MicrostructuralrefinementandhomogenizationofMg–SiCnanocomposites by cyclicextrusioncompression

Microstructural refinement and homogenization of Mg–SiC nanocompositesby cyclic extrusion compressionWei Guo a,b,Qudong Wang a,b,n,Bing Ye a,b,Xiaochun Li c,Xintao Liu a,Hao Zhou aa National Engineering Research Center of Light Alloy Net Forming,Shanghai Jiao Tong University,Shanghai200240,Chinab State Key Laboratory of Metal Matrix Composites,Shanghai Jiao Tong University,Shanghai200240,Chinac Department of Mechanical Engineering,University of Wisconsin-Madison,Madison,WI53706,USAa r t i c l e i n f oArticle history:Received10April2012Received in revised form21June2012Accepted28June2012Available online3July2012Keywords:NanocompositeMagnesiumCyclic extrusion compression(CEC)HomogenizationGrain refinementa b s t r a c tAn as-extruded Mg–1wt%SiC nanocomposite was processed by cyclic extrusion compression(CEC)at3501C.The homogeneity of grain and SiC nanoparticle($50nm average diameter)distribution duringthe processing was investigated.With the increasing number of CEC passes,afiner grain size and moreuniform particle distribution are obtained along with significant improvement in hardness.The matrixgrain size is reduced remarkably from$27.6m m to$6.5m m after8passes of CEC.Nanoparticledeclustering occurs through a mechanism of kneading caused by the intense turbulentflow of the Mgmatrix during CEC,and the SiC nanoparticles are dispersed into the original particle-free regions.Theproperty improvement is mainly attributed to Orowan strengthening and the Hall–Petch effect.&2012Elsevier B.V.All rights reserved.1.IntroductionMagnesium matrix composites are widely investigatedbecause of their attractive high specific properties,especially instrength and stiffness.Among various Mg-based composites,particle reinforced Mg matrix composites have the advantagesof lower cost,easier fabrication,and better machinability ascompared tofiber reinforced Mg matrix composites.For most ofthe particulate Mg-based composites,the sizes of reinforcingparticles are typically spanning from several microns to tens ofmicrons.Addition of micron-sized particulates,such as Ti parti-cles[1]or SiC particles[2–4],into Mg improves hardness,yieldstrength,wear resistance,as well as thermal stability.However,the improvement usually occurs at the cost of ductility or tensilestrength reduction.Recent studies have indicated that the use ofnano-sized reinforcements such as Al[5],Al2O3[6],SiC[7,8],Y2O3[9],and ZrO2[10],leads to a simultaneous increase in bothstrength and ductility of Mg composites.Nanoparticle reinforce-ments significantly enhance strength of the matrix by moreeffectively promoting particle hardening mechanisms thanmicron-sized particles.Afine and homogeneous dispersion ofnanoparticles provides a better balance between the reinforce-ment and interparticle spacing effects to maximize the yieldstrength and creep resistance[11].However,it is extremely difficult to distribute and dispersenanoscale particles uniformly in metal melts because of their highspecific area and the poor wettability of nanoparticles.The occur-rence of agglomeration and clusters[12]is detrimental to mechanicalproperties.In order to minimize these disadvantages,conventionalsecondary deformation processing methods such as hot extrusionhave been employed to improve the homogeneity of the nanoparticledistribution[13,14].However,these processing techniques are stilldifficult or impossible in the case of ultrafine particles,since veryhigh strains would be required[15].Severe plastic deformation(SPD)has been recognized as a promising method for fabricating bulkultrafine-grained materials,having very small grain sizes typicallyless than1m m and exhibiting high yield strength and hardness withfairly good ductility[16–18].Due to its unique shear deformationmode and extremely high accumulated strains,SPD of metal matrixcomposites is particularly attractive.After light metal matrix com-posites,such as Al–Si and Mg–Al–Mg2Si,were processed by cyclicextrusion