Surface Segregation during Directional Solidification of Ni-Base Superalloys
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INTRODUCTION
SUPERALLOYS are a class of materials that have been specifically developed for high-temperature applications. The enhanced high-temperature strength is principally derived from the ordered c¢ precipitates having optimum size and morphology.[1] The components of the high-pressure (HP) and intermediatepressure (IP) stages of the turbine need to possess prolonged high-temperature strength and thereby require a microstructure that shows excellent microstructural stability. This has been achieved through two ways: (1) a progressive migration in the processing route from equiaxed to directional and single-crystal solidification that has resulted in the single-crystal morphology, where elimination of grain boundary strengthening elements, such as Hf, B, enables complete solutioning of the c¢ phase during heat treatment; (2) in newer generation Ni-base superalloys, refractory additions, such as Ta, W, and Re, have been specifically added to improve solid solution hardening.[1,2] However, because of the preferential partitioning behavior of such elements, the as-cast structure is severely cored, resulting in the formation of nonequilibrium interdendritic G. BREWSTER, EngD Student, is with the Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, United Kingdom, and with Operations and Engineering and Technology, Rolls-Royce plc, Derby DE24 8BJ, United Kingdom. H.B. DONG, Royal Society Industry Fellow, Precision Casting Facility, Rolls-Royce plc, Derby, UK is on leave from the Department of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom. N.R. GREEN, Professor, is with the Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. N. D’SOUZA, Casting Specialist, is with the Precision Casting Facility, Rolls-Royce plc, Derby, UK Contact e-mail: [email protected] Manuscript submitted August 18, 2007. Article published online January 11, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
phases. The requirement of an optimum c¢ morphology for high-temperature strength therefore requires dissolution of this nonequilibrium eutectic and simultaneous homogenization of the segregated microstructure (termed the solutioning stage in heat treatment) prior to the quench and aging sequence. During the aging stage in heat treatment, the fine c¢ precipitates formed during quenching from the solutioning temperature coarsen, resulting in the so-called cuboidal morphology that is desired. In the process of investment casting as previously highlighted, there are two essential stages therefore to assess the metallurgical integrity of the component: (1) inspection of the grain structure to ensure a singlecrystal matrix and absence of randomly nucleated secondary grains and (2) the residual eutectic levels after heat treatment. A maximum residual eutectic fraction that can be tolerated without detrimental effects is usually specified. In particular, the latter operation that is destructive in nature is performed on dummy turbine bla
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