Quarternary and Quinary Additions to Directionally-Solidified X-X 3 Si Eutectics of Chromium and Vanadium
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Quarternary and Quinary Additions to Directionally-Solidified X-X3Si Eutectics of Chromium and Vanadium J Ang1, VA Vorontsov1, CL Hayward2, G Balakrishnan3, HJ Stone1 and CMF Rae1 Dept. of Materials Science, University of Cambridge, Pembroke St., Cambridge, CB2 3QZ, U.K. 2 School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, U.K. 3 Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K. 1
ABSTRACT An alternative high temperature structural alloy system based on the X-X3Si eutectic compositions of chromium and vanadium is put forward. These low-density (~6g/cm3) eutectics have a bcc solid-solution to increase alloy fracture toughness, and a A15 X3Si as the high temperature load-bearing phase. (½Cr,½V)-(½Cr,½V)3Si was used as the base alloy for further element additions, and is represented by the symbol 芄. 10at.% tantalum and aluminium were substituted for vanadium as quaternary and quinary alloy additions. Microstructure, elemental phase partitioning, compression creep and oxidation results will be discussed. Cr-Cr3Si has a tidy, fine lamellar microstructure. Vanadium coarsens and destabilises the lamellae to a limited extent. Tantalum addition causes two distinct populations of eutectic to form; one population having finer lamellae than the other. Aluminium does not coarsen or destabilise the lamellar microstructure. High temperature compression tests at 1200°C and 1300°C show that 芄 is stronger than the binary alloys, and of similar strength to the quaternary and quinary alloys. INTRODUCTION Over the last 70 years, marginal jet engine fuel efficiency has decreased with rising turbine inlet temperatures [1], due to the energy required to operate the intricate cooling systems required by nickel-based superalloys used in jet engine hot sections. Alternative materials that do not require cooling at current operating temperatures will result in higher engine efficiency. We would like to explore a low-density gas-turbine blade alloy based on the Cr-Cr3Si and V-V3Si eutectics, with the aim of developing an alloy that can operate at 1200°C /100MPa. Chromium silicides typically exhibit excellent corrosion and oxidation resistance, but are unacceptably brittle and retain insufficient high temperature strength [2-5]. It is hoped that the final alloy will form a dual-layer oxide with a Cr2O3 bottom layer to protect against corrosion, and a SiO2 top layer to protect against high temperature oxidation. The Cr-Cr3Si eutectic has a melting point at 1701qC [6]. It has a b.c.c. solid solution component to maximise fracture toughness, an A15 Cr3Si reinforcement phase, and a density of 6.81g/cm3. Chromium silicides are weaker at high temperature than the molybdenum and niobium silicides [2,3]. The analogous, low density V-V3Si system, has a eutectic temperature of 1870°C [7]. Vanadium has been chosen as an alloying addition due to its toughness and perfect miscibility with chromium. The A15 V3Si is reported to have superior high temperature hardness than Cr3Si [6], although the inability
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