jnanor.21.29age hardening behavior of carbon nanotube reinforced aluminum nanocomposites

8/13/2019 JNanoR.21.29Age Hardening Behavior of Carbon Nanotube Reinforced Aluminum Nanocomposites

http://slidepdf.com/reader/full/jnanor2129age-hardening-behavior-of-carbon-nanotube-reinforced-aluminum-nanocomposites 1/7

Age Hardening Behavior of Carbon Nanotube Reinforced AluminumNanocomposites

N. Saheb1,2,a, A. Khalil1,b, A.S. Hakeem2,c, T. Laoui1,2,d, N. Al-Aqeeli1,2,e, A.M. Al-Qutub1,f

1Dea!tme"t of Mecha"ical #"$i"ee!i"$

2%e"te! of #&celle"ce i" Na"otech"olo$'

Ki"$ (ahd )"i*e!+it' of et!oleum a"d Mi"e!al+, Dhah!a"-121, Saudi A!abia

a"oua!i/kfum.edu.+a, babdulkhaa/kfum.edu.+a 0co!!e+o"di"$ autho!,ca+hakeem/kfum.edu.+a, dtlaoui/kfum.edu.+a, e"aqeeli/kfum.edu.+a,

f aqutub/kfum.edu.+a

Keywords: Alumi"um "a"ocomo+ite, %a!bo" "a"otube+, A$e ha!de"i"$, Diffu+io"

Abstract. In the present work, age hardening behavior of CNT reinforced Al6061 and Al2124

nanocoposites, prepared b! ball illing and spark plasa sintering, was investigated" The effect

of CNT content, annealing tie and teperat#re on the age hardening behavior of the

nanocoposites was eval#ated and copared to the onolithic allo!s prepared and age hardened

#nder the sae conditions" It was fo#nd that CNTs have a negative infl#ence on the age hardening

of the allo!s" The allo!s displa!ed standard age hardening behavior i"e" a sharp increase in hardness

d#ring initial aging followed b! a stead! decrease in hardness" $hereas the nanocoposites did not

onl! displa! initial softening d#ring aging b#t also showed red#ced age hardening efficienc!" The

hardening efficienc! was fo#nd to decrease with increasing CNT content" The coplicated behaviorof nanocoposites was e%plained in ters of dislocation recover!, large theral isatch between

atri% and CNTs and b#lk icrostr#ct#re of the coposites"

Introduction

Age hardenable al#in# allo!s find iportant applications in a#toobile and aerospace

ind#stries" Age hardening &also called 'precipitation hardening( of soe al#in# allo!s belonging

to )***, 6*** and 2*** series, res#lts in the precipitation of etastable phases &depending

#pon allo! coposition( which offer resistance to dislocation oveent and hence strengthen the

allo!" The details of icrostr#ct#ral changes and strengthening echaniss which occ#r in these

allo!s d#ring age hardening are well doc#ented +1-."Al#in# etal atri% coposites &Al//Cs( have been proved as a proising sol#tion for

vario#s engineering applications where the light weight of al#in# allo!s is re#ired with

increased strength" The reinforceent phase which a! be in the for of particles, whiskers or

fibers and is #s#all! ceraic in nat#re, act as a barrier to dislocation otion and hence iparts

s#perior echanical properties to the atri%" t#dies have been e%tended to e%aine the age

hardening behavior of Al//Cs to take the d#al advantage of the e%ternal reinforceent phase as

well as the internal phases which a! be developed b! an appropriate heat treatent" atnaparkhi

et al" +4. showed that hardness of Al2124 allo! reinforced with 3 ol"5 iC whiskers can be

increased b! 10 7 thro#gh age hardening" 8an et al" +3. have deonstrated that significant

iproveent in wear resistance of Al2124 allo!, containing iC particles, is possible in the over

aged condition" Appendino et al" +6. reported an increase in hardness fro 900 7 to ore than1400 7 in Al6061 allo! containing iC particles" Increase in hardness b! alost 100 for

Al2024 allo! containing 20 ol"5 iC particles thro#gh age hardening was shown b! Abargho#ie

et al" +)." alvo et al" +:10. reported siilar res#lts for Al6061 allo! reinforced with iC and

