Abstract
Atom probe tomography (APT) and scanning transmission electron microscopy (STEM) techniques were used to probe the long-time thermal stability of nm-scale Mn-Ni-Si precipitates (MNSPs) formed in intermediate and high Ni reactor pressure vessel steels under high fluence neutron irradiation at ≈320 °C. Post irradiation annealing (PIA) at 425 °C for up to 57 weeks was used to determine if the MNSPs are: (a) non-equilibrium solute clusters formed and sustained by radiation induced segregation (RIS); or, (b) equilibrium G or Γ2 phases, that precipitate at accelerated rates due to radiation enhanced diffusion (RED). Note the latter is consistent with both thermodynamic models and x-ray diffraction (XRD) measurements. Both the experimental and an independently calibrated cluster dynamics (CD) model results show that the stability of the MNSPs is very sensitive to the alloy Ni and, to a lesser extent, Mn content. Thus, a small fraction of the largest MNSPs in the high Ni steel persist, and begin to coarsen at long times. These results suggest that the MNSPs remain a stable phase, even at 105 °C higher than they formed at, thus are most certainly equilibrium phases at much lower service relevant temperatures of ≈290 °C.
Highlights
A main challenge for RPV life extension is predicting embrittlement at low service-flux (φ) and high φt, where new embrittlement mechanisms may emerge
We first focus on the scanning transmission electron microscopy (STEM)-Energy Dispersive X-ray Spectroscopy (EDS) characterization of the 57 weeks Post irradiation annealing (PIA) condition, in order to significantly increase the sampling volume relative to the Atom probe tomography (APT) observations
The main conclusion of the STEM-EDS study is that sufficiently coarsened Mn-Ni-Si precipitates (MNSPs) are stable at near nominal amounts of Ni and Mn, even at a very long ta that is ≈8 times that required for full dissolution of the MNSPs in the medium Ni alloy
Summary
A main challenge for RPV life extension is predicting embrittlement at low service-flux (φ) and high φt, where new embrittlement mechanisms may emerge. Dissolution of what are argued to be RIS formed Mn-Ni-(Si) clusters following short term anneals at Ta from 450–500 °C10,28, or in low solute content model alloys at Ta = 400 °C14, does not prove that they are thermodynamically unstable at much lower service Ti ≈ 290 °C Due to their small radii (r) of ≈0.50– 2.5 nm, even if MNSPs are bulk equilibrium phases, they will dissolve at a higher Ta, due to the Gibbs-Thomson effect, if they are below the critical radius in a post-annealing, solute-depleted matrix. Achieving these fundamental objectives supports refining the predictive Mn-Ni-Si precipitation[21] and PIA models, including for application to guiding embrittlement predictions and annealing remediation treatments
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