• 한국세라믹학회지
• /
• 제28권7호
• /
• pp.549-555
• /
• 1991

Mullite-zirconia (0∼4 mol% yttria) composites were obtained by In-situ sintering of zircon and alumina mixture, and their mechanical properties were studied in conjuction with microstructure observation. Martensitic transformation temperature (Ms) of zirconia dispersed in the mullite matrix decreased with Y2O3 contents and was about 600$^{\circ}C$ for ZrO2 containing 4 mol% Y2O3. On cooling of this composites, tetragonal to monoclinic phase transformation induced microcracks at the grain boundary of mullite matrix. The microcracks seemed to absorb the fracture energy in stress field during mechanical tests. Therefore, toughening mechanisms of this composite were considered to nucleation and extension of microcrack, and crack deflection mechanism due to the difference of thermal expansion coefficient between matrix and dispersed phase.

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 2003년도 춘계학술대회 논문집
• /
• pp.426-429
• /
• 2003

In this study, phase transformation characteristics of Nitinol shape memory alloy with 54.5wt%Ni-45.5wt%Ti were investigated by varying with annealing treatment and cutting conditions through DSC(differential scanning calorimetry). Annealing treatment conditions were considered as heat treated time of 5 min, 15 min, 30 min, and 45 min, heat treated temperature of 40$0^{\circ}C$, 50$0^{\circ}C$, 5$25^{\circ}C$, 55$0^{\circ}C$, 575$^{\circ}C$, $600^{\circ}C$, $700^{\circ}C$, 80$0^{\circ}C$, and 90$0^{\circ}C$, and environmental condition of heat treatment under vacuum or air. Cutting conditions were considered as no cutting, one side cutting, and two side cutting. Tensile test was also conducted on Nitinol shape memory alloy to investigate thermomechanical characteristics by varying with annealing heat treatment histories. According to the results, annealing treatment and cutting conditions were found to significantly affect on phase transformation and thermomechanical characteristics of Nitinol shape memory alloy.

• 한국안전학회지
• /
• 제20권3호
• /
• pp.27-33
• /
• 2005

The shape memory effect in Ti-50.4at.%Ni alloy after solution treatment at 1273K for 2h and aged at 350, 450, $550^{\circ}C$ for 0.5, 1, 1.5, 2, 4, 10hrs had been investigated by differential scanning calorimetry measurement. It was found that ageing in the temperature range of $350^{\circ}C{\sim}550^{\cric}C$ induced complex transformation behavior, involving the R-phase and multiple-stage martensitic transformation. Usually aged Ni-rich NiTi alloys undergo martensitic transformation on cooling from high temperatures in two step : Austenite to R-phase and then R-phase to Martensite (normal behavior). In sample aged at $350^{\circ}C$ two distinct DSC peaks arised giving evidence of intermediate stages of martensite transformation. This results in the nucleation and growth of coherent $Ni_4Ti_3$-precipitate. These explain all features of the evolution of DSC charts during ageing including the number of distinct DS peaks and their positions.

• 한국분말야금학회:학술대회논문집
• /
• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
• /
• pp.469-470
• /
• 2006

[ $TiO_2$ ] nanoparticle was synthesized by the flame method, which was controlled by varying the ratio and flow rate of gas mixtures consisting of oxygen (oxidizer), methane (fuel) and nitrogen (carrier gas). The crystalline phases of $TiO_2$ nanoparticle depended strongly on the temperature distribution in the flame, whereas the morphology was not sensitive. We proved that the anatase phase formed without the phase transformation in the flame and the rutile phase generated through several phase transformations.

• 열처리공학회지
• /
• 제8권1호
• /
• pp.12-23
• /
• 1995

In this a set of constitutive equation relevant to the analysis of thermo-elasto-plastic materials with phase transformation during quenching process was presented on the basis of continuum thermo-dynamic. In calculating the transient thermal stresses, temperature between coolant and specimen(SM45C) surface was determined from the heat transfer coefficient. A calculation was made for specimen with 40mm in diameter quenched in coolant from $820^{\circ}C$ and the results are as follow. Stresses at starting point of transformation always show the maximum tensile value. Reverse of stresses takes place after completion of transformation of inner part at specimen.

• 열처리공학회지
• /
• 제17권5호
• /
• pp.271-277
• /
• 2004

Zr-xNb alloys (x = 0.2, 0.8, 1.5 wt.%) were prepared to study the characteristics of the phase transformation in Zr-Nb system. The samples were heat treated at ${\beta}$-temperature ($1020^{\circ}C$) for 20 min and then cooled with different cooling rate. The microstructures of the specimens having the same compositions were changed with cooling rate and Nb content. The Widmanst$\ddot{a}$tten structure was observed on the furnace-cooled sample. The relationship between ${\alpha}$-Widmanst$\ddot{a}$tten and ${\beta}$-phase was the {0001}${\alpha}$//{110}${\beta}$, <11$\bar{2}$0>//<111>. The ${\beta}$-phase in Widmanst$\ddot{a}$tten structure of Zr-Nb alloys containing Nb more than solubility limit was identified as ${\beta}_{Zr}$ phase which was a stable phase at high temperature. In the water quenched samples, two kinds of martensite structures were observed depending on the Nb-concentration. The lath martensite was formed in Zr-0.2, 0.8 wt.% Nb alloys and the plate martensite having twins was formed in Zr-1.5 wt.% Nb alloy.

