Eng Metallurgy 7 -5

8/8/2019 Eng Metallurgy 7 -5

1/31

SMVEC DOME [email protected]

UNIT 5

Plastic Deformation

Slip and Twinning

Hot, Cold and Warm working

Recovery and Recrystallization concepts

Introduction to Fracture Mechanics

Ductile to Brittle transition

Creep and Fatigue failures Testing.

Text Books :1. Raghavan V, Physical Metallugy Principles and Practice, Prentice Hall India Pvt. Ltd., New Delhi,

2006

2. S.H.Avner, Introduction to Physical Metallurgy, Tata-McGraw Hill Publishing Co., New Delhi, 2000.

3. G.E.Dieter, Mechanical Metallurgy, McGraw Hill Publishing Co., New York, 1988.

Reference Books :

1. Donald R. Askeland, The Science and Engineering of Materials, Chapman and Hall,1990.2. Raghavan V, Materials Science and Engineering, Prentice Hall India Pvt. Ltd.,New Delhi, 20073. Budinski and Budinski, Engineering Materials Properties and Selection, Prentice Hall India

Pvt.Ltd.,2005


2/31


DEFORMATION:

When force is applied on a metal piece, then the size and/or shape will be altered. Any

change in the size and/or shape of the metal is called as deformation of metal. Deformations may

be temporary or permanent depending on the existence of change. If change is there even after

removal of load then it is permanent; if change disappears after loading, then it is temporary

deformation.

.

Based on the nature of strain produced during deformation is classified into

(1) ELASTIC DEFORMATION

(2) PLASTIC DEFORMATION

1. ELASTIC DEFORMATION:

It is the deformation of a body which completely disappears as soon as the external load is

removed from the body. It is fully recoverable and time independent.

2. PLASTIC DEFORMATION:

It is the deformation of the body which remains even after removing the external load from

the body. The plastic deformation may occur under the tensile, compressive or torsional stresses.

Ex: forging, milling, turning.

Plastic deformation may occur by slip, twinning or a combination of both.Plastic deformation of a

body remains after removal of external load from the body. In crystalline materials, at

temperatures lower than 0.4Tm, the permanent deformation is called plastic deformation, where


3/31


Tm is the melting point in degrees absolute. This may occur under tensile, compressive and

torsional stresses. Plastic deformation is a function of stress, temperature and rate of straining. It

occurs by the process of slip and twinning. In elastoplastic material, elastic deformation is

followed by plastic deformation before failure occurs. This property makes the metal suitable for

various forming processes. The operations such as turning, milling, sawing involve localized

plastic deformation in the region of tool activity and metal removal.

SLIP AND TWINNING:

SLIP:

Relative displacement of one part of the crystal with respect to the rest due to elongation

caused due to tensile stresses that crosses elastic limit is known as slip.Further increase in load

will cause movement of another parallel place. Each successive elongation requires higher stress

and results in the appearance of another step, which is actually the intersection of a slip plane with

the surface of the crystal. Progressive increase of the load leads to fracture of metal.

.

It is a flow that depends upon perfectly repetitive structure of the crystal which allows the

atoms in one face of a slip plane to shear away from their original neighbors in the other face, toslide in an organized way along with face, carrying their own half of the crystal with them, and

finally to join up again with new set of neighbors as nearly perfect as before.

The plane about which this sliding occurs is known as slip plane.

Resolution of axial tensile load F gives sin & cos components.

(Along slip plane)


4/31


(Perpendicular to slip plane)

Shear stress = =

Normal stress =

Or is max when = 45

Differently oriented crystals of a given metal will begin to slip when different axial stresses

are applied but that the critical resolved shear stress, that is, the stress required to initiate slip, is

always same.

.

If the slip planes are either parallel or perpendicular to the direction of applied stress, slip

cannot occur, and either the material deforms by twinning or it fractures. As deformation proceeds

and the tensile load remains axial, both the plane of slip and the direction of slip tend to rotate into

axis of tension.

