Viewpoint PaperHigh-temperature deformation and creep in Mg wrought alloysStefano Spigarelli *and Mohamad El MehtediDipartimento di Meccanica,Universita`Politecnica delle Marche,via Brecce bianche,60131Ancona,Italy Received 4November 2009;revised 14December 2009;accepted 17December 2009Available online 23December 2009Abstract—The high-temperature deformation data obtained for a number of different Mg alloys by torsion and tension creep are discussed.ZM21alloy exhibited peak flow stresses comparable to that observed in AZ31,and the equivalent strain to fracture in torsion for the two alloys was similar.In the cases of ZEK200and ZM21,creep was found to obey the same constitutive equations as hot working,suggesting a continuity of mechanical parameters and substructure in the two regimes.Ó2009Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.Keywords:Magnesium alloys;High-temperature deformation;Creep test1.IntroductionInterpretation of the high-temperature deformation of magnesium alloys has been the subject of much atten-tion in recent years,due to the attractive properties of these materials.In particular,the present authors have studied at length the effect of the initial state on the hot workability of AZ31alloy [1–3];in the same context,other research has aimed at investigating the hot work-ability of materials of the Mg–Zn group [3–7].In addi-tion,the creep properties of Mg–Zn wrought alloys have been analysed in some detail in recent papers [7–9].Although a range of data has been discussed in some detail in the papers cited above,a comprehensive comparative analysis of the behaviour of these alloys is still lacking.The aim of the present study is thus to present and discuss the torsion and creep data obtained by the authors,in order to obtain new significant infor-mation that could be useful for the processing and/or applications of these alloys.2.Materials and methodsThe materials considered in this study are summa-rized in Table 1.Three different batches of AZ31were tested in torsion.The first batch was produced by ingot technology,and underwent different pre-treatments,identified by the suffixes I1H,E1,E1H and E1HSC;asecond batch was produced by direct chill casting (DCC)and was tested in the as-cast condition (I2);the third batch was die cast (DC)and rolled (R1).The other Zn-containing Mg alloys considered here included DCC ZM21tested in two different initial states (I1,E1),DCC ZK60,DCC ZEK200(Mg–1.52%Zn–0.27%Nd,0.19%Y–0.38%Zr)tested after extrusion,and,last but not least,an alloy identified here as AMZ110Ca (0.75%Al–0.79%Mn–0.55%Zn–0.17%Ca),again produced by DCC and then extruded.The hot workability of these materials was tested in torsion;the relationships used to convert the raw data obtained in torsion to equivalent stress and equivalent strain may be obtained from each of the studies mentioned in Table 1.Constant-load creep tests were carried out between 100and 125°C on the ZM21-E1[8],ZEK200-E1[9]and AMZ110+Ca-E1[7]alloys;details on the experi-mental techniques for creep testing are given in Refs.[8,9].2.1.Elevated temperature deformation:hot workability As pointed out by McQueen and Ryan in their com-prehensive study of the constitutive analysis of hot working [10],the constitutive equations relating stress and testing parameters are used to calculate the process-ing forces at given strain rate;in addition,the extensive use of finite-element modelling (FEM)is still largely based on the input of these relationships correlating peak flow stress,temperature and strain rates.The most widely used constitutive equation is:1359-6462/$-see front matter Ó2009Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.scriptamat.2009.12.034*Corresponding author.E-mail:s.spigarelli@univpm.itScripta Materialia 63(2010)704–709/locate/scriptamatanisms operating during deformation,in practice this parameter is strongly sensitive to the scatter of the experimental data.In the case of the AZ31,the Q HW cal-culated values range from135kJ molÀ1(a value equiva-lent to that of self-diffusion in Mg),to224kJ molÀ1, even though the majority of the data is reasonably close to150–160kJ molÀ1.An average value of Q HW=164 kJ molÀ1was used to plot Z as a function of peakflow stress in Figure1.Analysis of thisfigure clearly shows that almost all the data collapse on a relatively narrow scatter band,delimitated by the two curves(broken lines),that,for a given strain rate,identify a lower (Eq.