Lec4-Mechanism in Metals

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L-04

ENGINEERINGMATERIALS

MECHANISM IN METALS

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MECHANISMS IN METALS-L-04

Methods have been devised to modify the yield

strength, ductility, and toughness of both

crystalline and amorphous materials.

These strengthening mechanisms give engineers

the ability to tailor the mechanical properties ofmaterials to suit a variety of different

applications.

For example, the favorable properties of steelresult from interstitial incorporation ofcarbon

into the iron lattice.

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http://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Toughnesshttp://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Amorphoushttp://en.wikipedia.org/wiki/Interstitial_defecthttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Interstitial_defecthttp://en.wikipedia.org/wiki/Amorphoushttp://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Toughnesshttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Yield_strength


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MECHANISM IN METALS

Brass, a binary alloy ofcopper and zinc, has

superior mechanical properties compared toits constituent metals due to solution

strengthening.

Work hardening (such as beating a red-hotpiece of metal on anvil) has also been used for

centuries by blacksmiths to introduce

dislocations into materials, increasing theiryield strengths.

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http://en.wikipedia.org/wiki/Brasshttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Dislocationshttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Dislocationshttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Brass


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What is Strengthening? Plastic deformation occurs when large

numbers ofdislocations move and multiply soas to result in macroscopic deformation.

In other words, it is the movement ofdislocations in the material which allows fordeformation.

If we want to enhance a material's mechanicalproperties (i.e. increase the yield and tensile

strength), we simply need to introduce amechanism which prohibits the mobility ofthese dislocations.

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http://en.wikipedia.org/wiki/Plastic_deformation_in_solidshttp://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Plastic_deformation_in_solids


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What is Strengthening? Whatever the mechanism may be, (work

hardening, grain size reduction, etc.) they allhinder dislocation motion and render thematerial stronger than previously.

The stress required to cause dislocation motion is

orders of magnitude lower than the theoreticalstress required to shift an entire plane of atoms,so this mode of stress relief is energeticallyfavorable.

Hence, the hardness and strength (both yield andtensile) critically depend on the ease with whichdislocations move.

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What is Strengthening?

Pinning points, or locations in the crystal that

oppose the motion of dislocations, can beintroduced into the lattice to reducedislocation mobility, thereby increasing

mechanical strength. Dislocations may be pinned due to stress field

interactions with other dislocations and soluteparticles, creating physical barriers from

second phase precipitates forming along grainboundaries.

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What is Strengthening? In amorphous materials such as polymers,

amorphous ceramics (glass), and amorphous

metals, the lack of long range order leads to

yielding via mechanisms such as brittle

fracture, crazing, and shear band formation.

In these systems, strengthening mechanisms

do not involve dislocations, but rather consist

of modifications to the chemical structure andprocessing of the constituent material.

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What is Strengthening?

Unfortunately, strength of materials cannot

infinitely increase. Each of the mechanismselaborated below involves some trade off by

which other material properties are

compromised in the process of strengthening.

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Strengthening Mechanisms in Metals 1-Work hardening: Work hardening, also known as

strain hardening or cold working, is thestrengthening of a metal by plastic deformation.

Or is the phenomenon whereby a ductile metalbecomes harder and stronger as it is plastically

deformed. This strengthening occurs because ofdislocation

movements within the crystal structure of thematerial.

Any material with a reasonably high melting pointsuch as metals and alloys can be strengthened inthis fashion

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1-Work hardening Alloys not amenable to heat treatment,

including low-carbon steel, are often work-hardened.

Some materials cannot be work-hardened at

normal ambient temperatures, such asindium, however others can only be

strengthened via work hardening, such as

pure copper and aluminum. The primary species responsible for work

hardening are dislocations.11/25/2012 11
http://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Indiumhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Aluminumhttp://en.wikipedia.org/wiki/Work_hardeninghttp://en.wikipedia.org/wiki/Work_hardeninghttp://en.wikipedia.org/wiki/Work_hardeninghttp://en.wikipedia.org/wiki/Work_hardeninghttp://en.wikipedia.org/wiki/Aluminumhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Indiumhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Heat_treatment


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1-Work hardening Dislocations interact with each other by

generating stress fields in the material. The interaction between the stress fields of

dislocations can impede dislocation motion byrepulsive or attractive interactions.

