1. Atomistic Simulations Ju Li, Libor Kovarik

2. 8 nm Mishin, Acta Mater. 52 (2004) 1451 Ardell & Ozolins, Nature Mater. 4 (2005) 309

6. (NT  ) ensemble with two vacancies

8. Transition pathways obtained using Nudged Elastic Band (NEB) method. Henkelman & Jonsson, J. Chem. Phys. 113 (2000) 9901; ibid 113 (2000) 9978. 2D activation 3D activation sorta too long

9. fNfN kNkN A new NEB method connecting to unstable final state “Free-end” algorithm: last node constrained to move only along energy contour

10. T. Zhu, J. Li, A. Samanta, H.G. Kim, S. Suresh, “Interfacial plasticity governs strain rate sensitivity and ductility in nanostructured metals,” PNAS 104 (2007) 3031.Interfacial plasticity governs strain rate sensitivity and ductility in nanostructured metals Lu et al., Acta Mater. 53 (2005) 2169. dislocation transmission? Lu et al., Science 287 (2000) 1463; 304 (2004) 422.

11. First time atomistic calculation provides strain-rate sensitivity information, at experimentally realistic strain rate of ~10 -4 /s.

12. avg. shear stress = 750 MPa

13. initial equilibrium free-end node node 2 node 3 node 4

14. constant supercell calculation saddle-point configuration

16. [112]/6 pseudo-twin layer true-twin layer pure Ni column half Ni column push in pop out slightly tilted view red: Alblack: Ni Libor vacancy reordering mechanism

17. Vacancy-aided reordering in 2-layer pseudo-twin long behind dislocations For comparison, V Ni migration barrier in perfect Ni 3 Al is 1.24 eV.

18. shear stress = 900 MPa

20. Peter Sarosi

21. High Tensile Strength and Ductility of Cu with Nano-Sized Twins Lu et al., Science 287 (2000) 1463; 304 (2004) 422.

22. Lu et al., Acta Mater. 53 (2005) 2169. dislocation transmission?

23. Lu et al., Acta Mater. 53 (2005) 2169. Like other nanocrystals, nanotwinned Cu shows increased strain-rate sensitivity (~0.03) and small activation volume (~12b 3 ) Can atomistic calculation provide strain-rate sensitivity (m) and activation volume (v*) information of experimental relevance?

24. stress  Activation energy Q(  ) athermal threshold  ath 0.7eV 0eV very likely to happen in 1s very unlikely to happen in 1s 11 22 large  2 small thermal uncertainty small  1 large thermal uncertainty process 1 process 2 Stress-driven activated process Larger  means the activation is more “collective”, less thermal uncertainty & the process more “athermal”. point defect diffusion: ~0.02-0.1b 3 forest dislocation cutting: ~10 3 b 3 J. Li, “The Mechanics and Physics of Defect Nucleation,”The Mechanics and Physics of Defect Nucleation MRS Bulletin 32 (2007) 151-159.

25.  = 252MPa Q tms =0.67eV Q abs =0.49eV Q des ~5eV

26. In experiment, stress applied is uniaxial tension, not pure shear → Taylor factor M ≈ 3.1 to convert shear stress  to uniaxial stress  :  = M  We’ve computed  tms ≈79b 3,  abs ≈  des ≈43b 3 at  = 252MPa.