A technique for characterizing microsegregation in multicomponent alloys and its application to single-crystal superallo
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I. INTRODUCTION
INCREASED component lifetime at reduced scrap rate and cost are just some of the conflicting requirements facing designers of aerospace and industrial gas turbine blades. Substantial gains in operating temperatures are achieved using complex cast blade shapes with intricate air passageways[1] and complex nickel-based superalloy compositions. These alloys contain a large fraction of refractory elements such as Mo, Ta, W, and Re, making them more expensive and difficult to cast.[2,3] These elements are characterized by their low mobilities and provide solid-solution strengthening to both and phases, enhancing the creep-rupture lives of single-crystal (SX) superalloy components.[4] The refractory elements also segregate severely during casting,[5–8] leading to a number of problems, including (1) the formation of low melting point or brittle phases, (2) a nonuniform distribution of strengthening precipitates, (3) interdendritic porosity,[4] (4) misoriented grains,[2,8] (5) freckle formation,[9,10] (6) solidification cracking,[11] and (7) localized phase instability.[12] While some of these problems can be addressed, at increased cost, using long duration stepwise homogenization heat treatments or hot-isostatic pressing, most result in increased scrap rates. Therefore, when process route changes[13,14] or new alloy chemistries[15,16] are evaluated, it is usual to characterize the extent of microsegregation using a ranking scheme of randomly sampled electron microanalysis data. Thermodynamic quantities are often calculated from measurements of the as-cast segregation profile, in particular, the partition coefficient, k j, of each solute element, j, as given by C Sj kj j [1] CL M. GANESAN, Postgraduate Student, D. DYE, Lecturer, and P.D. LEE, Reader, are with the Department of Materials, Imperial College, London SW7 2AZ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted October 20, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
j
where C S and C Lj are the solid (S) and liquid (L) compositions at the solidification front. These thermodynamic quantities are used for alloy development programs[17] and in casting process models[18,19] both as input properties and for validation. A well-founded technique is thus imperative for evaluating compositional data from X-ray microanalysis. In the present paper, two new sorting approaches are described that consider contributions from every element present in order to assign each measured location a unique fraction solid. By illustrative application to four successive generations of commercial SX nickel-based superalloys, it is demonstrated that, in comparison to other sorting schemes, these methods minimize noise in the composition-fraction solid profile when taken across all components and that they estimate the segregation parameters in a reasonable way. A methodology is introduced for evaluating sorting schemes. Particular attention is paid to the implications for alloy design and process optimization. Where relevant, compariso
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