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MaterialsScienceand EngineeringA530 (2011) 304-314

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Materials Science and Engineering AELSEVIER journal homepage: www.elsevier.comllocate/msea

Effect of eutectic Si on surface nanocrystallization of AI-Si alloys by surfacemechanical attrition treatmentH.-W. Chang a, P.M. Kelly a, Y.-N. Shi b, M.-X. Zhang"-", Div is ion of Materials . School of Mechanical and Mining Engineering. Univers ity of Queensland. St Luda. Brisbane. QLD 4072. Australiab Shenyang National Laboratory for Materials Sdence . Insti tu te of Metal Research. Chinese Academy of Sdences . Shenyang 110016. China

ARTICLE INFO ABSTRACTPure Al (AA196) and Al-Si (A356) alloy were subjected to surface nanocrystallization through sur-face mechanical attrition treatment (SMAT).Strain induced microstructure evolution, including grainrefinement of Al matrix and Si particles, was examined using transmission electron microscopy.Nanocrystall ization of Al matrix in both pure Aland A356 alloy occurs through formation of disloca-tion cells separated by dense dislocation walls and dislocation tangle within the original coarse grains orsubdivided subgrains. During SMAT,the brittle Siphase ispreferentially refined through direct breakage.The Almatrix refinement process is greatly facilitated by the dispersed small Siparticles, because suchparticles can not only act as obstacles for dislocation movement, but also promote dislocation genera-tion and multiplication. Comparing to pure AI,thicker nanocrystalline zone can be obtained with muchsmaller stabilized nano-metered grains in the Al-Si alloy. Due to the low diffusion rate ofSi inAl matrixat low temperature, there is no obvious re-dissolve ofSi particles into Alduring SMATprocess.

2011 Elsevier B.V.Allrights reserved.

Article history:Received18 April2011Receivedin revisedform 14 July2011Accepted25 September2011Availableonline 1October2011Keywords:SMATAl-SicastalloyNanocrystallinematerialGrainrefinementDislocationgliding

1. Introduction

Severe surface plastic deformation is effective in refining sur-face microstructures of metals and alloys, hence enhancing theirsurface durability [1-4]. Based on this principle, various methodshave been developed to produce nanometer scaled grains on thesurface of alloys, including ultrasonic shot peening (USSP) [5], highenergy shot peening (HESP) [6,7], etc. Among the currently avail-able methods, surface mechanical attrition treatment (SMAT) isregarded asthe most effective one to generate nanostructured layeron a coarse-grained bulk metal [8,9]. In the past decade, the grainrefinement mechanism during severe plastic deformation processhas been widely investigated in various systems [10-15]. Gener-ally, it is considered that dislocation activit ies [10-13], mechanicaltwinning [14] and dynamic recrystallization [15] are responsiblefor the grain refining. The dislocation mechanism operates in met-a~s that contain large number of slip systems with high stackingfault energy. Formation of high density dislocation walls and dis-location tangles subdivides the coarse grains into smaller onesfollowed by grain rotations. The twinning mechanism occurs inmetals that have low stacking fault energy and associate withless slip systems. Coarse grains are initially divided by mechanicaltwins followed by further refining through formation of dislocation

* Correspondingauthor.Tel.:+61 733468709; fax:+61 73365 3888.E-mail address:mingxing.zhang@uq.edu.auM.-X.Zhang).

0921-5093/$ - seefrontmatter 2011 ElsevierB.V.Allrightsreserved.doi:10.1016/j.msea.2011.09.090

arrays, dislocation walls and tangles at higher plastic deformation.The dynamic recrystallization mechanism is valid in metals thathave lower recrystallization temperature. If the temperature risingresulted from plastic deformation is high enough for recrystal-lization, defect-free nanocrystalline structure forms on the metalsurface through recrystallization process. However, as previouswork mainly focused on single-phase alloys, these proposed grainrefining mechanism is only valid for this type of metals. In addition,to achieve high strength, alloys used in industry normally containmultiple phases. Thus, understanding of the effect of the secondaryphase on the mechanism of surface nanocrystallization is particu-larly important. Unfortunately, owing to the difficulty of taking thesecondary phase into account in elucidating the nanocrystall izationmechanism, limited work was done on multiple phase alloys. Luand co-authors' workofSMATon a steel with spheroidal cementite[16] showed the complexity of the nanocrystallization mechanismcompared with the single-phased systems. In addition to the grainrefining of both the ferrite and cementite phases during the plas-tic deformation process, it was also found that, at some regions,cementite dissolves into the ferrite matrix.The aims of the present work are to investigate the microstruc-

ture evolution of cast AI-Si alloys and study the effect of eutectic Sion the grain refinement mechanism in SAMT process. As the mostcommon cast Al alloys, AI-Si alloys are widely used in industriesbecause of their good mechanical properties and high castabil-ity resulting from the presence of the AI-Si eutectic that reducesthe shrinkage during solidification and the coefficient of thermal

