- Email: [email protected]
Mechanical properties and texture evolution during rolling process of an AZ31 Mg alloy
Journal of Alloys and Compounds 472 (2009) 127–132
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mechanical properties and texture evolution during rolling process of an AZ31 Mg alloy Shujin Liang ∗ , Hongfei Sun, Zuyan Liu, Erde Wang School of Materials Science and Engineering, 166# Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 April 2008 Received in revised form 29 April 2008 Accepted 30 April 2008 Available online 13 June 2008 Keywords: AZ31 Mechanical properties Texture Rolling
a b s t r a c t AZ31 Mg alloys were subjected to the rolling process in this paper. Microstructures of the as-rolled sheets were studied by electron backscattered diffraction method (EBSD) and then tensile testes were carried out to investigate the relationship between mechanical properties and microstructural parameters that include grain size and texture distribution. The grains of the as-rolled sheets were effectively refined after rolling, which resulted in the significant enhancement of mechanical properties. Basal texture was discovered in the as-rolled sheets and the intensity of the basal texture increased with raising the rolling reduction. At the same time, the incline of the basal poles away from normal direction of the sheet was obvious and it is believed that this was related to the mechanical anisotropy of the sheet. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are attractive materials due to their good specific properties [1], and have received great attention especially in the automobile industry in recent years. However, their low ductility at room temperature, due to the hexagonal close packed structure, is a major problem that must be overcome in order to increase the usage. It is well accepted that grain refinement can significantly improve the mechanical properties of the alloys based on Hall–Petch relation [2]. As it is known, plastic deformation is an effective method to refine grains. There is a large number of reports [3–8] on extrusion, rolling and forging processes of the alloys, which indicated that the grains were refined and the room temperature plasticity was increased. In this experiment, rolling was adopted as a refinement method to enhance mechanical properties of the AZ31 Mg alloy sheet. Compared with some extraordinary and expensive methods such as powder metallurgy technique, rapid solidification and equalchannel-angular extrusion, rolling process was a common way that can fabricate large dimension products. This processing may enlarge the industrial usage of Mg alloys in the future. In the present study, hot rolled AZ31 Mg alloy sheets were subjected to rolling processing. Mechanical properties of the as-rolled sheets were examined at room temperature. Texture distributions of the
as-received and as-rolled sheets were analyzed by EBSD method. Then the relationship between mechanical properties and texture distribution was discussed. 2. Experimental procedure The rolled AZ31 Mg alloy sheets with thickness of 9.8 mm were used as billets in these experiments. The alloy composition is given in Table 1. The original sheets were then machined into small slices with dimensions of 80 mm × 50 mm × 9.8 mm for use in rolling. In the rolling process, the roller temperature was selected as 673 K and the original sheets were maintained at room temperature. The rolling experiments consisted of one rolling pass to different final thicknesses. The thickness reduction was 20%, 30% and 40%, respectively. The rolling speed was 5 m/min. The microstructural examination of the as-received sheet was performed on OLYMPUS GX71 optical microscopy. Surface preparation consisted of grinding with progressively finer SiC papers and mechanical polishing. Revealing the grain structure was achieved by subsequent etching at room temperature during 1 min in a solution of picric acid (5.5 g), acetic acid (2 ml), water (10 ml) and ethanol (90 ml). Tensile specimens were machined along rolling direction (RD) and transverse direction (TD) of the sheets. Tensile tests were carried out on INSTRON5569 at room temperature with a strain rate of 1 × 10−3 s−1 . EBSD measurements were performed on the as-received and as-rolled samples in JEOL 733 electron probe equipped with HKL Channel 5 system. The observation plane was perpendicular to RD. Sample preparations for EBSD were difficult. It consisted of grinding with progressively SiC papers of grit size 600, 800, 1000, followed by electropolishing. The electropolishing process was carried out with a solution of 37.5% orthophosphoric acid and 62.5% ethanol at room temperature and using a voltage of 2.5–5 V. Finally, samples were cleaned with methanol and dried with filter paper.
