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Ameliorating the mechanical properties of magnesium alloy: Role of texture
Materials Science & Engineering A 689 (2017) 395–403
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Ameliorating the mechanical properties of magnesium alloy: Role of texture a
Qinghang Wang , Bin Jiang Bo Liud, Fusheng Pana,b
a,b,⁎
a,⁎⁎
, Aitao Tang
a
c
MARK
a
, Shaoxing Ma , Zhongtao Jiang , Yanfu Chai ,
a
State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Chongqing Academy of Science and Technology, Chongqing 401123, China Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China d Chongqing Chang-an Automobile Co., Ltd., Chongqing 400023, China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Differential speed extrusion Asymmetric shear deformation Grain refinement Texture weakening Formability
AZ31 magnesium alloy sheet was fabricated by two different approaches, namely conventional extrusion (CE) and differential speed extrusion (DSE), under the same process conditions and extrusion ratio. The microstructures, textures and mechanical properties of AZ31é magnesium alloy sheets processed by the CE and DSE processes were investigated. It was found that with the DSE process more uniform and finer microstructures were obtained. The DSE sheet exhibited the lower yield strength and yield ratio, but the larger n value combined with the lower r value, which made the DSE sheet achieve an excellent formability. Those results were mainly attributed to a tilted weak texture resulted from the differential flow speed of alloy at the severe deformation zone.
1. Introduction Magnesium (Mg) alloys possessing qualities of low density, high specific strength and specific stiffness have, over the past decade, created considerable interest in automotive application and aircraft industries to improve fuel efficiency and reduce CO2 emissions [1]. Mg alloy products mainly focus on strips, bars and profiles fabricated by primary processing, such as hot-extrusion or rolling. However, these products tend to present a strong basal texture whose basal plane is parallel to extrusion or rolling direction, resulting in a poor formability during second processing at room temperature [2,3]. Therefore, tailoring basal texture is considered as an effective way to ameliorate the formability of Mg alloys. Recently, the severe plastic deformation technologies aiming to refine grains and modify a strong basal texture of Mg alloys have received great attentions. They have been developed rapidly and widely used in preparing the high-performance Mg alloy. These technologies include twist extrusion (TE) [4], simple shear extrusion (SSE) [5] as well as dual equal channel lateral extrusion (DECLE) [6], torsionalequal channel angular pressing (T-ECAP) [7] and high-pressure tube twisting (HPTT) [8]. However, they are only applicable for bulk products limiting the promising of the thin sheets. Given that the mentioned disadvantage, Chang et al. [9] and Yang et al. [10–12] reported some new extrusion methods to manufacture
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high-performance thin Mg alloy sheets. Those approaches were collectively entitled as “Asymmetric Extrusion”, such as differential speed extrusion (DSE) [10], trapezoid extrusion (TE) [11] and asymmetric extrusion (ASE) [12]. Inspiration of differential speed extrusion (DSE) derived from differential speed rolling (DSR) which could effectively alter the texture and enhance mechanical properties of the rolled sheets. This change was mainly attributed to an asymmetric shear deformation in the whole thickness of the sheets giving rise to the difference of velocity between the top and bottom surface. The principle of DSE process was that altering a flow passage forms asymmetric shear deformation in the whole thickness of the DSE sheet, causing a grain refinement, a tilted weak basal texture and the improvement of mechanical properties. Yang et al. [10] only focused on the effect of the microstructure and texture characteristics on the final mechanical properties of the DSE sheet, but there was no systematic investigation into the microstructure and texture evolution of a workpiece during the whole extrusion process. Therefore, the present work was concentrated on the influence of microstructures and texture evolutions of the workpieces during the CE and DSE processes on the final mechanical properties of Mg alloy sheet. Additionally, the formability of both sheets was performed by the Erichsen cupping test.
Corresponding author at: State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (A. Tang).
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http://dx.doi.org/10.1016/j.msea.2017.02.067 Received 12 October 2016; Received in revised form 16 January 2017; Accepted 17 February 2017 Available online 20 February 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic sectional view of the extrusion die: (a) the conventional extrusion (CE) die and (c) the differential speed extrusion (DSE) die. Illustration of the flow passage for both dies: (b) the CE die and (d) the DSE die in details [10].
diameter of 20 mm at a punch speed of 5 mm/min at room temperature. Each sample of Erichsen cupping test was machined into a block with 60 mm in length, 60 mm in width and 1 mm in thickness. The tests were repeated three times for each sheet. DEFORM-3D simulation was employed to analyze the extrusion process in the present study. In order to clearly demonstrate the metal flow during the extrusion process, 1/2 symmetric geometry model was established. The geometry models of the workpieces (AZ31 alloy), the CE and DSE dies made by Unigraphics NX software were imported to DEFORM-3D system. In this simulation, the workpieces (AZ31 alloy) were set as plastic body, while the dies were defined as the rigid ones. The friction coefficient between workpiece (AZ31 alloy) and die was set as 0.25. The temperature and speed of each extrusion process were set as 430 ℃ and 10 mm/s, respectively.
