7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 1/35
TO MAKE STRONGER…
1
Sub-topics
The modulus – strength chrtMnipulting strength
!rdening – strengthening
Grin boundr" design
#old $or%ing
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 2/35
Many engineering materials can be strengthened
through various hardening mechanisms – however,
an increase in strength almost always results
in a decrease in ductility
STRENGTHENED MTER!"S
&
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 3/35
H#$ T# STRENGTHEN MTER!"S%
o
Solid&solution strengthening'oStrain (wor)* hardening'
oGrain si+e reduction
'
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 4/35
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
*
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 5/35
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
+
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 6/35
!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<
,
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 7/35
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
/
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 8/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 9/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 10/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 11/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 12/35
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&
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 13/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 14/35
$#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*
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 15/35
$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+
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 16/35
$#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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 17/35
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§ional
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 18/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 19/35
14
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 20/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 21/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 22/35
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<
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 23/35
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<
&'
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 24/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 25/35
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<
$hat is the total O cold wor)%
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 26/35
DR$!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<
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 27/35
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<
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 28/35
$#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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 29/35
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
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 30/35
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§ion
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 31/35
MET" EQTR-S!#N
'1
$or)ing metal is considerably
more com2le7 than s8uee+ing
tooth2aste, but the 2rinci2les
are the same<
billet o/ material is 2laced in a cavity with a die at one end<
The die has an o2ening cut into it in the sha2e o/ the 2ro/ile thats to bee7truded<
!/ the ob>ective is to e7trude a length o/ - sha2ed channel, then the die
will have an o2ening in the sha2e o/ the -<
t the o22osite end o/ the cavity a ram s8uee+es the metal, 2ushing it
through the die<
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 32/35
SHEET MET"$#R!NG
shearing
drawing
'&
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 33/35
$#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<
''
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 34/35
:#"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<
7/23/2019 L7 Strengthening 131
http://slidepdf.com/reader/full/l7-strengthening-131 35/35
M !NG STR#NGER
As the percent of Cold
Working increases:
'+