3. Results ∗ Corresponding author. Tel.: +86 451 86417629; fax: +86 451 86412064. E-mail addresses: [email protected], [email protected] (S. Liang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.093
Fig. 1(a) shows the microstructure of the as-received sheet. It can be seen that the microstructure was homogeneous and
128
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Table 1 Chemical composition of AZ31 used for present study (mass%) Al Zn Mn Fe Si Cu Ni Ca Mg
2.9339 0.9014 0.0021 0.0063 0.0103 0.0006 0.0001 0.0445 Bal.
full of equiaxed grains. The average grain size was 30 m measured by the linear intercept method (d¯ = 1.74L, L is the liner intercept size.). Fig. 1(b) shows the frequency–misorientation map of the as-received sample. The frequency values of high angle grain boundaries (HAGB > 15◦ ) and low angle grain boundaries (LAGB < 15◦ ) detected by EBSD method were 93% and 7%, respectively. From Fig. 1(a) and (b), it is obvious that dynamic recrystallization (DRX) has taken place during the previous hot rolling process. Fig. 1(c) illustrates {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-received sheet. {0 0 0 2} 1 0 1¯ 0 type texture was discovered and the maximum intensity of basal texture was 8.12. Fig. 2 gives the orientation imaging maps (OIM) and frequency–misorientation maps of the as-rolled sheets. The different colors represent different orientations of the grains. The nearly identical colored grains mean that the misorientations between
these grains are not large. The HAGBs and LAGBs were characterized by black and red lines, respectively. It is obvious that the grains were refined after rolling. The average grain sizes were 14 m, 11 m and 20 m, corresponding to the 20%-rolled, 30%-rolled and 40%rolled sheets. Table 2 gives the frequency values of HAGBs and LAGBs. It demonstrates that the frequency of HAGBs increased, while the LAGBs frequency decreased with raising deformation ratio. Fig. 3 illustrates {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-rolled sheets. As it is shown in Fig. 3(a), basal texture has been formed during 20% rolling process. The basal poles tilted a small angle away from ND. The f(g) of basal texture was 9.83. Fig. 3(b) shows the pole figures of the 30%-rolled sheet. It can be seen that basal planes were almost parallel to rolling plane. There are two intensity peaks in {0 0 0 2} pole figure. The highest intensity of the poles was located ∼10◦ away from ND and the f(g) was 10.48. As the arrows shown in {1 0 1¯ 0} and {1 1 2¯ 0} pole figures, it obviously demonstrates that weak {0 0 0 2} 1 1 2¯ 0 type texture has been formed during 30% rolling process. Fig. 3(c) is the pole figures of the 40%-rolled sheet. Basal texture can be easily found in {0 0 0 2} pole figure. But the texture is complex and several intensity peaks are discovered. The highest intensity was located ∼13◦ away from ND and the f(g) was 12.54. The relationship between mechanical properties and deformation ratio has been drawn into curves shown in Fig. 4. The strength and elongation of the as-rolled sheets are significantly raised due to the grain size refinement. The 30%-rolled sheet exhibited the best performance. The YS and EL were 180 MPa, 32% and 165 MPa, 34%,
Fig. 1. (a) Microstructure of the as-received sheet; (b) frequency–misorientation map of the as-received sheet; (c) {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figure of the as-received sheet.
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
129
Fig. 2. Orientation imaging maps (OIM) and frequency–misorietation maps (FM) of the as-rolled sheets. (a), (c) and (e) are OIMs of the 20%-rolled, 30%-rolled and 40%-rolled sheets, respectively. (b), (d) and (f) are FMs of the 20%-rolled, 30%-rolled and 40%-rolled sheets, respectively.
corresponding to the specimens machined along TD and RD of the sheets. The curves also demonstrate the mechanical anisotropy was obvious. This may be related to the texture distribution and will be discussed in the next section.
Table 2 Frequency of LAGBs and HAGBs in the as-received and as-rolled sheets Misorientation angles
As-received
20%-Rolled
30%-Rolled
40%-Rolled
LAGBs (≤15◦ ) HAGBs (>15◦ )
7% 93%
11% 89%
9.4% 90.6%
8% 92%
4. Discussion In these rolling experiments, the temperatures of the as-rolled sheets were tested as soon as they were rolled. The results revealed that the sheet temperatures were all higher than 573 K. At this point of view, this kind of rolling process also should be categorized into thermomechanical processing. Because of the intermediate value of stacking fault energy, limited slip systems and high grainboundary diffusion speed of magnesium alloy, DRX usually takes place during hot working process. With increasing rolling reduction, the frequency of HAGBs was raised. This indicates DRX was more and more sufficient. From Fig. 2, it clearly demonstrates that
130
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Fig. 3. {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-rolled sheets. (a) The 20%-rolled sheet; (b) the 30%-rolled sheet; (c) the 40%-rolled sheet.
