Effect of Nd and Y on the microstructure and mechanical.pdf

Materials Science and Engineering A 445 446 2007 1 6 Effect of Nd and Y on the microstructure and mechanical properties of ZK60 alloy H T Zhoua Z D Zhanga C M Liua Q W Wangb aSchool of Materials Science and Engineering Central South University Changsha 410083 PR China bSchool of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200030 PR China Received 26 July 2005 received in revised form 29 March 2006 accepted 12 April 2006 Abstract The effect of neodium Nd and yttrium Y on the microstructure and tensile properties of ZK60 alloy are investigated Experimental results show that an addition of neodium and yttrium both brings about precipitation of a new Mg41Nd5and Mg3Zn6 Y I phases and refi ne the as cast grains After hot extrusion the alloy added with Nd and the alloy with Nd and Y are greatly refi ned through dynamic recrystallization by means of the pining effect of particles or precipitates As a result very fi ner grains with size of 4 8 m are obtained in the alloy with Nd and Y However the grainsizeofthealloywithNdisrelativelylarge ThissuggestedthatthecombinationofNdandYadditionhasagreateffectongrainrefi ningduring dynamic recrystallization and leads to either the increase of both the melting temperatures of the eutectic phases and the melting temperature of the 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 of both 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 properties 1 Introduction Mg alloys are the lightest structural alloy and hence they are likely to be applicable to many structural parts in auto motive and aero industries due to high specifi c strength high specifi c stiffness and good damping capacity 1 2 However strength of most current Mg alloys cannot meet the strength requirements of general structures because of some undesirable properties Therefore applications of Mg alloys as structural parts are still very limited In order to overcome these draw backs and widen the application fi elds of Mg alloys researchers are trying any kinds of methods It has been demonstrated that mechanical properties of Mg alloys are signifi cantly improved by grain refi nement through adding rare earth metals RE and hot working 3 4 As well known ZK60 alloy has highest mechanical properties among all the Mg alloys such as high strengthatroomtemperatureandelevatedtemperature 5 How ever its strength at room temperature and elevated temperature Corresponding author Tel 86 731 8830257 fax 86 731 8830257 E mail address htzhou web H T Zhou stillislowcomparedtoaluminumalloy Fromthispointofview many researchers devote their efforts to improve its mechanical properties Recently it is reported that extruded Mg alloys con taining RE exhibit excellent mechanical properties 6 7 For instance Ma et al studied on extruded ZK60 RE alloys and suggested that hot extrusion could improve tensile properties of ZK60 RE 8 Singh and Tsai 9 and Zhang et al 10 studied the effect of Y on microstructure and mechanical properties of ZK60 alloy They point out that Y enhances the yield strength and elevated temperature strength by forming new phases of W Mg3ZnY2 and I Mg3Zn6Y which have high hardiness thermal stability high corrosion resistance low coeffi cient of friction lowinterfacialenergy etc 11 12 Subsequently these newphasescaneffectivelyobstructtheslipofdislocationduring hot deformation Although the mechanical properties of ZK60 alloy could be improved by an addition of Y the expected prop erties is not reachable Therefore in this study we initialize this article to study the effects of Nd addition and combination addition of Nd and Y on microstructure and tensile properties of ZK60 alloy Furthermore the relationship between the ten silepropertiesandmicrostructureisinvestigatedinhotextruded alloys 0921 5093 see front matter 2006 Published by Elsevier B V doi 10 1016 j msea 2006 04 028 2H T Zhou et al Materials Science and Engineering A 445 446 2007 1 6 Table 1 Chemical composition Composition wt MgZnZrYNd Alloy ABulk5 540 54 Alloy BBulk5 530 552 13 Alloy CBulk5 560 531 542 14 2 Experimental procedures The chemical compositions of the studied alloys are listed in Table1 Thealloyswaspreparedinfurnaceunderprotectionofa mixed gas atmosphere of SF6 1vol and CO2 BAL When themoltenalloyreaches780 C itispurredforabout300s After purring the molten alloy is hold for 15min to allow inclusions to settle to the bottom of the crucible Then the metal is poured into a medium furnace At 680 C the molten metal is poured into ingots with size of 90mm The ingots are solutionzed at 420 C for 18h They are extruded into long rods of 20mm at 390 C respectively with an extrusion ratio of 20 1 Tensile specimensof5mmdiameterand66mmlengtharemachinedout