Texture Modification During Extrusion of Some Mg Alloys

Texture Modification During Extrusion of Some Mg Alloys

LI JIN, RAJA K. MISHRA, and ANIL K. SACHDEV

The effects of extrusion ratio and alloying addition on the microstructure of Mg-0.2 wt pct Cealloys are investigated by electron backscatter diffraction. The results show that in this alloy,texture randomization does not occur at high or low extrusion ratios but at a ratio of 25:1 at400 �C. When extruded at the same temperature and extrusion ratio, Ca addition to Mg resultsin a weak nonbasal texture. In contrast, Mg-Al and Mg-3 wt pct Al-0.2 wt pct Ce alloys do notexhibit texture modification in single-pass extrusion. In the Mg-Al-Ce alloy, Ce and Al formAl11Ce3 particles, leaving little Ce solute in the matrix. The texture modifications in Mg-Ce orMg-Ca alloys are related to the nature of the solid solution and consistent with dynamic strainaging during extrusion.

DOI: 10.1007/s11661-011-0994-3� The Minerals, Metals & Materials Society and ASM International 2011

I. INTRODUCTION

DEFORMATION structures produced during high-temperature extrusion of Mg rods recrystallize toproduce a ring fiber texture with the c-axes of the grainsperpendicular to the extrusion direction (ED).[1,2] Suchextrusions exhibit poor ductility and strong anisotropy[3]

in their mechanical properties. Texture weakening orrandomization is a desirable approach to obtain highductility Mg alloys, and a number of cases of weak ornonbasal textures were reported recently for Mg alloysheets and extrusions.[3–7] Various authors have sug-gested that the texture could be weakened via dynamicrecrystallization during processing with the addition ofY, Ce, and other rare earth elements. Mishra et al.[5]

showed that tensile ductility of extruded Mg-0.2 wt pctCe is significantly enhanced due to texture randomiza-tion, where the basal planes become oriented at 40 to50 deg to the ED in the recrystallized microstructure.Mackenzie and Pekguleryuz pointed out[7] the weaken-ing of the basal texture in Mg-Zn-Ce alloy sheet with Ceaddition. No weakening of the basal texture in Mg-Al-Ce tubes was observed.[8]

Texture weakening in Mg alloy was associated withvarious phenomena, including abnormal graingrowth,[9,10] particle-stimulated nucleation (PSN),[3,6]

particle pinning,[11] solute drag,[4] dynamic strainaging,[12] and heterogeneous deformation leading to easeof shear band formation.[6,7] To clarify the effect of strainon the recrystallization texture after extrusion and tooptimize the processing conditions for producing ran-dom texture in the Mg-Ce extrusions, the microstructureof Mg-0.2 wt pct Ce alloy rod at various extrusion ratioswas investigated in this study. In addition, at the

optimized extrusion ratio identified for Mg-Ce, themicrostructures of Mg-Ca, Mg-Al, and Mg-Al-Ce alloyrods were processed to develop solution-strengthenedand texture-modified alloys for better strength-ductilitycombinations. The approach for developing wroughtMgalloys with texture randomization and strengthening viaalloying and extrusion process optimization is discussedon the basis of the results.The choice of extrusion ratio used in this study

was determined using the findings reported byJiang et al.,[12] who show that Mg-Ce alloy exhibitsdynamic strain aging in a narrow window of tempera-ture and strain rate; and if the extrusion conditionscorrespond to this window of strain rate and tempera-ture, texture randomization can occur. If dynamic strainaging is suppressed by extruding the material outsidethis strain rate–temperature window, the same alloyshould exhibit no texture randomization. As shown byJiang et al. and reported by Mishra et al.,[5] the extru-sion of 75-mm-diameter billet at 673 K (400 �C) at anextrusion ratio of 25:1 and extrusion speed of 10 mm/sproduces texture randomization in Mg-Ce alloys, andthis could be related to dynamic strain aging. In thisstudy, the extrusion parameters for a Mg-0.5 pct Cealloy billet are maintained at the same values except forthe extrusion ratios, which are chosen to be 9:1 and 36:1to bracket the dynamic strain aging regime reported byJiang et al.[12] so that the validity of this approach canbe confirmed and further extended. Since rare earthatoms are believed to be responsible for the occurrenceof this phenomenon, the influence of other alloyingelements besides rare earth atoms on the texturerandomization at this extrusion ratio (one falling inthe DSA regime) are also examined by choosing thesame extrusion ratio.