MECHANISM OF SLIP:

Portions of the crystal on either side of a specific slip plane move in opposite directions

and come to rest with the atoms in nearly equilibrium positions, so that there is very little changein the lattice orientation. By application of the shear force, an extra plane of atoms (called

dislocation) has been formed above the slip plane. This dislocation moves across the slip plane and

leaves a step when it comes out at the surface of the crystal. Each time the dislocation moves

across the slip plane, the crystal moves one atom spacing. Since the atoms do not end up in

exactly normal positions after the passage of the dislocation, subsequent movement of the

dislocation across the same slip plane encounters greater resistance.


5/31


Eventually, this resistance or distortion of the slip plane becomes great enough to lock the

dislocation in the crystal structure, and the movement stops. Further deformation will require

movement on another slip plane.Although the distortion is greatest on the active slip plane, its

effects is felt thought the lattice structure, and the applied load must be increased to cause

movement on another slip plane.

TWINNING:

Twinning is a movement of planes of atoms in the lattice parallel to a specific (twinning)

plane so that the lattice is divided into two symmetrical parts which are differently oriented.In

twinning, each atom moves by only a fraction of an inter atomic distance relative to its neighbors.

The orientation of the twinned region is different from the untwined region. Generally, twinning is

operative at lower temperatures and higher strain rates. Twinning happens in a region that involves

the movement of a large number of atoms, and usually it appears microscopically as a broad line

or band.

.

KINDS OF TWINS:

1) Mechanical twins: Happens by deformation (CPH, BCC metals)

2).Annealing twins: Happens by reheating. Change in normal growth mechanism.

SLIP Vs TWINNING:

Sl.

DESCRIPTION SLIP TWINNING

1. Amount of movement Moves whole number of inter atomic spacings

Moves fractional amountof inter atomic spacings

2. Microscopic appearance Broad lines or bands


6/31


Thin line

3.

Lattice orientation

Very little change in

orientation, evidence is

temporary

Different lattice

orientation, evidence is

permanent

DIFFERENCE BETWEEN SLIP AND TWINNING:

SL.NO. SLIP TWINNING

1. Slip occurs along individual slip planesTwinning occurs over severalcrystallographic planes

2. Slip lines or bands disappear after grinding or

other surface finishing operations

Two lines run though the whole

depth of the material and hence

do not disappear on grinding etc.,

3.

Slip lines may be present in even or odd

numbers

Twin lines always occur is pairs.

4. Slip lines are open ended

Twin lines are close ended,

making boat shaped formation

5. Slip lines do not appear during any heat

treatment

Twin lines appear during the

annealing operation of some

materials

6. Atomic displacement in case of slip is a

multiple of atomic distance

In case of twinning atoms maymove a distance which is more or

less then the inter atomic spacing.

7. There is no change in the orientation of the

atoms after slip has occurred

Twinned atoms undergo a change

in their orientation and becomemirror of the untwined atom.

HOT COLD AND WARM WORKING:

HOT WORKING:

Hot working or metals takes place above the re-crystallization of work hardening range.

But in hot working process, the metals are given desired shape by subjecting them to forces

which cause them to undergo plastic deformation at the temperature above the re-

crystallization ranges. The re-crystallization temperature of steel is 800c. Although most hot

working on steel is done at temperatures above this temperature. Some metals such as lead, tin

and zinc have a low crystallization range and can be hot worked at room temperature. Most


7/31


commercial metals require some heating. During all hot working operations, the metal is in a

plastic state and is readily formed by pressure. In hot working processes, due to the high

temperature of metals there is a rapid oxidation or sealing of the surface. Hot worked

components have poor surface finish and close tolerances on dimensions cannot be

maintained. The cost of tooling and handling is high but it is a rapid process.

ADVANTAGES:

1. Porosity in metal can be largely eliminated

2. Coarse grains are refined to obtain fine grain structure

3. Impurities in the form of inclusion are broken up and distributed though the metal

4. Physical properties are generally improved

5. Ductility and resistance to impact are improved

6. Strength increases and greater homogeneity is developed in the metal

7. Directional property resulting from the fiber structure is obtained

8. It is quick and economical process

DISADVANTAGES:

1. Tooling and handling costs are high

2. Poor surface finish due to rapid oxidation at high temperature

3. Close tolerances on dimensions cannot be maintained

4. The life of the tools used in this process is less because of high temperature

VARIOUS PROCESSES:

1. Forging

2. Welding

3. Rolling

4. Piercing

5. Spinning


8/31


6. Extruding

7. Drawing/cupping

COLD WORKING:

If the mechanical work is done on the metal below its re-crystallization temperature then it

is known as cold working. There will not be any grain growth; but there is grain disintegrationand elongation. Cold working process is limited to some metals only.

