current ratio of 1.7 × 10⁶ with a width/length of 2000 μm/200 μm. (Received October 18, 2016; Accpeted April 13, 2017)

keyword : thin films, sol-gel, surface, electrical, indium-zinc oxide TFTs

To realize focused design near-field conduction noise absorbers for sub-GHz frequencies, this study proposes screen-printed conductive grid films. The films` advantages include large-scale production and easy control of electrical properties. A conductive grid film with a resistance value of approximately 70 Ω was screen printed on polyimide film substrate and its conduction noise absorption was evaluated using a microstrip line. High power absorption was experimentally verified at low frequencies below 1 GHz by varying the grid width (w). The dominant loss mechanism was ohmic loss. Approximately 85% power absorption was obtained at 0.5 GHz for a grid film of w = 1.5 mm. Compared with a conventional magnetic sheet absorber, the conductive grid film exhibited a superior power absorption particularly at sub-GHz frequencies, and at a much reduced thickness of approximately 20 μm. (Received March 30, 2017; Accepted April 28, 2017)

keyword : conductive grid films, screen printing, electromagnetic noise, absorbers, electric properties, magnetic properties

The near-infrared (NIR) reflecting properties of TiO₂ /Ag /TiO₂ (TAT) multilayers deposited using DC/RF magnetron sputtering were investigated. For high transmittance and reflectance in the visible and NIR ranges, respectively, the thickness of each layer was theoretically optimized by the index matching method. As the Ag layer thickness increased, the reflectance of the TAT multilayers in the NIR range increased significantly because of surface plasmon resonance (SPR). It was confirmed that the TAT multilayer with a 12 nm-thick Ag layer showed both good transmittance and reflectance. After irradiation with an IR lamp for 10 min, the temperature of the bare glass increased from 27 to 41 ℃, while that of the TAT films increased by only 6 ℃. This is because of the enhancement of the SPR effect with the increase in carrier density and mobility. The TAT multilayers were found to be suitable for NIR reflecting windows. (Received December 20, 2016; Accepted March 14, 2017)

keyword : NIR reflecting coating, energy saving, multilayer, O/M/O structure, sputtering

The photoreflectance (PR) method is advantageous in that it is more convenient to use at low temperature and high temperature than electroreflectance (ER) due to the modulation of the electric field. This is caused by the recombination of the surface state charge and the minority carriers generated by the laser light. For this reason, PR makes it easier to study the microstructure of semiconductors using non-contact and non-destructive characteristics on the surface of the sample. The position of the Fermi level of the semiconductor can also be determined and the surface electric field can be optically determined. We studied the photoreflectance characteristics of the Al_0.21Ga_0.79As/GaAs heterostructure. We observed the A region (1.49∼1.53 eV) in the PR spectra by varying the pumping intensity in the sample. The room temperature PR spectra exhibited A region oscillations. This was found to be due to the increasing electric field intensity. (Received February 3, 2017; Accepted March 3, 2017)

keyword : AlGaAs, GaAs, photoreflectance, intensity