• The korean journal of orthodontics
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• v.28 no.2 s.67
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• pp.329-341
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• 1998

Sterilization has received much attention in orthodontic practices over the past several years. The present study was undertaken to investigate the effects of sterilization on the physical properties of orthodontic pliers-AEZ, Unitek, and Dentronix ligature cutters. This study was designed to examine the tips of ligature cutters before and after 200 and 400 sterilization cycles using the Bowmar RHT-1000, the Dentronix DDS-5000, and the Eschmann SES-2000. The tip surface and the fracture surface were observed with a scanning electron microscope. The microstructure was observed with an optical microscope. The hardness test was carried out with the mic개-Vickers hardness tester and the Rockwell C Scale hardness tester. The chemical composition was analyzed by energy dispersive X-ray spectrometer. The results of this study were as follows : 1. The number and the size of corrosion products on the tip surface and the proportion of cleavage planes in fractured specimen increased, but the hardness of the tip decreased in proportion to sterilization cycles. From these observations, it was considered that mechanical properities decreased in proportion to sterilization cycles. 2. The number and the size of chromium carbides increased in proportion to sterilization cycles. Coarse microstructure decreased mechanical properities. 3. The AEZ and Unitek ligature cutters were Fe-Cr stainless steels, but the Dentronix ligature cutter was Co-Cr alloy. There were many differences among manufactures, but the chemical composition was .not changed after sterilization cycles. 4. The tip edge of ligature cutter used in a clinic revealed microcracks with the SEM observation. Clinical experience confirmed that ligature cutters were gradually degraded by sterilization.

• Journal of Korea Foundry Society
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• v.24 no.1
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• pp.40-44
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• 2004

The influence of Pd and Ag additions on the thermal stability, the glass forming ability (GFA) and mechanical property of $Zr_{55}Al_{10}Cu_{20}Ni_{10}Pb_{(5-x)}Ag_x$ (x = $0{\sim}5at%$) alloys obtained by melt spun and injection casting method have been investigated by using of X-ray diffraction, thermal analysis (DTA, DSC) and micro-Vickers hardness(Hv) testing. The thermal properties of melt-spun $Zr_{55}Al_{10}Cu_{20}Ni_{10}Pb_{(5-x)}Ag_x$ (x = $0{\sim}5at%$) alloys exhibit a supercooled liquid region(${\Delta}T_x$) exceeding 91 K before crystallization. The largest ${\Delta}T_x$ reaches as large as 126 K for the $Zr_{55}Al_{10}Cu_{20}Ni_{10}Pb_5$ alloy. The reduced glass transition temperature, $T_{rg}$ increased with increasing Ag content. The largest $T_{rg}$ is obtained for the $Zr_{55}Al_{10}Cu_{10}Ni_{10}Ag_5$ alloy. The $Zr_{55}Al_{10}Cu_{10}Ni_{10}Ag_5$ bulk amorphous alloy rod with 3 mm in diameter was fabricated by injection casting. Hv increased with increasing Ag content and the largest value was obtained for the $Zr_{55}Al_{10}Cu_{10}Ni_{10}Ag_5$ bulk amorphous alloy.

• Proceedings of the Materials Research Society of Korea Conference
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• 1994.11a
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• pp.1-1
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• 1994

;The electronic state calculations for various types of ceramic materials have beell performed by the use of $DV-X\;{\alpha}$ cluster method. The molecular orbital levels and wave functions for model clusters have been computed to study the electronic properties ami chemical bonding of the ceramics. For ${\beta}-sialon(Si_{6-z}Al_zO_zN_{8-z})$ which is a high temperature structural material based on ${\beta}-Si_3N_4$, we have made model cluster calculations to estimate the strength of chemical bonding between atoms by the Mulliken population analysis. It is found that the covalent bonding between Si and N atoms is very strong in pure ${\beta}-Si_3N_4$, but the covalency around solute atom is considerably weakened when Si atom is substituted by AI. This tendency is enhanced by an additional substitution of oxygen atom for N. The result calculated can well explain the experimental data of changes in mechanical properties such as the reductions of Young's modulus and Vickers hardness with increment of z-value in ${\beta}-sialon$. Various model clusters for transition metal oxides which show many interesting physical and chemical properties have also been calculated. High-valent perovskite-type iron oxides EMFe0_3E(M=Ca and Sr) possess very interesting magnetic and chemical properties. In these oxides, iron exists as $Fe^{4+}$ state, but the experimental measurement of Mossba~er effect suggests that disproportionation $2Fe^{4+}=Fe^{3+}+Fe^{5+}$ takes place for $CaFe0_3$ at low temperatures. The model cluster calculations for these compounds indicated the existence of considerably strong covalent bonding of Fe-O. The calculations of hyperfine interaction at iron neucleus show very good agreement with the experimental Mossbauer measurements. The result calculated also implies that the disproportionation reaction is strongly possible by assuming the quenching of breathing phonon mode at low temperatures.tures.

• Korean Journal of Materials Research
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• v.30 no.6
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• pp.321-325
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• 2020

The present study demonstrates the effect of magnetic pulse compaction and spark plasma sintering on the microstructure and mechanical property of a sintered W body. The relative density of green specimens prepared by magnetic pulse compaction increases with increase in applied pressure, but when the applied pressure is 3.4 GPa or more, some cracks in the specimen are observed. The pressureless-sintered W shows neck growth between W particles, but there are still many pores. The sintered body fabricated by spark plasma sintering exhibits a relative density of above 90 %, and the specimen sintered at 1,600 ℃ after magnetic pulse compaction shows the highest density, with a relative density of 93.6 %. Compared to the specimen for which the W powder is directly sintered, the specimen sintered after magnetic pulse compaction shows a smaller crystal grain size, which is explained by the reduced W particle size and microstructure homogenization during the magnetic pulse compaction process. Sintering at 1,600 ℃ led to the largest Vickers hardness value, but the value is slightly lower than that of the conventional W sintered body, which is attributed mainly to the increased grain size and low sintering density.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.5
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• pp.377-383
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• 2016

The effect of long-term aging degradation on magnetic properties of ferritic 11Cr low-carbon steel was investigated. Coercivity and hysteresis loss measured from the hysteresis loops decreased with long-term aging time and showed that the relation was well fitted by a second order exponential function. Vickers hardness also decreased with aging time and resulted in mechanical softening. In addition, the microstructural evolution was observed by the scanning electron microscopy, backscattered electron image and X-ray diffraction. The $Cr_{23}C_6$ precipitates along grain boundary grew fast and Laves ($Fe_2W$) phase on martensitic lath boundaries in interior grains was developed. The solid solution atoms depleted in matrix and lath subgrains recovered owing to precipitate coarsening with long-term aging degradation. There was a close relation with softening of magnetic and mechanical properties.

• Journal of Powder Materials
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• v.16 no.5
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• pp.326-335
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• 2009

Fe based (Fe$_{68.2}$C$_{5.9}$Si$_{3.5}$B$_{6.7}$P$_{9.6}$Cr$_{2.1}$Mo$_{2.0}$Al$_{2.0}$) amorphous powder, which is a composition of iron blast cast slag, were produced by a gas atomization process, and sequently mixed with ductile Cu powder by a mechanical ball milling process. The Fe-based amorphous powders and the Fe-Cu composite powders were compacted by a spark plasma sintering (SPS) process. Densification of the Fe amorphous-Cu composited powders by spark plasma sintering of was occurred through a plastic deformation of the each amorphous powder and Cu phase. The SPS samples milled by AGO-2 under 500 rpm had the best homogeneity of Cu phase and showed the smallest Cu pool size. Micro-Vickers hardness of the as-SPSed specimens was changed with the milling processes.