The metals are crystalline in nature and are made up or irregularly shaped grains of

different sizes. Each grain is made up of atoms in an orderly arrangement. The orientation of

the atoms in a particular grain is uniform but differs from adjacent grains. In cold working of

metals, the grain structure changes resulting in the grain fragmentation, movement of atoms

and lattice distortion. Slip planes develop this lattice structure at points where atomic bonds of

attraction are the weakest, thus the whole block of atoms is displaced. But the orientation of

the atoms is not changed when slip occurs. Twinning occurs where the atoms are reoriented. In

twinning, the lattice on one side of the plane is oriented in a different fashion from the other.

Cold working requires greater pressure than hot working. As the metal is in a move rigid

state, it is not permanent by deformed until the stress exceeds the elastic limit. In cold working

range, there is no re-crystallization of grains and hence there is no recovery from grain

distortion or fragmentation. Tensile strength and hardness increase with corresponding

decrease in ductility by the phenomenon known as work hardening or strain hardening. This is

due to resistance built up in the grains by atomic dislocation of fragmentation or lattice

distortion. During cold working, the metal is deformed which gives rise to severe stresses,

known as residual stresses inside the metal. These stresses are undesirable and can be removed

by suitable heat treatment process. This heating of metal will not have any appreciable changein physical properties or grain structure of the metal. Further heating in the crystallization

range will eliminate the effect of cold working and restore the metal to its original condition.At

room temperature most cold working processes can be performed. The cold working merely

distorts the grains and does little towards reducing its size.

In cold working, use of soaking pits and furnaces as well as the handling of heated material

are avoided and hence it results in to faster production. The deformation of metals is brought

about by the method of slip of planes in the process of cold working. Also the force required

for hot working of metals, because in cold working the metals are not deformed permanently

till the elastic limit is exceeded. Cold working produces an improved surface finish and closertolerance on dimensions. For metals that do not respond to heat treatment, cold working is

possible method used to increase hardness. It is also useful in the forming of many articles by

extension of the ductile materials. As shaping process, it is limited to ductile materials.

COLD WORKING PROCESSES:

1. Drawing

2. Squeezing


9/31


3. Bending

4. Shearing

5. Extruding

6. Hobbing

EFFECT OF COLD WORKING ON METALS:

1. Ductility, tensile strength, hardness increases

2. Residual stresses are formed

3. A distorted grain structure is formed

4. Surface finish is improved

5. Re-crystallization temperature for steel is raised

DIFFERENCE BETWEEN HOT WORKING AND COLD WORKING:


10/31


WARM

WORKING:

Warm working is plastic deformation of a metal at temperatures below the temperature

range for re-crystallization and above room temperature. It attempts to combine the advantage

of both hot and cold working into one operation. Warm working has been applied most

extensively to steel forging process, where it offers the potential of fewer forging steps,

reduced forging loads and energy savings compared with cold forging.

ADVANTAGES:

1. Consumes less mechanical energy

2. Improved dimensional control

3. High surface finish

LIMITATIONS:

1. Special die required

2. Lubricant required

3. Cant be used for all metals

Sl.No.

Point Hot working Cold working

1. Working temperatureAlways above re-crystallization

temperature

Always below

2. Residual stresses Not produced Always produced

3. Hardness Not affected Increases

4. Strain hardening Not produced Produced

5. Energy required Less More

6. Directional properties

Produced in the direction

of flow

Not produced

7. Oxidation

Produced hence not

suitable for reactivematerials

Prevented and

suitable forreactive metals

8. Surface finish Not so good Better

9. Applicability

To all metals that

becomes plastic onheating

To all ductile

materials only


11/31


RECOVERY AND RECRYSTALLISATION:

RECOVERY:

Recovery has been defined as the process of releasing internal stresses in metal at relatively low

temperature which are below its re-crystallization temperature. Recovery hardly affects the

structure and mechanical properties of the metal. It reduces internal stresses and increases tensilestrength. Electrical conductivity is also slightly increases. Recovery is an important method for

relieving internal stresses in castings; forgings welded and fabricated equipments, cartridge cases,

and boiler tubes without lowering the strength acquired during cold working. The recovery is a

process of stress relieving treatment.

RECOVERY MECHANISM:

1. AT LOW TEMPERATURE: (vacancy motion)

(i) Migration of point defects to grain boundaries and dislocations

(ii) Combination of point defects

2. AT INTERMEDIATE TEMPERATURE: (dislocation movement)

(i) Arrangement of dislocation with in tangle

(ii) Annihilation of dislocations

(iii) Grain growth

3. AT HIGH TEMPERATURE: (dislocation climb)

(i) Disappearance of boundary between sub grains

(ii) Polygonisation

(iii) Dislocation climb

DISAPPEARANCE OF BOUNDARY:

In cold worked metals, for doing recovery process, the heating is at low temperature. By

this, the vacancies, interstitials and dislocations undergo rearrangement in the lattice vacancies and

interstitials are eliminated first and then some dislocations of opposite sign are annihilated.However, majority of dislocations are not removed by usual recovery treatments. The minor

structural changes during recovery have pronounced effect on residual stresses and on electrical

properties.


12/31


RECRYSTALLISATION:

As the upper temperature of the recovery range is reached, minute new crystals appear in

the microstructure. These new crystals have the same composition and lattice structure as the

original undeformed grains and are not elongated but are equiaxed. The new crystals generally

appear at the most drastically deformed portions of the grain, usually the grain boundaries and slip

planes. The duster of the atoms from which the new grains are formed is called a nucleus. Re-crystallization takes place by a combination of nucleation of strain free grain and the growth of

these nuclei to absorb the entire cold worked material.

Formation of new grains and thus the metal is said to re-crystallize. This is usually

followed by grain growth during which growth of the larger re-crystallized grains occurs at the

expense of smaller ones, obeying the tendency of polycrystalline material to reduce its total

interfacial surface energy. The re-crystallization process causes a rapid change in the mechanical

properties and microstructure of the metal. The strength and hardness are greatly reduced, ductility

increases and complete stress relief takes place distorted, elongated grains disappear and new

grains are formed. Re-crystallization temperature depends on the degree of deformation of coldworked metal. The greater the degree of work hardening, the lower will be the temperature.

COMPARISON BETWEEN RECOVERY AND RECRYSTALLISATION:

FACTORS THAT CONTROL RE-CRYSTALLIZATION TEMPERATURE:

1. Temperature of heating

2. Heating time

3. Amount of previous cold work

MECHANISM OF RE-CRYSTALLIZATION:

1. Pre-existing grain boundaries

2. Sub grain boundaries resulting from deformation

Sl.No.

Metal Re-crystallizationtemperature (c)

Melting temperature(c)

1. Aluminum(Al) 150 660

2. Copper(cu) 200 1083

3. Iron(Fe) 450 1535

4. Nickel(Ni) 620 14525. Magnesium(Mg) 150 651

6. Tungsten(w) 1210 3400

7. Silver(Ag) 100 960.5


13/31


GROWTH OF PRE-EXISTING GRAIN BOUNDRIES:

The boundary between a grain of high dislocation density and a grain of low dislocation

density suddenly grows. Thus the nucleation is essentially a growth phenomenon. We can observe

through the electron microscope that this mobile boundary is pinned at two points as it bulges out.

The nucleation by this growth mechanism will occur at boundaries having grain boundary

mobility. Ex. High angle boundaries.

GROWTH OF SUB-GRAIN BOUNDRIES:

The sudden growth may be due to either by coalescence mechanism or by grain boundary

migration. High mobility boundary forms a high angle boundary, which suddenly grows out.

Adjustment between sub-boundaries occurs on atomic scale. These adjustments modify high

mobility boundary and thus nucleation occurs. This mechanism is more common in highly

deformed metals large deformation produces large micro orientations between sub-grains and high

angle sub boundaries.