fromtheseextrudedrods Thesizeoftensiletestingspecimensis 10mm wide and 66mm long The microstructure of specimens are analyzed by a light microscopy OM LEICA MEF4M and phase analysis is performed by means of a D MAS IIIA X ray diffractometer XRD All the specimens are etched with 4 HNO3solution in alcohol 3 Results 3 1 Microstructure of as cast ZK60 alloys Fig 1 shows microstructures of as cast A B and C alloys respectively It can be seen from Fig 1a that A ZK60 alloy is composed of primary Mg matrix and eutectic Mg2Zn3 phase The phase precipitates as discontinuous network pri marily at grain boundaries When there is Nd addition named as B alloy more second phase precipitated as shown in Fig 1b Meanwhile NdandYareaddedtogethertoZK60alloycalledC alloy it seems to be that much more compounds appear and the size of the compounds is smaller than that of A and B alloys as shown in Fig 1c Consequently different grain sizes can be found among A B and C alloys in the sequence of 90 60 and 40 m respectively Therefore it could be concluded that Nd and Y have an effect on refi nement of ZK60 alloy This is constant with the result of Luo 13 Fig 2 shows SEM microstructure images of B and C alloys It is found that there are some cluster compounds at triple grain boundaries as seen A of Fig 2a EDAX analysis indicates that its chemical composition formula is Mg41Nd5 nMg nNd 1 25 0 14 10 8 1 2 ThisisconformedbyXRDseen in Fig 3 When Nd and Y are added together into ZK60 alloy much more cluster compounds appear at triple grain boundaries in which there are some paralleled laths They are identifi ed by XRD for C alloy It can be seen that there exists Mg41Nd5 Fig 1 Microstructure in as cast a A alloy b B alloy and c C alloy phase and I phase Mg3Zn6Y further identifying C alloy has I phase Mg3Zn6Y icosahedral quasicrystal structure except Mg41Nd5 The formation of cluster compounds can be ascribed to the increase of total amount of Nd and Y 14 However W phase Mg3Zn3Y2 andZphase Mg12ZnY cannotbefoundby XRD and EDAX analysis in this experiment Clearly yttrium addition brought about the formation of I phase in alloy C and surpassed the formation of W phase Mg3Zn3Y2 and Z phase Mg12ZnY H T Zhou et al Materials Science and Engineering A 445 446 2007 1 63 Fig 2 SEM images a B alloy and b C alloy Fig 4 shows the map distribution of B and C alloys for Nd and Y It is found that Nd and Y exist both in grain boundaries and in matrix However in some area the content of Nd or Y is very high As seen from Fig 4a and c it suggests that the secondary phases are likely to contain more Nd or yttrium than the matrix This may be used to explain the phenomenon that certainamountofsecondaryphasesexistsinalloysBandC This further conforms the results are agreement with XRD results Fig 5 shows the DTA analysis results of alloys B and C It is foundthatthefi rstendothermicpeakappearedatthetemperature ofabout463 3 CforalloyBand485 7 CforalloyC whilethe second peak appeared at 617 5 C for B alloy and 615 C for C alloy The fi rst peaks can be thought as the melting temperatures of the eutectic phases and the second peaks can be thought as the melting temperature of the alloys solution temperature of alloy We can conclude that the combined addition of Nd and Y to ZK60 alloy greatly increases the eutectic temperature This is agreement with yttrium can greatly increase the eutectic temperatureovertheeutectictemperatureofMg Znbinaryalloy 340 C 10 15 ResultsoftheDTAanalysisofthisexperiment further suggest that the eutectic temperature of Mg Zn Zr alloy increases with increasing total content of Nd and Y 3 2 Microstructure evolution of the hot extruded alloys Fig 6showsopticalmicrostructuresofalloysA BandCthat were extruded at 390 C It is found that all the three alloys have occurred dynamic recrystallization DRX However the grain size and the amount distribution of second phase are different In hot extruded alloys A as shown in Fig 6a there is no second phaseonthematrix ThesizeofDRXgrainislargecomparedto thatofalloysBandC Thereseemsalittlegrowthofgrainseven atthistemperature InthealloysBandC DRXgrainsizeisvery small and DRX also completed Full details on matrix can be foundwithcharacteristicsofsomesecondphases Thegrainsize of alloy C is the smallest among the three alloys This suggests that the combined addition of Nd and Y plays an important role during process of dynamic recrystallization On the other hand DRX grains of alloy C with an average size of about 4 m are very fi ne and uniform This may relate to the fact that the pinning effects of both broken secondary phase particles and fi ne precipitates can suppress the growth of DRX grains It can be concluded that grain refi nement by dynamic recrystallization