Trattamenti Superficiali e Resistenza alla Ossidazione di una Lega Y-a2 TiAl (Ti- 30.5 Al 9W 0.4 Si)

TiAl alloys arc potentially interesting for structural applications at temperatures in excess of 800° C. Their specific mechanical resistance at high temperatures is very good and this fact, with an acceptable oxidation resistance, suggests the use of these materials in applications such as automotive components and turbogas engine blades. When exposed to oxidising atmospheres, these alloys normally form a poorly protective TiO2 rich scale. As matter of fact the inadequate oxidation resistance is one of the main life limiting factors of these materials. In this study, an oxidation testing of a TiAl alloy has been performed in air over a temperature range from 750 °C to 1000 °C. We have followed two strategics for improving oxidation resistance of the alloy: surface nanocrystallisation via laser ablation and Cr surface enrichment. In the first methodology the extremely high cooling rate (1010 Ks-1 is the calculated value) following surface melting, generates a very thin nanocrystallised layer, in which nanometric grains are visible, as indicated by the TEM micrograph in figure 3. In this case the diffusivity of aluminium increases by more than hundred times and becomes comparable to the diffusivity of titanium (figure 9). An increased diffusivity helps the oxidation of Al, which would not be the kinetically favoured element for oxidation. Although, this same treatment results in a diffuse cracking of the alloy surface, as displayed by SEM image in figure 4, the formation of a more protective alumina richer scale becomes possible nonetheless. The second treatment involves the deposition on the alloy surface of a 5-7 micron thick layer of pure chromium, with a subsequent annealing to obtain interdiffusion, performed at a temperature of 900 °C in inert gas for 10h. The expected concentration profile for chromium is depicted in figure 1, whereas the actual microstructure of the deposited surface layer is displayed by figure 2 and the actual chromium concentration profile is shown by figure 7 . This approach, exploiting the so-called chromium gettering effect, that reduces the critical aluminium concentration for an outward oxidation, was very successful. The oxidation kinetic of the alloy is more than a hundred times lower (figure 8). The laser ablation nanocrystallisation results are not as good as the ones obtained with chromium treated samples, but still provide indications for an improved oxidational behaviour of the alloy with respect to the untreated condition. The relationship between diffusivity and oxidation rate was thoroughly studied.