Effect of the third elements on high temperature oxidation resistance of TiAl3 intermetallic compounds

The brittleness of TiAl3 is improved by the addition of third elements such as manganese, silver, chromium and so on. The effects of the third elements on oxidation resistance of TiAl3 have been investigated in the point of view of the microstructure and products of oxidation layer. The oxidation test was carried out at 1000degreesC for 25 h or 30 d in air. For the copper-substituted alloy, Ti(Al, Cu)(3), the oxidation layer consisted of TiO2, whose microstructure was porous. Thus Ti(Al, Cu)(3) showed very low oxidation resistance. While, for the manganese, silver, iron or chromium-substituted alloys, these oxidation layers consisted of only Al2O3. These alloys showed the oxidation resistance. It is not expected that the iron- or chromium-substituted alloys show the oxidation resistance of long duration, because oxygen atom was detected by EDS analysis in AI-depleted layer after the oxidation test for 30 d. In the oxidation test at 1000degreesC, the oxidation layer of the iron- or chromium-substituted alloys was constituted of alpha-Al2O3 and theta-Al2O3. On the other hand, that of the manganese- or silver-substituted alloys was constituted of only alpha-Al2O3 and their oxidation layer is dense. From these results, it was understood that the substitution elements would influence on the temperature of theta --> alpha transformation. That is, Fe and Cr would solute to theta-Al2O3, and then theta --> alpha transformation temperature would increase. While, Mn and Ag would scarcely solute to theta-Al2O3, and then theta --> alpha transformation temperature would be almost the same as that of pure theta-Al2O3. In the oxidation test at 1000degreesC, the residence of theta-Al2O3 decides oxidation resistance of Ti(Al, X)(3) materials. The choice of the substitution element promoting the transformation from theta-Al2O3 to alpha-Al2O3 during heating process is important for improving in the oxidation resistance.