Additionally, if two dislocations cross,dislocation line entanglement occurs, causingthe formation of a jog which opposes

dislocation motion. These entanglements and jogs act as pinning

points, which oppose dislocation motion.11/25/2012 12


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1-Work hardening As both of these processes are more likely to

occur when more dislocations are present, there

is a correlation between dislocation density and

yield strength, where G is the shear modulus, b is the Burgers

vector, and is the dislocation density.

Increasing the dislocation density increases theyield strength which results in a higher shear

stress required to move the dislocations.11/25/2012 13
http://en.wikipedia.org/wiki/Shear_modulushttp://en.wikipedia.org/wiki/Burgers_vectorhttp://en.wikipedia.org/wiki/Burgers_vectorhttp://en.wikipedia.org/wiki/Burgers_vectorhttp://en.wikipedia.org/wiki/Burgers_vectorhttp://en.wikipedia.org/wiki/Shear_modulus


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1-Work hardening This process is easily observed while working a

material.

Theoretically, the strength of a material withno dislocations will be extremely high (=G/2)

because plastic deformation would requirethe breaking of many bonds simultaneously.

, at moderate dislocation density values ofaround 107-109 dislocations/m2, the materialwill exhibit a significantly lower mechanicalstrength.

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1-Work hardening Analogously, it is easier to move a rubber rug

across a surface by propagating a small ripple

through it than by dragging the whole rug.

At dislocation densities of 1014 dislocations/m2

or higher, the strength of the material

becomes high once again. It should be noted

that the dislocation density can't be infinitely

high because then the material would lose itscrystalline structure.

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2-Solid Solution Strengthening/Alloying For this strengthening mechanism, solute

atoms of one element are added to another,

resulting in either substitutional or interstitial

point defects in the crystal (see Figure 1).

The solute atoms cause lattice distortions that

impede dislocation motion, increasing the

yield stress of the material.

Solute atoms have stress fields around themwhich can interact with those of dislocations.

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2-Solid Solution Strengthening/Alloying The presence of solute atoms impart

compressive or tensile stresses to the lattice,depending on solute size, which interfere withnearby dislocations, causing the solute atomsto act as potential barriers to dislocationpropagation and/or multiplication.

The shear stress required to move dislocationsin a material is:

where cis the solute concentration and isthe strain on the material caused by thesolute.

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2-Solid Solution Strengthening/Alloying Increasing the concentration of the solute atoms

will increase the yield strength of a material, butthere is a limit to the amount of solute that can be

added, and one should look at the phase diagram

for the material and the alloy to make sure that a

second phase is not created.

In general, the solid solution strengthening depends

on the concentration of the solute atoms, shear

modulus of the solute atoms, size of solute atoms,valency of solute atoms (for ionic materials), and

the symmetry of the solute stress field.11/25/2012 19


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2-Solid Solution Strengthening/Alloying Note that the magnitude of strengthening is

higher for non-symmetric stress fields because

these solutes can interact with both edge and

screw dislocations whereas symmetric stressfields, which cause only volume change and

not shape change, can only interact with edge

dislocations.11/25/2012 20


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3-Precipitation Hardening/Age Hardening precipitation hardening -a process in which alloys

are strengthened by the formation, in theirlattice, of a fine dispersion of one componentwhen the metal is quenched from a hightemperature and aged at an intermediate

temperature. Precipitation hardening, also called age

hardening, is a heat treatment technique used to

increase the yield strength ofmalleable materials,including most structural alloys ofaluminium,magnesium, nickel and titanium, and somestainless steels.

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http://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Malleablehttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Malleablehttp://en.wikipedia.org/wiki/Heat_treatment


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3-Precipitation Hardening/Age Hardening

It relies on changes in solid solubility with

temperature to produce fine particles of animpurity phase, which impede the movement of

dislocations, or defects in a crystal's lattice.

Since dislocations are often the dominant

carriers ofplasticity, this serves to harden the

material.

The impurities play the same role as the particle

substances in particle-reinforced composite

materials.

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http://en.wikipedia.org/wiki/Solubilityhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Solubility


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Just as the formation of ice in air can produce

clouds, snow, or hail, depending upon thethermal history of a given portion of the

atmosphere, precipitation in solids can

produce many different sizes of particles,which have radically different properties.

Unlike ordinary tempering, alloys must be

kept at elevated temperature for hours to

allow precipitation to take place.

This time delay is called aging.

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Solution treatment and aging is sometimes

abbreviated "STA" in metals specs and certs.

