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)

Space Lattice

Unit Cell Cubic

Hexagonal

FCCBCC

HCP

Crystals can have various structures: FCC, BCC and HCP are common to metals

Coordination # = 8Atomic Packing Factor = 0.68

Atomic Packing Factor = 0.74

Coordination # = # of nearest toughing atoms for each atom in the structure = higher APF

Coordination # = 12Atomic Packing Factor = 0.74

Hexagonal Close-Packed (HCP)

For a good visualization of the three crystals, check out this animation online:

http://www.youtube.com/watch?v=Rm-i1c7zr6Q&feature=related

Close-packed Planes & Miller Indices

Close-packed planes: BCC Primary Slip Plane: [110] = normal vector

Primary Slip Directions: <111>

Close-packed planes: FCCPrimary Slip Plane: [111] = normal vector

Primary Slip Directions: <110>

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)

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

Point Defects: Impurity Atoms

Play a role in strengthening mechanisms by inhibiting the movement of dislocations

Octahedral and tetrahedral sites

Linear Defects: Dislocations

Edge dislocations: Burges vector is perpendicular to dislocationScrew dislocation: Burges vector is parallel to dislocation

Planar Defect: Grain Boundaries

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)