Claude Carry

Effect of grain size on yttrium grain boundary segregation in fine-grained alumina

Abstract In this work, the effects of the mean grain size and of the total yttrium content on the distribution of this element in fine-grained alumina are studied. The yttrium distribution is determined in the TEM using energy dispersive X-ray analysis by measuring the mean grain boundary composition as well as the mean size and number of precipitates per unit volume. Two different types of microstructure are observed, depending on both the grain size and the total yttrium content: in the first, only grain boundary segregation is observed, whereas in the second, the grain boundaries are saturated with yttrium, resulting in the intergranular precipitation of an yttrium-rich second phase. For a given composition, the microstructure changes from the first kind of microstructure to the second kind during grain growth. Predictions using a simple geometrical model based on the mass conservation of yttrium are in good agreement with the experimental results for both types of microstructure. It is shown that the kinetics of grain growth are not significantly affected by the yttrium when it is used along with magnesia in alumina.

Reaction sintering of aluminium titanate: I—Effect of MgO addition

Abstract The effect of 2 wt% MgO addition on the reactive sintering of Al 2 TiO 5 was studied starting from commercially available alumina and rutile powders. The formation of titanate, the successive sintering process and the corresponding microstructure evolution have been investigated using differential dilatometry, X-ray powder diffraction and scanning electron microscopy. For pure Al 2 TiO 5 the reaction occurs initially by a nucleation and rapid growth process and, afterwards, by the slow conversion of unreacted oxide particles by solid-state diffusion. The addition of MgO changes the mechanism of titanate formation, probably by making easy the nucleation of the new phase through the intermediate formation of MgAl 2 O 4 , followed by the growth of a Mgrich solid solution. The main result is a strong reduction of the average grain size and remarkable increase of the density of the final material.

Low temperature superplastic flow of yttria stabilized tetragonal zirconia polycrystals

Very fine-grained (0·2 um), dense Y-TZP ceramics have been produced by free sintering at 1150°C. The superplastic deformation of these materials is studied in compression at low temperatures (1100-1300°C). A significant enhancement in initial strain rates was observed compared to a coarser-grained (0·4 ?m) commercially available material (Tosoh, Japan). Doping with small amounts of Fe2O3 led to a further enhancement of strain rate. Deformation occurred via interface reaction controlled grain boundary sliding. Prior to deformation a continuous glassy silicate film was observed at the grain boundaries. The applicability of interface reaction controlled solution-precipitation creep models is discussed. The absence of a steady-state during deformation at low stresses is attributed to dewetting of the silicate film.

Reaction sintering of aluminium titanate. II: Effect of different alumina powders

Abstract Reaction sintering of aluminium titanate was studied starting from different commercial alumina powders and rutile. The formation of titanate, the successive sintering process and the corresponding microstructure evolution have been investigated using differential dilatometry, X-ray powder diffraction and scanning electron microscopy. Reaction sintering of pure Al 2 TiO 5 results in a coarse-grained material with low density; the dependence of the average grain size of the resulting material from the level of impurities of the starting alumina is qualitatively discussed; the purest aluminas result in the largest grains. Addition of 2 wt% of MgO changes the mechanism of titanate formation, resulting in a strong reduction of the average grain size and remarkable influence on densification of the final material. Since final microstructure is mainly determined by the amount of MgO added, the influence of the nature of alumina powder is less pronounced in comparison with pure Al 2 TiO 5 .

Processing of SiC whisker-reinforced zirconia by extrusion at elevated temperature

Abstract The extrusion of a zirconia matrix reinforced with 13 vol.% SiC whiskers has been studied at 1550 and 1600°C. This method uses the superplasticity properties of the matrix and allowed fully densified composites to be obtained inside which the whisker orientation approximated the extrusion direction. Microstructures are characterized in terms of whisker alignment and zirconia grain shapes. The room-temperature mechanical properties of extruded material are remarkably improved compared to those of the same composition obtained by hot-pressing. The reinforcement effect, related to the nearly uniaxial whisker alignment, is essentially due to whisker pull-out phenomena even if pull-out lengths did not exceed a few whisker radii.