Comparison of Nb- and Ta-effectiveness for improvement of the cyclic oxidation resistance of TiAl-based intermetallics
High-temperature cyclic oxidation resistance of Ti–45.2Al–7.2Nb and Ti–44.8Al–6.6Ta (in at.%) alloys is studied. The alloys were oxidized at 800–1000 °C for up to 380 h and oxidation kinetics was expressed as a function of weight gain versus time. Both as-cast and oxidized alloys were examined by op...
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Veröffentlicht in: | Intermetallics 2011-04, Vol.19 (4), p.493-501 |
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Sprache: | eng |
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Zusammenfassung: | High-temperature cyclic oxidation resistance of Ti–45.2Al–7.2Nb and Ti–44.8Al–6.6Ta (in at.%) alloys is studied. The alloys were oxidized at 800–1000 °C for up to 380 h and oxidation kinetics was expressed as a function of weight gain versus time. Both as-cast and oxidized alloys were examined by optical and scanning electron microscopy, energy- and wave-dispersion spectrometry, X-ray diffraction and X-ray fluorescence spectrometry. In contrast to previously reported studies, tantalum is found to retard oxidation more effectively than niobium, despite its lower content in the investigated alloy. It also shows a significantly better adherence of oxide scales to the substrate, i.e. a lower tendency to scale spallation, particularly at 1000 °C. Chemical and phase analysis of oxidized alloys reveals considerable differences in element and phase distribution in oxidation-affected zones: 1. Niobium is observed to increase alumina amount in scales more effectively than tantalum. 2. Concentration of tantalum in scales is lower than of niobium. 3. Tantalum causes the occurrence of nitrides both in scales and in metallic substrate, while niobium supports formation of nitrides only in metallic substrate beneath scales. Differences between the investigated alloys are discussed in relation to oxidation mechanism, stability of Ta- and Nb-containing compounds.
► Oxidation of Ti-45.2Al-7.2Nb and Ti-44.8Al-6.6Ta (in at. %) alloys at 800-1000°C in air was studied. ► It was observed that both alloys show similar and excellent oxidation resistance at 800-900°C. ► At 1000°C, a strong oxide spallation is observed, mainly on the Nb-containing alloy. The Ta-containing alloy shows significantly better oxidation resistance at 1000°C. ► Despite the similarity of both ternary additives, chemical microanalysis of scales revealed that there are significant differences in element distributions between the alloys. ► Tantalum was shown to slow down the oxidation more efficiently than niobium. |
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ISSN: | 0966-9795 1879-0216 |
DOI: | 10.1016/j.intermet.2010.11.025 |