Strengthening in nanostructured 2024aluminum alloy and its composites containing carbon nanotubes
H.J.Choi,B.H.Min,J.H.Shin,D.H.Bae ?
Department of Materials Science and Engineering,Yonsei University,134Shinchondong,Seodaemungu,Seoul 120-749,Republic of Korea
a r t i c l e i n f o Article history:
Received 14January 2011
Received in revised form 5May 2011Accepted 20June 2011
Available online 28June 2011Keywords:
A.Metal–matrix composites A.Nano-structures
B.Mechanical properties E.Powder processing
a b s t r a c t
The ultra?ne-grained 2024aluminum alloy (A2024)and A2024matrix composites containing carbon nanotubes (CNTs)are developed.Three strengthening strategies of grain boundary hardening,age hard-ening and hardening by CNTs are employed.First,grain size of A2024is effectively reduced using a ball-milling technique and A2024with a grain size of 100nm exhibits a yield stress of $560MPa,exhibiting a well agreement with the Hall–Petch https://www.doczj.com/doc/e810245113.html,Ts also have a great effect for strengthening.The A2024matrix composite containing 3vol.%CNTs shows a yield stress of $780MPa with 2%tensile elongation to failure.Hardness is further increased after aging.The nanostructured composite shows its peak hardness after 4h of aging because the re?ned grain boundaries and CNTs act as a pathway for diffusion of atoms with stimulating the aging process.The composite in the present study has great potential for application as structural materials in industry.
ó2011Elsevier Ltd.All rights reserved.
1.Introduction
2024Aluminum alloy (A2024)as a heat-treatable metal has received a great deal of attention for past decades,due to their high speci?c strength and stiffness [1–4].The use of A2024,therefore,has been growing gradually in industry as a material of aeroplane constructions,automobiles,and pulling wheels [3].
Recently,a continued demand for high performance structural metals has stimulated the development of nanostructured alloys with superior strength [4].It has been recognized that nanocrystal-line metals show much improved strength,as compared to that of microcrystalline counterparts [5,6].The relations between grain size and the ?ow stress can be clari?ed with the Hall–Petch rela-tionship [7].Here pile-up of dislocations at grain boundaries is envisioned as a key mechanistic process underlying an enhanced resistance to plastic ?ow with grain re?nement.When grain size is relatively large,greater stresses can be concentrated near the adjacent grains due to the presence of multiple pile-up disloca-tions,leading to yield stress to be decreased.As grain size is smaller than a certain value (generally 20–30nm [8]),however,nanocrystalline metals show an inverse Hall–Petch behavior because intragranular slip is limited and atomic shuf?ing at grain boundaries or stress-assisted free volume migration gradually con-tribute to plastic deformation [9,10].Furthermore,the absence of work hardening capacity in nanocrystalline metals leads to low
ductility [11].Hence,breakthroughs to overcome these limitations are emerging in nanocrystalline metals and alloys.
Carbon nanotubes (CNTs)have captured a great deal of attention as a reinforcing agent for metal matrix composites [12–21].Agar-wal and coworkers have published a fair number of reports [12,13]on Al–Si alloy matrix composites containing CNTs,produced by spraying the ball-milled mixture on a substrate.The composites show enhanced wear resistances due to the superior mechanical properties of CNTs.Gupta and coworkers have devel-oped the spraying process termed disintegrated melt deposition (DMD);a mechanically stirred molten composite is disintegrated and then deposited onto a substrate in the form of a bulk ingot [14,15].The magnesium matrix composite containing 1.3wt.%CNTs exhibits simultaneous increase in yield stress and ductility [15].Ball-milling techniques followed by hot-consolidation pro-cesses such as spark plasma sintering [16],spark plasma extrusion [17],hot-extrusion [18],and hot-rolling [19]have generally been utilized to develop metal matrix composites containing CNTs.Zhong et al.[20]have demonstrated that the aluminum matrix composite reinforced by 5wt.%CNTs showed around two times higher hardness value.Furthermore,the aluminum matrix composite reinforced by 2wt.%CNTs,which was fabricated by a powder metallurgy route followed by annealing,shows $350MPa in tensile strength and $8%in tensile elongation to failure [21].With this scope,the ?rst aim of the present study is to develop the nanostructured A2024/CNTs composite with superior mechan-ical properties.Here,three strengthening strategies are involved in terms of grain boundary hardening,age hardening,and hardening by CNTs.The strengthening behaviors are investigated by
1359-835X/$-see front matter ó2011Elsevier Ltd.All rights reserved.doi:10.1016/https://www.doczj.com/doc/e810245113.html,positesa.2011.06.008
Corresponding author.Tel.:+82221235831;fax:+8223125375.
