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

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

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

For a good visualization of the three crystals, check out this animation online: i1c7zr6Q&feature=related

Close-packed Planes & Miller Indices

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

Close-packed planes: FCC Primary Slip Plane: [111] = normal vector Primary Slip Directions:

Close-packed planes: HCP Primary Slip Plane: [001] = normal vector Primary Slip Directions: 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. PointLinearPlanar Impurity 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 dislocation Screw 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? PointLinearPlanar Defect AddedImpurity 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)