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

Materials Science and Engineering A 445446 (2007) 16 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 48?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 strengthatroomtemperatureandelevatedtemperature5.How- ever, its strength at room temperature and elevated temperature Corresponding author. Tel.: +86 731 8830257; fax: +86 731 8830257. E-mail address: htzhouweb- (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 (WMg3ZnY2) and (IMg3Zn6Y) 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 445446 (2007) 16 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 themoltenalloyreaches780C,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 680C, the molten metal is poured into ingots with size of 90mm. The ingots are solutionzed at 420C for 18h. They are extruded into long rods of 20mm at 390C, 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 445446 (2007) 163 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 ofabout463.3CforalloyBand485.7CforalloyC,whilethe second peak appeared at 617.5C for B alloy and 615C 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 temperatureovertheeutectictemperatureofMgZnbinaryalloy (340C)10,15.ResultsoftheDTAanalysisofthisexperiment further suggest that the eutectic temperature of MgZnZr 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 390C. 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% proofofstressstronglydependsonthegrainsizeinMgalloy16 and obey the role of HallPetch relationship y=0+Kd1/2, 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 445446 (2007) 16 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 agreementwithexperimentobservations17,18.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/interfaceisrelativelystrong12sothatIphase 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 445446 (2007) 165 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 MgZnZr alloys, especially that the I phase exhibit obviously a strong strengthen effect 10. According to the well known HallPetch relation, the yield strength depends on the grain size as follows 16: ?0.2= Kd1/2(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, Mg6Zn0.5Zr2Nd and Mg6Zn0.5Zr2Nd1.5Y 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 390C: (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. / Materials Science and Engineering A 445446 (2007) 16 Fig. 7. The tensile properties of the extruded alloys at 390C: A alloy, B alloy and C alloy. temperature. In contrast, the elongation of