Fig. 4. The relationship between mechanical properties and deformation ratio. (a) Strength–deformation ratio curves; (b) elongation–deformation ratio curve.
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Fig. 5. The relationship between yield strength and the grain size for the as-rolled sheets.
the grains were first refined when the rolling reduction was lower than 30%, and then grew up when the deformation ratio reached 40%. The growth of the grains was related to the high grain boundary energy provided by the high working temperature and large strain deformation. As it is known, the yield strength as a function of grain size can be represented as Hall–Petch equation: s = 0 + Kd−1/2
(1)
where s is the yield stress, 0 the yield stress of a single crystal, K a constant and d is the grain size. The strength grows with grain size refinement. In this research, the grains were refined by rolling and the YS of the as-rolled sheets were larger, compared with the as-received sheet. When the rolling reduction was 40%, the average grain size was grown up to 20 m. So the YS of the 40%-rolled sheet was lower than that of the 20%-rolled (14 m) and 30%-rolled (11 m) sheets, but higher than the original sheet (40 m). The relationship between grain size and YS according well with the Hall–Petch relation. Fig. 5 shows the linear fit lines of the YS against grain size along TD and RD. It can be clearly observed that the two lines were almost parallel. There was only a little difference between 0 . The value of 0 along TD and RD was 44 MPa and 49 MPa, respectively. Koike and Kobayashi [9] have reported that 0 was approximately 60 MPa when the grain size was the only effective factor. However, the values obtained in this study was smaller than the value Koike et al. got. It is believed that the basal slip operated easily because of the basal planes incline [10]. Thus, the easy basal slip has resulted in the reduction of 0 . The different values of 0 between the RD-tensioned and TD-tensioned specimens may be ascribed to the different incline scope of the basal planes. The curves in Fig. 4 clearly demonstrate that the mechanical anisotropy was obvious in the as-rolled sheets. The YS of the specimens tensioned along RD was lower, but the EL was higher than that of the specimens tensioned along RD. Yoshida et al. [10,11] have reported that crystallographic orientation has a profound effect on the tensile properties of AZ31 alloy. They have found that when the basal planes were inclined at 45◦ to the extrusion direction, the YS was lower than that of the specimens which possessed the basal planes parallel to the extrusion direction. In the present study, the shapes of the basal texture distribution lines of the as-rolled sheets were elongated along RD. This obviously demonstrates that the basal planes with same texture intensity inclined larger angles along RD than that inclined along TD. When the tensile tests were carried out at room temperature, the Schmid factors for basal slip
131
of the RD-tensioned specimens were bigger than the TD-tensioned specimens. Therefore, the RD-tensioned specimens exhibit lower YS and higher EL. Rolling texture has been studied in many literatures [12–21]. Basal texture is the main character for hot rolled Mg alloys because the basal slip systems are easily activated. Some researchers also reported that twinning played an important role in the formation of basal texture because twinning of the type {1 0 1¯ 2} reoriented the c-axis parallel to the compression axis [21]. When the rolling reduction was 20%, it is clearly found that basal texture was formed. The formation of the texture was mainly due to the activity of basal slip. There is no obvious evidence of twins because twinning is usually restrained at high working temperature. In the 30%-rolled and 40%-rolled sheets, basal texture with splitting was discovered. This kind of texture distribution is typical from rolled and extruded Mg alloys [12,22,23]. However, the microscopic mechanisms responsible for the formation of this texture are still not clear. Wang and Huang [24] reported simulated and experimental texture results of cold rolled hexagonal metals. They found that Ti tended to form textures with basal poles tilted ±20–40◦ away from ND toward TD and slip on prismatic planes was responsible for it. They also suggested that Zn tended to exhibit textures with basal poles tilted ±15–25◦ away from ND toward RD and such textures were due to combination of basal slip and large-scale twinning. Styczynski et al. [13] gave clear evidence that the activity of the pyramidal c + a slip system was responsible for the basal splitting by simulating method. In the one pass 30%-reduction rolling process, twinning was restrained and only basal slip cannot fully accommodate intermediate strain deformation [8]. At the hot working process, the critical resolved shear stress (CRSS) of prismatic a and non-basal c + a slips were getting smaller and they could be easily activated. So, the prismatic a and non-basal c + a slips may be responsible for the basal texture splitting.
