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Influence of the Initial Texture on Texture Formation of High Temperature Deformation in AZ80 Magnesium Alloy

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In

uence of the Initial Texture on Texture Formation

of High Temperature Deformation in AZ80 Magnesium Alloy

+

Kwon-Hoo Kim

1

, Kazuto Okayasu

2

and Hiroshi Fukutomi

2

1Department of Metallurgical Engineering, Pukyong National University, Busan 608-739, Korea

2Faculty of Engineering, Graduate School of Yokohama National University, Yokohama 240-8501, Japan

The texture formation mechanism during high temperature deformation is investigated on AZ80 magnesium alloy. Three kinds of specimens with different initial textures were machined out from rolled plate having a©0001ªtexture. Plane strain compression tests were conducted at 723 K, 5.0©10¹2s¹1and strains ranging from¹0.4 to¹1.0. Development ofð0001Þh1010icomponent is confirmed regardless of the initial texture. It is concluded that the development ofð0001Þh1010icomponent can be attributed to the grain boundary migration during deformation. Besidesð0001Þh1010icomponent, several texture components appeared depending on the initial texture.

[doi:10.2320/matertrans.L-M2014840]

(Received June 2, 2014; Accepted October 9, 2014; Published December 12, 2014)

Keywords: magnesium alloy, texture, plane strain compression

1. Introduction

When solid solution alloys are deformed at high temper-atures, the resulting texture often differs from that obtained by the deformation at room temperature. The authors have investigated the behavior of texture formation under high-temperature uniaxial and plane strain compression deforma-tion of Al-Mg,1,2) Al-Cu,3)and Fe-Si alloys.4) It was found that the {001}(compression plane) texture appeared with increasing strain, after the formation of deformation texture which was usually observed at room temperature deformation of FCC crystals at the beginning of deformation. This texture change was confirmed experimentally to be due to the growth of{001}orientated crystal grains, consuming the grains with the other orientations.5) This behavior was considered to originate from the enhancement of the orientation depend-ence of stored energy caused by the uniform distribution of dislocations which was the result of dislocation motion controlled by the solute atom atmosphere.1­4)

Wrought AZ31, AZ61 and AZ80 magnesium alloys have poor ductility at room temperature, and are therefore usually formed at high temperatures. The texture which develops during high temperature deformation affects their subsequent workability. The basal texture frequently appears after hot rolling might decrease the workability of sheet material, and thus understanding the formation mechanism of the basal texture is an important issue. The magnesium alloys mentioned above show a strong solid solution hardening, and therefore textures originating from the mechanism same as reported on solid solution alloys with BCC and FCC structure might develop. The studies on Al-Mg alloys showed that the characteristics of texture development became apparent with an increase in the solute concentration. Therefore, an AZ80 alloy with solute concentration higher than AZ31 and AZ61 alloys is investigated in this study.

Specimens with various initial textures are prepared from a extruded bar with ah1010ifiber texture by changing cutting

geometry. Deformation behavior and characteristics of textures are investigated on these three kinds of specimens. It was found that: (1) basal texture is formed regardless of the initial texture; (2) the generation of (0001) orientated crystal grains due to deformation twins may contribute to the development of basal texture; (3) several components of the initial texture remain, even after substantial strain; (4) all the retained orientation of the texture components after deforma-tion seem to be stable for deformadeforma-tion.6)

The initial orientations in the previous study, however, were limited and hence further examination of the effect of initial texture on the texture development was necessary. In this study, specimens with initial textures different from the previous study are examined for the discussion on the mechanism of texture formation during high-temperature deformation more in detail.

2. Experimental Procedure

The commercial AZ80 cast billet is used as a starting material. A 60 mm©60 mm©40 mm rectangular plate was cut by machining, and subsequently rolled at 673 K with a rolling reduction of 30%. Figure 1 depicts the texture of the samples after rolling, in which pole densities are projected onto the compression plane. The mean pole density is used as a unit for drawing the contour lines. It is seen that the pole density is distributed in a concentric circular manner, suggesting the formation of (0001)fiber texture that reported frequently for rolled material.7)

Three kinds of plane strain compression specimens with different preferred orientations were machined by changing the cut direction in the rolled plate with texture; their dimensions being 10 mm©10 mm©6.7 mm. Figure 2 shows the geometric arrangement of the specimens; in which RD indicates the direction of extension at the plane strain compression deformation. The compression plane is indi-cated by shading. As a continuation of previous research, the samples are denoted as D, E, and F. The compression direction of specimen D, the direction in which the deformation is restricted in specimen E (The TD direction +This Paper was Originally Published in Japanese in J. JILM 63(2013)

212­217.

