Dislocations and Strengthening Mechanisms

Plastic deformation corresponds to the motion of large numbers of dislocations.

An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line

Dislocation densities as low as 10^3 mm^-2 are typically found in carefully solidified metal crystals. For heavily deformed metals, the density may run as high as 10^9 to 10^10 mm^-2.

By way of contrast, a typical dislocation density for ceramic materials is between 10^2 and 10^4 mm^-2 for silicon single crystals used in integrated circuits the value normally lies between 0.1 and 1 mm^-2.

The units of dislocation density are millimeters of dislocation per cubic millimeter or just per square millimeter.

Dislocation densities as low as 10^3 mm^-2 are typically found in carefully solidified metal crystals. For heavily deformed metals, the density may run as high as 10^9 to 10^10 mm^-2.

On a microscopic scale, plastic deformation corresponds to the net movement of large numbers of atoms in response to an applied stress.

During this process, interatomic bonds must be rup- tured and then re-formed. In crystalline solids, plastic deformation most often in- volves the motion of dislocations, linear crystalline defects that were introduced in Section 4.5.

The slip system depends on the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum.

For a particular crystal structure, the slip plane is the plane that has the most dense atomic packing—that is, has the greatest planar density. The slip direction corresponds to the direction, in this plane, that is most closely packed with atoms—that is, has the highest linear density.

Edge and screw are the two fundamental dislocation types.

In an edge dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms, which also defines the dislocation line. A screw dislocation may be thought of as resulting from shear distortion; its dislocation line passes through the center of a spiral, atomic plane ramp

SLIP SYSTEMS Dislocations do not move with the same degree of ease on all crystallographic planes of atoms and in all crystallographic directions.

Ordinarily there is a preferred plane, and in that plane there are specific directions along which dislocation motion occurs. This plane is called the slip plane; it follows that the direction of movement is called the slip direction. This combination of the slip plane and the slip direction is termed the slip system.

Chapter 6 explained that materials may experience two kinds of deformation: elastic and plastic.

Plastic deformation is permanent, and strength and hardness are measures of a material's resistance to this deformation.

All metals and alloys contain some dislocations that were introduced during solidification, during plastic deformation, and as a consequence of thermal stresses that result from rapid cooling.

The number of dislocations, or dislocation density in a material, is expressed as the total dislocation length per unit volume or, equivalently, the number of dislocations that intersect a unit area of a random section.

During plastic deformation, the number of dislocations increases dramatically.

We know that the dislocation density in a metal that has been highly deformed may be as high as 10^10 mm^-2. One important source of these new dislocations is exist- ing dislocations, which multiply; furthermore, grain boundaries, as well as internal defects and surface irregularities such as scratches and nicks, which act as stress concentrations, may serve as dislocation formation sites during deformation.

The process by which plastic deformation is produced by dislocation motion is termed slip

the crystallographic plane along which the dislocation line traverses is the slip plane Macroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations, or slip, in response to an applied shear stress

When metals are plastically deformed, some fraction of the deformation energy (approximately 5%) is retained internally;

the remainder is dissipated as heat. The major portion of this stored energy is as strain energy associated with dislocations.