Phase stability and decomposition processes in TiAl based intermetallics

The high-temperature phase equilibria and the phase decomposition of α and β phases were studied by crystallographic analysis of the solidification microstructures of Ti-48at.%Al and Ti-48at.%Al-2at.%X (X = Mn, Cr, Mo) alloys. The effects on the phase stability of Zr and O atoms penetrating from the specimen surface were also examined for Ti-48at.%Al and Ti-50at.%Al alloys. The third elements Cr and Mo shift the β phase region to higher Al concentrations, and the β phase is ordered to the β2 phase. The Zr and O atoms stabilize β and α phases respectively. In the Zr-stabilized β phase, α2 laths form with accompanying surface relief, and stacking faults which relax the elastic strain owing to lattice deformation are introduced after formation of α2 order domains. Thus shear is thought to operate after the phase transition from β to α2 by short-range diffusion. A similar analysis was conducted for the TiAl binary system, and the transformation was interpreted from the CCT diagram constructed qualitatively.