Initial stages of oxidation of Ti45Al7Nb0.4Y alloy at 900 °C in air

The initial stage of oxidation of Ti45Al7Nb0.4Y alloy (at.%) oxidized at 900 °C in air was investigated by using x-ray photoelectron spectroscopy and Auger electron spectroscopy. Experimental results revealed that YAl₂ segregated along the grain boundaries preferentially oxidized to Y₂O₃ and Al₂O₃ due to strong affinity of Y to oxygen. Oxides grew faster at the grain boundaries than in lamellar colonies. As a result, Y and Al oxides pegs protruded into the substrate, which can increase the contact areas of oxide scale and substrate. Moreover, inward diffusion of oxygen more easily occurred along the grain boundaries. As a result, it promoted the external oxidation of Al within the grains due to lower inward diffusion flux of oxygen. And coarse-grained Y₂O₃ blocked the cationic intergranular diffusion. Therefore, Y addition can effectively enhance the Al₂O₃ layer and suppress the TiO₂ outermost layer.