A Study of Quaternary Cr-Cr 2 Ta Alloys - Microstructure and Mechanical Properties
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A Study of Quaternary Cr-Cr2Ta Alloys - Microstructure and Mechanical Properties
Varun Choda1, Ayan Bhowmik1, Ian M. Edmonds2, C. Neil Jones2 and Howard J. Stone1 1 Department of Materials Science and Metallurgy, Pembroke Street, University of Cambridge, Cambridge CB2 3QZ, United Kingdom 2 Rolls-Royce plc, P. O. Box 31, Derby DE24 8BJ, United Kingdom
ABSTRACT Six alloys based on Cr-10Ta-7Si (by at.%) with quaternary additions of 0.5Ag, 5Ti, 1Hf, 3Mo, 3Al, or 3Re (by at.%) substituted for Cr were produced by vacuum arc-melting. The microstructures of the alloys were found to predominantly consist of a eutectic mixture of an A2 Cr-based solid solution and a C14 Cr2Ta Laves phase along with proeutectic Cr2Ta dendrites. Microstructural macro- and micro-scale inhomogeneities were observed in all alloy ingots, which were attributed to the non-equilibrium arc-melting process. The measured lattice parameters of the constituent phases and the elemental partitioning behaviour between the phases have been correlated with the respective covalent atomic radii. The bulk hardnesses of the alloys, along with the hardness of individual phases, have also been reported. INTRODUCTION Recently there has been renewed interest in developing Cr based alloys for high temperature structural applications, for example as turbine blades in gas turbine engines [1-3]. This interest has arisen as a result of the higher melting temperatures, higher elastic moduli, and lower densities offered by these alloys in comparison to nickel-base superalloys currently used for these applications [4]. In this study, two-phase Cr-Cr2Ta alloys based on Cr-10Ta have been considered. These are hypereutectic alloys, close to the eutectic composition of Cr-9.6Ta [5]. In these alloys, the Cr2Ta Laves phase possesses a high melting point and considerable high temperature strength. However, it also exhibits unacceptably low fracture toughness at room temperature. The presence of the A2 Cr-solid solution offers some ductile phase toughening; however, this is limited at lower temperatures due to interstitial embrittlement. The high Cr contents in these alloys also offer the potential of forming a protective chromia scale at elevated temperatures, although improvements are still required for these alloys to be competitive with nickel-base superalloys. Previous work on these alloys has therefore focused on improving their room temperature fracture toughness and elevated temperature oxidation resistance [6-10]. EXPERIMENT Vacuum arc-melting and characterisation Solid pieces of high purity elements (99.9 wt.%+) were arc-melted in a water-cooled copper hearth into cylindrical ingots of approximately 40g mass. The details of the melting process have been outlined elsewhere [11, 12]. The nominal compositions of the alloys investigated are given in Table I. Ingots were then encapsulated in quartz tubes under an Ar-atmosphere, and heat treated at 1100 C for 72 hours to reduce micro-segregation and to relieve residual solidification stresses induced during processing. The impuri
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