Materials and Design 40 (2012) 488–496

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Materials and Design

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Microstructure development and tensile mechanical properties of Mg–Zn–RE–Zrmagnesium alloy

Qiang Chen a,⇑, Dayu Shu a, Zude Zhao a, Zhixiang Zhao a, Yanbin Wang a, Baoguo Yuan b

a Southwest Technique and Engineering Institute, Chongqing 400039, PR Chinab School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 January 2012Accepted 30 March 2012Available online 19 April 2012

Keywords:F. MicrostructureMechanical propertiesAlloysA. non-ferros alloysC. extrusion

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.03.059

⇑ Corresponding author. Tel.: +86 023 68792286.E-mail addresses: 2009chenqiang@hfut.edu.cn

(Q. Chen).

The Mg–5.3 wt.%Zn–1.13 wt.%Nd–0.51 wt.%La–0.28 wt.%Pr–0.79 wt.%Zr alloy prepared by direct chillcasting is subjected to hot extrusion. The effects of extrusion ratio and temperature on microstructureand tensile mechanical properties have been studied. The results indicate coarse grains of as-cast alloysare refined with extrusion ratio increasing from 0 to 9. The eutectic constituents are elongated alongextrusion direction. However, further increase of extrusion ratio has a little influence on grain refinementand the improvement of mechanical properties of the alloy. Dynamic recrystallisation is the main mech-anism of grain refinement during hot extrusion. Raising extrusion temperature results in grain coarsen-ing. Grain shape becomes more equiaxed-like with raising extrusion temperature. At the same time,mechanical properties decrease with the increase of extrusion temperature.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Magnesium alloys have drawn a great deal of attention fromautomotive and aerospace industries due to their low density, highspecific rigidity and good damping capacity [1]. Magnesium alloyscan be broadly classified as Al-bearing or Al-free alloys [2]. Thewidely used magnesium alloys belong to Mg–Al series, such asAZ91 and AM60, which has good combination of castability andlow cost [3]. However, the application of these magnesium alloyshas been limited because of their poor mechanical properties andthermal stability [4]. Compared with Mg–Al series, the Mg–Zn ser-ies, such as ZK60, have great potential for the development of lowcost magnesium alloys with higher strength [5].

ZK60 alloy has relatively good mechanical properties among allmagnesium alloys such as high strength at room temperature andelevated temperature [6]. However, its strength at room tempera-ture and elevated temperature is still relatively low compared toaluminum alloys. Recently, it has been reported that rare earth(RE) additions can improve the mechanical properties of ZK60 alloy[7–9]. Zhou et al. [7] studied the effect of Nd and Y on the micro-structure and mechanical properties of ZK60 alloy. The combina-tion of Nd and Y addition had a great effect on grain refinementduring dynamic recrystallisation. Moreover, the combination ofNd and Y addition resulted in the increase of yield strength (YS)and ultimate tensile strength (UTS). He et al. [8] determined the

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effect of Gd on the microstructure and mechanical properties ofZK60 alloy. The addition of Gd led to a considerable reduction ofthe age-hardening response and a small decrease of YS and UTS.However, the grain refinement caused by the addition of Gd com-pensated partly for the loss of YS and UTS. Zhang et al. [9] reportedthat the ZK60 alloy with the addition of Er exhibited a considerablyimproved deformability, fine and uniform microstructure, as wellas good mechanical properties.

In this study, Mg–5.3Zn–1.13Nd–0.51La–0.28Pr–0.79Zr wasprepared by direct chill (DC) casting. Moreover, the effects ofextrusion ratio and temperature on Mg–5.3Zn–1.13Nd–0.51La–0.28Pr–0.79Zr are also presented.