• 한국표면공학회지
• /
• 제34권5호
• /
• pp.486-493
• /
• 2001

Partially stabilized zirconia (PSZ) is an attractive material for thermal barrier coating. Zirconia exists in three crystallographic phases: cubic, tetragonal and monoclinic. Especially, the phase transformation of tetragonal phase to monoclinic phase accompanies significant volume expansion, so this transition generally results in cracking and contributes to the failure of the TBC system. Both the plasma sprayed ZrO$_2$-8Y$_2$O$_3$ (YSZ) coat and the ZrO$_2$,-25CeO$_2$,-2.5Y$_2$O$_3$ (CYSZ) coat are isothermally heat -treated at 130$0^{\circ}C$ and 150$0^{\circ}C$ for 100hr and cooled at different cooling rates. The monoclinic phase is not discovered in all the CYSZ annealed at 130$0^{\circ}C$ and 150$0^{\circ}C$. In the 150$0^{\circ}C$ heat-treated specimens, the YSZ contains some monoclinic phase while none exists in the 130$0^{\circ}C$ heat-treated YSZ coat. For the YSZ, the different phase transformation behaviors at the two temperatures are due to the stabilizer concentration of high temperature phases and grain growth. For the YSZ with 150$0^{\circ}C$-100hr annealing, the amount of monoclinic phase increased with the slower cooling rate. The extra oxygen vacancy, thermal stress, and c to t'phase transformation might suppress the t to m martensitic phase transformation.

• 열처리공학회지
• /
• 제4권4호
• /
• pp.34-43
• /
• 1991

A small amount of misch metal and/or Zr was added as a dopant to 70.5wt----Cu-26wt----Zn-3.5wt----Al shape memory alloy in order to study the effect of grain refinement and heat treatments on the transformation behavior, stabilization of martensite, and shape memory ability. It was found that the addition of misch metal and Zr was very effective for reducing the grain size. The fracture mode has been changed from intergranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal and Zr. Aging of the ${\beta}$-phase decreases the $M_s$ temperature, but that of the martensite phase increases the $A_s$ temperature. The hysteresis of transformation temperature ${\Delta}T(A_s-M_s)$ has an increasing tendancy by grain refinement. The crystal structure of martensite was identified as monoclinic structure. As the grain size decreased, martensite stabilization more easily occured and the shape, memory ability has been reduced by the grain size refined.

• Restorative Dentistry and Endodontics
• /
• 제46권4호
• /
• pp.54.1-54.10
• /
• 2021

Objectives: The purpose of this study was to quantify phase transformation after hydrofluoric acid (HF) etching at various concentrations on the surface of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), and to evaluate changes in bonding strength before and after thermal cycling. Materials and Methods: A group whose Y-TZP surface was treated with tribochemical silica abrasion (TS) was used as the control. Y-TZP specimens from each experimental group were etched with 5%, 10%, 20%, and 40% HF solutions at room temperature for 10 minutes. First, to quantify the phase transformation, Y-TZP specimens (n = 5) treated with TS, 5%, 10%, 20% and 40% HF solutions were subjected to X-ray diffraction. Second, to evaluate the change in bond strength before and after thermal cycling, zirconia primer and MDP-containing resin cement were sequentially applied to the Y-TZP specimen. After 5,000 thermal cycles for half of the Y-TZP specimens, shear bond strength was measured for all experimental groups (n = 10). Results: The monoclinic phase content in the 40% HF-treated group was higher than that of the 5%, 10%, and 20% HF-treated groups, but lower than that of TS-treated group (p < 0.05). The 40% HF-treated group showed significantly higher bonding strength than the TS, 5%, and 10% HF-treated groups, even after thermal cycling (p < 0.05). Conclusions: Through this experiment, the group treated with SiO₂ containing air-borne abrasion on the Y-TZP surface showed higher phase transformation and higher reduction in bonding strength after thermal cycling compared to the group treated with high concentration HF.

• 한국결정성장학회지
• /
• 제18권1호
• /
• pp.15-21
• /
• 2008

탄화규소(SiC)는 산화저항성, 내식성, 고온 강도 및 열전도 특성 등의 기계적 특성이 매우 우수한 재료로 알려져 있지만, 강한 공유결합성으로 인하여 그 소결이 매우 어려운 재료이다. 본 연구에서는 치밀한 탄화규소 소결체를 제조하기 위하여 카본 및 보론을 소결 첨가제로하여 열간 가압 소결법을 적용하여 탄화규소 소결체를 제작하여 그 특성을 평가하였다. 카본의 첨가는 탄화규소의 소결을 촉진하는 역할을 하여 비정상 입성장을 억제하기 때문에 미세하고 균일한 미세구조를 형성하였기 때문에 탄화규소 소결체의 기계적 특성을 향상시키는 것을 확인 할 수 있었다. 또한 차본의 첨가는 소결 중 보론의 첨가에 의해 발생하는 탄화규소의 6H 구조에서 4H 구조로의 상전이를 억제함을 알 수 있었다.