Edge & screw dislocations, slip systems, Frank-Read source multiplication
Dislocations are line defects in crystals. Edge dislocations have Burgers vector b perpendicular to the dislocation line; screw dislocations have b parallel. Under applied shear stress τ, dislocations glide on slip planes. A Frank-Read source (pinned segment) bows out and multiplies, producing dislocation loops. Hall-Petch: σ_y = σ₀ + k/√d — smaller grains block dislocation motion, increasing strength.