| Directionally solidified eutectic oxide ceramics (DSEO) are very attractive as ultra-high-temperature structural materials with long-term service due to the excellent mechanical properties, thermal stability, creep resistance and oxidation resistance in atmosphere at high temperature. Eutectic ceramics have good microstructural stability and the application temperature can reach1600-1700℃. Among processing techniques for the eutectic oxide ceramic, only Bridgman technique with relatively modest thermal gradient is the the optimum method for the fabrication of large volume samples. However, this method implies low growth rates and consequently large interphase spacing samples with low strength. Other methods can obtain samples with finer interspacing size but the products are limited to several millimeters due to the large temperature gradients. From the processing viewpoint, the relationships among thermal gradient, growth rate and micro structure size impose limitations on the sample processing. Hence, it is a problem for eutectic ceramic processing technique that how to prepare large dimession sample and control the microstructural characteristics simultaneously, which limits the materials developing and engeering application in components.Present investigation focuses on the above aspect of Al2O3/ZrO2/YAG searching method to prepare samples which combines large dimension as well as fine microstructure. High temperature mlet-grown method with modest thermal gradient was adopted to prepare large dimension eutectic bulk. Combining with rapid cooling or melt superheating treatment, the solidification microstructure can be optimized by controlling technological parameters which affeact the solidification path and crystal growth rate. Solidification microstructure evolutions were studied systematically and discussed in terms of the classical nucleation mechanism and Jackson-Hunt model. The primary conclusions are as follows:By increasing the cooling rate, Al2O3/ZrO2/YAG eutectic bulks with the interspacing of microstructure in submicron level were fabricated successfully. With increasing the cooling rate, the microstructure characteristic scale of melt-grown samples decreases and the structure develops from colony to dendrite and finally to cell. The solidification Microstructure transitions were interpreted according to morphology variation of solid-liquid interfaces as a result of the interplay between solute diffusion and the curvature effects. Meanwhile, a mathematical model for ternary eutectic coupled growth was developed based on binary eutectic. With the mathematical model, solute, curvature, and kinetic undercoolings were deduced and calculated as a function of growth velocity.By controlling melt temperature and holding time, Al2O3/ZrO2/YAG eutectic rods with30mm in diameter were produced. The solidification microstructure is homogenous and the minimum interspacing is only350nm. Making using of the classical nucleation mechanism, solidification microstructure transitions under different melt temperatures on was discussed in terms of the variations about atom clusters distribution in melts and the passivation of the heterogeneous nucleation sites, which affect the melt nucleation undercooling, solidification path and the final solidification microstructure. The melt temperature can be divided into three stages according to its role on microstructural change. The first stage:the microstructure refinement with the melt temperature increasing or with the holding time prolonging, is due to the decay of atom clusters which homogenizes the melt and makes an increase of the melt nucleation undercooling. The second stage:the higher melt temperature results in the eutectic micrsostructure further refined because of the passivation of the heterogeneous nucleation sites increasing the melt nucleation undercooling. The third stage:the melt temperature beyonds a certain crtical value Tc the melt-grown sample is Al2O3-ZrO2-YAP metastable composite because the passivation of the heterogeneous nucleation sites is serious and the YAP nucleates instead of YAG.A method referred to as "pressure assistaned-solidification formation" was first suggested for the shape-forming of melt growth eutectic ceramics, and practically implemented for Al2O3/YAG/ZrO2composite. A component with crucible shape was prepared successfully. It was found the defects in the intercolony region and on the surface reduce dramatically, which is beneficial to improve the mechnical properties. |
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