2. Materials and methods The as-received AZ31 (Mg-2.78 Al-0.63 Zn-0.21 Mn in wt%) cast billets with 85 mm in diameter were cut into ingots with Φ 80 mm×60 mm. The billets then were homogenized at 400 ℃ for 12 h and cooled in air to obtain supersaturated solution. The AZ31 Mg alloy billets were extruded into the CE and DSE sheets with 1 mm in thickness and 60 mm in width at 430 ℃ with an extrusion ratio of 101:1. The extruded sheets were allowed to cool naturally in air after exiting the dies. The schematic section of flow passage in CE and DSE dies was shown in Fig. 1 [10]. The DSE die was equipped with a different parallel flow passage length (L=4 mm) [10]. Specimens remaining in the CE and DSE dies were split after extrusion processes. The longitudinal sections of specimens were etched or electro-polished, and observed using Optical Microscopy (OM) and electro backscatter diffraction (EBSD). The sample preparation for EBSD consisted of grinding on SiC papers of grit size 400, 600, 800, 1000, 1200, 2000 and electro-polishing at a voltage of 20 V for 120 s at a temperature of −10 ℃ with a special electrolyte named as AC2. The analyses of EBSD were done in a FEI Nova 400 FEG-SEM equipped with an EBSD detector. EBSD was performed at the operating voltage of 20 kV, 15 mm working distance and a 70° tilt. Scan steps for the sample CE and DSE were set as 1 µm. The EBSD data were evaluated by orientation imaging microscopy (OIM, HKL-channel 5) software. To support the result of EBSD data, the macro-textures of the CE and DSE sheets from the extrusion direction (ED)-the transverse direction (TD) plane were carried out by X-ray Diffraction (XRD, Rigaku D/Max 2500) using Cu Kαradiation at a wavelength of 0.15406 nm. To investigate the anisotropy of the mechanical properties, tensile specimens with 10 mm in initial gauge length, 6 mm in gauge width and 1 mm in gauge thickness were machined from the CE and DSE sheets in the tensile directions of 0, 45 and 90°. Tensile tests were carried out by a CMT6305–300 kN universal testing machine with a strain rate of 10−3 S−1 at room temperature. Tensile tests in the different tensile directions were repeated three times to ensure repeatability and to confirm the consistency of the results. However, a representative curve was shown for each test. The Lankford coefficient (r value) was examined at a permanent strain of 10% in each tensile direction. The strain hardening exponent value (n value) was obtained within a uniform strain using power law regression. The fractured tensile samples of the CE and DSE sheets were observed by the OM in the tensile directions of 0, 45 and 90°. Erichsen cupping tests were carried out to determine the formability of the CE and DSE sheets using a hemispherical punch with a
3. Results 3.1. Microstructures and textures EBSD inverse pole figure (IPF) maps and (0002) pole figures of typical locations of the specimen CE and DSE are demonstrated in Fig. 2. Low angle grain boundaries (LAGBs) are defined as boundaries with an angle θ such that 2°＜ θ ＜15° and are indicated in the figures by white lines, and high angle grain boundaries (HAGBs) are defined as boundaries such that 15° ≤θ and are indicated by black lines. It is apparent that from positionⅠto position Ⅱ of the specimen CE, coarse and elongated grains gradually are refined and grain boundaries become smoother in Fig. 2a (Ⅱ). When an AZ31 alloy is extruded from position Ⅱ to position Ⅲ, grains are further refined into fine dynamic recrystallized ones in Fig. 2a (Ⅲ). This microstructural feature is one of the typical characteristics of Mg alloys that underwent complete dynamic recrystallization [13]. This is ascribed to continuous production and absorption of dislocation in low angle grain boundaries, as well as progressive transformation to high angle grain boundaries [14]. As regards to the specimen DSE, the microstructure evolution shows a similar process (Fig. 2b). Compared with the specimen CE, however, the specimen DSE exhibits relative microstructural homogeneity and fine grain size at the corresponding positions. The average grain size of each position for the specimens CE and DSE are summarized in Table 1. This finding indicates that the DSE process is in favor of refining the microstructure of extruded sheet. Besides the microstructure evolution of extrusion process, the texture evolution is also a significant factor on the mechanical property of Mg alloy sheet. The (0002) pole figures reveal that both the specimens CE and DSE have a typical basal texture feature, despite