(1)with n=5,a=0.028MPaÀ1and A=1Â1011sÀ1)and a higher(Eq.(1)with n=5,a=0.018 MPaÀ1and A=2Â1011sÀ1)limit of the peak stress. Most of the experimental data align on the same master curve(solid line)described by Eq.(1)with n=5, a=0.019MPaÀ1and A=3Â1011sÀ1.Figure1also shows that the extruded alloy data overlap on the master curve in the high-Z regime;as temperature increases and/or strain rate decreases,i.e.Z decreases,the data for the extruded alloy gradually drift toward the lower limit of r.A possible explanation for this behaviour lies in the lower grain size of extruded alloys.Dynamic recrystallization(DRX),in AZ31as in many other Mg alloys,involves the creation of new grains,frequently nucleating at previous grain boundaries(Table2),fol-lowing a scheme that has been analysed by McQueen and Konopleva[11]:twinning takes place at low strains to reorient grains not suited for slip,preceding the other mechanisms except basal glide that occurs in favourably oriented grains.As strain reaches a critical level,DRX starts where high misorientations have been created by accumulation of dislocations,i.e.in those points(near grain boundaries and twins)where slip has occurred on several slip systems.The newfine grains form a“neck-lace”along grain boundaries and deform more easily than the grain core,thus repeatedly undergoing recrystalliza-tion.Thesefine grains can coarsen when testing tempera-ture is very high,but impingements of recrystallized necklace grains on the opposite side of the original de-formed grain could occur only when the initial structure wasfine-grained;in this case the necklace grainsfill the volume microstructure so that afine equiaxed DRX struc-ture results[12].On the other hand,a coarse initial grain size[11]should lead to only partially recrystallized micro-structures,and,since DRX swept out dislocations and is expected to reduceflow stress,to higher working loads.Table1.Summary of the hot-workability parameters for the alloys considered in this study.For the less-common alloys the chemical composition is also given(wt.%).Material Condition Ref.a[MPaÀ1]n Q HW[kJ molÀ1] AZ31-I1H a Ingot+heat treated(500°C/2h quench)[1]0.020 5.5224AZ31-E1a Ingot+extruded[2]0.020 3.9152AZ31-E1H a Ingot+extruded+heat treated(500°C/2h quench)[2]0.020 4.2157AZ31-E1HSC a Ingot+extruded+heat treated(500°C/2h slow cooling)[1]0.020 3.6135AZ31-I2Direct chill cast(DCC)[1]0.020 4.7150AZ31-R1Die cast–rolled[3]0.020 4.2168AMZ110Ca-E1b DCC–extruded[7]0.010 6.2201ZM21-I1DCC[4]0.024 4.7173ZM21-E1DCC–extruded[3]0.049 3.3201 cS.Spigarelli,M.El Mehtedi/Scripta Materialia63(2010)704–709705Figure 2shows the Z parameter as a function of the peak flow stress for the other wrought Mg–Zn alloys con-sidered in the present study and for an additional sand-cast ZM21alloy (ZM21-I2),tested by Sivakesavam and Prasad [13]in compression.The experimental values of the activation energy range from 173to 201kJ mol À1(only ZK60exhibits a lower Q HW value:115kJ mol À1;see Table 2),but to make comparison easier,the value of 164kJ mol À1was used in Figure 2.Figure 2a plotsregime (corresponding to temperatures of 450and 500°C,i.e.well above the maximum testing temperature usually used for Al-containing Mg alloys).The similitude in strength between AZ31and ZM21suggests that a mere 1%Zn and 0.8%Mn (the AZ31in general contains 0.2%Mn)are equivalent,in terms of strengthening effect,to 3%Al;this observation thus seems to confirm that Zn acts as solid solution strengthener,as already postulated in other studies (see,for a discussion on this subject,the references Table 2.Details on the microstructure before and after high-temperature deformation for some of the alloys investigated.Material Notes on the initial microstructure Notes on the final microstructure (after rupture)AZ31-E1EQM;GS =5l m0.05s À1:early