• Journal of the Korean Society for Heat Treatment
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• v.31 no.6
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• pp.307-314
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• 2018

Simple tensile tests and microstructural investigations have been performed on extrudates of high strength aluminum alloys such as 7075, 7021 and 7xxx(Sc) to understand correlation between extruding conditions and extruded properties. Tensile specimens which were taken from different locations at the same cross section of an extrudate were tested at room temperature and with a strain rate of $8.9{\times}10^{-5}/s$. The microstructures according to the locations at the cross section have been observed using optical microscopy and electron back-scattered diffraction (EBSD) mapping to characterize the effect on stress-strain curve. The results could be classified in three types independent of alloying contents and extusion methods. The fine differences in the stress-strain curves were resulted from inhomogenity in the microstructures according to locations of an extrudate which were performed through instantaneous extruding conditions such as temperature, strain rate and strain.

• Analytical Science and Technology
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• v.14 no.1
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• pp.50-58
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• 2001

The effect of cryogenic heavy rolling method on grain refinement has been studied and compared with those obtained by previous methods of cryogenic rolling and heavy rolling developed separately. Both grain structure and grain size were analyzed using electron back scattered diffraction (EBSD), which can distinguish grains each other by means of crystallographic misorientations between them. The cryogenic rolling and heavy rolling exhibit reduction in grain size of 39% and 87%, respectively, and the cryogenic heavy rolling shows reduced grain size similar to that observed in the heavy rolling. The heavy rolling was proved more effective in grain size refinement than the cryogenic rolling.

• Journal of Korea Foundry Society
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• v.32 no.1
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• pp.32-37
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• 2012

In this study, microstructural evaluation was carried out on secondary hardening type ultrahigh strength steel, Fe-Co-Ni composition. This paper as a first part of whole research presented the microstructural behavior by cyclic heat treatment. The cyclic heat treatment method includes normalizing, stress relieving, solution treatment and aging. Especially, solution treatments performed triple times to get maximized solution hardening. Phase transformation and microstructure were observed by using optical microscope (OM), Electron back-scattered diffraction (EBSD) and X-ray stress analyzer. During the triple solution treatment, size of grain boundary was dramatically decreased by generating a packet from the martensite transformation of residual austenite in the inner part of grain, whereas the hardness increase was not significant.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.106.1-106.1
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• 2012

EBSD(Electron BackScattered Diffraction)분석은 주사전자현미경에서 관찰되는 비교적 넓은 영역의 결정 방위를 측정하여 집합조직을 해석하는 동시에 결정 방위의 변화를 기준으로 결정립계를 구분 지어 미세조직의 정량 분석도 가능하기 때문에 많은 연구자들이 사용하고 있다. 그러나 EBSD의 Kikuchi 패턴은 시편 표면으로부터 30~50nm 깊이 범위의 표면층으로부터 방출되기 때문에 EBSD 분석 결과는 시편의 표면 처리 상태에 크게 영향을 받아 적절한 시편준비법이 요구된다. 시편 준비 과정 중에 생기는 변형층, 산화층이나 오염층이 10nm 이내로 제어되지 못하면 명확한 패턴을 얻지 못하여 분석이 어려운 경우가 많으므로, 시료의 절단과 연마 과정 중에 변형층을 되도록 적게 만들고 표면의 산화나 오염을 최대한 방지해야 한다. 또한 EBSD 분석 특성상 시편을 70도로 기울이기 때문에 시편의 요철이 심하면 볼록한 영역에 의해 오목한 영역의 패턴이 가려져 결정방위 정보를 얻기 힘들다. 이런 이유로 시편을 최대한 평평하게 하고 요철이 생기지 않게 시편 준비를 하는 것이 관건이다. 금속재료의 EBSD 시편준비법으로는 일반적으로 기계적 연마법과 전해연마법이 주로 쓰인다. 경한 석출물이나 개재물이 연한 기지에 분산되어 있는 시편이나 이종 소재 접합재의 경우는 전해연마법을 사용하면 특정 상(혹은 합금)이 먼저 연마되어 큰 단차가 생기거나 석출물에 의해 요철이 심해져서 정량적인 EBSD 분석이 어렵게 된다. 이 연구에서는 시편 준비가 어렵다고 알려진 다상 금속재료에서의 EBSD 분석 사례를 소개한다. Ti-6Al-4Fe-0.25Si 시효처리합금, 알루미늄 기지 복합재료, 마찰교반용접한 알루미늄-타이타늄합금의 EBSD 시편준비법과 그 분석 결과를 고찰한다.

• Journal of Korea Foundry Society
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• v.30 no.6
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• pp.210-216
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• 2010

The aim of this study is to characterize a thixoextruded 7075 Al wrought alloy bar in terms of its isotropic behavior through the optical microscope, mechanical test and electron back scattered diffraction. It is also discussed of the extrudability improvement for 7075 Al wrought alloy by thixoextrusion, with emphasis on controlling thixoextrusion parameters. Hot extrusion shows that the maximum extrusion pressure depends on their characteristics in terms of flow stress and hot workability. In the contrary, thixoextrusion demonstrates that the maximum extrusion pressure is almost uniform regardless of the experimental parameters, such as initial ram speed, die bearing length and thixoextrusion temperature. The hot extruded microstructures become elongated to extrusion direction, while the thixoextruded microstructures are isotropic and homogeneously distributed due to the existence of liquid phase between solid grains during the process. The grain refinement due to dynamic recrystallization during thixoextrusion has been also occurred. Subsequent recrystallization would lead to the strengthening of mechanical properties, as observed in the study. The important point is that the values of tensile, yield strength and elongation of the thixoextruded bar without plastic deformation are similar to those of the hot extruded bar with severe plastic deformation.

