The processing techniques, microstructures, and mechanical properties of directionally solidified eutectic ceramics are reviewed. It is considered the main methods for preparing of eutectic ceramics and the relationships between thermal gradient, growth rate, and microstructure parameters. Some principles of coupled eutectic growth, main types of eutectic microstructure and the relationship between the eutectic microstructure and the mechanical properties of directionally solidified eutectics at ambient and high temperatures are briefly described. The mechanical behavior and main toughening mechanisms of these materials in a wide temperature range are discussed. It is shown that the strength at high temperatures mainly depends on the plasticity of the phase components. By analyzing the dislocation structure, the occurrence of strain hardening in single crystalline phases during high-temperature deformation is revealed. The creep resistance of eutectic composites is superior to that of the sintered samples due to the absence of glassy phases at the interfaces, and the strain has to be accommodated by plastic deformation within the domains rather than by interfacial sliding. The microstructural and chemical stability of the directionally solidified eutectic ceramics at high temperatures are discussed. The aligned eutectic microstructures show limited phase coarsening up to the eutectic point and excellent chemical resistance. Directionally solidified eutectics, especially oxides, revealed an excellent oxidation resistance at elevated temperatures. It is shown sufficient potential of these materials for high-temperature applications.
TopProcessing Techniques
Directionally solidified eutectic (DSE) ceramics are highly structured composite materials with a dense and homogeneous microstructure, which determines the mechanical and functional properties. The key rule for regular homogeneous growth is to keep micro- and macroscopically flat solid–liquid interfaces during growth, thus preventing constitutional supercooling and cellular growth. This in turn requires large thermal gradients (Ashbrook, 1977; Llorca & Orera, 2006). These conditions are essentially the same as those required for the growth of single crystals from the melt, and consequently similar directional solidification procedures are used to grow ceramic eutectics. The growth methods can be classified in two groups: directional solidification in a container and crucibleless zone melting.
Among the former methods, the Bridgman-Stockbarger technique is suitable for growing bulk samples of large size, the ingot volume being limited only by crucible size. The apparent thermal gradients in the Bridgman method are generally below 102 K/cm and consequently the growth rates have to be relatively low to avoid cellular growth, usually they are less than 100 mm/h. In the Czochralski method, a container for the melt is also needed but direct contact between crucible and grown material is avoided since the eutectic is pulled out from the melt pool. Thick rods of about 6 cm diameter can be grown by this method.
Larger thermal gradients, and consequently faster growth rates, can be attained in the melt zone methods. The processing techniques based on floating zone or on pedestal growth (PG) are crucibleless methods in which a relatively small amount of sample volume is melted by the different types of heaters.
Growth thermal gradients of up to 104 K/cm can be obtained by laser-heated floating-zone method (Farmer & Sayir, 2002; Argon & Sayir, 2001; Sayir & Farmer, 2000; de la Fuente, Diez, Angurel, Peña, Sotelo and Navarro, 1995; Peña, Merino, Harlan, Larrea, de la Fuente and Orera, 2002). Other crucibleless methods are based on solid pulling from a wetting shaper as in the Stepanov or edge-defined film-growth and micro-pulling down methods (Rudolph & Fukuda, 1999; Rudolph, Yoshikawa, and Fukuda, 2000; Lee, Yoshikawa, Fukuda and Waku, 2001, Borodin, Reznikov, Starostin, Steriopolo, Tatarchenko, Chernyshova, & Yalovets, 1987). The thermal stresses associated with large axial thermal gradients also limit the sample diameter in the such methods, although eutectic rods thicker than single crystal rods can be processed thanks to the good thermo-mechanical properties of DSE. For example, rapid solidification allows to produce the DSEs with extremely small interphase spacing but the high thermal stresses induced by quenching limited the sample size to a few mm3 (Calderón-Moreno & Yoshimura, 2001).
The one of a recent root for fabrication of DSEs with large dimensions is consolidation of eutectic powders by the spark plasma sintering and by hot pressing (Isobe, Omori, Uchida, Sato, & Hirai, 2002; Bogomol, Grasso, Nishimura, Sakka, Loboda, & Vasylkiv, 2012). The arc plasma spraying of eutectic powders is also used for producing of dense ceramics with nanometric microstructural size (Balasubramaniam, Keshavan, & Cannon, 2005; Suffner, Sieger, Hahn, Dosta, Cano, Guilemany, Klimczyk, & Jaworska, 2009).