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 hcp solid solution by the diffusion of Al into Ti, the first phase nucleated at the Ti/Al interface is a (partially) L1(2)-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.