MICROSTRUCTURAL AND MECHANICAL PROPERTIES OF DIRECTIONALLY SOLIDIFIED CERAMIC IN Al{sub}2O{sub}3-Al{sub}2TiO{sub}5 SYSTEM

摘要

The mechanical properties of two-phase and poly-phase eutectics can be superior to that of either constituent alone due to the strong constraining effects of the interlocking microstructure. The present work focuses on the solidification characteristics and mechanical properties of Al{sub}2O{sub}3-Al{sub}2TiO{sub}5 system. The challenge for the development of Al{sub}2O{sub}3-Al{sub}2TiO{sub}5 system is to improve the mechanical strength and toughness concurrently with a high resistance to thermal decomposition. The solidification at the invariant eutectic point and Al{sub}2O{sub}3 rich region of off-eutectic compositions was studied. Critical to this effort is the correlation of mechanical properties with eutectic growth data. The strength is strongly related to the starting volume fraction of the minor phase. High strength in the order of 340 MPa is associated with high Al{sub}2O{sub}3 content. Bend tests showed a large displacement for all compositions studied. The off-eutectic composition had Al{sub}2O{sub}3 phase, new Al{sub}6Ti{sub}2O{sub}13 phase as a major phase and Al{sub}2TiO{sub}5 as a minor phase as determined by the WDX and FEG-TEM-STEM techniques. Samples from eutectic region consisted only Al{sub}6Ti{sub}2O{sub}13 and Al{sub}2TiO{sub}5 phases that bear a resemblance to layered structured materials. The proposed structure of new Al{sub}6Ti{sub}2O{sub}3 phase contained one more AlO{sub}6 octahedra along [010] direction and the proposed structure was confirmed by the x-ray, HRTEM, STEM and WDX analysis. The spatial arrangement of the new Al{sub}6Ti{sub}2O{sub}13 phase and Al{sub}2TiO{sub}5 phase around the reinforcing Al{sub}2O{sub}3 dendrites and microcracking were responsible for the improved toughness.