Clase Recristallization

8/16/2019 Clase Recristallization

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Dislocations and Strengthening Mechanisms


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OR

Why the strength of metals is

much lower than the value predicted from the metallic bond?

For example, the theoretical shear

strength is well over 1000MPa actual

shear strength of Cu is at least an

Order of magnitude less than this.

You can this way:

Atomic bond breaks


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Slip and Slip Plane

The process by which a dislocation moves and causes a material to deform is called slip.

The crystallographic plane along which the dislocation line traverses is the slip plane.

Shear stress


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Slip Direction

• The direction in which the dislocation moves is called slip direction.

For edge dislocations, it is the direction of Burgers vector.


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The motion of an edge dislocation:

The dislocation line motion

is parallel to the direction ofthe applied shear stress.

The motion of a screw dislocation:

The dislocation line motionis perpendicular to the

stress direction


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Slip System

The combination of slip direction and slip plane is the slip system.


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Dislocation slip is more difficult along a low-atomic-density

plane than along a high-atomic-density plan.


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An FCC unit cell and its slip system

{111} – type planes (4 planes) have

all atoms closely packed.

Slip occurs along – type directions

(A-B,A-C,D-E) within the {111} planes.

FCC has 12 slip systems - 4 {111} planes and each has 3 directions


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* Generally, the metals having larger number of slip systems are more ductile than the

one has smaller number of slip system. Thus, FCC & BCC metals are generally ductile

and HCP is brittle.


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Slip in Single Crystals

Resolved shear stress (τR) and critical resolved shear stress (τcrss )

τR is the resolved shear stress is given by:

F R = F cos λ and AR = A/cos

τR = F R / AR = σ cos cos λ

τ

crss

represents the minimum shear stressrequired to initiate slip and is a material’s

property, which determines when yieldingoccurs.

τcrss

σy =

(cos cos λ )max

at = λ = 45 °, we have: σy = 2 τcrssIt is the minimum stress necessary to cause yielding.

F R AR

λ


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Slip occurs along a number of equivalent

and most favorably orientated planes and

directions in a single-crystal specimen


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Plastic deformation of polycrystalline materials

Before deformation-

the grains are equiaxed. After deformation-

the grains are elongated.Slip lines in deformed grains


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Deformation by twinning - In addition to slip plastic

deformation in some metallic

materials can occur by the

formation of mechanical twins,

or twinning.

Deformation by twinning

Deformation by Slip

Crystallographic orientation above and below a slip is the same before and after the

deformation. But, for twinning, there will be a reorientation across the twin plan, which

may place new slip systems for slip to occur.


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OR

Why the strength of metals is

much lower than the value predicted from the metallic bond?

For example, the theoretical shear

strength is well over 1000MPa actual

shear strength of Cu is at least an

Order of magnitude less than this.

You can this way:

Atomic bond breaks


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Strengthening by grain size reduction

σy = σ0 + k y d-1/2

Where: 0 and k y are constants

(1) A dislocation passing into a grain having different

orientation will have to change its direction of motion;

thus grain boundary becomes a barrier.

(2) The disorder in a grain boundary will result in a

discontinuity of slip planes from one grain into the

other.

(3) Reducing grain size or increasing grain boundary is

a strengthening means for polycrystalline metals


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Strengthening by solid-solution

Tensile lattice strain on host atoms by a

smaller substitutional impurity atom.

Edge dislocation

Compressive lattice strain on host atoms by alarger substitutional impurity atom.

Edge dislocation


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Pure Metal Larger impurity atoms

Interstitial impurity atomsSmaller impurity atoms


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Strain Hardening


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Strain Hardening


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Strain Hardening

σT = K εT n


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Strain hardening and cold work

Cold work

%CW = (A0 -Ad )/A0 x 100


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Mechanisms of StrainHardening (cold-work)

in Metals

A dislocation

Bowing of

the dislocation

under stress

Formation of a new dislocation

More dislocation we have, the more likely they are interfere with one another and the stronger the metal becomes.


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Mechanisms of Strain

Hardening in Polymers

Polymer chain alignment occurs

under applied stress.


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Strain hardening by cold work - Strength and ductility relationship


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Example Question


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Anisotropic behavior induced by cold work


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Annealing is a heat treatment designed to eliminate the effects of cold working.

The properties of a metal may revert back to the precold-worked states by

Annealing, through recovery, recrystallization and grain growth.

Recovery : the relief of some of the internal strain energy of a previously cold-worked material.

Recrystallization: the formation of a new set of strain-free grains within a previously cold-worked material.

Grain Growth: the increase in average grain size of a polycrystalline material.

An elevated temperature heat treatment (annealing) is needed for these 3-processes.


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Recrystallization

and grain growth

of brass

b

(a) Cold-worked

(b) Initial stage of

recrystallization(3 s at 580 C)

(c) Partial replacement

of cold-worked

grains by

recrystallized ones

(4 s at 580

C)(d) Complete

recrystallization

(8 s at 580 C)

(e) Grain growth after

15 min at 580 C.

(f) Grain growth after 10 min at 700 C

ca

d e


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Annealing temperature

and Mechanical Properties

for a Brass Alloy


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Grain Growthd n – d 0 n = Kt


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Recrystallization proceeds more rapid in pure metals that in alloys.

For pure metals, the recrystallization temperature is about 0.3T m (T m is absolute melting temp.)

For some alloys, the recrystallization temperature can be as high as 0.7T m.

Di l i d S h i M h i


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Homework for this Chapter

Questions: 7.12

7.147.18

7.27

7.287.D4

The homework is due one week from Today

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Clase Recristallization