Kinetics of formation of the TiAl3 phase in the Ti-Al system

Abstract

In a general case the solid-phase reaction of aluminum with titanium leading to the formation of TiAl3 is controlled by dual kinetics. In the initial period the rate of the TiAl3 is controlled by dual kinetics. In the initial period the rate of the TiAl3 formation process at the interface between titanium and aluminum is constant with time. The temperature dependence of the formation rate constant under kinetic conditions obeys Arrhenius' equation. The energies of activation of TiAl3 formation in the linear stage, Es = 170 ± 30 in the solid-phase reaction and e1 = 127 ± 30 kJ/mole in the reaction of titanium with liquid aluminum, match, allowing for errors in the determination of Es and e1, the standard heat of formation of TiAl3, ΔH298 = 142 ± 4 kJ/mole [11]. It is therefore reasonable to conclude that the mechanism of contact reaction in the linear stage of layer growth is the same in both cases and is determined not by diffusional transport but by chemical kinetics. Differences between values of rate constants of the reactions of titanium with solid and liquid aluminum are apparently mainly due to the method employed in processing experimental data. The true area of the reaction surface between titanium and liquid aluminum is considerably larger than the surface area of the starting titanium specimen. Consequently, calculation in this case yields larger values of reaction rate constants. During the reaction of titanium with solid aluminum the growing thickness of the TiAl3 phase layer increasingly hinders the supply of aluminum to the reaction front, and this then becomes the limiting stage of the process. As a result, the conditions of layer growth change from kinetic to diffusional. When titanium reacts with liquid aluminum, the thickness of the thin layer of columnar TiAl3 crystals adjacent to titanium and the number of capillaries crossing this layer do not vary as functions of reaction time (up to 5.5 h at 850°C). The rate of growth of this layer is therefore equal to the rate of its disintegration on the outer boundary. In this case, since the length of the capillaries does not vary owing to constancy of the layer thickness, the flow of aluminum to the titanium surface remains unchanged. Thus, during the reaction of titanium with liquid aluminum the intermetallic compound layer grows according to a law which is always linear, never changing to parabolic.