2. Experimental method

High purity Mg (99.95%), Zn (99.99%) and Mg–30Zr (wt.%) wereemployed in the experiment. Rare earth (RE) elements in the formof intermediate alloys were added to the metal at 760 �C under theprotection of SF6 and CO2 gas mixture [10]. The molten alloy wasfirstly stirred and kept at 730 �C for 25 min. Then the molten alloywas poured into cylindrical ingots of 90 mm diameter and 400 mmheight. The chemical composition of the alloy was determined bymeans of inductively coupled plasma atomic emission spectrumapparatus. The composition of the alloy was estimated to be Mg–5.3 wt.%Zn–1.13 wt.%Nd–0.51 wt.%La–0.28 wt.%Pr–0.79 wt.%Zr.Some as-cast billets were extruded into bars at 350 �C with extru-sion ratios of 9, 16, 25 and 100, respectively. Some other as-castbillets were hot extruded into bars of 12.5 mm diameter at temper-atures of 250 �C, 300 �C, 350 �C, 400 �C and 450 �C, respectively. For

Fig. 1. X-ray diffraction patterns of Mg–Zn–Nd–La–Pr–Zr.

Table 1EDS analysis of the as-cast Mg–Zn–Nd–La–Pr–Zr alloy. Letters A–C refer to therepresentative regions shown in Fig. 2b.

Mg (wt.%) Zn (wt.%) Zr (wt.%) Nd (wt.%) La (wt.%) Pr (wt.%)

A 20.04 78.62 0.25 0.59 0.23 0.27B 95.01 3.49 0.24 0.54 0.25 0.47C 37.14 38.99 0.25 12.98 6.82 3.82

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each extrusion, samples and the die were coated with a lubricant ofmolybdenum disulfide. All extrusions were conducted at a con-stant ram velocity (the velocity at which billet is pushed againstthe die) of 80 mm/min. This corresponds to a higher extrusionvelocity for higher extrusion ratio. According to ASTM E 92, mea-surements of the Vickers microhardness of as-cast and extruded al-loys using a microhardness tester equipped with a diamondpyramidal indenter [11]. Each hardness value was the average ofat least ten measurements. According to ASTM B557, the mechan-ical properties of as-cast and extruded alloys were characterizedwith tests carried out at room temperature with an Instron mate-rial testing machine using the standard 6 mm diameter sampleswith 50 mm gauge length [12]. The crosshead speed was 2 mm/min.

Microstructures were examined by optical microscopy (OM),scanning electron microscopy (SEM) and X-ray diffractometry(XRD). The samples for OM and SEM were prepared by the stan-dard technique for grinding with SiC abrasive paper and polishingwith an Al2O3 suspension solution, followed by etching in an aque-ous solution of 4 vol.% concentrated HNO3. An optical microscopewas used for OM observations. While a scanning electron micros-copy, equipped with energy dispersive spectroscopy (EDS), wasused to preform the SEM examinations. Grain sizes were measuredusing a mean linear intercept method.

Fig. 3. WDX line scan of the grain and the grain boundary in the as-cast condition.

3. Results

3.1. Microstructure of as-cast alloys

XRD analysis indicates that the as-cast Mg–Zn–Nd–La–Pr–Zr al-loy mainly consisted of a-Mg matrix and the MgZn2 phase (Fig. 1).The optical micrography of the as-cast Mg–Zn–RE–Zr alloy reveals

Fig. 2. Microstructure of as-cast Mg–Zn–Nd–La–Pr–Zr alloy: (a) optical micrography aanalysis.

an average grain size of �92 lm and the eutectic constituents atthe grain boundaries (Fig. 2a). These constituents can be betterappreciated in the SEM image. The grain boundary particles areindicated by arrows and designated A in Fig. 2b. Table 1 is EDSanalysis of the as-cast Mg–Zn–Nd–La–Pr–Zr alloy. Letters A–C referto the representative regions shown in Fig. 2b. EDS analysis

nd (b) scanning electron micrograph. Representative regions A–C refer to the EDS

Fig. 4. DSC curve of the as-cast Mg–Zn–Nd–La–Pr–Zr alloy.

Fig. 6. The original particles were destroyed and broken into small particles afterextrusion with extrusion ratio of 9 at 350 �C.

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indicates that the Mg/Zn ratio (20.04/78.62) of the MgZn2 phasewas found to be a higher than the stoichiometric ratio, 0.185 (1/2). The excessive Mg content in the spectrum could be attributedto the contribution of the excess Mg by a-Mg matrix. In region B,Zn atoms dissolved into a-Mg matrix with the concentration of3.49 wt.%, which was about 66%of the average Zn content of the al-loy. EDS analysis in Fig. 2b also indicated that some Nd, La and Prelements dissolved into a-Mg matrix. The compounds designatedC in Fig. 2b, was determined by EDS to contain 12.98 wt.%Nd,6.82 wt.%La, 3.82 wt.%Pr and 0.25 wt.%Zr in the as-cast Mg–Zn–RE–Zr alloy, besides Zn and Mg elements.

