Dislocation And Strengthening Mechanisms Plastic deformation through dislocation: Slip Ideal shear stress d a   ~ G d/a ~ 10 6 psi (calc.) ~ 10~10 3 psi (measured !) FCC This is due to other mechanisms ~ dislocation movement or fracture propagation

Dislocation Movement Take much less energy !

Lattice strains in dislocation

Slip systems Slip direction : closest distance or highest linear atomic density Slip plane : planes with a highest atomic density Easier deformation (higher ductility) Brittle !

Slip systems Slip distance Slip plane Slip distance FCC HCP A combination of close-packed planes and close-packed directions on those planes where slip occurs.

Burgers vectors and slip systems in FCC

Slip in ionic materials NiO V 3 is favorable, since it is the shortest vector connecting crystallographically equivalent potions (Lowest-energy Burgers vector)

Critical Resolve Shear Stress (CRSS) Or Yield strength Zn single crystal CRSS: min stress required for slip Polycrystalline Cu Polycrystalline Cu

Plastic Deformation Of Polycrystalline Materials After deformation Equiaxed Before deformation Slip band

Deformation By Twining BCC and HCP BCC: (112) [111] for twining

Strengthening Mechanisms Grain size reduction GB 阻斷 Slip movement Solid solution hardening Hall-Petch Eq:

Strain Hardening Cold work

Recovery, Recrystallization, Grain Growth Recovery: The stored energy is relieved by dislocation motion at the elevated temperature. Some physcail properties, such as electrical conductivity, are improved. Recrystalliztaion: After recovery, the residual strain is further reduced by the formation of strain-free ann equiaxed grains.

Tensile strength Ductility Grain size Annealing temperature Recovery Recrystallization Grain growth

Grain growth: Grain growth to reduce the interfacial energy