Narihito Nakagawa

High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350∼400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics.

Sapphire matrix composites reinforced with single crystal YAG phases

An investigation of fabrication technology on eutectic composites consisting of Al2O3 phases and YAG (Y3Al5O12) phases was carried out by applying the unidirectional solidification process. Unidirectionally solidified eutectic composites consisting of 〈110〉 sapphire phases and 〈420〉 single crystal YAG phases could be fabricated successfully by lowering a Mo crucible at a speed of 5 mm h−1 under a pressure of 10−5 mmHg of argon. These eutectic composites have excellent high-temperature properties up to 1973 K. For example, the flexural strength is 360–500 MPa independent of testing temperature from room temperature to 1973 K. Oxidation resistance at 1973 K in an air atmosphere is superior to SiC and Si3N4 and the microstructure of these eutectic composites is stable even after heat treatment at 1773 K for 50 h in an air atmosphere.

The creep and thermal stability characteristics of a unidirectionally solidified Al2O3/YAG eutectic composite

Compressive creep characteristics at 1773, 1873, and 1973 K, oxidation resistance over 1000 h at a temperature of 1973 K in ambient air, and the thermal stability characteristics at 1973 K in ambient air of a unidirectionally solidified Al2O3/YAG eutectic composite were evaluated. At a test temperature of 1873 K and a strain rate of 10−4/s, the compressive creep strength of a eutectic composite manufactured by the unidirectional solidification method is approximately 13 times higher than that of a sintered composite with the same chemical composition. The insite eutectic composite also showed greater thermal stability, with no change in mass after an exposure of 1000 hours at 1973 K in ambient air. The superior high-temperature characteristics are closely related to such factors as (1) the in-situ eutectic composite having a microstructure, in which single crystal Al2O3 and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries and (2) no amorphous phase is formed at the interface between the Al2O3 and the YAG phases.

Deformation and fracture behavior of an Al2O3/YAG composite from room temperature to 2023 K

Abstract The deformation and fracture behavior of a newly developed Al 2 O 3 /YAG composite, fabricated by the unidirectional solidification of a eutectic composition, was investigated by means of bending tests between room temperature and 2023 K, on specimens whose longitudinal directions were parallel (L specimen) and perpendicular (T specimen) to the direction of solidification. At low temperatures, the composite fractured in a brittle manner at all displacement speeds. At high temperatures, it fractured in a brittle manner at high displacement speed, but in a ductile manner with accompanying plastic deformation at low speed for both L and T specimens. The brittle-ductile transition temperature became higher at higher displacement speeds in both L and T specimens, while it was slightly higher in the latter specimen. The crack propagated dominantly through YAG with lower ductility than Al 2 O 3 , as a consequence of which the fraction of YAG in the fracture surface was higher than the structural value estimated from a polished surface. The strength at room temperature of both L and T specimens was maintained up to 2023 K at high displacement speeds. At low displacement speeds, the strength decreased beyond 1900 K as a result of the enhanced plastic deformation. The stress exponent for plastic deformation at high temperatures (1823–2023 K) was 5–6, suggesting that the plastic deformation is controlled by a dislocation mechanism during bending as well as in compressive and tensile loading. The fracture toughness tended to increase with increasing temperature and with decreasing displacement speed, especially in L specimens.

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.

Selective Emission of Al2O3/Er3Al5O12 Eutectic Composite for Thermophotovoltaic Generation of Electricity

The emissive properties of Al2O3/Er3Al5O12 eutectic composite were measured in the temperature range of 1000 to 1700 K. It was confirmed that the Al2O3/Er3Al5O12 eutectic composite has selective emission bands at a wavelength of 1.5 µm attributable to Er3+ ions. It is found that the intensity obeys the T4-law. The emittance of over 0.8 is observed in the selective region. Since these emission bands match up the sensitive region of the GaSb PV cell spectrally, the Al2O3/Er3Al5O12 eutectic composite is a suitable emitter material for using in thermophotovoltaic generation systems. The effects of temperature and thickness on the selective emission efficiency have been studied and discussed.

High-Temperature Strength of Directionally Solidified Al2O3/YAG/ZrO2 Eutectic Composite

A2O3/YAG/ZrO2 eutectic Melt-Growth-Composites (MGCs) were unidirectionally solidified by the modified-pulling-down method (MPD) and the Bridgman type method, in which a crucible was brought down at different speeds. The microstructures and crystallographic textures were studied by field emission scanning electron microscopy (FE-SEM) and electron backscattered pattern (EBSP) method. The high-temperature strength was investigated by compression tests. All MGC rods show strong preferred growing orientation, although the structural size of eutectic microstructure among MGC rods was different. The high-temperature strength of MGC rods is dependent on orientation, compression temperature and strain rate. The high-temperature strength of MGC rods is controlled by the anisotropic strength of constituent Al2O3, as well as the structural size of eutectic microstructure.

A New Unidirectional Solidified Ceramic Eutectic with High Strength at High Temperature

Abstract Abstract New eutectic composites such as Al2O3/Er3Al5O12 or Al2O3/GdAlO3 have recently been fabricated by unidirectional solidification. The eutectic composite has a new microstructure, in which continuous networks of Al2O3 phases and oxide compounds (Er3Al5O12 or GdAlO3) interpenetrate without grain boundaries. The eutectic composite is thermally stable and has the following properties. In case of Al2O3/Er3Al5O12 composite, 1. The flexural strength at room temperature can be maintained up to 2073 K (just below its melting point of about 2130 K), 2. The compressive flow stress at 1873 K and a strain rate of 10−4/sec is about 8 times higher than that of sintered composites of the same composition, 3. It shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 500 hours. In case of Al2O3/GdAlO3 composite, and 4. It shows substantial plastic deformation at 1873 K with a flexural yield stress of about 700 MPa. It is found that the plastic deformation occurred by dislocation motion in each phase.

Microstructure and High Temperature Strength Characteristics of Unidirectionally Solidified Al2 O3 /GdAlO3 Eutectic Composite

Enploying unidirectional solidification, Al2O3/GdAlO3(Gadolinium Aluminum Perovskite:GAP) eutectic composite was fabricated. The Al2O3/GAP eutectic composite obtained has a flexural strength of over 600Mpa above 1500C. The composite also showed plastic deformation above 1550C on the flexural test. The effect of microstructure on mechanical strength at high temperature was investigated. Al2O3/GAP eutectic composites having different microstructure were fabricated by controlling the temperature gradient on unidirectional solidification. It was found that the flexural strength at high temperature of the composite became higher with refining the microstructure.

Innovative Manufacturing Process of MGC Components for Ultra-High Efficiency Gas Turbine Systems

Much attention has been paid to unidirectionally solidified ceramic composites as candidates for high-temperature structural materials. Eutectic composites, known as Melt Growth Composites (MGCs), have recently been developed. The binary MGCs (Al2 O3 /YAG and Al2 O3 /GAP binary systems) have a novel microstructure, in which continuous networks of single-crystal Al2 O3 phases and single crystal oxide compounds (YAG or GAP) interpenetrate without grain boundaries. Therefore, the MGCs have excellent high temperature strength characteristics, creep resistance, superior oxidation resistance and thermal stability in the air atmosphere at very high temperatures. Manufacturing processes for the MGCs are being examined under a Japanese national project, scheduled from 2001–2005. To achieve higher thermal efficiency for gas turbine systems, uncooled turbine nozzle vanes have been fabricated on an experimental basis. Novel manufacturing processes for MGC gas turbine components are proposed.Copyright