5. Conclusions The mechanical properties and texture evolution of the as-rolled AZ31 sheets has been studied. The following conclusions can be drawn from this study:
1. The significant grain size refinement has been achieved by hot rolling. When the deformation ratio was lower than 30%, the average grain size decreased with increasing rolling reduction. When the rolling ratio reached 40%, the grains grew up due to the high grain boundary energy aroused by high working temperature and large strain deformation. 2. Mechanical properties were improved due to the grain size refinement. The 30%-rolled sheet exhibited the best mechanical properties. The yield strength and elongation were 180 MPa, 32% and 165 MPa, 34%, corresponding to the specimens machined from the transverse direction and rolling direction of the sheet. 3. The relationship between yield strength and grain size was in a good agreement with Hall–Petch relation. The different values of 0 along the transverse direction and rolling direction of the sheet were related to the different texture distributions. 4. The intensities of the basal texture increased with the rolling reduction raised. The basal planes inclined away from normal direction of the sheet were related to the activities of the prismatic a and non-basal c + a slips. 5. The mechanical anisotropy was obvious in the sheet along the transverse direction and rolling direction. This was ascribed to the different texture distributions.
132
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11]
B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37. Y. Chino, M. Mabuchi, Adv. Eng. Mater. 2 (12) (2001) 981. Y. Chino, M. Mabuchi, K. Shimojima, Mater. Trans. 42 (3) (2001) 414. J. Xing, X.Y. Yang, H. Miura, Mater. Sci. Forum 488/489 (2005) 597. T. Murai, S. Matsuoka, S. Miyamoto, J. Mater. Process. Technol. 141 (2) (2003) 207. X.L. Wang, Y. Yu, E.D. Wang, Mater. Sci. Forum 488/489 (2005) 535. T.C. Chang, J.Y. Wang, C.M. O, S. Lee, J. Mater. Process. Technol. 140 (2003) 588. ´ J.A. del Valle, M.T. Perez-Prado, O.A. Ruano, Mater. Sci. Eng. A 355 (2003) 68. J. Koike, K. Kobayashi, Proceedings of a subcommittee symposium of the Japan Inst. Metals “Fundamentals of magnesium and industrial applications”, The Japan Inst. Metals, 2001, pp. 1–3. Y. Yoshida, L. Cisar, S. Kamado, Y. Kojima, Metal. Trans. 44 (2003) 468. L. Cisar, Y. Yoshida, S. Kamado, Y. Kojima, F. Watanabe, Metal. Trans. 44 (2003) 476.
´ [12] M.T. Perez-Prado, J.A. del Valle, J.M. Contreras, O.A. Ruano, Scripta Mater. 50 (2004) 661. [13] A. Styczynski, Ch. Hartig, J. Bohlen, D. Letzig, Scripta Mater. 50 (2004) 943. [14] W.J. Kim, J.B. Lee, W.Y. Kim, H.T. Jeong, H.G. Jeong, Scripta Mater. 56 (2007) 309. [15] Q.L. Jin, S.Y. Shim, S.G. Lim, Scripta Mater. 55 (2006) 843. [16] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Mater. Sci. Eng. A 441 (2006) 349. ´ [17] M. Eddahbi, J.A. del Valle, M.T. Perez-Prado, O.A. Ruano, Mater. Sci. Eng. A 410/411 (2005) 308. ˜ O.A. Ruano, Acta Mater. 54 (2006) 4247. [18] J.A. del Valle, F. Carreno, [19] M.R. Barnett, M.D. Nave, C.J. Bettles, Mater. Sci. Eng. A 386 (2004) 205. ˇ F. Kaiser, D. Letzig, P. Lukaˇ ´ c, K.U. Kainer, J. Alloys [20] J. Bohlen, F. Chmel´ık, P. Dobron, Compd. 378 (2004) 207. ´ Acta Mater. 49 (2001) 4277. [21] S.R. Agnew, M.H. Yoo, C.N. Tome, [22] C.S. Roberts, Magnesium and its Alloys, John Wiley & Sons, 1960, pp. 190. [23] M.J. Philippe, Mater. Sci. Forum 157–162 (1994) 1337. [24] Y.N. Wang, J.C. Huang, Mater. Chem. Phys. 81 (2003) 11.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mechanical properties and texture evolution during rolling process of an AZ31 Mg alloy Shujin Liang ∗ , Hongfei Sun, Zuyan Liu, Erde Wang School of Materials Science and Engineering, 166# Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 April 2008 Received in revised form 29 April 2008 Accepted 30 April 2008 Available online 13 June 2008 Keywords: AZ31 Mechanical properties Texture Rolling