TYPES OF RE-CRYSTALLIZATION:

1. Primary re-crystallization

2. Secondary re-crystallization

PRIMARY RE-CRYSTALLIZATION:

It occurs when cold worked metal is heated. It is defined as the nucleation and growth of

strain-free grains, from the matrix of cold worked metal. When primary re-crystallization occurs,

there is some degree of recovery and sub grain formation. Primary re-crystallization is of much

importance because the properties of an alloy, after properties it has before cold working. Consider

a cold working operation of an alloy. (Example: Deep drawing).

During cold working, it becomes hard and less ductile. It becomes hard and less ductile. It

becomes difficult to continue the forming operation. A partial forming operation is done first and

then the alloy is given a re-crystallization (annealing) treatment. This partially formed alloy

regains its original ductility and hardness (of easy deformation). Now the job can be given for next

forming operation for deep drawing process.

SECONDARY RE-CRYSTALLIZATION (OR) COARSENING:

When annealing of a deformed sample is continued, beyond that stage of primary re-

crystallization, or sample is heated at higher temperature, after primary re-crystallization is

complete; the usual grain growth is interrupted. Some grains suddenly experience very rapid

growth. The dimensions of these rapidly grown grains may be of the order of few centimeters

while the rest of the grains remain small. These grains expand at the cost of other grains. This is


14/31


called secondary re-crystallization. The large grains are not freshly nucleated; there are only

particular grains of primary structure which are grown large. (At temperature

(i) It shows the grain that rapidly grows in size. It is not clear that which primary grain

will grow in size, but the grains that grow is larger than mean primary grain size.

(At

(ii) Further growth in size of grains. (At

LAWS OF RE-CRYSTALLIZATION:

(i) A minimum of 2 to 8 % deformation is necessary for re-crystallization.

(ii) The smaller the degree of deformation, the higher the temperature required to initiate re-

crystallization

(iii) Increase in annealing time decreases the temperature required for re-crystallization

(iv)When degree of deformation is more and annealing temperature is less, the crystallized

grain size will be smaller

(v) New grains do not grow into deformed grains

(vi)Continued heating after primary re-crystallization causes grain size to increase

TEMPERATURE OF RE-CRYSTALLIZATION AND %DEFORMATION ARE HAVING INVERSE RELATIONS

CHANGE IN VARIOUS PROPERTIES PRODUCED BY HEATING OF COLD WORKED METAL


15/31


DIFFERENCE BETWEEN RECOVERY AND RE-CRYSTALLIZATION:

Sl.

No. Point Recovery Re-crystallization

1.Working temperature range Low (between room and High (at

2.Mechanical properties Not affected Affected

3.

Process Relieve internal stresses Rapid change in

microstructure and

properties

4. Enhanced property Tensile strength increases Ductility increases

DIRECTIONAL PROPERTIES:

In rolling and forging operation, the crystals of the metals get elongated along the direction

of plastic flow. The polyhedral grains are converted in to fibers at low temperature. The structure

is known as fibrous structure. The direction of the plastic flow is determined always in the

direction in which the material elongates during mechanical shaping. For example, if we take two

hooks, one made by rolling process and another one by forging process. The plastic flow lineshows that Forged hook is stronger than rolled hook because flow lines are perpendicular to stress

direction. But rolled hook shear stress acts in the direction parallel to fibers, weak, easily fails

during service.

WORK HARDENING OR STRAIN HARDENING:

When metals are deformed at room temperature by the application of load, they offer more

and more resistance to further deformation. The tensile strength and hardness of the metal

increases but ductility decreases. The phenomena are known as the work hardening or strain

hardening.

FRACTURE MECHANICS

FRACTURE:

Fracture is the mechanical failure of the material which will produce the separation or

fragmentation of a solid into two or more parts under the action of stresses. Crack initiation and

crack propagation are components of fracture


16/31


Fracture: Types

1.Brittle 2.Ductile 3.Creep 4.Fatigue 5.Shearing

BRITTLE FRACTURE: (CLEAVAGE FRACTURE)

It is defined as a fracture which takes place by the rapid propagation of crack with a

negligible deformation.Example: glass

In crystallography, the fracture takes place normal to the specific crystallographic planes,called cleavage planes. In polycrystalline materials the fracture takes place along the grain

boundaries. The tendency of the brittle fracture is increased with decreasing temperature and

increasing strain rate.

