The Effect of Low Concentrations Nb and C on the Structure and High-Temperature Strength of Fe 3 Al Aluminide
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TRODUCTION
IRON aluminide-based alloys are ideal candidates for the development of new structural materials with improved performance in petrochemical, power-generation, and aeronautical applications.[1,2] They have excellent resistance to oxidation and sulfidation. Their density is about two thirds of the steel density. Moreover, they have high electrical resistance. The input raw materials are relatively cheap due to their occurrence in the earth’s crust. The main drawbacks of these alloys are a bad workability at room temperature and low high-temperature (HT) strength. The low-temperature plasticity can be improved by the off-stoichiometric compositions, ternary additives (chromium, molybdenum, and manganese), and grain-refining agents (TiB2 and Ce). Improvements in the creep resistance and strength are given by solid solution hardening and/or precipitation hardening. The HT mechanical properties of Fe3Al-type alloys can be enhanced using a variety of methods (for an overview see e.g., References 3 and 4). Especially, effective method is the addition of elements with a low solubility in the Fe3Al-matrix. Niobium has a potential for both solid solution hardening and precipitation strengthening of Fe3Al aluminide. The beneficial effect of niobium addition on
PETR KRATOCHVI´L is with the Faculty of Mathematics and Physics, Charles University of Prague, Ke Karlovu 5, 121 16 Prag, Czech Republic. MARTIN SˇVEC and VEˇRA VODICˇKOVA´ are with the Department of Material Science, Faculty of Engineering, Technical University of Liberec, Studentska´ 1402/2, 46117 Liberec, Czech Republic. Contact e-mail: [email protected] Manuscript submitted February 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
e.g., creep resistance was first reported by McKamey and Maziasz.[5–7] The results obtained in a complex alloy FA-180 containing 28Al, 5Nb, 0.8Mo, 0.025Zr, 0.05C, 0.005B (atomic percent is given throughout) are summarized in Reference 7. The annealing for 1 hour at 1423 K (1150 °C) followed by quenching was successfully used to obtain the longest creep rupture live-time and the slowest minimum creep rate in creep tests at 866 K (593 °C). 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. Extensive study of niobium additions to Fe3Al-based alloys was reported by Morris et al.[8–11] The improvement of mechanical properties at elevated temperatures was disappointingly small. On the other hand, it was shown by Yu and Sun[12] that addition of 0.5 at. pct Nb to Fe-28Al-4Cr (at. pct) alloy increased creep rupture live-time by one order of magnitude. A similar beneficial effect on high-temperature strength and creep resistance was observed for a small (0.83 at. pct) addition of niobium to Fe-16Al-0.43C (at. pct) alloy.[13] On the contrary, the same author reported that addition of niobium up to 1.1 at. pct to Fe-19Al-3.65C (at. pct) alloy ‘‘did not exhibit any significant improvement in either creep life or minimum creep rate.’’[14] T
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