Oxidation Characteristics of Ti-14Al-21Nb(mass%) Alloy in N2-O2 at High Temperatures
Makoto Akai, Shigeji Taniguchi, Toshio Shibata
The oxidation characteristics of Ti-14Al-21Nb (mass%) were studied in a temperature range 1000K to 1300K in O2, N2-21 vol%O2 and Ar-21 vol%O2 under atmospheric pressure. The oxidation rates in N2-O2 are lower than those in O2. The oxidation rates in Ar-O2 are almost equal to those in N2-O2 below 1200K. However, at 1300K the oxidation rate in Ar-O2 is nearly equal to that in O2. The oxidation curves are almost parabolic, except for those in O2 and Ar-O2 at 1300K, for which temporal acceleration was observed. The scale consists of three layers: outermost layer of sintered rutile grains, intermediate alumina rich layer, and innermost layer consisting of a mixture of rutile, alumina and niobium oxide grains. In N2-O2, a nitride layer (TiN), an Nb-rich layer (Nb2Al) and an Al-rich layer (TiAl) are additionally formed in this order beneath the three layered scale. These differences in the oxidation characteristics are attributable to the folowing observations;
1. A more continuous alumina rich layer is formed because of the reduced oxygen pressure below 1200K.
2. A continuous TiN layer is formed near the scale/substrate interface.
3. By the selective nitridation of Ti near the scale/substrate interface, Al is enriched beneath TiN, and TiAl is formed.