a b s t r a c t AZ31 Mg alloys were subjected to the rolling process in this paper. Microstructures of the as-rolled sheets were studied by electron backscattered diffraction method (EBSD) and then tensile testes were carried out to investigate the relationship between mechanical properties and microstructural parameters that include grain size and texture distribution. The grains of the as-rolled sheets were effectively refined after rolling, which resulted in the significant enhancement of mechanical properties. Basal texture was discovered in the as-rolled sheets and the intensity of the basal texture increased with raising the rolling reduction. At the same time, the incline of the basal poles away from normal direction of the sheet was obvious and it is believed that this was related to the mechanical anisotropy of the sheet. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are attractive materials due to their good specific properties [1], and have received great attention especially in the automobile industry in recent years. However, their low ductility at room temperature, due to the hexagonal close packed structure, is a major problem that must be overcome in order to increase the usage. It is well accepted that grain refinement can significantly improve the mechanical properties of the alloys based on Hall–Petch relation [2]. As it is known, plastic deformation is an effective method to refine grains. There is a large number of reports [3–8] on extrusion, rolling and forging processes of the alloys, which indicated that the grains were refined and the room temperature plasticity was increased. In this experiment, rolling was adopted as a refinement method to enhance mechanical properties of the AZ31 Mg alloy sheet. Compared with some extraordinary and expensive methods such as powder metallurgy technique, rapid solidification and equalchannel-angular extrusion, rolling process was a common way that can fabricate large dimension products. This processing may enlarge the industrial usage of Mg alloys in the future. In the present study, hot rolled AZ31 Mg alloy sheets were subjected to rolling processing. Mechanical properties of the as-rolled sheets were examined at room temperature. Texture distributions of the
as-received and as-rolled sheets were analyzed by EBSD method. Then the relationship between mechanical properties and texture distribution was discussed. 2. Experimental procedure The rolled AZ31 Mg alloy sheets with thickness of 9.8 mm were used as billets in these experiments. The alloy composition is given in Table 1. The original sheets were then machined into small slices with dimensions of 80 mm × 50 mm × 9.8 mm for use in rolling. In the rolling process, the roller temperature was selected as 673 K and the original sheets were maintained at room temperature. The rolling experiments consisted of one rolling pass to different final thicknesses. The thickness reduction was 20%, 30% and 40%, respectively. The rolling speed was 5 m/min. The microstructural examination of the as-received sheet was performed on OLYMPUS GX71 optical microscopy. Surface preparation consisted of grinding with progressively finer SiC papers and mechanical polishing. Revealing the grain structure was achieved by subsequent etching at room temperature during 1 min in a solution of picric acid (5.5 g), acetic acid (2 ml), water (10 ml) and ethanol (90 ml). Tensile specimens were machined along rolling direction (RD) and transverse direction (TD) of the sheets. Tensile tests were carried out on INSTRON5569 at room temperature with a strain rate of 1 × 10−3 s−1 . EBSD measurements were performed on the as-received and as-rolled samples in JEOL 733 electron probe equipped with HKL Channel 5 system. The observation plane was perpendicular to RD. Sample preparations for EBSD were difficult. It consisted of grinding with progressively SiC papers of grit size 600, 800, 1000, followed by electropolishing. The electropolishing process was carried out with a solution of 37.5% orthophosphoric acid and 62.5% ethanol at room temperature and using a voltage of 2.5–5 V. Finally, samples were cleaned with methanol and dried with filter paper.