Journal of Nano Research Vol. 21 (2013) pp 29-35© (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/JNanoR.21.29

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 212.26.4.107-13/12/12,16:56:19)

http://www.ttp.net/

http://www.ttp.net/



al#ina particles" ong et al" +11. also showed that wear resistance of Al6061 and Al2014 allo!s

containing iC particles can be considerabl! enhanced thro#gh age hardening" It has also been

reported +12,1-. that the age hardening efficienc! of h!brid coposite of Al allo! containing iC in

both partic#late as well as whisker for is #ch higher as copared to a onolithic allo! and the

allo! containing a single t!pe of reinforceent" Age hardening of Al allo!s is also accelerated b!

the presence of ceraic fibers as reported elsewhere +14." The concept behind accelerated agehardening of Al//Cs is the increased dislocation densit! developed d#ring sol#tion treatent d#e

to isatch between the coefficient of theral e%pansion &CT;( of the soft atri% and the hard

reinforceent phase" This increased dislocation densit! prootes the n#cleation of strengthening

precipitates and th#s accelerates the aging of the coposite +13,16."

Carbon nanot#bes &CNTs( have been one of the candidate aterials to ipart high strengthlow

weight characteristics to al#in# allo!s" /an! papers have been p#blished on the effect of CNTs

on vario#s ph!sical and echanical properties of al#in# allo!s" owever, the literat#re

regarding the age hardening behavior of AlCNT nanocoposites is scarce and onl! recentl! the

age hardening behavior of Al2024 allo! containing CNTs was investigated b! Choi et al" +1). who

fo#nd that age hardening efficienc! is increased with increasing CNT content" It was proposed that

CNTs act as a pathwa! for diff#sion of copper atos which sti#lates the foration of precipitatesnearb! to CNTs" It was also proposed that the high dislocation densit! at the tips of CNTs a!

sti#late the precipitation kinetics" ence, these nanoscale precipitates a! for coherent or sei

coherent interface str#ct#res with the atri%, res#lting in significant increase of hardness"

The ai of this work is to investigate the age hardening behavior of CNT reinforced Al6061 and

Al2124 nanocoposites prepared tho#gh ball illing and spark plasa sintering"

Materials and Methods

The Al6061 and Al2124 preallo!ed powders #sed in this st#d! were s#pplied b! the Al#ini#

8owder Co" <td", =>" CNTs having an average diaeter of 24 n and a length of few icrons

were #sed as reinforceent" Al2124 and Al6061 nanocoposite powders containing 0"3 and 1 wt"5 CNTs were prepared b! sonication followed b! wet ball illing" /ore details on the cheical

coposition of the powders and preparation of coposites can be fo#nd elsewhere +1:." A f#ll!

a#toated &?CT s!ste, @eran!( spark plasa sintering e#ipent odel 8 3 was #sed to

sinter the saples +19." canning electron icroscope &;/( was #sed to characteriBe the

icrostr#ct#re of sintered speciens" ?or st#d!ing the age hardening behavior, sintered saples

were sol#tion heat treated at 320 0C for 10 ho#rs in a f#rnace #nder an inert environent and

iediatel! water #enched at roo teperat#re followed b! annealing at 1600C and 2000C for ,

1, -, 6 and 10 ho#rs" A 7#ehler //T- digital icrohardness tester was #sed for eas#ring the

hardness of the speciens with an applied load of 100 gf and indenter dwell tie of 12 sec" The

eas#reents were taken at 10 different locations on the specien and the arithetic ean of these

10 readings was reported with a standard deviation"

Results and Discussion

?ig#re 1 shows ;/ icrographs of sintered speciens" 8orosit! is evident in the coposites,

however, the onolithic allo!s do not reveal an! porosit!, this is in agreeent with the densit!

res#lts reported elsewhere +1:, 19." ?ig#re 2 &a( and &b( shows the age hardening behavior of

Al6061 onolithic allo! and Al6061DCNT coposites at an annealing teperat#re of 1600C and

