KINETICS AND DEFORMATION DURING THE REACTION OF YTTRIUM-ALUMINUM PEROVSKITE AND ALUMINA TO YTTRIUM-ALUMINUM-GARNET

The kinetics of the reaction of directionally solidified eutectics of yttrium-aluminum perovskite and alumina to yttrium-aluminum garnet were measured between 1150-degrees and 1550-degrees-C for times as long as 20000 min. Deformation from the 11 vol% increase was observed. Samples were characterized by optical microscopy, SEM, and extensively by TEM. Surface displacements from self-stress relaxation were measured by atomic force microscopy. YAG nucleated at yttrium silicate impurities as 20-nm-thick plates. For nucleation to occur, the 20-GPa self-stress must relax. YAG growth was parabolic with a 550-kJ/mol activation energy. Self-stress relaxation caused creep polygonization in YAG, forming short-circuit diffusion pathways that may have increased the growth rate. The maximum flow stress in YAG was estimated to be 1-3 GPa and decreased as the reaction progressed. The inferred flow stresses should not have significantly decreased the reaction driving force. Eventually, off-axis creep of alumina dominated self-stress relaxation because its flow stress was much lower than that of YAG.