Structural features of Y-saturated and supersaturated grain boundaries in alumina

Abstract

Grain boundary segregation of Y in α-Al 2O 3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α-Al 2O 3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y 3Al 5O 12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α-Al 2O 3, and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y 2O 3. This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.