L7 Strengthening 131

7/23/2019 L7 Strengthening 131

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TO MAKE STRONGER…

1

Sub-topics

The modulus – strength chrtMnipulting strength

!rdening – strengthening

Grin boundr" design

#old $or%ing



Many engineering materials can be strengthened

through various hardening mechanisms – however,

an increase in strength almost always results

in a decrease in ductility

STRENGTHENED MTER!"S

&



H#$ T# STRENGTHEN MTER!"S%

o

Solid&solution strengthening'oStrain (wor)* hardening'

oGrain si+e reduction

'



GR!N #-NDR!ES

.olume de/ect &0 disordered solid

!nternal sur/ace &0 higher energy regions

Grain is a virtual single crystal

1olycrystalline materials consist

o/ perfect crystals and boundaries

G – tra2 /or im2urities

arrest dislocations

G

engineering

The way to ma)e materials

stronger is to ma)e it

hrder (or disloctions

mo)ements

*



GR!N #-NDR3 HRDEN!NG

Grain si+e D is ty2ically 45&455 6m

Dislocations cannot sim2ly slide /rom

one grain to the ne7t because the

sli2 2lanes do not line u2

E//ect o/ grain boundaries on the

shear strength re8uired /or a

dislocation to move

+



!S GR!N #-NDR3 RR!ER 9#R

D!S"#:T!#N M#T!#N%

The grain boundary acts as a barrier to dislocation motion /or two

reasons;

4< Since the two grains are o/ di//erent orientations, a dislocation2assing into grain will have to change its direction o/ motion'

this becomes more di//icult as the crystallogra2hic misorientation

increases<

=< The atomic disorder within a grain boundary region will result in a

discontinuity o/ sli2 2lanes /rom one grain into the other<

Note; /or high-ngle grain

boundaries, it may not be the

case that dislocations traverse

grain boundaries during

de/ormation'

rather, a stress concentration

ahead o/ a sli2 2lane in one

grain may cti)te sources

o/ new dislocations in anad>acent grain<

,



D!S"#:T!#NS ND GR!N

#-NDR!ES

Dislocations are 2ositioned

closer together anddislocations movement in

the net is hindered by

interaction between them<

Together with the reducedelastic strain energy, this

/act results in dislocations

that are relatively immobileand the imposed stress

necessr" to de(orm

mteril increses $ith

decrese in grin sie<

.o$-ngle

grin

boundries

!igh-nglegrin

boundries

/



H ""&1ET:H E?-T!#N

The relation between yield stress and grain si+e is

described mathematically by

the !ll-0etch eution

where k y is the strengthening

coefficient (a constant uniqueto each material), σ o is amaterials constant for the

starting stress for dislocationmovement (or the resistanceof the lattice to dislocation

motion), d is the graindiameter, and σ y is the yield

stress.

$hy is a /ine&grained material is harder and stronger than coarse grained%

2

Material @o AM1aB k AM1a m4C=B

:o22er = 5<44

Titanium 5 5<F5

Mild steel 5 5<FNi l 55 4<5

Hall&1etch constants



H ""

&1ET:H STRENGTHEN!NG "!M!T

Strengthening islimited by the

sie o( disloctions3

#nce the grain si+e

reaches about 45 nm,grain boundaries start

to slide<

D I 45 nm

:an we mani2ulate

with grain si+e%

Grain si+e may be regulated by

the rate o/ solidi/ication /rom

the li8uid 2hase, and also by

2lastic de/ormation /ollowed byan a22ro2riate heat treatment

4



GR!N #-NDR3 ENG!NEER!NG

Grin boundr" strengthening

(or !ll-0etch strengthening* is

a method o/ strengtheningmaterials by changing their

average grain si+e<

!t is based on the observation thatgrain boundaries impede

dislocation movement and

that the number o/ dislocationswithin a grain have an e//ect on

how easily dislocations can

traverse grain boundaries and

travel /rom grain to grain<

The in/luence o/

grain si+e on the yield strength

o/ a 5 :u–5 Jn brass alloy<

15



GR!N #-NDR3 STRENGTHEN!NG

By changing grain size one can influence dislocationmovement and yield strength.

This is a schematic roughly illustrating the

conce2t o/ dislocation 2ile u2 and how it

e//ects the strength o/ the material<

material with larger grain si+e is able tohave more dislocation to 2ile u2 leading to

a bigger driving /orce /or dislocations to

move /rom one grain to another<

Thus you will have to a22ly less /orce tomove a dislocation /rom a larger than /rom

a smaller grain, leading materials with

smaller grains to e7hibit higher yield

stress<

htt2;CCen<wi)i2edia<orgCwi)iCHall&1etch

11



M-"T!1"!:T!#N #9 D!S"#:T!#NS

9ran) K Read 2ro2osed that dislocations

could be generated /rom e7isting

dislocations

The dislocation line bulges out( and are anchored by im2urities*

and 2roduces sli2 as the shear

stress L is a22lied<

The ma7imum L /or

semicircle dislocation

bulge

eyond this 2oint, the dislocation loo2

continues to e72and till 2arts m and n

meet and annihilate each other to /orm a

large loo2 and a new dislocation<

Note; Re2eating o/ this 2rocess 2roducing a dislocation

loo2, which 2roduces sli2 o/ one urgers vector alongthe sli2 2lane

1&



T$o (ctors determine the in/luence o/ obstacles

on dislocation movement;

