  The isothermal tetragonal-to-monoclinic transformation of 3Y-TZP ceramics
sintered at two different temperatures (1450{\deg}C and 1550{\deg}C) and
duration (2 and 5 h) is investigated at 134{\deg}C in steam. Particular
attention is paid to the presence of a cubic phase and its effect on isothermal
aging. Sintering at 1550{\deg}C can result in a significant amount of large
cubic grains in the specimens, that have a detrimental impact on aging
resistance, especially for the first stage of the aging process. Cubic grains
appear to be enriched in yttrium, which in turn leads to a depletion of yttrium
in the neighboring tetragonal grains. These grains will act as nucleation sites
for tetragonal-to-monoclinic transformation. Even for specimens sintered at
lower temperature, i.e. 1450{\deg}C, the presence of a cubic phase is expected
from the phase diagram, leading to a significant effect on aging sensitivity.
