Elastic and Damping Properties of AZ31 Magnesium Alloy Sheet

Processed by High-Temperature Rolling

Kazutaka Suzuki1;*, Yasumasa Chino1, Xinsheng Huang1 and Mamoru Mabuchi2

1Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology,Nagoya 463-8560, Japan2Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan

Elastic and damping properties of AZ31 magnesium alloy sheet processed by high-temperature rolling were investigated. The specimenrolled at high temperature (798 K) exhibited the significant low basal texture intensity and the wide spread of the basal pole toward the RD andTD, compared with the specimen rolled at 573 K. Young’s modulus of the specimen rolled at 798 K had smaller value at all angles than that ofthe specimen rolled at 573 K. Besides, the specimen rolled at 798 K exhibited slightly higher internal friction compared with the specimen rolledat 573 K over the strain amplitude range investigated. The suppression of the strong basal texture formation in the specimen rolled at 798 K likelycontributed to a reduction of Young’s modulus and an increase in internal friction, because the breakaway stress, which is closely related to themacro-yield stress, decreases with an increase in the Schmid’s factor of basal slip. [doi:10.2320/matertrans.L-M2011824]

(Received March 28, 2011; Accepted July 11, 2011; Published October 25, 2011)

Keywords: magnesium alloy, rolling, texture, damping, elastic modulus

1. Introduction

Magnesium alloys are promising light structural materialsbecause of their excellent properties such as high specificstrength and high specific stiffness. For their wider ap-plicability, magnesium alloy sheets with high room temper-ature formability should be developed. However, (0002)basal planes are aligned parallel to the rolling direction (RD),and the intense basal plane texture is formed during rolling,resulting in the poor formability of rolled magnesium alloysheets at room temperature.

Recently, magnesium alloy sheets showing excellentroom temperature formability have been developed.1–12)

One method for enhancement of room temperature form-ability of magnesium alloy sheets is the use of improvedrolling technologies such as shear rolling1–7) and high-temperature rolling.3,4,6,9–12) High-temperature rolling issuggested to be one of the attractive rolling technologies,because a conventional rolling machine is directly available,if rolling temperature can be set to more than 723 K. Themagnesium alloy sheet processed by the high-temperaturerolling exhibits a significant weak basal texture, resultingin the excellent room temperature formability comparableto those of aluminum alloy sheets.

It is well known that pure magnesium and magnesiumalloys have high damping properties compared with variousmetallic materials.13–19) However, damping property ofcommercial magnesium alloys such as Mg-Al-Zn alloys isnot so high compared with that of pure magnesium,15,17) thus,application of magnesium alloy sheets for damping materialsis still limited.20) In general, elastic and damping properties ofmetals are recognized to be dependent on microstructuralfactors such as grain size, precipitates, texture and so on.Among the limited literatures, Sugimoto et al.13) investigatedthe effects of crystal orientation on the elastic and dampingproperties of pure magnesium, and suggested that elastic and

damping properties strongly depend on the crystal orientationaccording to the Schmid’s law. These reports imply thatmagnesium alloy sheets with significant weak basal texturemay exhibit different elastic and damping properties withthose of the conventional magnesium alloy sheets with strongbasal texture. However, studies on elastic and dampingproperties of rolled magnesium alloy sheets with differenttexture are very few. Especially, there is no study reportingthe effects of texture distribution on the elastic and dampingproperties of rolled magnesium alloy sheets. Thus, in thepresent paper, AZ31 (Mg-2.7Zn-0.8Al-0.4Mn in mass%)alloy sheets were processed by high-temperature rolling,and the elastic and damping properties of the sheets wereinvestigated.

2. Experimental Procedure

A specimen with 120 mm length, 80 mm width and 5 mmthickness was machined from an as-received extrusion ofAZ31 (Mg-2.7Al-0.8Zn-0.4Mn in mass%) alloy, and thenwas homogenized at 723 K for 20 h prior to rolling. Thespecimen was rolled from 5 to 1.26 mm in thickness by 6passes at 723 K, and was subsequently rolled down to 1.0 mmby a single pass at 798 K, which is close to the solidustemperature (839 K) of the AZ31 alloy.21) The reduction perpass was 20% for each pass and the total reduction was 80%.As a reference, the specimen rolled at low temperature wasprepared, where the specimen was rolled from 5 to 1.0 mmby 7 passes at 573 K at a rolling reduction of 20%. Finally,the rolled specimens were fully annealed at 623 K for5:4� 103 s in order to minimize dislocation density in therolled specimens. Here after, the specimen rolled at 798 K atfinal rolling pass is called the specimen rolled at 798 K, andthe specimen rolled at 573 K through the rolling scheduleis called the specimen rolled at 573 K.

