The relation between the improvement of the thermomechanical properties of Mg-partially stabilised zirconia (Mg-PSZ) and the segregation of tetragonal precipitates after the appropriate thermal treatment –usually annealing below the eutectoid temperature of 1407 °C– has been known for more than 30 years [1]. The enhancement of the fracture toughness is particularly remarkable when segregation involves the creation of the metastable δ-phase, a trigonally distorted fluorite-like lattice with ordered oxygen vacancies and composition Mg₂Zr₅O₁₂. Most studies in this subject have been conducted in polycrystalline materials produced by solid state reaction methods. In this work we analyse the segregation behaviour in MgSZ-MgO eutectic composites produced by laser-assisted directional solidification. The microstructure of these eutectic bicrystals consists of ordered lamellae or fibres of the minority phase (MgO) alternating with (or surrounded by) the major phase, a cubic Mg-stabilised zirconia with ∼18 mol% MgO. Submitting the MgSZ-MgO eutectics to prolonged annealing at sub-eutectoid temperatures (typically 24h at 1200 °C) we have achieved full phase segregation within the MgSZ matrix, in the form of a mixture of monoclinic, tetragonal and δ phases with no remaining cubic phase. The structure, microstructure and composition of the segregated phases are studied by XRD, SEM/EDX and Raman spectroscopy, and mechanical properties are preliminary studied by the Vickers indentation method. The fracture toughness coefficient K_IC is more than doubled in the annealed lamellar sample with respect to the as-processed eutectic, whereas the hardness remains constant or decreases slightly. The dependence of the results on the growth rate as well as on the fibrilar or lamellar character is also discussed. To our knowledge, there is only one previous study in annealed MgSZ-MgO eutectics [2], but it was conducted in samples submitted to a different thermal treatment, not yielding the δ-phase.
Acknowledgements: The authors acknowledge financial support through grant PID2021-124863OB-I00 funded by MCIN/AEI/10.13039/501100011033.
[1] Richard H. J. Hannink, Christopher J. Howard, Erich H. Kisi and Michael V. Swain, Relationship between Fracture Toughness and Phase Assemblage in Mg-PSZ, J. Am. Ceram. Soc. 77, 571-79 (1994).
[2] Jun-ichi Echigoya, Kiyotaka Sasai and Hajime Suto, Decomposition of Cubic ZrO₂ in MgO-ZrO₂ Eutectic, Transactions of the Japan Institute of Metals 27, 247 – 253 (1986).