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Microstructural and textural evolution of AZ31 magnesium alloy during differential speed rolling
Journal of Alloys and Compounds 479 (2009) 726–731
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructural and textural evolution of AZ31 magnesium alloy during differential speed rolling Xinsheng Huang ∗ , Kazutaka Suzuki, Akira Watazu, Ichinori Shigematsu, Naobumi Saito Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, , Aichi 463-8560, Japan
a r t i c l e
i n f o
Article history: Received 26 August 2008 Received in revised form 8 January 2009 Accepted 18 January 2009 Available online 20 February 2009 Keywords: Magnesium alloy Asymmetric rolling Microstructure Texture Shear band
a b s t r a c t The microstructural and textural evolution of the hot-extruded AZ31 alloy plate during differential speed rolling (DSR) has been investigated. For the hot-extruded plate, a large texture gradient exists in the thickness direction while the grain size distribution is nearly homogenous. The textural evolutions at near-surface and mid-layer regions are different between the DSR processed and the normal symmetric rolled sheets. The inclination of basal pole at the mid-layer increases at an accelerating rate with progress of the DSR processing due to the enhancement of the unidirectional shear bands. The deformation tends to concentrate on the previously formed shear bands, which exhibit a favored crystalline orientation for further shear localization. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium (Mg) wrought alloys are the lightest commercial structural alloys and have a great potential for applications in automotive and electronic industries due to their low density, high specific strength, good damping characteristics, good electromagnetic shielding capability and easiness of recycling [1]. However, the wrought Mg alloys generally exhibit poor cold-formability due to their limited slip systems and strong basal texture formed during thermo-mechanical processing including rolling [2,3]. A strong basal texture increases the difficulty in a deformation accompanied with thickness reduction and thus leads to a very limited formability near room temperature. Therefore, the plastic behavior of the Mg alloys is significantly affected by the texture at ambient temperature and a remarkable enhancement of formability can be achieved by texture modification [4,5]. Differential speed rolling (DSR) is a process carried out at different rotation speeds for upper and lower rolls so that intense shear deformation can be introduced throughout the sheet thickness. This intense shear strain in the whole deformed part may be utilized for achieving a fine-grained microstructure and a texture control. Recently, it has been reported that the DSR processing is effective in improvements in both tensile elongation and press formability by the texture modification [6–9], indicating the potential of the DSR processing as a practicable technique for enhancing the formabil-
∗ Corresponding author. Tel.: +81 52 736 7195; fax: +81 52 736 7406. E-mail address: [email protected] (X. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.046
ity. In the previous work, we reported that a large inclination of basal pole at about 15◦ can be achieved by the DSR processing [10]. However, it is still lacking in understanding the microstructural and textural evolution during the DSR processing, even though several studies have been carried out on the microstructural and textural evolution of the Mg alloys during normal symmetric rolling [11–16]. A detailed understanding of how the microstructure and the texture change during the DSR processing is beneficial for an effective texture control. In this study, the DSR processing was carried out on an AZ31 Mg alloy and the microstructural and textural development during the DSR processing was investigated. 2. Experimental procedure The starting billets for rolling were cut from the commercial hot-extruded plates of the AZ31B (Mg–3.10Al–1.06Zn–0.35Mn in wt.%) alloy with a thickness of 4 mm. The hot-extruded plate was used as starting billet because it generally exhibit a better workability compared with the as-cast Mg alloy block due to much smaller grain size and is beneficial for preventing the DSR processed sheet from edge or surface cracking. The DSR processing conditions were similar to the previously reported ones [10]. The DSR processing was conducted without lubrication on roll surfaces at a rotation speed ratio of 1.167. Rolling direction was parallel to the extrusion direction (ED). The AZ31 alloy billets were rolled from 4 mm to 2 mm in thickness by 6 passes at 703 K, and then were rolled down to 1 mm by 8 passes at 573 K with a total reduction of 75%. The reductions per pass at 703 K and at 573 K were about 11% and 8%, respectively. Both rolls were heated to 573 K during the rolling process. The sheet was rotated and reversed after each pass so that the shear strain was introduced unidirectionally throughout the processing. For comparison, normal symmetric rolling was conducted under the same rolling schedule and the rolling direction was reversed after each pass. The sheets were quenched into water immediately after 6-pass at 703 K, 2-pass, 4-pass, 6-pass and 8-pass at 573 K and then one part of the sheet was cut for subsequent investigations.
X. Huang et al. / Journal of Alloys and Compounds 479 (2009) 726–731
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The polished samples were etched using a solution of picric acid, acetic acid, distilled water and ethanol for optical microscopic observation. The average grain size was determined by analyzing the optical micrographs with a line-intercept method. The X-ray texture analysis was performed by the Schulz reflection method at ␣-angles ranging from 15◦ to 90◦ for acquiring (0 0 0 2) incomplete pole figures using a Rigaku RINT2000. The pole figures of the rolled sheets were measured at the mid-layer and the region after 0.1 mm in thickness being removed from the surface (denoted as near-surface) by grinding. For the DSR processed sheets, the pole figures were measured on the mid-layer and the near-surface of the same sample from the two sides which contacted the high speed and the low speed rolls at the last rolling pass, respectively. Hereafter, RD, TD and ND denote the rolling direction, the transverse direction and the normal direction, respectively.