E-mail address:donghyun@yonsei.ac.kr (D.H.Bae).
comparing the experimental data with theoretical expectations. Furthermore,the effects of grain size and the presence of CNTs on precipitation processes are investigated.
2.Experimental
A2024and A2024/CNT composite specimens were fabricated by hot-rolling the ball-milled powder.The A2024powder was pro-duced by ball-milling the chips of the as-cast A2024ingot with an initial composition as shown in Table1.Prior to chipping,the A2024ingot was solution-treated at530±5°C for2h and then water-quenched.The A2024chips were subsequently ball-milled using an attrition mill at500rpm in speed under an argon atmo-sphere for up to48h(i.e.,12,24,and48h).The ball-to-chip weight ratio was15:1.Prior to milling,a control agent of1.0wt.%stearic acid was added to the chips to prevent agglomeration and exces-sive cold welding among chips and powder.During milling,the chips were broken into powder and moreover the grain size of the power is reduced effectively.The composite powder was pro-duced by ball-milling a mixture of the18-h-milled A2024powder and multi-walled CNTs(MWCNTs,diameter$20nm,length $5l m,supplied from Applied Carbon Nano Co.Ltd.)for6h under the same milling condition.By varying the volume fraction of MWCNTs(i.e.,0,1,2and3vol.%),four types of specimens were produced.
Hot-rolling was carried out to consolidate the ball-milled pow-der.Prior to rolling,the ball-milled powder was containerized in a copper tube(￡30mm?150mm, 1.5mm in thickness),com-pacted under pressure,and then sealed.The specimen was heated to a temperature of450°C and then hot rolled with every12% reduction per a pass;the initial thickness was$20mm and the ?nal thickness was$1mm.Gas-atomized A2024powder was also hot-rolled under the same condition as a reference starting mate-rial.After rolling,the specimens were solid-solution-treated at 530±5°C for2h to dissolve precipitates and second-phases which were produced during milling and rolling processes,and then quenched using cold water.Aging of the solution-treated speci-mens was carried out at180°C for up to60h.
The grain size of the specimens were analyzed using X-ray dif-fraction(XRD,Rigaku,CN2301)with a Cu K a radiation source. From the XRD patterns,average grain size can be determined using the Scherrer formula as[22];
b ge2hT?0:9k
e1T
where b g(2h)is the breadth at half the maximum intensity of the peak(excluding instrumental broadening),k is the wave length of the X-ray radiation,h is the Bragg angle,and D is the average grain size.The grain size is also estimated using a statistic analysis of sev-eral transmission electron microscopy(TEM,JEOL2000)images. The microstructure of the specimens was observed using high-resolution TEM(HRTEM,JEOL2000).Thin foil specimens from the sheets were carefully prepared by an ion beam milling method (Gatan,Model600,Oxford,UK)for TEM analysis.
The tensile properties of the specimens were evaluated using an Instron-type machine under a constant cross-head speed condition of an initial strain rate of1?10à4sà1at room temperature.Tensile specimens(thickness:1mm,gauge length:5.75mm,gauge width: 2mm,and effective grip length:30mm for both upper and lower grips)were prepared from the hot-rolled sheets,and then loaded in the rolling direction.The variation of the Vickers’hardness dur-ing aging was measured for the specimens with an indenter load of 300gf.
3.Results and discussion
3.1.Mechanical properties of2024aluminum alloy
3.1.1.Grain re?nement during milling
Fig.1a shows XRD patterns of the original A2024powder and powders ball-milled for12,24,and48h,respectively.The average grain size of the powder is effectively reduced as an increase in milling time up to24h;it is$92nm after12-h-milling and $50nm after24-h-milling.However,it increases again when the powder is further ball-milled;it increases to be$90nm after 48-h-milling.The grain growth during further milling beyond 24h may arise because of the thermal recovery process induced by the excessive milling energy.Hence,the milling time is limited to24h in the present study.Fig.1b and c show bright-?eld TEM images of hot-rolled sheets fabricated using A2024powder
Table1
Chemical compositions of a starting A2024cast.
