The early stages of the solid-state reaction between Ti and Al are characterized by a distinct phase selection in favor of TiAl3. In order to understand the selection mechanism which leads to the absence of phases other than TiAl3, trilayers were prepared by sputtering with a layer of α2-Ti3Al or γ-TiA1 being sandwiched between the Ti and Al layers. The growth of TiA13 in competition with α2 or γ was observed by transmission electron microscopy investigations. Upon annealing, the α2 and the γ layers were consumed by growth of TiA13.
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 a 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 rates in the competing phases.