Deformation mechanism at impact test of Al- 1 1 % Si alloy processed by.pdf

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Trans. Nonferrous Met. SOC.China 17(2007) 104-109

Transactions ofNonferrous MetalsSociety of China~~www.csu.edu.cn/ysxb/

Deformation mechanism at impact test of Al-11% Si alloy processed byequal-channel angular pressing with rotary dieMAAi-bin(gg%$) ' , Y. NISHIDA2, JIANG Jing-hua(&@%)', N. SAIT0 2, I. SHIGEMATSU2,A. WATAZU2

1. Department o f Materials Science and En gineering, Hoh ai University, Nanjing 2 10098 , China;2. National Institute of Advanced Industrial Science and Technology, Shimoshidami, Moriyama-ku,

Nagoya 463-8560, JapanReceived 29 May 2006; accepted 15 September 2006

Abstract: Al-ll%Si (mass fraction) alloy was transformed into a ductile material by equal-channel angular pressing (ECAP) with arotary die. Two mechanisms at impact test, slip deformation by dislocation motion and grain boundary sliding, were discussed. Theultrafine grains with modified grain boundaries and the high content of fine particles (< pm) were necessary for attaining highabsorbed energy. The results contradict the condition of slip deformation by dislocation motion and coincide with that of grainboundary sliding. Many fine zigzag lines like a mosaic were observed on the side surface of th e tested specimens. These observedlines may show grain boundaries appeared by the sliding of grains.Ke y words: aluminum alloys; impact test; equal-channel angular pressing(ECAP); microstructures; deformation mechanism; grainboundary sliding

Corresponding author: MA Ai-bin; TeliFax: +86-25-83787239; E-mail: [email protected];[email protected]

1 IntroductionA1-Si eutectic alloys are important for industry,

especially for automobile industries. However, the lowfracture toughness of these alloys limits broaderindustrial app lication. The microstructures of these alloysconsist of eutectic silicon crystals and alum inum crystals.The silicon crystals have three-dimensionally complexshapes and are very brittle. The low fracture toughnessoriginates in the microstructures of these alloys. Toimprove the microstructure, some technique, for example,addition of an element like sodium, stibium, strontium,are used in industry. The improvement of toughness,however, is limited and it is believed that the brittleproperty of those alloys is inherent. To improve themicrostructures of alloys, there are several routes such asrapid solidification, heat treatment, plastic deformation.Among them, plastic deformation is an energy efficientprocess. One of plastic deformation processes is theequal-channel angular pressing(ECAP) that has beenproposed recently and has become very popular, becausewe can give unlimited strain to bulk materials withoutany dimension al changes[ 1-51. To improve the

efficiency of the ECAP, we have developed a new ECAPprocess that uses a rotary die, namely rotary-dieequal-channel angular pressing (RD-ECAP) [6-71, bywhich the billet does not need to be removed from thedie and re-inserted into the die for the next pass. Wereported that we could attain the high toughness of anAl-11% Si alloy by this RD-ECAP[X]. Th e mechanism ofthe high toughness, however, has not yet been m ade veryclear. The purpose of the present work is to discuss thedeformation mechanism at Charpy impact test of theAl-1 1%Si alloy processed by R D-ECAP.2 Experimental

The Al-ll%Si (mass fraction) alloy contains11.3%Si-1.13%Mg-1.00%Cu-1.10%Ni and 0.277%Fe asimpurities (designated as Al-11 %Si alloy). The details ofthe RD-ECAP have been described elsewhere[6]. TheRD-ECAP die and the holder were held in an electricfurnace and the processes were carried out inside thefurnace. The billet size of samples for RD-ECAP was19.5 mm in diameter and 40 mm in length. The billetswere pressed by RD-ECAP for 16 and 32 passes at62 3 K and 673 K. Impact toughness test specimens were


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M A Ai-bin. et aliTrans. Nonferrous Met. Soc. China 17(2007) I05made from RD-ECAP processed billets by machiningalong the longitudinal direction. The size of the testspecimens was 3 mmxJ mm in cross-section and 34 mmin length, with a U-notch of 1.5 mm in width and 1.5 mmin depth. Six test specimens were made from one billet.A computer-aided instrumented Charpy impact testsystem was used for measuring thc impact toughness ofthe samples at room temperature. Un-pressed samples ofthe alloy were also tested for comparison. Opticalmicroscopy was used to observe the microstructures. AJEOL 201 0 transmission electron microscope(TEM) anda JEOL JSM-5200 scanning electron microscope ( SE M )were employed for observation of grain boundaries andthe polished side surface of impact test specimen near tothe fractured surface.3 Resu lts and discussion

The microstructures of un-pressed and RD-ECAPprocessed samples observed by optical microscopy areshown in Fig.1. The microstructure of the un-pressedsample, as shown in Fi g.l (a ), mainly consists ofdendritic aluminum primary crystals (wh ite crysta l) andeutectic structure (plate let silicon and alum inum ).Samples o f Figs.1 (b) and (c )were prepared by 16 passesand 32 passes at 623 K. respectively. By pressing, siliconcrystals are broken into fine particles and disperse in thealuminum matrix homogeneously. However. many largesilicon particles of 1-5 pm remain. Fig.2 shows the TEMmicrostructure of aluminum part in the Al-l 1% Si alloyprocessed by RD-ECAP for 32 passes at 623 K. Thegrain or grain fragment size is estimated to be about 300nm by the microstructure.

The absorbed energies by Charpy impact testing ofthe AI-lI%Si alloy ha\.e been reported in our previouspaper[8]. The absorbed energies increase with theincrease in number of passes and finally reach 10 Jicm'after 32 passes at 623 K. which is 10 times higher thanthe value. 0.9 J:cm'. of the un-pressed alloy.Appearances of test specimens are shown in Fig.3. Thesetest sp ecimens of Fi gs.3(a) and ( b ) are un-pressed andpressed for 32 passes at 623 K. respectively. The testspecimens of the un-pressed sample fracture in a brittlemanner. The test specimens without notch, however,deform plastically and do not fracture as shown inFig.3(b ). The un-pressed sa mples of this alloy are veryhrittle. However, the ahcorhcd cnrrgics of thc spccimcnsprocessed by RD-ECAP are very high. Most of energiesare absorbed for plastic deformation of specim ens duringimpact testing as shown in Fig.3. Therefore, it is ofimportance to reveal the mechanism of the plasticdeformation. The microstructure of' this alloy consists ofp r i m a r y a l u m i n u m c r y s t a l s . e u t e c t ic s i l i c o n a n d

Fig.1 Microstructures of Al- l l%Si alloy observed by opticalmicroscopy: (a ) Un-pressed; (b) 16 times pressed at 623 K ; (c)32 times presscd at 623 K

F i g 2 T E M micrograph of aluminum alloy matrix in Al- 1 1%Sialloy processed by RD-ECAP for 32 times at 62 3 K


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106 MA Ai-bin, et al/Trans.Nonferrous Met. SOC . hina 17(2007)

F i g 3 Appearance of test specimens: (a) Un-pressed;(b ) Pressed for 32 times at 623 K (These test specimens withouta notch deform plastically and do not fracture completely)aluminum crystals, other intermetallic compounds,CuAl2, Mg2 Si, etc, and needle-likc iron compounds(FeAI3) as impurity. However, aluminum crystals an dsilicon eutectic crystals should play important roles inthe plastic deformation. By RD-ECAP, the shape ofsilicon crystals simply changes from the networks ofplatelet eutectic silicon crystals into simple particles. Thetest specimens of Charpy impact testing are plasticallydeformed greatly, though the volume fraction of siliconcrystals remains unchanged even after RD-ECAPprocessing and is equal to the volume fraction of theun-pressed sample of this alloy. It is observed by theoptical microscopy that when un-pressed samplesfracture, fracture cracks advance along eutectic siliconcrystal networks[9]. Since silicon crystals are brittle,many silicon crystals are broken during RD-ECAPprocessing and usually several voids are formed inside oraround silicon crystals[9-10]. It is expected that ifseveral voids are present in or around silicon particles,these voids w ill provide paths for fracture cracks and thespecimens will fracture easily. The experimental results,however, are opposite. The absorbed energy of Charpyimpact testing increases with increasing number ofRD-ECAP passes.

