Microstructure and Creep Behavior of Fe-27Al-1Nb Alloys with Added Carbon
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GH-TEMPERATURE creep resistance and tensile strength of Fe-Al-based alloys can be improved by adding various elements, such as Si, Ta, Ce, Zr, Mo, Hf or Nb.[1] There are several possible advantages of adding niobium: 1. Niobium increases the temperature for the D03-B2 transformation.[2] There is evidence for a positive correlation between the solution strengthening and an increase in this transformation temperature.[3] 2. The solubility of Nb in Fe-Al is relatively low, and strengthening due to precipitating particles of the Laves phase is expected at niobium concentration greater than 1 at. pct.[4] 3. Niobium increases the B2-A2 order–disorder transformation temperature.[2] Both diffusion and creep are slower in an ordered phase. 4. Niobium readily reacts with carbon, which is typically present in iron charges to form carbides. 5. The addition of Nb should result in the formation of a passive layer, which would inhibit the entry of hydrogen into the iron aluminide matrix.[5]
FERDINAND DOBESˇ, Senior Researcher, is with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zˇizˇkova 22, 616 62 Brno, Czech Republic. Contact e-mail: dobes@ ipm.cz PETR KRATOCHVI´L, Professor, and JOSEF PESˇICˇKA, Associate Professor, are with the Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2, Czech Republic. VEˇRA VODICˇKOVA´, Senior Assistant Professor, is with the Department of Material Science, Faculty of Mechanical Engineering, Technical University of Liberec, Ha´lkova 6, 461 17 Liberec 1, Czech Republic. Manuscript submitted August 5, 2014. Article published online 31 January 2015 1580—VOLUME 46A, APRIL 2015
The beneficial effect of adding 1 pct niobium (atomic percent is given throughout this paper) to a binary Fe3Al alloy on the creep resistance was first reported by McKamey et al.[6] A number of alloys based on the Fe28 pct Al-5 pct Cr alloy with additions of 0.2 pct or 0.5 pct Nb were produced in the same laboratory in further developmental efforts.[7] It was shown that 0.5 pct Nb increased the strength. The results obtained in the complex alloy FA-180, which contained 28 pct Al, 5 pct Cr, 0.5 pct Nb, 0.8 pct Mo, 0.025 pct Zr, 0.05 pct C, and 0.005 pct B are summarized in Reference 8. The microstructural analysis revealed that the strengthening was due to a dispersion of fine Nb- and Zr-rich carbides in the matrix and along grain boundaries. Several Fe-Al-based alloys with Nb additions in a range of 2 to 5 pct were studied by Morris et al.[9–12] A promising high-temperature strength was obtained in the alloy with 25 pct Al and 2 pct Nb.[9] Creep tests results were reported only for the ferritic alloy, Fe15 pct Al-5 pct Nb.[12] The creep of ferritic Fe-Al alloys with niobium additions was also studied by Baligidad.[13,14] The beneficial effect on the high-temperature strength and creep resistance was observed for a small (0.83 pct) addition of niobium to the Fe-16 pct Al-0.43 pct C alloy.[13] Contrary to this optimistic ob
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