CYCLIC-OXIDATION BEHAVIOR OF TIAL AND OF TIAL ALLOYS

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000-degrees-C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO2-Al2O3 and an outward-growing coarse-grain TiO2 layer or TiO2 + Al2O3 mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al2O3 layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclic-oxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al2O3 layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.