Fig. 3 shows the wavelength dispersive X-ray (WDX) analysisline scan of the grain and the grain boundary in the as-cast condi-tion. As shown in Fig. 3, the concentrations of Zn, La, Pr, Nd variedwith the grain and the grain boundary. The concentrations of Zn,

Fig. 5. Optical micrographs of extruded Mg–Zn–Nd–La–Pr–Zr alloy with v

La, Pr, Nd at the grain boundary were higher than those within grain.In comparison, Zr concentration remained fairly consistent withinthe grain and the grain boundary. Fig. 4 show the DSC curve of as-cast Mg–Zn–Nd–La–Pr–Zr alloy. There are two endothermic peaksemerging in the DSC curve. It is found that the first endothermicpeak appeared that the temperature of about 547 �C. It correspondsto the dissolving temperature of eutectic phases. The second peakcould be thought as the melting temperature of the alloy.

arious extrusion ratios: (a) k = 9, (b) k = 16, (c) k = 25 and (d) k = 100.

Fig. 7. SEM of Mg–Zn–Nd–La–Pr–Zr alloy with the extrusion ratios: (a) k = 9, (b) k = 100.

Table 2EDS analysis of the extruded Mg–Zn–Nd–La–Pr–Zr alloy with extrusion ratios of 9 and100. Letters A and B refer to the representative regions shown in Fig. 7.

Mg (wt.%) Zn (wt.%) Zr (wt.%) Nd (wt.%) La (wt.%) Pr (wt.%)

A 38.78 38.52 0.60 12.34 6.57 3.19B 38.56 38.17 0.37 11.17 8.53 3.20

Fig. 8. Equivalent strain (extrusion ratio) dependence of average grain in the wholearea of Mg–Zn–Nd–La–Pr–Zr alloy after extrusion.

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3.2. Microstructure evolution of as-extruded alloys

3.2.1. Effect of extrusion ratio and temperature on microstructuredevelopment

Fig. 5 shows the optical micrograph of the extruded Mg–Zn–RE–Zr alloy at various extrusion ratios. Compared with the initialmicrostructure with coarse grains (Fig. 2), the whole matrix ofspecimen was almost taken up by new grains, which indicated thatdynamic recrystallisation has been completed after extrusion withextrusion ratio of 9 (Fig. 5a). However, the microstructure wasinhomogeneous to some extent. The recrystallised grains hadtwo populations of grain sizes. The relatively coarse grains wereof 10–15 lm size while fine grains are 4–8 lm. When extrusion ra-tio reaches 16, fine and equiaxed recrystallised grains uniformlydistributed in the microstructure (Fig. 5b). After that the averagegrain size changed a little with increasing the extrusion ratio(Fig. 5c and d). After extrusion, the original particles, which pre-sented along the grain boundaries, were destroyed and broken intosmall particles (Fig. 6).

Fig. 7 reveals that in extruded condition, there were bands ofstringers parallel to the extrusion direction present at extrusionratios of 9 and 100. Table 2 is EDS analysis of the extrudedMg–Zn–Nd–La–Pr–Zr alloy with extrusion ratios of 9 and 100.Letters A and B refer to the representative regions shown in Fig. 7.The EDS analysis reveals that these stringers mainly consisted ofMg–Zn–RE compounds. In comparison to extrusion ratio 9, thesestringers were finer in the case of extrusion ratio 100. Fig. 8 showsthat equivalent strain (extrusion ratio) dependence of the averagegrain size measured over the whole area. As shown in Fig. 8, theaverage grain size obviously decreased with increasing equivalentstrain (extrusion ratio). At the equivalent strain higher than 2.77,the microstructure was dominated by homogeneous fine grains.Fig. 9 shows the optical micrographs after extrusion at various

temperatures. Fig. 10 shows the average grain size for each extru-sion temperature. Raising extrusion temperature leads to the fol-lowing changes in microstructures. (a) The average size increased,and a significant increase occurred at 400 �C. (b) The shape of thegrains became more equiaxed-like.

