Which of the following statements concerning dislocations is FALSE? Dislocation

Question

Which of the following statements concerning dislocations is FALSE?

Dislocation density (count per volume) generally increases during plastic deformation at room temperature. Dislocations move in response to local shear stresses within a crystal. Dislocation motion is generally more dif ficult in BCC metals versus HCP metals. Dislocations that are in close proximity tend to mutually prevent one another's motion. The energy required to move a dislocation increases as the magnitude of the Burger's vector associated with a given slip direction increases in a crystal. Dislocations prefer to slip on high density planes and in high density directions within a crystal. While dislocations can exist in ionic and covalent ceramic materials, their motion in these materials is extremely dif ficult compared to metals. Dislocations feature strain fields that interact with other strain fields in a crystal (such as those produced by other crystalline defects).

Explanation / Answer

E = G/2 *b^2*d

b = burgers vector

d = length of dislocation

Thus energy required to move the dislocation increases with the square of burgers vector.

5. True. The dislocation require energy to move and it is given by the application of shear stresses. For largely spaced atoms in a crystal, this energy requirement increases. Thus the dislocations prefer to move along closely spaced atom planes.

6. True. The dislocations have to overcome the atoms electric charges in case of ionic bond which create additional strain fields in the material. In case of covalent bonds there is directional bonding between the atoms of the crystal. Thus movement of dislocations are difficult.

7. True. The dislocations have their own strain field. When they interact with other dislocations they interact with their strain fields which prevent their further motion.

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