3. Results ∗ Corresponding author. Tel.: +86 451 86417629; fax: +86 451 86412064. E-mail addresses: [email protected], [email protected] (S. Liang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.093
Fig. 1(a) shows the microstructure of the as-received sheet. It can be seen that the microstructure was homogeneous and
128
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Table 1 Chemical composition of AZ31 used for present study (mass%) Al Zn Mn Fe Si Cu Ni Ca Mg
2.9339 0.9014 0.0021 0.0063 0.0103 0.0006 0.0001 0.0445 Bal.
full of equiaxed grains. The average grain size was 30 m measured by the linear intercept method (d¯ = 1.74L, L is the liner intercept size.). Fig. 1(b) shows the frequency–misorientation map of the as-received sample. The frequency values of high angle grain boundaries (HAGB > 15◦ ) and low angle grain boundaries (LAGB < 15◦ ) detected by EBSD method were 93% and 7%, respectively. From Fig. 1(a) and (b), it is obvious that dynamic recrystallization (DRX) has taken place during the previous hot rolling process. Fig. 1(c) illustrates {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-received sheet. {0 0 0 2} 1 0 1¯ 0 type texture was discovered and the maximum intensity of basal texture was 8.12. Fig. 2 gives the orientation imaging maps (OIM) and frequency–misorientation maps of the as-rolled sheets. The different colors represent different orientations of the grains. The nearly identical colored grains mean that the misorientations between
these grains are not large. The HAGBs and LAGBs were characterized by black and red lines, respectively. It is obvious that the grains were refined after rolling. The average grain sizes were 14 m, 11 m and 20 m, corresponding to the 20%-rolled, 30%-rolled and 40%rolled sheets. Table 2 gives the frequency values of HAGBs and LAGBs. It demonstrates that the frequency of HAGBs increased, while the LAGBs frequency decreased with raising deformation ratio. Fig. 3 illustrates {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-rolled sheets. As it is shown in Fig. 3(a), basal texture has been formed during 20% rolling process. The basal poles tilted a small angle away from ND. The f(g) of basal texture was 9.83. Fig. 3(b) shows the pole figures of the 30%-rolled sheet. It can be seen that basal planes were almost parallel to rolling plane. There are two intensity peaks in {0 0 0 2} pole figure. The highest intensity of the poles was located ∼10◦ away from ND and the f(g) was 10.48. As the arrows shown in {1 0 1¯ 0} and {1 1 2¯ 0} pole figures, it obviously demonstrates that weak {0 0 0 2} 1 1 2¯ 0 type texture has been formed during 30% rolling process. Fig. 3(c) is the pole figures of the 40%-rolled sheet. Basal texture can be easily found in {0 0 0 2} pole figure. But the texture is complex and several intensity peaks are discovered. The highest intensity was located ∼13◦ away from ND and the f(g) was 12.54. The relationship between mechanical properties and deformation ratio has been drawn into curves shown in Fig. 4. The strength and elongation of the as-rolled sheets are significantly raised due to the grain size refinement. The 30%-rolled sheet exhibited the best performance. The YS and EL were 180 MPa, 32% and 165 MPa, 34%,
Fig. 1. (a) Microstructure of the as-received sheet; (b) frequency–misorientation map of the as-received sheet; (c) {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figure of the as-received sheet.
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
129
Fig. 2. Orientation imaging maps (OIM) and frequency–misorietation maps (FM) of the as-rolled sheets. (a), (c) and (e) are OIMs of the 20%-rolled, 30%-rolled and 40%-rolled sheets, respectively. (b), (d) and (f) are FMs of the 20%-rolled, 30%-rolled and 40%-rolled sheets, respectively.
corresponding to the specimens machined along TD and RD of the sheets. The curves also demonstrate the mechanical anisotropy was obvious. This may be related to the texture distribution and will be discussed in the next section.
Table 2 Frequency of LAGBs and HAGBs in the as-received and as-rolled sheets Misorientation angles
As-received
20%-Rolled
30%-Rolled
40%-Rolled
LAGBs (≤15◦ ) HAGBs (>15◦ )
7% 93%
11% 89%
9.4% 90.6%
8% 92%
4. Discussion In these rolling experiments, the temperatures of the as-rolled sheets were tested as soon as they were rolled. The results revealed that the sheet temperatures were all higher than 573 K. At this point of view, this kind of rolling process also should be categorized into thermomechanical processing. Because of the intermediate value of stacking fault energy, limited slip systems and high grainboundary diffusion speed of magnesium alloy, DRX usually takes place during hot working process. With increasing rolling reduction, the frequency of HAGBs was raised. This indicates DRX was more and more sufficient. From Fig. 2, it clearly demonstrates that
130
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Fig. 3. {0 0 0 2}, {1 0 1¯ 0} and {1 1 2¯ 0} pole figures of the as-rolled sheets. (a) The 20%-rolled sheet; (b) the 30%-rolled sheet; (c) the 40%-rolled sheet.
