Mechanism for Formation of Surface Scale during Directional Solidification of Ni-Base Superalloys
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SUPERALLOYS are a class of materials that have been developed specifically for high-temperature applications for use in aeroengines and land-based power generation. The alloy chemistry has evolved during the last four decades, primarily characterized by the addition of increasing refractory additions such as Ta, W, and Re to improve solid-solution hardening and to enhance microstructural stability at increased temperatures.[1–9] Simultaneously, developments in alloy chemistry have been accompanied by innovations in materials processing routes, with a progressive migration from equiaxed (polycrystalline) to directional (polycrystalline and aligned grain morphology) and single-crystal (SX) solidification (single crystal with aligned grain morphology) where the primary orientation of the grain is aligned along the turbine blade or stress axis.[10–14] However, changes in alloy chemistry and solidification structure have made the control of the crystal orientation and the prevention of defect formation in modern turbine components increasingly challenging. One of the most commonly occurring casting defects in SX castings is surface scale, which is a thin, ‘‘fishscale’’ layer. It always occurs across the aerofoil under the shoulder of turbine blades (TBs), and the extent of surface coverage is dependent on the geometry of the G. BREWSTER, Technologist, Surface Engineering, and N. D’SOUZA, Casting Specialist, are with Rolls-Royce plc, Derby DE24 8BJ, UK. K.S. RYDER, Reader, and S. SIMMONDS, PhD Student, are with the Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK. H.B. DONG, Reader, is with the Department of Engineering. Contact e-mail: [email protected] Manuscript submitted May 12, 2011. Article published online December 21, 2011 1288—VOLUME 43A, APRIL 2012
blades. Because of this texture, it is commonly referred to as ‘‘surface scale’’ in casting foundries. In the case of low-pressure and intermediate-pressure TBs with an appreciably greater length of aerofoil compared with the high-pressure TBs, the extent of surface scale coverage across the length of the aerofoil is more severe. The nature of scale is also dependent on the alloy chemistry. Two examples are presented for illustrative purposes encompassing differing turbine blade aerofoil geometries and alloy chemistries. Figure 1(a) is a schematic diagram of a turbine blade directionally solidified with the 1st-generation, Ni-base superalloy SRR99, and the inset shows a portion of the as-cast aerofoil near the top end (i.e., last to solidify). The surface scale here is clearly ‘‘tongue shaped’’ in spatial extent and has a blue and gold coloration. The scale is most notably present on the convex surface of the aerofoil. In contrast, the remainder of the aerofoil does not exhibit such a coloration and is therefore referred to as ‘‘unscaled.’’ Likewise, a schematic of a turbine blade directionally solidified with the Ni-base superalloy CMSX10N is shown in Figure 1(b), and the inset shows a portion of the as-cast aerofoil near the top end. Like in Figure
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