We investigated the influence of growth rate and Ce concentration on colony structure variation in Al2O3/YAG:Ce eutectic systems. The distance between boundary zones in the colony structure decreased with increases in either growth rate or Ce concentration. The eutectic spacing in the coarse microstructure in the boundary zone decreased with increasing growth rate but increased with increasing Ce concentration. We conclude that the colony structure is formed by cellular growth driven by constitutional supercooling with an interface instability due to Ce atom accumulation, so that the distance between boundary zones depends on both the growth rate and Ce concentration, and the coarse microstructure in the boundary zone depends on the solidification rate perpendicular to the growth interface at the cell bottom of the microscopic growth interface shape in the cellular growth. (C) 2016 Elsevier B.V. All rights reserved.