Author's personal copy Modeling of yttrium, oxygen atoms and vacancies in c-iron lattice Aleksejs Gopejenko a,⇑ , Yuri F. Zhukovskii a , Pavel V. Vladimirov b , Eugene A. Kotomin a , Anton Möslang b a Institute of Solid State Physics, University of Latvia, Kengaraga Str. 8, LV-1063 Riga, Latvia b Karlsruhe Institute of Technology, Institute for Materials Research-I, P.O. Box 3640, 76021 Karlsruhe, Germany article info Article history: Available online 13 December 2010 abstract Development of the oxide dispersion strengthened (ODS) steels for fission and fusion reactors requires a deep understanding of the mechanism and kinetics of Y 2 O 3 nanoparticle precipitation in the steel matrix. Therefore, it is necessary to perform a large-scale theoretical modeling of the Y 2 O 3 formation. In the cur- rent study, a series of first-principles calculations have been performed on different elementary clusters consisting of pair and triple solute atoms and containing: (i) the Y–Fe-vacancy pairs, (ii) the two Y atoms substituted for Fe lattice atoms and (iii) the O impurity atoms dissolved in the steel matrix. The latter is represented by a face-centered cubic c-Fe single crystal. This structure is relevant because a transition to c-phase occurs in low Cr ferritic–martensitic steels at typically hot isostatic pressing temperatures. The results clearly demonstrate a certain attraction between the Y substitute and Fe vacancy whereas no binding has been found between the two Y substitute atoms. Results of calculations on different Y–O– Y cluster configurations in lattice show that not only a presence of oxygen atom favors a certain binding between the impurity atoms inside the c-Fe lattice but also the increased concentration of Fe vacancies is required for the growth of the Y 2 O 3 precipitates within the iron crystalline matrix. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Reduced activation steels strengthened by yttria precipitates are considered as promising construction materials for fusion- and advanced fission-reactors. In particular, their use for fusion reactor blanket structure would allow to increase the operation temperature by 100 K [1,2]. Both size and spatial distributions of Y 2 O 3 nanoparticles drastically affect both mechanical properties and radiation resistance of ODS steels [3]. They are usually pro- duced by mechanical alloying for several tens of hours followed by hot isostatic pressing (HIPping) at temperature around 1275– 1475 K and pressure 100 MPa. This process is being continuously refined and optimized [1], to obtain better mechanical properties at high operation temperature and excellent radiation resistance. Further optimization of the manufacture process requires a deep understanding of atomic scale mechanism of oxide nanoparticles growth. To understand the structure and composition of ODS par- ticles, thorough TEM studies were performed on different ODS steels [4,5]. Specific orientation relationship was found between the atomistic structures of Y 2 O 3 nanoparticles and the steel matrix. Such relationship could be either formed during particle precipita- tion or by surface reconstruction of ODS particle during its growth under hipping conditions. Recent experimental studies suggest that the chemical composition and the dispersion of ODS particles might be affected by minor alloying elements contained in the steel matrix [6]. This fact could also indirectly support the idea of Y 2 O 3 particle ‘dissolution’ during mechanical alloying and subse- quent precipitation during hipping [7–9]. In this case the kinetics of nanoparticle growth would be controlled by both diffusion of solute atoms and their chemical affinity to oxygen. Unlike other publications [8,11] considering formation of yttria particles within a-Fe lattice, which is relevant either for high-Cr steels or for hipping temperatures below the temperature of a ? c transition, this paper is focused on the formation of Y 2 O 3 particles on c-Fe lattice, which is the case for low-Cr steels hipped above the transition temperature. At 1373 K c-Fe is paramagnetic and, hence, the total magnetization is zero due to random distri- bution of individual magnetic moments. Therefore, large-scale computer simulation of kinetics of O and Y precipitation on c-Fe lattice must be performed for further improvement and optimiza- tion of low chromium ODS steel production technology. The first-principles calculations of the kinetic parameters of yttrium and oxygen precipitation in a c-Fe lattice, stable at high- temperature, can be considered as a first stage of multi-scale mod- eling aimed at clarification of the role of various solute elements in this process. This paper presents supercell models of single Fe vacancies, O and Y impurity atoms as well as their small aggregates in a c-Fe lattice, which allow us to estimate the inter-atomic bind- ing energies and migration barriers for diffusion of impurity atoms [10]. These values can be obtained from the first-principles calcu- lations and used then for atomistic kinetic Monte Carlo simulations of Y 2 O 3 nanoparticle formation, which is a second stage of multi- scale modeling. 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.11.088 ⇑ Corresponding author. Tel.: +371 67187816; fax: +371 67132778. E-mail addresses: gopeenko@latnet.lv, agopejen@inbox.lv (A. Gopejenko). Journal of Nuclear Materials 416 (2011) 40–44 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat