COMPETITION BETWEEN STABLE AND METASTABLE PHASES DURING MECHANICAL ALLOYING AND BALL MILLING

The Ti-Al and the Ti-Si system are chosen as model systems to study the origin of metastable phase formation during mechanical alloying of elemental powder blends with a negative heat of mixing. The results demonstrate that kinetics play only a minor role in the phase selection, in particular for the final state. Instead, the intermetallic phases are energetically destabilized with respect to the solid solutions and the amorphous phase due to the destruction of the chemical long-range order upon milling. Thus the latter phases tend to form during mechanical alloying of elemental powder blends as well as during milling of powders of the stable intermetallic compounds. The metastable phases can be predicted by the free energy curves which are calculated by the CALPHAD method. In case of the Ti-Al and Ti-Si alloys, metastable structures and grain sizes in the nm range can be obtained which promise improved mechanical behaviour with respect to the processing of light-weight intermetallic compounds for high-temperature applications.