Mechanical Alloying of Ti–Al Powder Mixtures under Nitrogen Atmosphere

Abstract

Elemental titanium powder and Ti100−xAlx elemental powder mixtures were milled in a vibrational ball mill, Spex 8000, under nitrogen atmosphere, and the nitrogen absorption behavior during milling and the structures of milled powders were examined by means of X-ray diffraction measurements, SEM and HRTEM observations. The titanium powder absorbed 50 at% of nitrogen during milling and was nitrided completely to form TiN. The Ti–Al powder mixtures with compositions X≤50 absorbed also about 50 at% of nitrogen and were nitrided completely to form a solid solution nitride (Ti, Al)N with the NaCl-type structure through an amorphous-like solid solution. Each of the TiN and (Ti, Al)N powder particles had a nanocrystalline structure consisting of about 5 nm size grains. In the case of the Ti100−xAlx powder mixtures with compositions 60≤X≤70, 35-15 at% of nitrogen were absorbed in the powders and each of the milled powders had a dual-phase structure consisting of about 5 nm size (Ti, Al)N and Al–Ti solid solution phases. The Ti–Al powder mixtures with compositions 80≤X≤100 scarcely absorbed nitrogen and therefore no nitrides formed. The main reason why the aluminum rich Al–Ti powder mixtures scarecely absorb nitrogen may be ascribed to the fact that aluminum powder is too soft to be-refined by milling.