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in plane strain compression), and the direction of extension in specimen F (The RD direction in plane strain compression) are the directions parallel to ©0001ª of the rolled plate. All specimens were tested after annealing at 723 K for 1 h.

Figure 3 shows (0001) pole figures derived from EBSD (electron back-scattered diffraction) measurements on the specimens D, E and F before deformation. The pole densities are projected onto the compression plane.

Plane strain compression tests were conducted at 723 K with a strain rate of 5.0©10¹2s¹1. In Fig. 4, the outline of the testing system is shown. The specimens were immedi-ately quenched in oil to prevent further change in micro-structure after deformation. After preparing the mid-plane section of the specimen by polishing, the surface wasfinished using Emery paper and a silica particle (0.04 µm) suspension for microstructural observation. Then the texture was measured by the Schulz reflection method using Cu K¡ radiation. Five pole figures of f1010g, {0001}, f1011g,

f1012gand f1120gwere measured. From these pole figures, crystal orientation distribution function (ODF) was calculated by the Dahms and Bunge method.8) The main component and levels of the texture developments were evaluated on the basis of ODF. In addition to the X-ray measurement, observation by EBSD technique was also carried out on the specimens prepared by electrolytic polishing.

3. Experimental Results

[image:2.595.72.262.183.369.2]

3.1 Behavior of Deformation

Figure 5 shows the true stress­true strain curves obtained by a deformation to a maximum strain of ¹1.0. Work softening is observed in all the deformation conditions 1

1

1

3

3 5

Fig. 1 (0001) pole figure of the AZ80 rolled plate. Pole densities are projected onto the rolling plane. Mean pole density is used as a unit.

Rolling direction

F

RD

D

E

RD

Compression plane

RD

Fig. 2 Preparation of the specimens for plane strain compression.

Levels:1, 2, 4

Specimen D

1 2

4 (a)

Specimen E

1

2 4

1 2

4 (b)

Specimen F

1

2 4

1 2

4 (c)

Fig. 3 (0001) polefigures showing the crystallographic characteristics of the specimens. (a), (b) and (c) show the specimens of D, E and F, respectively. Pole densities are projected onto the compression plane. Mean pole density is used as a unit.

Pressing plate

Die

Sample Punch

Compression rod

[image:2.595.318.538.380.554.2] [image:2.595.63.276.418.554.2] [image:2.595.82.512.594.761.2]
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similar to the deformation of AZ31 and AZ80 with the textures different from the present specimens. The maximum value of stress varies depending on the deformation condition. Flow stresses of specimen D and F are nearly identical. Specimen E exhibits a much lower stress. An increase in flow stress is observed in all the samples above 0.6 in true strain. This increase is considered to be due to an increase in friction caused by theflattening of the sample.

3.2 Textures

3.2.1 The microstructure and texture of Specimen D

The (0001) pole figures in Fig. 6(a)­(c) show the change in texture with increasing strain in specimen D. The pole densities are projected onto the compression plane. The mean pole density is used as a unit of the contours. Prior to the deformation, as shown in Fig. 3(a), pole density about four times as high as the mean pole density is observed at the center of the pole figure. This accumulation expands to left and right ends by the deformation up to the true strain of 0.4. When the strain is increased to 1.0, the pole density at the center of the pole figure increases to over 16 times of the mean, and the pole densities of left and right ends increases to 8 times of the mean level. Although there is no continuous distribution of pole density between the two accumulated pole densities, a spread of accumulated pole density in the left and right direction is observed in the center, and both of

the left and right ends of the sample. Grain structure maps based on EBSD measurements taken at 4 µm intervals are shown in Fig. 7(a) and (b). The crystal orientation of the compression plane is given by the color shown in the inverse pole figure. Thick black lines express the grain boundaries with a misorientation higher than 15°. The top and bottom directions of these figures are the directions of extension in plane strain compression deformation. The crystal grains are not elongated in this direction, suggesting the occurrence of grain boundary migration. The frequency of grains close to (0001) in the deformed state is similar to that before deformation. But a difference in the shape of the crystal grains is seen. Whereas equiaxed grains are predominantly observed prior to the deformation, a number of grains seems to be connected by the deformation, with the shape indicating that a single grain enlarges and incorporates surrounding grains by grain boundary migration as indicated by arrows. This suggests that a stored energy difference provides the driving force for grain boundary migration, which is derived from the difference in dislocation densities in crystal grains.