• Korean Journal of Materials Research
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• v.21 no.7
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• pp.410-414
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• 2011

This study was carried out to evaluate the microstructures and mechanical properties of a friction stir welded Ni based alloy. Inconel 600 (single phase type) alloy was selected as an experimental material. For this material, friction stir welding (FSW) was performed at a constant tool rotation speed of 400 rpm and a welding speed of 150~200 mm/min by a FSW machine, and argon shielding gas was utilized to prevent surface oxidation of the weld material. At all conditions, sound friction stir welds without any weld defects were obtained. The electron back-scattered diffraction (EBSD) method was used to analyze the grain boundary character distributions (GBCDs) of the welds. As a result, dynamic recrystallization was observed at all conditions. In addition, grain refinement was achieved in the stir zone, gradually accelerating from 19 ${\mu}m$ in average grain size of the base material to 5.5 ${\mu}m$ (150 mm/min) and 4.1 ${\mu}m$ (200 mm/min) in the stir zone with increasing welding speed. Grain refinement also led to enhancement of the mechanical properties: the 200 mm/min friction stir welded zone showed 25% higher microhardness and 15% higher tensile strength relative to the base material.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.6
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• pp.319-326
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• 2022

Direct extrusions of an aluminum 7075 alloy were carried out using 1500 ton machine with and without die cooling system. Cooling of extrusion die has been performed by the flow of liquid nitrogen and controlled by laser thermometer. Billet was 180 mm in diameter and 500 mm in length. The preheating temperatures of billet, container and die were 390℃, 400℃ and 450℃, respectively. Ram speed was kept with 1.25 mm/sec first. The change of ram speed was carried out during extrusion according to the observation of surface defects such as crack or tearing. Extrudates of 8.3 m in length, 100 mm in width and 15 mm in thickness were obtained to observe and analyze surface defects by optical microscopy and EBSD (Electron BackScattered Diffraction). In case of extrusion without die cooling cracks on the surface and tearing in the corner of extrudate occurred in the middle stage and developed in size and frequency during the late stage of extrusion. At the extrusion with die cooling the occurrence of defects could be suppressed on the most part of extrudate. EBSD micrographs showed that cracks and tearings have been resulted from the same origin. Surface defects were generated at the boundaries of grains formed by secondary recrystallization due to surface overheating during extrusion.

• Nuclear Engineering and Technology
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• v.55 no.10
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• pp.3725-3731
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• 2023

Thorium dioxide (ThO₂)-silicon carbide (SiC) composite fuel pellets were fabricated via the spark plasma-sintering (SPS) method to investigate the role of the addition of SiC in enhancing the thermal conductivity of ThO₂ fuel. SiC particles with an average size of 1㎛ in 10 and 15 vol% were used to manufacture the composite pellets. The changes in the composites' densification, microstructure and thermal conductivity were explored by comparing them with pure ThO₂ pellets. The structural and microstructural characterization of the composite pellets has revealed that SPS could manufacture high-quality composite pellets without having any reaction products or intermetallic. The density measurement by the Archimedes principles and the grain size from the electron back-scattered diffraction (EBSD) analysis has indicated that the composites have higher densities and smaller grain sizes than the pellets without SiC addition. Finally, thermal conductivity as a function of temperature has revealed that sintered ThO₂-SiC composites showed an increase of up to 56% in thermal conductivity compared to pristine ThO₂ pellets.

• Journal of Powder Materials
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• v.31 no.1
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• pp.1-7
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• 2024

This study explores the profound impact of varying oxygen content on microstructural and mechanical properties in specimens HO and LO. The higher oxygen concentration in specimen HO is found to significantly influence alpha lath sizes, resulting in a size of 0.5-1 ㎛, contrasting with the 1-1.5 ㎛ size observed in specimen LO. Pore fraction, governed by oxygen concentration, is high in specimen HO, registering a value of 0.11%, whereas specimen LO exhibits a lower pore fraction (0.02%). Varied pore types in each specimen further underscore the role of oxygen concentration in shaping microstructural morphology. Despite these microstructural variations, the average hardness remains consistent at ~370 HV. This study emphasizes the pivotal role of oxygen content in influencing microstructural features, contributing to a comprehensive understanding of the intricate interplay between elemental composition and material properties.

• Archives of Metallurgy and Materials
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• v.64 no.2
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• pp.607-611
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• 2019

Thermal/cold spray deposition were used for additive manufacture of oxide dispersion strengthened (ODS) steel layers. Mechanically alloyed F/M ODS steel powders (Fe(bal.)-10Cr-1Mo-0.25Ti-0.35Y2O3 in wt.%) were sprayed by a high velocity oxygen fuel (HVOF) and cold spray methods. HVOF, as a thermal method, was used for manufacturing a 1 mm-thick ODS steel layer with a ~95% density. The source to objective distance (SOD) and feeding rate were controlled to achieve sound manufacturing. Y₂Ti₂O₇ nano-particles were preserved in the HVOF sprayed layer; however, unexpected Cr₂O₃ phases were frequently observed at the boundary area of the powders. A cold spray was used for manufacturing the Cr₂O₃-free layer and showed great feasibility. The density and yield of the cold spray were roughly 80% and 45%, respectively. The softening of ODS powders before the cold spray was conducted using a tube furnace of up to 1200℃. Microstructural characteristics of the cold sprayed layer were investigated by electron back-scattered diffraction (EBSD), the uniformity of deformation amount inside powders was observed.