Interplay of valence and oxygen vacancy formation energy towards oxidation response of γ-TiAl alloyed with Group V and Period V refractory elements

Abstract

Oxidation response of Group V (V, Nb and Ta) and Period V (Zr, Nb and Mo) refractory metals alloyed γ-TiAl phase has been studied at 1000 °C in air. Growth of oxide zones involve two regimes, linear and parabolic for all the alloys except for the Ta-alloyed one where the initial segment had a growth exponent of 1.5 followed by the parabolic behavior. An external oxide zone develops for all the alloys comprising of three distinctive layers: (i) innermost (Al₂O₃ dominantly) (ii) intermediate (Al₂O₃ + TiO₂) and (iii) outermost (TiO₂ + small pockets of Al₂O₃). Tiny amounts of refractory based oxides of Nb₂O₅, Ta₂O₅, ZrO₂ and MoO₂ also formed for the respective alloyed γ-TiAl. An internal oxide region of (Al₂O₃ + Ti₃Al) developed in the case of binary γ-TiAl and Nb-alloyed, while (Ti, Zr) complex oxides had formed in the Zr alloyed γ-TiAl. Mo- and Ta-alloyed γ-TiAl produced no internal oxide layer, but segregation of Mo and Ta was observed at the alloy/oxide interface. Group V elements had good solubility in TiO₂ and oxidation resistance of the alloys increased down the group in the sequence of Ta > Nb > V. While a sequence of Nb > Mo > Zr was observed for the Period V elements with no specific trend. The Ta-alloyed γ-TiAl exhibited the highest oxidation resistance because of an increase in the vacancy formation energy of oxygen in TiO₂. The oxygen vacancy formation energy of TiO₂ was concluded to be the most crucial factor governing the oxidation resistance of alloyed γ-TiAl phase.