ROLE OF SEGREGATING DOPANTS ON THE IMPROVED CREEP RESISTANCE OF ALUMINUM OXIDE J. CHO, C. M. WANG, H. M. CHAN, J. M. RICKMAN and M. P. HARMER{ Materials Research Center, Whitaker Laboratory, Lehigh University, Bethlehem, PA 18015, U.S.A. AbstractÐRecent studies have demonstrated that p.p.m. levels of rare-earth dopant ions (e.g. Y, La, Nd) wield a bene®cial and highly potent in¯uence on the creep properties of alumina. In addition, codoping with ions of disparate sizes (Nd, Zr) resulted in even further enhancement of the creep behavior. In all cases, the dopant ions were found to strongly segregate to grain boundaries. Creep rates were not in¯u- enced by the presence of second phase precipitates, verifying that the creep improvement is a solid solution eect. In an attempt to clarify the exact mechanism(s) that controls creep behavior of the doped aluminas, various advanced characterization techniques have been applied including: secondary ion mass spec- trometry, scanning transmission electron microscopy, orientation image microscopy, and extended X-ray absorption ®ne structure as well as atomistic computer simulation and studies of the creep kinetics. Although no de®nitive mechanism has been established, a logical explanation is that outsize ions segregate to more energetically favorable grain boundary sites, and improve creep resistance by blocking a few criti- cal diusive pathways. This mechanism is suciently general that it may be applicable to other ceramic systems. # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Oxides; Interface diusion; Grain boundaries; Segregation; Creep 1. INTRODUCTION It has been discovered recently that the addition of trace amounts (<1000 p.p.m.) of rare-earth dopants such as yttrium, lanthanum or neodymium, to ®ne-grained alumina (1±2 mm grain size) can reduce the creep rate by several orders of magnitude [1±3]. In order to fully exploit this opportunity for developing polycrystalline materials with signi®- cantly enhanced creep resistance, over the past several years we have been conducting a compre- hensive study aimed at elucidating the underlying mechanism(s) associated with this improved mech- anical response. This has included kinetic studies of the tensile creep rate as a function of temperature, applied stress, grain size, dopant type and concen- tration [3, 4]; detailed microstructural and micro- chemical analyses of grain boundaries using advanced electron microscopy techniques, high res- olution secondary ion mass spectrometry (SIMS) and orientation image mapping [5±7]; a study of the atomic structural environments around Y and Zr segregants using extended X-ray absorption ®ne structure (EXAFS) [8]; atomistic computer simulation of dopant segregation to surfaces and grain boundaries, and calculations of the migration energy for grain boundary diusion along grain boundaries with and without segregants [9]. The objective of the present work is to assimilate the ®ndings of these earlier component studies, along with other related studies in the literature, with a view to identify or at least suggest the underlying scienti®c mechanism involved. What emerges is a clear correlation between creep retardation and the segregation of solute at the grain boundaries. The exact mechanism is still debatable; however, a role involving site blocking of a few critical diusive pathways is strongly implied. From that it becomes apparent that there is indeed a realistic potential for tailoring materials with superior creep resistance, guided by a mechanism-based approach. 2. CREEP BEHAVIOR Work by Carry and Mocellin showed that the ad- dition of Y reduced the strain rate during super- plastic deformation of ®ne-grained, MgO-doped alumina [10]. Independently, French et al. [1] demonstrated that Y doping resulted in two to three orders of magnitude improvements in the ten- sile creep behavior of alumina. Since that time, researchers at Lehigh have extended this study to encompass a wide range of both singly doped, and codoped systems. The specimen preparation and testing procedures have been described in detail elsewhere [1±3], and hence will only be mentioned brie¯y here. It should be noted, however, that ultra- high purity starting powders were used throughout, and extreme care was taken to avoid the introduc- tion of impurities (aside from the intended dopant ions). Dog-bone shaped specimens were machined from fully dense, hot-pressed billets. Tensile creep tests were undertaken under various temperatures Acta mater. Vol. 47, Nos 15, pp. 4197±4207, 1999 # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain 1359-6454/99 $20.00 + 0.00 PII: S1359-6454(99)00278-5 {To whom all correspondence should be addressed. 4197