## 3 Results and Discussion
### MgO-MgSZ Eutectic  
Rods of MgO-MgSZ were obtained by directional solidification of precursor rods prepared according to the eutectic composition of the system MgO:ZrO\({}_{2}\) (53:47 in mol%). Figure 6 represents FESEM micrographs of the transverse (a) and longitudinal cross-sections (b) in the region of the center and edge of a sample solidified at 50 mm/h. The eutectic material was composed of two phases, one of magnesium oxide (MgO) and other of magnesium stabilized zirconia (Mg\({}_{0.2}\)Zr\({}_{0.8}\)O\({}_{1.8}\)) with a volume fraction of 0.28 and 0.72%, respectively. At this rate the microstructure consisted of grains of thin MgO fibers embedded in a zirconia matrix and oriented in the rod axis, and alternating lamellae oriented in different directions, extended throughout the cross section of sample. The longitudinal section indicated that the phases are mainly oriented perpendicular to the solidification front. Figure 5: Optical image of a longitudinal section of a glazed eutectic rod (**a**). Crack arrest through superficial laser melting (**b**). Cross section of a eutectic rod with multiple flaws (**c**). Details of the microstructure at the center and edge of the rod are shown. Fracture section of a glazed eutectic rod showing the crack causing the failure (**d**). At 150 mm/h, the resulting microstructure was mainly lamellar, as shown in Figure 7a,b. However, at 300 mm/h colonies started to develop in the entire section of the sample, Figure 7c,d, due to the breakdown of planar solid-liquid interface. A total change to cellular structure occurred at solidification rate of 750 mm/h, Figure 7e.f. The cells presented round shape in the center of the sample and were formed by an ordered dispersion of MgO fibers. At the edge, the microstructure was fibrillar (see insert in Figure 7f). The cells were surrounded by a thick intercellular region formed by coarse MgO and MgSZ particles of irregular shape. It is worth noting that at faster solidification rates, the microstructure is mainly fibrillar. However, at slow growth rates, there are grains with a fibrillar phase distribution and others with a lamellar phase arrangement. Fibrous structures are found in eutectics in which the minority phase corresponds to a volume fraction less than 28%. Above this percentage, the phase distribution is laminar, and the MgO volume fraction in this eutectic is close to the transition from fibrillar to lamellar geometries. Thus, very small changes in the solidification process could produce this transition, being more frequent in the center of the cylinder where the gradients are not as high as at the edge. The formation of colonies is explained by the rupture of the flat solidification front due to the high growth rates. A good agreement of the calculated eutectic spacing \(\lambda\) with the Jackson and Hunt theory was observed, for which \(\lambda\) = kV\({}^{-1/2}\), where V is the growth rate and k a constant of proportionality, which was calculated based on the experimental, obtaining a value of 8.24 um\({}^{3/2}\) s\({}^{-1/2}\), close to the one reported by [PERSON] [10] for MgO-MgSZ eutectics (\(\sim\)7 um\({}^{3/2}\) s\({}^{-1/2}\)). This constant depends on the individual partition coefficient for each phase (k\({}_{\alpha}\), k\({}_{\beta}\)) and also on the difference between the two phases (e.g., k\({}_{\beta}\)-k\({}_{\alpha}\)) and indicates the correlation of phase spacing and growth rate. The constant makes it possible to predict the phase separation for a determined solidification rate under the minimal undercooling conditions. Figure 6: FESEM micrographs of the transverse and longitudinal cross-sections of the specimens grown at 50 mm/h in the region of the center (**a,c**) and the edge of the sample (**b,d**). Dark phase corresponds to magnesium oxide and bright phase to magnesium-stabilized zirconia. Hardness and indentation fracture toughness were obtained from Vickers microhardness tests on polished transverse cross sections.