Effect of rapid solidification on microstructural evolution in MgO-MgAl2O4

The response of the MgO-20 and 30 mol% Al2O3 Compositions to rapid solidifi cation has been studied. The oxides were twin-roller quenched and the resul ting flakes were characterized by X-ray diffraction and transmission electr on microscopy. The results indicate that metastable extensions of spinel an d periclase occurred and the microstructural pathway was determined from th e final microstructure. The flakes having MgO-20 mol % Al2O3 show a dendrit ic structure consisting of periclase and spinel. In the MgO-30 mol % Al2O3 composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and ther modynamic aspects of phase selection have been rationalized based on the me tastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinet are stabilized be cause of the formation of disordered phases with a continuous range of orde r parameter on the tetrahedral sublattice.