Interface structure and defects in directionally solidified oxide–oxide eutectic composites

Interfaces in ternary oxide–oxide eutectic composites prepared by directional solidification are investigated at the atomic scale. Two systems are studied that both display a Chinese script interconnected microstructure, associating three monocrystalline phases, alumina (A), yttrium aluminium garnet (Y) phase or a perovskite phase SmAlO3 (S), and zirconia (Z), i.e. Al2O3–Y3Al5O12–ZrO2 (A–Y–Z) or Al2O3–SmAlO3–ZrO2 (A–S–Z). Most interfaces are semicoherent. The curved interfaces between A and Z nodules are strongly faceted and parallel to dense planes of both crystals or to dense planes of alumina. The same features occur for A–S interfaces, which show longer facets. The steps between facets accommodate the misfits. Y–A and Y–Z interfaces are often parallel to a dense plane of one crystal owing to a slight deviation of the Y phase from the perfect orientation relationship (OR). The OR between Y and A phases found in binary eutectic is modified by the presence of Z. Similarly, although the same OR occurs between A and Z in binary and ternary eutectics with the Y phase, this OR is modified by the presence of S. The discussion emphasises the predominant role of interfacial order on the final orientation relationships.