Korea Institute of Material Science

Changwon, South Korea

Korea Institute of Material Science

Changwon, South Korea
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Ramana C.V.,University of Texas at El Paso | Kolekar Y.D.,University of Texas at El Paso | Kamala Bharathi K.,University of Texas at El Paso | Sinha B.,Korea Institute of Material Science | Ghosh K.,Missouri State University
Journal of Applied Physics | Year: 2013

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMn xO4, referred to CFMO) were synthesized and their structural, magnetic, and dielectric properties were evaluated. X-ray diffraction measurements coupled with Rietveld refinement indicate that the CFMO materials crystallize in the inverse cubic spinel phase. Temperature (T = 300 K and 10 K) dependent magnetization (M(H)) measurements indicate the long range ferromagnetic ordering in CoFe2-xMnxO4 (x = 0.00-0.15) ferrites. The cubic anisotropy constant (K1(T)) and saturation magnetization (Ms(T)) were derived by using the "law of approach" to saturation that describes the field dependence of M(H) for magnetic fields much higher than the coercive field (Hc). Saturation magnetization (Ms), obtained from the model, decreases with increasing temperature. For CoFe2O4, Ms decreases from 3.63 μB per formula unit (f.u.) to 3.47 μB/f.u. with increasing temperature from 10 to 300 K. CFMO (0.00-0.15) exhibit the similar trend while the magnitude of Ms is dependent on Mn-concentration. Ms-T functional relationship obeys the Bloch's law. The lattice parameter and magnetic moment calculated for CFMO reveals that Mn ions occupying the Fe and Co position at the octahedral site in the inverse cubic spinel phase. The structure and magnetism in CFMO are further corroborated by bond length and bond angle calculations. The dielectric constant dispersion of CFMO in the frequency range of 20 Hz-1 MHz fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived from modeling the experimental data are ∼10-4 s and ∼0.35(±0.05), respectively. © 2013 AIP Publishing LLC.


Yoon J.,Korea Institute of Material Science | Cazacu O.,University of Florida | Mishra R.K.,General Motors
Materials Science and Engineering A | Year: 2013

In this paper an anisotropic model that accounts for the anisotropy and strong tension-compression asymmetry observed in AZ31 magnesium sheet is presented. Key in the formulation is the incorporation of distortional hardening, and consequently modeling of the effects of evolving anisotropy and evolving tension-compression asymmetry on the macroscopic response. Furthermore, application of the model to the simulation of axial crushing of a tube made of the same material reveals that strong differences between the response of AZ31 magnesium and that of usual steel qualities and aluminum should be expected. © 2012 Elsevier B.V..


Yoon J.,Korea Institute of Material Science | Jeon H.,Korea Institute of Material Science | Lee J.,Korea Institute of Material Science
Materials and Design | Year: 2013

A seat recliner is a device that connects the seat back with the seat frame. It can be used to adjust the lean angle. An important component of a seat recliner is the sector tooth, which has thus far been manufactured by the fine blanking process. In this study, we propose the use of the cold-forging process as an alternative in order to achieve improved mechanical stiffness and reduced weight. To apply the cold-forging process, the entire forming process is divided into two stages in order to improve the dimensional accuracy of the tooth part of the sector tooth. Uniform plastic deformation is induced by the design of the preform on the contacting area in the sector tooth by simultaneously using a bending and a forging mechanism. By the proposed process design for the two-step cold forging process, the mechanical stiffness and surface roughness of the test product can be improved. Furthermore, the weight can be reduced by 10% by applying a 3.6-mm-thick plate instead of a 4.0-mm-thick one. Finally, the number of forming stages can be reduced without sacrificing the dimensional accuracy of the sector tooth and increasing the punch load. © 2013 Elsevier Ltd.


Yoon J.,Korea Institute of Material Science | Park S.,Korea Institute of Material Science
Materials and Design | Year: 2014

Magnesium (Mg) alloys have been thoroughly researched to replace steel or aluminum parts in automotives for reducing weight without sacrificing their strength. The widespread use of Mg alloys has been limited by its insufficient formability, which results from a lack of active slip systems at room temperature. It leads to a hot forming process for Mg alloys to enhance the formability and plastic workability. In addition, forged or formed parts of Mg alloys should have the reliable initial yield and ultimate tensile strength after hot working processes since its material properties should be compatible with other parts thereby guaranteeing structural safety against external load and crash. In this research, an optimal warm forming condition for applying extruded Mg-Sn-Al-Zn (TAZ) Mg alloys into automotive parts is proposed based on T-shape forging tests and the feasibility of forged parts is evaluated by measuring the initial yield strength and investigating the grain size in orientation imaging microscopy (OIM) maps. © 2013 Elsevier Ltd.


Kim S.-J.,Korea Institute of Material Science | Lee Y.-S.,Korea Institute of Material Science | Kim D.,Korea Institute of Material Science
Materials Characterization | Year: 2016

The formability of sheets of the Ca-added magnesium alloy AZX311 was analyzed. The parameters affecting the sheet formability, such as the strain-hardening rate and the strain-rate sensitivity, did not seem to be higher in the alloy AZX311 at temperatures of room temperature (RT) and 200°C. In addition, the critical stress for fracture at RT was lower in AZX311 than in AZ31. However, AZX311 exhibited higher stretchability and formability at low temperatures than AZ31. Electron back-scattered diffraction microscopy revealed that AZX311 had a weaker basal texture as well as broadened basal poles along the transverse direction. Polycrystal plasticity simulations confirmed that this weaker basal texture increases the activity of basal slip over thickness strain, resulting in the higher formability of AZX311. © 2016 Elsevier Inc. All rights reserved.