Brittle fracture is characterized by rapid rate of crack propagation with minimal energy

absorption. A brittle fracture occurs by separation normal to tensile stress.Brittle fractures have

been observed in BCC and HCP crystals but not in FCC metals. There is very little mass

deformation but there is no gross deformation. Brittle fracture normally follows grain boundaries.

Tendency of brittle fracture is increased with

(i) Decreasing temperature

(ii) Increasing strain rate

(iii) Tri axial stress condition

Brittle fracture is to be avoided because it may cause heavy damage because it does not give any

prior warning.


17/31


Example: pressure vessels, bridges, ships

DUCTILE FRACTURE:

It is the fracture which takes place by a slow propagation of crack with appreciable plastic

deformation. It always preceded by localized deformation called NECKING.
http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Pedalarm_Bruch.jpg/180px-Pedalarm_Bruch.jpg


18/31


Specimen of very ductile metals (Au, Pb...) may actually be drawn down to a point before

they rupture. In moderately ductile metals, fracture takes place through local necking which

caused after appreciable plastic deformation. The failure of most polycrystalline materials occurswith cup and cone ductile fracture.

(Fig) ductile fracture with cup-cone formation

(Fig) completely ductile fracture

PREVENTION OF DUCTILE FRACTURE:

1. High hardness metals

2. Fine grained structure

3. High YMand cohesive energy

4. Defect/dislocation free

COMPARISON BETWEEN BRITTLE AND DUCTILE FRACTURE:

Sl.no. Brittle fracture Ductile fracture

1. It occurs with negligible plastic deformation It occurs with large plastic deformation

2. It occurs at the point where micro cracks is

more

It occurs in some localized region where

deformation is large

3. The rate of crack propagation is rapid Slow

4. Failure is due to the direct stress Failure is due to shear stress
http://wpcontent.answers.com/wikipedia/commons/thumb/b/be/Ductility.svg/157px-Ductility.svg.png


19/31


5. It is characterized by separation of normal totensile stress

It is characterized by the formation of cupand core

6. Fractured surface shows a sharp planar face Fractured surface is rough dirty grey contour

7. Increasing factors:

1. Increasing temperature2. Increasing strain rate

3. Work hardening

Increasing factor:

1. Dislocations2. Other defects

MECHANISM OF FRACTURE:

1. Brittle fracture : (i) Griffith crack theory (ii) cleavage

2. Ductile fracture : (i) cup and core formation

BRITTLE FRACTURE: (a) GRIFFITH THEORY

It is proved that the stress at which a material fractures is far below the lower value of the

ideal breaking strength calculated from the atomic strength. In other words, the fracture strength of

real materials is far lower than ( the theoretical minimum value for an ideal solid.

According to Griffith, the discrepancy between the strengths of real and ideal materials is

due to many fine cracks which act to concentrate the stress at their tips or ends. The micro cracks

in the metals that cause local concentration of stress to values high enough to propagate the crack

and eventually to fracture of metals.

GRIFFITH THEORY:

A crack will propagate when the decrease in elastic strain energy is at least equal to the

energy required to create the new crack surface.ie. in a brittle fracture, there are many fine cracks.

These cracks concentrate the applied stress at their tips or ends. When the stress at the tips of acrack exceeds the theoretical stress values, the crack expands and fracture occurs.

EXPLANATION OF MECHANISM OF BRITTLE FRACTURE:


20/31


Let us consider a crack of elliptical cross-section in a rectangular specimen

Let,

Radius of curvature at the ends of the ellipse

Maximum stress at the tip of the crack

Tensile stress applied to the specimen

C Half length of the crack

It is observed that when a tensile stress is applied to the specimen, then the applied stress is

distributed about the crack in such a way that the maximum stress occurs at its tips. The maximum

stress ( ) at the tip of the crack is given by


21/31


It is understood that when an elastic material is stressed, potential energy is stored in thematerial before crack occurs. This stored energy is known as elastic strain energy. When a crack

begins propagating, elastic energy is released. It is also understood that as the crack propagates,

new surfaces are created and a certain amount of energy, called surface energy, must be provided

to create them. Griffith supposed that the crack propagates when the released strain energy is just

sufficient to provide the surface energy necessary for the creation of the new surface.


