Question

Discuss dislocations, their types and their role in plastic deformation of metals. What microscopy technique can be used to detect dislocations.

Answer 1

Ans

1. Dislocations are type of defect in the crystal structure . Those are places in crystal structure where atoms are out of position from their regular position. These occurs when stress is applied on crystal .

Types of dislocations :

There are mainly two types of dislocations in a crystal structure .

1. Edge dislocation:

It is also called as line defect as the locus of the dislocated points are in a line . It occurs when an extra half plane of atoms is introduced in the lattice of crystal structure. The inter atomic bonds are significantly get distorted due to this.understanding movement in the dislocation allows us to understand how deformation occurs in the Crystal structure at very low stresses. If the extra half plane is inserted from above or top side it is positive edge dislocation and if inserted from bottom side is called as negative edge dislocation. Both are given in figure below.

2. Screw dislocation :

In screw dislocation ,when shear stress is applied ,the movement of the defect line is perpendicular to Direction of applied stress and movement of displaced atoms .

To visualize this , consider a wide metal strip , and you are about to rip it into 2 strips . When you start to Rip it ,atoms at the near end starts to dislocate whereas atoms at far end are still in their respective position and middle atoms are moving or about to move .This whole scenario forms a somewhat helical structure hence called as screw dislocation . The dislocation is given by burgers vector which shows the slip occurred in dislocation. Figure for screw dislocation is given below

3. The role of dislocation in plastic deformation:

Cumulative movements in dislocation leads to the increase in gross plastic deformation. At microscopic level , rupture and reforming of inter atomic bonds occurs .It plays key role in plastic deformation. One dimensional crystal dislocations plays important role in studying the plastic deformation in various studies .

4. Microscopic Techniques used to detect dislocation :

Transmission electron microscopy :

Usually transmission electron microscopy is used to detect various dislocations . In this thin foils of material are rendered to the electron beams . An image is get formed due to the interaction of electrons With the sample material when subjected to electron beam which is shown on screen . This technology is used to find out Burgers Vector b for the dislocations . TEM allows to magnify materials about several nanometers .

Schematic ray dia. For TEM is given below.