Composition Cu Mg Mn Al
Weight(%) 4.5 1.50.5Balance 100 nm 100 nm
H.J.Choi et al./Composites:Part A42(2011)1438–14441439
ball-milled for12h(Fig.1b)and24h(Fig.1c),respectively.The plane-view average grain sizes are$250and$100nm,respec-tively.Specimens were exposed to a high temperature of450°C (that is,$0.9T m where T m is a melting point of A2024)for$1h during hot-rolling and hence A2024sheets have larger grain sizes than those of as-milled powder,respectively.
3.1.2.Strengthening by grain re?nement
Fig.2a shows tensile true stress(r)–true strain(e)curves of the hot-rolled A2024specimens with$100and$250nm in grain sizes together with A2024starting material.As grain size decreases,the yield stress of A2024is dramatically enhanced.The A2024speci-mens with mean grain sizes of250and100nm show430and 560MPa in yield stress and10and8%in elongation,respectively. These yield stress values are3–4times larger than that ($140MPa)of the starting A2024.Work hardening rate(n)is also calculated using the classical Hollomon law through relation be-tween true stress and true strain[23]as;
r?K e ne2TThe calculated values are noted in Fig.2a.As shown,the work hardening rate is signi?cantly reduced as grain size decreases pos-sibly due to the lower density of dislocations in grain interior[7]. Due to the negligible work hardening capacity,the ultra?ne-grained A2024specimens exhibit early plastic instability during tensile deformation and less ductility,as shown in Fig.2a.
The variation of yield stress(r y)depending on grain size(d)of the specimens is plotted in Fig.2b,where the strengthening by grain re?nement is compared to literature data[24]and the theo-retical expectation based on the Hall–Petch relation[23]as;
r y?r0tkdà1=2e3Twhere k is a constant(0.13MPa/mm1/2)and r0is the intrinsic stress (170MPa)[4].The Hall–Petch model is based on the hypothesis that grain boundaries act as obstacles in dislocation movements. That is to say,as the grain size is reduced,dislocation movements are disturbed by grain boundaries,so yield stress of specimen in-creases.In the present study,the yield stress of A2024specimens is well matched with the theoretical expectation by the Hall–Petch relation.In this grain size regime(grain sizes larger than100nm), activities of forest dislocations may still be a dominant deformation mechanism.Other mechanisms that have been known for nano-crystalline metals(e.g.,activities of perfect or partial dislocations [9],grain boundary sliding[8],and twining[25])are thought to be diminishing in this grain size regime.
3.1.3.Precipitation hardening depending on grain size
Fig.3a shows the hardness of the starting A2024and the ultra-?ne-grained A2024with a grain size of$100nm,varied according to aging time up to60h.Both specimens show much increased hardness by up to1.2times after aging.The hardness peak for the ultra?ne-grained A2024is observed at4h in aging time whereas it reveals at18h in aging time for the starting A2024. The nucleation and growth of precipitates may progress much rap-idly as grain size is reduced because the?ne-structures enhance aging kinetics[26].Furthermore,the hardness increases rapidly at the beginning and then decreases with a high rate for the ultra-?ne-grained A2024whereas the starting A2024shows a broad hardness peak.