4* S2acing

=* Strength

"; distance between obstacle and the sli2 2laneN"; number o/ obstacles touching unit length o/ dislocation line

2; 2inning /orce e7erted by obstacle on dislocation line

α; dimensionless constant characteri+ing the strength o/ obstacle

The shear stress needed to /orce

the dislocation through a /ield

o/ obstacles



$#R HRDEN!NG & S!:S

The strain hardening

2henomenon is e72lained on

the basis o/6isloction–disloction

strin (ield interctions

ccumulation o/ dislocations

generated by 2lastic de/ormation

6isloction densit";"ength o/ dislocation lines C

unit volume (mCm*

Dislocations start moving when

the /orce τb 2er unit lengthe7ceed the lattice resistance f

τ b ≥ f

"ine tension; T ≈ ½ Eb2

s the dislocation density increases, this resistance to dislocation motion by

other dislocations becomes more 2ronounced< Thus, the imposed stressnecessr" to de(orm metl increses $ith incresing cold $or%3

1*



$HT T# D# !9 STR#NG MTER!" !S

NEEDED%

ecause macrosco2ic 2lastic de/ormationcorres2onds to the motion o/ large numbers o/dislocations, the ability of a metal to plasticallydeform depends on the ability of dislocations to move.

Since hardness and strength (both yield and tensile*

are related to the ease with which 2lasticde/ormation can be made to occur, by reducing themobility o/ dislocations, the mechanical strengthmay be enhanced' that is, greater mechanical /orces

will be re8uired to initiate 2lastic de/ormation< The more unconstrained the dislocation motion, the

greater is the /acility with which a metal mayde/orm, and the so/ter and wea)er it becomes< 1+



$#R HRDEN!NG – D!S"#:T!#NS !NTER:T!#N

moving dislocation /inds that its sli2 2lane is 2enetrated by a /orest o/

intersecting dislocations<

!/ a moving dislocation advances, it shears the

material above the sli2 2lane relative to that

below, and that creates a little ste2 – a >og – ineach /orest dislocation<

0inning (orce

on a moving

dislocation

τ ≈ ½ Eb√ρ

1,

1inning /orce e7erted

on dislocations by >ogs



STRENGTHEN!NG 3 GR!N S!JE RED-:T!#N

4< Materials are sub>ected to the im2osition o/ )er"

lrge strins without the introduction o/ concomitant changes in cross&sectional

dimensions o/ the sam2les<

=< Materials 2roduced by S1D techni8ues have

grin sies in the range o/ 5–4555 nm<

Now there are several S1D 2rocessing available;

equal-channel angular pressing (ECAP)

high-pressure torsion

accumulative roll-bonding

repetitive corrugationand friction stir processing.

1/

Se)ere plstic de(ormtion



SE.ERE 1"ST!: DE9#RMT!#N; E:1

Sketch of an E!" tool andmaterials deformation

!chematic model of dislocation structure evolution at different stages during

severe plastic deformation (ada2ted /rom R<J< .aliev, R<< !slamgaliev, !< le7androv<

"ulk nanostructured materials from severe plastic deformation Progress in #at. !ci.

=555, v< F, 45–4*

12



14



SE.ERE 1"ST!: DE9#RMT!#N; T#RS!#N

During torsion straining at room tem2erature,high 2ressure can 2rovide a rather high density

that may be close to 455O in the 2rocessed dis)

sam2le<

During torsion straining at room tem2erature,high 2ressure can 2rovide a rather high density

that may be close to 455O in the 2rocessed dis)

sam2le<

&5



9#RM!NG #1ERT!#NS

#old $or%ing 2roduces n increse in strength with the

attendant decrese in ductilit", since the metal strain hardens<&1



9#RMED 1RTS !N T31!:" -T#M#!"E

&& 7or%bilit" (and (ormbilit"* shows ma7 amount o/ de/ormation

a material can withstand without /racture in /orming 2rocess<



9#RG!NG

8orging denotes a /amily o/ metalwor)ing

2rocesses in which de/ormation o/ wor)2iece is

carried out by com2ressive /orces a22lied

through a set dies<

9orging 2rocess can be carried out at room and elevated T<

&'



R#""!NG

Rolling, the most widely used de/ormation 2rocess, consists o/

2assing a 2iece o/ metal between two rolls' a reduction in thic)ness

results /rom com2ressive stresses e7erted by the rolls<

#old rolling may be used in the 2roduction o/ sheet, stri2, and /oil with

high 8uality sur/ace /inish<

:ircular sha2es as well as !&beams and railroad rails are /abricated using

grooved rolls<

&*

htt2;CCwww<youtube<comCwatch%vI)Pi#DHlaP83K/eatureIrelated



R#""!NG M!"" :#N9!G-RT!#NS

9rom $ndustrial #aterials $ Colling et al.

