Separation of Phases by Spinodal Decomposition in the Systems Al2O3−Cr2O3 and Al2O3−Cr2O3−Fe2O3

Abstract

Separation of phases was investigated in the hexagonal (rhombohedral) systems Al2O3−Cr2O3 and Al2O3−Cr2O3−Fe2O3. The binary system shows a miscibility gap with a Tc of 950°C; the miscibility gap for the ternary system was determined for a constant Cr2O3 content of 16.6 mol%. Dark field transmission electron microscopy of solid solutions annealed within the miscibility gap showed dark and light lamellas ∼50 to 200 Å thick. X‐ray diffraction results for the solid solutions in the ternary system indicated that, in the early stages of annealing, broadening occurred only on (hkl) reflections where l≠0. There was no major change in the X‐ray diffraction patterns of the annealed solid solutions in the binary system. Electron diffraction results indicated, however, that phase separation in both systems proceeded in the [001] direction. Solid solutions in the binary system separated very slowly; the separation could be enhanced hydrothermally. The mechanism of the separation of phases in both systems is spinodal and proceeds as follows: solid solution→intermediate modulated phase→equilibrium phases.