Microstructure and Oxidation Resistance of Cr-Ta-Si alloys
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Microstructure and Oxidation Resistance of Cr-Ta-Si alloys 1
Ayan Bhowmik , Hon Tong Pang1, Steffen Neumeier1, Howard J. Stone1, Ian Edmonds2 1
Rolls-Royce UTP, Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom 2 Rolls-Royce plc, P.O.Box 31, Derby, DE24 8BJ, United Kingdom
ABSTRACT The phase equilibria and oxidation resistance of alloys lying near the Cr-rich end of the CrTa-Si system have been investigated. Samples were prepared by arc-melting and homogenized at 1300°C for 500hrs. Identification of the phases present and their compositions were carried out using x-ray diffraction and electron probe micro-analysis and the ternary phase diagram on the Cr-rich end was plotted. A three-phase equilibria was found to exist between an A2 Cr-solid solution, a hexagonal Laves phase and the A15 Cr3Si intermetallic phase for alloys with higher contents of Si. Thermo-gravimetric analysis of the alloys at 1100°C demonstrated an improvement in the oxidation resistance of the ternary alloys with increasing Si-content. The microstructures of the oxidized samples revealed the formation of a thick chromia layer on top of a Cr,Ta-mixed oxide layer and an internal oxidation zone for all the alloys. A protective silica layer was not observed to form in any of the alloys tested.
INTRODUCTION The drive for improved performance from gas turbine engines has lead to extensive research into high temperature intermetallic compounds [1, 2]. Among the many alloys that have been considered, refractory metal Laves phase alloys have shown substantial promise for such applications due to their high melting temperatures and good strength and damage tolerance at elevated temperatures [3-5]. However, these materials suffer from severe brittleness and low fracture toughness at lower temperatures, which has prohibited their use in monolithic form. As a result, it is generally accepted that Laves phases must be present in conjunction with a relatively ‘tougher’ phase, which would be capable of rendering adequate damage tolerance to the alloy. Another prerequisite for such turbine engine applications is good environmental resistance under service conditions. This has prompted the selection of alloys whch possess high levels of chromium. In the binary systems that exist between Cr and X (where X= Ta, Nb, Zr and Hf), a Cr2X Laves phase is typically observed to exist in equilibrium with a Cr-solid solution in the Cr-rich end. [6-9]. Alloys based upon these equilibria have been widely investigated for their suitability for high temperature service with the most encouraging results being obtained from alloys based upon the Cr-Cr2Ta and Cr-Cr2Nb systems. In the present study, the phase equilibria and oxidation resistance of alloys based upon the Cr-Ta-Si system have been investigated. In a previous study, the addition of Si has been found to improve the fracture toughness relative to binary Cr-Cr2Ta alloys [10]. However, the effect of alloying with Si on the phase equilibria in the C
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