Phase evolution during crystallization of sputter-deposited amorphous titanium-aluminium alloy thin films: dimensional and solute effects

Ti-Al thin films have been deposited by magnetron sputtering on oxidized Si substrates. The targets consisted of a binary Ti-48 at.% Al alloy and a qu aternary Ti-48 at.% Al-2 at.% Nb2 at.% Mn alloy. In the as-deposited condit ion, the thin film grown from the quaternary alloy was completely amorphous whereas the film grown from the binary alloy consisted of crystalline alph a-Ti(Al) precipitates embedded in an amorphous matrix. In order to study th e microstructural evolution during crystallization of these thin films two types of annealing experiment have been performed: ex situ in a furnace und er a protective Ar atmosphere and in situ on a hot stage in the transmissio n electron microscope. Both the binary and the quaternary films crystallize d into a two-phase mixture of gamma-TiAl and alpha(2)-Ti3Al as a result of ex situ crystallization. A thickness dependence in the evolution of microst ructure has been observed during the in situ annealing. Thus, thick regions on the binary film evolved into the two phase microstructure during in sit u annealing, which was similar to that observed in the ex situ annealed sam ples. In contrast, the thin regions of the binary thin film specimen exhibi ted growth of the pre-existing alpha-Ti(Al) crystallites into large grains and the formation of a new ordered tetragonal phase. The details of these i nvestigations are presented in this paper.