Phase Equilibria and Phase Transformation in the Aluminum Nitride‐Aluminum Oxycarbide Pseudobinary System

Abstract

Samples in the pseudobinary AIN–AI 2 OC were fabricated by hot‐pressing mixtures of ANI, AI 4 C 3 , and AI 2 O 3 powders in graphite dies in an atmosphere of nitrogen. The resulting dense samples were subjected to thermal treatments over a range of temperatures from 1550° to 1950°C. The hot‐pressed as well as annealed samples were examined using optical microscopy, scanning transmission electron microscopy, and X‐ray diffraction. An electron microprobe was used to determine composition. X‐ray diffraction showed that AI 2 OC dissolved in AIN up to 44 mol% at 1800°C. Thermal treatment at lower temperatures led to the decomposition of the solid solution into two isostructural phases. In samples containing ∼14 to 60 mol% AIN, the morphology of the precipitates was lenticular. Diffraction contrast analysis showed that the precipitates were rich in ANI. Lattice images showed that the (001) planes of the 2H structure were continuous between the matrix and the precipitates. These precipitates appear to be similar to Guinier‐Preston zones observed in metallic alloys. When annealed for long periods of time, interface dislocations formed, signifying partial loss of coherency. In some compositions (∼61 to 64 mol% AIN), a lamellar microstructure developed similar to that observed in cellular phase separation. Also, in some of the compositions, an additional phase was observed whose composition and structure were not determined.