MECHANICAL ALLOYING OF THE TI-AL SYSTEM

Ti-Al alloys have been prepared by mechanical alloying (MA) of pure titanium and aluminium powders for four compositions: Ti80Al20, Ti75Al25, Ti50Al50, and Ti40Al60. The 50 < Ti (at %) < 80 compositions could be fully amorphized with a milling time strongly dependent on the starting chemical composition. For the Ti40Al60 composition, only partial amorphization was observed. The investigation on the early stage of MA shows that the different systems amorphized through two different paths. On the titanium rich side (Ti = 75,80 at %), the MA first leads to the formation of an h.c.p. alpha-Ti(Al) solid solution with an aluminium content of 14-16%, which subsequently collapses into the amorphous state. For the aluminium rich side (Al = 50,60 at %), MA sooner promotes the nucleations of disordered forms of Ti3Al and TiAl intermetallic compounds, respectively. The inhibition of the ordering transition of the observed intermetallic phases is ascribed to the low temperature at which the SSR takes place. The complete (Ti50Al50) or partial (Ti40Al60) amorphization of the powder is then attained through a destabilization of the disordered Ti3Al and TiAl phases. The present results confirm the existence of a metastable f.c.c. phase as final alloying stage for the Ti75Al25 and Ti40Al60 compositions.