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H.-w. Chang et al. / Materials Science and Engineering A 530 (2011) 304-314Table 1Chemical composi tions of A356 and AA196 cast ing al loys.Alloy Si Zn Tie C u Mn MgA356AA196

7.80.D15

0.60.D15

0.250.005

0.35 0.2-0.4 0.1 0.2

expansion of the cast products. In some cases, higher surface dura-bility, including wear resistance, is required when the operationinvolves surface contact and relative motion, such as the enginecylinder and piston. Hence, it has both scientific and technologi-cal significance to study the surface nanocrystallization of this typeof alloys. Although grain refining mechanism of the single phasealuminum alloy during SMAT was studied [13] on a wrought 7075alloy at T4 condition, the microstructure evaluation of as-cast AI-Sialloys during surface nanocrystallization process induced by plasticdeformation is unknown. In the present work, the microstructurevariations in the surface layer of an as-cast AI-Si alloys and anas-cast pure aluminum after SMATwill be investigated using opti-cal microscopy, X-ray diffraction system (XRD) and transmissionelectron microscopy (TEM).2. Experimental

The alloys used were commercial AI-7.S wt% Si cast alloy(A356) and as-cast pure aluminum (AA196). Their chemical com-positions are listed in Table 1. Plate samples with dimensions100mm x 100mm x 10mm were cut from as-received ingots. Thetwo broad surfaces of the plates were polished with 600-grade SiCsand papers. This gives a surface finish good enough for surfacemechanical attrition treatment (SMAT).

SMAT process was carried out in a SNC-II surface nanocrystal-lization machine under vacuum conditions at room temperature.The machine was operated at a vibration frequency of 50 Hz. 10 mmin diameter bearing steel (Cr15) balls were used to create severeplastic deformation on the surface of the samples through repeatedimpaction from multiple directions. Reference [13] provides moredetails about the SMAT technique. In the present study, the dura-tions of SMAT were 5, 15, 30 and 60min for each alloy. To avoidoverheating, SMAT process was stopped every 5 min followed bycooling under vacuum for 30 min for the samples that were plannedfor over 5 min SMAT. After SMAT, the treated surface was coveredwith epoxy to prevent surface contamination introduced duringsample preparation for optical and TEM observations. The epoxycan be easily removed in acetone before examination.

Cross sectional optical microscopy specimens were preparedthrough electrochemical polishing in an electrolyte of 2.6vol.%HBF4 water solution operated at voltage of 20V at room tem-perature. The process of transmission electron microscopy (TEM)specimen preparation is as follows:To examine the outermost surface, (1) 1.0 mm thick thin sheet

containing the top surface was cut from the SMATed plat using a lowspeed diamond saw; (2) the plates were mechanically ground, onthe cutting surface (opposite to the SMATed surface) only, down to150 J-Lmthick foils, and 3 mm in diameter discs were punched fromthe thin foils; (3) these discs were further mechanically polishedto 70-S0 J-Lmthick from the cutting surface; then, (4) the epoxy onthe SMATed surfaces of the discs were removed by washing in ace-tone and then were re-covered with lacquer to prevent the SMATedsurfaces from thinning during jet-polishing; (5) all discs were elec-trochemically polished in an electrolyte of 33% HN03 + 67%CH30Hat 243 K (-30C) in Tenupol-5 twin-jet polisher with operationvoltage of 30V; (6) after perforation, the lacquer on the discs wasremoved by acetone. To examine the sub-micron layer that is about50-150 J-Lmbelow the topmost SMATed surface, and the SMATaffected layer that is 200-2000 J-Lmbelow the top surface, before

305

plate cutting from the SMATed samples, layers ofSO-100 J-Lmhick,and around 300 J-Lmthick were carefully removed from the top ofthe SMATed surface by mechanical grinding. Then, the same proce-dure described as above will be used to prepare the TEM specimens.For the high Si containing alloys, due to the difficulty in polish-ing the Si by twin-jet polishing, precision ion polishing system(PIPS) was used at the final stage to perforate the TEM specimens.TEM examination was conducted in aJEOL-2100 TEM operated at200kV.The grain sizes on the topmost surface of SMATed samples weremeasured using two different methods. One is direct measure-

ment on TEM micrographs using the linear intercept method. Theaverage values obtained from 10 to 15 micrographs are treatedas the final results. Another is X-ray diffraction (XRD) analysis onthe surface layer of the SMATed Al samples. XRD was carried outin a Rigaku Miniflex diffractometer with a cobalt source and ina Bruker OS diffractometer with Cu-K, radiation. Small angularsteps of28 =0.02 were used to measure the intensity of each Braggdiffraction peak. The average grain size was calculated from linebroadeningof(111)Al,(200)Al,(311)Aland(222)AIBraggdiffrac-tion peaks, using the Scherrer and Wilson method [17].The microhardness variation along the depth from the SMATed

surface was measured in a Struers DURAMIN 20 hardness testerwith a load of 109 for loading time of lOs.