Fig. 9. Optical micrographs of extruded Mg–Zn–Nd–La–Pr–Zr alloy with various extrusion temperatures: (a) 250 �C, (b) 300 �C, (c) 350 �C, (d) 400 �C and (e) 450 �C.

Fig. 10. Effect of extrusion temperature on average grain size.Fig. 11. Tensile mechanical properties of extruded Mg–Zn–Nd–La–Pr–Zr alloy atvarious extrusion ratios.

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Fig. 12. Micro-hardness of as-cast and extruded Mg–Zn–Nd–La–Pr–Zr alloy as afunction of extrusion ratio.

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3.2.2. Effect of extrusion ratio and temperature on mechanicalproperties

Fig. 11 shows the tensile mechanical properties of extrudedMg–Zn–Nd–La–Pr–Zr alloy at various extrusion ratios. The resultsindicate that the UTS, YS and elongation to fracture were influ-enced by extrusion ratio, which could be divided into two stages.In the first stage, the tensile mechanical properties obviously in-creased with increase of extrusion ratio. For instance, as the extru-sion ratio increased from 0 to 9, the UTS of as-cast sample increasesfrom 169 MPa to 309 MPa. In the second stage, at higher extrusionratio, the tensile mechanical properties of Mg–Zn–Nd–La–Pr–Zr al-loy were slightly improved with an increase of extrusion ratio.Fig. 12 shows the micro-hardness of the as-cast and extrudedmaterials as a function of extrusion ratio. The variable tendencyof micro-hardness was the same as that of mechanical properties.Hardness increased remarkably with the increase of extrusion ratiofrom 0 to 9. However, higher extrusion ratio resulted in a slight in-

Fig. 13. Tensile fracture of Mg–Zn–Nd–La–Pr–Zr alloys deformed at 350 �C wi

crease in hardness. The SEM images of Mg–Zn–Nd–La–Pr–Zr alloywith various extrusion ratios showed ductile fracture, in which alot of deep dimples and tear ridges could be observed (Fig. 13).Therefore, the extruded Mg–Zn–Nd–La–Pr–Zr alloy presented goodductility. Moreover, careful examination of the failure surface indi-cated that some cracked particles were RE-containing intermetal-lics (Fig. 14). Due to the fragile characteristic of RE-containingcompounds, the particles were broken during the tensile deforma-tion. Fig. 15 shows the tensile mechanical properties of extrudedMg–Zn–Nd–La–Pr–Zr alloy at various extrusion temperatures.The results show that both strength and elongation to fracture de-creased with the increase of extrusion temperature. It should benote that the UTS, YS and elongation to fracture of Mg–Zn–Nd–La–Pr–Zr alloy obviously decreased from 324 MPa, 278 MPa and12% to 267 MPa, 208 MPa and 5%, respectively, as the extrusiontemperature increased from 350 �C to 400 �C. Fig. 16 shows theinfluence of extrusion ratio on hardness of extruded Mg–Zn–Nd–La–Pr–Zr alloy. Slight decrease in hardness could be obtained forextruded alloy with the increase of extrusion temperature from250 �C to 350 �C. However, further increase of extrusion tempera-ture resulted in remarkable decrease of hardness. Fig. 16 showsSEM fractography of extruded Mg–Zn–Nd–La–Pr–Zr alloy at differ-ent extrusion temperatures. As shown in Fig. 17a–c, the fracturesurface mainly consisted of tearing edges and micro-cavities.Micro-cavities distributed unevenly among the tearing edges.However, as the extrusion temperature further increased, thedecohesion surface presents an interface between a-Mg and inter-metallic compound (Fig. 17c and d).

4. Discussion

Microstructure observations by SEM revealed that as-castMg–Zn–RE–Zr alloy consisted of a-Mg matrix, MgZn2 and eutecticconstituents (Fig. 2). During the solidification, the refinement of

th various extrusion ratios: (a) k = 9, (b) k = 16, (c) k = 25 and (d) k = 100.