The specimen for the measurement of elastic and dampingproperties with a 52 mm length, a 10 mm width and a 1 mmthickness was machined from the rolled specimen. Young’s*Corresponding author, E-mail: kztk.suzuki@aist.go.jp

Materials Transactions, Vol. 52, No. 11 (2011) pp. 2040 to 2044#2011 The Japan Institute of Light Metals

modulus (E) was measured by the free resonance transversevibration method, and the internal friction (Q�1) of therolled specimens was measured by the resonance transversevibration method by using cantilevered beam specimen,where the angles between the longitudinal direction of thespecimen and the RD were set to 0�, 45� and 90� for themeasurement of Young’s modulus and 0� and 90� for themeasurement of internal friction.

Tensile specimens with a 12 mm gage length, a 4 mm gagewidth and a 1 mm gage thickness were machined from therolled specimens. Tensile tests were carried out with an initialstrain rate of 3� 10�3 s�1, where the angles between thetensile direction and the RD were set to 0�, 45� and 90�. Themicrostructures of the rolled specimens were investigatedby optical microscopy, and the (0002) plane pole figure ofthe rolled specimens at the center through a thickness wasinvestigated by Schulz reflection method.

3. Results and Discussion

The microstructures of the specimens rolled at 798 and573 K in the RD-ND plane are shown in Fig. 1, where the NDis the normal direction. The average grain sizes were 15.4 mm

for the specimen rolled at 798 K and 14.7 mm for thespecimen rolled at 573 K. There was a little difference ingrain size between them.

The (0002) plane pole figures of the specimens rolled at798 and 573 K are summarized in Fig. 2. Note that the basaltexture intensity of the specimen rolled at 798 K wassignificantly lower than that of the specimen rolled at573 K. The (0002) plane of the specimen rolled at 573 Kintensively distributed parallel to the sheet plane in whichthere was a tendency to align the basal poles with the ND, andthere was a slight spreading of the basal poles toward the RD.The similar tendency was observed in the specimen rolled at798 K, although there were large differences, namely, therewas a symmetrical splitting of the peaks in intensity by �20�

from the ND toward the RD, and also there was a widerspread of the basal pole toward the transverse direction (TD).The significant reduction of basal texture intensity and thewide spread of the basal pole toward the RD and TD in thespecimen rolled at 798 K are likely ascribed to the occurrenceof discontinuous static recrystallization during annealing.6,12)

Tensile properties including ultimate tensile strength(UTS), 0.2% proof stress (YS), fracture elongation (FE),uniform elongation (UE), r-value and strain hardening

(b)(a)

Fig. 1 Optical micrographs of rolled and subsequently annealed AZ31 alloy sheets: (a) specimen rolled at 798 K and (b) specimen rolled

at 573 K.

MAX: 9.4

RD

TD

(b)

MAX: 2.9

RD

TD

(a)

MIN MAX

Fig. 2 (0002) pole figures of rolled and subsequently annealed AZ31 alloy sheets: (a) specimen rolled at 798 K and (b) specimen rolled at

573 K.

Elastic and Damping Properties of AZ31 Magnesium Alloy Sheet Processed by High-Temperature Rolling 2041

exponent value (n-value) of the rolled specimens aresummarized in Table 1. The YS and r-value increased withincreasing the angle between the RD and the tensile direction,while the n-value showed a converse behavior. This is due tothe spread of the (0002) orientation and the inclination ofbasal poles in the RD, which are favored for basal slip duringdeformation in the tensile direction. It is noted that thespecimen rolled at 798 K exhibited the lower YS and r-valueand higher FE, UE and n-value than the specimen rolled at573 K, suggesting that the suppression of the strong basaltexture formation was attributed to the low yield stress andenhanced ductility of the specimen rolled at 798 K.

