• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 한국소성가공학회 2005년도 춘계학술대회 논문집
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• pp.295-298
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• 2005

High temperature deformation behavior of $\alpha\;and\;\beta$ phase of Ti-6Al-4V was investigated within the framework of a self-consistent approach at various temperature ranges. To examine the flow behavior of u-phase, Ti-7.0Al-1.5V alloy was used, whose chemical composition is close to that of the $\alpha$ phase in Ti-6Al-4V at hot working temperatures. The flow stress of $\beta$ phase was predicted by using self-consistent approach. The flow stress of $\alpha$ phase was higher than that of $\beta$ phase above $750^{\circ}C$, while the $\beta$ phase revealed higher flow stress than a phase at $650^{\circ}C$. It was found that the relative strength and strain rate ratio between $\alpha\;and\;\beta$ phase significantly varied with temperature. From this approach, the mode for grain matrix deformation was proposed as a mixed type of both iso-stress and iso-strain rate modes.

• Journal of the Korean Ceramic Society
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• 제34권7호
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• pp.713-721
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• 1997

Molecular dynamic (MD) simulations with new interatomic potential function including the covalent bond were performed on the phase transition of $\alpha$-quartz-type GeO2 under high pressure. The optimized crystal structure and the pressure dependence of the lattice constant showed higher reproducibility than the previous models and were in very good agreement with the experimental data. A phase transition of $\alpha$-quartz and $\alpha$-quartz-type GeO2 by simulation was found approximately 24 GPa and 6-7 GPa, respectively. This phase transition involved an abrupt volume shrinkage and showed 4-6 coordination mixed structure with the increasing in the coordination number of cation.

• Journal of the Korean Ceramic Society
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• 제34권6호
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• pp.553-560
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• 1997

A fine $\alpha$-Al2O3 powder was prepared by sol-gel process for membrane application. And it was carried out by adding 1.5wt% $\alpha$-Al2O3 powders(mean size : 87 nm) as seeds to the prepared sols and by controlling the heating schedule (the heating rate and the soaking time) to prevent the microstructural change, which occured during $\theta$-to $\alpha$Al2O3 phase transformation. The seeded $\alpha$-Al2O3 particles acted as the heterogeneous nucleation sites for the $\alpha$-Al2O3 nucleation during the transformation of $\theta$- to $\alpha$-Al2O3 and resulted in increasing the driving force of phase transformation to activate the formation of $\alpha$-Al2O3 phase at 82$0^{\circ}C$. By $\alpha$-Al2O3 seeding and controlling of heating condition the phase transformation of $\theta$- to $\alpha$-Al2O3 was accomplished at low temperature and the grain growth process was depressed. Therefore, the unsupported membrane could be fabricated in $\alpha$-Al2O3 . The average diameter of pores in the fabricated membrane was 7 nm and the porosity was 47%.

• Journal of the Korean Ceramic Society
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• 제25권5호
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• pp.502-508
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• 1988

The $\alpha$ to $\beta$ phase transformation of $\alpha$-Si3N4 whisker and related microstructural changes have been investigated. When only $\alpha$-Si3N4 whisker was heat treated in the range 1650~175$0^{\circ}C$, the $\alpha$ to $\beta$ phase transformation occured. In this case, it eas suggested that the oxygen content in $\alpha$-Si3N4 whisker affected the transformation behavior. Although $\alpha$-Si3N4 whisker with Si was heat treated under the same condition, however, the variation of $\beta$- fraction had a similar tendency with heat treating time. Therfore, it was considered that the oxygen content in $\alpha$-Si3N4 whisker affected the transformation behavior dominently rather than the content of added Si. The added $\beta$ phase did not affect the transformation behaviro of $\alpha$-Si3N4 whisker.

• Journal of Powder Materials
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• 제10권2호
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• pp.118-122
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• 2003

Alpha-SiAlON ceramics having various compositions and modifying cations were investigated with respect to their phase stability, transformation kinetics. and resulting microstructures. Each composition was heat treated at 150$0^{\circ}C$ for 1h and measured the $\alpha$-SiAlON transformation. The phase-boundary composition in the single-phase $\alpha$-SiAlON region showed sluggish transformation from $\alpha$-$Si_3N_4$ to $\alpha$-SiAlON compared to the phase-center composition in the diagram. Using the different rare earth modifying cations, dependence of transformation kinetics on the phase stability in a fixed composition was also explained. By changing size of the stable u-phase region with exchanging cations, systematic change in transformation was observed. Transformation rate of $\alpha$-SiAlON at low temperature has an important role on controlling the final microstructure. Less transformation gives more chances to develop elongated grain in the microstructure.

