Question

Which one of the following statements is true? Please explain each choice: why it is true/false.

(a) The Burgers vector of an edge dislocation is normal to the dislocation line and normal to the slip plane.

(b) The Burgers vector of a screw dislocation is normal to the dislocation line and parallel to the slip plane.

(c) When a screw dislocation enables slip in a single crystal, the dislocation line remains in the slip plane.

(d) BCC crystals do not contain dislocations.

(e) Fine-grained samples of γ-iron contain fewer dislocations than coarse-grained samples of γ-iron.

Answer 1

Answers :

Which one of the following statements is true :

the Burgers vector, is a vector that represents the magnitude and direction of the lattice distortion resulting from a dislocation in a crystal lattice.The vector's magnitude and direction are best understood when the dislocation-bearing crystal structure is first visualized without the dislocation, that is, the perfect crystal structure. In this perfect crystal structure, a rectangle whose lengths and widths are integer multiples of "a" (the unit cell edge length) is drawn encompassing the site of the original dislocation's origin.

(a) The Burgers vector of an edge dislocation is normal to the dislocation line and normal to the slip plane. [ FALSE ]

Explanation : The Burgers vector of an edge dislocation lies in (is parallel to) the slip plane.

(b) The Burgers vector of a screw dislocation is normal to the dislocation line and parallel to the slip plane. [ FALSE ]

Explanation : The Burgers vector of a screw dislocation is parallel to the dislocation line.

(c) When a screw dislocation enables slip in a single crystal, the dislocation line remains in the slip plane. [ TRUE]

Explanation : The dislocation line of a screw dislocation lies in the slip plane.​​​​​​​

(d) BCC crystals do not contain dislocations. [ FALSE ]

Explanation : All crystals contain dislocations. BCC crystal have no truly close-packed planes struture slip only occur along the plane of shortest burgers vector. but slip system in bcc requires heat to activate.

(e) Fine-grained samples of γ-iron contain fewer dislocations than coarse-grained samples of γ-iron. [ FALSE ]

Explanation : ​​​​​​​The grain size affects the mobility of dislocations, not the number of dislocations.​​​​​​​