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(α,α)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.

Reactive synthesis and characterization of titanium aluminides produced from elemental powder mixtures

“…Similar behavior has been attributed to the formation of pores during the synthesis of various aluminides from powder mixtures [31][32][33][34][35][36]. This is also in agreement with studies suggesting that the diffusion of aluminum atoms into titanium particles is the predominant mechanism governing the formation of titanium aluminides [21,22,[25][26][27]. The globular morphology of the TiAl 3 phase obtained in this study is quite similar to what Xu et al [29] observed for the TiAl 3 phase in diffusion couple experiments.…”

“…This observation has been explained on the basis of the higher diffusivity of aluminum ions in TiAl 3 in comparison with other aluminides, which in any case might be formed in small amounts [23]. Formation of TiAl 3 on the titanium side of the diffusion couple, together with the movement of markers, proved that aluminum is the dominant diffusing specie [4,21,22,[25][26][27]. However, diffusion of titanium has also been reported by some workers [28,29].…”

Interreactions of TiAl<SUB>3</SUB> Thin Film on Bulk &gamma;-TiAl and on Bulk &alpha;<SUB>2</SUB>-Ti<SUB>3</SUB>Al Alloys at 700&mdash;1000&deg;C

“…From Table 3, the Q k values of TiAl 2 and -TiAl phases formation in this study are very similar in magnitude with those of the TiAl 3 phase in the Ti-Al thin film diffusion systems, [9][10][11][12] but are lower than those of Ti-Al product formation in Ti-Al bulk systems. [6][7][8] A possible explanation in Q k for this discrepancy could be that the intrinsic defects, voids and the residual stress inherent in the thin film are more than those in the bulk form.…”

“…Moreover, from many reported diffusion couples tests in the Ti-Al system, the TiAl 3 layer is observed to grow rapidly and complies with the parabolic time dependence. 6,[9][10][11][12] In our cases, we suggest that the TiAl 3 growth in Fig. 1 is also following the parabolic law, although its growth kinetics is not investigated in this study.…”

“…In addition, the growth of the TiAl 3 phase in the Ti-Al thin film couple system obeys the diffusion controlled kinetics with activation energies varying from about 150 to 190 kJ/mol according to different researchers. [9][10][11][12] To our best knowledge, most investigations on phase interreactions in the Ti-Al system have concentrated on the TiAl 3 formation of the thin film diffusion couple or the bulk diffusion couple. In the case of the diffusion couple of TiAl 3 thin film on -TiAl or on 2 -Ti 3 Al bulk alloys, their interreactions to form TiAl 2 andTiAl phases at high temperature have not previously been reported.…”