Precipitate Evolution and Creep Behavior of a W-Free Co-based Superalloy
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UCTION
NI-BASED superalloys are utilized for turbine disks, blades, and jet engines owing to their high oxidation and corrosion resistance, and their excellent strength and creep behavior at ambient and elevated temperatures.[1,2] Ni-based superalloys are strengthened by coherent (L12 ) precipitates (c0 ) with a base stoichiometry of Ni3 Al in a (fcc) Ni-rich matrix (c).[3,4] Challenges remain, however, in both high-temperature strength[5] and hot-corrosion resistance[6,7] for Ni-based superalloys, indicating a need for new high-temperature alloys. In 2006, the c0 -Co3 (Al,W) phase, with a solvus temperature up to 1273 K (1000 °C), was reported in a new Co-based superalloy with the base composition Co-9.2Al-9W at. pct,[8] analogous to the c0 -Ni3 (Al,Ti) of Ni-based superalloys. Subsequent studies examined additions of Ti and Ta, which increase the solvus temperature and volume fraction of c0 .[9] Ductility is also enhanced in polycrystalline material with boron additions that strengthen the grain boundaries and suppresses intergranular fracture.[10] Creep resistance of Co-based alloys is increased with the addition of e.g., Cr, Mo, Ti, and Ta.[11–13] Furthermore, additions of QINYUAN LIU, Ph.D. Student, and DAVID C. DUNAND, Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108. JAMES COAKLEY, Research Fellow, is with the Department of Materials Science and Engineering, Northwestern University, and also with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Contact e-mail: james.coakley@ northwestern.edu DAVID N. SEIDMAN, Professor, is with the Department of Materials Science and Engineering, Northwestern University, and also with the Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, IL, 60208. Manuscript submitted April 20, 2016. Article published online September 20, 2016 6090—VOLUME 47A, DECEMBER 2016
Ta,[14] Si[15], and Cr[16] increase oxidation resistance. The addition, however, of large quantities of W in these Co-based superalloys results in drawbacks. First, the low diffusivity of W[17] makes it difficult to homogenize the alloy. Second, the strength-to-weight ratio, which is essential for turbine blades, is lower due to the high density of W (yielding densities >9:3 gcm3 ). Finally, the presence of W results in deleterious precipitates, such as DO19 (Co3 W).[18] In 2015, a new class of W-free Co-based superalloys was described[19,20] with a base composition Co-10Al-5Mo-2Nb at. pct. The microstructure of this alloy is analogous to Co-Al-W alloys with cuboidal c0 [Co3 (Al, Mo, Nb)] precipitates coherently distributed in a c-fcc Co-based matrix. With additions of Ni up to 30 at. pct, the solvus temperature is increased from 1159 K (886 °C) to 1263 K (990 °C), and the c0 volume fraction is increased from 54 to 76 pct.[20] The density of the W-free alloys is decreased to 8:4gcm3 when compared to 9:2gcm3 for Co-7Al-7W at. pct.[20] The specific strength
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