MOLECULAR DYNAMICS SIMULATION OF STRESS INDUCED GRAIN BOUNDARY MIGRATION IN NICKEL

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MOLECULAR DYNAMICS SIMULATION OF STRESS INDUCED
GRAIN BOUNDARY MIGRATION IN NICKEL
Hao Zhang, Mikhail I. Mendelev, David J.
Srolovitz Department of Mechanical and Aerospace
Engineering, Princeton University, Princeton, NJ
08540

• Background
• Goal Determine grain boundary mobility from
  atomistic simulations
• Methods based upon capillarity driving force are
  useful, but not sufficient
• gives reduced mobility, MM(g g), rather than
  M
• boundary stiffness gg not readily available
  from atomistic simulations
• average over all inclinations
• Flat boundary geometry can be used to directly
  determine mobility, but subtle (Schönfelder, et
  al.)

• Molecular Dynamics
• Velocity Verlet
• Voter-Chen EAM potential for Ni
• Periodic BC in X, Y, free BC in Z
• Hoover-Holian thermostat and velocity rescaling
• 12,000 - 48,000 atoms, 0.5-10 ns

Non-Linear Stress-Strain Response

• Strain energy density
• Apply strain exxeyye0 and szz0 to perfect
  crystals, measure stress vs. strain and integrate
  to get the strain contribution to free energy
• Includes non-linear contributions to elastic
  energy

• Typical strains
• as large as 4 (Schönfelder et al.)
• 1-2 here

• Conclusion
• Developed new method that allows for the accurate
  determination of grain boundary mobility as a
  function of misorientation, inclination and
  temperature
• Activation energy for grain boundary migration is
  finite grain boundary motion is a thermally
  activated process
• Activation energy is much smaller than found in
  experiment (present results 0.26 eV in Ni,
  experiment 2-3 eV in Al)
• The relation between driving force and applied
  strain2 and the relation between velocity and
  driving force are all non-linear
• Why is the velocity larger in tension than in
  compression?