Yoon J.,Korea Institute of Material Science | Lee J.,Korea Institute of Material Science
Materials Science and Engineering A | Year: 2013

This paper mainly concerns about the effect of T5 heat treatment on the microstructure and mechanical properties of the as-extruded as well as as-forged Mg-8Al-0.5Zn alloy which was produced by the warm forging process for the Mg tie-rod at the temperature of 523. K. In the as-forged microstructure, rate of hardness increase from the initial to the peak hardness is substantially slow compared with the as-extruded since it already has a large amount of the precipitates in the warm forging process, which reduces the rate of precipitate hardening. Both types of precipitates such as the discontinuous lamellar- and continuous particle-types are favored simultaneously with the increase of aging time even though it is aged at the lower temperature of 450. K. © 2013 Elsevier B.V.


Park S.H.,Korea Institute of Material Science | Kim H.S.,Korea Institute of Material Science | You B.S.,Korea Institute of Material Science
Journal of Korean Institute of Metals and Materials | Year: 2013

The extrudability and mechanical properties of the Mg-7Sn-IAl-IZn (TAZ711) alloy were investigated by performing indirect extrusion at various temperatures and speeds and tensile tests and comparing the results with those of the Mg-8Al-0.5Zn (AZ80) alloy. When the AZ80 alloy was extruded at a high extrusion temperature of 450 V, or at extrusion temperatures above 350 °C with a high extrusion speed of 12 m/min, surface hot cracking occurred, while the TAZ711 alloy was successfully extruded without any surface defects at a high temperature of 450 V, with a high speed of 12 m/min. With an increasing extrusion temperature and speed, the average grain size of the extruded AZ80 and TAZ711 alloys increased and the yield and tensile strength decreased. The TAZ711 alloy had a higher yield strength and lower tensile strength and elongation than the AZ80 alloy extruded under the same conditions. © The Korean Institute of Metals and Materials.


Lee S.-I.,Korea Institute of Material Science | Yoon J.,Korea Institute of Material Science | Lee J.,Korea Institute of Material Science
International Journal of Precision Engineering and Manufacturing | Year: 2014

This paper proposes the net-shape forging process for the magnesium (Mg) scroll which is a device for compressing air or a refrigerant in air-conditioning equipment or an automobile supercharger. Mg scrolls make it possible to reduce the total weight of the scroll compressor noticeably by replacing the aluminum scroll in the compressor fields. In order to manufacture the sound scroll with uniform wrap height, the optimum back-pressure is applied to the Mg scroll forging, which is calibrated from the finite element analysis using DEFORM-3D. It is concluded that dimensional accuracy of the final product of the Mg scroll is comparably good since the all wraps are extruded more than the minimum machining line. It is also noted that the experimental and simulation results are able to be the best agreement with consideration of the texture evolution and DRX of the Mg alloy during the warm forging process. © 2014 Korean Society for Precision Engineering and Springer-Verlag Berlin Heidelberg.


Park Y.-S.,Kyung Hee University | Choi K.-H.,Kyung Hee University | Kim H.-K.,Kyung Hee University | Kang J.-W.,Korea Institute of Material Science
Electrochemical and Solid-State Letters | Year: 2010

We investigated the stacking sequence effect of indium zinc oxide (IZO)-Ag and IZO-Ag-IZO on the characteristics of multistacked flexible transparent electrodes for organic photovoltaics. In spite of the similar sheet resistances of the IZO-Ag and IZO-Ag-IZO stacked electrodes, the optical transparency of the latter is much higher than that of the former in the wavelength region of 400-800 nm due to the effective antireflection in the symmetric oxide-Ag-oxide structure. Furthermore, the flexible organic solar cells (OSCs) fabricated on the IZO-Ag-IZO-stacked multilayer electrode showed a higher power conversion efficiency than those fabricated on the IZO-Ag-stacked multilayer electrode due to the higher optical transparency of the former electrode in the main absorption region of the poly(3-hexylthiophere) and 1-(3-methoxycarbonyl)- propyl-1-phenyl-(6,6) C61 layers. This indicates that the multistacked electrode with the IZO-Ag-IZO sequence is more beneficial than that with the IZO-Ag sequence for low resistance and high transparency electrodes in flexible OSCs. © 2010 The Electrochemical Society.


Park S.H.,Korea Institute of Material Science | Yu H.,Korea Institute of Material Science | Yu H.,Shandong University | Kim H.S.,Korea Institute of Material Science | And 3 more authors.
Journal of Korean Institute of Metals and Materials | Year: 2013

The effects of copper (Cu) addition on the microstructure and mechanical properties of ZK60 alloy were investigated using an optical microscope (OM), a scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) and by performing tensile tests of indirect-extruded ZK60 alloys with 0.5, 1.0, and 1.5 wt% Cu contents. The results revealed that the as-extruded ZK60 alloy had a bimodal grain structure composed of fine recrystallized grains and coarse unrecrystallized grains. The homogeneity of the microstructure was increased by Cu addition due to the promotion of dynamic recrystallization (DRX) by particle stimulated nucleation (PSN) at the Mg-Zn-Cu particles. This enhanced DRX behavior resulted In the reduction of the average grain size and weakening of the basal fiber texture of the as-extruded alloys. The yield and tensile strengths were improved by the Cu addition owing to the decreased grain size and increased number of particles, while the elongation was decreased due to the hard Mg-Zn-Cu particles. Copyright © The Korean Institute of Metals and Materials.

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