3. Results and discussions3.1. Microstructure evolution of cast pure Al3.1.1. Optical microscopy observations

The optical microstructures revealed through electrochemicalpolishing on cross sections of the samples after SMAT for differentdurations are shown in Fig. 1. The thickness of the SMAT affectedregions varies from ~SOO J-Lmto ~2000 J-Lmwith the treatmentduration increasing from 5 min to 60 min. One can observe thatthe SMAT affected zone for all treatment durations consists of twosub-regions. The region close to the surface contains fine grainsand the closer to surface and the finer the grains are. The secondsub-region only shows slip bands. As the SMAT duration increases,both sub-regions are thickened. This implies that the strain withinthe SMAT affected zone varies with depth from the outermost sur-face. In order to understand the grain refining mechanism duringSMAT, microstructure at various layers within the SMAT affectedzone should be examined. In the present work, the sample SMATedfor 5 min was used because this sample has a reasonably flat sur-face, which facilitates the TEM specimen preparation. The actualdepths selected forTEM examination are 20, SOand 120 J-Lmbelowthe outermost surface.

3.1.2. rEM observationsFig. 2 shows typical TEM micrographs obtained in the region

that is 100-150 J-Lmbelow the outermost surface of the Al sampleSMATed for 5 min. The average grain size observed in TEM at thisdepth is around 2 J-Lmas shown in Fig. 2(a). Similar to the previ-ously reported results in a 7075 alloy [13]. dislocation cells (DC)separated by dislocation arrays (DA), dislocation tangles (DT) anddense dislocation walls (DOW) within one original coarse grain canbe observed in most regions, as shown in Fig. 2(b). The selected areaelectron diffraction (SAED) patterns shown as the insert in Fig. 2(b)were taken within an area containing a number of DCs. The sim-plicity of this SAED patterns indicates that all the DCs are in thesame orientation and the DDWs, DCs and DTs have not turned theDCs into smaller grains at this depth.Typical TEM micrographs of samples taken from the region that

is 50-100 J-Lmbelow the SMATed surface of pure Al sample are

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Fig.t. Optical micrographs on the cross sections ofthe pure Alsamples SMATedfor different durations: (a) 5minSMAT; (b) 15min SMAT;(c)30minSMAT; (d) 60min SMAT.

Fig. 2. Typical TEMmicrographs taken from the region of 100-150 urn below treated surface of pure Al SMATedfor 5min, showing micro-sized subgrains with smallmisorientation divided bydifferent dislocation configurations and boundaries. (a) Micro-sized grains at 100-150 urn below the topmost surface and the corresponding SAEDpattern; (b) subgrain divided by DTsand the corresponding SAEDpattern.

shown in Fig. 3. The average grain size at this depth is below 1 J-Lm(Fig. 3(a)). The discontinuous circular SAEDpattern can be regardedas an evidence of the conversion of Des into subgrains. The largerstrain associated with this depth leads to accumulation of more

dislocations at the DDWs and DTs with increased energy. In orderto accommodate the high energy, subgrain rotation [13] occurs,which results in high misorientations between the Des and formssubgrains. However, within such subgrains, smaller Des separated

Fig.3. Typical TEMmicrographs taken from the region of50-100 urn below the top surface of pure AlSMATedfor 5min. (a) Bright fie ld image showing micrometer sizedsubgrains with small misorientation divided by different dislocation configurations and boundaries and SAEDpattern; (b) subgrains divided by DDWs and DTs and SAEDpattern.

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by DDWs and DTs can also be observed, as shown in Fig. 3(b). Thisprocess will continue until nanometer scaled grains with particularsize form with increase in plastic deformation.

Fig. 4 shows the typical TEM microstructure within the topmostsurface layer of the pure Al samples after 5m in SMAT. Both thebright field image and the SAED pattern (Fig. 4(a)) illustrate thenanocrystalline structure. The average grain size is around 150 nm.Fig. 4(b) is another TEM micrograph showing the nanocrystallinestructure in the topmost surface of the SMATed Al sample at highermagnification. Wu et al. [13] indicated that the grain subdivisiondoes not continue indefinitely and eventually, after a given amountof deformation, continued straining can no longer reduce the grainsize. In cast pure Al and at the current SMATconditions, the smallestnanocrystalline grains achieved are 150 nm.