2000C, respectivel!" At 1600C, the onolithic allo! reached its peak hardness after annealing for 1

ho#r whereas peak hardness was achieved within ho#r at 2000C" This is e%pected beca#se a

higher teperat#re will accelerate the diff#sion of atos res#lting in earlier foration of coherent

precipitates which are ainl! responsible for strengthening the aterial" ?or the coposites, it can be seen that CNTs do not onl! retard the age hardening rate b#t also red#ce the age hardening

efficienc! of the allo!" At lower annealing teperat#re, the coposites reached their peak hardness

in 6 ho#rs whereas at higher annealing teperat#re, peak hardness was achieved after - ho#rs"

30 Journal of Nano Research Vol. 21



/oreover, it can be seen that for all aging ties, the hardness reained lower for the coposites as

copared to onolithic allo! and the coposite containing 1 wt"5 CNTs displa!ed a lower

hardness as copared to the coposite containing 0"3 wt"5 CNTs" A siilar trend was displa!ed

b! onolithic Al2124 and Al2124DCNT coposites as evident fro ?ig#re -"

Fig 1 ;/ icrographs of 8Eed Al6061 containing &a( 0 &b( 0"3 &c( 1 wt"5 CNTs and 8Eed

Al2124 containing &d( 0 &e( 0"3 &f( 1 wt"5 CNTs

An iportant observation in case of coposites is the decrease in hardness d#ring initial aging

stage" As can be seen in ?ig#res 2 and -, the coposites displa!ed decreased hardness d#ring the

first ho#r of aging" This shows that soe softening echanis occ#rred in coposites d#ring initial

aging" An e%actl! siilar trend has been reported b! <in et al" +1-. who st#died age hardening of

Al2024 allo! reinforced with iC nanoparticles" The average siBe of reinforceent particles was -3

n" This decreasing trend of hardness d#ring initial stage was attrib#ted to the dislocation recover!,

which is ore predoinant in the initial stage #ntil which the F phase has not fored" As the aging

tie increases, F phase fors at a higher rate beca#se of the presence of a high dislocation densit!,

which are considered favorable sites for F phase n#clei foration, which is responsible for peak

hardening of the allo! +20." Altho#gh the F phase foration is ore specificall! related to C# based Al allo!s, the CNT containing Al6061 allo! displa!ed a siilar trend" The aging kinetics in

the presence of nanosiBed reinforceents is th#s cople% and ore research is needed to f#ll!

concept#aliBe this aspect" Another iportant finding is that the presence of CNTs res#lts in red#ced

age hardening efficienc! as well as retardation of the age hardening rate in Al allo!s" It has been

proposed +21,22. that the densit! of #enchedin vacancies is saller in the coposite owing to the

greater densit! of sinks &dislocations, atri%reinforceents interfaces( and since vacancies are

needed to n#cleate @8 Bones, the precipitation kinetics in the coposite is slowed down" It is well

known that Al allo!s containing /g and i are sensitive to age hardening owing to the possible

foration of /g2i precipitates" The coplete ageing se#ence in the allo! s!ste isG

s#persat#rated solid sol#tion H @8 Bones or partiall! coherent J needles H partiall! coherent F rods

H incoherent &/g2i( discs" The last two stages correspond to over aged conditions +9." In the presence of CNTs, the foration of @8 Bones or J phase a! be s#ppressed as CNTs a! act as

an ip#rit!, inhibiting the reaction between /g and i" In a siilar wa!, the foration of F phase

a! be inhibited in Al2124DCNT coposites" The coposites displa!ed a%i# age

a

d

cb

ef

1 !m

1 m

1 m

1 m

1 m

1 m

Journal of Nano Research Vol. 21 31



hardenabilit! for the aging ties of - and 6 ho#rs at the annealing teperat#res of 200 and 160 0C,

respectivel!" This increase in hardness is d#e to the contrib#tion of phases precipitated inside the

atri% onl! and apparentl! there is no contrib#tion of CNTs" The decrease in hardness of

coposites after achieving a%i# hardness a! be attrib#ted to the coarsening of precipitates,

especiall! near the atri%CNT interfaces where the dislocation densit! is high" This will decrease

