Optimization of growth theory of the directionally solidified alumina based eutectic ceramics

Directionally solidified Al2O3/Y3Al5O12 (YAG), Al2O3/YAG/ZrO2, Al2O3/GdAlO3 (GAP), Al2O3/GAP/ZrO2 and Al2O3/ZrO2 eutectic/non eutectic ceramics were prepared with laser zone melting. Their microstructure, phase component and shape were investigated. The results indicated that the molar ratio of Zr4+ to rare earth ions (Y3+, Gd3+) in the ZrO2 phase of the ternary eutectic was closed to 2:1. The free energy of chemical reaction was the key to determine the precipitation order of phase. The growth of the firstly-precipitated phase could be divided into the formation of ordered Bravais lattice composed by the oxygen ions, the diffusion of metal cations in this lattice and the change of phase morphology. The unfilled cation gap might be a fast channel for the ion transport. On the same growth plane, the nucleation of the later-precipitated phase began only after the firstly-precipitated phase was fully formed. The triangular was the main ion arrangement way for the stable nucleation and growth of phases. The effects of the precipitation temperature, the phase size, the eutectic type and the growth rate on the microstructure were discussed. This improved theory provided a more reasonable explanation for the formation of rod-shaped, layered, triangular, Chinese script (CS) and cellular microstructures, which might be of significance to regulate the morphology of microstructures.