Fig. 4. The relationship between mechanical properties and deformation ratio. (a) Strength–deformation ratio curves; (b) elongation–deformation ratio curve.
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
Fig. 5. The relationship between yield strength and the grain size for the as-rolled sheets.
the grains were first refined when the rolling reduction was lower than 30%, and then grew up when the deformation ratio reached 40%. The growth of the grains was related to the high grain boundary energy provided by the high working temperature and large strain deformation. As it is known, the yield strength as a function of grain size can be represented as Hall–Petch equation: s = 0 + Kd−1/2
(1)
where s is the yield stress, 0 the yield stress of a single crystal, K a constant and d is the grain size. The strength grows with grain size refinement. In this research, the grains were refined by rolling and the YS of the as-rolled sheets were larger, compared with the as-received sheet. When the rolling reduction was 40%, the average grain size was grown up to 20 m. So the YS of the 40%-rolled sheet was lower than that of the 20%-rolled (14 m) and 30%-rolled (11 m) sheets, but higher than the original sheet (40 m). The relationship between grain size and YS according well with the Hall–Petch relation. Fig. 5 shows the linear fit lines of the YS against grain size along TD and RD. It can be clearly observed that the two lines were almost parallel. There was only a little difference between 0 . The value of 0 along TD and RD was 44 MPa and 49 MPa, respectively. Koike and Kobayashi [9] have reported that 0 was approximately 60 MPa when the grain size was the only effective factor. However, the values obtained in this study was smaller than the value Koike et al. got. It is believed that the basal slip operated easily because of the basal planes incline [10]. Thus, the easy basal slip has resulted in the reduction of 0 . The different values of 0 between the RD-tensioned and TD-tensioned specimens may be ascribed to the different incline scope of the basal planes. The curves in Fig. 4 clearly demonstrate that the mechanical anisotropy was obvious in the as-rolled sheets. The YS of the specimens tensioned along RD was lower, but the EL was higher than that of the specimens tensioned along RD. Yoshida et al. [10,11] have reported that crystallographic orientation has a profound effect on the tensile properties of AZ31 alloy. They have found that when the basal planes were inclined at 45◦ to the extrusion direction, the YS was lower than that of the specimens which possessed the basal planes parallel to the extrusion direction. In the present study, the shapes of the basal texture distribution lines of the as-rolled sheets were elongated along RD. This obviously demonstrates that the basal planes with same texture intensity inclined larger angles along RD than that inclined along TD. When the tensile tests were carried out at room temperature, the Schmid factors for basal slip
131
of the RD-tensioned specimens were bigger than the TD-tensioned specimens. Therefore, the RD-tensioned specimens exhibit lower YS and higher EL. Rolling texture has been studied in many literatures [12–21]. Basal texture is the main character for hot rolled Mg alloys because the basal slip systems are easily activated. Some researchers also reported that twinning played an important role in the formation of basal texture because twinning of the type {1 0 1¯ 2} reoriented the c-axis parallel to the compression axis [21]. When the rolling reduction was 20%, it is clearly found that basal texture was formed. The formation of the texture was mainly due to the activity of basal slip. There is no obvious evidence of twins because twinning is usually restrained at high working temperature. In the 30%-rolled and 40%-rolled sheets, basal texture with splitting was discovered. This kind of texture distribution is typical from rolled and extruded Mg alloys [12,22,23]. However, the microscopic mechanisms responsible for the formation of this texture are still not clear. Wang and Huang [24] reported simulated and experimental texture results of cold rolled hexagonal metals. They found that Ti tended to form textures with basal poles tilted ±20–40◦ away from ND toward TD and slip on prismatic planes was responsible for it. They also suggested that Zn tended to exhibit textures with basal poles tilted ±15–25◦ away from ND toward RD and such textures were due to combination of basal slip and large-scale twinning. Styczynski et al. [13] gave clear evidence that the activity of the pyramidal c + a slip system was responsible for the basal splitting by simulating method. In the one pass 30%-reduction rolling process, twinning was restrained and only basal slip cannot fully accommodate intermediate strain deformation [8]. At the hot working process, the critical resolved shear stress (CRSS) of prismatic a and non-basal c + a slips were getting smaller and they could be easily activated. So, the prismatic a and non-basal c + a slips may be responsible for the basal texture splitting.