the coherence between the atri% and the precipitates res#lting in red#ced hardness +1)."It has been shown that CNTs displa! a negative coefficient of theral e%pansion &CT;( in the

teperat#re range of 30 to 600 > &22- to -2)0C( +2-." Also, the val#e of CT; for CNTs in this

teperat#re range was fo#nd to be of the order of 10 % 106 > 1 as copared to al#in# for

which CT; is D22 % 106 > 1" Altho#gh, the sol#tion treatent teperat#re #sed in the present

work i"e" 3200C is well be!ond this range, the annealing teperat#res i"e" 160 and 2000C lie within

this range" #ch large difference in CT; val#es co#ld be another iportant reason for the observed

trend" $hile annealing, the atri% particles #nderwent e%pansion whereas CNTs e%perienced

contraction" This will not onl! res#lt in decreased adhesion between the atri% and CNTs b#t will

also create icroscopic gaps between CNT cl#sters at a few locations and the atri% particles" The

#ltiate res#lt will be the degraded hardness of the coposite"

The res#lts of this work are contrar! to that obtained b! Choi et al" +1). where CNTs were fo#ndto have a positive ipact on the age hardening efficienc! of Al2024 allo!" Choi et al" #sed

nanocr!stalline atri% powder whose grain siBe was red#ced b! ball illing the powder for 24

ho#rs" It is known that the n#cleation and growth of precipitates a! progress #ch faster when

grain siBe is red#ced beca#se the fine str#ct#res ca#se enhanceent of aging kinetics +24." At the

sae tie, the grain bo#ndar! diff#sion will be higher in the case of saller grains d#e to an

increased densit! of grain bo#ndaries" Also, the nanocr!stalline atri% will posses a greater degree

of coherence with the CNTs d#e to coparative siBe" $hereas in the present work, as received

icron siBed atri% powders were #sed having a lesser degree of coherence with the CNTs d#e to

large difference of siBe" These aspects co#ld be the possible reasons for contrasting res#lts"

Altho#gh it has been proposed that addition of foreign particles iproves the age hardening rate

and efficienc! of the aterial d#e to increased dislocation densit! and conse#entl! increased

precipitation kinetics +414., this siplified stateent ight not be applicable to the aterials

containing nanoparticles s#ch as CNTs" It is a well established fact that aggloeration of CNTs

cannot be totall! avoided d#ring processing d#e to the high s#rface energ! of CNTs +23-1." This

aggloeration hinders proper densification of the atri% and hence the pores are developed inside

the aterial, as alread! shown in ?ig#re 1" These pores can be considered as acroscopic defects

inside the aterial which degrade the echanical properties of the aterial" These defects cannot

be copared with the point vacancies and atoic level dislocations which are considered as

precipitate accelerators d#ring age hardening" The pores or acroscopic defects, developed

priaril! d#e to CNT aggloeration, wo#ld rather decelerate the aging kinetics d#e to incoplete

diff#sion of atos which is andator! for aterial strengthening" Also, it is known that thestrengthening of aterial in the presence of second phase is governed b! the relationG

where τ is the !ield stress, K is the interparticle spacing, G is the shear energ! of dislocation and b

is the dislocationEs b#rger vector" igher porosit! in atri% eans an increased val#e of K, which

eans red#ced strength of the aterial" ence increased porosit! d#e to CNT aggloeration can

slow down the aging kinetics"

The res#lts of this work indicate that age hardening of Al nanocoposites is cople% and cannot

be e%plained p#rel! in the conte%t of conventional age hardening theories" Altho#gh process odels

have been developed +-2,--. which describe the aging phenoena in Al allo!s to a reasonablee%tent, the aging process in coposites and nanocoposites is far ore cople% and f#rther

research is needed to #nderstand and odel the age hardening response of Al etal atri%

coposites and especiall! the nanocoposites"




Fig 2 ardness of Al6061 and Al6061DCNT coposites as a f#nction of aging tie at an annealingteperat#re of &a( 1600C and &b( 2000C