II. EXPERIMENTAL

The materials studied in this work include nominallypure Mg (baseline), Mg-0.2 wt pct Ce, Mg-0.5 wt pctCe, Mg-0.2 wt pct Ca, Mg-1 wt pct Al, and Mg-3 pct

LI JIN, Associate Professor, is with the National EngineeringResearch Center of Light Alloy Net Forming, Shanghai Jiao TongUniversity, Shanghai 200240, P.R. China. Contact e-mail: [email protected] RAJA K. MISHRA, Staff Researcher, and ANIL K.SACHDEV, Lab Group Manager, are with General Motors GlobalResearch & Development, Warren, MI 48090-9055.

Manuscript submitted December 22, 2010.Article published online January 5, 2012

2148—VOLUME 43A, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

Al-0.2 wt pct Ce alloys. Unalloyed Mg and Mg alloymelts of about 100 kg were prepared using commerciallypure Mg ingots, pure aluminum, pure calcium, and Mg-20 pct Ce master alloy in a steel crucible with SF6/CO2

protective cover gas. Billets of 75-mm diameter and 230-mm length were cast at about 973 K (700 �C) into apermanent mold. The billets were preheated to 673 K(400 �C) for 2 hours and extruded in a WELLMANENEFCO* 500-ton multipurpose vertical hydraulic

press fitted with a circular die. Solid rods, approximately25, 15, and 12.5 mm in diameter, corresponding to anextrusion ratio (ER) of 9, 25, and 36, respectively, wereextruded using boron nitride lubricant at a billet speedof 10 mm s�1 and air cooled. The samples are referred toas ER9, ER25, and ER36, respectively. Other sampleswere extruded at only one extrusion ratio, ER25,corresponding to the extrusion ratio for optimaltexture randomization in Mg-Ce billets, as discussedsubsequently.

The samples for microstructure evaluation wereobtained by cutting the front 0.15 m of the extrudedrod, where it had attained a steady state of extrusion.Metallographic samples were prepared by standardmethods, and the polished samples were etched in asolution containing 20 mL glacial acetic acid, 50 mLpicric acid, 10 mL methanol, and 10 mL deionizedwater. The polished samples were examined using ascanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) in a LEO** 1450 SEM

operating at 20 kV fitted with a TSL� EBSD camera.[5]

The compositions of the particles in the Mg alloys wereanalyzed by electron probe microanalysis. EBSD scanswere obtained using a beam step size of 2 lm and acamera length of 18 cm from an area approximately1 mm 9 1 mm, and at least three different scans fromthe same sample were collected to ascertain the repeat-ability of the results. The inverse pole figure (IPF) map,image quality map with the twin boundaries outlined,and the (0001), (10-10), and (1-210) pole figures wereprocessed with TSL OIM commercial software.

III. RESULTS

A. Microstructure of Mg-0.2 Wt pct Ce Alloys

The EBSD data from Mg-0.2 wt pct Ce alloyextruded at various extrusion ratios is shown as IPFmaps in Figure 1. The samples were taken from transverse

sections (extrusion axis normal to the sample plane),including the area near the outer diameter and the centerof the rod. The figures show that the microstructure inall samples consists of fully recrystallized grains. Mostgrain boundaries are high-angle boundaries. The orien-tation distribution of grains in the IPF maps (accordingto the color key) shows noticeable differences fordifferent extrusion ratios, but is quite consistent in thesame sample corresponding to one extrusion ratio.While nearly most grains are oriented with their basalplanes parallel to the extrusion axis in the ER9 sample(blue- or green-colored grains in Figures 1(a) and (b)),this is not the case in the ER25 sample, which has ahigher proportion of pink, red, or yellow colored grainsin Figures 1(c) and (d). The proportion of basal (blue orgreen) grains deceases in the sample in the followingorder: ER9>ER36>ER25.Figure 1 also shows twins in the samples, but their

