Study of the high-temperature oxidation behaviour of Ti and Ti4Nb with SNMS using tracers

The effect of niobium and water on the high-temperature oxidation behaviour of pure titanium and Ti4Nb (at,%) was studied. An oxidation apparatus was designed that allowed not only the performance of two-stage oxidation exper iments using synthetic air isotopically enriched in O-18(2) and N-15(2) but also the control of humidity using either H-2 O-16 or H-2 O-18. Oxidation experiments were performed in dry and humid atmospheres at 800 degrees C, T he oxide scales formed were analysed by secondary neutral mass spectrometry , The results obtained show that niobium significantly reduced the oxidatio n rate of titanium and also suppressed the influence of water. Moreover, th e presence of niobium stabilizes the formation of a nitride layer formed at the scale/metal interface. In humid atmospheres water is the main oxygen s ource for the oxidation process, This phenomenon has to be taken into consi deration when interpreting two-stage oxidation experiments and discussing o xidation mechanisms. Copyright (C) 2000 John Wiley & Sons, Ltd.