PHASE-STABILITY AND DECOMPOSITION PROCESSES IN TI-AL BASED INTERMETALLICS

The high-temperature phase equilibria and the phase decomposition of a and beta 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 beta phase region to higher Al concentrations, and the beta phase is ordered to the beta(2) phase. The Zr and O atoms stabilize beta and a phases respectively. Ln the Zr-stabilized beta phase, alpha(2) laths form with accompanying surface relief, and stacking faults which relax the elastic strain owing to lattice deformation are introduced after formation of alpha(2) order domains. Thus shear is thought to operate after the phase transition from beta to alpha(2) 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.