• The Korean Journal of Ceramics
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• 제3권3호
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• pp.208-212
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• 1997

Shear-induced phase transformation behavior of chemically stabilized $\beta$-cristobalite was studied by the fiber push-out technique. To obtain the critical grain size for phase transformation, the hot-pressed polycrystalline $\beta$-cristobalite, which was used as the interphase between fiber and matrix, was annealed at $1300^{\circ}C$ for 10h. Two types of fibers, mullite and sapphire fiber, were used in this study. Debonding between mullite fiber and cristobalite interphase occurred at a critical load of 230 MPa. Static friction and fiber sliding were continuously followed by debonding. Shear-induced transformation induced cracks in the cristobalite interphase at the debonding stage. In the case of the sapphire fiber, the debonding occurred at a lower load of 180 MPa due to the residual stress in the interface caused by the difference in thermal expansion coefficients between the fiber and the cristobalite interphase. The load was insufficient for shear-induced phase transformation.

• 한국재료학회지
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• 제24권10호
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• pp.520-525
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• 2014

Effects of Cu and B on effective grain size and low-temperature toughness of thermo-mechanically processed high-strength bainitic steels were investigated in this study. The microstructure of the steel specimens was analyzed using optical, scanning, and transmission electron microscopy; their effective grain size was also characterized by electron back-scattered diffraction. To evaluate the strength and low-temperature toughness, tensile and Charpy impact tests were carried out. The specimens were composed of various low-temperature transformation products such as granular bainite (GB), degenerated upper bainite (DUB), lower bainite (LB), and lath marteniste (LM), dependent on the addition of Cu and B. The addition of Cu slightly increased the yield and tensile strength, but substantially deteriorated the low-temperature toughness because of the higher volume fraction of DUB with a large effective grain size. The specimen containing both Cu and B had the highest strength, but showed worse low-temperature toughness of higher ductile-brittle transition temperature (DBTT) and lower absorbed energy because it mostly consisted of LB and LM. In the B-added specimen, on the other hand, it was possible to obtain the best combination of high strength and good low-temperature toughness by decreasing the overall effective grain size via the appropriate formation of different low-temperature transformation products containing GB, DUB, and LB/LM.

• The Korean Journal of Ceramics
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• 제3권2호
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• pp.116-123
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• 1997

A chemically stabilized $\beta$-cristobalite, which is stabilized by stuffing cations of $Ca^{2+}$ and $Al^{3+}$, was prepared by a solution-polymerization route employing Pechini resin or PVA solution as a polymeric carrier. The polymeric carrier affected the crystallization temperature, morphology of calicined powder, and particle size distribution. In case of the polyvinyl alcohol (PVA) solution process, a fine $\beta$-cristobalite powder with a narrow particle size distribution (average particle size : 0.3$\mu\textrm{m}$) and a BET specific surface area of 72 $\m^2$/g was prepared by an attrition-milling for 1 h after calcination at 110$0^{\circ}C$ for 1h. Wider particle size distribution and higher specific surface area were observed for the $\beta$-cristobalite powder derived from Pechini resin. The cubie(P1-to-tetraganalb) phase transformation in polynystalline $\beta$-cristobalite was induced at approximately 18$0^{\circ}C$. Like other materials showing transformation toughening, a critical size effect controlled the $\beta$-to-$\alpha$ transformation. Densifed cristobalite sample had some cracks in its internal texture after annealing. The cracks, occurred spontaneoulsy on cooling, were observed in the sample with an average grain sizes of 4.0 $\mu\textrm{m}$ or above. In case of the sintered cristobalite having a composition of CaO.$2Al_2O_3$.40SiO$_2$, small amount of amorphous phase and slow grain growth during annealing were observed. Shear stress-induced transformation was also observed in ground specimen. Cristobalite having a composition of CaO.2Al2O3.80SiO2 showed a more sensitive response to shear stress than the CaO.$2Al_2O_3$.40SiO$_2$ type cristobalite. Shear-induced transformation resulted in an increase of volume about 13% in $\alpha$-cristobalite phase on annealing for above 10 h in the case of the former composition.

• 한국분말재료학회지
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• 제13권5호
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• pp.360-365
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• 2006

The phase stability of tetragonal phase in Y-TZP was investigated in terms of the distribution of grain sizes and heat-treating atmosphere. Y-TZP with various grain sizes were prepared using duration time at $1600^{\circ}C$ as experimental parameter. Accumulated grain size distributions were built from the SEM micrographs and the amount of tetragonal phase were measured using XRD. Both results were compared to determine the critical grain size before and after heat-treatment in vacuum. The critical grain size drastically decreased compared with the small increase of average grain size due to the autocatalytic effect which critically affects the tetragonal to monoclinic phase transformation. After heat-treatment in reductive atmosphere critical grain size relatively increased due to the stabilization of tetragonal phase. The formation of oxygen vacancies during heat-treatment was ascribed to the increase of stability.