5. Conclusions The mechanical properties and texture evolution of the as-rolled AZ31 sheets has been studied. The following conclusions can be drawn from this study:
1. The significant grain size refinement has been achieved by hot rolling. When the deformation ratio was lower than 30%, the average grain size decreased with increasing rolling reduction. When the rolling ratio reached 40%, the grains grew up due to the high grain boundary energy aroused by high working temperature and large strain deformation. 2. Mechanical properties were improved due to the grain size refinement. The 30%-rolled sheet exhibited the best mechanical properties. The yield strength and elongation were 180 MPa, 32% and 165 MPa, 34%, corresponding to the specimens machined from the transverse direction and rolling direction of the sheet. 3. The relationship between yield strength and grain size was in a good agreement with Hall–Petch relation. The different values of 0 along the transverse direction and rolling direction of the sheet were related to the different texture distributions. 4. The intensities of the basal texture increased with the rolling reduction raised. The basal planes inclined away from normal direction of the sheet were related to the activities of the prismatic a and non-basal c + a slips. 5. The mechanical anisotropy was obvious in the sheet along the transverse direction and rolling direction. This was ascribed to the different texture distributions.
132
S. Liang et al. / Journal of Alloys and Compounds 472 (2009) 127–132
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11]
B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37. Y. Chino, M. Mabuchi, Adv. Eng. Mater. 2 (12) (2001) 981. Y. Chino, M. Mabuchi, K. Shimojima, Mater. Trans. 42 (3) (2001) 414. J. Xing, X.Y. Yang, H. Miura, Mater. Sci. Forum 488/489 (2005) 597. T. Murai, S. Matsuoka, S. Miyamoto, J. Mater. Process. Technol. 141 (2) (2003) 207. X.L. Wang, Y. Yu, E.D. Wang, Mater. Sci. Forum 488/489 (2005) 535. T.C. Chang, J.Y. Wang, C.M. O, S. Lee, J. Mater. Process. Technol. 140 (2003) 588. ´ J.A. del Valle, M.T. Perez-Prado, O.A. Ruano, Mater. Sci. Eng. A 355 (2003) 68. J. Koike, K. Kobayashi, Proceedings of a subcommittee symposium of the Japan Inst. Metals “Fundamentals of magnesium and industrial applications”, The Japan Inst. Metals, 2001, pp. 1–3. Y. Yoshida, L. Cisar, S. Kamado, Y. Kojima, Metal. Trans. 44 (2003) 468. L. Cisar, Y. Yoshida, S. Kamado, Y. Kojima, F. Watanabe, Metal. Trans. 44 (2003) 476.
´ [12] M.T. Perez-Prado, J.A. del Valle, J.M. Contreras, O.A. Ruano, Scripta Mater. 50 (2004) 661. [13] A. Styczynski, Ch. Hartig, J. Bohlen, D. Letzig, Scripta Mater. 50 (2004) 943. [14] W.J. Kim, J.B. Lee, W.Y. Kim, H.T. Jeong, H.G. Jeong, Scripta Mater. 56 (2007) 309. [15] Q.L. Jin, S.Y. Shim, S.G. Lim, Scripta Mater. 55 (2006) 843. [16] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Mater. Sci. Eng. A 441 (2006) 349. ´ [17] M. Eddahbi, J.A. del Valle, M.T. Perez-Prado, O.A. Ruano, Mater. Sci. Eng. A 410/411 (2005) 308. ˜ O.A. Ruano, Acta Mater. 54 (2006) 4247. [18] J.A. del Valle, F. Carreno, [19] M.R. Barnett, M.D. Nave, C.J. Bettles, Mater. Sci. Eng. A 386 (2004) 205. ˇ F. Kaiser, D. Letzig, P. Lukaˇ ´ c, K.U. Kainer, J. Alloys [20] J. Bohlen, F. Chmel´ık, P. Dobron, Compd. 378 (2004) 207. ´ Acta Mater. 49 (2001) 4277. [21] S.R. Agnew, M.H. Yoo, C.N. Tome, [22] C.S. Roberts, Magnesium and its Alloys, John Wiley & Sons, 1960, pp. 190. [23] M.J. Philippe, Mater. Sci. Forum 157–162 (1994) 1337. [24] Y.N. Wang, J.C. Huang, Mater. Chem. Phys. 81 (2003) 11.