3.2.2 The microstructure and texture in specimen E

Figure 8 shows the process of texture development of specimen E. An accumulation of pole density appears in the center and the left and right ends similar to the specimen D after the deformation up to a strain of 1.0. The pole density at the center where no pole density is not seen before deformation increases to over 4 times of the mean density, and the pole densities of left and right ends increases 8 times of mean. A continuous distribution of pole density between the center and the two ends is observed.

Figure 9 shows the grain structure maps after the deformation up to a true strain of 1.0. Similar to the grains shown in Fig. 7, grains which seem to spread into the neighboring grains are seen as indicated by the arrow.

3.2.3 The microstructure and texture in specimen F

Figure 10 depicts the process of texture development in the specimen F. The accumulation of pole density existed only in the top and bottom ends before the deformation. The deformation up to a strain of 0.4 results in the spread of the area of high pole densities towards the center of the pole

figure. After the deformation up to a true strain of 1.0, accumulation of pole density appears at the top, bottom, left,

0

0.2

0.4

0.6

0.8

1

0

10

20

30

40

T

rue stress,

/ MPa

σ

True strain,

ε

Rolled AZ80

Plane strain compression

723K

D type, =5.0 x 10-2s-1 E type, =5.0 x 10-2s-1 F type,

ε ε

ε =5.0 x 10-2s-1

Fig. 5 True stress-true strain curves for the specimens of D, E and F.

Levels:1, 2, 4, 8, 16

(a) (b) (c)

1 1

2 4 8

1 2

2 4 8 2

1 21

4

1 2 4 8 16

1 24

[image:3.595.63.277.401.566.2] [image:3.595.79.512.616.763.2]
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right and center of the (0001) pole figures. The maximum value of pole density in Fig. 10(c) is over 8 times of the mean. The grain structure was similar to that shown in Fig. 7 and Fig. 9.

4. Discussion

4.1 Formation of (0001) oriented crystal grains

Three different specimens were investigated in the previous paper: (1) specimen A, in which the main texture component h1010i was in the direction of compression

deformation; (2) specimen B, in which the deformation to the transverse direction h1010i was not possible in plane strain compression deformation; and (3) specimen C, in which the extension direction in plane strain compression deformation is h1010i. A basal texture component over 4 times of the mean existed before deformation in specimens B and C, whereas the accumulation of pole density did not exist before deformation in the specimen A.

After the deformation at 723 K with a strain rate of 5.0©10¹2s¹1 same as this study, basal texture component was observed in all the specimens A, B and C. When the true

Levels:1, 2, 4, 8

(a) (b) (c)

1 2 4

4 2 1

4 2

1 84

21

4 2 1

48 2 1

4 8

2 1

48 2 1

4 2 1

Fig. 8 (0001) polefigures showing the effect of strain on the texture of the specimen E. Absolute values of true strains of (a), (b) and (c) are 0.4, 0.7 and 1.0, respectively.

TD RD

Fig. 9 Microstructure of specimen E observed by EBSD measurements after deformation up to a strain of 1.0.

TD RD

(a) (b)

Fig. 7 Microstructure of specimen D observed by EBSD measurements (a) before deformation and (b) after deformation up to a strain of 1.0.

Levels:1, 2, 4, 8

(a) (b) (c)

1 2 4

1 2 4

1 2 4 8

1 2 4 8

2 1

1 2 4

2

4 1

1

1 1

[image:4.595.316.546.68.214.2]

2 4 8

[image:4.595.49.298.78.206.2] [image:4.595.79.513.260.412.2] [image:4.595.82.513.461.607.2]
(5)

strain is 1.0, the pole density of the specimen A at the center of the (0001) pole figure was about 4 times of the mean, whereas the pole densities in B and C were 22 and 26 times, respectively. The basal texture develops by the gradual change of the pole density in B and C, while the distribution of pole density in A is discontinuous and the basal texture component appeared independently. This indicates that the formation of basal texture component in the specimen A can be attributed to deformation twins. At the same time, it was considered that the preexisting crystal grains with basal orientation developed in the specimens B and C. However, it was not possible to clarify whether the basal orientation components would form or not without basal oriented grains and grains generating basal orientation by twinning before plane strain compression.

