A study of nitride formation during the oxidation of titanium-tantalum alloys Page: 7 of 15
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usually contain Ta2O5 in contact with Ta rich 1-phase. The oxide overlaying the
nitride/oxide layer was a very fine grained mixture of TiO2 and Ta205. A bright field
TEM micrograph of the nitride layer along with a microdiffraction pattern taken from the
grain labeled in the micrograph is shown in figure 6. The nitride layer in this case was
identified from microdiffraction patterns as Ti2N. This identification was repeated several
times and no other nitride species was identified. This was further confirmed by the
quantitative PEELS results mentioned above.
Discussion
In both Ti5Ta and Ti40Ta the nitride layer was found in contact with the oxygen
stabalised a-phase layer in the substrate. The a layer in the substrate was continuous in
the Ti5Ta alloy. The scale in the Ti5Ta contains only rutile, and the substrate below the
metal oxide interface is continuous a-phase, suggesting that all points along the scale
metal interface are equivalent, particularly with respect to the activity of the variou
species. Therefore it is reasonable that the nitride layer in Ti5Ta is also found to be a
continuous single phase, in this case TiN. After 2 hours of exposure the substrate below
the scale metal interface of the Ti40Ta was a mixture of a and P phases, both of which
were saturated with oxygen. Therefore it is not surprising that the nitride layer is found
to be discontinuous. The fact that the nitride formed in this case is Ti2N rather than TiN
suggests that the reduced oxidation rate associated with the formation of the nitride layer,
cannot be attributed solely to the unique properties (ionic conductivity etc.) of a particular
nitride. Rather , the nitrogen effect must be associated more with the physical barrier that
the nitrogen rich layer presents to the transport of oxygen into the substrate. This may be
compared to the well known nitrogen effect observed in y-TiAl. In TiAl the presence of
nitrogen in the atmosphere results in accelerated oxidation.4 This effect has been
described by Rakowski et al.5 as resulting from the formation of a discontinuous layer of
TiN (in A12O3) at the scale/metal interface, which is subsequently oxidized to TiO2. This
mechanism seems applicable to the nitrogen effect in TiTa alloys as well, however in this
case since there is no element present which forms a slow growing protective oxide layer,
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Hanrahan, R. J., Jr.; Lu, Y. C.; Kung, H. & Butt, D. P. A study of nitride formation during the oxidation of titanium-tantalum alloys, report, December 1996; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc678256/m1/7/: accessed April 27, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.