Thermostability of Ti/Al composite powders prepared by mechanical milling

The crystalline size of both the titanium and the aluminum phase becomes finer and finer, and finally nano-crystalline and amorphous Ti/Al composite powders are formed by mechanical milling of the Ti and Al elemental powder mixture. However, no titanium aluminides are observed in the as-above milled powders. Differential thermal analysis (DTA) tests show that the composite powders prepared by mechanical milling for various times retain different internal energy. The longer the milling time, the more the stored internal energy within the milled powders, with the amorphous composite powders possessing the most internal energy, which can be attributed to the fact that the lattice defects due to mechanical milling increase significantly with prolonged milling time. Isothermal vacuum annealing treatments of the Ti/Al composite powders milled for various hours show that the longer the milling time, the greater the possibility for the solid state reaction between the constituent Ti and Al element to form titanium aluminides within the composite powders during annealing. For the nanocrystalline or amorphous composite powders mechanically milled no less than 75 h, the full titanium aluminides microstructure can be formed by 1 h annealing even at such a low temperature as 450°C.