EFFECT OF W-ADDITION ON THE OXIDATION BEHAVIOR OF TIAL INTERMETALLIC COMPOUND

Isothermal and cyclic oxidation behavior and scale structures of the W modified Ti-34.5 mass% Al alloys at 1073, 1173 and 1273 K in air were investigated comparing with those of a binary Ti-34.5 mass%Al. Isothermal oxidation resistance of TiAl was extremely improved by the W addition at temperatures from 1123 to 1273 K. Mass gains of the W-modified TiAls, which were isothermally oxidized below 1173 K, significantly decreased with increasing W content. However, at 1273 K, minimum appeared in the mass gain vs. W content curve at 4 mass%W. W was a beneficial element under the cyclic oxidation condition. Scale spallation did not occur. A multi-layered scale structure of the W-modified TiAl was observed to be in the following order from the outside: TiO2/Al2O3/TiO2 + Al2O3/Ti3Al/TiAl (matrix). The Al2O3 internal oxidation was suppressed by the reduction of the oxygen solid solubility caused by the W addition, which resulted in the external Al2O3 formation at the interface. The second Al2O3 layer and the inner Al2O3 layer which was observed to be interconnected were observed to be moe continuous than those of the binary TiAl, and must act as protective layers.