TiAl3 formation by furnace annealing of Ti/Al bilayers and the effect of impurities

Abstract

Reactions of Ti/Al couples induced by furnace annealing were investigated (at elevated temperature) using large-grained Al substrates and vacuum-evaporated bilayers of both sequences. 4 He MeV backscattering spectrometry was principally used to monitor the reactions. Profiles of oxygen impurity were obtained by elastic 16 O(alpha,alpha)16 O resonant scattering. In the range of 460–515 °C, TiAl3 forms as a laterally uniform layer at the Ti/Al interface. The thickness of this compound layer increases as (annealing time)1/2. The activation energy is 1.9–2.0±0.1 eV. For evaporated bilayers on an oxidized Si substrate, the sequence of the bilayers does not effect the growth mechanism of TiAl3 , but the growth rate of samples with the Ti on top is lower than that of samples with Al on top, that is, oxygen in Ti/Al samples can reduce the reaction rate by decreasing the pre-exponential factor. Oxygen already contained in the Ti film and oxygen from the annealing ambient are incorporated in the growing TiAl3 compound during thermal annealing. In addition, a TiAl3 layer also forms at the free Ti surface during vacuum annealing when the oxygen-containing contaminants in the ambient are minimized. So far, we succeeded in accomplishing this only for large-grained Al substrates. We conclude that the formation of the TiAl3 compound at the surface is controlled by nucleation and depends sensitively on the condition of the surface layer of the Ti film.