We can regard this RD-ECAP processed alloy as an11% silicon particle dispersed aluminum composite. If analuminum alloy contains about 1094 (volume fraction)

ceramic particles, the absorbed energy by Charp y impacttesting of the alloy generally decreases to about 115 thatof matrix alloy[ll]. The low fracture toughness andductility o f those composites are attributed to the limiteddislocation motion. Coarse Si crystals (1-5 pm indiameter) are dispersed in RD-ECAP processed samples,as shown in Figs.l(b) and (c). From thesemicrostructures, it is apparent that dislocation movementis extremely restricted and dislocation pile-up will becreated around particles. Th erefore, large strain could notbe obtained by intragranular dislocation motion.

In our previous paper[8], we discussed theimprovement of the impact tough ness o f the Al-1 1%Sialloy and obtained experimentally the following threeimportant results.

1) The improved impact toughness is related to: (a)the breakage of the large aluminum dendrites andinterdendritic networks of eutectic silicon in the firstseveral passes, (b) the modified grain or grain fragmentboundaries, (c) ultrafine grains or grain fragments, (d)the content of fine silicon particles (smaller than 1 pm)and (e) homogenized m icrostructure resulting frommultipass RD-ECAP.

2) The average size of the grains or grain fragmentsis about 300 nm after RD-ECAP. With an increasednumber of RD-ECAP passes. beyond about four, thegrain or grain fragment size docs not clearly decrease.

3) Modified grain boundaries. The results for EBSD(electron backscatter diffraction) show that the fractionsof high angle grain boundaries with 8> 15" are 65% and73% for the samples processed by RD-ECAP at 623 Kfor 8 and 16 passes, respectively.

The contribution of the breakage of large aluminumdendrites and interdendritic nctworks of eutectic siliconto the improvement of impact toughness has almostcompleted in the first several passes (about four passes).Therefore, the increase of the impact toughness beyondfour passes is based on other mechanisms. The absorbedenergy of the sample processed for 32 passes is threetimes higher than the value of the sample processed forfour passes. The results o f 2 ) mentioned above showsthat the average size of the grains or grain fragments isnot the important reason for the increase of the impacttoughness beyond four passes.

The average strain rate at Charpy impact testing isknown as about lo3 s-I. According to the conventionalclassification of deforma tion mech anism[ 121, thedeformation at Charpy impact testing should be the slipdeformation by dislocation motion. To make clear themechanisms, we performed other experiments and havereported in our previous paper[8]. A sample wasprocessed with 16 passes at 623 K by RD-ECAP andthen heat-treated at 793 K for 2 h followed by water


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M A Ai-bin, et alrrrans. Nonferrous Met. Soc. China 17(2007) 107quenching (solution treatment). B y this experiment,aluminum grain growth occurs and the a verage grain sizeincreases from 30 0 nm to about 8 pm. In addition. finesilicon particles (smaller than 1 pml disappear and onlyrough silicon particles (about 5 pm in diameter) remain.The absorbed energy of this sample falls from 7.2 Jicm'to 4 J/cm' by the solution treatment. though the samplewith larger grain and no fine silicon particles should begood for dislocation movement. Because grainboundaries and fine silicon particles should be barriers todislocation motion. Since the impact testing of thissolution treated sample was carried out at roomtemperature immediately after the heat treatment, theprecipitation of fine particles before impact testing canbe negligible.

As mentioned in the previous result[8], the highangle grain boundaries after RD-ECAP processingshould contribute to the plasticity of Charpy impact testspecimens, though the high angle grain boundary is abarrier to dislocation motion. Conclusively. ultrafinegrains with high angle grain boundaries and the largecontent of fine silicon particles (smaller than 1 pm) arenecessary for attaining large plastic deformation ofimpact test specimens. This conclusion contradicts thecondition of the slip deformation by dislocation motion.These experimental results remind us of anothermechanism, namely. the grain boundary sliding(GBS)and coincide with the condition of GBS. If GBS isdominant in the plastic deformation of present impacttest specimens. the ductility can be explained reasonably.GBS, however, has been observed only in superplasticdeformation[ 121 and creep at low strain rates and at hightemperatures (above 0.4T,,), where T,; is the meltingpoint. As for researches on G B S of ultrafine-grained

matcrials, computer simulation of GBS at the atomiclevel is prominent u sing mo lecular-dynamics[ 13-14] orthe Monte Carlo[lS], due to the difficulty ofexperimental techniq ue.