Based on the observation of microstructure evaluation of pureAl at different depth from the topmost surface, the grain refinementprocesses during SMAT can be summarized as follows:(1) Formation of dislocation cells within original grains. These cells

are separated by DDWs and DTs.(2) Dislocation cells convert into subgrains through subgrain rota-

tion at higher strain and smaller dislocations cells with DDWsand DTs form within the subgrains.

(3) Further plastic straining leads to formation of even smallerDCs and further rotation of subgrains and eventually nanocrys-talline structure is obtained. In cast pure Al and at the presentSMAT conditions, the smallest grains that can be achieved isaround 150 nm.Obviously, above observations of microstructure evaluation in

pure Al during SMAT and the proposed grain refining mechanismare consistent with previous work [13].3.2. Effect of eutectic Si on the microstructure evolution of castAI-Si alloy during SMAT

The cross-sectional optical micrographs of the as-castA356 alloybefore and after 60min SMAT process are shown in Fig. 5. The as-cast alloy shown in Fig. 5(a) contains primary AI(Si) solid solutiondendrites and AI-Si eutectic structure. After 60 min SMAT, fromFig. 5(b) one can see that the eutectic Siwithin the top surface layer(~150 J-Lmthick) of the SMATed A356 sample were smashed intosmall particles and the primary Al dendrites also disappeared. Suchsmall Siparticles will play an important role in the grain refinementof Al matrix within the surface layer during SMAT process. Fig. 6shows the variation of the nanocrystalline grain size with the SMATduration. The smallest grains produced by SMAT are within therange of 15-40 nm, which is much smaller than the nanocrystallinegrains obtained in pure Al (150 nm). These results are attributed tothe secondary Si phase in Al alloys.3.2.1. Microstructure evolution of the SMATed A356 alloy

As shown in Fig. 6, after SMAT for 30 or 60 min, the smallestgrains can be obtained in the A356 alloy. But, the SMAT affectedzone after 60min SMAT is thicker. Thus, the samples SMATed for60 min will be used to examine the microstructure evaluation inTEM.3.2.1.1. 100-150 p.m below the SMATed surface. Fig. 7 shows thetypical TEM micrographs within the region that is 100-150 J-Lmbelow the top surface of the A356 sample SMATed for 60 min. Thereis no typical eutectic structure can be observed. In addition to therefinement of Al matrix into sub-micron grains as evidenced by theSAED patterns, eutectic Si has been broken into micron particles.Although the deformation mechanism of Al matrix is similar to thatin pure AI, the hard Si particles significantly affect the dislocation

307

activities, which is similar to the behaviors of spheroidal cementitein a high carbon steel [16]. First, due to the strain incompatibilitybetween Al and Si, stress concentration takes place at AI/Si inter-faces. As a result, dislocations are generated from such interfaces.Arrow "A" in Fig. 7(a) and (b) points the generation of disloca-tions from the AI/Si interfaces. In addition, the broken Si particlescan also act as barriers to dislocation movement, leading to twoconsequences. One is the multiplication of dislocations. When a dis-location segment is pinned down by two Si particles, a Frank-Readsource forms and generates more dislocations. Such Frank-Readsources are pointed by Arrow Bin Fig. 7(a ) and (b). Development ofsmaller DCs separated by DDWs and DTs in between the Si particlesis another phenomenon in AI-Si alloys after SMAT.As shown in Fig. 7(c), the size of the DCs is about 500nm, which

is smaller than those in pure Al (~2 J-Lm).These DCs not only sub-divide the original Al grains into refined blocks but also have largermisorientation as evidenced by the discontinuous circular SAEDpattern in Fig. 7(c). This implies that subgrains have been producedat this depth in the AI-Si alloy. In pure AI, subgrains normally format higher strain region.

3.2.1.2. 50-100 tun below the SMATed surface. Fig. S shows the typ-ical TEM micrographs taken from the region that is 50-100 J-Lmbelow the top SMATed surface of the A356 alloy after 60 min SMAT.With increased strain within this layer, the eutectic Si i s further bro-ken down to smaller particles with shorter inter-particle distances,and more dislocations are generated. As a result, smaller DCs formin between the Si particles and more dislocations are trapped inthe DDWs or DTs that separate the DCs. The interaction of largenumber of dislocations with DDWs and DTs results in the conver-sion of DDWs into low angle subgrain boundaries, even high anglegrain boundaries, and then forming subgrains. Hence, at this depth,the subgrain size is even smaller (around 300 nm), and the misori-entation is larger. The discrete rings in the selected area electrondiffraction (SAED)patterns in Fig.S indicate that the misorientationbetween adjacent subgrains has become significant.

