Chapter 8 - 1 ISSUES TO ADDRESS... Why are the number of dislocations present greatest in metals ? How are strength and dislocation motion related? Why.

Chapter 8 - 1

ISSUES TO ADDRESS...

• Why are the number of dislocations present greatest in metals ?

• How are strength and dislocation motion related?

• Why does heating alter strength and other properties?

Chapter 8: Deformation & Strengthening

Mechanisms

Chapter 8 - 2

Dislocations & Materials Classes

• Covalent Ceramics (Si, diamond): Motion difficult - directional (angular) bonding

• Ionic Ceramics (NaCl): Motion difficult - need to avoid nearest neighbors of like sign (- and +)

+ + + +

+++

+ + + +

- - -

----

- - -

• Metals (Cu, Al): Dislocation motion easiest - non-directional bonding - close-packed directions for slip

electron cloud ion cores

++

++

++++++++ + + + + +

+++++++

Chapter 8 - 3

Dislocation Motion

Dislocation motion & plastic deformation• Metals - plastic deformation occurs by slip – an edge

dislocation (extra half-plane of atoms) slides over adjacent plane half-planes of atoms.

• If dislocations can't move, plastic deformation doesn't occur!

Adapted from Fig. 8.1, Callister & Rethwisch 3e.

Chapter 8 - 4

Dislocation Motion

• A dislocation moves along a slip plane in a slip direction perpendicular to the dislocation line

• The slip direction is the same as the Burgers vector direction

Edge dislocation

Screw dislocation

Adapted from Fig. 8.2, Callister & Rethwisch 3e.

Chapter 8 - 5

Slip System– Slip plane - plane on which easiest slippage occurs

• Highest planar densities (and large interplanar spacings)

– Slip directions - directions of movement • Highest linear densities

Deformation Mechanisms

Adapted from Fig. 8.6, Callister & Rethwisch 3e.

– FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed)

=> total of 12 slip systems in FCC– For BCC & HCP there are other slip systems.

Chapter 8 - 6

Stress and Dislocation Motion• Resolved shear stress, R

– results from applied tensile stresses

slip plane

normal, ns

Resolved shear stress: R =Fs/As

slip

directi

on

AS

R

R

FS

slip

directi

on

Relation between and R

R =FS /AS

Fcos A/cos

F

FS

nS

AS

A

Applied tensile stress: = F/A

slip

directi

on

FA

F

coscosR

Chapter 8 - 7

• Condition for dislocation motion: CRSS R

• Ease of dislocation motion depends on crystallographic orientation

10-4 GPa to 10-2 GPa

typically

coscosR

Critical Resolved Shear Stress

maximum at = = 45º

R = 0

=90°

R = /2=45°=45°

R = 0

=90°

Chapter 8 - 8

Single Crystal Slip

Adapted from Fig. 8.8, Callister & Rethwisch 3e.

Adapted from Fig. 8.9, Callister & Rethwisch 3e.

Chapter 8 - 9

Ex: Deformation of single crystal

So the applied stress of 6500 psi will not cause the crystal to yield.

MPa 20.7

cos cos

= 35°

= 60°crss = 20.7 MPa

a) Will the single crystal yield? b) If not, what stress is needed?

= 6500 psi

Adapted from Fig. 8.7, Callister & Rethwisch 3e.

(45 MPa)

(45 MPa) (0.41)

18.5 MPa crss

(cos35)

(cos60)

20.7 MPa

Chapter 8 - 10

Ex: Deformation of single crystal

psi 732541.0

psi 3000

coscoscrss

y

What stress is necessary (i.e., what is the yield stress, y)?

)41.0(cos cos psi 3000crss yy

psi 7325 y

So for deformation to occur the applied stress must be greater than or equal to the yield stress

Chapter 8 - 11

• Stronger - grain boundaries pin deformations

• Slip planes & directions (, ) change from one crystal to another.

• R will vary from one crystal to another.

• The crystal with the largest R yields first.

• Other (less favorably oriented) crystals yield later.

Adapted from Fig. 8.10, Callister & Rethwisch 3e.(Fig. 8.10 is courtesy of C. Brady, National Bureau of Standards [now the National Institute of Standards and Technology, Gaithersburg, MD].)

Slip Motion in Polycrystals

300 m

Chapter 8 - 12

• Can be induced by rolling a polycrystalline metal

- before rolling

235 m

- isotropic since grains are approx. spherical & randomly oriented.

- after rolling

- anisotropic since rolling affects grain orientation and shape.

rolling direction

Adapted from Fig. 8.11, Callister & Rethwisch 3e.(Fig. 8.11 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, Structure, p. 140, John Wiley and Sons, New York, 1964.)