The variation of the stress?eld near precipitates leads to the change in hardness during aging and the internal stresses are determined by the degree of coherency between the precipitate and the matrix.Generally,A2024has been reported to have two main types of precipitates with the following formation sequences [27]:
e1TGP I!GP IIeh00T!h0!heCuAl2T
e2TGPB I!GPB IIeS00T!S0!SeAl2CuMgT
Initially,coherent precipitates(GP I and GPB I)cause an in-crease in hardness.The hardness peak is observed at GP II and GPB II zones because the increased internal stresses in the crystal lattice.As precipitates grow up,they lose their degree of coherency with the matrix and the rate of increase in hardness slows down [28].Furthermore,as the coherent or semi-coherent phases such as h00or S00are transformed to the incoherent phases such as h0/h or S0/S,it is gradually accompanied by a decrease in hardness.Dur-ing the aging process,copper atoms diffuse from h00(S00)to h0(S0) and?nally to h[29].Therefore,diffusion on matrix is one of impor-tant factors in the nucleation and growth rates of precipitates.In general,at any temperature the magnitudes of D b(the grain boundary diffusivity)and D s(surface diffusivity)relative to D l (the lattice diffusivity)are such that
D s>D b>D le4T
Although surface diffusion plays an important role in many metallurgical phenomena,in the case of a bulk specimen grain boundary diffusion is usually most important.If the grain bound-ary has certain values of the effective thickness(d)and the grain size(d),the total?ux(J)will be given by
1440H.J.Choi et al./Composites:Part A42(2011)1438–1444
J ?eJ b d tJ l d T=d ?à
D b d tD l d d dC
dx
e5T
where J l is the ?ux of solute through the lattice,J b is the ?ux along
the boundary J b ,and C is concentration gradients.Thus the apparent diffusion coef?cient (D app )in this case,
D app D l
?1t
D b d
D l d e6T
It can be seen that the relative importance of lattice and grain
boundary diffusion depends on the ratio D b d =D l d .Hence,diffusion along grain boundary makes a considerable contribution to the total ?ux when grain size is reduced (i.e.D b d )D l d ).The ultra?ne-grained A2024has much larger value of D app as compare to the starting A2024and hence precipitation hardening of the ultra-?ne-grained A2024occurs even more rapidly than the starting A2024.
Figs.3b and c show bright-?eld TEM images of the ultra?ne-grained A2024after 4h (Fig.3b)and 28h of aging (Fig.3c).After
100 nm 500 nm
(b)(c)
20 nm
MWCNTs
b
(a)(b)
H.J.Choi et al./Composites:Part A 42(2011)1438–14441441
4h of aging(Fig.3b),minute precipitates($5nm in size,marked by open circles in Fig.3b)are created and distributed over the matrix. As previously mentioned,copper atoms may diffuse along grain boundaries as well as through lattice because grains are very small. Hence,nano-scale h00or S00precipitates would be formed after just 4h of aging.These nano-precipitates form coherent or semi-coher-ent interface structures with the matrix,giving rise to an increase of hardness due to the internal stress?eld near precipitates.With further aging(28h of aging,Fig.3c),however,these precipitates grow to be$100nm in size(marked by open circles in Fig.3c) and they lose coherency with the matrix,leading to a decrease in hardness.3.2.The A2024matrix composite containing CNTs
3.2.1.Microstructures
Fig.4a shows TEM image of the ultra?ne-grained A2024matrix composite containing3vol.%MWCNTs.MWCNTs seem to be dis-persed and aligned in the rolling direction.The magni?ed image of a rectangle in Fig.4a is shown in Fig.4b and it reveals that the interface between MWCNTs the matrix is very clean;no voids or carbides are observed at the interface.
3.2.2.Strengthening by CNTs
The tensile properties of the A2024/MWCNT composite are shown in Fig.5,where specimens were solid solution treated at 530°C for2h,respectively.Tensile properties of the monolithic A2024specimens which were ball-milled for24h are also included in Fig.5for comparison.By incorporating MWCNTs,the composite shows much enhanced yield stress but reduced ductility and work hardening https://www.doczj.com/doc/e810245113.html,Ts have sometimes been reported to play a role to increase tensile ductility as well as strength of composites particularly when tensile ductility of the matrix is very limited [15].For the A2024/MWCNT composite in the present study,how-ever,the A2024matrix already has a good tensile ductility and hence it decreases as MWCNTs are added.The composite contain-ing3vol.%of MWCNTs shows$780MPa of yield strength with2% plastic elongation to failure.Such a superior yield stress has rarely been reported for aluminum-based alloys and composites in a ten-sile mode.MWCNTs are found to be very effective reinforcing agent for strengthening metals.The strengthening ef?ciency of the discontinuous?bers can be calculated based on the assump-tion that the matrix transmits the load to the?ber by means of shear stresses that develop along the?ber–matrix interface.A force balance allows;
MWCNT MWCNT
1442H.J.Choi et al./Composites:Part A42(2011)1438–1444
r f p r2
4
?s0p r L c2e7T
where r f is the tensile stress of MWCNTs(30GPa[19]),r is the diameter of MWCNTs($20nm,obtained from TEM observations), and s0is the shear stress of the matrix($r m/2,r m is the yield stress of the matrix(480MPa for the ultra?ne-grained A2024annealed at 530°C for2h)).Moreover,L c is the critical length of MWCNTs, which can be expressed as;
L c?r
f
r
2s0
e8T
The calculated L c for the ultra?ne-grained A2024is about1.2l m.