Fig 3 ardness of Al2124 and Al2124DCNT coposites as a f#nction of aging tie at an annealing

teperat#re of &a( 1600C and &b( 2000C

Conclusion

The present e%perientation reveals that carbon nanot#bes &CNTs( retard the age hardening kinetics

in Al6061 and Al2124 al#in# allo!s" The CNT reinforced nanocoposites showed softening

d#ring initial aging which was attrib#ted to dislocation recover! before the foration of coherent

precipitates which are responsible for strengthening the aterial" ?or higher aging d#rations, the

nanocoposites displa!ed age hardenabilit! d#e to the sole contrib#tion of the atri%" The hardness

of nanocoposites reained lower than that of the onolithic allo!s for all aging ties d#e to the

wide difference between the theral properties of CNTs and atri%, scarcit! of #enchedin

vacancies, and an increase in the interparticle spacing ca#sed b! CNT aggloeration"

Acknowledgement

The a#thors wo#ld like to acknowledge the financial s#pport for this work fro >ing Abd#l ABiB

Cit! for cience and Technolog! &>ACT( thro#gh research proLect n#ber A82:122"




References

+1. "A" 8orter and >";" ;asterling, 8hase Transforations in /etals and Allo!s, CC 8ress, 1992"

+2. M"$" Christian, The Theor! of Transforations in /etals and Allo!s, 8ergaon, 2002"

+-. I"M" 8olear, <ight Allo!s, ;lsevier <td" 2003"

+4. 8"<" atnaparkhi and "M" ack, Aging effects on the fract#re to#ghness of iC whisker

reinforced 2*** al#in# allo!s, cripta /etall" 2- &19:9( 214-2146"+3. "/" 8an, /";" ?ine and "" Cheng, Aging effects on the wear behavior of 8O/ al#in#

allo! iC particle coposites, cripta /etall" /ater" 24 &1990( 1-411-43"

+6. 8" Appendino, C" 7adini, ?" /arino and A" Toasi, 6061 Al#in# allo!iC partic#late

coposite G a coparison between aging behavio#r in T4 and T6 treatents, /at" ci" ;ng" A" 1-3

&1991( 2)32)9"

+). "/""/" Abargho#ie and "/"" eihani, Aging behavior of a 2024 Al allo!iCp coposite,

/ater" esign" -1 &2010( 2-6:2-)4"

+:. <" alvo and /" #er!, ;ffect of reinforceent on age hardening of cast 6061 AliC and 6061

AlA120- partic#late coposites, /at" ci" ;ng" A" 1)) &1994( 192:"

+9. <" alvo, @"<" ;sperance, /" #er! and M"@" <ego#%, Interfacial reactions and age hardening in

A1/gi etal atri% coposites reinforced with iC particles, /at" ci" ;ng" A" 1)) &1994(

1)-1:-"

+10. <" alvo, /" #er!, " Towle and C"/" ?riend, Agehardening behavio#r of li#idprocessed

6061 allo! reinforced with partic#lates and short fibres, Copos" 8art A" 2) &1996( 12011210"

+11. $"P" ong, 8" >ra#klis, A"8" /o#ritB and " 7and!opadh!a!, The effect of theral ageing on

the abrasive wear behavio#r of agehardening 2014 AlOiC and 6061 AlOiC coposites, $ear" 1:3

&1993( 1231-0"

+12. 7"ch#l >o and "ch#l oo, The effect of aging treatent on the icrostr#ct#re and

echanical properties of AA2124 h!brid coposites reinforced with both iC whiskers and iC

particles, Copos" ci" and Technol" 39 &1999( ))3))9"

+1-. Q" *#enan, $" @#isong, Q" QhenBh# and *" 7in and @eng <in, ;ffect of aging treatent onechanical properties of & iCwDiCp(O2024A1 h!brid nanocoposites, T" Nonferr" /etal" oc" 16

&2006( -:)-91"

+14. "" #ang, N" ort and >"=" >ainer, Theral behavior of short fiber reinforced

Ali12C#/gNi piston allo!s, Copos" 8art A" -3 &2004( 24926-"

