Directional solidification of Al2O3-Al2TiO5 system

International audienceDirectional solidification of Al2O3–Al2TiO5 eutectic system was investigated to design in situ composites that could exhibit optimized properties of strength and toughness. Directional solidification of an alumina rich melt was expected to produce primary Al2O3 dendrites, these would act as load bearing component of the structure, separated by a Al2O3–Al2TiO5 eutectic matrix and. Additional aim was to utilize the differential expansion coefficient between the two phases to produce microcracks and thus weak interfaces for load deflection. Three different melt compositions were investigated; two off-eutectic compositions, at 11, 26 mol% TiO2, and one eutectic at 43.9 mol% TiO2. The crystallized phases were different than as one would expect from the published phase diagram. Off-eutectics were composed of Al2O3 dendrites separated by a Al6Ti2O13 matrix. Eutectic structure was composed of Al2TiO5 lamellae in an aluminum titanate matrix. This matrix consisted of a superstructure made of 5 cells of Al6Ti2O13 for one cell of Al2TiO5 along [0 0 1] direction. Al6TiO13 phase decomposed by a eutectoid reaction into Al2O3 and Al2TiO5. Eutectoid reaction occurred during post heat treatment at 1400 °C and decomposition of Al6Ti2O13 was completed after 10 h of annealing at 1500 °C. Based on these observations, a modification of the alumina rich part of Al2O3–TiO2 phase diagram was proposed. A second intermediate compound, Al6Ti2O13 is introduced as a high temperature phase. Two invariant points, a peritectic (L + Al2O3 → Al6Ti2O13) and a eutectic (L → Al6Ti2O13 + Al2TiO5) are also added between Al6Ti2O13 and Al2TiO5