Creep Resistance of the Directionally Solidified Ceramic Eutectic of Al 2 O 3 /c-ZrO 2 (Y 2 O 3 )
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Creep Resistance of the Directionally Solidified Ceramic Eutectic of Al2O3/c-ZrO2(Y2O3) Jin Yi1, Ali S. Argon1 and Ali Sayir2 1 Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA, 02139, U.S.A. 2 NASA - John Glenn Research Center, Cleveland, OH, 44135 ABSTRACT The creep resistance of the directionally solidified ceramic eutectic of Al2O3/c-ZrO2(Y2O3) was studied in the temperature range of 1200-1520ºC both experimentally and by mechanistic dislocation models. The creep of the eutectic in its growth direction exhibits an initial transient that is 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. A detailed dislocation model of the creep rate indicates 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 repeatly circumvent the c-ZrO2(Y2O3) domains acting as dispersoids resulting in the stress exponents larger than 3. The creep model is in very good agreement with the experiments. INTRODUCTION It is necessary to consider compounds such as oxides, carbides, borides, etc. to reach service temperatures in the 1400-1600ºC range [1]. Their high temperature performance is largely governed by their creep resistance and fracture resistance. It is creep resistance that is of principal concern here to us. Early experiments of Firestone and Heuer [2] on [0001] axis-oriented-sapphire in the 16001800ºC range produced evidence that such sapphire crystals creep entirely by the climb of the 1 / 3 < 1 101 > pyramidal edge dislocations, with no slip line or stereo-TEM evidence of glide of such dislocations on any of the possible pyramidal planes available. The stress dependence of the creep rates of [0001] oriented sapphire crystals and their governing activation energy of oxygen ion diffusion in Al2O3 have all been consistent with a pure climb mode of creep [2]. A series of directionally solidified ceramic eutectics, consisting largely of an Al2O3 component together with compatible stable oxides of ZrO2 or YAG combine many of the advantages of sapphire fiber with morphological stability at elevated temperatures and relative ease of production. Of these, the Al2O3, ZrO2 system eutectic, with additional 4.2 mol% Y2O3 modification to create cubic ZrO2, has received considerable attention. This eutectic has demonstrated some attractive characteristics. In the Laser Heated Float Zone (LHFZ)-produced eutectic in the form of fibers or slender rods the Al2O3/c- ZrO2(Y2O3)* eutectics possess a sub*
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