1. Copyright © 2012 SCKCEN Ab initio study of basic properties of minor alloying elements in high-Cr ferritic steels Alexander Bakaev University of Gent abakaev@sckcen.be

2. Copyright © 2012 SCKCEN Minor alloying elements of ferritic/martensitic steels The interaction of steel's alloying elements with radiation defects, the efficiency of their mass transport, their affinity to different microstructural features (GB, free surface, dislocations, etc.) determines their rearrangement in the course of the irradiation process F/M steels, beside Cr and C, can contain: Purpose of this study: to distinguish and separate contributions of alloying elements to different radiation-related degradation mechanisms provide a full and consistent set of ab initio data to be used for the parameterization and validation of upper-scale atomistic cohesive models for large scale simulations MoWNbTaVMnSi MagnetismPM M-58DM Transition metal++++++- T m, K2890365327413290216015171688

3. Copyright © 2012 SCKCEN Methodology. VASP parameterization VASP parameterization: PAW potential generated with GGA-PW91 [1] 300 eV cut-off energy, converged k-point mesh Constant volume Relaxation criterion: ionic relaxation stops if all forces on all nuclei are smaller than 0.03 eV/Å Structures: Cubic box (128 atoms) for point defect calculations Cubic slab for inter-row calculations Grain boundaries Free surfaces Screw dislocation [1] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, and C. Fiolhais. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B, 46:6671, 1992.

4. Copyright © 2012 SCKCEN Methodology. High angle tilt grain boundaries Σ3 {111} Σ3 {112} Σ5 {310}) 72 atoms 96 atoms 120 atoms 1.4 x 2.4 x 10.6 a 0 3 2.8 x 1.7 x 9.9 a 0 3 1.0 x 3.2 x 19.2 a 0 3 γ GB =-1.56 J/m 2 γ GB =-0.47 J/m 2 γ GB =-1.51 J/m 2 Pure Fe Same numbers denote equivalent sites Impurities inserted at 1

5. Copyright © 2012 SCKCEN Methodology. Screw dislocation Single screw dislocation 128 atoms in the box: 68 free of moving 40 fixed C – core position 1 – 1nn from core 2 – 2nn from core C 1 2 1 2 C 1 2

6. Copyright © 2012 SCKCEN Methodology. Inter-row potential The profile of inter-row potential correlates [1] with the screw dislocation core structure: a). Double hump → isotropic b). Single hump → degenerate c). Flat top profile → either degenerate or isotropic 36 atoms Box size: 4.2×4.2×0.9 a 0 3 Two cases studied: a). Drag of Fe (A) row near solute (B) b). Drag of solute (A) near solute (B) I B [1] Chiesa, S., Gilbert, M. R., Dudarev, S. L., Derlet, P. M. and Van Swygenhoven, H.(2009), Philosophical Magazine, 89: 34, 3235 — 3243

7. Copyright © 2012 SCKCEN Results. Behaviour as solute atoms Refractory metal atoms are oversized Mn & Si are undersized but do not produce contraction Mn-Mn interaction is the only attractive one Mn & Si are only atoms to be only weakly repelled by Cr atoms

8. Copyright © 2012 SCKCEN Results. Interaction with vacancy Mn & Si are the only elements which bind the vacancy up to 2nn distance  fulfill condition to be dragged Refractory metal atoms can only trap vacancies All atoms are bound to the divacancy, though V very weakly

9. Copyright © 2012 SCKCEN Results. Interaction with self-interstitial atom Only Mn forms a mixed dumbbell Only Mn and Si have attractive interaction with dumbbells in compressed location Only Si has slightly repulsive interaction in tensed location, but interaction is weak  no trapping

10. Copyright © 2012 SCKCEN Results. Interaction with self-interstitial atom Only Mn forms a (weak) Cr-Mn dumbbell Mn has attractive interaction with Cr in dumbbell at compressed location; Si too, on the other side Only Si (& V) have slightly repulsive interaction in tensed location, but weak interaction  no trapping Cr

11. Copyright © 2012 SCKCEN Results. Interaction with carbon atom Only Mn is not repelled by nearby C atom

12. Copyright © 2012 SCKCEN Results. Interaction with free surfaces Z vacuum = 10Å E i = E surf -E bulk Si, Mn & Nb have strong affinity for free surfaces

13. Copyright © 2012 SCKCEN Results. Interaction with grain boundaries Almost all elements have strong affinity for GB Mn & Si exhibit different trend along layers away from GB V is the element with the least affinity for GBs Strong variation of affinity depending on GB

14. Copyright © 2012 SCKCEN Results. Inter-row potential No significant effect of solutes on screw dislocation core shape Refractory metal atoms increase friction along Si and Mn decrease friction along

15. Copyright © 2012 SCKCEN Magnetic features of manganese in Fe Mössbauer experiments on Fe 5% Mn [1] Mn-Mn: 0.11-0.14 eV. binomial distribution of the Mn while solubility of Mn is less than 5% [2] in α-Fe. Vincent et al.[3] USPP DFT: repulsion 0.28; 015 eV at 1nn and 2nn, respectively E i (Mn-Mn;1nn)=-0.26 eV E i (Mn-Cr;1nn)=0.01 eV Attraction Lack of repulsion [1] J. Chojcan, J. Alloy. Compd., 264 (1998) 50-53. [2] V.T. Witusiewicz, F. Sommer, E.J. Mittemeijer, J. Phase Equilib. Diffus., 25 (2004) 346-354. [3] E. Vincent, C. Becquart, C. Domain, J. Nucl. Mater., 351 (2006) 88-99.

