Phase segregation of aged MgCSZ–MgO directionally solidified eutectic composites

Abstract 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 ZrO 2 –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 (Mg 2 Zr 5 O 12 ), 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.