Microstructural stability and mechanical properties of directionally solidified alumina/YAG eutectic monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the 'use temperature' of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments weve studied. Microstructural stability experiments were conducted in air from 1200 to 1500 degrees C and creep tests at temperatures of 1400 to 1700 degrees C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400 degrees C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior ave discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.