Creep deformation of single crystals of new Co–Al–W-based alloys with fcc/L1₂ two-phase microstructures

Abstract

The creep deformation behaviour of single crystals of Co–Al–W-based alloys with γ + γ′ two-phase microstructures has been investigated in tension under a constant stress of 137 MPa in air at 1000°C as a function of the γ′ solvus temperature and the volume fraction of the γ′ phase. When described by the creep strain rate versus time curve, the creep deformation of Co–Al–W-based alloys consists of transition and accelerating regions without a steady-state region, as observed in many modern nickel-based alloys. However, the creep strength of the present Co–Al–W-based alloys is comparable with nickel-based superalloys of the first generation but is much weaker than those of the second and higher generations. Unlike in nickel-based superalloys, the so-called p (parallel)-type raft structure, in which the γ′ phase is elongated along the tensile axis direction, is formed during creep in Co–Al–W-based alloys, being consistent with what is expected from the positive values of lattice misfit between the γ and γ′ phases. As a result, of the alloys investigated, the best creep properties are obtained with the alloy possessing the highest volume fraction (85%) of the γ′ phase, which is far larger than usual for nickel-based superalloys (55–60%).

Keywords:

• cobalt-based superalloys
• creep test
• microstructure
• raft structure

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research (A) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and in part by the Global COE (Center of Excellence) Program of International Center for Integrated Research and Advanced Education in Materials Science from the MEXT, Japan.

Notes

Note

1. A report has been published during the revisions of the present article 35.