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Discussion Notes
Farzana Ansari Feb 14 & 16, 2012
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From macroscale to nanoscale
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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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Space Lattice
Unit Cell Cubic
Hexagonal
FCCBCC
HCP
Crystals can have various structures: FCC, BCC and HCP are common to metals
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Coordination # = 8Atomic Packing Factor = 0.68
Atomic Packing Factor = 0.74
Coordination # = # of nearest toughing atoms for each atom in the structure = higher APF
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Coordination # = 12Atomic Packing Factor = 0.74
Hexagonal Close-Packed (HCP)
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For a good visualization of the three crystals, check out this animation online:
http://www.youtube.com/watch?v=Rm-i1c7zr6Q&feature=related
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Close-packed Planes & Miller Indices
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Close-packed planes: BCC Primary Slip Plane: [110] = normal vector
Primary Slip Directions: <111>
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Close-packed planes: FCCPrimary Slip Plane: [111] = normal vector
Primary Slip Directions: <110>
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Close-packed planes: HCPPrimary Slip Plane: [001] = normal vector
Primary Slip Directions: <110>
Note: A four-axis coordinate system (“Miller-Bravais”) is often used for hexagonal symmetry to account for some crystallographic equivalent directions.
Bottom line: HCP has LESS slip systems than FCC & BCC (only 3)
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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.
Point Linear PlanarImpurity Atoms (Interstitial or Substitution) Vacancies
Dislocations(“line defects”)
Grain Boundaries
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Point Defects: Impurity Atoms
Play a role in strengthening mechanisms by inhibiting the movement of dislocations
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Octahedral and tetrahedral sites
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Linear Defects: Dislocations
Edge dislocations: Burges vector is perpendicular to dislocationScrew dislocation: Burges vector is parallel to dislocation
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Planar Defect: Grain Boundaries
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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?
Point Linear PlanarDefect Added Impurity Atoms (Interstitial or
Substitution) Dislocations(“line defects”)
Grain Boundaries
Strengthening mechanisms
Solid-solution strengthening Precipitation hardening
Reduction in grain size (hot rolling)Strain hardening (work hardening)