Mechanical alloying of the Ti-Al system in atmosphere of hydrogen and argon

Three kinds of Ti-Al powders, Ti₇₂Al₂₈, Ti₅₇Al₄₃ and Ti₄₈Al₅₂, were mechanically alloyed by a planetary ball mill in atmosphere of argon or hydrogen gases (0.1 MPa) with alloying times up to 30 h. The mechanical alloying (MA) process as well as the phase variations of each powder after subsequent heating at 1173 K were investigated. About 5000 wppm hydrogen, which could be easily removed by a heat treatment at 800 K (heating rate was 20 K/min), was occluded in all powders during MA in the hydrogen atmosphere, whereas the mechanically alloyed powders in the argon atmosphere occluded about 1000 wppm hydrogen. In the hydrogen atmosphere, the titanium powder easily crumbled into finer particles, assisting the diffusion of aluminum into titanium (solid-solid reaction) at an early stage of the MA process and accelerating the formation of an amorphous-like phase at a longer MA process. The phase formation after heat treatment of MA powders at 1173 K could be estimated by the Ti-Al binary phase diagram without the effect of the gas atmosphere.