Oxidation oxide scale spallation resistance

Lee etal. studied the high temperature oxidation properties of SiC particle-reinforced Ti3Al and TiAl alloys, and Si3N4-dispersed TiAl alloys [163-165]. Their results showed that addition of these dispersions could improve the oxidation and scale spallation resistances, though the microstructure of the oxide scale was not significantly changed. The mechanisms proposed by these authors were as follows ... 

Similar calculations can be done for an oxide scale thickness of 1.5 pm. In this case, larger grains of around 80 pm size should be selected to reach a minimum of 17 wt % Cr in the bulk. It should be noted that chromia scale is the limiting conductive pathway between the functional anode and the power leads. Therefore, increasing the thickness of the oxide scale would contribute to increase cell resistance, and ultimately, to spallation of the layer. 

On the basis of the above survey surface application of Nb of a high concentration may be effective to enhance the formation of Al203.This hypothesis was confirmed by using an ion-implantation technique. The results are shown in Fig. 6 [40], where the Nb-implanted specimen with a maximum surface concentration of 35 mole% Nb shows an excellent oxidation resistance during thermal cycle with temperature varying between room temperature and 1200 K for at least up to 10 cycle (200 h). The scale was virtually AI203 and its structure was similar to that shown in Fig. 3, and did not show any spallation during the cyclic oxidation. 

The oxidation life of ODS FeCrAl alloys is determined by their ability to form or reform a protective alumina scale, and can be related to the time required for the aluminum content of the alloy to be depleted to some minimum level [2-5]. As a result, the service life is a function of the available aluminum content of the alloys and the minimum aluminum level at which breakaway oxidation occurs. Therefore, there is a minimum cross sectional thickness which can be safely employed at temperatures above 1200°C.The major factors that result in depiction of the reservoir of aluminum in the alloy are the inherent growth rate of the aluminum oxide and the tendency for the scale to spall, which results in a (temporary) increase in oxide growth rate in the area affected by spallation. Because of their significantly higher aluminum content >28 at% compared to 9 at%), alloys based on Fe3Al afford a potentially larger reservoir of aluminum to sustain oxidation resistance at higher temperatures and, therefore, offer a possible improvement over the currently-available ODS FeCrAl alloys [61-... 

---