Dictionary of Chemistry [6th Ed.]

153

c

these is called a dislocation.Dislocations are essentially line-defects; that is,

there is an incomplete plane of atoms in the crystal lattice. In 1934, Taylor,

Orowan, and Polanyi independently proposed the concept of the dislocation to

account for the mechanical strength of metal crystals. Their microscopic

studies revealed that when a metal crystal is plastically deformed, the

deformation does not occur by a separation of individual atoms but rather by a

slip of one plane of atoms over another plane. Dislocations provide a

mechanism for this slipping of planes that does not require the bulk

movement of crystal material. The passage of a dislocation in a crystal is

similar to the movement of a ruck in a carpet. A relatively large force is

required to slide the carpet as a whole. However, moving a ruck over the

carpet can inch it forward without needing such large forces. This movement

of dislocations is called plastic flow.

Strength of materials

In practice most metal samples are polycrystalline;that is they consist of many

small crystals or grains at different angles to each other. The boundary

between two such grains is called a grain boundary. The plastic flow of

dislocations may be hindered by the presence of grain boundaries, impurity

atoms, and other dislocations. Pure metals produced commercially are

generally too weak to be of much mechanical use. The weakness of these

samples can be attributed to the ease with which the dislocations are able to

move within the sample. Slip, and therefore deformation, can then occur

under relatively low stresses. Impurity atoms, other dislocations, and grain

boundaries can all act as obstructions to the slip of atomic planes.

Traditionally, methods of making metals stronger involved introducing defects

that provide regions of disorder in the material. For example, in an alloy, such

as steel, impurity atoms (e.g. carbon) are introduced into the lattice during the

forging process. The perfection of the iron lattice structure is disturbed and the

impurities oppose the dislocation motion. This makes for greater strength and

stiffness.

The complete elimination of dislocations may seem an obvious way to

strengthen materials. However, this has only proved possible for hair-like

single crystal specimens called whiskers.These whiskers are only a few

micrometers thick and are seldom more than a few millimetres long;

nevertheless their strength approaches the theoretical value.

H A

B D

G

C

F

AE

G

F

B D

C

Dislocation in a two-dimensional crystal.The extra plane of atoms AB causes strain at

bond CD. On breaking, the bond flips across to form CB. This incremental movement shifts the

dislocation across so that the overall effect is to slide the two planes BDG and CF over each

other.