compression(CEC),the Si or Mg2Si phases were refinedand distributed uniformly in the matrix[19–21].Therefore,it isanticipated that the SPD process may be a good candidate forproducing nanocomposites with a homogeneous dispersion of nano-particles.Nevertheless,few studies have been carried out on SPD ofnanocomposites.Goussous et al.mixed carbon nanoparticles withpure Al particles and consolidated them using equal channel angularpressing(ECAP)to produce Al–C nanocomposites[22].Asadi et al.added nanosized SiC and Al2O3particles to an as-cast AZ91magne-sium alloy,and produced surface nanocomposite layers via frictionstir processing(FSP)[23].However,the distribution of SiC and Al2O3particles was not uniform,and there were still clusters.Contents lists available at SciVerse ScienceDirectjournal homepage:/locate/mseaMaterials Science&Engineering A0921-5093/$-see front matter&2012Elsevier B.V.All rights reserved./10.1016/j.msea.2012.06.086n Corresponding author at:National Engineering Research Center of Light AlloyNet Forming,Shanghai Jiao Tong University,Shanghai200240,China.Tel.:þ862154742715;fax:þ862134202794.E-mail address:wangqudong@(Q.Wang).Materials Science&Engineering A556(2012)267–270Accordingly,this investigation aimed to study the effect of CEC on the homogeneity of nanoparticle distribution in a Mg–SiC composite.The influence of CEC on grain refinement and hardness was also investigated.2.Experimental proceduresThe material used in the present study was a Mg–1wt%SiC(i.e. Mg–0.54vol%SiC)nanocomposite fabricated by ultrasonic cavita-tion based solidification processing[24],and the average size of the SiC particles was$50nm.The ingots were extruded with an extrusion ratio of25:1.Both the as-extruded sample and the CEC die coated with a lubricant of graphite powder were held for10 min at1501C before heating to3501C for about30min.The CEC processing cycle was carried out by putting the sample into the upper chamber[25].After2,5and8passes of CEC processing,the corresponding equivalent strains were2.4,7.3and12.2,respec-tively[25].After processing,longitudinal sections of the specimen were sectioned for microstructure analyses by optical microscopy (OM),field emission scanning electron microscopy(FESEM),and transmission electron microscopy(TEM).The hardness tests were performed using a Vickers indenter under a test load of49N and a dwell time of15s.The uniformity of the nanoparticle distribution in the composite was studied by the quadrat method[26].The images to be studied were divided into a grid of square cells and the number of nanoparticles in each cell(N q)was counted. Twenty images at a magnification of100,000Âwere taken by the FESEM on each specimen.Each image was divided into54 quadrats and the size of each quadrat was set to a¼300nm.3.Results and discussionFig.1shows the evolution of the grain structure for the Mg–1wt%SiC nanocomposite after CEC processing from the as-extruded condition.The initial average grain size for the as-extruded condition is$27.6m m as shown in Fig.1a.After 2passes it is apparent that the microstructure consists of coarse grains surrounded by muchfiner grains but with the coarse grains occupying a significantly larger area fraction.The mean grain size is reduced to$14.2m m.After5passes(Fig.1c)there is again a mix of coarse andfine grains but the area fraction offiner grains less than8m m now becomes significant,and some coarse grains exhibit distorting with no refinement.Further deformation to 8passes makes the grain structure more homogeneous with approximately allfine grains with an average size of$6.5m m. The decreasing grain size with increasing deformation is consis-tent with the results obtained on pure Mg[27],AZ31alloy[28], and a Mg–Y2O3nanocomposite[13].Due to the relative high temperature of3501C(T$0.7T m)in the CEC condition,dynamic recrystallization could occur,resulting in the observed grain refinement.On the other hand,the SiC nanoparticles possess the ability of nucleating Mg grains during recrystallization,and they