3. Results and discussion Fig. 1 shows the optical micrographs of the near-surface region and the central region of the as-received material taken in the longitudinal section (ED-ND plane). No significant variation in grain size is observed along the through-thickness direction of the hotextruded plate. The near-surface region exhibits approximately the same grain size of 40 m as that (44 m) of the central region. Fig. 2 shows the (0 0 0 2) pole figures of the as-received material measured at the surface and at different thicknesses after removing the layers of 0.2 mm, 1 mm (quarter-layer) and 2 mm (mid-layer) in thickness from the surface. The hot-extruded plate exhibits quite different textures in the through-thickness direction even though no apparent grain size distribution exists. A weak prismatic component starts to appear in the sample after 0.2 mm in thickness being removed from the surface and it develops toward the mid-layer. For the quarter-layer and the mid-layer, the pole figures show the typi-
Fig. 1. Optical micrographs of (a) the near-surface region and (b) the central region of the as-received hot-extruded plate taken in the longitudinal section (ED-ND plane). ED is horizontal.
Fig. 2. (0 0 0 2) pole figures of the hot-extruded plate measured at (a) the surface and at different thicknesses after removing the layers of (b) 0.2 mm, (c) 1 mm and (d) 2 mm in thickness from the surface.
cal extrusion texture, i.e. a combination of a strong basal texture and a moderate prismatic component. The spread of (0 0 0 2) orientation in the TD may be associated with the activation of the prismatic slip during the extrusion [17]. No apparent inclination of basal pole is observed for the quarter-layer and the mid-layer. In contrast, the surface and the region after 0.2 mm being removed exhibit the basal poles tilting at about 20◦ and 10◦ toward the ED, respectively. In addition, the basal texture intensities are 7.9, 24.4, 15.4 and 10.4 at the surface and the regions after 0.2 mm, quarter and half of the thickness being removed from the surface, respectively, also significantly change along the through-thickness direction. The result shows that the basal texture weakens with approaching the midlayer except the thin surface region. These results indicate that it is insufficient to characterize the whole texture of the hot-extruded Mg alloy plate only by the information on the surface and the midlayer. The result on a weaker basal texture intensity at the surface compared with the mid-layer is consistent with that as indicated in Ref. [18]. However, a detailed report on texture gradient of hotextruded Mg alloy plate has not been published and the reason for the formation of the texture gradient is not clear. The texture gradient may be related to the frictional force between the extrusion die and the deformed material as well as the temperature gradient caused by the different temperatures between the extrusion die and the deformed material. The frictional force may introduce a large amount of shear strain and may be responsible for the inclination of basal pole in the near-surface region. A large frictional force at the surface may also induce friction heating and thus affect the temperature gradient throughout the thickness. It can be presumed that the shear strain at the surface and the temperature gradient in the deformed material result in the texture gradient. Fig. 3 shows the optical micrographs taken in the longitudinal section (RD-ND plane) of the normal rolled and the DSR processed sheets after 6-pass at 703 K and 2-pass, 4-pass, 6-pass and 8-pass at 573 K. The magnified sections of the sheets DSR processed after 6-pass at 703 K, 4-pass and 8-pass at 573 K are shown in Fig. 4. The shear bands exhibit well-equiaxed fine grains, while the coarse grains in the remaining region exhibit slight wavy grain boundaries and deformation twins especially for the sheets rolled at 573 K.
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Fig. 3. Optical micrographs taken in the longitudinal sections (RD-ND plane) of (a–e) the normal rolled sheet and (f–j) the DSR processed sheet, which were rolled after (a and f) 6-pass at 703 K and (b and g) 2-pass, (c and h) 4-pass, (d and i) 6-pass and (e and j) 8-pass at 573 K, respectively. For the DSR processed sheets, the upper and lower sides in the figure are the sides which contacted the high speed roll and the low speed roll at the last rolling pass, respectively.
Fig. 4. Optical micrographs (magnified sections) taken in the longitudinal sections (RD-ND plane) of the sheets DSR processed after (a) 6-pass at 703 K, (b) 4-pass and (c) 8-pass at 573 K, respectively. RD is horizontal.