onset of necklace recrystallization at 200°C(around 30%of recrystallized structure);FR structure at 250–400°C,presence of extensive twinning inside grains,and moderate grain coarsening at400°C;onset of DRX is retarded by increasing strain rate (no DRX at 5s À1,200°C)AZ31-E1H EQM;GS =12l m0.05s À1:twins on severely deformed grains at 200and 250°C;partly (40–50%)recrystallized structure (necklace DRX)at 300°C.FR structure at 350–400°C.Twinning observed inside recrystallized grains.Moderate grain coarsening at 400°C AMZ110Ca-E1AEQM;GS =3l m0.5s À1:severely deformed microstructure at 200°C;few recrystallized grains on grain boundaries of deformed grains at 250°C;AFR structure at 350°C;FR structure at 400°CZM21-I1EQM;GS =20l m 0.5s À1:severely deformed microstructure between 200and 300°C;bimodal distribution of large and fine grains produced by recrystallization at 350°C and aboveZM21-E1AEQM;GS =40l m 0.5–5s À1:severely deformed grains below 300°C;AFR grain structure at 400°C.FR structure and large grain growth at 500°CZEK200-E1EQM;GS =11l m0.5–5s À1:severely deformed grains below 300°C;early onset of recrystallization on grain boundaries at 350°C.AFR grain structure at 400°C.FR structure and moderate grain growth at 450°CEQM,equiaxed microstructure;AEQM,almost completely equiaxed microstructure;FR,fully recrystallized;AFR,almost fully recrystallized;GS,grain size;DRX,dynamic recrystallization.706S.Spigarelli,M.El Mehtedi /Scripta Materialia 63(2010)704–709precipitates.Figure 2b plots the data obtained by testing ZK60and ZEK200;the large scatter of the ZK60data points was expected,due to the large difference between the experimental value of the activation energy and the value used in Figure 2.The relatively high strength of ZEK200was also expected,due to its alloying with rare-earth (RE)elements;again,as in ZM21,extensive DRX was observed only in the very-high-temperature regime.The above analysis gives some interesting indications that could be used to predict the processability,in terms of working loads,of the set of investigated alloys.How-ever,Figures 1and 2do not give any information about the ductility,i.e.the maximum strain that could be accu-mulated at high temperature without cracking.Torsion testing,due to the pure shear condition,in itself cannot give quantitative information on this subject,except the equivalent strain to fracture.This parameter,obtained in all the investigated alloys by a common testing proce-dure,could nevertheless be used as a comparative index to analyse the ability of the different materials to deform without defects.Figure 3plots the equivalent strain to fracture as a function of Z for the AZ31(Fig.3a)and the Zn-containing alloys (Fig.3b).Analysis of Figure 3a clearly indicates that for AZ31,while in the case of peak stress values the differences in initial microstructure have a relatively minor effect,the same is not true for the strain to fracture;as a mattersidered,the strain to fracture was found to increase from ZEK200to ZK60.The relatively low values of the equivalent strain to fracture observed in ZEK200can be associated with the presence of RE elements forming RE-rich particles.Furthermore,the drop in strain to fracture at low-Z values in AMZ110+Ca (the four data points well below the band of the other experimental data)can easily be explained by the use of an excessively high testing temperature (450°C,equivalent to the eutectic temperature in Mg–Al binary system).2.2.Elevated temperature deformation:creep regime Figure 4plots the minimum creep rate values for the Zn-containing wrought alloys that have been tested by the authors of the present study between 100and 150°C [7–9];for all these materials,the value of the acti-vation energy for creep was found to be close to 180kJ mol À1,i.e.reasonably close the Q HW values re-ported in Table 1.This value was thus used to plot the Zener–Hollomon parameter as a function of applied stress.Figure 4also shows the straight lines representing the power-law equation:Z ¼_eexp ðQ =RT Þ¼A 0r n ð2Þconventionally used to described the minimum strain rate dependence on applied stress in creep (where A 0is a material parameter,Q is the activation energy for creep,and n is the creep exponent),for two cast alloys,AS21[14]and AE44[15].These two alloys were here considered as reference standards for commercial die-cast materials with acceptable (AS21)and excellent (AE44)creep response.In both cases their enhanced creep strength,when compared with the classical AZ91,is based on the presence of stable precipitates (Mg 2Si in AS21and RE-rich particles in AE44).The values of the stress exponent describing the data in Figure 4were 12.7,14.8and 18.9for AMZ1100+Ca,Figure 3.Equivalent strain to fracture in torsion as a function of Z ,for (a)AZ31and (b)the other Zn-containing alloys.S.Spigarelli,M.El Mehtedi /Scripta Materialia 63(2010)704–709707Even more remarkable is the equivalence of the creep response of the ZEK200and AMZ110+Ca;only at the highest temperature (150°C)does the RE-contain-ing alloy exhibit lower creep rates than the material con-taining 0.17%Ca.The increase in creep resistance when compared to ZM21is also in this case attributable to the presence of precipitates,i.e.Ca-and RE-rich particles,respectively.An interesting point to be considered is the contiguity and overlap of creep and hot-working data.In a recent paper,McQueen and Kassner [16]emphasized that in the traditional approach,different paradigms govern creep and hot workability.While creep studies aim at increasing the resistance to flow and reducing the accu-mulated strain,hot-working researches go for the oppo-site target;nevertheless,the continuity of steady-state parameters and structures between creep and hot-work-ing regimes has been confirmed in the case of Al [16].The situation should be significantly different in Mg,since above 225°C non-basal slip is theoretically ex-pected to act as rate-controlling mechanism [17],and hot-working studies are usually carried above this limit;in the case of the alloys considered here,for example,torsion testing was usually carried out between 200and 500°C,while the maximum testing temperature in creep was 150°C.In addition,the extended ductility in torsion leads to the onset of DRX,a mechanism never observed in creep.Therefore one might wonder whether creep and torsion data differ in their Z –r relations;the problem is addressed in Figure 5,where torsion and creep data were plotted for those alloys for which both sets were available.Analysis of Figure 5proves that the tensile creep and torsional data obtained by testing3.ConclusionsThe analysis of the torsion and creep data obtained by testing the wrought alloys considered in the present study led to the following conclusions:1.The effect of a different initial microstructure on the hot-working response of AZ31alloy mainly consists in a significant variation of the maximum attainable strain (equivalent strain to fracture in torsion);a fine grain size—obtained,for example,by extrusion—reduces the working loads and leads to an increase in the hot workability.2.The Mg–Zn alloys can be successfully deformed at higher temperatures when compared with Mg alloys containing Al and,in general,the working loads do not dramatically differ from those necessary for AZ31in the same temperature and strain rate range.The equivalent strain to fracture in torsion is slightly higher for ZM21than for AMZ110;ZEK200,as expected due to the presence of a RE addition,is the least ductile alloy investigated here.3.Necklace DRX is the typical recrystallization mecha-nism operating in all the investigated alloys;the lower temperature limit for the onset of DRX was found to vary with the presence of Al (which seems to extend DRX to lower temperatures).In addition,the initial grain size could significantly affect the DRX phenom-ena,since a fine initial grain size,for example,pre-vents the presence of unrecrystallized regions in the final structure.4.The creep response of wrought Mg alloys containing Zn is more than acceptable;ZM21is only marginally inferior to the AS21reference material,while 708S.Spigarelli,M.El Mehtedi /Scripta Materialia 63(2010)704–709References[1]M.El 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