Two&high mill

9our&high mill:luster mill

&+

0roblem3 sheet o/ alloy is cold&rolled =5 O to a thic)ness o/

<55 mm< The sheet is then /urther cold rolled to =<55 mm<

$hat is the total O cold wor)%



DR$!NG

reduction in cross section

resultsin a corres2onding

increse in length<

6r$ing is the 2ulling o/ a metal 2iece through a die having a

ta2ered bore by means o/ a tensile /orce that is a22lied on the

e7it side<

&,

0roblem; :alculate the 2ercent cold reduction when annealed co22er

wire is cold drawn /rom a diameter o/ 4<= mm to a diameter o/ 5<4 mm<

The total drawing o2eration may consist o/ a number o/ dies in a series

se8uence< Rod, wire, and tubing 2roducts are commonly /abricated in

this way<



THE $!RE&DR$!NG 1R#:ESS<

&/

0roblem3 Design a 2rocess to2roduce 5< cm diameter co22er

wire<

The starting diameter o/ the

co22er wires available in the stoc)is 4 cm' 5<= cm and 5< cm<

Strin hrdening is the

2henomenon whereby a ductile metal

becomes harder and stronger as it is

2lastically de/ormed<



$#R HRDEN!NG ND 1R#1ERT!ES

The increase in

yield strength

The increase in

tensile strength

The decrease in

ductility

n is called the strin hrdening e9ponent,

which is a measure o/ the bilit" o/ a metal to

strin hrden' the larger its magnitude, the

greater the strain hardening /or a given amount o/

2lastic strain<

&2



D.NTGES ND "!M!TT!#NS T#

STRENGTHEN!NG MET""!: MTER!" 3 :#"D

$#R!NG

$e can simultaneously strengthen the metallic material and2roduce the desired /inal sha2e<

$e can obtain e7cellent dimensionl tolernces nd sur(ce(inishes by the cold wor)ing 2rocess<

The cold&wor)ing 2rocess is an ine9pensi)e method /or 2roducinglarge numbers o/ small 2arts, since high /orces and e72ensive/orming e8ui2ment are not needed<

lso, no llo"ing elements are needed, which means lower&costraw materials can be used<

(&* Some metals, such as H:1 magnesium, have a limited numbero/ sli2 systems and are rather brittle at room tem2erature' thus,only a small degree o/ cold wor)ing can be accom2lished<

(&* Ductility, electrical conductivity, and corrosion resistance areim2aired by cold wor)ing<

(&* Since the e//ect o/ cold wor)ing is decreased or eliminated athigher tem2eratures, we cannot use cold wor)ing as astrengthening mechanism /or com2onents that will be sub>ected tohigh tem2eratures during a22lication or service<

&4



EQTR-S!#N 1R#:ESSES

(a*direct

e7trusion,

(b*indirect

e7trusion,

(c* hydrostatic

e7trusion,

(d* 2ierce and

e7trude

(9rom $ndustrial #aterials $ Colling et al.).

'5

2rocess o/ s8uee+ing material through ano2ening to 2roduce a long length with

a uni/orm cross&section



MET" EQTR-S!#N

'1

$or)ing metal is considerably

more com2le7 than s8uee+ing

tooth2aste, but the 2rinci2les

are the same<

billet o/ material is 2laced in a cavity with a die at one end<

The die has an o2ening cut into it in the sha2e o/ the 2ro/ile thats to bee7truded<

!/ the ob>ective is to e7trude a length o/ - sha2ed channel, then the die

will have an o2ening in the sha2e o/ the -<

t the o22osite end o/ the cavity a ram s8uee+es the metal, 2ushing it

through the die<



SHEET MET"$#R!NG

shearing

drawing

'&



$#R HRDEN!NG

(:#"D $#R!NG

* &S-MMR3

#old 7or%ing & de/orming o/ a

metal at low tem2eratures and

strengthening by dislocation/ormation<

The strengthening o/ a

metal during de/ormationis a result o/ the increase

in dislocations density<

Dislocations /ormed during cold wor)ing

strengthen a metal by storing some o(

the energ" a22lied, in the /orm o/

residul stress<

''



:#"D $#R!NG ND NNE"!NG

'*

Since cold wor)ing or strain hardening results /rom

increased dislocation density we can assume that any

treatment to rerrnge or annihilate dislocations would

begin to undo the e//ects o/ cold wor)ing<

Anneling is a heat treatment

used to eliminate some or all o/ the

e//ects o/ cold wor)ing<

/ter annealing, dditionl cold $or%

could be done, since the ductility is restored'by combining re2eated cycles o/ cold

wor)ing and annealing, lrge totl de(ormtions

may be achieved<



M !NG STR#NGER

As the percent of Cold

Working increases:

'+

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L7 Strengthening 131

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