Dislocations are linear defects in crystals affecting atomic alignment and facilitating plastic deformation. The movement of dislocation lines, influenced by applied stress, plays a critical role in the mechanical properties of metals, where restricting this motion increases strength. Fine-grained materials, with a larger total grain boundary area, result in reduced dislocation motion, leading to harder and stronger metals.

1. Dislocation

2. Linear defect around which some of the atoms are misaligned
•An extra plane of atoms
terminate within the crystals
•Called slip-plane
•Plastic deformation moves
around this plane

4. Dislocation lines move in the direction of applied stress
EDGE DISLOCATION

5. MOVEMENT OF EDGE DISLOCATION

6. Dislocation lines move perpendicular to applied stress
SCREW DISLOCATION

8. Internal Energy after plastic
deformation ( minus heat dissipation)
Lattice strains

10. •Plastic Deformation means motion of large number of
dislocations
•More the plastic deformation less is the hardness and tensile
strength of the material
•Reducing the mobility of dislocation means greater
mechanical forces are required to initiate the plastic
deformation
•It means increasing the strength of the metal
•Summary: Restricting dislocation motion
makes a metal stronger

11. Grain boundary: Barrier to dislocation motion

12. Fine-grained material=
greater total grain boundary area=
decreased dislocation motion =
harder & stronger metal