Question

Dislocation in metals

Describe edge dislocation motion in metals from an atomic perspective. Be sure to clearly illustrate the sequence in which bonds are broken and reformed.

Also from an atomic perspective, describe how macroscopic plastic deformation occurs by the motion of dislocations in response to an applied stress. Again, use words and visuals to explain.

Answer 1

In an edge dislocation distortion of atoms is there along an extra half plane of atoms. The burgers vector is perpendicular to dislocation line.

When a shear stress is applied, the extra half plane of atoms in plane A is pushed to right and a new half plane of atoms at B is formed. This continues by bonding and rupture of bonds and finally a unit step of slip is formed at right of crystal. Similarly for screw dislocation also due to applied stress a unit step of slip is produced.

We know that plastic deformation is accompanied by movement of dislocations known as slip. Slip occures through only some specific slip plane and slip direction in which planar density and linear density of atoms are higher. This slip plane and slip direction constitutes the slip system.

The net plastic deformation for screw dislocation and edge dislocation movements are same. Dislocation density increases as highas 10¹⁰ /mm² for heavilydeformedmaterials. Rupture of atomic bonds finally results in formation of small cracks. This cracks grow in size and coalesce to become a large crack and finally the fracture of surface by plastic deformation( ductile material)

But for brittle material propagation of cracks through grains and is transgranular and this type of fracture is known as cleavage.