Acta Materialia 50 (2002) 3453–3463 www.actamat-journals.com The effect of yttrium on densification and grain growth in α-alumina R. Voytovych a,1 , I. MacLaren * , M.A. Gu ¨lgu ¨n b , R.M. Cannon c , M. Ru ¨hle a a Max-Planck-Institut fu ¨r Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany b Sabanci University, 81474 Tuzla/Istanbul, Turkey c Evans Hall (MC 1760), Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 27 January 2002; received in revised form 11 April 2002; accepted 11 April 2002 Abstract The grain growth and densification have been investigated in very high-purity α-alumina doped with varying amounts of yttrium (0 to 3000 wt ppm of yttria) and sintered in air at 1450, 1550 and 1650 °C. Yttrium doping inhibited densification and coarsening at 1450 °C, but had very little effect at 1550 °C and no effect at 1650 °C. The change in densification behaviour is suggested to be related to the transition with increasing temperature from grain boundary diffusion to lattice diffusion controlled densification. The coarsening rate increases faster with temperature than the densification rate. This was correlated with a higher measured activation energy for grain growth than for the diffusion processes, which control the densification.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sintering; Grain growth; Kinetics; Ceramics; Alumina 1. Introduction Numerous cation impurities including Mg [1,2], Ca [2–5], Si [2,4,5], Ti [6], Y [7] and Zr [8] play a major role in modifying the microstructure and related properties of sintered polycrystalline α-alu- mina. Yttrium is of particular interest because this element dramatically decreases the creep rate in α- alumina [9–12]. Yttrium is known to have a very low solubility limit in α-alumina and segregates * Corresponding author. E-mail address: ian.maclaren@physics.org (I. MacLaren). 1 Current addrdress: LTPCM INPG, BP 75, 38402 Saint- Martin-D’Heres Cedex, France 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(02)00159-3 strongly to grain boundaries. Although there is a clear correlation between the improved creep properties and segregation of the solute to the grain boundaries, the exact mechanism is still under debate [11,12]. The first quantitative investigation of Y segre- gation behaviour was undertaken by Gruffel and Carry [13]. Using energy-dispersive X-ray spec- trometry (EDXS) in the scanning transmission electron microscope (STEM) mode of a combined TEM/STEM machine, they showed that as the grain size increased, the Y level at the boundary increased monotonically and then saturated at a certain level. At this level yttrium aluminium gar- net (YAG) precipitates start to form and further