Improved tensile creep properties of yttrium- and lanthanum-doped alumina: a solid solution effect
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The tensile creep behavior of yttrium- and lanthanum-doped alumina (at dopant levels below the solubility limit) was examined. Both compositions (100 ppm yttrium, 100 ppm lanthanum) exhibited a uniform microstructure consisting of fine, equiaxed grains. The creep resistance of both doped aluminas was enhanced, compared with undoped alumina, by about two orders of magnitude, which was almost the same degree of improvement as for materials with higher dopant levels (in excess of the solubility limit). In addition, measured creep rupture curves exhibited predominantly steady-state creep behavior. Our results, therefore, verified that the creep improvement in these rare-earth doped aluminas was primarily a solid-solution effect.
I. INTRODUCTION
Oxide ceramics have recently become promising candidate materials for fiber-reinforced ceramic-matrix composites (CMCs) for use in various applications including hot exhaust components in advanced engines and aircraft.1,2 However, the creep properties have limited their applicability. Given this constraint, it is of interest to identify oxides having superior creep resistance. We note that significant progress toward this goal has been achieved after it was discovered that the tensile creep rate of aluminum oxide (␣–Al2O3) was reduced by approximately two or three orders of magnitude by the addition of rare-earth ions (e.g., Lu3+, Y3+, Nd3+, and La3+).3–6 Furthermore, a small amount of Zr4+ was shown to enhance creep resistance of alumina by a factor of about 15.7 In these promising doped aluminas, emerging evidence suggests that grain boundaries play a dominant role in this behavior. One such observation is that oversized cation dopants strongly segregate to grain boundaries.8–10 Therefore, it has been suggested that oversized segregants selectively block rapid grain boundary diffusion paths and ultimately reduce creep rate.3 Previous studies on 1000 ppm yttrium (Y)- and 500 ppm lanthanum (La)-doped alumina, however, have shown that second-phase precipitates (i.e., Y3Al5O12 and LaAl11O18, respectively) were present in the microstructure. As a result, it was not clear whether these precipitates played a major role in the creep retardation. Wakai et al.7 showed creep improvement from the Zr4+ doping in alumina even below the solubility limit. Therefore, the objective of the present work was to measure the creep J. Mater. Res., Vol. 16, No. 2, Feb 2001
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rate of Y- and La-doped alumina samples doped below the solubility limit to discriminate solid-solution from second-phase effects. II. EXPERIMENTAL PROCEDURE
Y- and La-doped aluminas used in this study were prepared from commercial powders of ␣–Al2O3 (AKP53, Sumitomo Chemical America, New York, NY). The ␣–Al2O3 powder was dispersed in methanol approximately with equal mass of liquid and solid. The dopant ions were added to the suspension in the form of yttrium (or lanthanum) nitrate methanol solution. The amount of the dopant can be nominally controlled by weighing. The ind
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