is very effective even at this temperature in ZK60 alloy 3 3 Mechanical properties of extruded alloys Fig 7 shows the mechanical properties of the three extruded alloys at 390 C As shown in the fi gures the ultimate 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 B alloy 0 2 316 2MPa b 373 2MPa 8 C alloy 0 2 376 2MPa b 389 0MPa 6 Clearly the 0 2 proofofstressstronglydependsonthegrainsizeinMgalloy 16 and obey the role of Hall Petch relationship y 0 Kd 1 2 where yis the yield stress 0the lattice friction stress related to move individual dislocation K a constant and d is the grain size Thus this can explain why the tensile properties of alloys B and C are higher than that of A ZK60 alloy In addition the increase of ultimate tensile strength and yield strength of alloys B and C may be related to the second phase strengthening 4 Discussion Alloys A B and C have different microstructure both in as cast and in extruded state Subsequently it brings to different tensile 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 thenewphasesappearasMg41Nd5in alloy B and Mg41Nd5 Mg3Zn6 Y in alloy C This is identifi ed by XRD and SEM During solidifi cation process the peritectic reactionshouldoccurfi rst Owingtothenoequilibriumdistribu tion the solute atoms of Zn and RE are pushed to the front of the liquid solidinterfaceformedalongthegrainboundarieswhilein the interior of the grain only the Zr rich zone is present This is verifi ed by Fig 4 From Fig 1 we conclude that the grain refi n 4H T Zhou et al Materials Science and Engineering A 445 446 2007 1 6 Fig 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 good agreementwithexperimentobservations 17 18 Secondly asin extruded state Mg41Nd5in alloy B and Mg41Nd5 Mg3Zn6Y in alloy C are destroyed and broken into small particles Dur ing hot extrusion many fi ne particles homogeneously distribute across the matrix These thermally stable second phases with a relativelyhighmetingpointcanpingrainboundariesandimpede grain growth during hot deformation especially I phase Due to the low interfacial energy of I phase matrix interface the bond ingattheIphase interfaceisrelativelystrong 12 sothatIphase Fig 4 Map distribution a Nd alloy B b Nd alloy C and c Y alloy C and precipitates were relatively diffi cult to be moved during hot deformation Thirdly concentrated strain in the vicinity of sec ond phases can introduce sites of high dislocation density and largeorientationgradient particledeformationzone Suchsites are ideal for nucleation of recrystallized grains It is known that aparticledeformationzonemayextendtoadistanceofevenone diameterfromthesurfaceoftheparticlesandmaybedisoriented bytensofdegreesfromtheadjacentmatrix Inthesedeformation H T Zhou et al Materials Science and Engineering A 445 446 2007 1 65 Fig 5 DTA trace of as cast a alloy B and b alloy C zones secondparticlescanstimulatenucleationofrecrystallized grains 19 20 Thus nucleation of recrystallization can be pro moted by Nd and Y addition in ZK60 alloy through forming second phases In addition the second phases can hinder grain growth during recrstallization 20 As a result alloy C exhibits very fi ner grains This is attributed to much more disperse fi ner particles than alloys A and B Therefore the strength of alloys B and C is much higher This suggests that second phase except the effect of grain refi ning has a strong strengthening effect on the strength of Mg Zn Zr alloys especially that the I phase exhibit obviously a strong strengthen effect 10 According to the well known Hall Petch relation the yield strength depends on the grain size as follows 16 0 2 Kd 1 2 1 where 0 2 is the increase in yield stress due to grain refi ne ment Kaconstantanddisthegrainsize So grainrefi nementby the DRX process has an infl uence on alloys B and C are higher than that of ZK60 alloy 5 Summary The microstructure and mechanical properties of ZK60 Mg 6Zn 0 5Zr 2Nd and Mg 6Zn 0 5Zr 2Nd 1 5Y alloys are studied in this article Some neodium and yttrium brings about precipitation of a new Mg41Nd5and Mg3Zn6Y I phases and refi ne the as cast grains with an addition of Nd and Y The alloy added with Nd and the alloy with Nd and Y are refi ned through dynamic recrystallization by means of the pining effect of par ticles or precipitates This suggested that the combination of Fig 6 Optical microstructures extruded at 390 C a A alloy b B alloy and c C alloy Nd and Y addition has a great effect on grain refi ning during dynamic recrystallization and leads to either the increase of both the melting temperatures of the eutectic phases and the melting temperature of the alloys or the increase of the yield strength and tensile strength of the alloy with Nd and Y at room 6H T Zhou et al Mater