On the Elevated Temperature Thermal Stability of Nanoscale Mn-Ni-Si Precipitates Formed at Lower Temperature in Highly Irradiated Reactor Pressure Vessel Steels

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 Γ 2 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.

The mechanistic implications of the high temperature, long time thermal stability of nanoscale Mn-Ni-Si precipitates in irradiated reactor pressure vessel steels

“…While, the APT MNSP statistics were limited, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) characterization confirmed the stability of the larger MNSPs. These results are consistent with cluster dynamics and kinetic lattice Monte Carlo models [16,32], which predict a large reduction in precipitate number density (N) at small r < rc at the high PIA temperature of 425°C, in a solute depleted matrix. Thus to further explore the thermal stability of MNSPs, we designed and carried out a special heavy ion irradiation experiment for the same Cu free high Ni steel.…”

“…The previous PIA study of neutron irradiated CM6 showed that MNSPs below rc ≈ 2.2 nm redissolved in much shorter times at 425°C, assuring that the 52 week PIA results will not be affected by kinetics [16]. Standard FIB liftouts (see SI) were extracted and fabricated into APT tips and TEM foils at a depth of ≈ 400-600 nm from the surface, where the damage profile is relatively flat.…”

“…The underprediction of embrittlement at high fluence is primarily attributed to the delayed formation of large volume fractions (f) of Mn-Ni-Si precipitates (MNSPs), which are not accounted for in current regulatory models [4][5][6][7][8]. The existence of MNSPs has been demonstrated in both surveillance and test reactor irradiations [4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22], but their detailed character and formation mechanisms is one of the most highly debated issues in the field of radiation effects [4,14,[23][24][25][26]. The central question relates to the driving force for MNSP formation.…”

“…In a recent ,425°C PIA study up to 57 weeks, of a nearly fully phase separated highly 320°C neutron irradiated, Cu free, 1.6 at.% Ni RPV steel, atom probe tomography (APT) showed that most MNSPs dissolved, although a very low density of larger MNSPs with r > 2.2 nm, remained and appeared to coarsen at long times [16]. While, the APT MNSP statistics were limited, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) characterization confirmed the stability of the larger MNSPs.…”

“…The growth of the larger MNSPs is due to re-precipitation of solutes that initially dissolved during PIA. Note the actual rc varies with ta, since the dissolved solute concentrations change with dissolution and re-precipitation [16].…”