Y. Waku

Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro-pulling-down method

Al2O3/ZrO2 eutectic crystal fibers containing various amounts of Y2O3 were grown by the micro-pulling-down method. The eutectic microstructures and some mechanical properties were investigated as a function of growth rate and doping amount of Y2O3. Doped with Y2O3, Al2O3/ZrO2 eutectic fibers 0.3–2 mm in diameter and 500 mm in length have been grown over the range of pulling rate 0.1–15 mm/min. The dominant zirconia phase was changed from monoclinic to cubic as the doping amount of Y2O3 increased. The eutectic microstructures were oriented to a common direction which depended on the growth rate and doping amount of Y2O3. The uniform lamellar structure obtained at a lower growth rate below 1 mm/min transformed to an ellipsoidal cellular structure at a growth rate above 10 mm/min, and circular or triply facetted cellular patterns were observed at intermediate growth rates. The intercellular spacing increased as the doping amount of Y2O3 increased. The hardness value reached 20 GPa for 9 mol% Y2O3 doped fibers grown at a rate of 15 mm/min. The highest tensile strength reached 2000 MPa at room temperature and 560 MPa at 1500°C for the 3 mol% Y2O3 doped fibers grown at a rate of 15 mm/min.

Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites

New Al2O3/Y3Al5O12(YAG)/ZrO2 ternary Melt Growth Composites (MGCs) with a novel microstructure have been fabricated by unidirectional solidification. These MGCs displayed superior high-temperature strength characteristics. The flexural strength increases progressively in the range 650–800 MPa with a rise in temperature from room temperature up to 1873 K. These excellent high-temperature characteristics are closely linked to such factors as: a microstructure consisting of three-dimensionally continuous and complexly entangled single-crystal Al2O3 with a hexagonal structure, single-crystal YAG with a garnet structure and fine ZrO2 with a cubic structure; characteristic dimensions of the microstructure of Al2O3/YAG/ZrO2 ternary eutectic ceramics of around 2–3 μm for YAG, around 2–3 μm for Al2O3 and around 0.4–0.8 μm for ZrO2; and the fact that no amorphous phase is formed at interfaces between any of the phases.

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al2O3/Y3Al5O12 (YAG) or Al2O3/Er3Al5O12 (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al2O3 phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10−4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al2O3/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.

Phase identification of Al2O3/RE3Al5O12 and Al2O3/REAlO3 (RE = Sm-Lu, Y) eutectics

Abstract Growth of various kinds of eutectic fibers based on Al 2 O 3 and oxides of Y and rare earths from Sm to Lu using micro-pulling-down (μ-PD) method was investigated. The effect of rare-earth element substitution on growth, microstructure and mechanical properties are discussed. Eutectic materials were classified into Al 2 O 3 /garnet system and Al 2 O 3 /perovskite system types at the boundary between Gd and Tb. Al 2 O 3 /garnet eutectic fibers showed superior high-temperature strength properties. This is the first systematic study of the characteristics of these eutectic materials.

Growth of Al2O3/Y3Al5O12 Eutectic Fiber by Micro-Pulling-Down Method and Its High-Temperature Strength and Thermal Stability

Fiber growth of Y3Al5O12 (YAG) matrix composite reinforced with the sapphire phase was investigated to manufacture sapphire/YAG eutectic composites using the micro-pulling-down (µ-PD) method. Both the sapphire and YAG phases were crystalline. We have controlled the fiber diameter from 200 µm to 2 mm. The maximal length was about 500 mm. The eutectic fibers have superior high-temperature strength properties, 580 MPa at 1500°C. In the tensile test at this temperature, the fiber showed considerable (about 10%) plastic deformation due to dislocation motion. The microstructure of this eutectic fiber has a very high thermal stability. No grain growth was observed after even 75 h of heat treatment at 1500°C in air atmosphere.

Selection of eutectic systems in Al2O3–Y2O3 ceramics

Abstract There are two eutectic reactions in the Al203–rich portion of the AI2O3-Y2O3 pseudo–binary system; one is the equilibrium A1203–YAG eutectic reaction at 1826%C, and the other is the metastable A1203–YAP eutectic reaction at 1702%C. Selection of the A1203–YAG and the A1203–YAP eutectics was examined in terms of cooling rate, nucleation temperature and maximum melt temperature. When the melt was cooled from 2100%C at any cooling rate, it always nucleated below the Al203–YAP eutectic temperature, therefore the Al203–YAP eutectic was selected. The Al203–YAG eutectic was selected when the melt was cooled from 1900%C at a cooling rate of less than 1 K s-1. The selection of the two eutectic systems was determined by the nucleation temperature, although the maximum holding temperature of the melt and the cooling rate significantly affected the nucleation temperature. The structure of the melt, such as coordination of oxygen and chemical order when being heated to 2100%C may affect the nucleation behavior.