The Kelly–Tyson formula given as[23];
a c?V f r f
L
2L c
tV m r me9T
where r c is the strength of the composite,V f is the volume fraction of?ber,V m is the volume fraction of the matrix,and L is the average length of MWCNTs($800nm,obtained from TEM observations). Fig.6compares experimental data with theoretical calculations for the ultra?ne-grained A2024matrix composite,where the exper-imental data are quite well matched with the theoretical calcula-tions.Negative deviation of the yield stress from the theoretical expectations has sometimes been reported because of the relaxa-tion of the transmitted load.This may arise at the interface due to the presence of cracks or voids[30].For the composite in the pres-ent study,however,the energy relaxation at the interface is thought to be insigni?cant.
3.2.3.Precipitation hardening of the composite
Fig.7a shows the hardness of the monolithic A2024and the composite containing2or3vol.%MWCNTs,varied according to aging time.The composites exhibit their peak hardness at4h in aging time,where the peak hardness is$1.2times higher than ini-tial.Fig.7b and c show TEM images of the composite with3vol.% MWCNTs after4h(Fig.7b)and28h(Fig.7c)of aging.In Fig.7b, nano-scale precipitates(marked by dashed circles)are created par-ticularly nearby MWCNT(marked by arrows).MWCNTs as well as grain boundaries might act as a path way for diffusion of copper atoms and it may stimulate the formation of precipitates nearby tips of MWCNTs.Furthermore,the dislocation density is very high at tips of MWCNTs due to the high stress level[18,19]and the chemical composition could be modi?ed as a consequence of seg-regation.This may stimulate the kinetics of precipitation[31]. Hence,precipitates are readily nucleated at tips of MWCNTs as shown in Fig.7b and c.These nano-scale precipitates may form coherent or semi-coherent interface structures with the matrix, giving rise to a signi?cant increase of the hardness.After further aging(28h of aging,Fig.7c),however,the precipitates grow to be more than100nm near the tip of MWCNT.Hence,the interface between the precipitates and the matrix becomes more incoherent, providing a decrease in the hardness of the composite.As shown in Fig.7a,the hardness of the composite is more rapidly reduced as further annealing after the peak point as compared to that of the monolithic A2024.The presence of large precipitates grown nearby tips of MWCNTs could affect the stress?eld surrounding MWCNTs and hence these large precipitates may reduce the strengthening ef?ciency of MWCNTs as well as their own strengthening ef?ciency.
4.Conclusions
Strengthening behavior of the ultra?ne-grained A2024and the A2024/MWCNT composite,fabricated using a hot rolling process involving ball-milling techniques,is investigated.When milling is processed up to24h,grain size of A2024powder is effectively re-duced and the yield stress is dramatically enhanced.The ultra?ne-grained A2024with a grain size of100nm exhibits a yield stress of $560MPa with a tensile elongation of2%.This grain re?nement strengthening is well-matched with theoretical expectations based on the Hall–Petch relation,and hence activities of forest disloca-tions are thought to be a main deformation mechanism in the grain size regime.The re?ned grain structures also stimulate the precip-itation hardening because grain boundaries may contribute to dif-fusion of atoms;the ultra?ne-grained A2024shows the peak hardness after4h of aging whereas the starting A2024shows the peak hardness after18h of aging.MWCNTs are found to have a great effect for strengthening the ultra?ne-grained A2024;The A2024matrix composite containing3vol.%MWCNTs exhibits a yield stress of$780MPa in tensile mode.The further increase in hardness is achieved by age hardening.The composite shows the peak hardness after4h of aging because MWCNTs also act as a pathway for diffusion of atoms.The composite in the present study exhibits superior mechanical properties and hence it has great po-tential for application as structural materials in industry. Acknowledgements
This research was supported by the R&BD Program of Korean Institute for Advancement of Technology,Korea Science and Engi-neering Foundation(Grant No.2010-0016139)and the Second Stage of Brain Korea21Project in2010.
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