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suggests that dynamic recrystallization can effectively weaken the basal texture during extrusion process, which is in agreement with the result of Ma et al. [15]. The specimen DSE, however, exhibits much weaker scattered basal texture than the specimen CE. Besides, from position Ⅱ to Ⅲ, the specimen DSE still keeps a relative weak tilted basal texture, but with regard to the specimen CE a rotated texture gradually alters into one, whose basal plane is preferentially aligned parallel to the ED. The micro-textures measured by EBSD are supported by macrotexture measurements from XRD. The macro-textures of both sheets are shown in Fig. 3. A distinct texture characteristic is found. It is apparent that, for the CE sheet, c-axis is aligned along the ND and the texture component (Imax=16.586) almost focuses on the center of the (0002) pole figure. The DSE sheet is characterized by the inclination (~12°) of c-axis of grains from the ND to the negative ED, and Imax, which is 10.014, is found to be less than the one for the CE sheet. This result is in agreement with the observation of micro-texture from FEI measurement. Fig. 4 demonstrates EBSD analysis of Schmid factors for basal slip system (mbasal) of the CE and DSE sheets under a tension in the ED and the TD at room temperature. The average mbasal of the CE and DSE sheets are shown in Table 2, when applied the virtual tension along the ED and the TD. In Table 2, the mbasal of CE sheet is obviously low along the ED (0.221) and the TD (0.161). On the contrary, the DSE sheet exhibits the high mbasal along the ED and the TD. It is remarkably evident that the DSE process has a considerable effect on the activation of basal slip along the ED and the TD. This is ascribed to the increased fraction of grains with the mbasal of 0.4–0.5 along the ED and TD (Fig. 4). 3.2. Mechanical properties Tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° are shown in Fig. 5 and their corresponding tensile properties are listed in Table 3. The DSE sheet exhibits a lower yield strength (YS) and ultimate tensile strength (UTS) in the tensile directions of 0, 45 and 90° compared with the CE sheet, especially in the tensile direction of 0°. This result indicates that the larger mbasal of the DSE sheet, when applied the tension on the ED, is beneficial to the decrease of the YS in the tensile direction of 0°. In addition, the yield ratios (YS/UTS) of the DSE sheet are lower than that of the CE sheet in three tensile directions, especially, a maximum decrease of 0.06 is obtained in the tensile direction of 0°. Besides, the fracture elongation (εf) of the DSE sheet in the tensile direction of 0° is larger than that of the CE sheet. However, The εf for the DSE sheet are found to be less than that of the CE sheet in the tensile directions of 45° and 90°. A possible reason is described in “discussion” section. Additionally, in comparison with the CE sheet, the DSE sheet presents the lower r value and higher n value in the tensile directions of 0, 45 and 90°. The largest n and the lowest r value are measured in the tensile direction of 0° for the DSE sheet. Fig. 6 displays Erichsen values (IE) of the CE and DSE sheets from the Erichsen cupping tests at room temperature. The average IEs are 2.73 and 3.14 mm for the CE and DSE sheets, respectively. In contrast with the CE sheet, the average IE increased by ~15% for the DSE sheet. Besides, it is explicitly observed that the lengths of cracks for both Erichsen samples, which are formed by Erichsen cupping tests, are obviously different along the ED. The length of crack for the DSE sheet (~8.5 mm) is smaller than that of crack for the CE sheet (~12 mm). These results mean that the formability of Mg sheets can be effectively ameliorated by the DSE process.
Fig. 2. The EBSD inverse pole figure (IPF) maps and (0002) pole figures of typical positions of the specimens: (a) the specimen CE, (b) the specimen DSE.
Table 1. The average grain size of each position for the specimens CE and DSE. Specimen
Ave (Position Ⅰ)/μm
Ave (Position Ⅱ)/μm
Ave (Position Ⅲ)/μm
CE DSE
7.9 6.1
7.1 5.6
6.2 4.8
4. Discussion the basal plane rotated about a certain degree. In Fig. 2, for both the specimens CE and DSE, there is a trend of decreasing basal texture intensity with development of the extrusion process. This change
The metal flows during the CE and DSE processes shown in Fig. 7 can be divided into two distinct zones based on the metallographic change that occurs within them: a deformation zone and a shear strain 397
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Fig. 3. The (0002) pole figures of the sheets from XRD measurements (a) the CE sheet (b) the DSE sheet.
