Directionally solidified eutectic ceramic oxides

Alumina/yttrium aluminum garnet (YAG) eutectic rods of 1 mm in diameter were grown by the laser-heated floating zone method at different rates to obtain microstructures with the same morphology but of very different domain size. The mechanical properties of the rods (hardness, toughness, strength) were measured at ambient temperature in the longitudinal and transverse directions and, in addition, the longitudinal flexure strength was determined up to 1900 K. The fracture resistance and the hardness of the eutectics at ambient temperature were isotropic and independent of the domain size. On the contrary, the longitudinal strength was significantly higher than the transverse one and increased linearly with the growth rate, reaching almost 2 GPa in the rods grown at 750 mm/h, which presented a submicrometer homogeneous microstructure. The critical defect size was equivalent to that of Al2O3 and YAG domains in the microstructure, and the strength was proportional to the inverse of the square root of the domain size. In addition, the strength retention of the eutectics was remarkable, and the rods with the finest microstructure withstood 1.53 GPa at 1900 K. The moderate reduction in strength at very high temperature was induced by the homogeneous coarsening of the microstructure.

Mechanical Properties of Melt‐Grown Alumina–Yttrium Aluminum Garnet Eutectics up to 1900 K

Synthesis of an alumina–YAG nanopowder via sol–gel method

Study of the gas inclusions in Al2O3/Y3Al5O12 and Al2O3/Y3Al5O12/ZrO2 eutectic fibers grown by laser floating zone

Directionally solidified Al 2 O 3 –Er 3 Al 5 O 12 –ZrO 2 eutectic rods were processed using the laser floating zone method at growth rates of 25, 350 and 750 mm/h to obtain microstructures with different domain size The mechanical properties were investigated as a function of the processing rate The hardness, ∼156 GPa, and the fracture toughness, ∼4 MPa m 1/2 , obtained from Vickers indentation at room temperature were practically independent of the size of the eutectic phases However, the flexural strength increased as the domain size decreased, reaching outstanding strength values close to 3 GPa in the samples grown at 750 mm/h A high retention of the flexural strength was observed up to 1500 K in the materials processed at 25 and 350 mm/h, while superplastic behaviour was observed at 1700 K in the eutectic rods solidified at the highest rate of 750 mm/h

http://digital.csic.es/bitstream/10261/120817/1/Mechanical%20properties%20up.pdf

Mechanical properties up to 1900 K of Al2O3/Er3Al5O12/ZrO2 eutectic ceramics grown by the laser floating zone method

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Multi-phase—single crystal oxide fibers offer the best choice for reinforcing oxide matrix composites because they have superior creep resistance up to 1700 °C without significant strength loss at moderate temperatures due to growth of processing flaws. In this work, Directionally Solidified Al2O3–YAG eutectic fibers were grown at various rates by the Edge-defined, Film-fed Growth (EFG) method and their microstructure, microstructural stability and creep properties were studied. A methodology was developed in order to determine if the creep behavior of a fiber was affected by any heterogeneous coarsening defects. The creep behavior could be rationalized using a threshold stress concept with activation energy of 1100 kJ/mol K. TEM analysis of the crept fibers suggested that the Sapphire phase was deforming by a dislocation mechanism, while the YAG phase deformed by a diffusional mechanism. A creep model was developed which contained geometrical factors for describing the microstructure. Analysis of the data showed that the creep resistance would increase to single crystal values as the phase aspect ratio increased. Further, these two phases—single crystal structures exhibit a flaw-independent strength and are suggested to have a decrease in slow crack growth rate as the transverse phase size decreases.

N. Hecht

Papers

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Creep of directionally solidified alumina/YAG eutectic monofilaments