3.2.1.3. 30-50 tun below the SMATed surface. Within this region,higher strains are achieved. Both the Al matrix and the Si are fur-ther refined as shown in Fig. 9. The high stress leads to rising ofthe local stress concentration at the AI/Si interfaces. This leads tofurther breakage of the Si particles into sub-micron scale, as shownin Fig. 9(a). Between the refined Si particles, the Al grains are fur-ther refined to ~200 nm through formation of smaller DCs and highdensity dislocation walls and tangles. However, the refined Si par-ticles are not evenly distributed in the Al matrix. Thus, lamellarmicrobands are observed within the regions where the distancesbetween Siparticles are larger, as can be seen in Fig. 9(b). Comparedwith pure Al sample, where microbands are found in the layer thatis ~300 J-Lmbelow the surface, microbands form at higher strain inthe AI-Si alloy. In addition, the lamellar spacing is ~300-400 nm,which is smaller than that in the pure Al (~700-S00 nm). Thisdifference in micro bands formation and morphology in pure Aland AI-Si alloy is attributed to the strengthening effect of refinedSi particles in the AI-Si alloy. As microbands result from locallattice reorientation due to severe plastic deformation [lS]. theformation of micro bands is associated with large strains. Sincepure Al is ductile and soft, plastic deformation that is sufficientto produce microbands can be easily achieved in deeper regionsof the SMATed sample. But, the AI-Si alloy is much harder withlower ductility compared with the pure AI, the plastic deformationrequired to form microbands can only be achieved within the regionthat is close to the top surface, where more energy is absorbedduring SMAT. Furthermore, because the refined Si particles pro-vide additional precipitation-like strengthening, they will limit the

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Fig. 4. TEM micrographs of the outermost layer showing nanometer scaled grains. (a) Nanometer scaled equiaxed grains within the outermost surface layer and thecorresponding SAEDpattern; (b) nanocrystalline grains with well-defined subgrain boundaries and SAEDpattern.

Fig.5. Optical micrographs on the cross sections ofthe as-cast A356 before SMAT(a) and A356 SMATedfor 60min (b),

deformation. Hence, microbands can only be observed in regionswhere there are fewer Si particles.

patterns showing the nanocrystalline structure. The average Algrain size at this depth is around ~ 20-50 nm, which is much smallerthan that in the pure Al sample (~150 nm). This result is also con-sistent with the grain size determined using XRD as shown in Fig.6 .The boundaries of nanometer scaled grains are visible and welldefined. Inset is the ring-l ike SAEDpattern indicating highly disori-ented boundaries (Fig. 1O(a) and (b)). In addition, nanometer scaledSi particles is also visible as shown in Fig. 11.

3.2.1.4. The topmost SMATed surface. During SMAT, the strain andstrain rate are drastically increased in the topmost surface layer.Both the Si particles and the Al matrix have been refined downto nanometer scales within the layer that is 10 J-Lmbelow the topsurface. Fig. 10 is TEM micrographs and the corresponding SAED

50- --A356Ec 40 i1.1NC ;;C0 i i j 30_ ,e(1.1ClC G,_(1.1 20 - 1 - - - - - - - - 1< C'0(1.1. . . . . . ..!!= 10u'iij(J

0 0 10 20 30 40 50 60 70Treatment Duration (mins)

Fig.6. The variation ofthe average grain sizewithin the outermost layers, which was calculated from the XRDpatterns, with different SMATdurations.

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Fig.7. Typical TEMmicrographs taken from the region that is 100-150 urn below the top surface ofthe A356 alloy after SMATfor 60min. (a) Micrometer scaled subgrainswith small misorientation and generation ofdislocations from the AljSiinterface within Almatrix; (b) multiplication ofdislocations; (c) DDWsbetween Siparticles.

3.2.2. Effect of SMAT duration on the microstructure in thetopmost layer of the A356 alloyAs mentioned in Section 3.1, in pure Al the grains within the

topmost surface layer cannot be further refined into smaller grainsonce the SMAT duration is greater than 5 min. Longer time SMATprocess only results in larger amount of plastic deformation. In fact,after 30 min SMAT the 10 mm thick pure Al plate is fully curved.Previous SMAT work [13] on wrought 7075 alloy also concludedthat the nanocrystalline sub-grains cannot be indefinitely refinedat a given amount of deformation. However, in the A356 alloy, therefined Si particles not only arrest the dislocations, promoting their

accumulation and the formation of DDWs and DTs, but also con-tribute to generation and multiplication of dislocations. Hence, asshown in Fig. 6 the nanometre scaled grain size in the topmost layerof the A356 alloy decreases with the SMAT duration. Fig. 11 showsthe typical TEMmicrographs of the topmost layers of the A356 alloyafter SMATfor various times. After 5 min SMAT,the nanocrystallinegrain size is around 100 nm, as shown in Fig. 11(a). This is slightlysmaller than the grain size of the pure Al after the same time treat-ment. Further examination of the microstructure reveals that thebroken eutectic Si particles are within micro-meter scale, as indi-cated in Fig. 11 (b), and have less effect on the grain refinement

