Internal stresses and the creep resistance of the directionally solidified ceramic eutectics

The creep resistance of the directionally solidified (DS) ceramic eutectic of Al2O3/c-ZrO2(Y2O3) was studied in the temperature range of 1200–1520 °C. The DS eutectic morphology consists of a topologically continuous majority phase of Al2O3, with a growth texture of [0 0 0 1] and an encapsulated minority c-ZrO2(Y2O3) phase in a variety of morphologies having a nearly 〈1 1 2〉 texture. The two phases are separated by well-structured but incoherent interfaces.The creep of the eutectic in its growth direction exhibits an initial transient that is attributed to stress relaxation in the c-ZrO2 phase that also allows relaxation of large initial thermal misfit stresses between phases. In steady state creep, the DS eutectic 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. A finite element analysis of distribution of internal misfit stresses and those resulting from applied stresses in the two phases, together with a detailed dislocation model of the creep rate indicate that much of the nearly nano-scale encapsulated c-ZrO2 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 repeatly circumvent the c-ZrO2 domains acting like dispersoids resulting in the stress exponents larger than 3. The creep model is in good agreement with the experiments, and is generally supported by transmission electron microscopy (TEM) observations of dislocations in crept samples.