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The grain size is inhomogeneous and some grains are elongated along the RD especially in the sheets after 703 K due to incomplete dynamic recrystallization during rolling. The accumulated strain in the elongated grains may be insufficient to induce the dynamic recrystallization due to the high billet temperature of 703 K, which is more likely to cause only dynamic recovery. The amount of the elongated grains remarkably decreases with increasing the rolling pass at 573 K due to the dynamic recrystallization especially for the DSR processed sheet. The elongated grains almost disappear after 4-pass at 573 K in the DSR processed sheet, while a small amount of the elongated grains still remain in the normal rolled sheet even after 8-pass at 573 K, indicating the effect of the shear strain on enhancing the dynamic recrystallization. Another different aspect is shear banding, which is caused by localization of shear deformation. For the normal rolled sheet, only weak traces of the crossed shear bands can be seen. In contrast, the unidirectional shear bands are clearly visible due to a larger concentration of deformation for the DSR processed sheet. For the DSR processed sheet after 6-pass at 703 K, the shear bands are mainly formed at nearsurface region and transmit toward the mid-layer, indicating a more intense shear strain concentrated on the near-surface region due to the frictional effect between the roll and the sheet surface. A larger shear deformation within the near-surface region compared with the mid-layer has also been observed in an aluminium alloy
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sheet processed by asymmetric rolling using two rolls with different diameters [19,20]. No apparent increase in the amount of the shear bands is observed. The shear bands become darker and sharper due to the refined grains, and penetrate throughout the thickness with the progress of the DSR processing. This indicates that the strain tends to concentrate around the shear bands formed by the previous rolling pass. The inclination angle of the shear band is 20–30◦ and tends to decrease with proceeding the DSR processing due to extension and thinning of the sheet. The DSR processing conditions, e.g. reduction per pass, billet temperature, rotation speed ratio and initial sheet thickness might also affect the inclination angle of the shear band. As shown in Fig. 4, the grain sizes of the shear band of the DSR processed sheets after 6-pass at 703 K, 4-pass and 8-pass at 573 K are 6.4, 4.7 and 3.5 m, respectively, is decreasing with accumulating amount of deformation. This also indicates that the shear localization is likely to occur along the previously formed shear bands. The (0 0 0 2) pole figures of the normal rolled and the DSR processed sheets measured at the near-surface and the mid-layer are shown in Fig. 5. For the normal rolled sheet, the pole figures show a similar distribution of orientation between the near-surface and the mid-layer. A remarkable spread of the {0 0 0 1} orientation in the TD exists in the sample after 6-pass at 703 K, and it gradually disappears and becomes a roughly circle-shaped distribution with
Fig. 5. (0 0 0 2) pole figures of (a) the normal rolled sheet and (b) the DSR processed sheet measured at the near-surface (left part) and the mid-layer (right part), which were rolled after 6-pass at 703 K and 2-pass, 4-pass, 6-pass and 8-pass at 573 K, respectively.
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proceeding the rolling. The spread of the {0 0 0 1} orientation in the TD may be originated from the initial extrusion texture. In contrast, the DSR processed sheet showed a different characteristic in texture from the normal rolled sheet especially at the near-surface region. At the mid-layer, the DSR processed sheet after 6-pass at 703 K exhibits a similar texture (a dispersed {0 0 0 1} orientation in the TD without inclination of basal pole) as the normal rolled sheet, while the basal pole tilts in the RD with increasing the rolling pass at 573 K. However, at the near-surface region, the DSR processed sheet after 6-pass at 703 K shows a tilted basal pole with a roughly circle-shaped distribution and the {0 0 0 1} orientation disperses in the RD with proceeding the rolling. The basal pole inclines toward the RD at the near-surface while it inclines toward the opposite direction of the RD at the mid-layer. Because the pole figures of the near-surface and the mid-layer were measured from the opposite sides, the opposite inclination direction of the basal pole in the pole figures means that the c-axes in the two regions mostly incline at the same direction with respect to the RD.
Fig. 6. Changes in (a) basal texture intensity and (b) inclination angle of basal pole during rolling for the normal rolled (NR) and the DSR processed sheets. Here, 6-pass at 703 K is designated as 0-pass at 573 K.
