Some observations of microstructural changes in alumina induced by Ti inhomogeneities
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(Received 3 February 1988; accepted 10 March 1988) Analysis of Ti-containing heterogeneities in a hot-pressed A12O3 has revealed changes in microstructure caused by oxidation that provide some insight regarding the effects of Ti on local grain growth. The most consistent hypothesis is that Ti 4 + in solution compensates the Mg 2 + ion taken into solution from the MgO additive, probably by forming dipoles, and negates the effect of Mg 2 + on grain boundary drag.
I. INTRODUCTION Recent observations1 have revealed that A12O3 containing localized high concentrations of Ti exhibits large-grained regions, typically 100/urn in extent (Fig. 1). The present investigation is designed to address some issues concerning the role of Ti in promoting localized grain growth. The study reinforces and clarifies the fact that small concentrations of aliovalent ions can bring about profound changes in microstructure. There is extensive literature on the influence of Ti 4 + doping (both by itself and in combination with divalent cations) on the defect equilibria and sintering of A12O3 (Refs. 2-8). From such studies, it is well established that sintering is enhanced when small amounts of Ti 4 + are present, whereas codoping with Zr 4 + and Mg 2 + results in discontinuous grain growth.9 Such studies provide a useful basis for the present investigation. II. EXPERIMENTAL METHODS The samples (obtained from AVCO) studied were A12O3—0.25 wt. % MgO with the composition given in Table I. They were prepared by hot pressing in vacuum in graphite dies at 1600 °C. The major secondary component, apart from the MgO dopant, is Ni. In order to trace the evolution of the microstructure, as influenced by temperature and stress, samples were prepared for electron microscope. The heterogeneous regions of interest were less than 100 fim in extent and, furthermore, their average separation was 1-2 mm, such that a typical thin foil could contain only one or two such regions. Consequently, specimen preparation had to be carefully monitored from the initial cutting operation through to final ion-milling. Two types of specimens were prepared. In some cases, as-received material was chemically etched in boiling 85% H 3 PO 4 at 450 K for 2-4 min to delineate the grain boundaries. Suitable regions were mounted on Al rings and mechanically dimpled to a thickness of about 20 fim before ion thinning. In other instances, annealing was carried out in air at temperatures varying 764
J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
from 1375 to 1675 K for times ranging from 5 min to 12 h. Transmission electron microscopy (TEM) was used to identify the distribution and morphology of additives and impurities, such as Mg, Ni, and Ti, and to examine the amorphous phases present at grain boundaries. III. OBSERVATIONS A. As-processed material The coarse grains exhibited a number of distinctive features apart from their size. A small concentration of Ti(~0.2-0.3 at. %) was invariably dissolved in these grains, as was established by energy dispersive x-ray spec
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