THE FORMATION OF METASTABLE TI-AL SOLID-SOLUTIONS BY MECHANICAL ALLOYING AND BALL-MILLING

Elemental Ti-Al powder blends were mechanically alloyed in order to study phase formation during the alloying process. In addition, the stability of intermetallic phases upon milling was investigated separately in order to determine the origins of phase selection during the milling process. It was found that by mechanical alloying of powder blends, as well as by ball milling of Ti-aluminides for long milling times, the 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) analyses indicated that the process of mechanical alloying occurred via the diffusion of Al into Ti. By lowering the milling intensity, a two-phase mixture of the hcp solid solution and the amorphous phase was observed for Ti50Al50 and confirmed by transmission electron microscopy (TEM). The results show that phase selection in the final state during mechanical alloying of Ti-Al powder blends and milling of intermetallic compounds 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 decrease in long-range order upon milling. For the final milling stage, phase formation can be predicted by considering the relative stabilities of the respective phases calculated by the CALPHAD method using the available thermodynamic data for the Ti-Al system.