Anisotropy in y

Chapter 8 - 13

side view

1. Cylinder of tantalum machined from a rolled plate:

rolli

ng d

irect

ion

2. Fire cylinder at a target.

• The noncircular end view shows anisotropic deformation of rolled material.

endview

3. Deformed cylinder

platethicknessdirection

Photos courtesyof G.T. Gray III,Los AlamosNational Labs. Used withpermission.

Anisotropy in Deformation

Chapter 8 - 14

4 Strategies for Strengthening Metals: 1: Reduce Grain Size

• Grain boundaries are barriers to slip.• Barrier "strength" increases with Increasing angle of misorientation.• Smaller grain size: more barriers to slip.

• Hall-Petch Equation: 21 /yoyield dk

Adapted from Fig. 8.14, Callister & Rethwisch 3e. (Fig. 8.14 is from A Textbook of Materials Technology, by Van Vlack, Pearson Education, Inc., Upper Saddle River, NJ.)

Chapter 8 - 15

• Impurity atoms distort the lattice & generate stress.• Stress can produce a barrier to dislocation motion.

4 Strategies for Strengthening Metals: 2: Solid Solutions

• Smaller substitutional impurity

Impurity generates local stress at A and B that opposes dislocation motion to the right.

A

B

• Larger substitutional impurity

Impurity generates local stress at C and D that opposes dislocation motion to the right.

C

D

Chapter 8 - 16

Stress Concentration at Dislocations

Adapted from Fig. 8.4, Callister & Rethwisch 3e.

Chapter 8 - 17

Strengthening by Alloying

• small impurities tend to concentrate at dislocations• reduce mobility of dislocation increase strength

Adapted from Fig. 8.17, Callister & Rethwisch 3e.

Chapter 8 - 18

Strengthening by Alloying

• large impurities concentrate at dislocations on low density side

Adapted from Fig. 8.18, Callister & Rethwisch 3e.

Chapter 8 - 19

Ex: Solid SolutionStrengthening in Copper

• Tensile strength & yield strength increase with wt% Ni.

• Empirical relation:

• Alloying increases y and TS.

21 /y C~

Adapted from Fig. 8.16 (a) and (b), Callister & Rethwisch 3e.

Ten

sile

str

engt

h (M

Pa)

wt.% Ni, (Concentration C)

200

300

400

0 10 20 30 40 50 Yie

ld s

tren

gth

(MP

a)wt.%Ni, (Concentration C)

60

120

180

0 10 20 30 40 50

Chapter 8 - 20

• Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum).

• Result:S

~y

1

4 Strategies for Strengthening Metals: 3: Precipitation Strengthening

Large shear stress needed to move dislocation toward precipitate and shear it.

Dislocation “advances” but precipitates act as “pinning” sites with

S.spacing

Side View

precipitate

Top View

Slipped part of slip plane

Unslipped part of slip plane

Sspacing

Chapter 8 - 21

• Internal wing structure on Boeing 767

• Aluminum is strengthened with precipitates formed by alloying.

Adapted from chapter-opening photograph, Chapter 11, Callister & Rethwisch 3e. (courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

1.5m

Application: Precipitation Strengthening

Adapted from chapter-opening photograph, Chapter 11, Callister & Rethwisch 3e. (courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

Chapter 8 - 22

4 Strategies for Strengthening Metals: 4: Cold Work (%CW)

• Room temperature deformation.• Common forming operations change the cross sectional area:

Adapted from Fig. 14.2, Callister & Rethwisch 3e.

-Forging

Ao Ad

force

dieblank

force-Drawing

tensile force

AoAddie

die

-Extrusion

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

100 x %o

do

A

AACW

-Rolling

roll

AoAd

roll

Chapter 8 - 23

• Ti alloy after cold working:

• Dislocations entangle with one another during cold work.• Dislocation motion becomes more difficult.

Adapted from Fig. 5.11, Callister & Rethwisch 3e. (Fig. 5.11 is courtesy of M.R. Plichta, Michigan Technological University.)

Dislocations During Cold Work

0.9 m

Chapter 8 - 24

Result of Cold Work

Dislocation density =

– Carefully grown single crystal

ca. 103 mm-2

– Deforming sample increases density

109-1010 mm-2

– Heat treatment reduces density

105-106 mm-2

• Yield stress increases as d increases:

total dislocation lengthunit volume

large hardening

small hardening

y0 y1

Chapter 8 - 25

Effects of Stress at Dislocations

Adapted from Fig. 8.5, Callister & Rethwisch 3e.

Chapter 8 - 26

Impact of Cold Work

Adapted from Fig. 8.20, Callister & Rethwisch 3e.

• Yield strength (y) increases.• Tensile strength (TS) increases.• Ductility (%EL or %AR) decreases.

As cold work is increased

Chapter 8 - 27

• What is the tensile strength & ductility after cold working?

