Solidification Reaction Sequence of Co-Rich Nb-Al-Co Alloys
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TION
THE natural choice of materials for high-temperature applications, such as in turbines in aero-engine applications, is Ni-base superalloys owing to their excellent combination of strength, ductility, and high-temperature oxidation properties.[1,2] However, with increasing demands on efficiency and step increase in turbine entry temperatures, there has been a keen interest in the development of alternative alloy systems.[3] One alternative that has been explored refers to the RM-Al-X system, where RM = Ta, Nb and X = Pd, Fe, Ti. For instance, it has been demonstrated that in the Pd-Al-Nb system, the ordered B2 precipitates present in the disordered A2 matrix constitute the strengthening phase.[4] This principle, in fact, is similar to that in the Ni-base alloys, where the ordered c¢ (L12) precipitate in the disordered A1 fcc matrix provides the high-temperature strength, while the matrix is ductile.[5–7] Also, the beneficial role of Al is to impart oxidation resistance through the formation of a continuous layer of Al2O3. Alternatively, the intermetallic Laves phase in iron-aluminide alloys has also been identified as a promising candidate for structural applications.[8–10] Laves phases show outstanding strength at elevated temperatures above 1273 K (1000 C) but are
L.M. FEITOSA and H.B. DONG are with the Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK. Contact email: [email protected] N. D’SOUZA is with Rolls-Royce plc, Derby, DE24 8BJ, UK. G.D. WEST is with the Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7AL, UK. Manuscript submitted January 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
brittle at room temperature due to their structures.[9,11] It has been shown that Ta is an important alloying addition to Fe-Al, as it can form the ternary Laves phase with the hexagonal C14 structure.[9] Recent investigations indicate that, in addition to the formation of the (Fe,Al)2Ta Laves phase, when Ta is added to Fe3Al, there is the unexpected formation of the Heusler-type phase.[12] It is not surprising, therefore, that the Fe-Al-Ta system has been studied with an emphasis on the Laves and Heusler phases, and on the relationship existing between alloy composition, phase stability, microstructure, and mechanical properties.[13,14] Another ternary, the Co-Al-Nb alloy system, exists, in which both the ternary Laves phase and the Heusler phase, Co2AlNb, occur.[15–17] Additionally, unlike the Fe-Al-Ta ternary, the Co-Al-Nb system is of significant interest for the fundamental understanding of the stability of different types of poly types of Laves phase. This is because all three types of Laves phase, hexagonal C14 and C36 as well as cubic C15, are stable in the binary Co-Nb system.[18,19] It is this alloy system that is the subject of investigation in the current study. From metallographic observations of as-cast microstructures, composition analyses, and differential thermal analysis, a liquidus projection was determined in the ternary Co-Al-Nb system and the solidification p
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