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Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causesexcess irradiation embrittlement of reactor pressure vessel (RPV) steels athigh, extended-life fluences. Thus, a new and unique, semi-empirical clusterdynamics model was developed to study the evolution of MNSPs in low- Cu RPVsteels. The model is based on CALPHAD thermodynamics and radiation enhanceddiffusion ki- netics. The thermodynamics dictates the compositional andtemperature dependence of the free energy reductions that drive precipitation.The model treats both homogeneous and heterogeneous nucleation, where thelatter occurs on cascade damage, like dislocation loops. The model has onlyfour adjustable parameters that were fit to an atom probe tomography (APT)database. The model predictions are in semi-quantitative agreement withsystematic Mn, Ni and Si composition variations in alloys character- ized byAPT, including a sensitivity to local tip-to-tip variations even in the samesteel. The model predicts that heterogeneous nucleation plays a critical rolein MNSP formation in lower alloy Ni contents. Single variable assessments ofcompositional effects show that Ni plays a dominant role, while even smallvariations in irradiation temperature can have a large effect on theMNSPevolution. Within typical RPV steel ranges, Mn and Si have smaller effects. Thedelayed but then rapid growth of MNSPs to large volume fractions at highfluence is well predicted by the model. For purposes of illustration, theeffect of MNSPs on transition temperature shifts are presented based onwell-established microstructure-prop- erty and property-property models.

Thermodynamic and kinetic modeling of Mn-Ni-Si precipitates in low-Cu reactor pressure vessel steels