Fig. 6 shows the changes in basal texture intensity and inclination angle of basal pole during rolling at 573 K for the normal rolled and the DSR processed sheets. After 6-pass at 703 K, the basal texture intensities at the near-surface and the mid-layer of both sheets remarkably decrease from 24.4 to 7.1–9.4 and from 10.4 to 7.8–8.5, respectively. A larger decrease is observed at the nearsurface with an initial strong basal texture. The decrease in basal texture intensity may be attributed to rotational dynamic recrystallization (RDRX) [11,21]. The RDRX tends to take place at high temperature in a material in which the grains are not favorably orientated to accommodate the rolling strain. The new small recrystallizated grains are rotated away from the original coarse grains mostly belonging to the {0 0 0 1} fiber and thus the basal texture can be weakened [11]. The basal texture intensity of the mid-layer tends to increase with proceeding the rolling at 573 K. The grains with the basal plane parallel to the rolling plane are more stable during continuous dynamic recrystallization (CDRX) and the CDRX may be the reason for the strengthening of basal texture [15]. The basal texture intensity at the near-surface is stronger than that at the midlayer for the normal rolled sheet, while it is reverse for the DSR processed sheet. It has been reported that the shear deformation has effects on enlarging the fraction of high angle grain boundaries and misorientation angle [22], which contributes to the weakening of basal texture. Therefore, a weaker basal texture intensity at the near-surface of the DSR processed sheet can be attributable to the intense shear deformation localized in the near-surface, while this effect is weak for the normal rolled sheet. The larger basal texture intensity at the near-surface of the normal rolled sheet may be related to the initial texture gradient of the hot-extruded plate. For the normal rolled sheet, the textures at the mid-layer exhibit a typical rolling texture without an inclination of basal pole, while a slight inclination (2–3◦ ) of basal pole at the near-surface may be due to the shear effect originated from the frictional force on the sheet surface. In contrast, for the DSR processed sheet, the inclination of basal pole even exists at the mid-layer and the inclination angle increases at an accelerated rate from 4-pass at 573 K when the shear bands penetrate throughout the thickness. The early inclination of basal pole at the near-surface indicates that a stronger shear deformation occurs at the near-surface and it transmits toward the mid-layer. As mentioned above, the deformation seems to concentrate on the previously formed shear bands with progress of the DSR processing. In the previous work, we investigated the microtexture of the DSR processed sheet after annealing at 573 K using electron backscattered diffraction (EBSD) and found that the inclination directions of basal poles were opposite between the shear band and the remaining coarse grain region, and the latter with a much larger volume ratio dominated the macro-texture [10]. The basal planes in the shear bands tend to align parallel to the planes of the shear bands [10]. This crystalline orientation is favored for subsequent shear deformation and thus is likely to induce the deformation concentration on the existing shear bands during the next rolling pass. In contrast, the inclination of c-axes in the remaining coarse grain regions is not favored for the formation of the shear band due to the opposite inclination direction with respect to the unidirectional shear bands. This restricts the formation of the new shear bands in the coarse grain regions. The inclination of basal pole is closely related to the formation of unidirectional shear bands. It is suggested that the unidirectional oblique slide of the coarse grain regions along the shear bands leads to a rotation of the microstructure and thus the inclination of basal pole [10]. Therefore, it can be considered that the accelerated inclination of basal pole is mainly due to the enhancement and the penetration of the shear bands throughout the thickness. The developed shear bands accelerate the inclination of basal pole, while it also tends to induce cracking along the shear bands when the accumulated strain exceeds a tolerable
X. Huang et al. / Journal of Alloys and Compounds 479 (2009) 726–731
level. This inhomogeneous deformation is one reason for the poorer rollability compared with the normal rolling. For avoiding the cracking along the shear bands, it could be effective to introduce larger amount of shear bands aiming a more homogeneous deformation, which may be affected by the rolling parameters such as speed rotation ratio, thickness reduction per pass, rolling temperature, lubrication, etc. Further work is in progress for clarifying the relationship between the microstructure and the DSR processing parameters. 4. Conclusions The microstructural and textural evolution of the hot-extruded AZ31 alloy plate during the DSR processing has been investigated. The conclusions can be drawn as follows: (1) A large in-depth texture gradient in thickness exists in the hot-extruded plate even though the grain size is nearly homogenous. The basal texture is quite strong at the near-surface and weakens with approaching the mid-layer. (2) The basal texture intensity at the near-surface is stronger than that at the mid-layer for the normal rolled sheet, while it is reverse for the DSR processed sheet. (3) The effect of DSR on the inclination of basal pole is more remarkable for the sheet rolled at 573 K compared with that rolled at 703 K due to the enhancement of the unidirectional shear bands. (4) The early occurrence of inclination of basal pole at the nearsurface indicates that a stronger shear deformation occurs at the near-surface and it transmits toward the mid-layer. The inclination of basal pole at the mid-layer increases at an accelerating rate with progress of the DSR processing due to the enhancement of the unidirectional shear bands penetrating throughout the thickness of sheet.
