ABSTRACT
Protective alumina scales are formed on Nb-Al alloys by the addition of third elements that reduce the solubility and diffusivity of oxygen in the alloy and produce microstructures in which the diffusivity of Al is high. Titanium is the most effective third element addition and stabilizes a high temperature B2 phase on which alumina scales can be formed at an atomic fraction of aluminium (NAl) as low as 0.32. At temperatures below 1300°C, the alloy transforms to a two phase microstructure of γ-TiAl and σ-NbAl2 on which the formation of alumina scales is more difficult. A transition from external to internal oxidation of Al occurs and the alloy must be modified in composition and structure to promote the formation of alumina scales at lower temperatures. The effects of Cr, V, and Si additions, Al content, Nb:Ti ratio, and temperature on the transition from internal to external oxidation of Al have been studied by optical metallography and scanning electron microscopy. Reaction kinetics are correlated with alloy and oxide scale microstructures and the conditions under which protective alumina scales can be formed by the selective oxidation of Al are defined.
