M. Oehring et al., THE FORMATION OF METASTABLE TI-AL SOLID-SOLUTIONS BY MECHANICAL ALLOYING AND BALL-MILLING, Journal of materials research, 8(11), 1993, pp. 2819-2829
Elemental Ti-Al powder blends were mechanically alloyed in order to st
udy phase formation during the alloying process. In addition, the stab
ility of intermetallic phases upon milling was investigated separately
in order to determine the origins of phase selection during the milli
ng process. It was found that by mechanical alloying of powder blends,
as well as by ball milling of Ti-aluminides for long milling times, t
he same metastable phases were formed for corresponding compositions,
i.e., the hcp solid solution for Al concentrations up to 60 at. % and
the fcc solid solution for 75 at. % Al. X-ray diffraction (XRD) analys
es indicated that the process of mechanical alloying occurred via the
diffusion of Al into Ti. By lowering the milling intensity, a two-phas
e mixture of the hcp solid solution and the amorphous phase was observ
ed for Ti50Al50 and confirmed by transmission electron microscopy (TEM
). The results show that phase selection in the final state during mec
hanical alloying of Ti-Al powder blends and milling of intermetallic c
ompounds is mainly determined by the energetic destabilization of the
competing phases caused by the milling process. The destabilization is
most pronounced in the case of intermetallic compounds due to the dec
rease in long-range order upon milling. For the final milling stage, p
hase formation can be predicted by considering the relative stabilitie
s of the respective phases calculated by the CALPHAD method using the
available thermodynamic data for the Ti-Al system.