Oxide-oxide eutectic ceramic materials prepared by unidirectional solidification from the melt seem to be promising candidates for thermo-mechanical applications at high temperatures such as for gas turbine parts applications. They are prepared from Al₂O₃ – RE₂O₃ – ZrO₂ (RE = Y, Er, Sm) systems and associate 2 or 3 phases among: Al₂O₃ (corundum structure), RE₃Al₅O₁₂ (garnet structure), REAlO₃ (perovskite structure) and ZrO₂ (fluorite structure) phases. The microstructure consists of a three-dimensional interpenetrated network of single-crystal phases, free of grain boundary (fig. 1) and preferred orientation relationships occur between constituent phases¹. Consequently, they exhibit a noticeable thermal stability of the microstructure and good mechanical properties ^2,3.

Compressive tests were carried out at 1450°C and 1550°C under 100 MPa and 200 MPa. Low strain rates obtained confirm good creep resistance at high temperature of these materials. Deformation mechanisms were studied by TEM with conventional two-beam analysis. During creep tests, the specimens deform via micro-twinning in alumina and dislocation activity (climb in most cases) in all phases (fig. 2).

The interface nature of directionally solidified eutectic ceramics is the origin of their good properties in comparison with sintered equivalent ceramics. Thus, the different types of interfaces were studied at atomic scale by HRTEM (fig. 3). In a first approach, the chosen observation direction axis was parallel to the growth direction. Investigations were made on as-grown materials and after creep tests in order to:

     – understand interface structure and defects at the atomic scale in as-grown materials ;

     – analyse modification of interfacial structure after creep deformation ;

     – correlate microstructure and deformation mechanisms.

References:

1. Mazerolles, L. et al. New microstructures in ceramic materials from the melt for high temperature applications. Aerosp. Sci. Technol. 12, 499–505 (2008).

2. Waku, Y. et al. High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite. J. Mater. Sci. 33, 1217–1225 (1998).

3. Mazerolles, L., Perriere, L., Lartigue-Korinek, S., Piquet, N. & Parlier, M. Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites. J. Eur. Ceram. Soc. 28, 2301–2308 (2008).

Figures:

Bright field TEM image showing the microstructure of the solidified composite Al2O3 – Y3Al5O12 – ZrO2 (white: Al2O3, dark grey: Y3Al2O12, light grey: ZrO2)

Bright field TEM image showing dislocation activity in the solidified composite Al2O3 – Er3Al5O12 – ZrO2 after creep deformation at 1450°C under 200 MPa

HRTEM image of [10-10] Al2O3 // [001] ZrO2 interface in as-grown Al2O3 – Y3Al5O12 – ZrO2 composite

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