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Discussion Notes

Discussion Notes. Farzana Ansari Feb 14 & 16, 2012. From macroscale to nanoscale. Start small, work our way back up. Crystal Structure Defects Dislocation motion & plastic deformation Strengthening Mechanisms Using defects Using crystal structure modification (heat treatment).

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Discussion Notes

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  1. Discussion Notes Farzana Ansari Feb 14 & 16, 2012

  2. From macroscale to nanoscale

  3. Start small, work our way back up • Crystal Structure • Defects • Dislocation motion & plastic deformation • Strengthening Mechanisms • Using defects • Using crystal structure modification (heat treatment)

  4. Crystals can have various structures: FCC, BCC and HCP are common to metals Unit Cell Cubic FCC BCC Space Lattice Hexagonal HCP

  5. Coordination # = 8 Atomic Packing Factor = 0.68 Atomic Packing Factor = 0.74 Coordination # = # of nearest toughing atoms for each atom in the structure = higher APF

  6. Hexagonal Close-Packed (HCP) Coordination # = 12 Atomic Packing Factor = 0.74

  7. For a good visualization of the three crystals, check out this animation online: http://www.youtube.com/watch?v=Rm-i1c7zr6Q&feature=related

  8. Close-packed Planes & Miller Indices

  9. Close-packed planes: BCC Primary Slip Plane: [110] = normal vector Primary Slip Directions: <111>

  10. Close-packed planes: FCC Primary Slip Plane: [111] = normal vector Primary Slip Directions: <110>

  11. Close-packed planes: HCP Primary Slip Plane: [001] = normal vector Primary Slip Directions: <110> Bottom line: HCP has LESS slip systems than FCC & BCC (only 3) Note: A four-axis coordinate system (“Miller-Bravais”) is often used for hexagonal symmetry to account for some crystallographic equivalent directions.

  12. Crystal Structure & Defects • Crystal structure and slip planes play a role in how defects are understood with regard to both plastic deformation and strengthening mechanisms.

  13. Point Defects: Impurity Atoms Play a role in strengthening mechanisms by inhibiting the movement of dislocations

  14. Octahedral and tetrahedral sites

  15. Linear Defects: Dislocations Edge dislocations: Burges vector is perpendicular to dislocation Screw dislocation: Burges vector is parallel to dislocation

  16. Planar Defect: Grain Boundaries

  17. Strengthening • Macroscopic plastic deformation = motion of large number of dislocations • Restrict dislocation motion and you can make metal harder and stronger • How can you inhibit dislocation motion?

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