High-temperature Creep Resistance in Rare-earth-doped, Fine-grained Al 2 O 3
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High-temperature creep resistance in rare-earth-doped, fine-grained Al2 O3 H. Yoshida,a) Y. Ikuhara, and T. Sakuma Ceramic Materials Laboratory, Department of Material Science, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan (Received 23 June 1997; accepted 23 December 1997)
High-temperature creep in undoped Al2 O3 and La2 O3 - or Y2 O3 - or Lu2 O3 -doped Al2 O3 with a grain size of about 1 mm is examined in uniaxial compression testing at temperatures between 1150 and 1350 ±C. The high-temperature creep resistance in Al2 O3 is highly improved by the rare-earth oxide doping in the level of 0.045 mol %, and the creep rate is suppressed in the order La2 O3 , Y2 O3 , Lu2 O3 . Rare-earth ions in each doped Al2 O3 are found to segregate in Al2 O3 grain boundaries without forming amorphous phase or second-phase particles. The activation energy for creep in undoped Al2 O3 is estimated to be 410 kJymol, while it is about 800 kJymol in the three rare-earth oxide-doped Al2 O3 . The grain boundary diffusivity must be highly reduced by the segregation of the dopant cation in Al2 O3 grain boundaries. I. INTRODUCTION
In polycrystalline alumina, high-temperature creep deformation often occurs by diffusional flow.1–8 Heuer et al. reported that the operation of slip systems does not satisfy the von Mises criterion for homogeneous deformation in polycrystalline Al2 O3 , and that hightemperature creep deformation occurs by grain boundary diffusion process in fine-grained alumina.4 Heuer et al. found that the predominant deformation mechanism in Al2 O3 with a grain size of less than 10 mm is Coble creep at temperatures of 1100–1400 ±C and an applied stress of less than 100 MPa, but the grain boundary sliding becomes important as the grain size is smaller.2 Chokshi reported that grain boundary sliding contributes dominantly to high-temperature creep deformation in polycrystalline alumina with a grain size of 1.6 mm at 1673 K under 44 MPa.9 One of the striking facts in high-temperature creep or plastic flow in fine-grained Al2 O3 is a marked effect of small dopant cation. Cho et al. reported that 1000 ppm yttrium or 50 ppm lanthanum-doped Al2 O3 , which contain the second-phase precipitations, has improved creep resistance in comparison with pure Al2 O3 at 1200–1350 ±C under the stress of 50 MPa.10 An addition of 0.1 wt. % ZrO2 in Al2 O3 with a grain size of about 1 mm severely suppresses the creep rate by a factor of about 15 at 1250 ±C under the stress of 10–200 MPa.11 Such a dopant effect of Mg21 , Y31 , and Zr41 has also been observed in high-temperature flow stress of fine-grained Al2 O3 in conventional tensile testing.12,13
a)
Graduate Student, The University of Tokyo. J. Mater. Res., Vol. 13, No. 9, Sep 1998
http://journals.cambridge.org
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The present paper aims to report that the hightemperature creep resistance in Al2 O3 with a grain size of 1 mm is markedly improved by a doping of rareearth oxide such as La2 O3 ,
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