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The Ti-50%Al (molar fraction) composite powder was synthesized by high energy ball milling (HEBM). The characteristics of the composite particles, including surface morphology, structure and components, were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The mechanochemical reaction between Ti and Al gradually took place through intermediate stages during milling, and resulted in the formation of disordered Ti/Al phases. Dissolution of Al in Ti (after 9 h milling), nanocrystalline Ti/Al solid solution and high density of dislocations were caused by HEBM. The element distribution of the mechanically alloyed Ti-50%Al composite powder was close to the initial composition, but the distribution was inhomogeneous.

The evolution in microstructure and composition of the milled Ti-Al composite powder with different milling time were investigated. It shows that with the milling time increasing, the initial powder underwent a successive change in its morphology from a flattened shape (2 h) to a fine, equiaxed and uniform one (above 6 h). The milled Ti-Al composite powder was nanocrystalline with the average crystallite size of about 17 nm after milling for 8 h. The evolution mechanism of Ti-Al composite powder was elucidated. The Ti(Al) solid solution is formed through a gradual and progressive solution of Al into Ti lattice. By differential thermal analysis on the ignition temperature of the reaction between Ti and Al as a function of milling time, it indicates that mechanical milling of the powders significantly lowered the ignition temperature of the reaction by refining its Ti-Al composite structure.