Adapted from Fig. 8.19, Callister & Rethwisch 3e. (Fig. 8.19 is adapted from Metals Handbook: Properties and Selection: Iron and Steels, Vol. 1, 9th ed., B. Bardes (Ed.), American Society for Metals, 1978, p. 226; and Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979, p. 276 and 327.)

%6.35100 x %2

22

o

do

r

rrCW

Cold Work Analysis

% Cold Work

100

300

500

700

Cu

200 40 60

yield strength (MPa)

y = 300MPa

300MPa

% Cold Work

tensile strength (MPa)

200Cu

0

400

600

800

20 40 60

ductility (%EL)

% Cold Work

20

40

60

20 40 6000

Cu

Do =15.2mm

Cold Work

Dd =12.2mm

Copper

340MPa

TS = 340MPa

7%

%EL = 7%

Chapter 8 - 28

• Results for polycrystalline iron:

• y and TS decrease with increasing test temperature.• %EL increases with increasing test temperature.• Why? Vacancies help dislocations move past obstacles.

Adapted from Fig. 7.14, Callister & Rethwisch 3e.

- Behavior vs. Temperature

2. vacancies replace atoms on the disl. half plane

3. disl. glides past obstacle

-200C

-100C

25C

800

600

400

200

0

Strain

Str

ess

(M

Pa)

0 0.1 0.2 0.3 0.4 0.5

1. disl. trapped by obstacle

obstacle

Chapter 8 - 29

• 1 hour treatment at Tanneal... decreases TS and increases %EL.• Effects of cold work are reversed!

• 3 Annealing stages to discuss...

Adapted from Fig. 8.22, Callister & Rethwisch 3e. (Fig. 8.22 is adapted from G. Sachs and K.R. van Horn, Practical Metallurgy, Applied Metallurgy, and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys, American Society for Metals, 1940, p. 139.)

Effect of Heating After %CW te

nsi

le s

tre

ngth

(M

Pa)

duc

tility

(%

EL

)tensile strength

ductility

Recovery

Recrystallization

Grain Growth

600

300

400

500

60

50

40

30

20

annealing temperature (ºC)200100 300 400 500 600 700

Chapter 8 - 30

Annihilation reduces dislocation density.

Recovery

• Scenario 1Results from diffusion

• Scenario 2

4. opposite dislocations

meet and annihilate

Dislocations annihilate and form a perfect atomic plane.

extra half-plane of atoms

extra half-plane of atoms

atoms diffuse to regions of tension

2. grey atoms leave by vacancy diffusion allowing disl. to “climb”

R

1. dislocation blocked; can’t move to the right

Obstacle dislocation

3. “Climbed” disl. can now move on new slip plane

Chapter 8 - 31

• New grains are formed that: -- have a small dislocation density -- are small -- consume cold-worked grains.

Adapted from Fig. 8.21 (a),(b), Callister & Rethwisch 3e. (Fig. 8.21 (a),(b) are courtesy of J.E. Burke, General Electric Company.)

33% coldworkedbrass

New crystalsnucleate after3 sec. at 580C.

0.6 mm 0.6 mm

Recrystallization

Chapter 8 - 32

• All cold-worked grains are consumed.

Adapted from Fig. 8.21 (c),(d), Callister & Rethwisch 3e. (Fig. 8.21 (c),(d) are courtesy of J.E. Burke, General Electric Company.)

After 4seconds

After 8seconds

0.6 mm0.6 mm

Further Recrystallization

Chapter 8 - 33

• At longer times, larger grains consume smaller ones. • Why? Grain boundary area (and therefore energy) is reduced.

After 8 s,580ºC

After 15 min,580ºC

0.6 mm 0.6 mm

Adapted from Fig. 8.21 (d),(e), Callister & Rethwisch 3e. (Fig. 8.21 (d),(e) are courtesy of J.E. Burke, General Electric Company.)

Grain Growth

• Empirical Relation:

Ktdd no

n elapsed time

coefficient dependenton material and T.

grain diam.at time t.

exponent typ. ~ 2

Ostwald Ripening

Chapter 8 - 34

TR

Adapted from Fig. 8.22, Callister & Rethwisch 3e.

º

º

TR = recrystallization temperature

Chapter 8 - 35

Recrystallization Temperature, TR

TR = recrystallization temperature = point of highest rate of property change1. Tm => TR 0.3-0.6 Tm (K)

2. Due to diffusion annealing time TR = f(time) shorter annealing time => higher TR

3. Higher %CW => lower TR – strain hardening

4. Pure metals lower TR due to dislocation movements

• Easier to move in pure metals => lower TR

Chapter 8 - 36

Coldwork Calculations

A cylindrical rod of brass originally 0.40 in (10.2 mm) in diameter is to be cold worked by drawing. The circular cross section will be maintained during deformation. A cold-worked tensile strength in excess of 55,000 psi (380 MPa) and a ductility of at least 15 %EL are desired. Further more, the final diameter must be 0.30 in (7.6 mm). Explain how this may be accomplished.

