Deformation & Strengthening Mechanisms of Materials

Deformation & Strengthening Mechanisms of

Materials

Chapter 8

Deformation & Strengthening Mechanisms of Materials

Deformation of materials… - is refer to plastic deformation which is

occur due to motion of dislocations. -- consider metals only. -- show a significant plastic deformation before failure/rupture.Strengthening of materials… - restrict the dislocation motion & makes a material harder and stronger. -- 4 strategies/mechanisms to strengthen the materials.

Annealing of materials… - heat treatment for cold-worked metals. -- change mech. properties & microstructure.

Dislocation motion in selected materials…

1. Metals (Cu, Al): - dislocation motion easiest. -- non-directional bonding. -- close-packed directions for slip.

electron cloudion cores

+

+

+

+

+++++++

+ + + + + +

+++++++

2. Covalent Ceramics (Si, diamond): - dislocation motion difficult. -- directional (angular) bonding.

3. Ionic Ceramics (NaCl): - dislocation motion difficult. -- need to avoid nearest neighbors of like sign (- and +).

+ + + +

+++

+ + + +

- - -

----

- - -

slip

dislocation motion

me

cha

nis

ms

sta

ge

s

grain size reduction

solid solution strengthening

precipitation strengthening

strain hardening @ cold working

recovery

recrystallization

grain growth

deformation strengthening annealing


Deformation mechanisms…

Represent a slip plane & slip direction combination.Slip plane - plane on which easiest slippage occurs

-- Highest planar densities (and large interplanar spacings). Slip directions - directions of movement

-- Highest linear densities.

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

example: Dislocation cell structure in lightlydeformed Aluminum

Wall of high dislocation density

Deformation of materials… - plastic deformation which is occur due to dislocation motion. -- consider metals only. -- show a significant plastic deformation before failure/rupture.

(1) slip

- atom slide over each other (slip) due to movement of dislocations. -- exist external shear stress.- Occurs on specific crystallographic planes & within these planes only in certain direction.

Slip system…

Screw dislocation

Edge dislocation


Slip systems…Slip in crystals…- slip occurs in densely or close packed planes.- lower shear stress is required for slip to occur in densely packed planes.- if slip is restricted in close planes, then

less dense planes become operative. - Less energy is required to move atoms along denser planes.

Close packedplane

Non-close-packedplane

- Slip systems are combination of slip planes

& slip direction. - Each crystal has a number of

characteristic slip systems. - i.e: In FCC crystal, slip takes place in

{111} octahedral planes and <110> directions.

-- 4 (111) type planes and 3 [110] type directions. 4 x 3 = 12 slip systems.

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


Strengthening mechanisms…Strengthening of materials

(1) grain size reduction

(2) solid solution strengthening

(3) precipitation strengthening

- grain boundaries are barriers to slip.- barrier "strength" increases with

increasing angle of misorientation.- smaller grain size: more barriers to slip.

- impurity atoms distort the lattice & generate

stress.- stress can produce a barrier to

dislocation motion.

Smaller substitutionalimpurity

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

- hard precipitates are difficult to shear. i.e: ceramics in metals (SiC in Iron @ Aluminum).

Side View

precipitate

Top View

Slipped part of slip plane

Unslipped part of slip plane

S

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

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

S.spacing

- aim: -- restrict the dislocation motion. -- makes a material harder & stronger.


Strengthening mechanisms…Strengthening of materials

(4) strain hardening

- also known as cold work (%CW). -- low temperature deformation.- common forming operations change the cross sectional area. i.e: forging, extrusion, rolling & drawing process.

-Forging

Ao Ad

force

die

blank

force

-Drawing

tensile force

Ao

Addie

die

-Extrusion

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

-Rolling

roll

Ao

Adroll

100 x %o

do

A

AACW

- grain structure at different regions of brass rolled into a wedge. -- grains elongate in rolling direction. -- dislocations get rearranged.

Cold work (rolling process)

- aim: -- restrict the dislocation motion. -- makes a material harder & stronger.

cold-worked grains

- As cold work (%CW) is increased -- Yield strength (y) increases. -- Tensile strength (TS) increases. -- Ductility (%EL or %AR) decreases.

Impact of Cold Work


Materials


Strain hardening @ Cold work analysis

What is the yield strength, tensile strength & ductility of copper rod after cold working?

Do=15.2mm

Cold Work

Dd=12.2mm

Copper

%6.35100 x %2

22

o

do

r

rrCW

Answer:

Brass

Copper

1040 Steel

200

100

300

400

500

600

700

Yie

ld s

tren

gth

(MP

a)

800

10 20 30 40 50 60 700

Yie

ld s

tren

gth

(ksi

)

120

100

80

60

40

20

Percent cold work

Example 1:

900

800

700

600

500

400

300

200 10 20 30 40 50 60 700

Percent cold work

120

100

80

60

40

140

Tens

ile

stre

ngth

(M

Pa)

Tens

ile

stre

ngth

(ks

i)

Brass

Copper

1040 Steel

Brass

Copper

1040 Steel

10 20 30 40 50 60 700

10

0

20

30

40

50

60

70

Percent cold work

Duc

tili

ty (

%E

L)

Yield strength = 310 MPa

Tensile strength = 340 MPa

Ductility = 7%


Materials


Strain hardening @ Cold work analysis

Answer:

Brass

Copper

1040 Steel

200

100

300

400

500

600

700

Yie

ld s

tren

gth

(MP

a)

800

10 20 30 40 50 60 700

Yie

ld s

tren

gth

(ksi

)

120

100

80

60

40

20

Percent cold work

Example 2:

900

800

700

600

500

400

300

200 10 20 30 40 50 60 700

Percent cold work

120

100

80

60

40

140

Tens

ile

stre

ngth

(M

Pa)

Tens

ile

stre

ngth

(ks

i)

Brass

Copper

1040 Steel

Brass

Copper

1040 Steel

10 20 30 40 50 60 700

10

0

20

30

40

50

60

70

Percent cold work

Duc

tili

ty (

%E

L)

A cylindrical specimen of cold-worked copper has a tensile strength of 300 MPa and the cold-worked radius is 7.0 mm.(a) calculate its radius before deformation.(b) estimate its yield strength & ductility (% EL).

1002

22

o

do

r

rrCW%

100

720

2

22

o

o

r

r

(a) %CW = 20(b) Yield strength = 250 MPa

Ductility (% EL) = 17 %

ro = mm


Materials

Stages of annealing process…Annealing of materials

(1) recovery

(2) recrystallization

(3) grain growth

- relieve stored internal strain energy.- enhance atomic diffusion at the elevated temperature.- reduce the number of dislocations.

- new grains are formed at TR that: -- have a small dislocation density. -- are small. -- replace cold-worked grains.

- cold worked metals become brittle.- reheating, which increases ductility results in recovery, recrystallization & grain growth.- this is called annealing & changes material properties. -- reduce tensile strength. -- reduce hardness. -- increase ductility (% EL).

- further recrystallization -- all cold-worked grains are replaced.

After 4 seconds After 8 seconds

0.6 mm0.6 mm

i.e: 33% cold-worked brass is reheat at 580oC

new crystalsnucleate

0.6 mm 0.6 mm

After 3 seconds

- at longer times, larger grains consume smaller ones. - why? Grain boundary area & energy is reduced.

After 8 s After 15 min

0.6 mm 0.6 mm

º

º

If metal is held at recrystallization temperature, TR long enough, cold worked structure is completely replaced with recrystallized grains.

TR = recrystallization temperature

- point of highest rate of property change.

• 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

End of Chapter

8

Deformation & Strengthening Mechanisms of Materials

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