Fig.8. Typical TEMmicrographs taken from the region that is 50-100 urn below the top surface ofSMATedA356 alloy showing sub-micron scaled subgrains. (a) Subgrainswith small misorientation divided by different dislocation configuration and boundaries; (b) subgrains divided by subgrain boundaries or high angle grain boundaries andSAEDfromAI.

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Fig. 9. Typical TEMmicrographs taken from the regions that are 30-50 urn below the SMATed surface ofA356 sample showing (a) sub-micron Siparticles; (b) lamellarmicrobands and SAEDpattern.

process. Increasing the SMAT time to 15min, the Si particles arebroken into sub-micron and even nanometre scaled size. Such smallhard part icles have significantly effect on the dislocation activities,including generation, manipulation and suppressing their move-ment. As a result, the grains of Al matrix are further subdividedinto smaller ones close to 20-50 nm in the topmost layer. The TEMmicrograph and the SAED pattern in Fig. 11(c) demonstrate thenanocrystalline structure. However, nanocrystalline grains are sta-bilized and continuous SMAT does not further refine the grains. Asshown in Fig. 11(d), after 30min SMAT, the grain size at the samedepth from the topmost surface is still around 20-50 nm. When theSMAT duration is over 15 min, longer time treatment only causesmore plastic deformation of the specimen plate without furthergrain refinement. Furthermore, once the nanocrystalline structurein the topmost layer of SMATed samples is stabilized, the depth ofthe nanocrystalline layer keeps unchanging, even though the totalSMAT affected zone may be enlarged.The microstructure change after SMAT leads to variation of the

microhardness with the depth from the top surface. As shown inFig. 12, for all SMAT durations, the micro hardness of the nanocrys-talline layer (the topmost layer) is 60-80% higher than that ofthe substrate. Within the SMAT affected zone, the microhard-ness increases with the SMAT duration varying from 5 min to30 min. But, longer time SMATto 60 min does not further increasesthe hardness. This micro hardness variation is consistent withthe microstructure change during SMAT process. The increase inmicrohardness within the SMAT affected zone generally can be

considered as the result of two hardening mechanisms. One isthe grain refinement hardening and another is work hardening.Although it is difficult to actually distinguish the contributions ofthese two hardening effects from each other, the similar hardnessvalues after 30 min SMAT and after 60 min SMAT suggest that thegrain refinement hardening effect is greater than that of work hard-ening. This can be understood in terms of the fact that there is nofurther grain refining when SMAT time increases from 30min to60 min, but work hardening should increase. Further research workon the relationship between the mechanical property and SMAT isbeing undertaken.3.3. Discussion on the grain refinement of Al matrix in the A356alloy during SMATAs stated above, under the currently used SMAT conditions, Al

grains in both pure Al metal and the AI-Si alloy cannot be indef-initely refined. At a particular SMAT time, the nanostructure isstabilized and the grain size reaches the smallest. The stabilizednanocrystalline grain size in the pure Al is 150 nm obtained at5 min SMAT. In the AI-Si alloy, it is 20-50nm achieved at 15 minSMAT. Longer time treatment only leads to larger amount of plas-tic deformation rather than grain refinement. The difference in thestabilized nanocrystalline grain size obtained in pure Al and in theAI-Si alloy evidences the effect of Si on the grain refinement pro-cess. In the present work, as-cast A356 alloy was used. The castmicrostructure consists of primary Al dendrites, which is AI(Si)

Fig. 10. TEMmicrographs taken from the topmost surface ofthe SMATedA356 sample: (a) nanometer grained microstructure and the corresponding SAEDpattern; (b)subgrains with well-defined subgrain boundaries and SAEDpattern.

