Yoshiharu Waku

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.

Dislocation mechanism of deformation and strength of Al2O3–YAG single crystal composites at high temperatures above 1500°C

Abstract A new unidirectionally solidified eutectic Al 2 O 3 –YAG composite has recently been fabricated by accurately controlling the unidirectional solidification. The eutectic composite has a new microstructure, in which single crystal Al 2 O 3 and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries. The dislocation structure is observed in both single crystal Al 2 O 3 and YAG in the plastically deformed specimens in the tensile and compressive tests at high temperatures for the Al 2 O 3 –YAG single crystal composite, showing that the plastic deformation occurred by dislocation motion. The Al 2 O 3 –YAG single crystal composite fabricated has the following properties: (1) the flexural strength at room temperature can be maintained up to just below melting point (about 1830°C), (2) the compressive flow stress at 1600°C and a strain rate of 10 −4 /s is about 13 times higher than that of sintered composites of the same composition.

Ultra-high temperature compressive creep behavior of an in-situ Al2O3 single-crystal/YAG eutectic composite

Abstract Compressive creep tests were conducted for an in-situ single-crystal alumina/yttrium aluminum garnet [Al 2 O 3 /Y 3 Al 5 O 12 (YAG)] eutectic composite for a specimen with a compressive axis with 0 or 90° to the solidified direction at temperatures between 1723 and 1923 K under stress ranges of 140–450 MPa in air environments. The single-crystal Al 2 O 3 /YAG eutectic exhibited a stress exponent of 5.4–10, indicative of compressive creep behavior characterized by a dislocation mechanism. Activation energies for creep deformation were 810–1024 kJ/mol, close to that for 42° from c -axis single-crystal Al 2 O 3 or single-crystal YAG, in agreement with that of self-diffusion for c -axis single-crystal Al 2 O 3 . The 0° specimen exhibited a slight decrease in creep rate by about half for a 90° specimen corresponding to results from crystal anisotropy in single-crystal Al 2 O 3 . The Larson–Miller method provided an effective means of comparing creep resistance of oxides on the basis of results obtained under different stresses and temperatures.

Anisotropy in high-temperature deformation in unidirectionally solidified eutectic Al2O3–YAG single crystals

Abstract High-temperature deformation behavior in unidirectionally solidified eutectic Al 2 O 3 –Y 3 Al 5 O 12 single crystals for the specimen with a compressive axis inclined by 0°, 45° or 90° to the solidification direction was examined by uniaxial compression testing in argon flow at temperatures between 1773 and 1973 K.

Solidification and shape casting of Al2O3?YAG eutectic ceramics from the undercooled melt produced by melting Al2O3?YAP eutectics

Abstract There are two eutectic systems in the Al2O3-rich portion of the Al2O3–Y2O3 system: one is the Al2O3–YAG equilibrium eutectic system and the other is the Al2O3–YAP metastable eutectic system. Heating the Al2O3–YAP metastable eutectic structure up to temperatures above the metastable eutectic temperature but below the equilibrium eutectic temperature produced the undercooled melt. Solidification in the equilibrium path immediately followed the undercooled melt formation. The solidification in the equilibrium path along with the melting of the metastable eutectic structure resulted in a fine and uniform equilibrium eutectic structure with lamellar spacing of less than 1 μm. The equilibrium eutectic structure was not affected by the metastable eutectic structure used for the undercooled melt formation. Shape casting and joining of alumina rods were demonstrated using the undercooled melt produced by the melting of the Al2O3–YAP metastable eutectic structure. The fine eutectic structure was obtained throughout the castings. Coupling of the solidification and the melting achieved a higher growth rate than conventional solidification from the undercooled melt. The skewed coupled growth zone due to the kinetic effect contributed to formation of the fine eutectic structure at the metastable eutectic composition.

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al2O3–SiO2 (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO2 particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture.

Simultaneous improvement of the strength and fracture toughness of MgO-Al2O3-SiO2 glass/Mo composites by the microdispersion of flaky Mo particles

The effect of shape and volume percent of Mo particles on theflexural strength and fracture toughness of MgO-Al2O3-SiO2(MAS) glass/Mo composites was investigated. The flexural strengthand fracture toughness of composites depends heavily on Mo particleshapes, and there is greater improvement in composites reinforcedwith flaky rather than massive Mo particles. In the compositesreinforced with flaky Mo particles, fracture toughness increases withvolume percent of Mo and, at 50 vol% Mo, is 11.6 MPa√m,which is approximately 6.7 times higher than that of the matrix. Increases in fracture toughness of composites reinforced with flakyMo particles is greater than with SiC whiskers, SiC platelets, SiC particles or ZrO2 particles. Fabricating composites reinforcedwith flaky Mo particles is an effective toughening technique capableof simultaneously improving the strength and toughness of brittlematerials, such as monolithic Al2O3 and MAS glass, by utilizing plastic deformation of ductile phase.

Influence of particle size and volume percent of flaky mo particles on the mechanical properties of AI2O3/Mo composites

The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of A12O3 composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with A12O3 powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the A12O3 matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.