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(5) No apparent increase in the amount of the shear bands during the DSR processing indicates that the deformation tends to concentrate on the previously existed shear bands. It is speculated that the favored local crystalline orientation for further shear localization in the previously formed shear bands restricts the formation of the new shear bands. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
I.J. Polmear, Mater. Sci. Technol. 10 (1994) 1. M.H. Yoo, Metall. Trans. 12A (1981) 409. G.S. Rao, Y.V.R.K. Prasad, Metall. Trans. 13A (1982) 2219. E. Yukutake, J. Kaneko, M. Sugamata, Mater. Trans. 44 (2003) 452. K. Iwanaga, H. Tashiro, H. Okamoto, K. Shimizu, J. Mater. Process. Technol. 155–156 (2004) 1313. H. Watanabe, T. Mukai, K. Ishikawa, J. Mater. Sci. 39 (2004) 1477. W.J. Kim, J.B. Lee, W.Y. Kim, H.T. Jeong, H.G. Jeong, Scripta Mater. 56 (2007) 309. X.S. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, Mater. Sci. Eng. A 488 (2008) 214. X.S. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, J. Alloy Compd. 470 (2008) 263. X.S. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, J. Alloy Compd. 457 (2008) 408. J.A. del Valle, M.T. Pérez-Prado, O.A. Ruano, Mater. Sci. Eng. A 355 (2003) 68. J.A. del Valle, M.T. Pérez-Prado, J.R. Bartolome, O.A. Ruano, Mater. Trans. 44 (2003) 2625. M.T. Pérez-Prado, J.A. del Valle, J.M. Contreras, O.A. Ruano, Scripta Mater. 50 (2004) 661. A.K. Singh, R.A. Schwarzer, Z. Metallkd. 96 (2005) 345. Q.L. Jin, S.Y. Shim, S.G. Lim, Scripta Mater. 55 (2006) 843. A. Srinivasan, S.G. Chowdhury, V.C. Srivastava, Mater. Sci. Technol. 23 (2007) 1313. M.T. Pérez-Prado, J.A. del Valle, O.A. Ruano, Scripta Mater. 50 (2004) 667. M.T. Pérez-Prado, O.A. Ruano, Scripta Mater. 46 (2002) 149. S.B. Kang, B.K. Min, H.W. Kim, D.S. Wilkinson, J.D. Kang, Metall. Mater. Trans. A 36 (2005) 3141. J. Sidor, A. Miroux, R. Petrov, L. Kestens, Acta Mater. 56 (2008) 2495. S.E. Ion, F.J. Humphreys, S.H. White, Acta Metall. 30 (1982) 1909. J.Y. Cho, T. Inoue, F.X. Yin, K. Nagai, Mater. Trans. 45 (2004) 2966.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructural and textural evolution of AZ31 magnesium alloy during differential speed rolling Xinsheng Huang ∗ , Kazutaka Suzuki, Akira Watazu, Ichinori Shigematsu, Naobumi Saito Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, , Aichi 463-8560, Japan
a r t i c l e
i n f o
Article history: Received 26 August 2008 Received in revised form 8 January 2009 Accepted 18 January 2009 Available online 20 February 2009 Keywords: Magnesium alloy Asymmetric rolling Microstructure Texture Shear band
a b s t r a c t The microstructural and textural evolution of the hot-extruded AZ31 alloy plate during differential speed rolling (DSR) has been investigated. For the hot-extruded plate, a large texture gradient exists in the thickness direction while the grain size distribution is nearly homogenous. The textural evolutions at near-surface and mid-layer regions are different between the DSR processed and the normal symmetric rolled sheets. The inclination of basal pole at the mid-layer increases at an accelerating rate with progress of the DSR processing due to the enhancement of the unidirectional shear bands. The deformation tends to concentrate on the previously formed shear bands, which exhibit a favored crystalline orientation for further shear localization. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium (Mg) wrought alloys are the lightest commercial structural alloys and have a great potential for applications in automotive and electronic industries due to their low density, high specific strength, good damping characteristics, good electromagnetic shielding capability and easiness of recycling [1]. However, the wrought Mg alloys generally exhibit poor cold-formability due to their limited slip systems and strong basal texture formed during thermo-mechanical processing including rolling [2,3]. A strong basal texture increases the difficulty in a deformation accompanied with thickness reduction and thus leads to a very limited formability near room temperature. Therefore, the plastic behavior of the Mg alloys is significantly affected by the texture at ambient temperature and a remarkable enhancement of formability can be achieved by texture modification [4,5]. Differential speed rolling (DSR) is a process carried out at different rotation speeds for upper and lower rolls so that intense shear deformation can be introduced throughout the sheet thickness. This intense shear strain in the whole deformed part may be utilized for achieving a fine-grained microstructure and a texture control. Recently, it has been reported that the DSR processing is effective in improvements in both tensile elongation and press formability by the texture modification [6–9], indicating the potential of the DSR processing as a practicable technique for enhancing the formabil-
∗ Corresponding author. Tel.: +81 52 736 7195; fax: +81 52 736 7406. E-mail address: [email protected] (X. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.046
ity. In the previous work, we reported that a large inclination of basal pole at about 15◦ can be achieved by the DSR processing [10]. However, it is still lacking in understanding the microstructural and textural evolution during the DSR processing, even though several studies have been carried out on the microstructural and textural evolution of the Mg alloys during normal symmetric rolling [11–16]. A detailed understanding of how the microstructure and the texture change during the DSR processing is beneficial for an effective texture control. In this study, the DSR processing was carried out on an AZ31 Mg alloy and the microstructural and textural development during the DSR processing was investigated. 2. Experimental procedure The starting billets for rolling were cut from the commercial hot-extruded plates of the AZ31B (Mg–3.10Al–1.06Zn–0.35Mn in wt.%) alloy with a thickness of 4 mm. The hot-extruded plate was used as starting billet because it generally exhibit a better workability compared with the as-cast Mg alloy block due to much smaller grain size and is beneficial for preventing the DSR processed sheet from edge or surface cracking. The DSR processing conditions were similar to the previously reported ones [10]. The DSR processing was conducted without lubrication on roll surfaces at a rotation speed ratio of 1.167. Rolling direction was parallel to the extrusion direction (ED). The AZ31 alloy billets were rolled from 4 mm to 2 mm in thickness by 6 passes at 703 K, and then were rolled down to 1 mm by 8 passes at 573 K with a total reduction of 75%. The reductions per pass at 703 K and at 573 K were about 11% and 8%, respectively. Both rolls were heated to 573 K during the rolling process. The sheet was rotated and reversed after each pass so that the shear strain was introduced unidirectionally throughout the processing. For comparison, normal symmetric rolling was conducted under the same rolling schedule and the rolling direction was reversed after each pass. The sheets were quenched into water immediately after 6-pass at 703 K, 2-pass, 4-pass, 6-pass and 8-pass at 573 K and then one part of the sheet was cut for subsequent investigations.
