Creep resistance of the directionally solidified ceramic eutectic of Al2O3/c-ZrO2(Y2O3) : experiments and models

The creep resistance of the directionally solidified ceramic eutectic of Al2O3/c-ZrO2(Y2O3) was studied in the temperature range of 1200-1520 C both experimentally and by mechanistic dislocation models. The topologically continuous majority phase of Al2O3, has a nearly perfect growth texture in the [0001] direction and encapsulates the minor c-ZrO2(Y2O3) phase in a variety of morphologies. This encapsulated phase has a close to < 112 > growth texture, regardless of morphology. The two phases are separated by close to coherent and well structured interfaces. The creep of the eutectic in its growth direction shows an initial transient attributed to stress relaxation in the c-ZrO2(Y2O3) phase, but otherwise in steady state shows many of the same characteristics of creep in sapphire single crystals with c-axis orientation. The creep strain rate of the eutectic has stress exponents in the range of 4.5-5.0 and a temperature dependence suggesting a rate mechanism governed by oxygen ion diffusion in the Al2O3. While TEM evidence is still incomplete, finite element analysis of stress distribution in the two phases and a detailed dislocation model of the creep rate indicate that much of the nano-scale encapsulated c-ZrO2(Y2O3) is too small to deform by creep, so that the major contribution to the recorded creep strain is derived from the diffusion-controlled climb of pyramidal edge dislocations in the Al2O3 phase. The evidence suggests that the climbing dislocations in Al2O3 must repeatedly circumvent the c-ZrO2(Y2O3) domains acting as dispersoids resulting in stress exponents larger than 3. The model is in very good agreement with the experiments. 25 refs.