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Texture and Mechanical Properties of Mg 3Al 1Zn 0 5Mn 1 5Ca Alloy Produced by Torsion Extrusion

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Texture and Mechanical Properties of Mg-3Al-1Zn-0.5Mn-1.5Ca Alloy

Produced by Torsion Extrusion

Yasumasa Chino

1;*

, Xinsheng Huang

1

, Kazutaka Suzuki

1

, Kensuke Sassa

1

,

Michiru Sakamoto

1

and Mamoru Mabuchi

2

1Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology,

Nagoya 463-8560, Japan

2Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan

Torsion extrusion was carried out on Mg-3Al-1Zn-0.5Mn-1.5Ca (in mass%) alloy (AZ31+1.5Ca alloy). The annealed torsion-extruded AZ31+1.5Ca alloy exhibited higher elongation compared with the annealed extruded AZ31+1.5Ca alloy without torsion. The ductility enhancement was attributed to a unique texture withh10110idirection inclined 30from the extrusion direction and low pole density. Besides, the

annealed torsion-extruded AZ31+1.5Ca alloy exhibited higher strength compared with the annealed torsion-extruded Mg-3Al-1Zn-0.5Mn (in mass%) alloy (AZ31 alloy). In the extruded AZ31+1.5Ca alloy, the fine second phase particles, which contributed to a suppression of the grain growth during extrusion and annealing, were observed. The enhanced strength of the annealed torsion-extruded AZ31+1.5Ca alloy was likely attributed to not only grain refinement but also strengthening by the presence of the second phase particles.

[doi:10.2320/matertrans.MH200909]

(Received October 19, 2009; Accepted February 26, 2010; Published April 25, 2010)

Keywords: magnesium alloy, texture, mechanical properties, grain refinement, torsion extrusion

1. Introduction

Lightweight materials can greatly contribute to a reduction in environmental load through light construction. In partic-ular, Mg alloys have the high potential for improving fuel efficiency and reducing CO2emission of vehicles because of

their high specific strength and stiffness.1) In Mg, the ð0001Þh11220i basal slip takes place preferentially because the critical resolved shear stress (CRSS) for the basal slip is lower than those for the non-basal slips, that is, the prismatic and pyramidal slips.2) Five independent slip systems are

necessary to fulfill von Mises criterion.3)However, the basal

slip has only two independent slip systems.4)This gives rise

to the poor ductility and formability for a polycrystalline magnesium and its alloy, thereby limiting their applications. Mukaiet al.5)showed that an AZ31 alloy processed by a shear extrusion (equal channel angular extrusion (ECAE)) exhibited a significant enhancement of room temperature ductility. The ductility enhancement of the ECA-extruded Mg alloy was related to an unusual texture, where the basal planes were highly inclined (45) from the extrusion direction axis.6)Recently, authors newly focused on torsion

extrusion7–12) as another shear extrusion process. The

schematic view of torsion extrusion is shown in Fig. 1. The torsion extrusion is characterized by the rotation of a die during a hot extrusion. The torsion extrusion has been proposed in order to reduce an extrusion load and accumulate a plastic strain in metals.7–9)Mizunuma10,11)revealed that the

accumulation of plastic strain by torsion extrusion was an effective method for grain refinement of commercial Mg alloys. Furthermore, authors12) revealed that a commercial Mg alloy (Mg-Al-Zn alloy) torsion-extruded at 523 K tended to produce a unique texture with h10110i direction inclined

30 from the extrusion direction, resulting in significant enhancement of tensile ductility.

The aim of this study is to investigate mechanical properties of torsion-extruded Mg-Al-Ca alloy. It is known that an addition of Ca makes magnesium alloy noncombus-tible, which can facilitate casting preparation of raw Mg materials.13) Besides, insoluble second phase particles

(such as Al2Ca and Mg2Ca) in Mg-Al-Ca alloy is known to

serve as nucleation sites for recrystallization during hot deformation.14–16) Thus, it is expected that Ca addition in Mg alloy improves not only casting properties but also mechanical properties. In this study, the torsion extrusion was carried out on a Mg-Al-Ca alloy, and the relationships between microstructure, texture and mechanical properties were investigated.

Load

Load

Punch Container

Heater

Material

Die

(rotated by motor) Ram

Ram

Fig. 1 Schematic view of torsion extrusion.

*Corresponding author, E-mail: y-chino@aist.go.jp

Special Issue on Growth of Ecomaterials as a Key to Eco-Society IV

[image:1.595.347.501.373.569.2]
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2. Experimental Procedure

A Mg-2.9Al-1.1Zn-0.5Mn-1.5Ca alloy (AZ31+1.5Ca alloy) ingot with 26 mm in diameter and 18 mm in height was prepared. Microstructure of the as-received AZ31+1.5Ca alloy ingot is shown in Fig. 2. The micro-structure had equiaxed grains with a grain size of 47mm. The as-received AZ31+1.5Ca ingot was torsion-extruded at 523 K with the extrusion ratio of19 : 1, and the extrusions with 6 mm in diameter were obtained. The rotation rate of a die was fixed to 6 cycle/min, and the punch speed was fixed to 0.5 mm/min. It should be noted that the processing temperature increased from 523 to 553 K during the torsion extrusion due to an intense torsion strain. An extrusion without torsion was also processed as a reference specimen. The extrusion temperature for the reference specimen was fixed to 543 K. The temperature rise during an extrusion without torsion was not observed. Finally, the extrusions were annealed at 623 K for5:4103s.

Microstructures of AZ31+1.5Ca extrusions were inves-tigated by optical microscopy (OM) with a typical metallo-graphic etching technique. The grain size was measured by the line intercept method (d¼1:74L, wheredandLare the grain size and the line intercept, respectively). A Jeol JXA-8800RL electron probe microanalyzer (EPMA) was used to determine the distribution of the alloying elements in the AZ31+1.5Ca extrusions. The acceleration voltage was set to 40 kV and the probe current to 50 nA using a focused electron beam. The distribution of the alloying elements was determined with a qualitative mapping procedure.

The {0001},f10110g,f11220gandf10111gpole figures were measured by the Schulz reflection method using a X-ray diffractometer (Rigaku RINT ultima III) to observe the plane perpendicular to the extrusion direction, using Cu K radiation at 40 kV and 40 mA with a sample tilt angle ranging 0 to 75. The data were normalized by a powder data.17)

Since it is difficult to obtain a AZ31+1.5Ca alloy powder, data of a pure Mg powder was used for a substitute for the random data.

The specimens for tensile tests were machined from the annealed extrusions, where the tested specimens had a gauge length of 10 mm and a gauge diameter of 2.5 mm. Tensile

tests were carried out at room temperature and at the initial strain rate of1:7103s1.

3. Results and Discussion

The microstructures and macrostructure of the reference and torsion-extruded specimens before annealing are shown in Figs. 3 and 4, respectively, where the plane parallel to the extrusion direction was observed. Figure 3(a) exhibits the microstructure of the reference specimen at the entrance of the die. The equiexed grains were gradually elongated along the extrusion direction, and the fine recrystallized grains

(d¼2mm: recrystallized region) were formed around the

grain boundary with a necklace structure. The recrystalliza-tion was gradually promoted as the extruded material approached an exit of the die as shown in Fig. 3(b). In the case of the torsion extrusion, the coarse equiaxed grains were suddenly elongated along the rotation direction of a die, and the necklace type recrystallization was developed around grain boundary as shown in Fig. 4(a). Finally, the fine-grained microstructure with a grain size of 3mm (recrystal-lized region) was obtained as shown in Fig. 4(b). The difference in the deformation behavior between the reference and torsion-extruded specimens indicates that an intense torsion deformation was effectively imposed on the specimen during the torsion extrusion.

It is known that the grain size of metals can be often associated with Z (Zener-Hollomon) parameter:

d/ ""_exp Q

RT

1=p

ð1Þ

where d is the grain size, ""_ is the strain rate, Q is the activation energy, R is the gas constant, T is the absolute temperature and pis the grain size exponent (from 2 to 3 for AZ91 alloy).18)Maet al.9)reported that the equivalent strain

rate imposed on a specimen during the torsion extrusion was proportional to an angular velocity of a die, suggesting that

100

µ

m

Fig. 2 Microstructure of an as-received AZ31-1.5Ca alloy.

(a)

Extrusion direction 3mm (a)

(b)

20µm

20µm (b)

Radial direction CL

[image:2.595.61.276.71.234.2] [image:2.595.306.547.543.748.2]
(3)

an increase in the angular velocity contributes to the grain refinement. However, in our study, the torsion-extruded specimen showed a little larger grain size than the reference specimen contrary to expectation. The coarse microstructure of the torsion-extruded specimen is likely related to the competition of recrystallization and grain growth due to the heat generation during the torsion extrusion.

It is noted that the recrystallized grains of AZ31+1.5Ca reference and torsion-extruded specimens exhibited smaller grain size compared with AZ31 reference (d¼5mm) and torsion-extruded specimens (d¼7mm), whose extrusion condition was similar to the present investigation.12) As

shown in Figs. 3(a) and 4(a), in the AZ31+1.5Ca reference and torsion-extruded specimens, the second phase particles were distributed around the grain boundaries, and the recrystallization was promoted around second phase par-ticles. These experimental results suggest that second phase particles in the AZ31+1.5Ca alloys offered nucleation sites for recrystallization due to large strain in deformation zone around the particles, and second phase particles effectively pinned grain growth during extrusion.

[image:3.595.52.552.65.290.2]

Secondary electron image (SEI) and aluminum, calcium and manganese images in the annealed torsion-extruded AZ31+1.5Ca specimen observed by EPMA are shown in Fig. 5, where the plane parallel to the extrusion direction was observed. Strong aluminum and calcium peaks were dis-tributed parallel to the extrusion direction, and the strong aluminum and calcium peaks clearly corresponded to the sites of the second phase particles, indicating that composi-tion of the second phase particles corresponded to Al2Ca,

which were often observed in Mg-Al-Ca alloys.19,20)

Microstructures of the reference and torsion-extruded specimens after annealing are shown in Fig. 6, where the plane parallel to the extrusion direction was observed. Figure 6 reveals that second phase particles, whose diameter was from sub-micron to 2mm, were distributed along the extrusion direction in the case of the reference specimen. On the other hand, in the case of the torsion-extruded specimen,

second phase particles, whose size was almost the same as those of reference specimens, were not always distributed along the extrusion direction. The size and distribution of the second phase particles in AZ31+1.5Ca alloy did not vary with the annealing.

In the previous study,12) an annealed AZ31 specimen processed by the torsion extrusion exhibited a duplex microstructure composed of the coarse grains of 100mm and fine grains of 20mm. An appearance of the duplex microstructure in the annealed AZ31 torsion-extruded speci-men was recognized as an occurrence of the abnormal grain growth, which was often observed in a severe plastic-deformed specimen followed by an annealing.21,22) The

duplex microstructure was also observed in the annealed torsion-extruded AZ31+1.5Ca specimen (Fig. 6(b)). How-ever, grain size of the duplex microstructure (coarse grain: 25mmand fine grain: 6mm) was much smaller than that of the annealed torsion-extruded AZ31 specimen. Previous studies reported that an abnormal grain growth was observed in an annealed friction-stir-welded ZK60 alloy21)and an annealed friction-welded AZ31 alloy,22) and suggested that the abnormality in grain growth seemed to be attributed to the meso-scale variation in local second phase particle density.22) Thus, it can be presumed that suppression of the abnormal

SEI Al

Ca Mn

20µm Extrusion direction

Level Level

Level Level

658

100 698

1401

29 761

297

0 148 794

0 337

Fig. 5 SEI and Al, Ca and Mn images by EPMA for the annealed torsion-extruded AZ31+1.5Ca specimen, where the plane parallel to the extrusion direction was observed.

(b)

20µm

(a)

20µm

Extrusion direction

Fig. 6 Microstructures of the annealed AZ31+1.5Ca specimens: (a) reference specimen and (b) torsion-extruded specimen, where the plane parallel to the extrusion direction was observed.

(a)

Extrusion direction 3mm

(a)

(b)

20µm

20µm

(b)

CL

Radial direction

[image:3.595.48.291.72.270.2] [image:3.595.303.549.73.293.2] [image:3.595.306.548.357.442.2]
(4)

grain growth observed in the annealed torsion-extruded AZ31+1.5Ca alloy was closely related to the pinning effects by second phase particles in the specimen.

It is noted that grain size of the recrystallized grains in the annealed reference specimen (6mm) was almost the same as that of fine grains of the annealed torsion-extruded specimen

(6mm), and that elongated grains with length of 20–100mm

were observed in the annealed reference specimen. These elongated grains were not likely originated from the abnormal grain growth during annealing but originated from the un-recrystallization during extrusion (Fig. 3(b) and Fig. 6(a)).

The pole figures of the reference specimen before anneal-ing are shown in Fig. 7(a). The strong texture off10110gplane can be considered as having a strong h10110i==ED fiber texture with the (0001) basal planes parallel to the extrusion direction of the reference specimen. The pole figures of the torsion-extruded specimen before annealing are shown in Fig. 7(b). On the other hand, the torsion-extruded specimen had the h211113i==ED texture, and the prismatic pole and

basal pole were oriented to 30 and 60 from the extrusion direction, indicating that the inclination of prismatic pole was mainly caused by the inclination of basal pole. Additionally, it is observed that the f11220gplane was distributed around 0–20 from the extrusion direction, indicating that the inclination of prismatic pole was also caused by theh11220i== ED texture.

It should be noted that the prismatic pole density of the torsion-extruded specimen was much smaller than that of the reference specimen. The smaller pole density implies that a more random orientation was attained by the torsion extrusion. These tendencies of texture were similar to that of the ECA-extruded AZ31 specimen5) and that of the

torsion-extruded AZ31 specimen.12)It is also worthwhile to

note that thef10110gandf10111gpole figures of the annealed specimens showed almost the same tendency as those of the as-extruded specimens as shown in Fig. 8.

The engineering stress-strain curves for the annealed reference specimen and the annealed torsion-extruded speci-men are shown in Fig. 9. The 0.2% proof stress, ultimate tensile strength and elongation to failure were 181 MPa, 274 MPa and 34% for the torsion-extruded specimen and 278 MPa, 313 MPa and 12% for the reference specimen. The yield stress of the torsion-extruded specimen exhibited 35% smaller value compared with that of the reference specimen. Besides, it is noted that the elongation to failure of the torsion-extruded specimen exhibited 2.8 times higher value compared with that of the reference specimen.

In Mg, the basal slip is preferentially operative because the CRSS of basal slip is much lower than that of non-basal slips.2) In Fig. 7(b), the basal pole of the torsion-extruded specimen was distributed at a tilt of 60 to the extrusion direction. When tensile stress is applied along theh211113iaxis of a Mg crystal to cause slip on the ð0001Þh11220i system, the Schmid factor of the basal slip is calculated to 0.35. Thus, it is suggested that an increase of grains with high Schmid

MAX:2.1 MAX:27.8 1.2 1.8 1.5 1.2 1.5 1.8 21.3 14.8 8.3 1.9 MAX:1.7 MAX:3.4 (b) MAX:0.9 (a) MAX:0.8 0.7 0.5 0.5 0.3 0.2 0.6 0.6 0.5 0.3 0.2 0.2 0.3 1.4 1.0 0.7 1.0 2.72.1 1.4 0.7 2.7 2.1 1.4 0.7 {0001} {0001} {1010} {1010} {1120} {1120} 1.9 1.9 1.5 1.5

{1011} MAX:2.2 {1011} MAX:1.6

1.6 1.4 1.4 1.2 0.9 0.7 0.8 1.3 1.7 0.8 1.3 1.7 1.3 0.9 0.8 0.7

Fig. 7 Pole figures of as-extruded AZ31+1.5Ca specimens: (a) reference specimen and (b) torsion-extruded specimen. (The measured surface is normal to the extrusion axis.)

MAX: 2.1 (b) (a) MAX: 13.8 1.2 1.5 1.8 0.9 11.5 9.8 8.1 6.4 1.5 1.3 {1010} {1010}

{1011} {1011} MAX:

1.9 0.8 0.8 1.1 1.4 1.7 1.4 1.1 0.8 0.6 1.7 1.4 1.2 1.0 0.7

[image:4.595.52.288.73.481.2] [image:4.595.314.547.77.298.2]
(5)

factor may be one of the reasons for lower 0.2% proof stress and much higher elongation of the torsion-extruded specimen than those of the reference specimen. Besides, the torsion-extruded specimen exhibited more random orientation of the texture than the reference specimen with the strongh10110i== ED fiber texture. The more random orientation of the texture may be another reason for an increase of grains with high Schmid factor of the torsion-extruded specimen.

Koike et al.23) reported that the grain refinement was effective for an activation of the prismatic slip at grain boundaries, resulting in an enhancement of the ductility of Mg alloys. In our study, the ductility of the annealed torsion-extruded specimen was superior to the annealed reference specimen regardless of the similar microstructure. This fact denotes that the texture effect on ductility overwhelmed that of the grain refinement.

It is worthy of special mention that the tensile properties of torsion-extruded AZ31+1.5Ca alloy exhibited much higher 0.2% proof stress (181 MPa) and higher ultimate tensile strength (274 MPa) compared with those of the torsion-extruded AZ31 alloy (The 0.2% proof stress: 83 MPa, ultimate tensile strength: 238 MPa).12) One possible reason for higher strength of the torsion-extruded AZ31+1.5Ca alloy is a difference in grain size, because grain size of the annealed AZ31+1.5Ca alloy (25mmfor the coarse grains and 6mmfor the fine grains) exhibited much smaller grains than that of the annealed AZ31 alloy (100mmfor the coarse grains and 20mmfor the fine grains). Nussbaum et al.24)reported

that Hall-Petch slope of pure Mg and Mg-Al solid solution alloys was 0.28 MPa mm1=2at room temperature. This value

denotes that the corresponding strength increase in yield stress is approximately 35 MPa, when the grain size is assumed to be refined from 100mm to 20mm. This result suggests that one of the main strengthening mechanisms of the torsion-extruded AZ31+1.5Ca alloy is the grain refine-ment strengthening mechanism. However, a large difference of about 100 MPa in yield stress between the torsion extruded AZ31+1.5Ca and AZ31 alloys cannot be explained by only the grain refinement strengthening mechanism.

Mabuchi and Higashi25)studied mechanical properties of the extruded Mg alloy containing dispersed Mg2Si particles,

and revealed that high strength was attained by the presence of dispersed Mg2Si particles, which contributed to various

strengthening mechanisms such as load transfer and Orowan mechanisms. In the present study, second phase particles with sub-micron to 2mm were mainly distributed at grain boundary of coarse grains in the torsion-extruded specimen as shown in Fig. 6(b). Thus, it is suggested that the increase in yield stress and ultimate tensile strength of the extruded AZ31+1.5Ca alloy was likely attributed to not only grain refinement strengthening mechanism, but also the strength-ening mechanisms due to the presence of the second phase particles. Quantitative studies for the contribution of second phase particles to the strengthening of the extruded AZ31+1.5Ca alloy are still in progress.

Anyway, it is demonstrated that the annealed torsion-extruded AZ31+1.5Ca alloy exhibited not only superior elongation but also superior strength. The superior elongation was originated from the texture modification by the torsion deformation, and the superior strength was likely originated from not only strengthening by the grain refinement but also strengthening by the presence of the second phase particles.

4. Summary

The torsion extrusion was carried out on a AZ31+1.5Ca alloy, and the relationships between microstructure, texture and mechanical properties were investigated. It is demon-strated that the application of the torsion extrusion to AZ31+1.5Ca alloys promoted the formation of the unique texture withh10110idirection inclined 30from the extrusion direction and low pole density. The above texture modifi-cations were attributed to the enhancement of the Schmid factor of basal slip, resulting in the ductility enhancement of the annealed torsion-extruded specimen. Furthermore, the fine second phase particles were dispersed at grain boundary of coarse grains in the extruded AZ31+1.5Ca alloy. These second phase particles likely contributed to not only the suppression of grain growth but also the strength-ening due to the presence of the second particles, resulting in the enhanced-strength of the annealed torsion-extruded AZ31+1.5Ca alloy.

Acknowledgements

This study was partially supported by the Research for Promoting Technological Seeds from the Japan Science and Technology Agency (JST), Japan.

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Engineering strain

Engineering stress (MPa)

: reference specimen : torsion extruded specimen

0 0.1 0.2 0.3 0.4

350

280

210

140

70

[image:5.595.71.270.71.264.2]
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7) W. Bochniak and A. Korbel: Mater. Sci. Technol.16(2000) 664–669. 8) X. Ma, M. R. Barnett and Y. H. Kim: Int. J. Mech. Sci.46(2004) 449–

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Figure

Fig. 1Schematic view of torsion extrusion.
Fig. 2Microstructure of an as-received AZ31-1.5Ca alloy.
Fig. 4Microstructuresandmacrostructureofthetorsion-extrudedAZ31+1.5Ca specimen before annealing
Fig. 7Pole figures of as-extruded AZ31+1.5Ca specimens: (a) referencespecimen and (b) torsion-extruded specimen
+2

References

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