On the mechanism of heterogeneous reaction and phase formation in Ti/Al multilayer nanofilms

This article describes results obtained during the study of the combustion syntheses of intermetallic Al-Ti films, starting from elemental nanoscaled multilayers of Al and Ti. The original multilayers were prepared by plasma sputtering and the combustion synthesis was started by local or general heating, depending on the case. Atomic scanning tunneling microscopy, scanning and transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry have been used to study the slow and self propagating reaction modes. Self-sustained reaction starts at temperature 600 K, which is much lower than the value for powder mixtures of similar composition and may be explained by abnormally fast diffusion along grain boundaries. Formation of product "bridges" across the nanolayers was observed at the first stages of the reaction. The combustion product (TiAl) inherits the texture of initial Ti and Al layers. An interpretation of the reaction mechanisms for the formation of TiAl and TiAl3 is given and discussed.