We tnade an attempt to observe GBS at roomtemperature in the impact test specimens of theAl- l l%Si a l loy processed by RD-ECAP. We usedimpact test sp ecimens having polished side surfaces. Theside surface images of the tested specimen observed byscanning electron microscopy are shown in Fig.4, whichwas RD-ECAP processed 32 passes at 673 K. The sidesurface images of an un-pressed specimen are alsoshown in Fig.5 for a comparison. Fig.4(a) shows theappearance of the test specimen. Figs.4(b) and (c) areimages of a near and a distant locations from the fracturesurfa ce as marked in Fig.4(a).

As sho wn in Fig.5, there are many big cracks in thespecimen near the fracture surface of the un-pressedsample. The images show that the sample is very brittle.On the contrary, many fine lines are observed in bothFigs.4(b) and (c). There are few spaces, like cracks inFig.5, between these fine lines. Aluminum crystal hasfour slip plancs and 12 slip systems. If these lines are sliplines by dislocation movement, usually several parallellines should appear and they will be more straight lines.In addition, each line is composed of several f iner l ines,namely, lamellae slip band. Most of the observed lines,however, are very fine single zigzag lines and notstraight lines. These lines are closed ones like a mosaic.Therefore, these lines may be grain boundaries. Inaddition, these fine lines are not cracks. because thefracture surfacc of this sample shows a typical ductilecharacteristic with m any sma ll dimples as show n in Fig.6.Th i s r e su lt m a y show tha t a s e a c h g r a in ha s d i f f e r e n t

Fig.4 SE M images of side surface of samples aficr room tempcrature impact test, showing grain boundaries appearing on side surfaceof impact test specimen processed b> KD-ECAP for 32 t imes at 67 3 K: (a ) Appearance of impact test specimen; (b ) Place nearfracture surface: ( c , Distant place from fracture surface


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108 M A Ai-bin, et aliTrans. Nonferrous Met. Soc. China 17(2007)

0 0.1 mm 0.2 nim 0.3 mm 0.4 mmFig.5 SEM images showing some big cracks on side surface only appeared near fracture surface of impact test specimen ofun-pressed sample

Figd Typical ductile fracture surface of Al-1l%Si alloyprocessed by RD-ECAP at 67 3 K for 32 timesmigration distance in the perpendicular direction to theside surface, the slipped grain boundaries appear clearlyand we can observe the fine grain boundaries. In thelocation near to the fracture surface, as shown in Fig.4(b),most of fine zigzag lines observed clearly are parallel tothe strain direction. Several small spaces between theselines are also observed in the sam e image. As most o f thespaces are parallel to the strain direction, these spacesmay be formed during itnpact deformation whenaccommodation for stress relief at grain boundaries doesnot work well. Many fine lines parallel to the straindirection are also observed in Fig.4(c). There are,however, few small spaces between lines. This resultmay show that accommodation works well in this region,because strain is not so large in this region as in theregion o f Fig.4(b).

In this RD-ECAP processed sample, coarse Sicrystals coexist with fine aluminum grains. Since thegrain boundary diffusion of Si during impactdeformation will be negligible. aluminum atoms andgrains should play an important role in the grainboundary accommodation. The average aluminum grainsize (matrix) of the RD-ECAP processed sample is about

30 0 nm, as shown in Fig.2. This aluminum grain size ismuch smaller than grains observed in Fig.4. This mayimply that several grains move cooperatively making agroup and the relative sliding distances between them arevery small, which was predicted by simulation innanocry stalline structure[ 12.1. There fore, clearlyobserved fine lines appear to be the grain boundariesslipped for long distances.

