(h k i l)-specific G.P. zones in Sn- versus Zr-doped α-Al2O3 by solution annealing

The α-Al2O3 powder doped with Sn or Zr via a solid-state sintering route was annealed for a comparative study of Guinier-Preston (G.P.) zone by electron microscopy. The 2 at.% Sn-doped α-Al2O3 showed G.P. zones plates with coherent habit planes parallel to (0 0 0 1) and (1¯ 2 1¯ 0). The 0.6 at.% Zr-doped α-Al2O3 showed similar shaped G.P. zones but with more corrugated (0 0 0 1) plane. The habit plane selection can be rationalized by anisotropic solute expulsion along relatively close packed (0 0 0 1) and (1¯ 2 1¯ 0) planes under the influence of a precondensation effect analogous to that reported for a crystallization process. Solute-specific volume and charge-compensating defects account for the coherency strain difference of the G.P. zones in α-Al2O3.