Grain size and texture are significant factors for the mechanical properties of the Mg alloy sheet. How they affect the mechanical properties of the Mg alloy sheet can be illustrated as follows. The relationships between the YS and the average grain size of the CE and DSE sheets in the tensile directions of 0，45 and 90° are shown in Fig. 9. It is noticeable that the YS of the DSE sheet does not follow the standard Hall-Petch relationship in spite of grain refinement. According to the Hall-Petch relationship [16], the grain refinement increases the YS. However, there is a drop of the YS in three tensile directions, especially in the tensile direction of 0°. Kim et al. [17,18] also reported that the ECAP processed AZ31 and AZ61 rods exhibited a decrease in the YS against d−1/2. As shown in a previous report [16], in the case of ECAP processed AZ31 sheets, texture modification plays a more dominant role in the determination of the YS than grain refinement. In our work, when the CE sheet is applied a virtual tension along the ED, the basal planes of most of grains are almost parallel to the tensile direction, which leads to lower mbasal and difficulty of basal slip. Compared with the CE sheet, the DSE sheet exhibits a tilted weak basal texture. When the DSE sheet is applied a virtual tension along the ED, the basal planes of most of grains and the tensile direction are at the certain angles resulting in the higher mbasal. The relationship between the YS of the sheet and the mbasal is expressed as follows [19]:
zone. The highly plastically deformed shear zone is created throughout the region in contact with the die and follows the sub-surface region of the extruded material. It can also be clearly seen that the shear strain zone during the DSE process is non-symmetric along the ED, and its area ratio is higher than that of the shear zone during the CE process, as evidenced by the white arrows in Fig. 7. This is quite significant, as difference between the severely deformed sub-surface region and deformed center region is responsible for creating microstructural inhomogeneity. Red arrows represent coarse and elongated grains in Fig. 7. Clearly, there is a trend of decreasing the number of coarse and elongated grains and improving microstructural homogeneity in the specimen DSE, due to an asymmetric shear strain. Therefore, the DSE process can effectively obtain much more homogenous refined grains by the asymmetric shear strain. As mentioned above, the homogeneous and refined microstructure is achieved by altering the flow passage. Simultaneously, this change also has a significant impact on texture weakening. The DEFORM-3D simulation is employed to analyze the effect of alteration of the flow passage on crystallographic orientations during the extrusion process. Fig. 8 shows the metal flows at different steps during two extrusion processes. Positions of point tracking at the different steps during the metal flow are marked. Point P1 and P2, which are distributed symmetrically along the ED, are set on the workpiece before the extrusion process (as seen in step −1). With the performance of extrusion process, the point P1 and P2 gradually move. The point P1 keeps pace with the point P2 before entering the severe deformation zone during both extrusion processes (as seen in step 33). However, the point P1 is out of step with the point P2 after accessing to the severe deformation zone during the DSE process (as seen in step 106). Obviously, when the alloy flows into the severe deformation zone, the point P1 is prevented by the die, which gives rise to a low velocity of the point P1. Nevertheless, the point P2 moves until the alloy is extruded out the exit of die. With respect to the CE process, the points P1 and P2 still keep pace after entering the severe deformation zone (as seen in step 225). This difference indicates that altering the flow passage can significantly induce a difference of velocity during the metal flow, which further results in a variation of crystallographic orientation. As shown in Fig. 3, the basal plane of the DSE sheet rotated about 12° away from the ND to the negative ED, but the CE sheet is characterized by a strong basal texture. For the CE process, the point P1 and P2 remain relatively static during the metal flow causing the c-axis of grains to be almost parallel the ND of the sheet (Fig. 8c). On the contrary, for the DSE process, a relative motion is occurred between the points P1 and P2 after accessing the deformation zone resulting in crystal rotation away from the ND to the negative ED (Fig. 8d). It is explained why the DSE sheet presents a tilted weak basal texture.