amounts vary. There are more twins near the outersurface than near the center of the rod. Figure 2 showsthe image quality maps with the twin boundariesoutlined for ER9, ER25, and ER 36 samples near theouter edge. Only a few twins are observed in ER9 andER25 samples, including the extension, contraction, anddouble twins. In contrast, the microstructures in theER36 samples contain many twins and these twins arepredominantly {10-12} extension twins.Figure 3 shows the grain size distribution dates in the

center of the Mg-0.2 wt pct Ce alloy rods for differentextrusion ratios, as determined using the OIM software.The figure shows that grain size increases with increas-ing extrusion ratio. The average grain sizes were 38, 48,and 73 lm at the extrusion ratio of 9, 25, and 36,respectively.The textures of the Mg-0.2 wt pct Ce alloy rods

corresponding to the three extrusion ratios are presentedin Figure 4. The sample ER9 exhibits a ring basaltexture both near the outer edge and at the center(Figures 4(a) and (b)), but the texture in the center isweaker. The basal poles rotate by ~45 deg toward theED in the ER25 sample (Figures 4(c) and (d)), and thetexture in the center is weak. Figures 4(e) and (f) show anoticeable texture gradient in the ER36 sample; thebasal poles are parallel to the RD (Figure 4(e)) near theouter edge, while the sample exhibits a ring basal texture(Figure 4(f)) near the center. The ring basal texture inER36 is weaker than that in the ER9 sample. The resultssuggest that the texture evolution may be a strain- orstrain-rate- dependent phenomenon, and random orweak texture could be obtained in a narrow range,corresponding to the ER25 for the Mg-0.2 wt pct Cealloy at an extrusion temperature of 673 K (400 �C).

B. Microstructures of Mg Alloys with Al, Ca,and Ce Additions

Based on the results for randomizing the texture inER25 Mg-Ce samples, the microstructures of pure Mgand Mg alloy rods, with Ca, Ce, and Al additions, wereinvestigated in the ER25 samples only.Figure 5 shows the backscattered electron micro-

graphs along with the electron probe composition maps.

*WELLMAN ENEFCO is a trademark of WELLMAN ENEFCOLIMITED, West Yorkshire, UK.

**LEO is a trademark of Carl Zeiss Microscopy, Jena, Germany.

�TSL is a trademark of EDAX company, Mahwah, NJ.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 2012—2149

Fig. 1—IPF maps of the Mg-0.2 wt pct Ce alloy rods taken from transverse sections of the rods close to the outer surface (left) and the center(right) of the rods at various ERs.


The samples were taken from the longitudinal section(extrusion axis parallel to the sample plane). Thechemical composition data listed in Table I shows thatsmall intermetallic Mg12Ce phase particles[13] are dis-tributed homogeneously in the Mg-0.2 wt pct Ce alloy.Mg-0.5 wt pct Ce samples contain more of these inter-metallic particles compared to the Mg-0.2 wt pct Cesamples. There are few second-phase particles in Mg-0.2 wt pct Ca sample, mostly located in the grain interiorand aligned along the ED; these particles show thepresence of Fe and Si impurities besides some Ca. Thefew particles in Mg-1 wt pct Al alloy have a compositionclose to Mg17Al12 phase. In the Mg-3 wt pctAl-0.2 wt pct Ce sample, particles with a compositionclose to Al11Ce3 are seen aligned along the ED.[14]