Note that two different heat treatments

involving precipitates can alter the strength of

a material: solution heat treating andprecipitation heat treating.

Solid solution strengthening involves

formation of a single-phase solid solution viaquenching and leaves a material softer.

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Diffusion's exponential dependence upon

temperature makes precipitation strengthening,like all heat treatments, a fairly delicateprocess.

A large number of other constituents may beunintentional, but benign, or may be added forother purposes such as grain refinement orcorrosion resistance.

In some cases, such as many aluminum alloys,an increase in strength is achieved at theexpense of corrosion resistance.

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The addition of large amounts of nickel and

chromium needed for corrosion resistance instainless steels means that traditional hardeningand tempering methods are not effective.

However, precipitates of chromium, copper orother elements can strengthen the steel bysimilar amounts in comparison to hardening andtempering.

The strength can be tailored by adjusting theannealing process, with lower initialtemperatures resulting in higher strengths.

The lower initial temperature increase drivingforce of nucleation.

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More driving force means more nucleation sites,

and more sites, means more places fordislocations to be disrupted while the finished

part is in use.

Many alloy systems allow the aging temperatureto be adjusted.

For instance, some aluminium alloys used to

make rivets for aircraft construction are kept indry ice from their initial heat treatment until they

are installed in the structure.

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After this type of rivet is deformed into its

final shape, aging occurs at room temperatureand increases its strength, locking the

structure together.

Higher aging temperatures would risk over-aging other parts of the structure, and require

expensive post-assembly heat treatment.

Too high of an aging temperature promotesthe precipitate to grow too readily.

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In most binary systems, alloying above a

concentration given by the phase diagram will causethe formation of a second phase.

A second phase can also be created by mechanical

or thermal treatments.

The particles that compose the second phase

precipitates act as pinning points in a similar

manner to solutes, though the particles are not

necessarily single atoms.

The dislocations in a material can interact with the

precipitate atoms in one of two ways (see Figure 2).

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If the precipitate atoms are small, the

dislocations would cut through them.

As a result, new surfaces (b in Figure 2) of the

particle would get exposed to the matrix and

the particle/matrix interfacial energy wouldincrease.

For larger precipitate particles, looping or

bowing of the dislocations would occur whichresults in dislocations getting longer.

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For larger precipitate particles, looping or

bowing of the dislocations would occur whichresults in dislocations getting longer.

Hence, at a critical radius of about 5 nm,dislocations will preferably cut across theobstacle while for a radius of 30 nm, thedislocations will readily bow or loop toovercome the obstacle.

The mathematical descriptions are as follows: For Particle Bowing- For Particle Cutting-

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4. Grain Boundary Strengthening In a polycrystalline metal, grain size has a

tremendous influence on the mechanicalproperties.

Because grains usually have varyingcrystallographic orientations, grain boundaries

arise.

While an undergoing deformation, slip motionwill take place.

Grain boundaries act as an impediment todislocation motion for the following tworeasons:

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4. Grain Boundary Strengthening 1. Dislocation must change its direction of

motion due to the differing orientation ofgrains. 2. Discontinuity of slip planes fromgrain 1 to grain 2.

The stress required to move a dislocation fromone grain to another in order to plasticallydeform a material depends on the grain size.

The average number of dislocations per grain

decreases with average grain size (see Figure3).

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4. Grain Boundary Strengthening

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A lower number of

dislocations per grainresults in a lower

dislocation 'pressure'

building up at grainboundaries.

This makes it more

difficult fordislocations to move

into adjacent grains.35


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4. Grain Boundary Strengthening This relationship is the Hall-Petch Relationship

and can be mathematically described asfollows:

where kis a constant, dis the average graindiameter and y,0 is the original yield stress.

The fact that the yield strength increases withdecreasing grain size is accompanied by thecaveat that the grain size cannot be decreased

infinitely. As the grain size decreases, more free volume

is generated resulting in lattice mismatch.11/25/2012 36
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4. Grain Boundary Strengthening Below approximately 10 nm, the grain

boundaries will tend to slide instead; aphenomenon known as grain-boundary sliding.

If the grain size gets too small, it becomes more

difficult to fit the dislocations in the grain andthe stress required to move them is less.

It was not possible to produce materials with

grain sizes below 10 nm until recently, so thediscovery that strength decreases below a

critical grain size is still exciting.11/25/2012 37


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5. Transformation Hardening This is an assignment

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Lec4-Mechanism in Metals

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