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Materials Science and Engineering A 462 (2007) 334338Influence of processing technology on phase transformations in arare-earth-containing MgZnZr alloyJitka Pelcov a, Bohumil Smola, Ivana Stul kov aCharles University, Faculty of Mathematics and Physics, Ke Karlovu 5, CZ-121 16, Prague 2, Czech RepublicReceived 30 August 2005; received in revised form 13 December 2005; accepted 15 December 2005AbstractAn investigation was carried out into the effect of annealing on the precipitation processes and stability of the microstructure in the magnesiumalloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr produced by various solidification conditions. The alloy was produced by squeeze casting and sprayforming with and without subsequent extrusion. The phase transformations were studied by means of relative electrical resistivity changes duringisochronal annealing in the temperature range from 293 to 783K. The microstructure of selected states was analyzed by transmission electronmicroscopy. 2006 Elsevier B.V. All rights reserved.Keywords: Mg alloys; Precipitation; Electrical resistivity; Microstructure; Spray forming1. IntroductionMagnesium alloys are attractive for space, aeronautical,automobile or leisure and tool applications owing to theirspecific properties, such as low density, high specific strength,good machinability and availability. The use of low-costMg-base alloys is limited owing to their moderate mechanicaland creep properties at elevated temperatures. These couldbe improved by the use of modern processing technologies(composites, rapidly solidified alloys, nano-particle reinforcedalloys, etc.) or by using non-traditional alloying elements, suchas rare-earths (RE).Spray forming, as an example of the rapid solidificationprocessing technique, is one possibility of reducing the cost ofmanufactured components by reducing of the number of pro-ductionstepsnecessarytogetherwiththeadvantageofenhancedproperties associated with microstructural refinement and sup-pressed macro-segregation 1. This technique was used in thestudy of the alloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr. Zincis often used as an alloying element in commercial Mg alloys.DuringdecompositionofsupersaturatedsolidsolutionofbinaryMgZn alloys GuinierPreston (GP) zones and metastableMgZn, MgZn2, Mg2Zn3precipitates are observed. ZirconiumCorresponding author. Tel.: +420 2 21911372; fax: +420 2 21911618.E-mail address: pelcova.jemail.cz (J. Pelcov a).is added to refine the grain size and to participate in the devel-opment of phases leading to a higher ratio of proof stress totensile strength and increase the creep resistance. Most MgZralloys contain RE elements, such as cerium, neodymium andlanthanum, which form eutectic systems with magnesium andimprovethecastabilityowingtotheformationofgrainboundarynetworks of relatively low melting point eutectics. Continuousalloy development has led to major improvements in roomtemperature mechanical properties of MgZr based alloys. Thesignificant improvement in high-temperature properties enablesthemostrecentalloystobeusedupto573Kcomparedto423Kfor earlier MgZr alloys 2. MgZnZr alloys (known as ZKalloys) are widely used commercially for their high-strength,good plasticity and corrosion-resistance 3,4. A strengtheningeffect of RE on wrought MgZnZrRE, due to the formationof RE-containing particles, which suppresses dynamic recrys-tallisation during extrusion was found by Luo et al. 5. Theaddition of 3wt.% Nd to the MgZnZr alloy can effectivelyimprove the yield strength and ultimate tensile strength of thealloy at higher temperatures as a result of grain refinement andthe formation of the Mg12Nd phase 6. In MgZnRE(Zr)alloys, if the cooling rates are high enough, quasicrystals canbe formed. The ZnMgY icosahedral quasicrystal phasewith a five-fold symmetry was first reported by Luo et al.7 in a high-strength magnesium alloy containing Zn and Y.Following on from this, Niikura et al. 8 and Tsai et al. 9 havesynthesized a family of icosahedral quasicrystals with RE=Y,0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2005.12.110J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338335Table 1Composition of the alloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr (nominal composition)AlloyZn wt.%REawt.%Zr wt.%Mg wt.%Squeeze cast and extruded material4.190.980.5894.25Spray formed material3.411.190.3895.02Spray formed and extruded material3.080.910.3295.69aRE consists of Nd with a small amount of Y.Nd, Sm, Dy, Gd, Tb, Ho and Er. The presence of quasicrystalsbrings an improvement in mechanical properties, such as higherhardness, higher thermal stability due to stabilization of grainboundaries, higher corrosion-resistance and ductility, etc.In the work presented in this paper, the influence of pro-cessing technology on the phase transformations in Mg-3wt.%Zn-1wt.% Nd-0.5wt.