Deformation of Metal

8/2/2019 Deformation of Metal

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Deformation of metal

The change produced in the shape of the metal pieceunder the action of action of a single force or a set ofthe force is known as deformation or mechanicaldeformation.

Classification of metal deformation :Elastic deformation

Plastic deformation


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ELASTIC DEFORMATIONThe term elastic deformation may be defined as the

process of deformation which appears and disappearssimultaneously with the application and removal of

stress.It has been observed that whenever a stress of low

magnitude is applied to piece of metal , it causedisplacement of atoms from their original positions.

But on the removal of the stress, the atom takesspring back and occupy the original positions.In the elastic deformation the tensile strain is due to a

slight elongation of the unit cell in the direction of thetensile load.

The compressive strain is due to a slight contraction ofthe unit cell in the direction of the compressive load.


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ELSTIC DEFORMATION

With tensile load


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PLASTIC DEFORAMATION

The term plastic deformation may be defined asthe process of permanent deformation which existin a metal even after removal of the stress.

It is due to this property that the metal may besubjected to various operation like forging ,drawing, spinning etc.

The plastic deformation in crystalline materialoccurs at temperature lower than 0.4 Tm


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Comparison between Elastic and Plasticdeformation

Sr.

No.

Elastic deformation PlasticDeformation

1 It is a deformation which appears anddisappears with the application and removalof stress.

It is permanentdeformation which existeven after the removal ofstress.

2 The elastic deformation is the beginning of

the progress of deformation

The plastic deformation

takes place after theelastic deformation hasstopped

3 It takes place over short range of stressstrain curve.

Over a wide range ofstress strain diagram.


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Modes of plastic deformation: The plasticdeformation may occur,

Slip

Twinning

Slip: The term slip , may be defined as a shear

deformation ,which moves the atoms throughmany inter-atomic distances, relative to theirinitial positions. The deformation by slip is asshown in fig.


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Slip plane

Slip line

Before slip After slip


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Mechanics of Slip:

Fig shows the simplified mechanics of slip.in thiscase the slip is shown to occur by thetranslation of one plane of atom over another.

Fig(1) shows the position of atoms in twoadjacent planes of a hypothetical crystal before

slip.

While fig(2) shows the movement of atom of theupper plane towards left on the application of

shear stress that shows new position of atomafter the slip has taken place.


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The value of shear stress required for such movementof atoms in a perfect lattice (called theoretical shearstrength ) is approximately equal to the value of shearmodulus divided by 6

In actual practice the value of shear stress is requiredis 100 times less than the theoretical value.

The experimental evidence shows that the mechanism

of slip is actually due to the movement of dislocation inthe crystal lattice.

Since the mechanics of slip requires the growth andmovement of dislocation line ,there fore energy is

required for this purpose. The energy of a dislocationline is given by the relation,

E l.G.b2,

Wr, l= length of dislocation line, G= shear modulas ,

b=unit slip or berger vector.


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Twinning: The term twinning , may be defined asthe plastic deformation, which takes place alongtwo planes due to a set of forces applied on a

given metal piece.

The process of twinning shown in fig.

A

B

D

C


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It may be noted that process of deformationb/w the two planes AB and CD is similar tothat of slip .

Whereas the arrangement of the atoms oneither side of the twinning planes (i.e.,

towards left of AB and right CD) remains

unaffected . It has been observed that a metal , usually ,

deforms by twinning only if it is unable toslip.

Moreover , the deformation produced bytwinning is small.


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Mechanism of twinning: In twinning process ,the movement of atoms is only a fraction of inter-atomic

distance.

The line AB and CD represents the planes of symmetry, from wr thetwinning starts and ends respectively. These planes are known as twinningplanes.

It has been observed that the crystals twin about the twinning planes. Andthe atoms in the regions to the left of the twinning plane AB and right of thetwinning planes CD remains undisturbed wr as in the twinned region, each

atoms moves by a distance proportional to its distance from the twinningplane AB.


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Comparison between slip and twinning

Sr.No.

Slip Twinning

1 The deformation takes placedue to the sliding of atomicplanes over the others.

however the orientation of thecrystal above and below theslip plane is the same afterdeformation as before.