In the specimen D, basal oriented grains exist before deformation. Tensile twinning contributes to the formation of basal oriented grains in the specimen F. In the specimen E, the grains having basal orientation hardly exist before deformation, and the grains having basal orientation cannot be generated by deformation twinning.

As seen in Figs. 6, 7, and 9, however, the development of basal orientation is observed after deformation regardless of the texture that exists before deformation. At the stage of strain of 1.0, the pole density at the center of (0001) pole

figure is over 16 times in D, is over 4 times in E and is over 8 times in F. Examination on Fig. 8 for specimen E and Fig. 10 for specimen F, elucidates the development of pole density distribution toward (0001) by the deformation that is hidden the initial textures of specimens B and C in the previous paper. This suggests that the development of (0001) oriented grains originates not only from the growth of (0001) oriented crystal grains by consuming grains of other orientation, but also from the crystal rotation by deformation.

4.2 Orientation component of texture

Figure 11 depicts the ¤2=0° and ¤2=30° sections of specimens after deformation, which was used to clarify the orientation component of texture. Note that the present study uses a coordinate system wherein ð¤1;;¤2Þ ¼ ð0;0;0Þ, corresponds toð0001Þh1120i. As can be seen from thisfigure, there is afiber texture with (0001)(compression plane), and

f1120gh1010i, f1120gh0001i and f1010gh0001i orientation component in specimen F. On the order hand, a high accumulation of orientation density is confirmed at

ð0001Þh1010i and f1120gh1010i in specimens D and E. A comparison of the orientation density and orientation component before and after deformation is given in Table 1. Six different types of orientation were identified in specimens D, E, and F; with these all being identical to those previously identified, and categorized by symbols in Table 1 based on five levels of orientation density. It is found that

ð0001Þh1010iis developed by deformation regardless of the initial texture, with the orientation density more than 8 times of the average. More importantly, the significant development of ð0001Þh1120i reported by others for high temperature plane strain compression deformation9,10) was not observed in this study. Furthermore, with the exception of

ð0001Þh1010i, no common orientation with a high orientation density exists among the three kinds of specimens. However,

the initial texture does exert some influence over the orientation developed by deformation. For example, the

f1120gh0001iorientation is hardly observed in specimens D and E after deformation, but is present at 8 times the average orientation density in the case of specimen F. The ©0001ª

fiber texture, which is highly oriented in the direction of extension before deformation, is evident in specimen F.

f1120gh0001i also developed in the specimen B of the previous study. The specimen B is a specimen with a fiber texture having h1010i in the traverse direction. The crystal orientation common to specimens B and F isf1120gh0001i, and the development off1120gh0001i which was seen here, suggesting that this crystal grains of the crystal orientation is grown by consuming the grains of other orientation during deformation. Considering with the result of previous study,

f1010gh1120idevelop at the orientation which is formed by the growth of the crystal grains that were exited before deformation in specimen E. The development of this orientation component is also confirmed at the specimen A of previous study. A is a specimen with afiber texture that

h1010iis oriented at compression direction before deforma-tion. On the other hand, the fiber texture that ©0001ª is oriented in the traverse direction before deformation is existed at in specimen E. The crystal orientations common to both is to be f1010gh1120i. From this result, it is therefore considered that theð0001Þh1120iorientation in specimens D and F, and thef1020gh1010i orientation in specimen D, are developed through the growth of crystal grains that exist prior

(a) (b) (c)

(ϕ2=0°°)

(ϕ2=30°)

{0001}<1120> {1010}<1120> {0001}<1010>

[image:5.595.308.546.72.223.2]

{1010}<0001> {1120}<1010> {1120}<0001> ϕ1 φ 1 1 48 16 16 2 2 1 4 8 16 16 16 8 4 2 1 4 2 1 1 12 4 8 2 4 8 4 8 21 24 4 2 1 4 2 1 8 4 2 1 8 4 2 1 4 2 1 2 1

[image:5.595.306.550.293.412.2]

Fig. 11 ¤2 sections for specimens (a) D, (b) E and (c) F, after the deformation up to a true strain of 1.0.

Table 1 The summary of six texture component for AZ80 magnesium alloy deformed 723 K, 5.0©10¹2s¹1and strain up to¹1.0.

