Rs. Hay, KINETICS AND DEFORMATION DURING THE REACTION OF YTTRIUM-ALUMINUM PEROVSKITE AND ALUMINA TO YTTRIUM-ALUMINUM-GARNET, Journal of the American Ceramic Society, 77(6), 1994, pp. 1473-1485
The kinetics of the reaction of directionally solidified eutectics of
yttrium-aluminum perovskite and alumina to yttrium-aluminum garnet wer
e measured between 1150-degrees and 1550-degrees-C for times as long a
s 20000 min. Deformation from the 11 vol% increase was observed. Sampl
es were characterized by optical microscopy, SEM, and extensively by T
EM. 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-stres
s must relax. YAG growth was parabolic with a 550-kJ/mol activation en
ergy. Self-stress relaxation caused creep polygonization in YAG, formi
ng 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 shoul
d not have significantly decreased the reaction driving force. Eventua
lly, off-axis creep of alumina dominated self-stress relaxation becaus
e its flow stress was much lower than that of YAG.