The deformation takes place due toorientation of one part of the crystalwith respect to the other. the twinned

portion is mirror image of the originallattice.

2 the atomic movement are

over large distances.

The atomic movement are over a

fraction of atomic spacing.

3 It requires lower stress foratomic movements

It requires higher stress for atomicmovements.

4 It occurs on widely spaced It occurs on every atomic plane


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Work hardening:

it is also called strain hardening .

The term work hardening may be defined as a process of deforming a metalat room temperature to improve its hardness , tensile and fatigue strength

etc. .we know that whenever a metallic piece is subjected to a load , beyondits elastic limit , some plastic deformation takes place.

As matter of fact ,this deformation takes place due to slipping of the atomicplanes. it has been observed that the slipping takes place more easily inthose planes which has favorable orientation i.e.

This decreases the resistance of metal against plastic deformation and thework piece is known to be work hardened or strain hardened.

It has been observed that the work hardening produces internal stresses inthe metal.These stresses are produced due to :

pilling up of dislocations grain boundaries

Distorsion of grains due to loading.

It will be interesting to know that the work hardening improves the hardness,strength of metals like iron ,cu,Al ,nickel,etc.


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It decreases the ductility and electricalconductivity of the metals.


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Annealing of a cold worked metal During cold working a certain amount of work done on

the metal is stored internally in the form of strainenergy.

This energy produces internal stresses in a coldworked metal, which leads to the cracking of the

metals. In order to relieve the metal from internal stresses , a

particular process heat treatment called annealing isused.

In this process , the metal is heated to a temperaturebelow the melting point. In doing so the metal loses itsstored energy and comes back to its strain freeconditions.

It has been observed that the metal losses its storedenergy in the following three stages:


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Recovery

Recrystallisation

Grain growth


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Recovery

The term recovery may define as the process of removing internalstresses , in a metal by heating it to relatively low temperature which is, usually , below the melting point.

It has been observed that the recovery process does not effect thegrain structure but it removes the internal stresses only.

Moreover ,the recovery process does effect the hardness and strength.

But it increases the ductility of the material.

As a result of the cold working , the dislocation pileup at the grainboundaries.

During the process of recovery , dislocations start reducing and

rearranged themselves . They do so through mechanism known as polygonization.

In this mechanism ,the dislocation line out of their slip plane andrearranged themselves in a lower energy configuration.


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Recrystallisation The terms recrystallisation may be define as the process of forming strain

free new grains, in a metal , by heating it to a temperature known asrecrystallisation temperature.

It may be noted that the recrystallisation is , usually , the temperature atwhich abut 50% of the cold worked metal recrystallises in one hour.

It has been observed that the formation of new grains in recrystallisationtakes place through the following three processes.

NucleationPrimary grain growth

secondary grain growth

It will be interesting to know ,that during the nucleation , small strain freenuclei are formed at the grain boundaries wr the deformation is very high

these nuclei grow in to strain free grains during primary grain growth . These grains meet each other and replaced the old grains by the new ones.

during the secondary grain growth , these new grain grow at the expense ofothers . At the end of its stage , the grains are , usually , of very small sizebut of the same shape.


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The factors which affect the recystallisationprocess , are time of heating , temperatureprior to deformation , impurity and alloyetc


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Grain growth The term grain growth may define as the process of

forming strain free grains larger in size in metal by heatingit to a temperature above that of recrystallization.

It may be noted that the recrystallisation produces strainfree new grains. These grains are of smaller size , but of

equal shape. When the temperature is increased above recrystallisation

, these grains grow in size and the growth of grains takesplace even during the recrystallisation. but the growth rateis slow and becomes rapid with the increases of

temperature. The grain growth takes place due to thecombination of individuals grains, thereby reducing theirboundary area .

As a result of this ,the total energy decreases and thegrains become stable.


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The factors , which affect the growth rate ,are time of heating ,temperature , degree ofcold work and addition of impurities.

It has been observed that the grain growthresults in the decrease of hardness andstrength as well as increases in ductility.


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Variation of mechanical properties

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Deformation of Metal