Kinetics and Deformation during the Reaction of Yttrium‐Aluminum Perovskite and Alumina to Yttrium‐Aluminum Garnet

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Abstract
The kinetics of the reaction of directionally solidified eutec‐tics of yttrium‐aluminum perovskite and alumina to yttrium‐aluminum garnet were measured between 1150° and 1550°C for times as long as 20 000 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.