The Effect of Niobium-Ion Implantation on the Oxidation Behavior of γ -TiAl Alloys in Static and Flowing Air

Abstract

γ-TiAl (Ti–50Al at.%) alloys were implanted with Nb ions at an acceleration energy of 50 keV, at a dose of 1.2×1017 ions/cm2. The cyclic-oxidation behavior of the unimplanted and Nb+-implanted TiAl specimens was investigated at 850°C in static air and in air with a flow velocity of 12.0 m/s (1000 ml/min). In static air, the unimplanted TiAl specimen showed rapid oxidation during a transition period of about 80 hr, after which partial scale spallation occurred and a net mass loss was observed. In flowing air, the whole scale spalled off after each cycle. On the other hand, Nb-ion implantation led to the formation of an adherent protective Al2O3 scale during oxidation in both static and flowing air, thereby significantly improving the cyclic-oxidation resistance of γ-TiAl alloys. A remarkable deference in the initial-oxidation behavior between unimplanted and Nb+-implanted specimens was also observed. A mixed TiO2/A2O3 scale on the unimplanted specimen developed at a high growth rate during the very initial stage of oxidation. In contrast, the initial scale growth rate was significantly decreased by Nb-ion implantation and an Al2O3-rich layer was found present as the inner part of the initial scale on Nb+-implanted TiAl. Flowing air appeared to cause severe scale spallation during oxidation of unimplanted TiAl, but not to have any influence on the adhesion of the scale on Nb+-implanted TiAl.