Abstract
Nano-scale Ni-Mn-Si-rich precipitates formed in a reactor pressure vessel steel under high neutron fluence have been characterized using highly complimentary atom probe tomography (APT) and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) combined with STEM-EDS modeling. Using these techniques in a synergistic manner to overcome the well-known trajectory aberrations in APT data, the average upper limit Fe concentration within the precipitates was found to be ∼6 at.%. Using this knowledge, accurate compositions of the precipitates was determined and it was found that the spread of precipitate compositions was large, but mostly centered around the Γ2-and G-phases. The use of STEM-EDS also allowed for larger areas to be examined, and segregation of minor solutes was observed to occur on grain boundaries, along with Ni-Mn-Si-rich precipitates that were smaller in size than those in the matrix. Solute segregation at the grain boundaries is proposed to occur through a radiation induced segregation or radiation enhanced diffusion mechanism due to the presence of a denuded zone about the grain boundary. It is also proposed that the reduced precipitate size at the grain boundaries is due to the structure of the grain boundary. The lack of Ni-Mn-Si precipitates observed in larger Mo-rich precipitates is also discussed, and the absence of the minor solutes required to form the Ni-Mn-Si precipitates results in the lack of nucleation. This is in contrast to cementite phases in which Ni-Mn-Si precipitates have been observed to have formed. It was also determined through this work that the exclusion of all the Fe ions during atom probe analysis is a reasonable approximation.
| Original language | English |
|---|---|
| Pages (from-to) | 31-39 |
| Number of pages | 9 |
| Journal | Acta Materialia |
| Volume | 134 |
| DOIs | |
| State | Published - Aug 1 2017 |
Funding
Research at Oak Ridge National Laboratory was sponsored by the Light-Water Reactor Sustainability Program of the Office of Nuclear Energy. This research was performed, in part, using instrumentation (FEI Talos) provided by the Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science User Facilities. Atom probe tomography research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The authors thank Dr Brian Burgos formerly of Westinghouse Electric Co. for retrieving and shipping samples to ORNL; and to Mr William Server of ATI Consulting, Inc. for assistance in coordinating the specimen identification and retrieval effort. We also thank Dr Keith Leonard, LWRS Materials Pathway program manager, and the US Department of Energys Office of Nuclear Energy for financial support.
| Funders | Funder number |
|---|---|
| U.S. Department of Energy | |
| Office of Science | |
| Office of Nuclear Energy |
Keywords
- Atom probe tomography
- Multivariate statistical analysis
- Precipitation
- RPV steels
- Scanning/transmission electron microscopy