Figure 3 shows the Young’s modulus of the rolled speci-mens rolled at 798 and 573 K with different angles betweenthe longitudinal direction of the specimen and the RD. Inboth the specimens, the Young’s modulus increased with anincrease in the angles, implying that the spread of basal poletoward the RD promoted a decrease in Young’s modulus.It is noted that the Young’s modulus of the specimen rolledat 798 K had smaller value at all angles than that of the

specimen rolled at 573 K. This result indicates that the elasticproperties of the rolled specimen strongly depend on thebasal pole distribution.

Figure 4 shows the variation in internal friction of thespecimens rolled at 798 and 573 K as a function of strainamplitude ("). The range of strain amplitude was set to from0:5� 10�5 to 1:1� 10�3. The internal friction of both thespecimens exhibited strain-amplitude dependent with thesame manner as the previous studies,13,15,16) suggesting thatthe main damping mechanism is the dislocation damping,which is caused by the interaction between the dislocationand impurity atoms. In Fig. 4, the internal friction at thestrain amplitude of 5� 10�4 for the specimen rolled at798 K was 1:69� 10�3 at 0� and 1:68� 10�3 at 90�, andthat for the specimen rolled at 573 K was 1:63� 10�3 at 0�

and 1:57� 10�3 at 90�. Thus, the results in Fig. 4 indicatethat the specimen rolled at 798 K demonstrated slightlyhigher internal friction compared with the specimen rolledat 573 K over the strain amplitude range investigated. It issuggested that the internal friction has close relationships

Table 1 Tensile properties at room temperature of the rolled and

subsequently annealed AZ31 alloy sheets (UTS, ultimate tensile strength;

YS, 0.2% proof stress; FE, fracture elongation; UE, uniform elongation;

r-value, Lankford value; n-value, strain hardening exponent.)

Specimen OrientationUTS

(MPa)

YS

(MPa)

FE

(%)

UE

(%)r-value n-value

0� 258 143 29 22 1.1 0.26

Rolled at 798 K 45� 260 149 28 21 1.2 0.26

90� 262 153 28 20 1.3 0.25

0� 268 178 24 17 2.8 0.20

Rolled at 573 K 45� 269 182 24 16 3.0 0.19

90� 271 191 23 15 3.6 0.18

46

45

440° 45° 90°

Orientation

Youn

g’s

mod

ulus

, E/G

Pa

: Specimen rolled at 798 K

: Specimen rolled at 573 K

Fig. 3 Young’s modulus in the longitudinal direction of the specimen

machined from rolled and subsequently annealed AZ31 alloy sheets,

where the orientation indicates the angle between the longitudinal

direction of the specimen and the RD.

0.5

1.0

1.5

2.0

2.5

Strain amplitude, ε x 104

Inte

rnal

fric

tion,

Q- 1

x 10

3 (b)

0.5

1.0

1.5

2.0

2.5

3 6 9 120

Strain amplitude, ε x 104

Inte

rnal

fric

tion,

Q-1

x 10

3 (a)

: 0°: 90°

: 0°: 90°

3 6 9 120

Fig. 4 Variation in internal friction as a function of strain amplitude in rolled and subsequently annealed AZ31 alloy sheets: (a) specimen

rolled at 798 K and (b) specimen rolled at 573 K. The orientation indicates the angle between the longitudinal direction of the specimen

and the RD.

2042 K. Suzuki, Y. Chino, X. Huang and M. Mabuchi

with the basal pole distribution similar to the case of Young’smodulus.

