TEM characterization of modulated microstructures in CaO-Dy₂O₃ solid solutions

The lanthanide sesquioxides (e.g. Dy₂O₃) have been identified as potential high-temperature transformation tougheners alternative to zirconia (ZrO₂). The monoclinic (B) to cubic (C) transformation on cooling is accompanied by an 8% volume increase and shattering. In the course of work to stabilize the B phase by adding CaO to pure Dy₂O₃ and rapid cooling, modulated microstructures were found in B phases of CaO-Dy₂O₃ solid solutions. Primarily two unit-cell scale modulations were identified: q₁ (001-type, λ ≈ 9.0 Å) and q₂ (200-type, λ ≈ 7.5 Å). In the [010] orientation another modulatio n (q₃; λ ≈ 40 Å) occured in some samples. Higher-order satellite reflections of q₁ and q₂ were always present and their intensities gradually decreased as the order increased. However, q₃ showed strong first-order intensities but weak higher-order intensities. As the cooling rate increased, angles q₁<(00-)* and q₂<(600)* tended to increase, whereas the intensity of q₃ tended to decrease or diffuse. As the sintering temperature and time increased, the intensity of q₃ tended to increase. A structural model to explain the modulated structures was proposed. It was suggested that the origin of the q₁ and q₂ modulations was a crystallographic shearing of structural units. The systematic shearing locally generated domains similar to the C structure, which gave rise to the q₃ modulation.