Phase segregation of aged MgCSZ–MgO directionally solidified eutectic composites

The improvement of the thermomechanical properties of Mg-partially stabilized zirconia after appropriate thermal treatments is well known. Most studies in this subject have been conducted in polycrystalline materials produced by solid-state reaction. In this work, we analyze the segregation behavior and mechanical properties of solidified eutectic composites of the ZrO₂–MgO system, where MgO lamellae or fibers are embedded into a cubic Mg-stabilized zirconia (MgCSZ) containing ∼18 mol% MgO. The eutectics have been aged at 1240 or 1225°C for periods varying between 2 and 24 h. This treatment results in the progressive phase separation of the MgCSZ matrix into different mixtures of tetragonal (t), monoclinic (m), δ (Mg₂Zr₅O₁₂), and MgO phases. Two types of segregation are observed, according to whether the Mg-rich phase is the metastable δ phase (type I) or MgO (type II). Both types may coexist in the same sample but type I predominates at 1240°C and type II at 1225°C. Type I is mainly of the t + δ type, although for long ageing time the t precipitates transform to monoclinic symmetry, resulting in a final m + δ segregation scheme. In type II segregation, the monoclinic phase is the main Mg-poor product. Hardness and toughness have been studied by the Vickers indentation method. The indentation fracture toughness K_IC increases progressively with ageing time and for 24 h treatment is more than doubled with respect to the as-processed eutectic, while the hardness decreases slightly.