22/31


E Youngs modulus

Surface energy per unit area (J/

C Half length of crack

This is known as GRIFFITH FRACTURE EQUATION.

CLEAVAGE:

At low temperatures, brittle fractures takes place when a knife edge is hammered as shown

in fig. below, then the crystal will split into two parts with reference to a plane called cleavage

plane. This operation is called as cleavage.

Example: Fe (BCC) {110}

FCC metal do not develop cleavage

Zinc crystals are capable of being cleaved at room temperature but only with somedifficulty. Very nice cleavages are possible at -196c.

Cleavage of zinc produces distorted surfaces.

Alkali metals like NA, K, etc. do not cleave although they are BCC in structure.

DUCTILE FRACTURE: MECHANISM

(Cup and cone fracture)

The various stages at which ductile fracture takes place is mentioned in fig.

Fig:


23/31


(a) Formation of neck: when the tensile stress is increased beyond the ultimate tensile stress, a

neck is formed in the specimen

(b) Fine cavities: it happens when the plastic deformation continues.

(c) Central necking: cavities grow in size if the straining is continuous. A central neck is also

formed.

(d) Crack propagation: the crack grows in a direction perpendicular to the axis of the specimen

until it approaches the surface of the specimen. It then propagates to form the cone part of

fracture.

(e) Cup and cone: the central cup region of the fracture has very fibrous appearance.

(f)

DUCTILE TO BRITTLE TRANSITION:

When BCC metals (ex. Steels) are subjected to impact loads at comparatively low

temperatures, a transition occurs from ductile fracture (requiring high energies) to non ductile

(brittle) fracture (requiring lower energy). This transition can become quite important to the

engineer who is designing a structure which will be subjected to impact stresses. When the

transition temperature is below operating temperatures, brittle type fractures will not occur. Eachfracture crack originated at same point of stress concentration, probably a sharp corner or

fabrication defect, which crack then propagated around the entire structure of split.

Below a certain temperature, the energy to break a specimen (ex. Iron) under impact

loading decreases abruptly which corresponds to transition from ductile to brittle behaviors. This

temperature is known as transition temperature.

Example: BCC and HCP metals, ceramics, polymers


24/31


Titanic ship fracture

FCC dont show transition behavior

The ductile brittle transition temperature depends on

(i) Crystal structure

(ii) Alloying elements (ex. Carbon content in steel)

(iii) Rates of strain

(iv)Micro structure of material

(v) Smaller grain size

(vi)Impurities

(vii) Size and shape of specimen

(viii) Stress distribution

CREEP AND FATIGUE FAILURES:

CREEP:

It is defined as the property of a material by virtue of which it deforms continuously under

a steady load.

It is the permanent deformation of a material under a steady load as a function of time,

usually at higher temperature when a material is subjected to a constant loading, then the timedependant strain occurring under the constant stress is known as creep

Example: zinc, lead (at room temperature)

Fe, Ni, Cu (at elevated temperature)

FACTORS AFFECTING CREEP:

1).Grain size

2).Thermal stability of the microstructure

3).Chemical reactions


25/31


4).Prior strain

PREVENTION OF CREEP:

1. Use of coarse grained materials

2. Strain hardening

3. Free from residual stresses

4. Precipitation hardening

5. Proper heat treatment

CREEP CURVE:

1. Instantaneous elongation

2. Primary creep

3. Secondary creep

4. Tertiary creep

A creep test curve under constant nominal stress at constant temperature is shown above. Different

stages are

1. Primary creep

2. Secondary creep

3. Tertiary creep.


26/31


I. PRIMARY CREEP:

In this stage, the creep is mainly due to dislocation movement. The creep rate decreases

with time, during this stage, the recovery effect is less than the work hardening effect. Hence the

creep rate decreases logarithmically.