In addition, the electron probe maps of individualelement compositions in Figure 5 suggest that, in theMg-0.2 wt pct Ce and Mg-0.5 wt pct Ce samples, theelement Ce is distributed in the matrix while there is no

measurable Ce present in the matrix of the Mg-3 wt pctAl-0.2 wt pct Ce sample. In the Mg-0.2 wt pct Casample and in the Mg-1 wt pct Al alloy sample, Caand Al, respectively, are seen to be distributed in thematrix, mostly in bands running parallel to the extrusionaxis. In the Mg-3 wt pct Al-0.2 wt pct Ce sample, thedistribution of Al in the matrix is similar to that inMg-1 wt pct Al alloy.Figure 6 shows the IPF maps of pure Mg and Mg

alloys from the center of the transverse sections of theER25 samples. The IPF maps of pure Mg, Mg-1 wt pctAl, and Mg-3 wt pct Al-0.2 wt pct Ce alloy show thatthe c-axes of the grains are normal to the ED. In theMg-0.2 wt pct Ca, Mg-0.2 wt pct Ce, and Mg-0.5 wt pct Ce samples, c-axes of the grains are oriented40 to 50 deg from the extrusion axis and some evenparallel to the extrusion axis.Figure 7 shows the grain size distribution of these

samples, which indicates that there are different grainsize range in unalloyed Mg and other Mg alloys, withthe Ca addition, the grain size was increased and there islargest grain size range from 0 to 150 mm in Mg-0.2 pctCa alloy. The Al and Ce addition drastically refine thegrain size and the smallest grain size range from0 ~ 50 mm was gained in Mg-0.5 pct Ce alloy. Table IIalso lists the grain sizes of the extruded rods from thecenter of the rod using the TSL software for numberaverage (Num.), grain size, and Area average (Area)grain size. Generally, these two average values will beclose to each other for uniform grain size distributionand differ more and more when fewer large grains aresurrounded by many small grains. The Al and Ceaddition refines the grain size compared to that of pureMg. The smallest grain size is obtained in Mg-0.5 wt pctCe alloy, and the highest grain size is in Mg-0.2 wt pctCa alloy. Mg-3 wt pct Al-0.2 wt pct Ce alloy has a finergrain size, between that of Mg-0.5 wt pct Ce andMg-1 wt pct Al extrusions. The deviations of area averagegrain size from number average grain size in Table II

Fig. 2—Image quality map with the twin boundaries outlined for the OUTER sample of Mg-0.2 wt pct Ce: (a) ER9, (b) ER25, and (c) ER36.The {10-12} extension twin boundaries (86 deg<1-210> ± 5 deg) are outlined in red, the contraction twin boundaries (56 deg<1-210> ± 5 deg)in green, and the double twins boundaries (38 deg<1-210> ± 5 deg) in blue.

Fig. 3—Grain size distribution in the Mg-0.2 wt pct Ce alloy rods.The measured average grain sizes are 38, 48, and 73 lm for theER9, ER25, and ER36 samples, respectively.


suggests that there is inhomogeneous distribution of thegrain size in Mg-0.2 wt pct Ca, and close to homoge-neous grain size distribution in Mg-Al and Mg-Cealloys. In the Mg-0.5 wt pct Ce alloy, the grain sizedistribution seems to be nearly uniform.

Figure 8 shows a plot of the aspect ratio values(transverse to ED vs parallel to the ED) and theirfraction for all alloys. The fraction of grains with aspectratios higher than 0.6 goes up in the order Mg-1 wt pctAl, Mg-3 wt pct Al-0.2 wt pct Ce, Mg-0.5 wt pct Ce,Mg-0.2 wt pct Ce, and Mg-0.2 wt pct Ca. This indicatesthat the recrystallized microstructure has formed inbands running parallel to the ED with grains growing inwhat could have been shear bands during deformationin the extrusion chamber. Ce and Al addition seem tofavor their formation over Ca addition.

Figure 9 shows the pole figures of all the samplesanalyzed with orthotropic symmetry. For the unalloyedMg, c-axes lie perpendicular to the ED with a randomdistribution of the grains between radial and tangentialdirections. The texture of Mg-1 wt pct Al andMg-3 wt pct Al-0.2 wt pct Ce alloy are similar to thatof pure Mg, but the texture intensity in Mg-3 wt pctAl-0.2 wt pct Ce alloy is weaker. In the case ofMg-0.2 wt pct Ca alloy, the c-axes of the majority ofthe grains are perpendicular to the ED, but the radialcomponent is nearly absent and the intensity of the polesis lower than that in Mg. For the Mg-0.2 wt pct Ce

alloy, the c-axes are aligned at an angle to ED, theintensity peaks lying along the ED axis. InMg-0.5 wt pct Ce alloy, the intensity of basal poles islowest, and the spread of the basal poles is greatest. Theresults indicate that the texture of Mg alloy could berandomized with the Ca and Ce additions; however, theCe addition to Mg-3 wt pct Al alloy retains the pure Mgtexture and only slightly reduces the intensity comparedto that of Mg and Mg-1 wt pct Al alloy.

IV. DISCUSSION

Extruded Mg-0.2 pct Ce alloy produced fully recrys-tallized grains for all three extrusion ratios. The grainsizes in the center of the extrusions increased withextrusion ratio, but the texture modification showed adifferent trend—ER25 showed maximum randomiza-tion, which dropped off for lower (ER9) and higher(ER36) extrusion ratio samples. As will be discussedsubsequently, the governing mechanism for texturerandomization is related to strain rate/temperaturecombination, and ER25 seems to correspond to aprocessing window to achieve best texture randomiza-tion in Mg-0.2 pct Ce alloy. For other alloy composi-tions, in the interest of keeping the test matrixmanageable, it was decided to examine the texture ofthe extruded material at this extrusion ratio, ER25.

Fig. 4—(0001) pole figures of Mg-0.2 wt pct Ce alloy rod corresponding to Fig. 1. The outer edge pole figure is analyzed with orthotropic sym-metry, and the center pole figure is analyzed with both orthotropic and axial symmetry.


Deformed Mg acquires a preferred orientation ortexture because deformation occurs on the most favor-ably orientated slip or twinning systems that reorient thegrains.[15] The ring basal texture and rolling basaltexture are almost universal in extruded Mg rod[2,4,16]

and rolled Mg sheet,[16–18] respectively, because of thebasal slip and {10-12} extension twinning[19,20] domi-nating the deformation process. These textures areretained through the recrystallization stage. In thisstudy, the Mg-0.2 wt pct Ce alloy exhibited nonbasaltexture in the ER25 sample after recrystallization, e.g.,the basal poles were rotated ~45 deg toward the ED.The ER9 sample had a near basal texture, and anoticeable texture gradient existed in the ER36 sample.The ER36 sample also exhibited more (10-12) extensiontwins still retained near the sample surface after extru-sion. It is proposed that as the deformed material

undergoes recrystallization, the ER25 sample does notretain the deformed texture due to growth of nonbasalgrains, while in ER9 and ER36 samples, the nonbasalgrains grow preferentially as in AZ31 and AM30 alloys.This effect is consistent with the result of Jiang et al.[12]

that dynamic strain aging in the presence of Ceatoms[12,21] is in the strain rate and temperature windowattained during extrusion. In the ER36 sample, thetexture gradient between the center and the samplesurface region seems to be related to incompleterecrystallization, with retaining deformed twinned mate-rial near the surface.In earlier studies of the relationship between particle-

simulated nucleation (PSN) and recrystallization texturein Mg alloys, the weakening of the texture of WE54alloy[3,6] with 4 wt pct RE was ascribed to particle-simulated nucleation or PSN. More recent studies[4] of

Fig. 5—Microprobe analysis showing the backscattered images and the element distribution maps for ER25 samples. The insert shows a sche-matic drawing of the sample location.

Table I. EDS Analysis of Particles, as Shown in SEM Images in Fig. 5 (All Values in Weight Percent)

Alloy Particles Mg Al Ca Ce Fe Si

Mg-0.2 wt pct Ce I 99.74 — — 0.26 — —II 99.8 — — 0.2 — —

Mg-0.5 wt pct Ce III 99.67 — — 0.33 — —Mg-0.2 wt pct Ca IV 75.8 — 0.06 — 10.29 3.75Mg-1 wt pct Al V 87.74 0.74 — — — —Mg-3 wt pct Al-0.2 wt pct Ce VI 42.3 27.21 — 29.56 — —


Mg-1 wt pct Y alloy showed significant texture ran-domization even though this alloy is expected to containvery few particles.[22] Increasing the Mn content (i.e.,particle) in AZ31 does not lead to changes in the sheet

texture.[23] PSN alone cannot explain the differenttexture in AZ31 and ME10 alloy, although both alloyscontain particles.[24] In this study, weak or nonbasaltexture in the Mg alloy with 0.2 wt pct Ce, 0.5 wt pct

Fig. 6—IPF maps from the longitudinal section (shown in the insert in Fig. 5) of (a) pure Mg, (b) Mg-0.2 wt pct Ce, (c) Mg-0.5 wt pct Ce,(d) Mg-0.2 wt pct Ca, (e) Mg-1 wt pct Al, and (f) Mg-3 wt pct Al-0.2 wt pct Ce alloys. The IPF (h) represents the ND direction of the observedsurface.

Fig. 7—Grain size distribution of Mg and Mg-Al alloys with Ca and Ce additions. The average grain sizes are listed in Table II.


Table II. Average Grain Size (lm) of Mg and Mg Alloys

CalculatedMethods

UnalloyedMg

Mg-0.2Wt pct Ce

Mg-0.5Wt pct Ce

Mg-0.2Wt pct Ca

Mg-1Wt pct Al

Mg-3 Wt pctAl-0.2 Wt pct Ce

Area average 67.6 45.9 24.2 93.4 52.8 34.8Number average 52.8 21.8 18.5 53.4 36.5 22.6Difference 14.7 24.0 5.7 40.0 16.3 12.2

Fig. 8—Grain aspect ratios of Mg and Mg alloys.

Fig. 9—(0001), (10-10), and (1-210) pole figures of Mg and alloys from the longitudinal section. TD refers to the tangential direction and ED tothe extrusion direction. The sample surface normal is parallel to the radial direction: (a) unalloyed Mg, (b) Mg-0.2 wt pct Ce, (c) Mg-0.5 wt pctCe, (d) Mg-0.2 wt pct Ca, (e) Mg-1 wt pct Al, and (f) Mg-3 wt pct Al-0.2 wt pct Ce.


Ce, and 0.2 wt pct Ca addition, and typical ring basaltexture in the Mg-1 wt pct Al and Mg-3 wt pct-0.2 wt pct Ce alloys are observed. Microprobe analysisfor those samples (Figure 5) indicated that there aremany particles in Mg-0.2 wt pct Ce, Mg-0.2 wt pct Ce,and Mg-3 wt pct Al-0.2 wt pct Ce alloys, and fewparticles in Mg-0.2 wt pct Ca and Mg-1wt pct Al alloys.The results suggest that particles are not necessary forthe texture randomization in magnesium.

Comparing the solute (Ce and Al) distribution in theMg-0.2 wt pct Ce and Mg-3 wt pct Al-0.2 wt pct Ce(Figure 5), Ce is distributed in the magnesium matrix inMg-0.2 wt pct Ce alloy, but Ce is exhausted by theAl11Ce3 particles and no Ce solute is found in the matrixin Mg-3 wt pct-0.2 wt pct Ce alloy. The differencesuggests that Ce solute in the magnesium may benecessary for the texture randomization for the Mg-Cealloys. Al solute does not affect the texture according tothe microstructure in Mg-1 wt pct Al alloy. Ca isdistributed as solute in Mg-0.2 wt pct Ca alloy, whichshowed a weak texture in this study. This is most likelydue to the occurrence of DSA in the Mg-Ca system inthe extrusion window as well and will require separateinvestigation in the future.

Combined with the result discussed previously thatthe deformation mode can remain unchanged but thetexture can change in the Mg-0.2 wt pct Ce alloy withchanging extrusion ratio, this suggests that specificsolutes must alter boundary migration and dislocationmotion during recovery or recrystallization. Solute dragis known to influence both the grain boundary mobilityof different grain boundary orientations[15] and therecrystallization kinetics.[25] In this study, the grainstructures in Figures 6 through 9 show more equiaxedgrains and higher grain aspect ratio in the alloys with Ceand Ca in solution. This is consistent with Ce and Casolutes influencing the boundary mobility due to solutedrag to influence texture. Al solutes in magnesium donot affect the texture evolution during the hot extrusionas the solute drag effect could be absent at thetemperature/strain rate combination present duringextrusion. In Mg-3 wt pct-0.2 wt pct Ce alloy, the Ceand Al are tied up in the Al11Ce3 phase and no Ce soluteis present in the alloy, leading to the ring basal texture.

The particle distribution in these alloys seems tocorrelate with the grain size. Particles can affect thedifferent deformation structures around them,[15] which,in turn influences nucleation of new grains and increasesthe driving force for recrystallization in the deformationzone near particles. The particles also affect graingrowth by pinning the grain boundaries. The smallergrain size in Mg-0.5 wt pct Ce and Mg-3 wt pctAl-0.2 wt pct Ce extrusions, which have many particlescompared to Mg-0.2 wt pct Ce extrusions, suggests suchdependence. In the case of pure Mg, Mg-1 wt pct Al,and Mg-0.2 wt pct Ca extrusions, which have minimalparticle content, the grain sizes are larger. The largestgrain size and nearly equiaxed grain structure in theMg-Ca alloys suggest that Ca in the boundary could bea factor for enhanced grain boundary migration.

The preceding results point to the fact that Mg alloys,which are solute strengthened by Al addition, will not

benefit from texture modification for ductility enhance-ment, but such addition can influence grain refinementin lean Al alloys when Al content is below the solubilitylimit. Concurrent strengthening and texture modifica-tion in Mg alloys were reported in Mg-Zn-Ce alloyswhere Zn-Ce intermetallics were not reported.[7,26]

Ternary Mg alloys containing Ca can offer opportuni-ties for simultaneous texture weakening effect and grainsize refinement if second-phase particles can be formedto hinder grain boundary motion.

V. SUMMARY

In this study, the effect of extrusion ratios on thetexture evolution of Mg-0.2 wt pct Ce alloy rods, andthe microstructure of pure Mg and Mg alloyed with Ca,Al, and Ce addition, extruded at a given extrusion ratio,were investigated.For Mg-0.2 wt pct Ce alloy, an extrusion ratio of 25:1

yielded more random textures than that at extrusionratios of 9:1 and 36:1. Weak nonbasal texture was alsogenerated in extruded rods with 0.5 wt pct Ce and0.2 wt pct Ca at the extrusion ratio of 25:1. The texturein Mg-3 wt pct Al-0.2 wt pct Ce alloy did not exhibitobvious modification from the ring basal texture. InMg-3 wt pct Al-0.2 wt pct Ce alloy, Ce and Al formedAl11Ce3 intermetallic phase and no Ce solute was foundin the matrix. In Mg-0.2 wt pct Ce and Mg-0.5 wt pctCe alloy, many intermetallic particles were observed.The results suggest that the texture modification in

Mg-0.2 wt pct Ca, Mg-0.2 wt pct Ce, andMg-0.5 wt pctCe alloy and the absence of such modification inMg-1 wt pct Al andMg-3 wt pct Al-0.2 wt pct Ce alloysare related to the nature of the solid solution and theoccurrence of dynamic strain aging during extrusion andsubsequent cooling and not necessarily the presence ofparticles. Mechanisms such as solute drag may be respon-sible for the texture modification, while the presence ofparticles inMg-CeandMg-Al-Cealloysmaybe responsiblefor grain refinement while playing a minor role in texturemodification through particle-simulated nucleation.

ACKNOWLEDGMENTS

This research was sponsored by a General Motors(GM)–Shanghai Jiao Tong University research collab-oration. One of the authors (L. JIN) acknowledges thefinancial support of the National Natural SciencesFoundation of China (Grant No. 50901044). Theauthors thank Robert Kubic for technical assistance inEBSD data collection experiments.

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