Fig. 14. EDS analysis of Mg–Zn–Nd–La–Pr–Zr alloy deformed at 350 �C with aextrusion ratio of 9.

Fig. 15. Tensile mechanical properties of extruded Mg–Zn–Nd–La–Pr–Zr alloy atvarious extrusion temperatures.

Fig. 16. Micro-hardness of the extruded materials as a function of extrusiontemperature.

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grain size by Zr is generally attributed to the peritectic solidification.Zr-rich Mg solidifies first when nucleation starts at the primaryZr-rich particles. Not all the Zr-rich particles, but only those whichseparate from the Zr master alloy near the peritectic reaction tem-perature, give rise to such nucleation. RE elements, such as Nd, Laand Pr, are added in the form of intermediate alloys. The liquidustemperatures of the intermediate alloys are lower than the temper-ature of the melt. Therefore, RE elements can fully dissolve into theliquid and diffuse across the melt. Because Mg and RE elements ex-hibit an extensive mutual solubility, a little amount of RE elementsmay be soluble in Mg lattice during crystallization of Mg. However,a large number of RE elements and solute atom Zn are pushed andthen enriched at the front of solid–liquid interface. Therefore, REelements and Zn-bearing intermetallics are formed at the grainboundary areas (Figs. 2 and 3).

Upon extrusion, the nucleation of the new and fine grains indi-cates that dynamic recrystallisation occurs (Fig. 5). For dynamicrecrystallisation, new grains generally nucleate at the originalgrain boundaries; subsequently, more fine grains nucleate at theboundaries of the newly formed fine grains [13–15]. In this way,recrystallised fine grains are formed and broaden as dynamicrecrystallisation proceeds; eventually, recrystallised fine grainsreplace the original coarse grains [16,17]. It is generally believedthat the higher extrusion ratio sample gives a finer recrystallisedgrain size [18–31]. The higher the extrusion ratio results in thegreater the overall grain boundary and sub-grain boundary area.

This leads to greater potential for the development of recrystallisa-tion nuclei, and therefore a finer recrystallised grain size. Anothereffect of extruding the material to a higher extrusion ratio is thatmisorientation of the subgrain boundaries may increase, leadingto an increase in the amount of stored energy in the material andincreased driving force towards recrystallisation [32]. However,the present research results indicate that the effect of grain refine-ment was remarkable at lower equivalent strain (Figs. 5 and 8).With the increase of equivalent strain, the average grain size ofthe billet quickly reduced to a final-equilibrium fine-grained size.The present results also suggest that there might be a critical strainwhich is in the range of 2.77–3.21 and controls the degree ofhomogeneity and average grain size during its microstructure evo-lution process. The complete recrystallised microstructure can beattained when that applied equivalent strain exceeds the criticalequivalent. The similar cases had been reported by Pérez-Pradoet al. [33] and Guo et al. [34]. Once dynamic recrystallisation iscompleted, the sample is in a steady state and further extrusionof the sample cannot change the grain size. This is the one reasonthat the average grain size of a sample in the present research re-mained almost unchanged after deformation at the extrusion ratioof 16 (equivalent strain of 2.77).

The temperature at which the extrusion takes place also has aprominent role to play, due to the fact that the heat generation(during plastic deformation and due to friction) and the heat dissi-pation to surroundings, the actual extrusion is not the same as theinitial sample temperature [7]. According to the relationshipbetween the size of dynamic recrystallised grains and the Zener–

Fig. 17. Tensile fracture of Mg–Zn–Nd–La–Pr–Zr alloy with a extrusion ratio of 16 deformed at : (a) 250 �C, (b) 300 �C, (c) 350 �C, (d) 400 �C and (e) 450 �C.