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Fig. 11. Typical TEMmicrographs observed in the topmost surface layer of A356 samples after different SMATdurations. (a) SMATfor 5min; (b) Siparticle in the topmostsurface layer after 5min SMAT;(c) SMATfor 15min; (d) SMATfor 30min.

solid solution and AI-Si eutectic structure. Thus, the grain refine-ment of the A356 alloy during SMAT will be affected by both theeutectic structure and the Si in the Al solid solution, even thoughthe solid solubilityofSi inAI is very low. Previous work [13] and thepresent results indicate that nanocrystallization of Al in both pureAl metal and in AI-Si alloy occurs through formation of Des sepa-rated by DDWs and DTs within the original coarse grains. Formation

of DDWs and DTs relies on dislocation movement and dislocationinteraction with each other and with other defects. In pure AI, t heoriginal coarse grains are relatively "clean". Dislocation movementin such grains has less barriers and obstacles. At beginning of theSMAT, a large fraction of energy or momentum generated fromthe repeated impactions of balls has been consumed to promotethe movement of dislocations, which leads to plastic deformation,

14 0

13 0

12 01 ' : i0>;;. 11 0

U)U)III 10 0c"t:!. . .I'lls: 900. . .0~ 80

70

600

A3560 SMAT 5 rninse SMAT 15minsl', SMAT 30 minse SMAT60 mins

40 0 800 1200 1600Distance from surface D (IJm)

Fig. 12. Variation ofmicrohardness with depth from the top surface ofthe SMATedA356.

2000

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including the formation of microbands. Formation of DDWs andDTs, and then Des only occurs when multiple slip systems areactivated at higher strain through interactions among dislocationsgliding on different slip systems. Thus, the size of the Des formedis relatively large. However, in the Si-containing Al alloy, the Sisolute atoms inAI suppress the dislocation movement through solidsolution strengthening. Thus, it promotes the formation of DDWsand DTs and therefore leads to formation of smaller Des, and con-tributes to smaller grain size after SMAT.Furthermore, the eutecticSi has preference to be broken up into small particles, which actas effective obstacles for dislocation movement in Al matrix, ini-tially within the eutectic Al regions. As shown in Fig. 5(b), withincrease in plastic deformation and reduction of grain size withinthe surface layer, the broken small Si particles are trapped into theregions where were primary Al dendrites. As a result, the effect ofeutectic Si on the grain refinement process takes place within theentire Almatrix. Such Siparticles significantly reduce the velocity of

dislocation movement, further promoting the formation of DDWsand DTs and decreasing the size of Des. Longer time SMAT con-tinues reducing the size of the Si particles, and therefore shortensthe distance between the particles. In addition, these small par-ticles not only act as barriers for dislocation movement, but alsopromote dislocation generation and multiplication as stated above.Thus, the Des formed in the AI-Si alloy are much smaller. Hence,the nanocrystalline Al grains in the Si-containing Al alloy is muchsmaller than those in the pure AI.As a hypothesis, the present authors consider that the stabi-lization of nanocrystalline grains during SMAT is related to strain

rate. In SMAT process, the energy generated from the repeatedimpaction does the following work: (1) converting into heat, whichcan be released and raises the temperature of the samples; (2) caus-ing plastic deformation of the samples; (3) generating defects inthe sample; and (4) refining the grains in the surface layer of thesample and the energy is converted into grain boundary energy.

Po nt 2

Fig. 13. TEMmicrographs and EDXpattern taken from the region that is 50-100 urn below the top surface of the A356 alloy after SMATfor 60 min. (a) Twinning in a Siparticle; (b) EDXpatterns from Aland Siparticles; (c) HRTEMimages of the deformed Siparticle.

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In Al alloys, plastic deformation is a result of dislocation slippingout of the sample surface or to the grain boundaries. If t he disloca-tion movement in the grain is suppressed or arrested by interfaces,other dislocations and small particles, DCs form and eventually thegrain is refined into smaller ones. If t he dislocation moves out thesurface or to the grain boundaries, the grains and then the sam-ple is deformed. Hence, whether the refined grain can be furthersub-divided by DCs depends on whether DDWs and DTs can stillform. During SMATprocess, after the grains are small enough, suchas 150 nm in pure Al and 20-50 nm in the A356 alloy, a dislocationonly travels a shorter distance to the grain boundary in a shortertime, which causes deformation of the grain. Thus, there are less"opportunities" for the dislocations to interact and then to formDDWs and DTs. As a result, no grain refining occurs. Because thetime for a dislocation to slip through a grain relies on the dis-location velocity that is related to the strain rate by the Orowanequation (" y = pbli, where y is the shear strain rate, p is dislocationdensity, b is the Burgers vector, Ii is the average dislocation velocity)[19]. the distance that the a dislocation moves without suppress-ing is associated with the strain rate of the sample. At the currentSMAT condition, for a given metal, when the grains are refinedinto a particular size, there are no obstacles suppressing disloca-tion movement within the grains and the dislocations have justsufficient time to move to the grain boundaries before interactingwith other dislocations. Longer time SMAT can only lead to furtherplast ic deformation of the sample. At this stage the nanocrystallinegrains are stabilized. Iffurther grain refinement is required, the dis-location velocity should be reduced. This can be achieved throughincreasing the external straining rate applied to the sample so thatmore dislocations are generated to promote dislocation interaction.Furthermore, dislocation velocity can also be significantly reducedby secondary particles that suppress the dislocation movement,like the broken Si particles in the A356 alloy. Hence, the stabilizednanocrystalline grains in the A356 alloy are smaller than that in thepure AI.3.4. Refinement of Siparticles