X. Huang et al. / Journal of Alloys and Compounds 479 (2009) 726–731
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The polished samples were etched using a solution of picric acid, acetic acid, distilled water and ethanol for optical microscopic observation. The average grain size was determined by analyzing the optical micrographs with a line-intercept method. The X-ray texture analysis was performed by the Schulz reflection method at ␣-angles ranging from 15◦ to 90◦ for acquiring (0 0 0 2) incomplete pole figures using a Rigaku RINT2000. The pole figures of the rolled sheets were measured at the mid-layer and the region after 0.1 mm in thickness being removed from the surface (denoted as near-surface) by grinding. For the DSR processed sheets, the pole figures were measured on the mid-layer and the near-surface of the same sample from the two sides which contacted the high speed and the low speed rolls at the last rolling pass, respectively. Hereafter, RD, TD and ND denote the rolling direction, the transverse direction and the normal direction, respectively.
3. Results and discussion Fig. 1 shows the optical micrographs of the near-surface region and the central region of the as-received material taken in the longitudinal section (ED-ND plane). No significant variation in grain size is observed along the through-thickness direction of the hotextruded plate. The near-surface region exhibits approximately the same grain size of 40 m as that (44 m) of the central region. Fig. 2 shows the (0 0 0 2) pole figures of the as-received material measured at the surface and at different thicknesses after removing the layers of 0.2 mm, 1 mm (quarter-layer) and 2 mm (mid-layer) in thickness from the surface. The hot-extruded plate exhibits quite different textures in the through-thickness direction even though no apparent grain size distribution exists. A weak prismatic component starts to appear in the sample after 0.2 mm in thickness being removed from the surface and it develops toward the mid-layer. For the quarter-layer and the mid-layer, the pole figures show the typi-
Fig. 1. Optical micrographs of (a) the near-surface region and (b) the central region of the as-received hot-extruded plate taken in the longitudinal section (ED-ND plane). ED is horizontal.
Fig. 2. (0 0 0 2) pole figures of the hot-extruded plate measured at (a) the surface and at different thicknesses after removing the layers of (b) 0.2 mm, (c) 1 mm and (d) 2 mm in thickness from the surface.
cal extrusion texture, i.e. a combination of a strong basal texture and a moderate prismatic component. The spread of (0 0 0 2) orientation in the TD may be associated with the activation of the prismatic slip during the extrusion [17]. No apparent inclination of basal pole is observed for the quarter-layer and the mid-layer. In contrast, the surface and the region after 0.2 mm being removed exhibit the basal poles tilting at about 20◦ and 10◦ toward the ED, respectively. In addition, the basal texture intensities are 7.9, 24.4, 15.4 and 10.4 at the surface and the regions after 0.2 mm, quarter and half of the thickness being removed from the surface, respectively, also significantly change along the through-thickness direction. The result shows that the basal texture weakens with approaching the midlayer except the thin surface region. These results indicate that it is insufficient to characterize the whole texture of the hot-extruded Mg alloy plate only by the information on the surface and the midlayer. The result on a weaker basal texture intensity at the surface compared with the mid-layer is consistent with that as indicated in Ref. [18]. However, a detailed report on texture gradient of hotextruded Mg alloy plate has not been published and the reason for the formation of the texture gradient is not clear. The texture gradient may be related to the frictional force between the extrusion die and the deformed material as well as the temperature gradient caused by the different temperatures between the extrusion die and the deformed material. The frictional force may introduce a large amount of shear strain and may be responsible for the inclination of basal pole in the near-surface region. A large frictional force at the surface may also induce friction heating and thus affect the temperature gradient throughout the thickness. It can be presumed that the shear strain at the surface and the temperature gradient in the deformed material result in the texture gradient. Fig. 3 shows the optical micrographs taken in the longitudinal section (RD-ND plane) of the normal rolled and the DSR processed sheets after 6-pass at 703 K and 2-pass, 4-pass, 6-pass and 8-pass at 573 K. The magnified sections of the sheets DSR processed after 6-pass at 703 K, 4-pass and 8-pass at 573 K are shown in Fig. 4. The shear bands exhibit well-equiaxed fine grains, while the coarse grains in the remaining region exhibit slight wavy grain boundaries and deformation twins especially for the sheets rolled at 573 K.
