Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy

ABSTRACT

The compressive creep deformation behavior of a Co-base single-crystalline superalloy is studied in the high temperature / low stress regime at 950°C/150 MPa. Emphasis is placed on the mechanisms causing the double minimum creep behavior consisting of a local and a global creep rate minimum. The local minimum occurs at ∼0.2% strain with dislocation accumulation at γ/γ′ interfaces after bowing out of dislocations in horizontal matrix channels. Plate-like rafting occurring normal to the applied stress axis in the early creep stages is regarded as the primary reason for the decreasing creep rate until the global minimum takes place at ∼0.8% strain. In the final creep stage following the global minimum the creep rate accelerates due to coarsening and extensive precipitate shearing. The shearing by partial dislocations leads to the formation of numerous stacking faults, nano/microtwins, and occasionally to the formation of small amounts of ordered D0₁₉-Co₃W precipitates. The reasons for twinning and second phase formation as well as differences of the deformation mechanisms between Co- and Ni-base superalloys during double minimum creep are discussed in detail in this work.

KEYWORDS:

• Co-base superalloys
• directional coarsening
• creep
• deformation
• transmission electron microscopy

Acknowledgments

The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through projects B3 of the Collaborative Research Center SFB/TR 103: ‘From Atoms to Turbine Blades – A Scientific Approach for Developing the Next Generation of Single Crystal Superalloys’. One of the authors (FX) acknowledges funding by the Sino-German (CSC-DAAD) Postdoc Scholarship. One of the authors (CHZ) acknowledges funding by the Alexander von Humboldt Foundation (AvH) through a Feodor Lynen Research Fellowship.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

Note: Data marked with ‘–’ indicate that the creep stage was not reached or uncompleted.

Additional information

Funding

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through projects B3 of the Collaborative Research Center SFB/TR 103, the Sino-German (CSC-DAAD) Postdoc Scholarship and the Alexander von Humboldt Foundation (AvH) through a Feodor Lynen Research Fellowship.