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Atomistic Simulations

Atomistic Simulations. Ju Li, Libor Kovarik. 8 nm. Ardell & Ozolins, Nature Mater . 4 (2005) 309. Mishin, Acta Mater . 52 (2004) 1451. (NT t ) ensemble with two vacancies. 2D activation.

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Atomistic Simulations

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  1. Atomistic Simulations Ju Li, Libor Kovarik

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

  3. (NTt) ensemble with two vacancies

  4. 2D activation Transition pathwaysobtained using Nudged Elastic Band (NEB) method.Henkelman & Jonsson, J. Chem. Phys. 113 (2000) 9901; ibid113 (2000) 9978. 3D activation sorta too long

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

  6. Lu et al., Science287 (2000) 1463; 304 (2004) 422. dislocation transmission? Lu et al., Acta Mater. 53 (2005) 2169. T. Zhu, J. Li, A. Samanta, H.G. Kim, S. Suresh, “Interfacial plasticity governs strain rate sensitivity and ductility in nanostructured metals,” PNAS104 (2007) 3031.

  7. Firsttime atomistic calculation provides strain-rate sensitivity information, at experimentally realistic strain rate of ~10-4/s.

  8. avg. shear stress = 750 MPa

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

  10. constant supercell calculation saddle-point configuration

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

  12. Vacancy-aided reordering in 2-layer pseudo-twin long behind dislocations For comparison, VNi migration barrier in perfect Ni3Al is 1.24 eV.

  13. shear stress = 900 MPa

  14. Peter Sarosi

  15. High Tensile Strength and Ductility of Cu with Nano-Sized Twins Lu et al., Science287 (2000) 1463; 304 (2004) 422.

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

  17. Lu et al., Acta Mater. 53 (2005) 2169. Like other nanocrystals, nanotwinned Cu shows increased strain-rate sensitivity (~0.03) and small activation volume (~12b3) Can atomistic calculation provide strain-rate sensitivity (m) and activation volume (v*) information of experimental relevance?

  18. Stress-driven activated process very unlikely to happen in 1s small W1 large thermal uncertainty Larger W means the activation is more “collective”, less thermal uncertainty & the process more “athermal”. 0.7eV large W2 small thermal uncertainty Activation energy Q(s) process 1 W1 W2 process 2 athermal threshold very likely to happen in 1s 0eV sath stress s point defect diffusion: ~0.02-0.1b3 forest dislocation cutting: ~103b3 J. Li, “The Mechanics and Physics of Defect Nucleation,” MRS Bulletin32 (2007) 151-159.

  19. Qabs=0.49eV Qdes~5eV Qtms=0.67eV t = 252MPa

  20. In experiment, stress applied is uniaxial tension, not pure shear → Taylor factor M ≈ 3.1 to convert shear stress t to uniaxial stress s: s = Mt We’ve computed Wtms≈79b3, Wabs≈Wdes≈43b3 at t = 252MPa.

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