Effect of Cr additions on a γ-γ’ microstructure and creep behavior of a Co-based superalloy with low W content

Daniel S. Ng, Ding Wen Chung, Jacques P. Toinin, David N. Seidman, David C. Dunand, Eric A. Lass*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

Nickel- and cobalt-based superalloys derive their excellent oxidation and corrosion resistance from the surface oxidation of Al and Cr, which form protective oxide layers. Starting from an existing Cr-free cobalt-based superalloy (Co–30Ni–7Al–4Ti–3Mo–2W–1Nb–1Ta-0.1B at. %) with a γ-γ′ microstructure, we create three alloys with 4, 8 and 12 at. % Cr additions. Chromium lowers the γ′ solvus temperature (from 1129 °C for 0% Cr to 1075 °C for 12 at. % Cr) and alters the γ′ precipitate morphology (from cuboidal-to-spherical), but does not affect the coarsening kinetics (which follow the Lifshitz-Slyozov-Wagner model). The alloys with 8 and 12 at. % Cr exhibit minor amounts of a Mo-rich intermetallic phase at grain boundaries after homogenization, and the 12 at. % Cr alloy displays an additional ~3 vol % intragranular refractory-rich secondary precipitates upon aging at 850 °C for 168 h. Atom-probe tomography reveals that Cr partitions strongly to the γ-matrix, with a partitioning coefficient of κ γ'/γ = 0.33 and 0.26 for Cr additions of 4 and 8%, respectively. These additions halve the creep rate of the original alloy at stresses between 275 and 400 MPa, reflecting significant changes in γ’ precipitate composition, volume fraction, morphology, and rafting tendency.

Original languageEnglish (US)
Article number139108
JournalMaterials Science and Engineering A
Volume778
DOIs
StatePublished - Mar 19 2020

Keywords

  • Atom-probe tomography
  • Coarsening kinetics
  • Creep
  • Partitioning
  • Rafting
  • Superalloys

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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