A novel oxide composite reinforced with a ductile phase for very high temperature structural materials

Abstract  The strength of nearly all metallic and ceramic polycrystalline materials drops off rapidly with increases in temperature. Here we have developed a strong and tough oxide composite reinforced with a ductile phase at 1873 K with a different kind of microstructure, made by unidirectional solidification of an Al2O3-Y3Al5O12(YAG)-ZrO2 ternary eutectic mixture. This composite has a microstructure in which continuous networks of single-crystal Al2O3, single-crystal YAG and single-crystal c-ZrO2 interpenetrate without grain boundaries. No amorphous phases are observed at the interfaces between these phases and relatively compatible interfaces are formed. This material displays plastic deformation at 1873 K indicating high yielding flexural strength. This composite’s average flexural strength at 1873 K of around 850 MPa – approximately 57 times higher than that of the sintered composite with the same composition, and its ductile fracture at 1873 K have never been found up to now. Consequently several useful applications can be considered in mechanical engineering.

A jelly-like ceramic fiber at 1193 K

Abstract A high-strength ceramic Al31Gd9O60 continuous fiber with a fiber diameter of about 20 µm and an amorphous structure could be made successfully by using the melt extraction method. This fiber can be freely shaped by viscous flow deformation in the supercooled liquid state (about 1193 K). The fiber strength is about 2 GPa and this strength is maintained up to around 973 K. A high-strength ceramic continuous fiber with a uniform GdAlO3 nanocrystalline in an amorphous matrix can also be obtained with a suitable crystallization from the amorphous state by heat treatment. The heat resistance, Young’s modulus, and other properties are therefore improved. The amorphous ceramic fiber is promising as a ceramic that can be easily shaped at a relatively low temperatures (around 1193 K) in agreement with temperature range of superplastic processing of Ti alloys.

Strain determination of an eutectic single crystal composite Al2O3/Y3Al5O12 (YAG) by neutron diffraction techniques at reactor and pulsed sources

Abstract Bragg diffraction angle analysis with a high-resolution strain diffractometer and time-of-flight analysis with backscattering powder diffractometer were complementarily applied to neutron residual strain investigations in an ultra-high temperature resistant composite of eutectic two-phase Al2O3/Y3Al5O12 (YAG) single-crystal. It was found that the YAG phase was in tension and the Al2O3 phase was in compression which resulted in strains ∊ in the range of ∼10−4 at room temperature, when the measured lattice spacings of the sintered YAG and sintered Al2O3 were used as the reference values of strain-free materials. Discussions are given on the methodological points of both analyses with respect to evaluation of a strain value for each phase in the two-phase single crystal materials.

An amorphous ceramic Al32.4Er7.6O60 fiber with large viscous flow deformation and a high-strength nanocrystallized ceramic fiber

An amorphous ceramic Al32.4Er7.6O60 continuous fiber with a diameter of about 20 μm could be made successfully by using the melt extraction method. This fiber shows large viscous flow deformation at the supercooled liquid state (about 1273 K). The fibers tensile strength is about 900 MPa and this strength is maintained up to around 1100 K. A high-strength continuous ceramic fiber with a uniform Er3Al5O12 nanocrystalline phase in an amorphous matrix can also be obtained with suitable crystallization from the amorphous state by heat treatment. The heat resistance, Youngs modulus, and other properties are therefore improved. The nanocrystallized fiber which was heat-treated at 1373 K for 2 hours in an air atmosphere has a maximum room temperature tensile strength of 1.9 GPa, around twice that of an as-extracted amorphous fiber. The amorphous continuous ceramic fiber is promising as a ceramic that can be easily shaped at relatively low temperatures (about 1273 K), and as a reinforcing fiber for composites that can undergo secondary processing. Furthermore, this fiber can be considered as more superior to glass fibers because of its greater high-temperature strength and its high Youngs modulus.