It is known that the breakaway stress, which is defined asthe stress where the basal dislocations bow out to moveagainst the binding force of impurity, is closely related to themacro-yield stress, and it decreases with an increase in theSchmid’s factor.13) Sugimoto et al.13) investigated the effectsof crystal orientation on elastic and damping properties ofmono and polycrystalline pure magnesium, and suggestedthat the Young’s modulus and internal friction have theminimum and maximum values, when the Schmid’s factorfor basal slip has the maximum value of around 0.5,respectively. It is interest to note that an elastic constantof shear elasticity (C44) has a much lower value than thatof orthogonal elasticity (C11 and C33) in the case ofmagnesium crystal.22) Provided that the macroscopic elasticconstants are determined by basal texture distribution, thesuppression of basal texture formation may result in thedecrease in the elastic constants of the rolled specimen. Inthe present study, the specimen rolled at 798 K exhibitedsignificant reduction of basal texture intensity and the widespread of the basal pole toward the RD and TD. This factimplies that the Schmid’s factor for basal slip had a largevalue for the majority of grains in the specimen rolled at798 K. This may be one of the main reasons for the lowerYoung’s modulus and higher internal friction of the specimenrolled at 798 K. Thus, it is suggested that a suppression of thestrong basal texture formation by high-temperature rollingis effective for controlling elastic and damping propertiesof the rolled magnesium alloy sheets, at least in the strainamplitude range investigated.

According to the ‘‘Granato-Lucke’’ theory,23) a plot of theLn(Q�1

H � ") and the inverse of strain amplitude (1=") gives alinear relationship, if the main damping mechanism is thedislocation damping, where Q�1

H is the amplitude-dependentinternal friction. Figure 5 shows the ‘‘G-L plots’’ of thespecimens rolled at 798 and 573 K. The Q�1 is used as Q�1

H

since Q�1H is almost equal to Q�1 at comparatively high strain

amplitude. In general, the damping properties of magnesiumare weakly related to the strain amplitude at the low strainamplitude, while it is strongly related to the strain amplitudeat high strain amplitude.13,15,16,19) The linear relationshipin Fig. 5 can be roughly divided into two parts at around

5� 10�4 of strain amplitude (2000 of 1="), indicating thatthe effects of strain amplitude on the internal friction in thepresent study are qualitatively the same as the previousstudies. Nisiyama et al.16) suggested that the deviation fromthe straight line in the G-L plot is ascribed to the generationof deformation twins. Kageyama et al.15) pointed out thatthe deviation from the straight line in the G-L pot is likelybecause the effects of extended dislocation and thermalactivation process are not taken into consideration inGranato-Lucke theory. Identification of twinning and gen-eration of extended dislocation in the specimens rolled at798 and 573 K is still in progress. Further research isunderway to understand the role of damping mechanisms ofthe magnesium alloy sheets rolled at high temperature.

4. Summary

Elastic and damping properties of AZ31 magnesium alloysheet processed by high-temperature rolling were inves-tigated. The following results were obtained.(1) The specimen rolled at high temperature (798 K)

exhibited the significant reduction of basal textureintensity and the wide spread of the basal pole towardthe RD and TD compared with the specimen rolled at573 K. As a result of tensile test, the specimen rolled at798 K exhibited the lower 0.2% proof stress and r-valueand higher elongation and n-value than the specimenrolled at 573 K, suggesting that the suppression of thestrong basal texture formation was attributed to thelow yield stress and enhanced ductility of the specimenrolled at 798 K.

(2) Young’s modulus of the specimen rolled at 798 K hadsmaller value at all angles than that of the specimenrolled at 573 K. Besides, the specimen rolled at 798 Kdemonstrated slightly higher internal friction comparedwith the specimen rolled at 573 K over the strainamplitude range investigated. The lower Young’smodulus and higher internal friction in the specimenrolled at 798 K are likely ascribed to the suppression ofthe basal texture formation, because the breakawaystress, which is closely related to the macro-yield stress,decreases with an increase in the Schmid’s factor ofbasal slip.

-16

-15

-14

-13

-12

0 1000 2000 3000 4000 5000

ln (

QH

-1· ε

)

1/ε

-16

-15

-14

-13

-12

0 1000 2000 3000 4000 5000

ln (

QH

-1· ε

)

1/ε

: 0°: 90°

: 0°: 90°

(a) (b)

Fig. 5 G-L plots for the rolled and subsequently annealed AZ31 alloy sheets: (a) specimen rolled at 798 K and (b) specimen rolled at

573 K. The orientation indicates the angle between the longitudinal direction of the specimen and the RD.

Elastic and Damping Properties of AZ31 Magnesium Alloy Sheet Processed by High-Temperature Rolling 2043

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2044 K. Suzuki, Y. Chino, X. Huang and M. Mabuchi