16. Copyright © 2012 SCKCEN Comparison with experimental data 12/14 Solute-solutes [1] J. Chojcan Mossbauer spectroscopy. E i (V-V)= -0.01 to 0.01 eV. This work: repulsion of 0.23 eV (1nn) and 0.12 eV (2nn) [2] J. Chojcan Mossbauer spectroscopy Fe-3%V: repulsion V-V of 0.05 eV. => qualitative agreement between our results and experimental data Solute-vacancies [3] Nagai (PAS) and Möslang [4] (Muon Spin Rotation-γSR): Si-vacancy strong attractive interaction (confirmed); the formation of stable vacancy-Si pairs is expected. Möslang [4] could not reveal strong attractive interaction for a vacancy with Cr and Mn. [5] Maury (ERRS): Mn binds vacancies. (This work: Cr-Vac: ~0.05 eV (binding); Mn-Vac ~0.15 eV (binding)) SIAs [6] H. Abe (Electrical resistivity recovery spectra ERRS): Mo suppresses the migration of Fe-Fe SIAs and thus acts as a trap for SIAs. E trap = 0.06 eV. At the same time, Mo does not form the mixed dumbbell. Align with this work E i (Fe-Fe-M(T))= -0.08 eV excellent agreement [7] F.Maury (ERRS, isochronal recovery experiments): Si atoms show suppression of long-range migration of SIAs with increasing Si concentration. This has been interpreted as a multiple trapping effect, implying that trapped SIAs, instead of detrapping, may form mixed migrating dumbbells and then be bound to a second Si atom[6]. This work: Si acts as a relatively strong trap for Fe-Fe dumbbells (0.28 eV) and given the possible formation of mixed Si-Fe dumbbells, multiple trapping by Si is not excluded by this work [5] Maury (ERRS): Mn forms stable mixed dumbbells, which agrees with this work [1] J. Chojcan, J. Alloy. Compd., 264 (1998) 50-53. [2] J. Chojcan,, Hyperfine Interact., 156 (2004) 523-529. [3] Y. Nagai, K. Takadate, Z. Tang, H. Ohkubo, H. Sunaga, H. Takizawa, M. Hasegawa, Phys. Rev. B, 67 (2003). [4] A. Moslang, E. Albert, E. Recknagel, A. Weidinger, P. Moser,, Hyperfine Interact., 15 (1983) 409-412. [5] F. Maury, A. Lucasson, P. Lucasson, Y. Loreaux, P. Moser,, Journal of Physics F-Metal Physics, 16 (1986) 523-541. [6] H. Abe, E. Kuramoto,, J. Nucl. Mater., 271–272 (1999) 209-213 [7] F. Maury, A. Lucasson, P. Lucasson, P. Moser, Y. Loreaux,, Journal of Physics F-Metal Physics, 15 (1985) 1465-1484

17. Copyright © 2012 SCKCEN Summary Mn and SiRefractory metals and V Solute-vacancyMn;Si: attraction at 1nn and 2nn Attraction at 1nn and repulsion 2nn Solute-soluteMn: attraction at 1nnRepulsion at 1nn Solute-chromiumMn;Si: no repulsionStrong repulsion Fe-Solute mixed dumbbellFe-Mn: stable; Fe-Si: neutralNot formed Fe-Fe dumbbell solute(C)Fe;Mn: attractionRepulsion Solute - carbon (octa)Mn: attractionRepulsion Solute - free surfacesMn,Si (and Nb): affinity to FSAffinity to bulk (except for Nb/Ta) Solute - grain boundariesAttraction (correlated with size misfit) Inter-row potentialMn, Si: facilitateEnhance lattice friction (except for V) Core structure of ½ screw dislocation Si: strong distortion of the coreC position: core structure not altered 1nn: expansion of the core in a {110} plane

18. Copyright © 2012 SCKCEN Conclusions Mn and Si exhbit different behaviour from refractory metal atoms: -Solute-solute attraction (especially Mn) -Fulfill condition to be dragged by vacancy (binding up to 2nn) -Attracted quite strongly by SIA (Mn forms mixed Fe-Mn dumbbell and even Mn-Cr dumbbell in Fe) -Mn is not repelled by C -Both have strong affinity for free surfaces (as well as for GBs) -Decrease friction for motion along screw dislocation line  Will tend to segregate anywhere possible Refractory metal atoms: -May trap vacancies but will repel interstitials -Won’t accumulate at free surfaces and dislo lines but might at GBs

19. Copyright © 2012 SCKCEN Questions? Thank you for your attention Alexander Bakaev Nuclear Materials Science Institute Structural Materials abakaev@sckcen.be Contributions: D. Terentyev (SCKCEN, staff) L. Malerba (SCKCEN, staff) T. P. C. Klaver (Delft, staff) G. Bonny (SCKCEN, staff) P. Olsson (KTH, staff) D. Van Neck (UGent, staff)