• Journal of Technologic Dentistry
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• 제28권1호
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• pp.27-41
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• 2006

The physical property and phase transformation in a commercial dental casting high gold alloy was investigated as a function of ageing temperature and time using microvickers hardness tester, X-ray diffraction, optical and electron microscopy and EPMA analyser. 1. With increasing ageing time, the hardness of solution-treated gold alloys increased slowly at the initial stage of ageing treatment at an ageing temperature of $300{\sim}400^{\circ}C$, and it reached a maximum value of hardness at the medium stage. Finally, it decreased gradually during further ageing. The maximum value of hardness at was similar with that of the conventional materials and suitable for using as the crown & bridge. 2. During isothermal ageing at a temperature range of $300{\sim}400^{\circ}C$, three phases consisting of the Au-rich ${\alpha}_1$phase with a face-centered cubic structure, the Pt3Zn ${\alpha}_2$phase with an ordered AuCu3(L12) type(f.c.c.) and the Pt-rich ${\alpha}_3$phase with face-centered cubic structure in solution-treated gold alloys were transformed into different three phases consisting of the ${\alpha}_1$phase, the ${\alpha}_3$phase and the PtZn $\beta$phase with an ordered AuCu I(L10) type. 3. The hardening of gold alloys was attributed to the lattice strains of the matrix resulting from the transformation of the ${\alpha}_2$phase to the $\beta$phase. 4. The softening of gold alloys during over-ageing was attributed to the coarsening of the nodules consisting of the $\beta$phase and ${\alpha}_1$matrix.

• Journal of Powder Materials
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• 제5권1호
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• pp.15-21
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• 1998

The effect of $\alpha$/$\beta$ phase on the mechanical properties and contact damage of silicon nitrides $Si_3N_4$) was investigated. Silicon nitride materials were prepared from two starting powders, at selective increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse $\alpha$-phase powder, essentially equiaxed $\alpha$-$Si_3N_4$ grains, with limited, slow transformation to $\beta$-$Si_3N_4$ grain; (ii) from relatively fine $\alpha$-phase powder, a more rapid transformation to $\beta$-$Si_3N_4$, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of $\alpha$/$\beta$ phase ratios, grain sizes, and grain shapes. Fracture strength, hardness, and toughness were measured, and contact damage and strength degradation after indentation were investigated by Hertzian indentation using spherical indenter. A brittle to ductile transition in $Si_3N_4$ depended on $\alpha$/$\beta$ phase ratio as well as grain size. Silicon nitride with elongated $\beta$ grains showed a superior, contact damage resistance.

• Korean Journal of Materials Research
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• 제2권6호
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• pp.389-397
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• 1992

Workability and aging behavior of Mg-Li-Al alloys hate been investigated with variation of Li and Al contents. It was found that ${\alpha}$(HCP) + ${\beta}$(BCC) two phase structure reveals better workability than ${\alpha}$(HCP) single phase structure. Increase of workability is attributed to the additional slip on pyramidal and prism planes besides the slip on basal plane of HCP Mg. Workability of ${\alpha}$+${\beta}$ two phase structure is improved with increased amount of Al. Among alloys studied in the present study, reduction limit up to 62 % was obtained for Mg-8.08Li-4.26Al. MgL$i_2$Al(${\theta}$) aging precipitation was observed in ${\alpha}$+ ${\beta}$ two phase structure, while it was not in a single phase structure. Result of microhardness reveals peak hardness for ${\alpha}$ +${\beta}$ two phase structure due to $\theta$ precipitate in ${\theta}$ phase. In ${\alpha}$+${\beta}$ two phase Mg-Li-Al alloys, hardness increased with increasing amount of Al. It is believed that addition of Al raised the amount Al partition in u phase and ${\theta}$ precipitation in ${\beta}$ phase.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• 제14권8호
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• pp.629-633
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• 2001

Electrical properties of the $\alpha$-In$_2$Se$_3$ single crystals grown by use of the Bridgman technique were examined in the transition temperature range between $\alpha$-phase and $\beta$-phase. $\alpha$-In$_2$Se$_3$ single crystal has ' the rhombohedral structure and lattice constants are a=4.025 $\AA$, c=28.771 $\AA$ in c-axis. The transition temperatures of the stoichiometric $\alpha$-In$_2$Se$_3$ single crystal is 198.8$^{\circ}C$ according to the specimens. The temperature of $\alpha$longrightarrow$\beta$ phase transition decreased but the temperature of $\beta$longrightarrow$\alpha$ phase transition increased as the number of heating-cooling cycle increased.

• Transactions on Electrical and Electronic Materials
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• 제13권3호
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• pp.136-138
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• 2012

The electrical characteristics of complex films composed of poly(ethylene oxide) (PEO) and urea as a function of the urea concentration were examined in this study. Moreover, their structural characteristics were also compared. Depending on the urea concentration, the structural phases were classified as PEO+${\beta}$-phase composite, ${\beta}$-phase+${\alpha}$-phase composites, or ${\alpha}$-phase composite+urea. At urea concentrations below ~0.064 M, the ${\beta}$-phase was dominant in the complex film. Moreover, the conductance increased rapidly with an increase in the urea concentration. For urea concentrations ranging from ~0.064 to ~0.25 M, the ${\beta}$-phase was gradually substituted by the ${\alpha}$-phase. As the film was composed entirely of the ${\alpha}$-phase at urea concentrations greater than ~0.25 M, its conductance was decreased. In this study, the electrical characteristics observed for the different phases are analyzed and discussed.