II. SECONDAY CREEP:

During this stage, the rates of work hardening and recovery are equal, so the material

creeps at steady rate. For the above reason, secondary creep is usually termed as steady state

creep. Steady state creep may be viscous or plastic in character, depending upon the state level

and temperature. It is the important part of the creep curve which is used to estimate the service

life of the alloy.

III. TERTIARY CREEP:

In this stage creep rate increases with time until fracture occurs. Generally the tertiary

creep occurs due to necking of the specimen or grain boundary sliding.

MECHANISM OF CREEP FAILURE:

1. DISLOCATION CLIMB

(i) At high temperature

(ii) Atomic climb

2. VACCANCY DIFFUSION

(i) Movement of vacancy

3. GRAIN BOUNDARY SLIDING

(i) At low temperature

CREEP TESTS:

The purpose of creep tests is to determine the creep limit. The creep limit or the limiting

creep stress is defined as the stress that will not break the specimen when applied for an infinite

period at a specific constant temperature. In creep test, we measure stress, strain, temperature and

time. It is the tension test that is done at constant load and constant temperature value of strain of

the test piece is noted as a function of time. The test arrangement is as shown:


27/31


28/31


FATIGUE:

The behavior of the materials subjected to fluctuating or repeated loads is called fatigue.

FATIGUE FRACTURE:

The fatigue fracture is defined as the fracture which takes place under repeatedly applied

fatigue stresses.The fatigue fracture occurs at stresses well below the tensile stresses of the

materials.

Example: high speed machines, motor shafts, bolts, springs, gear teeth valves,turbine blades, air planes, automobile and gas engine parts, wire rope, suspension bridges

STRESS CYCLES:

(i) Reversed stress

(ii) Fluctuation stress

(iii) Irregular stress

FACTORS AFFECTING FARIGUE STRENGTH:

(i) Chemical composition, grain size, amount of cold working

(ii) Rise in temperature decreases fatigue strength

(iii) Corrosion decreases fatigue strength

(iv)Residual stress, stress gradients can be induce problem to fatigue strength. Proper design to

avoid those factors is important
http://www3.ntu.edu.sg/mae/research/labs/materials/_creep_test.jpg


29/31


PREVENTION OF FATIGUE FAILURE:

(i) Avoid sharp recesses and stress risers

(ii) Maintain good surface finish

(iii) Reduce corrosive environments

(iv)Fine grain materials

S-N DIAGRAM:

S-N diagram can be obtained by plotting the number of cycles of stress reversals (N)

required to cause fracture against the applied stress level(S).

Fatigue strength of steels in more compared to non-ferrous metals.

Fatigue stress (or) fatigue strength

Fatigue limit (or) endurance limit

Fatigue life

In high-cycle fatigue situations, materials performance is commonly characterized by an S-N

curve, also known as aWohlercurve. This is a graph of the magnitude of a cyclical stress (S)

against the logarithmic scale of cycles to failure (N).

FATIGUE TEST:
http://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/Logarithmic_scalehttp://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/Logarithmic_scale


30/31


Fatigue tests determine the resistance of material to repeated pulsating (or) fluctuating

loads.Fatigue tests are done using rotating beam fatigue testing machine as shown in fig.

below:

TESTING PROCEDURE:

The step by step procedure for fatigue testing is given below:

(i) The test specimen is placed on the machine

(ii) Now the specimen is rotated using an electric motor

(iii) When the specimen is rotating, it can be noted that the upper surface of the specimen is

subjected to tension and its lower surface experiences compression.

(iv)As the specimen rotates, there is sinusoidal variation of stress between a state of maximum

tensile stress and a state of maximum compressive stress

(v) The cycles of stress are applied until the specimen fractures. A reduction counter records

this number of stress cycles

(vi)Now a number of specimen of the same material( at least six specimens) are tested in the

same manner under different stress levels and the results are plotted on S-N graph

(vii) S-N graph is drawn and S-N curve is obtained

INFERENCES MADE:


31/31


(i) Life time of part

(ii) Maximum allowable load

(iii) To fix endurance limit

(iv)Fatigue life obtained (stress, time, cycles)

(v) Endurance ratio = endurance limit / tensile strength 0.5

Eng Metallurgy 7 -5

Documents