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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...

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Abstract

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The mechanical properties and radiation tolerance of nanostructured ferritic alloys rely on a dense population of nanometer-scale Y-Ti oxides. The stability of these nano-oxides during extended service is critical in high temperature applications. Here, a model framework is developed for the thermodynamics and kinetics of Y-Ti oxide nucleation, growth and coarsening. The model, which is based upon available thermodynamic and kinetic data as well as key density functional theory calculations, shows that nano-oxide nucleation and growth are highly driven and that pipe diffusion is the dominant mode of their coarsening, in agreement with previous analyses of experimental high temperature data. The model predicts that the nano-oxides are thermally stable for 80 or more years below 1175 K. This analysis also provides insights into the effect of O and Ti on nanooxide sizes, and on optimization of alloy microstructure.

Abstract

BackgroundRepeated PO dosing of anti‐epileptic drugs may contribute to poor compliance in treated cats. Intermediate‐release levetiracetam has been used safely in cats, but must be given q8h to maintain serum concentrations in the therapeutic interval for humans (5–45 μg/mL). Approved extended‐release levetiracetam (XRL) for human use may require less frequent dosing, but the large dosing unit has limited its use in cats.HypothesesIn healthy cats, serum levetiracetam concentration will remain above 5 μg/mL for at least 24 hours after administration of a single dose of XRL PO and will be well tolerated.Animals7 healthy cats.MethodsExtended‐release levetiracetam (500 mg) was administered PO. Blood was collected and neurologic examination findings recorded at scheduled times over 30 hours. Serum levetiracetam concentration was quantitated by an immunoassay validated in cats. Data were subjected to noncompartmental analysis. Descriptive statistics were reported.ResultsThe median dosage of 86.2 mg/kg, (range, 80–94.3) achieved a mean maximum concentration (C max) of 89.8 ± 25.8 μg/mL at 4.9 ±1.57 hours. Serum levetiracetam was >5 μg/mL in all cats by 90 minutes. Mean concentrations were 43.7 ± 18.4 and 4.9 ± 3.4 μg/mL at 12 and 24 hours, respectively. The half‐life was 4.1 ± 1.0 hours. The drug was well tolerated.Conclusions and Clinical ImportanceA single 500 mg PO dose of XRL safely maintained serum levetiracetam concentration ≥5 μg/mL in healthy cats for at least 21 hours. Clinical efficacy studies in epileptic cats receiving XRL are indicated; however, monitoring should be implemented for individual cats.