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Fig. 3. Optical micrographs taken in the longitudinal sections (RD-ND plane) of (a–e) the normal rolled sheet and (f–j) the DSR processed sheet, which were rolled after (a and f) 6-pass at 703 K and (b and g) 2-pass, (c and h) 4-pass, (d and i) 6-pass and (e and j) 8-pass at 573 K, respectively. For the DSR processed sheets, the upper and lower sides in the figure are the sides which contacted the high speed roll and the low speed roll at the last rolling pass, respectively.
Fig. 4. Optical micrographs (magnified sections) taken in the longitudinal sections (RD-ND plane) of the sheets DSR processed after (a) 6-pass at 703 K, (b) 4-pass and (c) 8-pass at 573 K, respectively. RD is horizontal.
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The grain size is inhomogeneous and some grains are elongated along the RD especially in the sheets after 703 K due to incomplete dynamic recrystallization during rolling. The accumulated strain in the elongated grains may be insufficient to induce the dynamic recrystallization due to the high billet temperature of 703 K, which is more likely to cause only dynamic recovery. The amount of the elongated grains remarkably decreases with increasing the rolling pass at 573 K due to the dynamic recrystallization especially for the DSR processed sheet. The elongated grains almost disappear after 4-pass at 573 K in the DSR processed sheet, while a small amount of the elongated grains still remain in the normal rolled sheet even after 8-pass at 573 K, indicating the effect of the shear strain on enhancing the dynamic recrystallization. Another different aspect is shear banding, which is caused by localization of shear deformation. For the normal rolled sheet, only weak traces of the crossed shear bands can be seen. In contrast, the unidirectional shear bands are clearly visible due to a larger concentration of deformation for the DSR processed sheet. For the DSR processed sheet after 6-pass at 703 K, the shear bands are mainly formed at nearsurface region and transmit toward the mid-layer, indicating a more intense shear strain concentrated on the near-surface region due to the frictional effect between the roll and the sheet surface. A larger shear deformation within the near-surface region compared with the mid-layer has also been observed in an aluminium alloy
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sheet processed by asymmetric rolling using two rolls with different diameters [19,20]. No apparent increase in the amount of the shear bands is observed. The shear bands become darker and sharper due to the refined grains, and penetrate throughout the thickness with the progress of the DSR processing. This indicates that the strain tends to concentrate around the shear bands formed by the previous rolling pass. The inclination angle of the shear band is 20–30◦ and tends to decrease with proceeding the DSR processing due to extension and thinning of the sheet. The DSR processing conditions, e.g. reduction per pass, billet temperature, rotation speed ratio and initial sheet thickness might also affect the inclination angle of the shear band. As shown in Fig. 4, the grain sizes of the shear band of the DSR processed sheets after 6-pass at 703 K, 4-pass and 8-pass at 573 K are 6.4, 4.7 and 3.5 m, respectively, is decreasing with accumulating amount of deformation. This also indicates that the shear localization is likely to occur along the previously formed shear bands. The (0 0 0 2) pole figures of the normal rolled and the DSR processed sheets measured at the near-surface and the mid-layer are shown in Fig. 5. For the normal rolled sheet, the pole figures show a similar distribution of orientation between the near-surface and the mid-layer. A remarkable spread of the {0 0 0 1} orientation in the TD exists in the sample after 6-pass at 703 K, and it gradually disappears and becomes a roughly circle-shaped distribution with
Fig. 5. (0 0 0 2) pole figures of (a) the normal rolled sheet and (b) the DSR processed sheet measured at the near-surface (left part) and the mid-layer (right part), which were rolled after 6-pass at 703 K and 2-pass, 4-pass, 6-pass and 8-pass at 573 K, respectively.
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proceeding the rolling. The spread of the {0 0 0 1} orientation in the TD may be originated from the initial extrusion texture. In contrast, the DSR processed sheet showed a different characteristic in texture from the normal rolled sheet especially at the near-surface region. At the mid-layer, the DSR processed sheet after 6-pass at 703 K exhibits a similar texture (a dispersed {0 0 0 1} orientation in the TD without inclination of basal pole) as the normal rolled sheet, while the basal pole tilts in the RD with increasing the rolling pass at 573 K. However, at the near-surface region, the DSR processed sheet after 6-pass at 703 K shows a tilted basal pole with a roughly circle-shaped distribution and the {0 0 0 1} orientation disperses in the RD with proceeding the rolling. The basal pole inclines toward the RD at the near-surface while it inclines toward the opposite direction of the RD at the mid-layer. Because the pole figures of the near-surface and the mid-layer were measured from the opposite sides, the opposite inclination direction of the basal pole in the pole figures means that the c-axes in the two regions mostly incline at the same direction with respect to the RD.
Fig. 6. Changes in (a) basal texture intensity and (b) inclination angle of basal pole during rolling for the normal rolled (NR) and the DSR processed sheets. Here, 6-pass at 703 K is designated as 0-pass at 573 K.
