DEFORMATION OF METAL
INTRODUCTION
The changes produced in the shape of the metal piece under the action of the single force or a set of force is called as deformation.
CLASSIFICATION OF METAL DEFORMATION
The metal deformation is classified into two types depending upon the nature of the strain produced during the deformation.
1. Elastic deformation 2. Plastic deformation.
Elastic deformation:
The term elastic deformation may be defined as the process of deformation, which appears and disappears simultaneously with the application and removal of stress according to Hook′s law.
It has been observed that whenever a stress of low magnitude is applied to a piece of a metal, it causes displacement of atoms from their original position. In elastic deformation the tensile strain is due to slight elongation of the unit cell in the direction of tensile load. Similarly the compressive strain is due to the compressive
load.
Fig 7.1
Plastic deformation:
The term “plastic deformation” may be defined as the process of permanent deformation, which exists in metal even after removal of the stress. It is due to this property that the metals may be subjected various operations like rolling, forging, drawing, etc.
COMPARISON OF ELASTIC AND PLASTIC DEFORMATION
The following table gives the important difference between the plastic and elastic deformation.
SL.NO
| ELASTIC DEFORMATION | PLASTIC DEFORMATION |
1
|
It is a deformation which appears and disappears whit the application and the removal of stress.
|
It is permanent deformation, which exists even after the removal of stress.
|
2
|
It is the beginning of process of deformation
|
It takes place after the elastic deformation has stopped.
|
3
|
It takes place over a short range stress strain curve
|
It takes place over a wide range of stress strain curve
|
4
|
In elastic deformation the strain reaches its maximum value after the stress has reached its maximum value.
|
In plastic deformation the strain occurs simultaneously with the application of stress.
|
- TYPES PLASTIC DEFORMATION
Though there are a number of plastic deformations, yet the following are important from the subject point of view:
1. Slipping 2. Twinning
Slipping:
The term slipping may be defined as the plastic deformation in which a crystal plane moves over another due to shear stress the process of slipping is shown in the fig 7.2
In fig a circle which indicates the arrangement of atoms in a crystals lattice. Whereas in the next fig the slipping of atoms along the slipping direction is shown. It has been observed that in slipping the movement of crystal plane is over a large inter atomic distance. It may be noted that the atoms do not come back to their original position even after the removal of stresses as the atoms now have new neighbors.
It has been found that the slipping occurs along the planes and direction, in which atoms are more closely spaced and the inter atomic spacing is least. Such planes are known as a slip planes, and the direction as slip direction. The combination of slip direction and slip plane is called as the slip system.
Mechanism of slipping:-
In the slipping process, all the atoms move simultaneously. In fig the circles indicates the atoms in the cubic lattice. The next fig shows the process of slipping due to shear stress. It may be noted that at any stage of slipping, a boundary can be drawn between the slipped & unslipped regions. More over, there will be a region of mis fit along this boundary. This misfit is called dislocation & boundary as dislocation lines. The movement of these dislocations produce further slip (fig 7.2)
Twinning:
The twinning may be defined as the plastic deformation which takes place along the two planes due to set of force applied on a given metal piece. The process of twinning is shown in the fig given below. In fig below the circles shows the arrangement of atoms before twinning, it may be noted that the process of deformation between the two planes AB & CD is similar to that of slipping. Where as the arrangement of atoms on either side of the twinning planes remain unaffected (fig 7.3)
It has been observed that the metal usually deform by twinning only if it is unable to slip. More over the deformation produced by twinning is very small. But the places the slip planes in more favorable orientation causing the deformation to take place through slip.
Mechanism of twinning:
In a twining process the movement of atoms is only a fraction of inter atomic distance. Fig shows the circles, which indicates the arrangement of atoms. The dashed lines AB & CD represent the plane of symmetry, from where the twinning starts and ends respectively. These planes are known as twinning planes or the application of shear stress.
It has been observed that the crystals twin about the twin planes, and the atoms in the region to the left of the twinning plane AB and right of the twinning plane CD remains undisturbed. Whereas in the twin region, each atoms move by a distance proportional to its distance from the twinning plane AB.
Comparison of slipping and twinning:
The following are the some of the important difference between slipping and twinning,
SL.NO | SLIPPING | TWINNING |
1
|
In this process, the deformation takes place due to the sliding of the atomic plane over the other.
|
In this process, the deformation takes place due to the orientation of one part of crystal with respect to the other part.
|
2
|
In this process, the atomic movements are over large atomic distance.
|
In this process, the atomic movements are over a fraction of atomic spacing.
|
3
|
It requires low stresses.
|
It requires higher stresses.
|
4
|
A threshold value of stress called resolved shear stress is required for slipping to take place.
|
There is no such threshold value of stress in this case.
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COMMENTS