• 한국분말재료학회지
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• 제17권1호
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• pp.52-58
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• 2010

Bulk nanostructured metallic materials are generally synthesized by bottom-up processing which starts from powders for assembling bulk materials. In this study, the bottom-up powder metallurgy and High Pressure Torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. After the HPT process at 473K, the disk samples reached a steady state condition when the microstructure and properties no longer evolve, and equilibrium boundaries with high angle grain boundaries (HAGBs) were dominant. The well dispersed alumina particles played important role of obstacles to dislocation glide and to grain growth, and thus, reduced the grain size at elevated temperature. The small grain size with HAGBs resulted in high strength and good ductility.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 추계학술대회 논문집
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• pp.452-455
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• 2008

In the present study, blow forming characteristics of commercially roiled AZ31 alloy sheets were investigated. Two different kinds of AZ31 sheets were originally fabricated by using direct casting and strip casting methods respectively. Both sheets have similar grain sizes of about $7{\mu}m$ with a relatively equiaxed structure after rolling. A series of tensile tests were carried out to get flow behavior in terms of temperature and strain rate. Also, grain size effect was investigated by annealing as-received sheet at elevated temperatures. Elongation increased with temperature increment as well expected. However, the differences in tensile test condition did not give much difference in elongation even at the temperature range where a large elongation would be expected with such as fine grain of $7{\mu}m$. Blow forming experiments showed that forming condition did not result in higher difference in dome height. However, the interesting feature from this study was that formability of this AZ31 alloy got different with stress condition. Firstly, biaxial stress condition might result in lower temperature and strain rate dependencies compared to uniaxial tension results for both DC and SC sheets. Secondly, DC showed slower grain growth in uniaxial tension than in biaxial stress state while SC has much higher grain growth rage in uniaxial tension than in bulging.

• Nuclear Engineering and Technology
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• 제35권1호
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• pp.14-24
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• 2003

This work investigates the effects of Cr$_2$O$_3$ and oxygen potential on grain growth and densification of UO$_2$ pellets. Powder mixtures of UO$_2$ and 0.03-0.4wt% Cr$_2$O$_3$ were pressed and sintered in 3 different gas atmospheres: the $H_2O$-to-H$_2$ ratios were 5$\times$10$^{-4}$ , 1$\times$10$^{-2}$ and 3$\times$10$^{-2}$ In the first gas atmosphere the Cr$_2$O$_3$ contents below 0.2 wt% have an insignificant effect on grain size, but the Cr$_2$O$_3$ contents more than 0.3 wt% promote grain growth in the inner zone of a pellet but not in the outer zone. In both the second and third atmospheres, the grain size increases with the Cr$_2$O$_3$ content. With the same level of Cr$_2$O$_3$ content the grain size is larger in the second atmosphere than in the third. Sintering behavior and developed microstructure are discussed in terms of the reduction of C$r^2$O$^3$ to Cr, the dissolution of C$r^2$O$^3$ in UO$_2$, and liquid phase sintering.

• 열처리공학회지
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• 제13권4호
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• pp.239-245
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• 2000

Effect of tin and iron content on the recrystallization behavior of temary Zr-0.8Sn-x%Nb alloys were studied. The specimens with 0.1, 0.2 and 0.4 wt.% of iron were prepared under various annealing temperatures from $400^{\circ}C$ to $800^{\circ}C$ and times from 30 to 5000 minutes after vacuum arc remelting. The recrystallization behavior was observed using a polarized optical microscope, TEM and micro-vickers hardness tester. The recrystallization temperature of the alloys slightly increased with iron content due to increase of activation energy. The grain growth of the alloys with 0.1 and 0.2 wt.% of iron occured rapidly, however, that of the alloys with 0.4 wt.% iron was gradually retarded due to precipitation. The hardness of the alloy with a high iron slightly increased by the precipitation of beta phase after annealing at $800^{\circ}C$.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 1996년도 추계학술강연 및 발표대회 강연 및 발표논문 초록집
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• pp.25-25
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• 1996

• 한국재료학회:학술대회논문집
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• 한국재료학회 1998년도 추계학술발표강연 및 논문개요집
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• pp.92-92
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• 1998

• 한국결정성장학회지
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• 제32권5호
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• pp.175-182
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• 2022

MLCC(Multi-Layer Ceramic Capacitor)의 유전체 층에 사용되는 BaTiO₃는 유전 특성의 온도안정성을 향상시키기 위해 첨가제로서 희토류 및 Mg를 사용한다. 이러한 첨가제는 소결 중 입자성장 및 치밀화 거동, 결국 유전 특성에 지대한 영향을 주게 되므로 조성에 따른 미세구조 발현 양상을 살펴보는 것이 중요하다. 본 연구는 95BaTiO₃-1Dy₂O₃-2MgO-2SiO₂(mol%)의 조성에서 온도 변화에 따른 결정구조, 입자성장 및 밀도 변화를 관찰하고 이러한 변화가 유전 상수에 미치는 영향을 관찰하였다. 1200~1300℃의 온도범위에서 소결 온도가 증가함에 따라, 평균 입도는 눈에 띄게 커지는 반면 밀도의 변화는 미미하여 입자크기가 주요한 미세구조적 요소임을 밝혔다. 본 실험에서 관찰된 입자크기의 온도의존성은 기존 입자성장 이론에서 설명한 온도 변화에 따른 입자성장 거동의 변화양상과 잘 부합하였으며, 이러한 이해는 향후 희토류가 첨가된 BaTiO₃에서 유전 특성 향상을 위한 소결 미세구조 제어에 유용하게 활용될 수 있을 것이다.

• 한국전기전자재료학회논문지
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• 제23권11호
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• pp.875-879
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• 2010

In this study, we have characterized the roles of $Cr_2O_3$ on the sintering and electrical properties of ZnO. The densification and grain growth of Cr-doped ZnO (ZCr) system was mainly influenced by Cr contents. In the beginning of sintering, the densification of ZnO was retarded as reducing the Zni concentration in ZnO lattice with Cr doping. And the densification and grain growth of ZnO was more retarded due to a formation of spinel phase with increasing the Cr contents. ZCr system revealed varistor behavior with nonlinear coefficient $\alpha$ of 3~23 depending on the sintering temperature, implying double Schottky barrier formation on the grain boundary of ZnO. Especially the best varistor characteristics should be developed with 0.1~0.5 at% Cr contents and under $1100^{\circ}C$ in ZCr systems.

• 한국결정성장학회지
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• 제32권4호
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• pp.136-142
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• 2022

Dy₂O₃ 첨가량에 따른 BaTiO₃의 결정구조, 입자성장 거동 및 유전특성에 대해 연구하였다. 고상합성법으로 (100-x) BaTiO₃-xDy₂O₃(mol%, x = 0, 0.5, 1.0, 2.0) 비율로 합성하고, 공기 중 1250℃에서 2시간 동안 소결하였다. Dy₂O₃가 첨가되면서 소결체의 결정구조는 정방정계 구조에서 입방정계 구조로 전이되어 tetragonality(c/a)가 감소하였다. 또한, Dy₂O₃가 첨가 시 Ba₁₂Dy_4.67Ti₈O₃₅은 이차상이 확인되었다. Dy₂O₃의 첨가량이 증가할수록 소결 후 평균입자의 크기가 감소하고 비정상 입자성장 거동을 보였다. 이를 통해 Dy₂O₃가 첨가된 BaTiO₃의 입자성장은 이차원 핵생성 및 성장에 의해 입자성장이 일어나고 계면 반응이 지배적인 것으로 판단할 수 있다. 또한, 결정구조 및 미세구조와 유전특성과의 상관관계에 대해서 고찰하였다.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.348-349
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• 2006

The plastic deformation behavior of cemented carbides is related to the WC grain boundary strength. Ab initio calculations predict that Co and Mn segregate to WC/WC grain boundaries. To experimentally study the effect of Mn, a WC-Co-Mn material was manufactured and compared to a WC-Co material. The microstructure was studied using scanning electron microscopy (SEM), including electron backscatter diffraction (EBSD). Special attention was paid to the WC grain size and the frequency of special low-energy grain boundaries. Mn was found to have negligible effect on both the WC grain growth and the fraction of $\sum2$ WC/WC boundaries in the as-sintered material.