Fig. 6 shows the changes in basal texture intensity and inclination angle of basal pole during rolling at 573 K for the normal rolled and the DSR processed sheets. After 6-pass at 703 K, the basal texture intensities at the near-surface and the mid-layer of both sheets remarkably decrease from 24.4 to 7.1–9.4 and from 10.4 to 7.8–8.5, respectively. A larger decrease is observed at the nearsurface with an initial strong basal texture. The decrease in basal texture intensity may be attributed to rotational dynamic recrystallization (RDRX) [11,21]. The RDRX tends to take place at high temperature in a material in which the grains are not favorably orientated to accommodate the rolling strain. The new small recrystallizated grains are rotated away from the original coarse grains mostly belonging to the {0 0 0 1} fiber and thus the basal texture can be weakened [11]. The basal texture intensity of the mid-layer tends to increase with proceeding the rolling at 573 K. The grains with the basal plane parallel to the rolling plane are more stable during continuous dynamic recrystallization (CDRX) and the CDRX may be the reason for the strengthening of basal texture [15]. The basal texture intensity at the near-surface is stronger than that at the midlayer for the normal rolled sheet, while it is reverse for the DSR processed sheet. It has been reported that the shear deformation has effects on enlarging the fraction of high angle grain boundaries and misorientation angle [22], which contributes to the weakening of basal texture. Therefore, a weaker basal texture intensity at the near-surface of the DSR processed sheet can be attributable to the intense shear deformation localized in the near-surface, while this effect is weak for the normal rolled sheet. The larger basal texture intensity at the near-surface of the normal rolled sheet may be related to the initial texture gradient of the hot-extruded plate. For the normal rolled sheet, the textures at the mid-layer exhibit a typical rolling texture without an inclination of basal pole, while a slight inclination (2–3◦ ) of basal pole at the near-surface may be due to the shear effect originated from the frictional force on the sheet surface. In contrast, for the DSR processed sheet, the inclination of basal pole even exists at the mid-layer and the inclination angle increases at an accelerated rate from 4-pass at 573 K when the shear bands penetrate throughout the thickness. The early inclination of basal pole at the near-surface indicates that a stronger shear deformation occurs at the near-surface and it transmits toward the mid-layer. As mentioned above, the deformation seems to concentrate on the previously formed shear bands with progress of the DSR processing. In the previous work, we investigated the microtexture of the DSR processed sheet after annealing at 573 K using electron backscattered diffraction (EBSD) and found that the inclination directions of basal poles were opposite between the shear band and the remaining coarse grain region, and the latter with a much larger volume ratio dominated the macro-texture [10]. The basal planes in the shear bands tend to align parallel to the planes of the shear bands [10]. This crystalline orientation is favored for subsequent shear deformation and thus is likely to induce the deformation concentration on the existing shear bands during the next rolling pass. In contrast, the inclination of c-axes in the remaining coarse grain regions is not favored for the formation of the shear band due to the opposite inclination direction with respect to the unidirectional shear bands. This restricts the formation of the new shear bands in the coarse grain regions. The inclination of basal pole is closely related to the formation of unidirectional shear bands. It is suggested that the unidirectional oblique slide of the coarse grain regions along the shear bands leads to a rotation of the microstructure and thus the inclination of basal pole [10]. Therefore, it can be considered that the accelerated inclination of basal pole is mainly due to the enhancement and the penetration of the shear bands throughout the thickness. The developed shear bands accelerate the inclination of basal pole, while it also tends to induce cracking along the shear bands when the accumulated strain exceeds a tolerable
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level. This inhomogeneous deformation is one reason for the poorer rollability compared with the normal rolling. For avoiding the cracking along the shear bands, it could be effective to introduce larger amount of shear bands aiming a more homogeneous deformation, which may be affected by the rolling parameters such as speed rotation ratio, thickness reduction per pass, rolling temperature, lubrication, etc. Further work is in progress for clarifying the relationship between the microstructure and the DSR processing parameters. 4. Conclusions The microstructural and textural evolution of the hot-extruded AZ31 alloy plate during the DSR processing has been investigated. The conclusions can be drawn as follows: (1) A large in-depth texture gradient in thickness exists in the hot-extruded plate even though the grain size is nearly homogenous. The basal texture is quite strong at the near-surface and weakens with approaching the mid-layer. (2) The basal texture intensity at the near-surface is stronger than that at the mid-layer for the normal rolled sheet, while it is reverse for the DSR processed sheet. (3) The effect of DSR on the inclination of basal pole is more remarkable for the sheet rolled at 573 K compared with that rolled at 703 K due to the enhancement of the unidirectional shear bands. (4) The early occurrence of inclination of basal pole at the nearsurface indicates that a stronger shear deformation occurs at the near-surface and it transmits toward the mid-layer. The inclination of basal pole at the mid-layer increases at an accelerating rate with progress of the DSR processing due to the enhancement of the unidirectional shear bands penetrating throughout the thickness of sheet.
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(5) No apparent increase in the amount of the shear bands during the DSR processing indicates that the deformation tends to concentrate on the previously existed shear bands. It is speculated that the favored local crystalline orientation for further shear localization in the previously formed shear bands restricts the formation of the new shear bands. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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