The effect of an Ni-Cr protective layer on cyclic oxidation of TiAl.

The effect of an 80Ni-20Cr (at.%) metallic coating on the cyclic oxidation behaviour of a Ti(3)Al-based alloy with the composition Ti-25Al-11Nb (at.%) was investigated in this study. Cyclic oxidation tests were carried out in air at 600 degrees C and 900 degrees C for 120 h. For one cycle test, the specimens were held for 24 h at test temperature and then furnace-cooled to room temperature. The oxidation rate was determined by plotting the mass gain per unit surface area of the specimen vs. exposure time. The morphology and composition of the oxidation products were characterized on the cross-section of the specimens by scanning electron microscopy, energy-dispersive X-ray spectroscopy and atomic force microscopy. The oxidation scale forms during exposure at both 600 degrees C and 900 degrees C. TiO(2) is the main oxide component, whereas the Al(2)O(3) layer appears only discontinuously. The remarkable improvement in oxidation resistance at 900 degrees C was attributed to the chemical composition and structure of the scale formed on the 80Ni-20Cr coating.