Examination with atom probe tomography (APT) of environmentally degraded grain boundaries from a turbine disk Ni-based superalloy ME3
EMSL Project ID
47633
Abstract
The focus of this work is to investigate the chemistry of interior grain boundaries from the fatigued volume of tested notched low cycle fatigue (NLCF) specimens of an advanced powder metallurgy superalloy ME3, which is used in turbine disk applications. Prior to NLCF testing at 704 degrees C in air, ME3 specimens were pre-exposed in vacuum at 815 °C for 440 hours and pre-oxidized in air at 704 degrees C, 760 degrees C, and 815 degrees C for exposures times up to 2,020 hours [Sudbrack, et al., Superalloys 2012 (Warrendale, PA: TMS)]; the air exposure conditions capture some aspects of the current and projected service conditions for the turbine disk rim. To summarize prior work, in agreement with unexposed specimens, specimens exposed in vacuum failed via a single crack that initiated and propagated transgranularly and had statistically the same fatigue life as the unexposed specimen. Prior exposures in air led to pronounced debits of up to 99 % in fatigue life, where fatigue life decreased inversely with exposure time. For moderate air exposures (~1 um thick oxide), primary cracks propagated intergranularly to depths comparable to the dissolution of strengthening (Mo,Cr,W)23C6 carbides at the near surface grain boundaries before transitioning to transgranular crack propagation. For aggressive air exposures (greater than or equal to 2 um thick oxide), however, the intergranular cracking extended well beyond the carbide dissolution layer.
To study the mechanisms associated with the environmental degradation, we propose to use atom probe tomography (APT) to analyze the chemistry of interior grain boundaries at comparable depths for: 1) an aggressive exposure in vacuum, 2) an aggressive exposure in air and 3) a moderate exposure in air. Analysis of degraded grain boundaries that are brittle and contain ceramic phases with APT is challenging due to the potential for premature fracture during data acquisition. Thus preliminary results obtained from this rapid access proposal will serve to confirm the feasibility to successfully analyze our samples and provide motivation for further studies. We will employ site-specific FIB lift-out techniques to prepare APT specimens. The support of NASA Aviation Safety program and NASA Subsonic Fixed Wing program is acknowledged.
Project Details
Project type
Limited Scope
Start Date
2012-06-01
End Date
2012-08-01
Status
Closed
Released Data Link
Team
Principal Investigator