• 열처리공학회지
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• 제7권4호
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• pp.277-287
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• 1994

The effect of grain size on the tensile properties and fatigue behavior of austenitic high Mn steel has been investigated. The recrystallized austenite grain size of the cold rolled high Mn steel was increased as the annealing temperature increased from $600^{\circ}C$ to $1000^{\circ}C$. Larger austenite grain size decreased the yield strength and the tensile strength, and increased the uniform elongation due to transformation of some austenite into twins or E-martensite phase during deformation. Austenite grain refinement increased the tendency to form dislocation cells, instead. The specimen annealed at $1000^{\circ}C$ with large grain size showed lower fatigue crack propagation rate in low ${\Delta}K$ region due to rougher fracture surface caused by formation of deformation twins during fatigue at the crack tip region.

• 한국세라믹학회지
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• 제37권5호
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• pp.418-424
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• 2000

Effects of Ar ion damage prior to the phase transformation from pyrochlore to perovskite structure of PZT thin films have been investigated. As the degree of damage increased by increasing the acceleration voltage in the ion mass doping system, the phase transformation temperature decreased such that the temperature could be lowered down to 550$^{\circ}C$ when the film was damaged at 15 kV for 5 minutes. When the film was damaged prior to the heat treatment grain size of the perovskite thin films became less than 300${\AA}$. It turned out that relatively high value of the remanent polarization (about 30${\mu}$C/$\textrm{cm}^2$) as well as improvement of the fatigue characteristics to a large extent is closely related to the fine grain size of thus obtained PZT films.

• 한국분말재료학회지
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• 제17권2호
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• pp.154-159
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• 2010

In this study, the sintering behavior of (Nd, Dy)-Fe-B powder which fabricated by strip-casting was investigated with various sintering temperatures and holding times. The relative density over 99% could be obtained by both sintering at $1070^{\circ}C$ for 1h and sintering at $970^{\circ}C$ for 20h. The grain growth was observed in sintered specimen at $1050^{\circ}C$ compared to one at $970^{\circ}C$. The isothermal sintering process below $1000^{\circ}C$ led to suppress grain growth showing the improved magnetic properties. The phase transformation of Nd-rich was confirmed by X-ray diffraction pattern.

• 한국세라믹학회지
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• 제42권2호
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• pp.140-144
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• 2005

The microstructures and mechanical properties of Tetragonal Zirconia Polycrystals (TZP) sintered bodies, which made by pressureless and spark plasma sintering techniques, were investigated using XRD, SEM, and TEM techniques. In the spark plasma sintered samples, the TZP grains were equiaxed type including many sub-grain boundaries regardless of sintering conditions. The biaxial strength of TZP having an average of 80 nm grains in diameter was high in value with 1025 MPa, but fracture toughness showed a low value due to the absence of a fracture toughening mechanism such as transformation toughening. In the Pressureless Sintered (PLSed) samples, the grain size of TZP was strongly dependent on the sintering temperature; i.e., it gradually increased as the sintering temperature increased. The value of fracture toughness increased as the grain size increased by the stress-induced phase transformation and Borne crack deflection.

• Journal of Welding and Joining
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• 제33권2호
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• pp.1-7
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• 2015

This article describes the basic technical concepts for applying the friction stir welding (FSW) process to titanium and its alloys. Titanium and its alloys are demanding applications of FSW. During FSW, a protective atmosphere is needed at the welding region to prevent the joints from oxidation due to the absorption of interstitial elements (O, N, and H) at high temperature. The process parameters for FSW have great influence on the microstructure and properties of the joints. No phase transformation occurred in CP Ti because FSW was achieved below the ${\beta}$-transus temperature. Therefore, the mechanical properties of the joints with CP Ti were governed by recrystallization and grain refinement. Furthermore, the strong crystallographic texture indicating <0001>//ND formed in the stir zone. On the other hands, the phase transformation occurred in Ti-6Al-4V alloy because the process temperature reached above ${\beta}$-transus temperature. For this reason, the mechanical properties of the joints with Ti-6Al-4V alloy were altered by not only recry stallization and grain refinement but also phase transformation during FSW. Engineers who want to get sound FSW joints with Ti-6Al-4V alloy have to pay attention to the control about process conditions.

• 한국산학기술학회논문지
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• 제4권3호
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• pp.155-158
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• 2003

자동차 부품 T/P housing과 valve에 사용되는 S20C 계열의 강에 Mn과 V 첨가에 따른 냉각속도의 영향과 조직의 거동, 단조 시 변형속도와 경도 변화를 조사하였다 냉각속도를 증가하면 오스테나이트 결정립에 의한 변태 지연에 의해 변태개시 온도가 저하되었다 미소한 Mn 함량의 증가는 냉각속도를 민감하게 하여 변태개시 온도를 감소 시켜 결정립 미세화를 일으킨다. 또한, 성형압이 클수록 내부보다 표면이 큰 경도값이 나타났다.