Line defects |Line imperfections in crystals
When defect in crystal is centered around a line or the lattice distortion is centered around a line then the type of defect generated is called line defect.
Few reasons of formation of line defects are solidification of solid crystal, plastic deformation of crystals and vacancy condensation.
Types of line defects / Line imperfections:
- Edge dislocation
- Screw dislocation
Edge dislocation: When defect in a crystal occurs due to absence of extra half plane of atoms or presence of half plane of atoms then this type of line defect is called edge dislocation or edge imperfection.
Edge defect causes the nearby lattice structure to distort towards or away from the dislocation line. The displacement distance of atoms around the dislocation line is called Burgers vector and is perpendicular to edge dislocation line.
Dislocations are non-equilibrium defects and store energy in the distorted region of the crystal lattice around the dislocation.Edge dislocation has a region of compressive strain where the extra half of plane is present and a region of tensile strain below the extra half plane of atoms.
Screw dislocation: Screw dislocation can be formed in a crystal structure by applying upward and downward shear stress to regions of a perfect crystal which have been separated by a cutting plane. In screw dislocation plane of the crystal lattice trace a helical path around the dislocation line.
The burgers vector of screw dislocation is parallel to the dislocation line and when stress is applied on crystal having this defect then the dislocation area moves perpendicular to the direction of stress.