D E F

¾ ¾ ¾

0 ¹1.0 0 ¹1.0 0 ¹1.0

ð0001Þh1010i ð0001Þh1120i f1010gh0001i f1010gh1120i f1120gh1010i f1120gh0001i

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to deformation. Furthermore, these specific orientations can be regarded as being stable for deformation.

4.3 Control of basal texture

The plane strain deformation and resulting texture for the three different initial orientations of specimens D­F, was compared with the additional three orientations of specimens A­C from a previous study.6)From this, it was found that the

ð0001Þh1010i orientation is formed without exception. It is therefore considered that this orientation is not only stable during deformation; but does in fact grow through consuming crystal grains with other orientations, and is also likely to be generated by crystal rotation. Since this basal texture orientation is not suitable for subsequent forming, it is desirable tofind a suitable means to control it. However, such control is made difficult by the deformation conditions investigated in this study, as the basal texture is generated by deformation twins and slip deformation.9,11) Consequently, control over the development of basal texture during high temperature processing is incomplete without also taking into consideration the relative activity ratio of the plural-active slip systems, or changing the activity of the deformation twin. It is therefore necessary to investigate the texture formation behavior resulting from varying the temperature and strain rate used for deformation; which is currently being under-taken by the authors.

5. Conclusions

In order to clarify the formation mechanism of texture by high temperature deformation in an AZ80 alloy, three specimens with different initial textures were prepared from a rolled plate. Plane strain compression deformation was conducted at 723 K and a strain rate of 5.0©10¹2s¹1, with subsequent observation of the microstructure and a texture analysis being performed for each specimen. The results obtained were compared with those of a previous report concerning specimens prepared from extruded bars. Major results are summarized as follow.

(1) Basal texture is formed in all of the specimens. In the microstructure observation by EBSD, the consumption

of adjacent grains by (0001)(compression plane) orient-ed crystal grains was confirmed. Basal texture is considered to be formed by grain boundary migration. (2) (0001) oriented crystal grain is also likely of being

generated by the crystal rotation from deformation other than twin.

(3) A total of six different crystal orientation compo-nents have been identified after deformation, with

ð0001Þh1010i the only orientation found to be formed regardless of the initial texture.

(4) The formation of texture components other than

ð0001Þh1010i is dependent on the texture that exists prior to deformation. These additional components are believed to be crystal orientations that initially exist in the specimen, and are stable during deformation.

Acknowledgements

The authors are grateful to The Japan Light Metal Educational Foundation, Inc. for their financial support. The authors thank Mr. Hisashi Yoshida for his help in experiments and analysis of experimental results.

REFERENCES

1) K. Okayasu, H. Sakakibara and H. Fukutomi:Mater. Process. Texture: Ceram. Trans.200(2008) 679­685.

2) K. Okayasu, H. Takekoshi, M. Sakakibara and H. Fukutomi:J. Japan Inst. Metals73(2009) 58­63.

3) K. Okayasu, S. Takahata and H. Fukutomi: Mat. Sci. Forum495­497

(2012) 336­339.

4) S. Onuki, K. Okayasu and H. Fukutomi:Tetsu-to-Hagane98(2012) 27­33.

5) R. Horiuchi and M. Otsuka: J. Japan Inst. Metals35(1971) 406­415. 6) J. Kim, K. Okayasu and H. Fukutomi:Mater. Trans.53(2012) 1870­

1875.

7) A. Galiyev, R. Kaibyshev and G. Gottstein:Acta Mater. 49(2001) 1199­1207.

8) M. Dahms and H. J. Bunge:J. Appl. Crystallogr.22(1989) 439­447.

9) R. Gehrmann, M. M. Frommert and G. Gottstein:Mater. Sci. Eng. A

395(2005) 338­349.

10) G. Gottstein and T. Al-Samman:Mater. Sci. Forum495­497(2005) 623­632.

Figure

Figure 5 shows the true stress­by a deformation to a maximum strain oftrue strain curves obtained ¹1.0
Fig. 6(0001) pole figures showing the effect of strain on the texture of the specimen D
Fig. 9Microstructure of specimen E observed by EBSD measurementsafter deformation up to a strain of 1.0.
Fig. 11¤2 sections for specimens (a) D, (b) E and (c) F, after thedeformation up to a true strain of 1.0.

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