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Molecular Dynamics Simulation of Crack Propagation in Nanocrystalline Materials

Moradi, Masoud | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48188 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. Nanocrystalline metals and alloys have some appealing characteristics with significance potential compared to their microcrystalline counterparts for engineering applications. These include ultra-high yield and fracture strengths, decreased elongation and toughness, superior wear resistance, and the promise of enhanced superplastic formability at lower temperatures and faster strain rates. This leads us to study the effects of different nanocrystalline parameters on crack propagation process in these materials. In the present study, the behavior of a crack in a columnar nanocrystalline structure is examined. One of the methods of modelling nanocrystals primary structures is the Voronoi tessellation method. This method produces high-angle grain boundaires using random seeds and crystallography angles. At least ten degrees difference in crystallography angle is regarded for neighboring grains. After the initial modeling, nanocrystal structure is annealed. Then, a crack is created in the nanocrystal and the process of crack growth is simulated by molecular dynamics method with the help of LAMMPS software. The effect of size and crystallography angles of grains and grain boundaries form on the process of crack growth in nanocrystals with less than 15 nanometers average grain size is studied. The results of this study showed that in nanocrystalline metals with the average grain size of less than 10 nanometers, crack initiation requires higher stresses and the grain crystallography angles and grain boundaries form do not have any effect on the crack growth mechanism. On the other hand, in nanocrystalline metals with coarser grains, the crack initiate under lower stresses, whereas the presence of grain boundaries and emitted stacking faults can slow down or stop the crack growth
  9. Keywords:
  10. Crack Growth ; Molecular Dynamics ; Grain Size ; Grain Boundaries ; Atomistic Simulation ; Nanocrystalline Metals

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