Phase selection governed by different growth velocities in the early stages of the Ti/Al phase reaction

The solid-state reaction between Ti and Al is characterized by a distinct phase selection in favor of TiAl3. The absence of phases other than TiAl3 was investigated in order to understand the selection mechanism using synthetically prepared trilayers with a layer of α2-Ti3Al or γ-TiAl sandwiched between Ti and Al layers. The growth of TiAl3 in competition with α2 or γ was observed by transmission electron microscopy. Upon annealing, the α2 and the γ phases are completely consumed by growth of TiAl3. The results indicate that the absence of phases other than TiAl3 is not due to the existence of nucleation barriers for α2 and γ, but is caused by growth selection. A kinetic model is presented which quantitatively explains the observed phase selection on the basis of different growth velocities which result from different diffusion fluxes in the competing phases.