Effect of Nd and Y on the microstructure and mechanical.pdf
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Materials Science and Engineering A 445446 (2007) 16Effect of Nd and Y on the microstructure and mechanicalproperties of ZK60 alloyH.T. Zhoua, Z.D. Zhanga, C.M. Liua, Q.W. WangbaSchool of Materials Science and Engineering, Central South University, Changsha 410083, PR ChinabSchool of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR ChinaReceived 26 July 2005; received in revised form 29 March 2006; accepted 12 April 2006AbstractThe effect of neodium (Nd) and yttrium (Y) on the microstructure and tensile properties of ZK60 alloy are investigated. Experimental resultsshow that an addition of neodium and yttrium both brings about precipitation of a new Mg41Nd5and Mg3Zn6Y (I) phases and refine the as-castgrains. After hot extrusion, the alloy added with Nd and the alloy with Nd and Y are greatly refined through dynamic recrystallization by means ofthe pining effect of particles or precipitates. As a result, very finer grains with size of 48?m are obtained in the alloy with Nd and Y. However, thegrainsizeofthealloywithNdisrelativelylarge.ThissuggestedthatthecombinationofNdandYadditionhasagreateffectongrainrefiningduringdynamic recrystallization, and leads to either the increase of both the melting temperatures of the eutectic phases and the melting temperature ofthe alloys or the increase of the yield strength and tensile strength of the alloy with Nd and Y at room temperature. In contrast, the elongation ofboth ZK60 and the alloy with Nd are higher than that of the alloy with Nd and Y. 2006 Published by Elsevier B.V.Keywords: ZK60 alloy; Neodium; Yttrium; Extrusion; Tensile properties1. IntroductionMg alloys are the lightest structural alloy, and hence theyare likely to be applicable to many structural parts in auto-motive and aero industries due to high specific strength, highspecific stiffness and good damping capacity 1,2. However,strength of most current Mg alloys cannot meet the strengthrequirements of general structures because of some undesirableproperties. Therefore, applications of Mg alloys as structuralparts are still very limited. In order to overcome these draw-backs and widen the application fields of Mg alloys, researchersare trying any kinds of methods. It has been demonstrated thatmechanical properties of Mg alloys are significantly improvedby grain refinement through adding rare earth metals (RE) andhot working 3,4. As well known, ZK60 alloy has highestmechanical properties among all the Mg alloys such as highstrengthatroomtemperatureandelevatedtemperature5.How-ever, its strength at room temperature and elevated temperatureCorresponding author. Tel.: +86 731 8830257; fax: +86 731 8830257.E-mail address: (H.T. Zhou).stillislowcomparedtoaluminumalloy.Fromthispointofview,many researchers devote their efforts to improve its mechanicalproperties. Recently, it is reported that extruded Mg alloys con-taining RE exhibit excellent mechanical properties 6,7. Forinstance, Ma et al. studied on extruded ZK60-RE alloys andsuggested that hot extrusion could improve tensile properties ofZK60-RE 8. Singh and Tsai 9 and Zhang et al. 10 studiedthe effect of Y on microstructure and mechanical properties ofZK60 alloy. They point out that Y enhances the yield strengthand elevated temperature strength by forming new phases of(WMg3ZnY2) and (IMg3Zn6Y) which have high hardiness,thermal stability, high corrosion resistance, low coefficient offriction,lowinterfacialenergy,etc.11,12.Subsequently,thesenewphasescaneffectivelyobstructtheslipofdislocationduringhot deformation. Although the mechanical properties of ZK60alloy could be improved by an addition of Y, the expected prop-erties is not reachable. Therefore, in this study, we initializethis article to study the effects of Nd addition and combinationaddition of Nd and Y on microstructure and tensile propertiesof ZK60 alloy. Furthermore, the relationship between the ten-silepropertiesandmicrostructureisinvestigatedinhotextrudedalloys.0921-5093/$ see front matter 2006 Published by Elsevier B.V.doi:10.1016/j.msea.2006.04.0282H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16Table 1Chemical compositionComposition (wt.%)MgZnZrYNdAlloy ABulk5.540.54Alloy BBulk5.530.552.13Alloy CBulk5.560.531.542.142. Experimental proceduresThe chemical compositions of the studied alloys are listed inTable1.Thealloyswaspreparedinfurnaceunderprotectionofamixed gas atmosphere of SF6 (1vol.%) and CO2(BAL). Whenthemoltenalloyreaches780C,itispurredforabout300s.Afterpurring, the molten alloy is hold for 15min to allow inclusionsto settle to the bottom of the crucible. Then, the metal is pouredinto a medium furnace. At 680C, the molten metal is pouredinto ingots with size of 90mm. The ingots are solutionzed at420C for 18h. They are extruded into long rods of 20mmat 390C, respectively, with an extrusion ratio of 20:1. Tensilespecimensof5mmdiameterand66mmlengtharemachinedoutfromtheseextrudedrods.Thesizeoftensiletestingspecimensis10mm wide and 66mm long. The microstructure of specimensare analyzed by a light microscopy (OM, LEICA MEF4M), andphase analysis is performed by means of a D/MAS-IIIA X-raydiffractometer (XRD). All the specimens are etched with 4%HNO3solution in alcohol.3. Results3.1. Microstructure of as-cast ZK60 alloysFig. 