The isothermal and cyclic high temperature oxidation behaviour of Ti-48Al-2Mn-2Nb compared with Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr

The isothermal and cyclic oxidation behaviour of Ti-48Al-2Mn-2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic a lloys Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr. Tests were performed in air between 800 and 900 degrees C. At 800 degrees C Ti-48Al-2Mn-2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable wi th Ti-48Al-2Cr-2Nb, and superior to Ti-48Al-2Cr. At 900 degrees C the isoth ermal oxidation rate of Ti-48Al-2Mn-2Nb was similar as found for Ti-48Al-2C r-2Nb, but much lower as that of Ti-48Al-2Cr. Upon cooling the oxide scale formed on Ti-48Al-2Mn-2Nb was prone to spallation. During the cyclic oxidat ion at 900 degrees C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti-48Al-2 Mn-2Nb was higher than that of Ti-48Al-2Cr-2Nb, indicative of a higher susc eptibility for spallation. During the initial stage of oxidation of all tes ted materials a complex multi-phased and multi-layered scale was formed con sisting of alpha-Al2O3, TiO2 (rutile), TiN and Ti2AlN. After longer exposur e times the outer scale was dominated by TiO2. In case of the niobium conta ining materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO2 layer (under isothermal conditions). Two-stag e oxidation experiments with isotope tracers were performed to study the ox idation mechanism in more detail. (C) 1999 Elsevier Science Ltd. All rights reserved.