• Journal of the Korean Society for Heat Treatment
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• v.33 no.3
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• pp.99-106
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• 2020

Selective laser melting (SLM) is an additive manufacturing process by melting metallic powders and stacking into layers, and can product complex shapes or near-net-shape (NNS) that are difficult to product by conventional processes. Also, SLM process is able to raise the efficiency of production by creating a streamlined manufacturing process. For manufacturing in SLM process using Ti-6Al-4V powder, analysis of microstructural evolution and evaluation of mechanical properties are essential because of rapid melting and solidification process of powders according to high laser power and rapid scan speed. In addition, it requires a post-processing because the soundness and mechanical properties are degraded by defects such as pore, un-melted powder, lack-of-fusion, etc. In this study, hot isostatic press (HIP) was conducted as a post-processing on SLM-printed Ti-6Al-4V alloy. Microstructure of post-processed Ti-6Al-4V alloy was compared to as-built Ti-6Al-4V, and the evolution of quasi-static (Vickers hardness, room temperature tensile characteristic) and dynamic (high-cycle fatigue characteristic) mechanical properties were analyzed.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1398-1401
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• 2005

The nano/microstructure, the aging response (in T5 heat treatment), and the mechanical/tribological properties of the eutectic regions in squeeze-cast A356 alloy were investigated using nano/micro-indentation and mechanical scratching, combined wit optical microscopy and atomic force microscope(AFM). Most eutectic Si crystals in the A356 alloy showed a modified morphology as fine-fibers. The loading curve for the eutectic region was more irregular than that of the primary Al region due to the presence of various particles of varying strength. In addition, the eutectic region showed lower pile-up and higher elastic recovery than the primary Al region. The aging responses of the eutectic regions in the squeeze-cast A356 alloys aged at $150^{\circ}C$ for different times(0, 2, 4, 8, 10, 16, 24, 36 and 72 h) were investigated. As the aging time increased, acicular Si particles in the eutectic regions gradually came to a fine structure. Both Vickers hardness ($H_V$) and indentation ($H_{IT}$) test results showed almost the same trend of aging curves, and the peak was obtained at the same aging time of 10 h. A remarkable size-dependence of the tests was found. The friction coefficient for the eutectic region was lower than that for the primary Al region.

• Korean Journal of Materials Research
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• v.9 no.1
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• pp.8-13
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• 1999

$ZrB _2$ was prepared from a mixture of $ZrO_2$, $B_2$$O_3$and Mg by SHS method. The combustion products were successfully obtained from a mixture of $Zro_2$:$B_2$$O_3$:Mg=1:2.0:8.5(molar ratio). MgO, by-product, was removed to 92.7% by leaching with 1M HCl solution at 9$0^{\circ}C$, for 10 hours. After leaching, the mean particle size of the resultant $ZrB_2$powders was 23.6$\mu\textrm{m}$. $ZrB_2$-ZrC composite was suitably obtained from a mixture of C/Zr=1.2 molar ratio by arc-melting method. The density of arc-melted specimen increased by adding excess zirconium content(x). The bulk density was 6.17g/㎤ for x=0, and 6.37g/㎤ x=4. Vickers hardness of arc-melted specimen was /$1290kg\textrm{mm}^2$ for x=0, and fracture toughness increased to 4.2MPa.m\ulcornerforx=4 compared to 3.4MPa.m\ulcornerfor x=0.

• Korean Journal of Materials Research
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• v.21 no.5
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• pp.263-267
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• 2011

The present study was carried out to evaluate the microstructural and mechanical properties of cross-roll rolled pure copper sheets, and the results were compared with those obtained for conventionally rolled sheets. For this work, pure copper (99.99 mass%) sheets with thickness of 5 mm were prepared as the starting material. The sheets were cold rolled to 90% thickness reduction and subsequently annealed at $400^{\circ}C$ for 30 min. Also, to analyze the grain boundary character distributions (GBCDs) on the materials, the electron back-scattered diffraction (EBSD) technique was introduced. The resulting cold-rolled and annealed sheets had considerably finer grains than the initial sheets with an average size of 100 ${\mu}M$. In particular, the average grain size became smaller by cross-roll rolling (6.5 ${\mu}M$) than by conventional rolling (9.8 ${\mu}M$). These grain refinements directly led to enhanced mechanical properties such as Vickers micro-hardness and tensile strength, and thus the values showed greater increases upon cross-roll rolling process than after conventional rolling. Furthermore, the texture development of <112>//ND in the cross-roll rolling processed material provided greater enhancement of mechanical properties relative to the case of the conventional rolling processed material. In the present study, we systematically discuss the enhancement of mechanical properties in terms of grain refinement and texture distribution developed by the different rolling processes.