THE EARLY STAGES OF PHASE FORMATION DURING MECHANICAL ALLOYING OF TI-AL

The early stages of phase formation during mechanical alloying of Ti/Al powder blends were investigated using x-ray diffraction and transmission electron microscopy. After the initial formation of an hep solid solution by the diffusion of Al into Ti, the first phase nucleated at the Ti/Al interface is a (partially) Ll2-ordered fcc phase with a crystallite size of about 10–30 nm. This observation is ascribed to the kinetics of phase formation and the energetic destabilization of the equilibrium intermetallic compounds during milling due to chemical and structural disordering. A mechanism of phase formation during mechanical alloying is proposed, and comparisons are made with the thermal annealing behavior of Ti/Al multilayered thin films.