Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causes excess irradiation embrittlement of reactor pressure vessel (RPV) steels at high, extended-life fluences. Thus, a new and unique, semi-empirical cluster dynamics model was developed to study the evolution of MNSPs in low-Cu RPV steels. The model is based on CALPHAD thermodynamics and radiation enhanced diffusion kinetics. The thermodynamics dictates the compositional and temperature dependence of the free energy reductions that drive precipitation. The model treats both homogeneous and heterogeneous nucleation, where the latter occurs on cascade damage, like dislocation loops. The model has only four adjustable parameters that were fit to an atom probe tomography (APT) database. The model predictions are in semi-quantitative agreement with systematic Mn, Ni and Si composition variations in alloys characterized by APT, including a sensitivity to local tip-to-tip variations even in the same steel. The model predicts that 1 Notice of Copyright This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).recognized that the copper rich precipitates (CRPs) are highly alloyed with Mn and Ni [10,11].Mn-Ni synergisms rationalized the strong effects of the alloying element Ni, as well as Cu, on hardening and embrittlement [12]. In the early 1990s Odette and coworkers carried out CALPHAD [10] based thermodynamic calculations, suggesting that Mn-Ni precipitates could form even in low Cu steels, which would otherwise be relatively insensitive to embrittlement.The Mn-Ni precipitates were predicted to be slow to nucleate and grow, thus they were dubbed late blooming phases (LBP) at that time.These models equilibrated Mn, Ni and Cu in solution with these solutes in a specified number density of CRPs and included the effects of composition on the interface energy [10]. At sufficiently high Ni and at lower temperatures, the precipitates contain more Mn and Ni than Cu.This mean-field thermodynamic modeling was later extended to include Si in Lattice Monte Carlo (LMC) simulations, based on pair-bond energy estimates extracted from CALPHAD, that predicted Cu-rich core and Mn-Ni-Si-rich shell precipitate structures [12]. Such core shell structures were also observed in atom probe tomography studies [15][16][17][18]. The early models predicted that low irradiation temperatures, high Ni and even small amounts of Cu enhance the formation of Mn-Ni and MNSPs. It was also...

Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPV-type steels, with an unusually wide range of dissolved Mn (0.06-1.34 at.%) and Ni (0.19-3.50 at.%) contents, were irradiated at ≈ 290°C to ≈ 1.4x10 20 n/cm 2 at an accelerated test reactor flux of ≈ 3.6x10 12 n/cm 2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with ≈ Ni 1.6 Mn 0.8 . Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Nisilicide phase type compositions are observed.

Massive, thick-walled pressure vessels are permanent nuclear reactor structures that are exposed to a damaging flux of neutrons from the adjacent core. The neutrons cause embrittlement of the vessel steel that growswith dose (fluence), as manifested by an increasing ductile-to-brittle fracture transitiontemperature. Extending reactor life requires demonstrating that large safety margins against brittle fracture are maintained at the higher neutron fluence associated with beyond 60years of service.Here synchrotron-based x-ray diffraction and small angle x-ray scattering measurements are used to characterize highly embrittling nm-scale Mn-Ni-Si precipitates that develop in the irradiated steels at high fluence. These precipitates lead to severe embrittlement that is not accounted for in current regulatory models. Application of the complementary techniques has, for the very first time, successfully identified the crystal structures of the nanoprecipitates, while also yielding self-consistent compositions, volume fractions and size distributions. I. INTRODUCTIONReactor pressure vessels (RPVs) are the primary permanent component of light water reactors (LWRs). RPVs experience irradiation embrittlement that increases with neutron fluence (see Refs[1-3] for overviews of the embrittlement phenomena and underlying mechanisms). Ensuring that large safety margins are maintained in the face of embrittlement is required to extend LWR service life to beyond 60 years. Embrittlement is marked by increases in the ductile-to-brittle transition fracture temperature (ΔT) of RPV steels. The ΔT is primarily caused by irradiation hardening (Δσ y ) associated with the formation of nm-scale precipitates and solute-defect complexes that act as obstacles to dislocation glide.At low to intermediate neutron fluence, significant hardening and embrittlement is primarily caused by the formation of coherent, transition phase copper rich precipitates (CRP). Trace impurity amounts ofCu(< 0.35 at.%) are insoluble in steels and rapidly phase separate due to radiation enhanced diffusion (RED)at © 2015. This manuscript version is made available under the Elsevier user license

What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys of two precipitating phases, with the fast-precipitating phase acting as heterogeneous nucleation sites for a second phase manifesting slower kinetics. Kinetic lattice Monte Carlo simulations show that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly affect the selection of core-shell verses appendage morphologies. We study a FeCuMnNiSi alloy using the combination of atom probe tomography and simulations, and show that the ordering energy reduction of the MnNiSi phase heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the energy penalty of a predominantly Fe/Cu interface, leading to initial appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around and through the Cu core towards the ordered phase results in subsequent appendage growth. We further show that in cases with higher primary precipitate interface energies and/or suppressed ordering, the coreshell morphology is favored.3

Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (> 3 wt.%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs.

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.

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