% Zr alloy was studied by means of rela-tiveresistivitychangesduringisochronalannealingupto783K.The microstructure of selected states was studied by transmis-sion electron microscopy (TEM).2. Experimental detailsA detailed investigation of the microstructural developmentand phase transformations in the course of isochronally increas-ing temperature (273783K) was carried out using electricalresistometry. The results of electrical resistometry were corre-lated with those of microhardness measurement carried out inthe same way as the resistometry measurements. An analysis ofthemicrostructurewasundertakenusingTEMonselectedstatesof the material.The alloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr was pre-pared by squeeze casting or the spray forming technique withand without subsequent extrusion. The temperature of the meltinsprayformingwas1013K,theprocessgasbeingAr+1vol.%O2. Squeeze casting was undertaken in a protective atmosphereofAr+1%SF6.Extrusionwascarriedoutat623Kwithareduc-tionof50:1afteraone-hourpreheatat573K.Thecompositionsof the alloys studied are listed in Table 1.Thechangesinrelativeresistivityduetoisochronalannealingweredeterminedintherange293783Kinstepsof30K/30min.Each annealing step was followed by quenching in liquid nitro-gen for annealing temperatures up to 513K and in water atroomtemperatureforhigherannealingtemperatures.Heattreat-ment was carried out in a stirred oil bath up to 513K and in afurnace with an argon protective atmosphere at higher temper-atures. The four contact specimens in the shape of a letter Hwere used in the resistivity measurements at 77K after eachheating step. Relative resistivity changes ?/ were obtainedto an accuracy of 104using the dc four-point method witha dummy specimen in series. The effect of parasitic thermo-Table 2Measured and calculated density of the alloys studiedAlloyMeasured densitykg/m3Calculateddensity kg/m3Squeeze cast and extruded material1828 51819Spray formed material1598 31813Spray formed and extruded material1813 51803electromotive forces was suppressed by current reversal. Thevalue of the electrical resistivity was measured also at 293Kin selected states of the material to obtain the residual resis-tivity ratio, RRR=(293K)/(77K), which does not depend onthespecimenform-factorandincreaseswithincreasingmaterialpurity.ChangesinthemicrohardnessHV0.1(Vickershardnesswith0.1kg load) were measured following the same treatment asresistivity measurements to reveal the thermal stability of themechanical properties related to the microstructural develop-ment.Transmission electron microscopy, electron diffraction (ED)and X-ray microanalysis (EDX) were used to determine thestructure and morphological characteristics of the phases pre-cipitated (using a JEOL JEM 2000FX electron microscope andaLinkAN10000microanalyzer).ThespecimensforTEMwereprepared by the same isochronal annealing procedure as thosefor electrical resistivity and hardness measurements.3. Results and discussionThe grain size was about 1?m for spray formed alloys. Thevalues of density are listed in Table 2 and compared to the cal-culated values. The high discrepancy between the measured andcalculated density of the spray formed material indicates a largeporosity content (about 12vol.%), which induces extreme brit-tleness. Table 3 summarizes the values of the RRR parameter inthe denoted states of heat treatment and Vickers microhardnessHV0.1 in the as prepared state. The lowest values of micro-hardness and the RRR parameter in the spray formed alloy alsosupport the presence of voids. The microhardness value is con-siderably higher and comparable for both extruded alloys theTable 3Values of the RRR parameter and microhardness HV0.1AlloyRRR (as prepared state)RRR (minimum of resistivity)RRR (after annealing up to 783K)HV0.1 (as prepared)Squeeze cast and extruded material3.529(3.823)633K2.48795 3Spray formed material2.6653.08452 4Spray formed and extruded material3.120(3.593)693K2.67286 3336J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338Fig. 1. Response of relative resistivity changes to isochronal annealing up to783K in MgZnREZr alloy with various preparation conditions (?, sprayformed and extruded; ?, spray formed non-extruded; ?, squeeze cast andextruded; ?, squeeze cast and extruded 2nd run and; ?-, spray formed andextruded 2nd run).squeeze cast as well as the spray formed alloy in the initialstate.Relative resistivity changes ?/0owing to isochronalannealing annealing curves in the alloys investigated arecompared in Fig. 1. The resistivity annealing curve of the sprayformed alloy shows a negligible increase during isochronalannealing up to 603K followed by a continuous decrease inresistivity to 783K (14%). This is, most probably, caused bya precipitation process leading to the purification of the matrix,which is verified by the increasing RRR value (Table 3).The spray formed and extruded material responds to anneal-ing by a decrease in resistivity in two temperature ranges(423543K decrease of 8% and 603693K decrease of 18%).Conglomerates of rectangular particles (sizel?m) of acomplex phase containing Zn and Nd were observed in the aspreparedstateofthesprayformedandextrudedalloy,seeFig.2a.ED patterns could not be indexed on the basis of any knownphase of MgZnNd- and Zr-containing alloy. The C-base-centred orthorhombic (cbco) reciprocal lattice was constructedTable 4Measured angles of goniometer position and those between the poles (in deg)Position123Gonio angle ?11.015.50.0Gonio angle ?1.07.38.0127.313.0227.321.3313.021.3Table 5Angles between