One of typical low toughness materials is metalmatrix composite. The low toughness limits its industrialapplication. The absorbed energies of the un-pressedsample used for this work are as low as those ofcomposite materials. The value, however, increasesgreatly with the increase in n umber of RD-ECAP passes.4 Conclusions

The slip deformation by dislocation motion and thegrain boundary sliding were discussed as the d eformationmechanism of Charpy impact test specimen of anAl-1 1%Si alloy pressed for 32 passes b y RD-ECAP andit was tried to observe the defiirmation phenomena. Theultrafine grain with high angle grain boundaries and thehigh content of fine silicon particles (smaller than 1 pm)are necessary for attaining high absorbed energy atCharpy impact testing. These experimental resultscontradict the condition of the slip deformation bydislocation motion and coincide with that of grainboundary sliding. Many fine zigzag lines like a mosaicare observed on the side surface of the Charpy impacttested specimens of the alloy pressed for 32 passes byRD-ECAP. These observed lines may show grainbound aries appeared by the sliding o f grains.References[ I ] SEGAL V M. Materials processing by simple shear [J]. Mater SciEn g A, 1995, 197: 157-164.


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MA Ai-bin, et al/Trans. Nonferrous Met. SOC. hina 17(2007) 109LANGDON T G. FURUKAWA M. NEMOTO M, HORITA Z.Using equal-channel angular pressing for refining grain size [J]. JOM.2000.52: 30-33.STOICA G 31. L1.4u' P K. Progress in equal-channel angularprocessing [J]. JOM. 2001. 53: 36-40.LIU Z. LIANG G. WANG WANG E. WANG Z. Effect ofequal-channel angular pressing on structure of Al alloy 2024 [J].Trans Nonferrous Met SO CChina, 1997. 7(2): 160-162.WANG L Z. WANG J T. GUO C. CHEN J D. Observation ofmacroscopic shear band in aluminum-based alloy dunngequal-channel angular prcssing [J]. Trans Nonferrous Met SOCChina,2004. 14(5) : 957-960.KISHIDA Y . ARlMA H. KIM J C, A N D 0 T. Rotaly-dieequal-channel angular pressing of an Al-7mass%Mg alloy [J].Scripta Mater, 200 . 45: 26 1-266.MA A B. NISHIDA Y. SUZUKI K. SHIGEMATUS I, SAITO N.Characteristics of plastic deformation by rotary-die equal-channelangular pressing [J]. Scripta Mater, 2005. 5 2: 433-437.MA A B. SUZUKI K. NISHIDA Y. SA ITO N. SHIGEM ATSU I,TAKAGI M, WATA H. WATAZU A. IMURA T. mpact toughnessof an ultrafine-grained Al- ImassO/aSi lloy processed by rotary-dieequal-channel angular pressing [J]. Acta Mater. 2005, 53 : 21 1-220.MA A B. SAITO N. TAKAGI M. NISHIDA Y . SUZUKI K,SHIGEMATSU I. IWAT.4 H. WATAZU A . Effect of severe plastic

deformation on tensile properties of a cast Al-1 Imass%Si alloy [J].Mater Sci Eng A, 2005, 395: 70-76.MA A B, SUZUKI K, SAITO N, NISHIDA Y, TAKAGI M,SHIGEMATSU I, IWATA H. Impact toughnesa of an ingothypereutectic Al-23mass%Si alloy improved by rotary-dieequal-channel angular pressing [J]. Mater Sci Ehg A, 2005, 399:181-189.KOBAYASHI T, NIINOMI M. Fracture toughness and fatiguecharacteristics of aluminium casting alloy [J]. J Japan Inst LightMetals, 1991, 41: 398-405.NIEH T G, WADSWORTH J, SHERBY 0 D. Superplasticity inMetals and Ceramics [MI. Cambridge: Cambridge University Press,1997: 183.VAN SWYG ENHOVEN H, SPACZER M, FARKAS D, CA R0 A.Role of grain size and the presence of low and high angle grainboundaries in the deformation mechanism of nanophase Ni: Amolecular dynamics computer simulation [J:I. NanostructuredMaterials, 199 9, 12(1): 323-326.VAN SWYGENHOVEN H. Plastic deformation in metals withnanosized grains: Atomistic simulations and experiments [J]. MaterSci Forum. 2004,447144 8: 3-10.BA LL0 P, SLUGEN V. Grain boundary sliding and migration incoppor: the effect of vacancies [J]. Compu tational Materials Science,2005, 33(4): 491-498.

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Deformation mechanism at impact test of Al- 1 1 % Si alloy processed by.pdf