Chapter 8 - 37

Coldwork Calculations Solution

If we directly draw to the final diameter what happens?

%CW Ao Af

Ao

x 100 1 Af

Ao

x 100

1 Df2 4

Do2 4

x 100 1 0.30

0.40

2

x 100 43.8%

Do = 0.40 in

BrassCold Work

Df = 0.30 in

Chapter 8 - 38

Coldwork Calc Solution: Cont.

• For %CW = 43.8%

540420

y = 420 MPa– TS = 540 MPa > 380 MPa

6

– %EL = 6 < 15• This doesn’t satisfy criteria…… what can we do?

Adapted from Fig. 8.19, Callister & Rethwisch 3e.

Chapter 8 - 39

Coldwork Calc Solution: Cont.

380

12

15

27

For %EL > 15

For TS > 380 MPa > 12 %CW

< 27 %CW

our working range is limited to %CW = 12-27

Adapted from Fig. 8.19, Callister & Rethwisch 3e.

Chapter 8 - 40

Coldwork Calc Soln: RecrystallizationCold draw-anneal-cold draw again• For objective we need a cold work of %CW 12-27

– We’ll use %CW = 20• Diameter after first cold draw (before 2nd cold draw)?

– must be calculated as follows:

100

%1 100 1% 2

02

22

202

22 CW

D

Dx

D

DCW ff

50

02

2

100

%1

.

f CW

D

D

50

202

100%

1.

f

CW

DD

m 335010020

130050

021 ..DD.

f

Intermediate diameter =

Chapter 8 - 41

Coldwork Calculations Solution

Summary:

1. Cold work D01= 0.40 in Df1 = 0.335 m

2. Anneal above D02 = Df1

3. Cold work D02= 0.335 in Df 2 =0.30 m

Therefore, meets all requirements

20100 3350

301%

2

2

x

.

.CW

24%

MPa 400

MPa 340

EL

TSy

%CW1 10.335

0.4

2

x 100 30

Fig 7.19

Chapter 8 - 42

Rate of Recrystallization

• Hot work above TR

• Cold work below TR

• Smaller grains – stronger at low temperature– weaker at high temperature

t/RT

BCt

kT

ERtR

1:note

log

logloglog 0

RT1

log t

start

finish

50%

Chapter 8 - 4343

Mechanical Properties of Polymers – Stress-Strain Behavior

• Fracture strengths of polymers ~ 10% of those for metals

• Deformation strains for polymers > 1000%

– for most metals, deformation strains < 10%

brittle polymer

plasticelastomer

elastic moduli – less than for metals Adapted from Fig. 7.22,

Callister & Rethwisch 3e.

Chapter 8 - 4444

Mechanisms of Deformation—Brittle Crosslinked and Network Polymers

brittle failure

plastic failure

(MPa)

x

x

aligned, crosslinkedpolymer Stress-strain curves adapted from Fig. 7.22,

Callister & Rethwisch 3e.

InitialNear

Failure Initial

network polymer

NearFailure

Chapter 8 - 4545

Mechanisms of Deformation — Semicrystalline (Plastic) Polymers

brittle failure

plastic failure

(MPa)

x

x

crystallineblock segments

separate

fibrillar structure

near failure

crystalline regions align

onset of necking

undeformedstructure amorphous

regionselongate

unload/reload

Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along plastic response curve adapted from Figs. 8.27 & 8.28, Callister & Rethwisch 3e. (Figs. 8.27 & 8.28 are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

Chapter 8 - 4646

Predeformation by Drawing

• Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction• Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL)• Annealing after drawing... -- decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL)• Contrast to effects of cold working in metals!

Adapted from Fig. 8.28, Callister & Rethwisch 3e. (Fig. 8.28 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

Chapter 8 - 4747

• Compare elastic behavior of elastomers with the: -- brittle behavior (of aligned, crosslinked & network polymers), and -- plastic behavior (of semicrystalline polymers)

(as shown on previous slides)

Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along elastomer curve (green) adapted from Fig. 8.30, Callister & Rethwisch 3e. (Fig. 8.30 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)

Mechanisms of Deformation—Elastomers

(MPa)

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straighter,

stillcross-linked

elastomer

deformation is reversible (elastic)!

brittle failure

plastic failurex

x

Chapter 8 - 48

• Dislocations are observed primarily in metals and alloys.• Strength is increased by making dislocation motion difficult.

• Particular ways to increase strength are to: -- decrease grain size -- solid solution strengthening -- precipitate strengthening -- cold work

• Heating (annealing) can reduce dislocation density and increase grain size. This decreases the strength.

Summary