Microstructural stability and mechanical behavior of a Co–20Ni–7Al–7W–4Ti at.% superalloy

Chunan Li, Ding Wen Chung, David C. Dunand, David N. Seidman*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


The temporal evolution of the microstructure and microhardness of a polycrystalline L12-strengthened Co-based superalloy (Co–19.6Ni–7.6Al–7.4W–4.1Ti, at.%) was investigated for an aging temperature of 900 °C. The γ(f.c.c.)/γ′(L12) structure remained stable throughout aging to the longest aging time of 2000 h (83 days), with no additional phases present. While the volume fraction of γ′(L12)-precipitates reached an almost constant value of 64% at 16 h, the temporal evolution of the γ′(L12)-precipitates’ mean-radius and their number density (temporal exponent of p = 3.49 ± 0.18 for the equivalent radius, and q = −0.74 ± 0.04 for the volumetric number density) deviated somewhat from the Lifshitz-Slyozov (LS) predictions (p = 3 and q = −1), suggesting that the alloy had not achieved a true stationary-state at 2000 h. At ambient temperature, a peak Vickers microhardness of 3.80 ± 0.10 GPa was achieved after 240 h of aging. As the testing temperature was increased, an anomalous increase in flow-stress was also observed, with a peak flow-stress value of 705 MPa measured at 750 °C, 30 MPa higher than an archival literature value with about half the Ni concentration (10 at.%). Creep resistance at 850 °C was greater than an archival literature value for a ternary Co–9Al–9W alloy. After 200 h of compressive creep at 850 °C the microstructure evolved as follows: (i) the volume fraction of the γ′(L12)-phase increased from 64% to 79%; and (ii) some grains exhibited rafts of γ′(L12)-precipitates along ⟨100⟩-directions. While the first change is expected to improve creep resistance, the second one is deleterious, as evidenced by a comparison with the creep resistance of a sample without rafts of γ′(L12)-precipitates tested for the same conditions.

Original languageEnglish (US)
Article number156378
JournalJournal of Alloys and Compounds
StatePublished - Dec 25 2020


  • Coarsening
  • Cobalt alloys
  • Creep
  • Rafting
  • Superalloy

ASJC Scopus subject areas

  • Mechanics of Materials
  • Mechanical Engineering
  • Metals and Alloys
  • Materials Chemistry

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