Coatings for TiAl
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MRS BULLETIN/OCTOBER 1994
away kinetics have often been reported after long oxidation times. The oxidation products are TiO2 (rutile) and A12O3 (aalumina), and formation of Ti3 Al (a 2 -phase) in the substrate near the scale/substrate interface has been observed. A typical scale structure is shown in Figure 1, a fracture section of a TiAl coupon specimen oxidized at 1027°C for 100 ks. Figure 2 shows a model for development of the scale and structure of the substrate
Figure 1. Fractured section of a TiAl coupon specimen oxidized in oxygen at 1027°C for 100 ks, showing the scale structure.
near the scale/substrate interface for TiAl. In general, the oxide scale consists of two layers: an outer layer which is mainly TiO2 grains, and a porous inner layer which is a mixture of TiO2 and A12O3 grains. The inner layer contains many small uniformly distributed pores. A12O3 grains are particularly numerous near the interface between the two layers (Figure 2d). However, they are not sufficiently continuous to become a protective layer. In addition, there are large voids near the interface between the two layers (Figures 1 and 2). The development of the scale structure can be summarized in the following way (cf. Figure 2): 1. During the very initial period, both titanium and aluminum are oxidized to form TiO2 and A12O3 grains, respectively (Figure 2a). 2. Immediately after this, the TiO2 grains overgrow the A12O3 grains because of the difference in the growth rates, resulting in an outer TiO2 layer and an inner porous layer (Figures 2b to 2d). The sequence of void formation can be explained in terms of the initial growth of the TiO2 grains almost normal to the specimen surface, followed by their lateral growth. The formation of thin A12O3 platelets as internal oxides in the substrate is noteworthy (Figure 2d). Their growth direction is almost normal to the scale/substrate interface. The areas between the platelets are enriched in titanium, resulting in a transformation to Ti3Al. These areas are oxidized as the oxidation process proceeds. The lamellar structure thus developed is incorporated into the scale near the scale/substrate interface (Figure 2e). During steady-state oxidation, the scale retains a structure similar to that shown in Figure 2d. 3. After longer oxidation periods, several small cracks appear near the scale/substrate interface, as shown in Figure 2e. One possible reason for this is the growth of thin A12O3 platelets in the substrate. This induces stresses1314 owing to the volume increase accompanying oxide formation. The rate-controlling step for steadystate oxidation is ionic diffusion through the scale, provided the outer TiO2 layer is dense. The deviation from exact parabolic kinetics is attributable to the formation of small cracks in the scale which allow inward gaseous diffusion of oxygen to a small degree. The TiO2 grains provide more rapid diffusion paths because ionic diffusion through A12O3 grains is much slower. It has been reported15 that both oxygen ions and titanium ions diffuse through TiO2 gr
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