σs = τ / ms
(1)
where σs is yield strength, τ is resolved shear stress, ms is Schmid factor for basal slip. The σs decreases with increasing the ms. Therefore, the DSE sheet with the higher mbasal can exhibits the lower YS. In our study, the texture weakening and grain refinement are acquired simultaneously during the DSE process and the final yield strength of the sheet was decreased, suggesting that the effect of the texture weakening on declining the yield strength is greater than that of the grain refinement on enhancing the yield strength. The UTS of the DSE sheet seems also related to the texture weakening. In spite of the grain refinement, the UTS presents an obviously decreasing trend due to the weak tilted basal texture. This result is in consistent with a report that the AZ31 Mg alloy with weak basal texture simultaneously exhibited the low UTS by continuous extrusion forming process [20]. Such phenomenon was considered as a result of texture softening induced by changing crystal orientation [20]. Much attention is also paid to the change of the εf. As shown in previous study [21], for the composite extruded AZ31 alloy sheet, the εf was improved from 18.5% to 22.5%, which is related to a tilted weak basal texture. Kim et al. [22] also reported that tensile-ductility improvement in the asymmetrically rolled AZ31 Mg alloys could be attributed to a weakening of the basal texture during the DSR. In our work, the DSE sheet exhibits an obvious higher ductility at room
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Fig. 4. The EBSD analysis of Schmid factors for basal slip of the CE and DSE sheets: (a, b) the Schmid factor maps of the CE and DSE sheets, when applied the virtual tension parallel to the ED, (c, d) the Schmid factor maps of the CE and DSE sheets, when applied the virtual tension parallel to the TD, (e) the average Schmid factors along the ED and the TD.
Table 2. The average Schmid factors for basal slip system of the CE and DSE sheets, when applied a virtual tension along the ED and TD. Sheet
mbasal along the ED
mbasal along the TD
CE DSE
0.221 0.271
0.161 0.242
temperature than the CE sheet in the 0° tensile direction due to an EDtilted weak basal texture. However, there is an opposite result for the DSE sheet in the tensile directions of 45 and 90°. We believe that twinning may induce the formation of cracks resulting in the decrease ductility of the DSE sheet in the tensile direction of 45 and 90°. The optional microscopy of the fractures of the tensile samples in the 45 and 90° tensile directions are shown in Fig. 10. 399
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Fig. 5. The tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° at room temperature: (a) the true stress-strain curves, (b) the 0.2% proof yield strength (YS) and the ultimate tensile strength (UTS), (c) the yield ratio, (d) the n value and r value. Table 3. The tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° at room temperature. Sheet
Tension direction
YS/(MPa)
UTS/(MPa)
εf (%)
Yield ratio
n value
r value
CE
0° 45° 90°
226.8 235.6 245.3
427.8 414.9 387.2
19.3 21.7 19.6
0.53 0.56 0.63
0.24 0.22 0.20
2.01 2.19 2.86
DSE
0° 45° 90°
180.5 206.4 205.6
378.5 376.7 332.9
22.0 20.1 18.8
0.47 0.54 0.61
0.27 0.23 0.19
1.31 1.90 2.71
As shown in Fig. 10, there are a large number of twins occur in the fractures of the DSE sheet compared with that of the CE sheet, despite the average grain size of the DSE sheet is slight lower than that of the CE sheet. It is well known that the twins were easily formed within the grains with larger grain size rather than within the smaller grains. Apparently, this result is not in agreement with our finding. It is indicated that the grain size is a factor to affect the formation of twins, but it may be not a primary factor. In our paper, the DSE sheet exhibits higher mbasal compared with the CE sheet in the 0 and 90° tensile directions. When it is applied a virtual tension in the 45 and 90° tensile directions, there is a tendency for the extension of the c-axis of grains. This trend is beneficial to the formation of twins at the late stage of tension with increasing the strain, in spite of small grain size of the DSE sheet. For the CE sheet, there is not the formation of twins due to a strong basal texture. It is suggested that the formation of twins is mainly attributed to the crystallographic orientation. In Fig. 10, it is found that the micro-cracks initiated at twins formed within grain of the DSE sheet. As we know, the formation of the micro-cracks indicates
Fig. 6. The Erichsen values (IE) and Erichsen samples of the CE and DSE sheets after Erichsen cupping tests.
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Fig. 7. The metal flows of the exit of the extrusion dies during the two processes consist of two zones: shear strain zone (white arrow) and the deformation zone. Additionally, red arrows represent coarse and elongated grains. (a) The CE process (b) the DSE process.