also restrict the growth of the recrystallized Mg grains due to the pinning effect[29].Fig.2illustrates the improvement of the nanoparticle distribu-tion after8CEC passes.In the as-extruded condition,diffused clusters elongated in the extrusion direction are observed (Fig.2a).The clusters have a size of up to500nm in the direction perpendicular to the extrusion rge nanoparticle free zones around the clusters are also observed.It is clearly seen that the boundaries between the nanoparticle clusters and the particle-free matrix are sharp.After8passes of CEC,the SiC nanoparticles have debonded from the cluster and moved into the particle-free matrix.Thus,the large particle clusters dissociate and the particle distribution appears homogeneous(Fig.2b). However,a few dense particle clusters remain visible in the longitudinal section,ascribing to the high surface energy associated with the nanoparticles.It should be noted that CEC does not degrade the microstructure of the investigated nano-composite.No cavity formation is observed inside the specimen. The TEM study also reveals the uniform distribution of SiC nanoparticles(Fig.2c)and good interfacial integrity between SiC nanoparticles and the Mg matrix(Fig.2d)after8CEC passes.The frequency histograms of the number of SiC nanoparticles per quadrat,Nq,for the as-extruded and8passesCEC-processed Fig.1.Micrographs showing grain morphology of the Mg–1wt%SiC nanocomposite:(a)as-extruded,and after(b)2,(c)5,and(d)8passes of CEC processing at3501C.W.Guo et al./Materials Science&Engineering A556(2012)267–270268condition are plotted in Fig.3.The histogram for the as-extruded condition indicates a very inhomogeneous particle distribution (Fig.3a).After 8CEC passes,the frequencies of empty quadrats and quadrats containing 6or more nanoparticles decrease remarkably,and the frequencies of quadrats containing 1,2,or 3nanoparticles increase significantly.The improvement in the distribution of SiC nanoparticles can be attributed to intense turbulent flow of the nanocomposite matrix during CEC.Mutual intersection between a -Mg and SiC nanoparticles is active in the deformation exerted by CEC.The repetitive kneading effect may easily intersperse the clusters and redistribute SiC nanopar-ticles.It is expected that any two neighbor regions in the sample could intermix with each other after enough CEC passes.Thus,the SiC nanoparticles are dispersed into the original particle-free regions in the matrix.In other words,SiC nanoparticle clusters are sheared open into much smaller clusters or single nanopar-ticles,which are further ‘‘engulfed’’by the flowing matrix during CEC.Therefore,microstructural studies conducted on the specimens after CEC show fairly uniform distribution of SiC nanoparticles with a very limited amount of clustering in the matrix.The results of the hardness measurement reveals that the hardness of the Mg–1wt%SiC nanocomposite increases with increasing strains (see Fig.4).These results are consistent withthe findings reported in the literature [13].The hardness shows a sharp rise when the pass number increases from 0to 2,but rises only slightly when the pass number increases from 2to 8.The enhanced hardness can be attributed primarily to (a)Orowan strengthening and (b)grain size strengthening.After 8passes,the contribution to yield strength from Orowan strengthening canbeFig.2.Micrographs showing the distribution of SiC nanoparticles in the Mg–1wt%SiC nanocomposite:(a)SEM image after extrusion only,(b)SEM image after 8CEC passes,and (c and d)TEM images after 8CECpasses.Fig.3.The distribution of nanoparticles in each cell from the quadrat analysis of the Mg–1wt%SiC nanocomposite:(a)as-extruded and (b)after 8passes ofCEC.Fig.4.Effect of CEC pass number on hardness of Mg–1wt%SiC nanocomposite.W.Guo et al./Materials Science &Engineering A 556(2012)267–270269expressed as [30]:D s Orowan ¼ð0:4MGb =p l ln d r =b ÀÁ=ð1Àn Mg Þ1=2ð1Þwhere M (¼6.5)is the mean orientation (Taylor)factor for Mg,G (¼17.3GPa)is the shear modulus,b (¼0.321nm)is the