Oxidation of TiAl based intermetallics

Abstract

The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO2. In all the alloys a layer of porosity is created just below the outer TiO2 layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies.