Atomistic insights into H diffusion and trapping

Transcription

1 Atomistic insights into diffusion and trapping Matous Mrovec, Davide Di Stefano, Christian Elsässer Fraunhofer Institute for Mechanics of Materials IWM, Freiburg. Germany Roman Nazarov, Tilmann ickel Max-Planck Institute for Iron Research, Düsseldorf, Germany

2 Mutiy Scheme of the multiscale approach 2

3 Mutiy Scheme of the multiscale approach 3

4 Interfaces between bcc Fe and TiC Where and how strongly is trapped? Atom probe tomography: Takahashi. et al., Scripta Mater. (2010) 4

5 [001] Carbide particles in bcc Fe Basic geometrical considerations Size and morphology depends on heat treatment The typical shape is a thin platelet Broad interface has a (001) Fe /(001) TiC orientation. The lateral face can be either round (total incoherent) or faceted with (110) Fe /(100) TiC orientation. Side view Wei et al. (2006) Fe TiC Top view <100> 5

6 trapping in Fe/TiC system Possible trapping sites Fe matrix TiC Particle Coherent Interface Misfit dislocations Vacancies [001] 6

7 trapping in Fe/TiC system Atomistic model Fe matrix TiC Particle [001] 7

8 trapping in Fe/TiC system Perfect interface Fe matrix TiC Particle trapping 0.3 ev 0.9 ev Perfect interface [001] 8

9 trapping in Fe/TiC system Misfit dislocation cores Fe matrix TiC Particle trapping Misfit dislocation cores: 0.49 ev [001] 9

10 trapping in Fe/TiC system Vacancies at interface Fe matrix TiC Particle trapping Vacancy at interface 0.49 ev [001] 10

11 trapping in Fe/TiC system Vacancies in TiC particle Fe matrix TiC Particle trapping Vacancy in TiC bulk 0.9eV [001] 1.8eV 11

12 [001] trapping in Fe/TiC system Trap energies summary Fe matrix TiC Particle trapping Perfect interface: 32 kj/mol [001] Dislocation core: Vacancy at interface: Vacancy in bulk TiC: 49 kj/mol 49 kj/mol 180 kj/mol (110) side Interface: 23 kj/mol 12

13 trapping in Fe/TiC system Comparison with experiments C vacancies Incoherent interface Semi-coherent interface Perfect coherent interface, dislocations, grain boundaries 13

14 trapping in Fe/TiC system Comparison with experiments C vacancies Incoherent interface Semi-coherent interface Perfect coherent interface, dislocations, grain boundaries 14

15 ydrogen at Ni grain boundaries Trapping, diffusion and cohesion Oudriss et al., Acta Mat. (2012) Bechtle et al., Acta Mat. 57 (2009) 15

16 5 (210) grain boundary in Ni A representative general GB Bulk GB 16

17 5 (210) grain boundary in Ni Diffusion towards the GB Energy pathways towards the GB GB Octahedral site Saddle point site Tetrahedral site GB site 17

18 5 (210) grain boundary in Ni Diffusion in the GB plane Potential energy surface of the GB plane Energy pathways along the GB plane

19 3 (111) grain boundary in Ni Negligible trapping B C A B C A C B A C B A GB the most perfect GB in FCC metals local CP packing of close-packed {111} planes at the GB trapping energy at the special 3 GB only 0.02 ev compared to 0.2 ev for the 5 general GB GB presents an obstacle for diffusing with 20% higher diffusion barrier then in bulk GB plane diffusion across the GB plane 19

20 Effective diffusion coefficient Influence of microstructure D 5 ~ 10 4 D bulk vs D 3 ~10-1 D bulk 20

21 Effective diffusion coefficient Influence of microstructure D 5 ~ 10 4 D bulk vs D 3 ~10-1 D bulk polycrystalline materials are characterized by effective diffusivity D eff D eff depends on: grains size GB type GB interconnectivity D eff can be obtained using kinetic Monte Carlo (kmc) approach EBSD of polycrystalline Ni 21

22 Kinetic Monte Carlo Toy model: influence of microstructure fast GBs only D eff 1200D blk Bulk-like diffusivity igh diffusivity GB Low diffusivity GB 22

23 Kinetic Monte Carlo Toy model: combination of GB types 100% fast GB D eff 1200D blk 66% fast GBs Not connected D eff D blk Bulk-like diffusivity igh diffusivity GB Low diffusivity GB 23

24 Kinetic Monte Carlo Toy model: influence of GB connectivity 100% fast GB D eff 1200D blk 66% fast GBs Not connected D eff D blk Bulk-like diffusivity igh diffusivity GB Low diffusivity GB 66% fast GBs Connected D eff 1500D blk 24

25 Kinetic Monte Carlo Toy model: influence of GB connectivity 100% fast GB D eff 1200D blk 66% fast GBs Not connected D eff D blk Bulk-like diffusivity igh diffusivity GB Low diffusivity GB D66% eff 1800D fast GBs blk Connected D eff 1500D blk 25

26 26 Influence of on GB cohesion ow much can be there?

27 ydrogen concentration at GBs Filling the 5 (210) GB with ~7 at/nm 2 For grain size of ~10μm this corresponds to c 0.04 appm 27

28 ydrogen concentration at GBs Filling the 5 (210) GB with ~50 at/nm 2 For grain size of ~10μm this corresponds to c 0.3 appm 28

29 Tensile test Stress-Strain curves Clean GB Charged GB 25% 32% 29

30 Conclusions There is no single trapping energy associated with TiC particles but the trapping energies depend sensitively on the trapping site diffusion and trapping at GBs in Ni depends on GB type but general GBs are likely to act as both trapping sites and fast diffusion channels for ; segregated can decrease the GB strength by more than 30% Complementary methodologies are needed for description of polycrystalline materials with realistic microstructures 30