Nanostructure evolution under irradiation in FeMnNi alloys: A ‘‘grey alloy’’ object kinetic Monte Carlo model M. Chiapetto a,b,⇑ , L. Malerba a , C.S. Becquart b a SCKCEN, Nuclear Materials Science Institute, Boeretang 200, B-2400 Mol, Belgium b Unité Matériaux Et Transformations (UMET), UMR 8207, Université de Lille 1, ENSCL, F-59600 Villeneuve d’Ascq Cedex, France article info Article history: Received 15 July 2014 Accepted 25 March 2015 Available online 28 March 2015 abstract This work extends the object kinetic Monte Carlo model for neutron irradiation-induced nanostructure evolution in Fe–C binary alloys developed in [1], introducing the effects of substitutional solutes like Mn and Ni. The objective is to develop a model able to describe the nanostructural evolution of both vacancy and self-interstitial atom (SIA) defect cluster populations in Fe(C)MnNi neutron-irradiated model alloys at the operational temperature of light water reactors (300 °C), by simulating specific reference irradiation experiments. To do this, the effects of the substitutional solutes of interest are introduced, under simplifying assumptions, using a ‘‘grey alloy’’ scheme. Mn and Ni solute atoms are not explicitly introduced in the model, which therefore cannot describe their redistribution under irradiation, but their effect is introduced by modifying the parameters that govern the mobility of both SIA and vacancy clusters. In particular, the reduction of the mobility of point-defect clusters as a consequence of the presence of solutes proved to be key to explain the experimentally observed disappearance of detectable defect clusters with increasing solute content. Solute concentration is explicitly taken into account in the model as a variable determining the slowing down of self-interstitial clusters; small vacancy clusters, on the other hand, are assumed to be significantly slowed down by the presence of solutes, while for clusters bigger than 10 vacancies their complete immobility is postulated. The model, which is fully based on physical considerations and only uses a few parameters for calibration, is found to be capable of reproducing the experimental trends in terms of density and size distribution of the irradiation-induced defect populations with dose, as compared to the reference experiment, thereby providing insight into the physical mechanisms that influence the nanostructural evolution undergone by this material during irradiation. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Low-alloy bainitic steels are used to fabricate the reactor pres- sure vessel (RPV) of most commercial light water nuclear power plants. The vessel is an irreplaceable component that constitutes the main barrier to impede the release of radioactive substances present in the core of the reactor and its capability to maintain integrity also in case of an accident determines therefore the life- time of the installation. The main threat to the vessel integrity comes from the fact that exposure to prolonged neutron irradiation causes hardening and embrittlement of RPV steels. Changes in mechanical properties induced by irradiation can be understood in terms of changes in the micro- or nanostructure of the material. A neutron flux impinging on a metal creates a large amount of point-defects (vacancies, V, and self-interstitial atoms, SIA) and relevant clusters within displacements cascades, which may evolve to form voids and dislocation loops (so-called matrix damage). These radiation-induced defects are also the main actors responsible for the accelerated diffusion of solute atoms within the matrix, leading to the formation of solute-rich clusters (precipi- tates) in irradiated steels. The formation of matrix damage and solute clusters or precipitates changes the mechanical properties of the steels in nuclear reactors, because these constitute obstacles to the movement of dislocations, increasing yield strength and reducing ductility, thereby contributing to an increase of the probability of brittle fracture. While for a long time the precipitation of the highly insoluble Cu has been known to be mainly responsible for embrittlement of RPV steels [2], the continuously increasing embrittlement observed also in low Cu steels up to high fluence is attributed to the formation of Mn and Ni-rich clusters, the existence of which was theoretically anticipated [3,4] and experimentally observed, http://dx.doi.org/10.1016/j.jnucmat.2015.03.045 0022-3115/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: SCKCEN, Nuclear Materials Science Institute, Boeretang 200, B-2400 Mol, Belgium. Tel.: +32 (0)14 33 31 81. E-mail address: mchiapet@sckcen.be (M. Chiapetto). Journal of Nuclear Materials 462 (2015) 91–99 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat