Microstructure and Mechanical Properties of Alternately-CompressedAZ31 Mg Alloy in Two Orthogonal Directions

Hyukjoon Kwon1,+, Mamoru Mabuchi1 and Yasumasa Chino2

1Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan2National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan

AZ31 Mg alloy was compressed alternately in two orthogonal directions at room temperature. Many {10�11} twins connected by {10�12}twins were found in the alternately-compressed specimens. The compressed specimens exhibited much lower ductility than the non-compressedspecimen, and their ductility decreased with an increase in the number of compression. It is therefore suggested that many triple junctions of{10�11} and {10�12} twin boundaries induced initiation of fracture, resulting in low ductility of the alternately-compressed specimens.[doi:10.2320/matertrans.MBW201305]

(Received October 8, 2013; Accepted January 15, 2014; Published February 28, 2014)

Keywords: magnesium alloys, mechanical properties, alternate compression, twinning

1. Introduction

Magnesium (Mg) draws much attention because it has highspecific strength and specific elastic modulus. However, Mgshows low ductility at room temperature due to its limitednumber of slip systems. Because non-basal slips hardly occurat room temperature in Mg, deformation twinning often playsan important role in plastic deformation. There are two maindeformation twins in Mg:1,2) {10�12} extension twin3,4) and{10�11} contraction twin.5) A {10�12} twin occurs at an initialstage of deformation or at a small strain because its criticalresolved shear stress (CRSS) is relatively low (about 2­3MPa2)) and it grows readily.6,7) On the other hand, a{10�11} twin occurs at a large strain, indicating that its CRSSis large. The twins affect mechanical properties of Mg.For example, {10�12} twins cause large strain-hardening,3,4)

and {10�11} twins are related to premature fracture because{10�11}-{10�12} double twins provide sites for nucleationof cracks.5) Recently, it was found that {10�12} twinning-detwinning readily occurs during multiple deformation suchas compressive pre-deformation on subsequent tensiledeformation in a small strain region.8) To date, there hasbeen much understanding on influence of multiple deforma-tion on {10�12} twins.8­14) However, there are few studiesabout influence of multiple deformation on {10�11} twins. Forpractical applications, it is important to investigate mechani-cal properties of Mg from the viewpoint of influences ofmultiple deformation on {10�11} twins.

Because a large strain is needed for formation of {10�11}twins, it is difficult to investigate influences of multipledeformation on {10�11} twins by compressive-tensile defor-mation. In the present work, alternate compression in twoorthogonal directions was repeated on AZ31 Mg alloy, and itsmicrostructure and compressive properties were investigated.

2. Experimental Procedures

In the present work, a Mn-less AZ31 magnesium alloy (Mg­2.8mass%Al­0.88mass%Zn­0.003mass%Mn) was used to

obtain a Mg specimen with a large grain size. An ingot ofthe Mg alloy was extruded at 773K with the extrusion ratioof 16 : 1 and the extrusion rate of 1mm/min, and an extrudedbar with 10mm in diameter was fabricated. The averagegrain size of the extrusion, which was measured by a linearinterception method, was 350 µm. The specimens foralternate compression were machined from the extrusion.The specimens had a cubic shape with dimensions of 6.5 ©6.5 © 6.5mm (Fig. 1). Alternate compression was performedat room temperature on the cubic specimens by compressionalong the extrusion direction (ED) and subsequent compres-sion along the transverse direction (TD), as shown in Fig. 1.The strain by each compression was 12%, and the initialstrain rate of compression was 1 © 10¹2 s¹1. The alternatecompression process was repeated 2, 4, 7 and 10 times. In thepresent paper, the specimens alternately-compressed 2, 4, 7and 10 times are called 2, 4, 7 and 10 times alternately-compressed specimens, respectively. For comparison, an as-extruded specimen, which is called the non-compressedspecimen, was also investigated.

To investigate mechanical properties, compressive testswere carried out at room temperature on the alternately-compressed and the non-compressed specimens, where thecompressive direction was parallel to the ED. In addition,

ED

1set

AZ31

TD

Fig. 1 Schematic illustration of a specimen and repeated alternatecompression. ED is extrusion direction and TD is transverse direction.

+Graduate Student, Kyoto University

Materials Transactions, Vol. 55, No. 4 (2014) pp. 739 to 741©2014 The Japan Institute of Metals and Materials RAPID PUBLICATION

their microstructures were investigated by an optical micro-scope (OM), a scanning electron microscope (SEM) and anelectron back-scattered secondary diffraction (EBSD).

3. Results and Discussion

Figure 2 shows OM images of the 2 and 7 timesalternately-compressed specimens. An inspection of Fig. 2revealed that many twins with narrow-band morphology wereformed in the alternately-compressed specimens. Usually, a{10�12} twin has lens-shape morphology and a {10�11} twinhas narrow-band morphology.2,15) Therefore, it is suggestedthat many {10�11} twins were generated by the repetition ofalternate compression. The number of twins increased withthe number of alternate compression.

Figure 3 shows a SEM image of the 7 times alternately-compressed specimen. Many twins with narrow-bandmorphology were observed, corresponding to Fig. 2. Thetwins with narrow-band morphology were {10�11} twins, asshown later. Also, white bands, which are shown by arrowsin Fig. 3, were found in matrix grains. The white bands wereconnected to the {10�11} twins.

The results of EBSD analyses are shown in Figs. 4 and 5.It was found from EBSD analyses that the misorientation ofbasal plane between the grain matrix and the twin withnarrow-band morphology, which is shown by a white bar inFig. 4, was 54°, indicating that the twin with narrow-bandmorphology is a {10�11} twin. Besides, EBSD analysesshowed that the misorientation between the grain matrix andthe band connecting to the {10�11} twins, which is shown bya white bar in Fig. 5, was 82°. This indicates that the bandsconnecting to the {10�11} twins are {10�12} twins.

Figure 6 shows the stress­strain curves in the alternately-compressed and the non-compressed specimens. Because astrain of approximately 0.05 was generated by the repeatedalternate compression, the stress­strain curves of alternately-compressed specimens were drawn from the strain ofapproximately 0.05. The results in Fig. 6 are summarizedin Table 1. The alternately-compressed specimens exhibitedhigher ultimate strengths and yield stresses than the non-compressed specimen. Also, the alternately-compressedspecimens showed larger strain hardening than the non-compressed specimen. However, the fracture strain was lowerfor the alternately-compressed specimens than for the non-compressed specimen. The effect of number of alternatecompression on the ductility for the 2, 4, 7 and 10 timesalternately-compressed specimens was not as large as theeffect of alternate compression for the non-compressedspecimen and the 2 times alternately-compressed specimen.

200μμm 200μm

(a) (b)

Fig. 2 Optical microscope images, (a) 2 times alternately-compressedspecimen and (b) 7 times alternately-compressed specimen.

5μμm

Fig. 3 A scanning electron microscope image of the 10 times alternately-compressed specimen. White bands, which are shown by arrows, areconnected to {10�11} twins. As shown later, the white bands are {10�12}

twins.

5μm

ED

ND

1010

0001 1210

Fig. 4 An electron back-scattered secondary diffraction (EBSD) map in the7 times alternately-compressed specimen after compression in extrusiondirection. EBSD analyses showed that the misorientation of basal planebetween the grain matrix and the twin with narrow-band morphology,which is shown by a white bar, is 54°.

5µm

ED

ND

1010

0001 1210

Fig. 5 An electron back-scattered secondary diffraction (EBSD) map in the7 times alternately-compressed specimen after compression in transversedirection. EBSD analyses showed that the misorientation of basal planebetween the grain matrix and the band connecting to the {10�11} twins,which is shown by a white bar, is 82°.

0.1 0.20

100

200

300

400

0.3

Tru

e St

ress

, σ /

MPa

True Strain, ε

2times4times

non-compressed

alternately-compressed

10times7times

0

Fig. 6 Stress­strain curves of the alternately-compressed and the non-compressed specimens.

H. Kwon, M. Mabuchi and Y. Chino740

It has been found in many previous investigations5,16,17)

that {10�11}-{10�12} double twins provide sites for fracture.A {10�11} twin boundary tends to absorb dislocations, whilea {10�12} twin boundary has a repulsive interaction withdislocations. Hence, dislocations absorbed in a {10�11} twinboundary can pile up at a {10�12} twin boundary. Recently,Yuasa et al.18) suggested that the stress concentration isgenerated by pileup of twinning dislocations around triplejunctions in a {10�11}-{10�12} double twin, resulting ininitiation of fracture at the double twin. In the present work,{10�12} twins connected to {10�11} twins were formed in thealternately-compressed specimens. Therefore, it is suggestedthat the low ductility of the alternately-compressed specimensis related to {10�11} twins connected by {10�12} twins.

4. Conclusions

AZ31 Mg alloy was compressed alternately in twoorthogonal directions (ED and TD) at room temperature,and its microstructure and compressive properties wereinvestigated. The alternately-compressed specimens exhib-ited much lower ductility than the non-compressed specimen,and their ductility decreased with an increase in the numberof alternate compression. Many {10�11} twins connected by{10�12} twins were formed in the alternately-compressed

specimens, and triple junctions consisting of {10�11} and{10�12} twin boundaries were formed in the alternately-compressed specimens. Therefore, it is suggested that thetriple junctions induce initiation of fracture, resulting in lowductility of the alternately-compressed specimens.

Acknowledgment

The authors thank Dr. T. Hirata (Technology ResearchInstitute of Osaka Prefecture) for EBSD analyses.

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Table 1 Mechanical properties of the alternately-compressed specimensand the non-compressed specimen.

Type of compressionUltimate strength,

MPaYield stress,

MPaFracture strain,

%

Alternately-compressed

2 times 333 91 16

4 times 332 84 15

7 times 318 83 15

10 times 319 82 15

Non-compressed 299 57 21

Microstructure and Mechanical Properties of Alternately-Compressed AZ31 Mg Alloy in Two Orthogonal Directions 741