Crystallization Process in Amorphous Al<SUB>2</SUB>O<SUB>3</SUB> and Al-Ti-O-N Mixed Powders Prepared by Mechanical Alloying

Powders consisting of amorphous Al2O3 and Al-Ti-O-N phases were prepared from Al-12.4 at%Ti mixed powders by mechanical alloying in an atmosphere containing oxygen and nitrogen, and crystallization process upon annealing in the amorphous powders was investigated by means of X-ray diffraction, DTA, SEM and TEM (HREM). Al3Ti was detected after annealing for 3.6 ks in the temperature range of 673 to 1273 K and crystal growth of Al3Ti was accerelated at about 1073 K and declined gradually with increasing temperature. Al3Ti was not detected after annealing above 1373 K. γ-Al2O3 was detected after annealing ranging from 1123 to 1373 K. The crystallization temperature of γ-Al2O3 was about 1120∼1180 K and the apparent activation energy for crystal growth of γ-Al2O3 was 317 kJ/mol. Above 1223 K, the formation of α-Al2O3 and TiN proceeded preferentially. AlN was formed at temperatures above 1373 K. The size of the various crystalline phases in the annealed powders was about 5∼30 nm. No melt phase was observed during annealing at temperatures below 1573 K.