Question

Mark the following statements as True or False.

1) Substitutional vacancy atoms are present in the region of the dislocation where compressive stresses are present.

2) If the Burgers vector of a screw dislocation is along the y - direction, then displacements surrounding the stationary screw dislocation are also in the same direction.

3) External tensile stresses generates vacancies in the material when edge dislocations are present.

Answer 1

1) True

In the case of an impurity, the atom is often occupied at a regular atomic site in the crystal structure. This is neither a vacant site nor is the atom on an interstitial site .This type of defect is substitutional defect. In this the  atom is not supposed to be present anywhere in the crystal, and is thus an impurity. The compressive stress in a element  would decrease the equilibrium molar fraction of vacancy while a tensile stress would increase it. And thus Vacancy type substitutional atoms reside in the region of the dislocation where compressive stresses are present.

2) True

In case of screw dislocation , the displacement and the direction of slip are parallel to one another whereas in case of  the edge dislocation , the slip direction and displacement are perpendicular to one another . Also in case of screw dislocation burger vector is parallel to the slip location and hence we can conclude that If the Burgers vector of a screw dislocation is along the y - direction, then displacements surrounding the stationary screw dislocation are also in the same direction.

3) True

Application of external tensile stresses generates the defects in the material during deformation .There defects in the material arise due to the vacancies when edge dislocations are present . Thus External tensile stresses generates vacancies in the material when edge dislocations are present