During the surface nanocrystallization through SMAT, boththe eutectic Si and the Al matrix are refined. Unlike the refine-ment of Al grains by means of dislocation activities, the brittle Siphase is refined through direct breakage. Refined Si particles canbe observed at the region that is around 100 J-Lmbelow the topsurface of a SMATed AI-Si alloy (as shown in Fig. 5). At the begin-ning of the SMAT process, coarse eutectic Si can be easily brokendown into small pieces by the impacts. As refinement of Al grainsprogresses with increase of strain, the Al matrix is substantiallywork-hardened. Consequently, stress concentrations at the AI/Siinterfaces are built up. When the local stress concentration exceedsthe critical shear strength of Si, twinning occurs within the Si par-ticles. Fig. 13(a) shows deformation twins in a Si particle. Due tothe high brittleness of Si, the twinning readily leads to fracture ofa bigger Si particle into smaller ones. EDX analysis illustrated inFig. 13(b) indicates that particle 1 is Al and particle 2 is a Si par-ticle containing deformation twins. Refined Al grains with sizesof 50-100 nm are found surrounding this Si particle. Fig. 13( c ) isa high resolution image showing the deformation of a Si particleby twinning. Further increase in the stress concentration betweenAI/Si interfaces allows fracture of the Si particle to take place.

Based on our experimental results, there are no obvious evi-dences showing that the Si particles re-dissolve into Al subjectto severe plastic deformation. Not like the cementite in steel,results from Lu's group [16] indicated that the detectable amountof cementite is reduced in the region close to the topmost surfaceof a SMATed steel sample. The area fraction of cementite is ~16%in the deep matrix, but it reduces to ~ 7% at a depth of 60 J-Lm.

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Two possible mechanisms can be attributed to the dissolution ofcementite. One is that moving dislocations may trap Catoms whilecutting cementite during plastic deformation. Another one is thatthe dissolution takes place by carbon diffusion through B /a inter-face driven by the Gibbs- Thomson effect. However, inAI-Si system,due to the low solid solubility of Si in Al and the big atom size ofSi, the substitutional solutes cannot be arrested by dislocations inthe Al matrix. Furthermore, as substitutional solute, the diffusion ofSi in Al matrix is also quite hard, particularly at low temperatures.Hence, it is unlikely for Si to re-dissolve into Al during SMAT.4. Conclusions1. Surface mechanical attrition treatment effectively results in sur-face nanocrystallization of both cast A356 alloy and AA196 pureAI.The smallest grains produced within the topmost surface lay-ers are 150nm in the pure Al and 20 ~50nm in the A356 alloy.The nanocrystalline surface shows 80% increase in micro hard-ness.

2. Nanocrystallization of Al matrix in both pure Al and A356 alloyoccurs through formation of DCs separated by DDWs and DTswithin the original coarse grains or subdivided subgrains. But therefining of the eutectic Si in A356 alloy occurs through twiningthat results in direct breakage of the particles.3. During SMAT process, the eutectic Si has preference to bebroken up into small particles distributed within the originalgrains. Those Si particles not only act as obstacles for dislocationmovement, but also promote dislocation generation and multi-plication, and significantly contribute to further refining of theAl matrix. Therefore, the nanocrystalline grains in the A356 alloyare much smaller than those in the pure AI.

4. Under the present SMAT conditions, for a particular alloy thegrains within the top nanocrystalline region cannot be refinedindefinitely. There is a minimum stabilized grain size withnanometre scale. Once this minimum grain size is achieved, fur-ther SMAT will not lead to further grain refinement, but merelyincrease the plastic deformation. The value of the stabilized min-imum grain size not only depends on the properties and originalmicrostructure of the alloys, but also relies on the strain rateapplied to the sample.

AcknowledgementsThe authors are very thankful to the Australian Research Council

(ARC) for funding support. Acknowledgement is also to Dr. DongQiu for assistance in TEM experiments and to Ms. Lihui Zheng forhelp in anodizing of Al samples.References[1] Z.B.Wang, N.R Tao, S.Li,W. Wang, G.Liu,j. Lu, K. Lu,Materials Science andEngineering A352 (1-2) (2003) 144-149.[2] j.W. Tian, j.C Villegas, w. Yuan, D. Fielden, L.Shaw, P.K.Liaw, D.L.Klarstrom,

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