Metal deformation and Crystal Defect and imperfection

Metal deformation

Introduction

The changes in shape of metal piece under the action of a single force or set of forces os known as deformation or mechanical deformation.

Classification of metal 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.

It has been observed that whenever a stress of low magnitude is applied to a piece of metal it causes displacement of atoms from their original positions

But on the removal of stress, the atoms spring back and occupy their original positions.

      

 Plastic deformation

The term plastic deformation may be defined as the process of permanent deformation, which exist in a metal, even after the removal of the stress.

Example- Rolling, Forging, Drawing, spinning

The plastic deformation in crystalline materials occurs at temperature lower than 0.4 Tm (Tm is the melting temperature in Kelvin)

Difference in Elastic & Plastic deformation

Elastic Deformation

1. It is deformation which appears and disappears with the application and removal of stress.

2. The elastic deformation is the beginning of the progress of deformation.

3. It takes place over a short range of strass strain curve.

4. In elastic deformation the strain reaches its maximum value after the stress has reached its maximum value.

Plastic deformation

1. It is permanent deformation which exists even after the removal of stress.

2. The plastic deformation takes place after the elastic deformation has stopped.

3. It takes place over a wide range of stress strain curve.

4. In plastic deformation, the strain occurs simultaneously with the application of stress.

Modes of plastic deformation

Modes of plastic deformation

1. Slip

2. Twinning

Slip:- The term “slip” may be defined as a shear deformation, Which moves the atoms through many inter eratomic distances relative to their initial position.

Figure shows the adjacent planes of a hypothetical crystal. A shearing stress, acting as indicated by the arrow, tends to move the atoms of the upper planes to the right.

The movement of atoms or slip occurs only when the shear stress exceeds a critical values.

The atoms move an integral number of atomic distance along the slip plane and a step is produced as shown in fig. b

This movement can only possible to view with microscope only.

Twinning:

Twinning takes place along two plane due to a set of force applied on metal. process shown in the figure. 

Work Hardening:- The term work hardening may be defined as a process of deforming a metal at room temperature to improve its hardness, tensile & fatigue strength etc.

Whenever a metallic piece is subjected to a load, beyond its elastic limit, some plastic deformation takes place

As a matter of fact, this deformation takes place due to slipping of the atomic planes.

Cold working:-

The term cold working may be defined as the process of deforming plastically, a piece of metal below its recrystallization temperature. The recrystallization of a metal, usually, takes place within the temperature range of 0.3 to 0.5 times the melting temperature of a metal.

Example- Drawing, Squeezing, Shearing, Bending, Extruding etc.

Advantage of Cold work

1. It improves the mechanical properties like hardness, tensile, fatigue strength.

2. It prevents the loss of metal due to oxidation.

3. The cold worked metals have good surface finish.

4. It maintains a closer tolerance on dimension of a metal.

Disadvantage of cold work

1. It decreases ductility and creep resistance of a metal.

2. It requires greater energy to deform a metal plastically.

3. It produces internal stresses in a metal.

4. It produces distortion in the grain structure of a metal.

Recovery :- The term recovery may be defined as the process of removing internal stresses, in a metal by heating it to a relatively low temperature, which is usually below the melting point.

Crystal Defect and imperfection

Crystals are rarely found to be perfect. The atoms do not have their full quota of elements of electrons in the lowest energy level. But the atoms do not vibrate due to thermal effect, and the electrons also change their position.

There are many other types of defect found in the structure of the crystal.

1. Point defect

2. Line defect

3. Surface Defect

Point defect

The defect, which take place due to imperfect packing of atoms during crystallization, are known as point defect.

The Point defects also take place due to vibrations of atoms at high temperature. Following type of point defects are important from the subject point of view.

1. Vacancy defect

2. Interstitial defect

3. Frenkel defect

4. Substitutional Defect

5. Schotty Defect

Vacancy Defect

Whenever one or more atoms are missing from a normally occupied position as shown in figure. The defect caused is known as vacancy.

Such defect is a result of imperfect packing during formation of crystal. It also arise from thermal vibration of atoms at high temperature.

Type of vacancy

1. Single vacancy

2. Di-Vacancy

3. Tri-vacancy

Interstitial defect

Whenever an extra atoms occupies interstitial position in the crystal system, without dislodging the parent atom, the defect is known as interstitial defect.

It is noted that the interstitial position is generally smaller than the parent atoms.

Frenkel defect

Whenever a missing atom occupies interstitial position (Responsible or interstitial defect) as shown in figure, the defect caused is known as Frenkel defect.

Schottky Defect

Whenever a pair of positive and negative ions is missing from a crystal, as shown in figure, the defect is known as schottky defect.

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