1 shows microstructures of as-cast A, B and C alloys,respectively. It can be seen from Fig. 1a that A (ZK60) alloy iscomposed of primary ? (Mg) matrix and eutectic ? (Mg2Zn3)phase. The ? phase precipitates as discontinuous network pri-marily at grain boundaries. When there is Nd addition namedas B alloy, more second phase precipitated, as shown in Fig. 1b.Meanwhile,NdandYareaddedtogethertoZK60alloycalledCalloy, it seems to be that much more compounds appear, and thesize of the compounds is smaller than that of A and B alloysas shown in Fig. 1c. Consequently, different grain sizes canbe found among A, B and C alloys in the sequence of 90, 60and 40?m, respectively. Therefore, it could be concluded thatNd and Y have an effect on refinement of ZK60 alloy. This isconstant with the result of Luo 13.Fig. 2 shows SEM microstructure images of B and Calloys. It is found that there are some cluster compounds attriple grain boundaries as seen A of Fig. 2a. EDAX analysisindicates that its chemical composition formula is Mg41Nd5(nMg:nNd=1.25:0.14/10.8:1.2).ThisisconformedbyXRDseenin Fig. 3. When Nd and Y are added together into ZK60 alloy,much more cluster compounds appear at triple grain boundariesin which there are some paralleled laths. They are identifiedby XRD for C alloy. It can be seen that there exists Mg41Nd5Fig. 1. Microstructure in as-cast: (a) A alloy, (b) B alloy and (c) C alloy.phase and I phase (Mg3Zn6Y), further identifying C alloy hasI phase (Mg3Zn6Y, icosahedral quasicrystal structure) exceptMg41Nd5. The formation of cluster compounds can be ascribedto the increase of total amount of Nd and Y 14. However, Wphase(Mg3Zn3Y2)andZphase(Mg12ZnY)cannotbefoundbyXRD and EDAX analysis in this experiment. Clearly, yttriumaddition brought about the formation of I phase in alloy C, andsurpassed the formation of W phase (Mg3Zn3Y2) and Z phase(Mg12ZnY).H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 163Fig. 2. SEM images: (a) B alloy and (b) C alloy.Fig. 4 shows the map distribution of B and C alloys for Ndand Y. It is found that Nd and Y exist both in grain boundariesand in matrix. However, in some area, the content of Nd or Yis very high. As seen from Fig. 4a and c, it suggests that thesecondary phases are likely to contain more Nd or yttrium thanthe matrix. This may be used to explain the phenomenon thatcertainamountofsecondaryphasesexistsinalloysBandC.Thisfurther conforms the results are agreement with XRD results.Fig. 5 shows the DTA analysis results of alloys B and C. It isfoundthatthefirstendothermicpeakappearedatthetemperatureofabout463.3CforalloyBand485.7CforalloyC,whilethesecond peak appeared at 617.5C for B alloy and 615C for Calloy The first peaks can be thought as the melting temperaturesof the eutectic phases, and the second peaks can be thoughtas the melting temperature of the alloys (solution temperatureof alloy). We can conclude that the combined addition of Ndand Y to ZK60 alloy greatly increases the eutectic temperature.This is agreement with yttrium can greatly increase the eutectictemperatureovertheeutectictemperatureofMgZnbinaryalloy(340C)10,15.ResultsoftheDTAanalysisofthisexperimentfurther suggest that the eutectic temperature of MgZnZr alloyincreases with increasing total content of Nd and Y.3.2. Microstructure evolution of the hot extruded alloysFig.6showsopticalmicrostructuresofalloysA,BandCthatwere extruded at 390C. It is found that all the three alloys haveoccurred dynamic recrystallization (DRX). However, the grainsize and the amount distribution of second phase are different.In hot extruded alloys A as shown in Fig. 6a, there is no secondphaseonthematrix.ThesizeofDRXgrainislargecomparedtothatofalloysBandC.Thereseemsalittlegrowthofgrainsevenatthistemperature.InthealloysBandC,DRXgrainsizeisverysmall, and DRX also completed. Full details on matrix can befoundwithcharacteristicsofsomesecondphases.Thegrainsizeof alloy C is the smallest among the three alloys. This suggeststhat the combined addition of Nd and Y plays an importantrole during process of dynamic recrystallization. On the otherhand, DRX grains of alloy C with an average size of about 4?mare very fine and uniform. This may relate to the fact that thepinning effects of both broken secondary phase particles andfine precipitates can suppress the growth of DRX grains. It canbe concluded that grain refinement by dynamic recrystallizationis very effective even at this temperature in ZK60 alloy.3.3. Mechanical properties of extruded alloysFig. 