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Hollomon parameter (d = AZ�n), the size of dynamic recrystallisedgrains is controlled by the strain rate and the extrusion tempera-ture. Z is defined as = _e expðQ=RTÞ, where _e is strain rate, Q is acti-vation energy similar to that of self-diffusion and R is gas constant.On the one hand, keeping the ram velocity constant, higher extru-sion ratio corresponds to higher strain rate. On the other hand,keeping the initial sample temperature and the ram velocity con-stant, higher extrusion ratio corresponds to higher actual extrusiontemperature, because of generated heat caused by friction. The in-crease of strain rate results in decrease of recrystallised grain size,while the increase of deformation temperature results in increaseof recrystallised grain size. Strain rate and raising deformationinfluence recrystallised grain size from two opposite aspects.When the effect caused by increase of strain rate equals to theeffect caused by raising deformation temperature, the effect ofextrusion ratio on grain refinement is limited. This is also the rea-son why a finer grain size has not been observed for the higherextrusion ratio (Figs. 5 and 8).

During extrusion, at high extrusion temperatures, new andrecrystallised grains nucleate at the original grain boundaries;while at low extrusion temperature, there is an enhanced activity

of deformation twinning and new grains also nucleate at the twininterfaces [7,8]. Therefore, lowering of the extrusion temperatureprovides additional nucleation sites and reduces the grain size(Figs. 9 and 10). During extrusion, intermetallic compounds arefurther broken into small particles, which distribute across the ma-trix (Figs. 6 and 14). These second phases with a relatively highmelting point can pin grain boundaries and impede grain coarsen-ing during hot extrusion. In this case, the tendency of grain coars-ening is not obvious, as the extrusion temperature increases from250 �C to 350 �C. However, when the extrusion temperature in-creases to 400 �C, the thermal stability of intermetallic compoundsdecreases and the pinning effect of particles is eliminated. There-fore, a significant grain coarsening remarkably occurs at 400 �C.Recrystallisation is both temperature and time dependent. Thetemperature determines the nucleation rate. The amount of graingrowth is controlled by the length of time at temperature. The in-crease of extrusion temperature favors the formation of equiaxed-like grains (Fig. 9).

The effect of extrusion ratio on the evolution of tensile propertiescan be explained by examining the microstructures of Mg–Zn–Nd–La–Pr–Zr alloy at various extrusion ratios. According to the famous

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Hall–Petch relationship [35], a finer grain size results in higherstrengths. In this case, more grains contribute to the macroscopicdeformation and the stress concentrations are reduced and spreadover a wider area. Therefore, the improved ductility is expected.However, the ductility will be little affected by extrusion ratio whengrain size tends to reach a limited value. To produce ideal fine-grained structure, extrusion parameters need to be controlled prop-erly. As for good mechanical properties, our results suggest thathigher extrusion temperature should be avoided. As for grain shapeconcern, high extrusion temperature favors the formation of equi-axed-like grains.

5. Conclusions

Mg–Zn–RE–Zr magnesium alloy prepared by direct chill castingis subjected to hot extrusion. The extrusion ratio and temperaturehave exerted influences on microstructure and mechanical proper-ties. They are as follows.

1. During extrusion, hot extrusion results in dynamic recrystal-lisation and initial grains are refined. There is a critical strain(extrusion ratio) which is in the range of 2.77–3.21 (extru-sion ratio: 9–16) and controls the degree of homogeneityand average grain size during its microstructure evolutionprocess. The complete recrystallised microstructure can beattained when that applied equivalent strain exceeds thecritical equivalent. Once the critical minimum grain size isachieved, excessive equivalent strains (extrusion ratios) pro-vide no effect on grain refinement. Dynamic recrystallisationis main mechanism of grain refinement during extrusion.

2. The ultimate tensile strength, yield strength and elongationto fracture of Mg–Zn–Nd–La–Pr–Zr alloy increase remark-ably with extrusion ratio increasing from 0 to 9, then theUTS, YS and elongation to fracture are litter influenced.

3. The average grain size gradually increases between 250 and350 �C, a sudden increase of grain size occurs at 400 �C. Rais-ing extrusion temperature causes grain shape to becomemore equiaxed-like. A low extrusion temperature is desir-able for producing a fine-grained Mg–Zn–Nd–La–Pr–Zralloy, which has a high strength accompanied by reasonablygood tensile ductility.

Acknowledgements

We are grateful for the support of the National Natural ScienceFoundation of China (NSFC) for support under Grant No. 51005217.Dr. Chen is grateful for the support from China PostdoctoralScience Foundation Grant No. 20100480677 and from Chong QingPostdoctoral Science Foundation Grant No. Yu RC2011013.

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