Damage mechanism of nickel-based creep-resistant alloys strengthened by the Laves phase at the grain boundary

ABSTRACT

This study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 10⁵h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100 MPa and rupture elongation >20% at 800°C and 10⁵h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.

KEYWORDS:

• Laves phase
• precipitation-free zone
• creep degradation

Acknowledgments

The authors would like to express their gratitude to Mr. M. Kinoshita and Mr. S. Kuroki of Nippon Steel & Sumitomo Metal Corporation as well as Ms. S. Kodama and Mr. T. Ueyama of Nippon Steel & Sumikin Technology Corporation for their technical support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors are also grateful for the financial support provided by New Energy and Industrial Technology Development Organization (NEDO) and express their sincere gratitude to all the members for their sincere collaborative activities in the NEDO project.