poles of the cbco phase (in deg)Position123Pole10200121610227.113.400127.121.521613.421.50using four ED patterns obtained from different particles. Latticeparametersofthecbcophasewereestimatedtobea=0.997nm,b=1.149nm and c=0.974nm. This interpretation was verifiedby a tilting experiment on a single coarsened particle of thisphase in the spray formed and extruded alloy after isochronalannealing up to 543K. Three ED patterns were indexed unam-biguously as 102, 001 and 216 pole patterns of the cbcophase Fig. 3. The measured and calculated angles between thepoles agree very well with each other within the experimentalerror 0.5, see Tables 4 and 5.A relatively large dislocation density was observed in somegrains of the spray formed and extruded material in the as pre-pared state. Fine particles of a complex phase containing Y andZn in grain interiors (up to 30nm) as well as at the grain bound-aries (up to 50nm) were present in this material, see Fig. 2b.Coarsening of the cbco phase and recovery of the dislocationsubstructure was observed after annealing up to 543K, whichleads to a slight decrease in HV0.1 (7%). The develop-ment of fine, dense dispersed precipitates of Zn-, Y- and a Nd-Fig. 2. Structure of spray formed and extruded MgZnREZr alloy in as prepared state, (a) dark conglomerates of rectangular particles cbco ZnNd(Mg) phase,(b) fine particles of YNd containing phase. (Bright-field TEM).J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338337Fig. 3. Diffraction patterns of a cbco particle in spray formed and extruded MgZnREZr alloy isochronally annealed up to 543K. (a) pole 102, (b) pole 001,(c) pole 216.containing phase (Fig. 4) was detected after annealing to 693K,which resulted in a resistivity decrease in the range 603693K.A slight increase in microhardness was associated with this pro-cess (+7%). The increase in the RRR value after annealingto 693K, Table 3 confirms the higher effective purity of thematrix due to the precipitation processes. Unlike annealing ofnon-extruded material, annealing of spray formed and extrudedmaterial to higher temperatures leads to an increase of morethan 25% above the initial resistivity value. An increase abovethe initial resistivity value was not observed in spray formedmaterial supporting the assumption of a higher concentration ofsolutes in the as prepared state but not in the extruded material,where precipitation could occur during the thermal treatment at573623K. The increase above the as prepared value in sprayformed and extruded alloys is caused, most probably, by thedissolution of precipitates containing Nd and simultaneous pre-cipitationofthephasecontainingZnZr(needlesandellipsoids)observed in TEM specimens after annealing up to 753K, seeFig. 4. Fine precipitates of ZnY phase in spray formed and extrudedMgZnREZralloyafterisochronalannealingupto693K.(Bright-fieldTEM).Fig. 5. Experimental data show that a contribution of 1at.% Ndin the Mg matrix to the residual resistivity is relatively high(77n?m/at.% 10, 79.4n?m/at.% 11, 95n?m/at.% 12 at77K). It cannot only compensate the decrease caused by thedepletion of Zn and Zr solute in the matrix due to precipitationof ZrZn phase but can also lead to the pronounced resistivityincrease observed.The response of relative resistivity changes to isochronalannealing of the squeeze cast and extruded specimen can bedescribed by a continuous slight decrease of resistivity valuesup to 603K (minimum 13%). At higher annealing temper-atures (above 633K), a very pronounced increase is observed(more than 60% above the value of as prepared alloy) similar tothat in the spray formed and extruded material. It indicates anincreasingconcentrationofsolutesinthematrixafterdissolutionof precipitated phases as confirmed by the decrease in the RRRvalue(Table3).Thisresultinvokedthemeasurementofso-calledsecond runs, in which the stepwise isochronally annealed spec-imens of extruded materials (spray formed as well as squeezecast) were again isochronally annealed from 293 to 783K.Fig.5. FineneedlesofZnZrcontainingphaseinthesprayformedandextrudedMgZnREZr alloy isochronally annealed up to 753K. (Bright-field TEM).338J. Pelcov a et al. / Materials Science and Engineering A 462 (2007) 334338Relative resistivity changes during the second run (Fig. 1,dashed lines) are substantially larger than those in the first runand the shape of resistivity annealing curves is similar. Thisresult clearly proves that the isochronal heat treatment up tohigher temperatures significantly suppress the influence of pro-cessing technology. The supersaturation provided by the firstisochronal annealing up to 783K enables the development ofmetastablephasesduringthesecondrunofisochronalannealingfrom room temperature to 523K. The shape of the main resis-tivity decrease (513603K) indicates that several precipitationprocesses take place simultaneously. Depending on the resistiv-ity response, the temperature range of precipitation processesdiffers substantially in spray formed specimen after extrusionand the same specimen exposed to a second run with the sameisochronal heat treatment.4. ConclusionThe alloy Mg-3wt.% Zn-1wt.% Nd-0.5wt.% Zr alloy pre-pared by spray forming shows a high porosity (about 12%)that causes extreme brittlen