Mechanisms of Topologically Close-Packed Phase Suppression in an Experimental Ruthenium-Bearing Single-Crystal Nickel-Ba
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HIGH fuel costs and increasing environmental pressures demand unprecedented and ambitious performance, efficiency, and emission targets from the next generation of gas turbine engines. The development of more temperature-resistant single-crystal Ni-base superalloys for turbine blade applications is critical to the delivery of this technology because this dictates the turbine entry temperature, and therefore thermal efficiency of the engine. To date single-crystal Ni-base superalloys have been through three generations of development; the major advance of the third generation alloys was achieved through rhenium (Re) additions, which contributed to significant gains in creep capability over earlier generation alloys.[1–3] However, the difference between the crystal structures and atomic sizes of Re (hcp) and Ni (fcc) limits the solubility of the former in the latter. Moreover, the low diffusivity of Re, which partitions strongly to the dendrite core during solidification, makes homogenization of the microstructure during solution heat treatment difficult.[4,5] In certain instances, local supersaturation of the c matrix caused by residual Re microsegregation is relieved by the precipitation of deleterious topologically close-packed (TCP) phases following prolonged exposure to elevated temperatures.[2,6,7] The major detriment caused by the formation
of TCP phases is the depletion of the important strengthening elements, such as Cr, Mo, W, and Re from the desired alloy phases and their concentration in the brittle intermetallic TCP phases; this adversely affects the mechanical properties, most notably the high-temperature creep rupture strength.[6,8–10] Microstructural instability therefore limits the degree to which Ni-base superalloys can be alloyed and, as a consequence, is one of the main factors impeding continued increases in their maximum operating temperature capability. Recent initiatives to address these microstructural stability limitations have involved the addition of the refractory platinum group metal ruthenium (Ru) to single-crystal Ni-base superalloys. The improvement in stability afforded by Ru additions not only extends the creep capability of single-crystal superalloys to higher temperatures,[10–15] but it also allows a more holistic approach to alloy design, where the relative concentrations of the constituents are tailored to achieve a balanced alloy that more adequately satisfies the long list of turbine blade material requirements, not simply that of creep temperature capability. However, the mechanism(s) by which Ru enhances microstructural stability remains debatable.[6,10–12,16,17] This article attempts to bridge this gap in understanding.
II. R.A. HOBBS is a Turbine Aerofoil Materials Specialist with RollsRoyce plc, Derby DE24 8BJ, United Kingdom. Contact e-mail: [email protected] L. ZHANG, Research Fellow, and C.M.F. RAE, Lecturer, are with Department of Materials Science & Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom. S. TIN, Associate Professor, is with
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