7 shows the mechanical properties of the threeextruded alloys at 390C. As shown in the figures, theultimate tensile strength and yield strength of alloys A,B and C increase, while the ductility of them decreases(Aalloy:0.2=270.2MPa,b=320.5MPa,=12%;Balloy: 0.2=316.2MPa, b=373.2MPa, =8%; C alloy:0.2=376.2MPa, b=389.0MPa, =6%). Clearly, the 0.2%proofofstressstronglydependsonthegrainsizeinMgalloy16and obey the role of HallPetch relationship y=0+Kd1/2,where yis the yield stress, 0the lattice friction stress relatedto move individual dislocation, K a constant and d is the grainsize. Thus, this can explain why the tensile properties of alloysB and C are higher than that of A (ZK60) alloy. In addition, theincrease of ultimate tensile strength and yield strength of alloysB and C may be related to the second phase strengthening.4. DiscussionAlloys A, B and C have different microstructure both in as-cast and in extruded state. Subsequently, it brings to differenttensile properties. Firstly, as in as-cast state, alloy A consists of? Mg and ? (Mg2Zn3) phase. When Nd is added into A alloy,andNdwithYistogetheraddedintoAformingCalloy,inaddi-tionto?(Mg2Zn3)phase,thenewphasesappearasMg41Nd5inalloy B, and Mg41Nd5+Mg3Zn6Y in alloy C. This is identifiedby XRD and SEM. During solidification process the peritecticreactionshouldoccurfirst.Owingtothenoequilibriumdistribu-tion the solute atoms of Zn and RE are pushed to the front of theliquid/solidinterfaceformedalongthegrainboundarieswhileinthe interior of the grain only the Zr-rich zone is present. This isverified by Fig. 4. From Fig. 1, we conclude that the grain refin-4H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16Fig. 3. XRD diffraction: (a) A alloy, (b) B alloy and (c) C alloy.ing effect of Nd and Y element exists in Mg alloy. This is goodagreementwithexperimentobservations17,18.Secondly,asinextruded state, Mg41Nd5in alloy B, and Mg41Nd5+Mg3Zn6Yin alloy C are destroyed and broken into small particles. Dur-ing hot extrusion, many fine particles homogeneously distributeacross the matrix. These thermally stable second phases with arelativelyhighmetingpointcanpingrainboundariesandimpedegrain growth during hot deformation, especially I phase. Due tothe low interfacial energy of I phase/matrix interface, the bond-ingattheIphase/interfaceisrelativelystrong12sothatIphaseFig. 4. Map distribution: (a) Nd alloy B, (b) Nd alloy C and (c) Y alloy C.and precipitates were relatively difficult to be moved during hotdeformation. Thirdly, concentrated strain in the vicinity of sec-ond phases can introduce sites of high dislocation density andlargeorientationgradient(particledeformationzone).Suchsitesare ideal for nucleation of recrystallized grains. It is known thataparticledeformationzonemayextendtoadistanceofevenonediameterfromthesurfaceoftheparticlesandmaybedisorientedbytensofdegreesfromtheadjacentmatrix.InthesedeformationH.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 165Fig. 5. DTA trace of as-cast: (a) alloy B and (b) alloy C.zones,secondparticlescanstimulatenucleationofrecrystallizedgrains 19,20. Thus, nucleation of recrystallization can be pro-moted by Nd and Y addition in ZK60 alloy through formingsecond phases. In addition, the second phases can hinder graingrowth during recrstallization 20. As a result, alloy C exhibitsvery finer grains. This is attributed to much more disperse finerparticles than alloys A and B. Therefore, the strength of alloysB and C is much higher. This suggests that second phase, exceptthe effect of grain refining, has a strong strengthening effecton the strength of MgZnZr alloys, especially that the I phaseexhibit obviously a strong strengthen effect 10. According tothe well known HallPetch relation, the yield strength dependson the grain size as follows 16:?0.2= Kd1/2(1)where ?0.2is the increase in yield stress due to grain refine-ment,Kaconstantanddisthegrainsize.So,grainrefinementbythe DRX process has an influence on alloys B and C are higherthan that of ZK60 alloy.5. SummaryThe microstructure and mechanical properties of ZK60,Mg6Zn0.5Zr2Nd and Mg6Zn0.5Zr2Nd1.5Y alloys arestudied in this article. Some neodium and yttrium brings aboutprecipitation of a new Mg41Nd5and Mg3Zn6Y (I) phases andrefine the as-cast grains with an addition of Nd and Y. The alloyadded with Nd and the alloy with Nd and Y are refined throughdynamic recrystallization by means of the pining effect of par-ticles or precipitates. This suggested that the combination ofFig. 6. Optical microstructures extruded at 390C: (a) A alloy, (b) B alloy and(c) C alloy.Nd and Y addition has a great effect on grain refining duringdynamic recrystallization, and leads to either the increase ofboth the melting temperatures of the eutectic phases and themelting temperature of the alloys or the increase of the yieldstrength and tensile strength of the alloy with Nd and Y at room6H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16Fig. 7. The tensile properties of the extruded alloys at 390C: A alloy, B alloyand C alloy.temperature. In contrast, the elongation of
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