The effects of process control agents on mechanical alloying mechanisms in the Ti---Al system

During the mechanical alloying (MA) of ductile materials, it is often found that welding becomes so dominant that a very fine layered structure or homogeneously alloyed powders cannot be obtained. It is often desired to add an appropriate amount of process control agent (PCA) to powders in order to retain the equilibrium state between welding and fracturing processes. The present work investigated the effects of PCAs on MA mechanisms in the Ti---Al system. It was found that the amount of PCA and the energy transfer from ball to powder during MA influenced the mechanism of MA. As the amount of PCA and/or energy transferred from ball to powder increased, the mechanism of MA changed from a substitutional diffusion to a penetration of metallic atoms into interstitial sites. The penetration of metallic atoms seems to play an important role in the formation of the metastable f.c.c. phase, for which the lattice parameter is about 4.2 Å in the Ti---Al system.