Abstract
The γ′ volume fraction is a key parameter in precipitation-strengthened Co- and Ni-base superalloys and mainly determines the alloys’ properties. However, systematic studies with varying γ′ volume fractions are rare and the influence on thermal expansion has not been studied in detail. Therefore, a series of six Ta-containing Co-based alloys was designed with compositions on a γ–γ′ tie-line, where the γ′ volume fraction changes systematically. During solidification, Laves (C14-type) and µ (D85-type) phases formed in alloys with high levels of W and Ta. Single-phase γ or two-phase γ/γ′ microstructures were obtained in four experimental alloys after heat treatment as designed, whereas secondary precipitates, such as χ (D019-type), Laves, and μ, existed in alloys containing high levels of γ′-forming elements. However, long-term heat treatments for 1000 hours revealed the formation of the χ phase also in the former χ-free alloys. The investigation of the thermal expansion behavior revealed a significant anomaly related to the dissolution of γ′, which can be used to determine the γ′ solvus temperature with high accuracy. Compared to thermodynamic calculations, differential scanning calorimetry (DSC) and thermal expansion analysis revealed a larger increase of the γ′ solvus temperatures and a lesser decline of the solidus temperatures when the alloy composition approached the composition of the pure γ′ phase.
Highlights
PRECIPITATION-STRENGTHENED Co–Al–W-based alloys exhibit superior high-temperature strength compared to conventional carbide and solid-solution-strengthened Co-based alloys
We found that the strengthening contribution of the c¢ phase correlates with its volume fraction, that is, an increasing c¢ volume fraction provides a higher strengthening contribution and, subsequently, a higher creep resistance
The actual compositions are generally consistent with the nominal contents, but the Al and Ta contents in all alloys are slightly lower than the designed ones with an increasing deviation from VF0 (~ 0.4 at. pct) to VF100 (~ 1 at. pct)
Summary
Based alloys exhibit superior high-temperature strength compared to conventional carbide and solid-solution-strengthened Co-based alloys. Their strength even exceeds that of some precipitation-strengthened Ni-base superalloys.[1,2,3] they are regarded as potential structural materials for future turbine engine applications.[4,5,6] this new alloy system was rediscovered just over 1 decade ago,[4] the knowledge regarding the effect of alloying elements,[7,8,9,10,11] oxidation,[12,13,14] and Manuscript submitted March 19, 2021, accepted June 5, 2021. There are still open questions regarding this new class of alloys
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