+13. M"8" Cott#, M"M" Co#derc, 7" ig#ier and <" 7ernard, Infl#ence of iC reinforceent on

precipitation and hardening of a etal atri% coposite, M" /ater" ci" 2) &1992( -06:-0)4"

+16. T"" rivatsan and M" /attingl!, Infl#ence of heat treatent on the tensile properties and

fract#re behavio#r of an al#ini# allo!ceraic particle coposite, M" /ater" ci" 2:9 &199-( 611

620"

+1). "M" Choi, 7"" /in, M"" hin and "" 7ae, trengthening in nanostr#ct#red 2024 al#in#

allo! and its coposites containing carbon nanot#bes, Copos" 8art A" 42 &2011( 14-:1444"+1:. N" aheb, A" >halil, A"" akee, N" AlAeeli, T" <ao#i and A">" P#t#bG #bitted to

Mo#rnal of Coposite /aterials"

+19. A" >halil, A"" akee and N" aheb, RptiiBation of 8rocess 8araeters in park 8lasa

intering Al6061 and Al2124 Al#in# Allo!s, Adv" /ater" es" -2:--0 &2011( 131)1322"

+20. "C" $ang and /"M" tarink, Two t!pes of phase precipitates in AlC#/g allo!s, Acta

/ater" 33 &200)( 9--941"

+21. "M" ack, 8roc" Conf" ispersion trengthened Al#in# Allo!s, /inerals, /etals and

/aterials ociet!, $arrendale, 8A &19::( 649"

+22. " ibes, <" alvo and /" #er!G 8roc" Conf" )th International Conference on Coposite

/aterials,S China ociet! of Aerona#tics and Astrona#tics, 7eiLing, China &19:9( )03"

+2-. N" 8rakash, /aster of cience Thesis, epartent of /echanical ;ngineering, ?lorida tate=niversit! &2003("




+24. " /aBBini and MC Caretti, ;ffect of deforation at elevated teperat#re before agehardening

on the echanical properties of 2024 coercial al#ini# allo!, cripta /etall" /ater" 23 &1991(

19:)1990"

+23. C" eng, " $ang, *" Qhang and A" <i, 8rocessing and properties of carbon nanot#bes

reinforced al#in# coposites, /at" ci" ;ng" A" 444 &200)( 1-:143"

+26. $" alas, N"@" Alba7aena and <";" /#rr, ;%plosive hock$ave Consolidation ofAl#in# 8owderOCarbon Nanot#be Aggregate /i%t#resG Rptical and ;lectron /etallograph!,

/etall" /ater" Trans" A" -: &200)( 292:29-3"

+2). A" ;sawi and /" ;lborad!, Carbon nanot#bereinforced al#ini# strips, Copos" ci"

Technol" 6: &200:( 4:6492"

+2:. I"" >i, M"" <ee, @"" <ee, "" 7aik, "M" >i and "Q" <ee, ?riction and wear

characteristics of the carbon nanot#be al#in# coposites with different an#fact#ring

conditions, $ear" 26) &2009( 39-39:"

+29. "M" Choi, "/" <ee and "" 7ae, $ear characteristic of al#in#based coposites

containing #ltiwalled carbon nanot#bes, $ear" 2)0 &2010( 121:"

+-0. A"/">" ;sawi, >" /orsi, A" a!ed, /" Taher and " <anka, ;ffect of carbon nanot#be &CNT(

content on the echanical properties of CNTreinforced al#ini# coposites Copos" ci"Technol" )0 &2010( 22-)2241"

+-1. M"Bhi <iao, /"M" Tan and I" ridhar, park plasa sintered #ltiwall carbon nanot#be

reinforced al#in# atri% coposites, /ater" esign" -1 &2010( 96100"

+-2. "" hercliff and /"?" Ashb!, A process odel for age hardening of al#ini# allo!sUI"

The odel, Acta /etall" /ater" -: &1990( 1):91:02"

+--. "" hercliff and /"?" Ashb!, A process odel for age hardening of al#ini# allo!sUII"

Applications of the odel, Acta /etall" /ater" -: &1990( 1:0-1:12"


jnanor.21.29age hardening behavior of carbon nanotube reinforced aluminum nanocomposites

Documents