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Investigation of Nano Scale Cementite Workability by Using Molecular Dynamics Simulation

Ghaffarian, Hadi | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47336 (07)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. In this study, the nano-scale deformation mechanism of cementite was investigated with aid of Molecular dynamics (MD) simulation using MEAM potential. For this purpose, flow localization was evaluated in various structures such as single crystal cementite, nanocrystalline (NC) cementite, and lamellar cementite at various temperatures and different stress conditions. In order to understand the deformation mechanism in the cementite single crystal, two cylindrical samples with 15 nm length and 5 nm diameter oriented along [001] and [011] were constructed. The result of tensile test simulation at 100 K, 300 K, 700 K and 1100 K revealed that the deformation mechanism in single crystal cementite is depended on the loading direction. In low temperature regime, a ductile fracture was observed at [011] loading direction (due to activity of <100>{010} slip systems}, while a brittle fracture was occurred at [001] loading direction (due to zero Schmid factor of <100>{010} slip systems). Brittle to ductile transition (BDT) was only observed in [001] oriented samples due to higher dislocation activity at elevated temperature. In order to clarify the role of grain boundary on deformation mechanism of cementite, three cubic nanocrystallie cementite samples were generated by Voronoi construction method with 7.6, 19, and 30 initial edge lengths (providing an average grain size of 3.4, 9.7, and 16.4, respectively). The results of tensile MD simulation at 100 K, 300 K, 700 K, and 1100 K showed that at low temperature and large grain size, dislocation glide acts as the preferred deformation mechanism. Due to the limited number of slip systems at low temperature, NC cementite breaks by forming voids at grain boundaries upon tensile loading. When the temperature rises or the grain size reduces, grain boundary sliding becomes the primary mechanism and plastic deformation is accommodated effectively with BDT. Four nano-composite ferrite/cementite samples with different cementite thickness were constructed in order to investigate the cementite deformation in lamellar pearlite structure. Performing nanoindentation test at 100 K, 300 K and 700 K (spherical indenter of radius R = 3 nm with a constant velocity of 10 m/s along y direction) revealed that the cementite layer act as a barrier of deformation from one ferrite layer to the next. Increasing temperature causes more distributive plastic deformation in the ferrite layer and leads to increase the pearlite workability. It also was found from the results of nano-composite tensile test (at 100 K, 300 K, and 700 K) that the loading direction affects on the dislocation distribution and nano-composite deformation mechanism. Flow localization was observed in cementite layer during tension test along x direction while the uniform deformation was observed along z loading direction. Lower activated slip systems along z loading direction causes to decrease cross slipping and increase dislocation cancelation probability. It causes to postpone the formation of shear band in ferrite. It was found that an increase in temperature leads to formation of necking regions in the x-oriented samples while the temperature does not affect on the z-oriented samples
  9. Keywords:
  10. Molecular Dynamic Simulation ; Cementite ; Workability ; WORKABILITY ; Flow Localization ; Brittle To Ductile Transition

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