magni-tude of the burgers vector [30],d r (¼50nm)is the diameter of SiC particles,n Mg (¼0.35)is Poisson’s ratio,and l is the mean inter-particle distance given by [15]:l ¼d r ½ðp =6V f Þ1=3À1ð2Þwhere V f (¼0.54%)is volume fraction of SiC particles.After 8passes,Hall–Petch strengthening due to grain size is given by [30]:D s Hall-Petch ¼k y d Mg À0:5ð3Þwhere k y (¼280MPa m m 1/2)is the Hall–Petch coefficient,and d Mg is the average Mg grain size.Therefore,the comparison of Orowan strengthening and grain size strengthening after 8passes can be described as D s Orowan /D s Hall–Petch ¼1.46.4.ConclusionsThe effect of severe deformation by CEC on the grain structure and nanoparticle distribution is investigated for a Mg–1wt%SiC nanocomposite.The results are summarized as follows:(1)CEC significantly improves the uniformity of the SiC nanoparti-cle distribution.After 8CEC passes,the large particle clusters resolve and only a few much smaller clusters are visible.(2)The nanoparticle declustering occurs through a mechanism ofkneading caused by the intense turbulent flow of matrix during CEC.The micro-clusters are thus dispersed,and the nanoparticles move into the particle-free matrix.(3)As the number of CEC passes increases,a more homogeneousgrain structure with finer mean grain size is obtained.(4)Hardness increases with increasing pass number.The incre-ment attributes to the microstructural homogenization in terms of grain size and nanoparticle distribution.AcknowledgmentsThis study was supported by the National Natural Science Foundation of China (NSFC)under Grant nos.50674067and51074106,National Key Technology R&D Program of China under Grant no.2011BAE22B01-5,and the Science and Technology Com-mission of Shanghai Municipality under Grant no.09JC1408200.References[1]S.F.Hassan,M.Gupta,J.Alloys Compd.345(2002)246–251.[2]M.K.Surappa,R.A.Saravanan,Mater.Sci.Eng.A 276(2000)108–116.[3]T.S.Srivatsan,S.Seshan,M.Jayamathy,S.V.Kailas,Mater.Sci.Eng.A 363(2003)345–351.[4]C.Y.H.Lim,S.C.Lim,M.Gupta,Mater.Sci.Forum 437–438(2003)447–450.[5]M.Gupta,X.L.Zhong,W.L.E.Wong,Acta Mater.55(2007)6338–6344.[6]M.Gupta,M.Paramsothy,S.F.Hassan,N.Srikanth,J.Mater.Sci.44(2009)4860–4873.[7]G.Cao,J.Kobliska,H.Konishi,X.Li,Metall.Mater.Trans.A 39A (2008)880–886.[8]M.De Cicco,H.Konishi,G.P.Cao,H.S.Choi,L.S.Turng,J.H.Perepezko,S.Kou,kes,X.C.Li,Metall.Mater.Trans.A 40(2009)3038–3045.[9]C.S.Goh,J.Wei,L.C.Lee,A.Gupta,Acta Mater.55(2007)5115–5121.[10]S.F.Hassan,M.Gupta,J.Mater.Sci.41(2006)2229–2236.[11]X.Li,G.Cao,H.Konishi,Mater.Sci.Eng.A 486(2008)357–362.[12]D.V.Dudina,Y.S.Kwon,M.A.Korchagin,O.I.Lomovsky,J.Mater.Sci.39(2004)5325–5331.[13]M.Gupta,K.S.Tun,J.Mater.Sci.43(2008)4503–4511.[14]K.B.Nie,X.J.Wang,L.Xu,K.Wu,X.S.Hu,M.Y.Zheng.,J.Alloys Compd.512(2012)355.[15]M.J.Tan,X.Zhang,Mater.Sci.Eng.A 244(1998)80–85.[16]Q.D.Wang,Y.J.Chen,H.J.Roven,M.P.Liu,M.Karlsen,Y.D.Yu,J.Hjelen,Scr.Mater.58(2008)311–314.[17]Q.D.Wang,T.Peng,J.B.Lin,M.P.Liu,H.J.Roven,Mater.Sci.Eng.A 528(2011)1143–1148.[18]Y.J.Chen,Q.D.Wang,H.J.Roven,M.Karlsen,Y.D.Yu,M.P.Liu,J.Hjelen,J.Alloys Compd.462(2008)192–200.[19]J.W.Yeh,S.Y.Yuan,C.H.Peng,Mater.Sci.Eng.A 252(1998)212–221.[20]J.W.Yeh,S.Y.Yuan,C.H.Peng,Metall.Mater.Trans.A 30(1999)2503–2512.[21]Z.M.Zhang,C.J.Xu,X.F.Guo,S.Z.Jia,Acta Metall.Sin.21(2008)169–177.[22]S.Goussous,W.Xu,X.Wu,K.Xia,Compos.Sci.Technol.69(2009)1997–2001.[23]P.Asadi,G.Faraji,A.Masoumi,M.K.Besharati Givi,Metall.Mater.Trans.A42A (2011)2820–2832.[24]X.C.Li,H.Choi,N.Alba-Baena,S.Nimityongskul,M.Jones,T.Wood,M.Sahoo,kes,S.D.Kou,J.Mater.Sci.46(2011)2991–2997.[25]Y.J.Chen,Q.D.Wang,J.B.Lin,L.J.Zhang,C.Q.Zhai,J.Mater.Sci.42(2007)7601–7603.[26]P.A.Karnezis,G.Durrant,B.Cantor,Mater.Charact.40(1998)97–109.[27]W.M.Gan,M.Y.Zheng,H.Chang,X.J.Wang,X.G.Qiao,K.Wu,B.Schwebke,H.G.Brokmeier,J.Alloys Compd.470(2009)256–262.[28]H.K.Kim,W.J.Kim,Mater.Sci.Eng.A 385(2004)300–308.[29]A.Gupta,S.F.Hassan,Mater.Sci.Eng.A 392(2005)163–168.[30]B.Q.Han,D.C.Dunand,Mater.Sci.Eng.A 277(2000)297–304.W.Guo et al./Materials Science &Engineering A 556(2012)267–270270。