Fig. 8. The metal flow at different stages during the CE process (a) and the DSE process (b), red squares represent local magnification at entering the severe deformation zone; Schematic diagram of crystal rotation after entering the deformation zone during the CE process (c) and the DSE process (d).
the CE sheet. The high n value results in a low sensitivity to strain localization in the form of necking and is responsible for the increase of uniform elongation. Magnesium alloy with a lower basal texture would have a high n value since a large number of grains would be favorably oriented for deformation [25]. Huang et al. [26] suggested that the lower basal texture intensity increased the n-value due to restricted dynamic recovery and activated {10−12} extension twinning, suggesting a better stretch formability. Additionally, it is important to note that the r value related to the texture is the dominant factor to enhance the formability. The width strain can contribute to the total strain for uniaxial tensile extension, while the thickness strain is most necessary at a biaxial tension stress state of stretch forming [27]. A smaller r value may enhance the deformation capacity of sheet thinning, due to a weak texture [26]. In our work, it is confirmed that the DSE process is
a poor ductility for the sheet. Therefore, we believe that the decrease of ductility for the DSE sheet in the 45 and 90° tensile directions is related to the formation of twins. Jiang et al. [23] also reported that the ductility of Mg-1.58 Zn-0.52 Gd alloy without pre-forging was lower than that of this alloy with pre-forging. This result was attributed to the formation of micro-cracks initiated at twins on the fracture surface. Our finding is in agreement with the report of Jiang et al. The formability of Mg alloy sheet is strongly connected with the yield ratio, the r value and the n value. Han et al. [24] reported that a sheet with the low yield ratio could improve its stretch formability. Besides, they indicate that the stretch formability was mainly dependent on r value and n value [24]. Although the lower YS and UTS occurred in the DSE sheet, the yield ratios are lower than that of the CE sheet, indicating that the DSE sheet can present better formability than
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altering the flow passage. 2. Texture evolutions of the CE and DSE specimens were strikingly different. Compared with the CE specimen, the DSE specimen showed the lower texture intensity at the corresponding position. Besides, the tilted weak basal texture occurred in the DSE sheet, but for the CE sheet a strong basal texture was obtained. This result was the asymmetric flow passage, in which the differential flow speed of the alloy was occurred and the crystallographic orientation was changed into the ED-tilted weak basal one. 3. The formability of DSE sheet was effectively enhanced, which was attributed to the low yield strength and yield ratio, but the larger n value together with the lower r value. Those ameliorated properties were mainly owed to the weak basal texture. Acknowledgements The authors are grateful for the financial supports from Chongqing Science and Technology Commission [CSTC2014jcyjjq0041, cstc2015zdcy-ztzx50003], National Natural Science Foundation of China [51474043,51531002], and The National Key Research and Development Program of China (2016YFB0301104), and Education
Fig. 9. The relationships between the YS and the average grain size of the CE and DSE sheets in the tensile directions of 0，45 and 90°.
Fig. 10. The optional microscopy of the fractured tensile samples in the 45 and 90° tensile directions (a, c) the CE sheet along the 45 and 90° tensile directions, respectively (b, d) the DSE sheet along the 45 and 90° tensile directions, respectively (The red arrows indicate the micro-cracks initiated at twins formed within grains.).
an effective one to modify the basal texture and the DSE sheet exhibits superior formability owing to the lower yield strength, the lower yield ratio and the smaller r value together with the larger n value.
Commission of Chongqing Municipality (KJZH14101), and the Fundamental Research Funds the Central Universities (106112016CDJZR138801).
5. Conclusions
Appendix A. Supplementary material
In the present study, the comparison of the microstructure, texture and mechanical properties of the sheet fabricated by the CE and DSE processes were performed. The results were summarized as follows:
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.msea.2017.02.067. References
1. Through observation of microstructure evolution during two extrusion processes, more effective grain refinement was achieved during the DSE process in comparison with the CE process. A critical reason was that extra asymmetric shear deformation was introduced by
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texture, recrystallization and mechanical property of Mg–Zn alloys, J. Magnes. Alloy. 4 (2016) 188–196. M. Kavyania, G.R. Ebrahimia, M. Sanjarib, M. Haghshenasc, Texture evaluation in warm deformation of an extruded Mg–6Al–3Zn alloy, J. Magnes. Alloy. 4 (2016) 89–98. Y. Beygelzimer, V. Varyukhin, S. Synkov, D. Orlov, Mater. Sci. Eng. A 503 (2009) 14–17. N. Pardis, R. Ebrahimi, Mater. Sci. Eng. A 527 (2009) 355–360. B. Talebanpour, R. Ebrahimi, K. Janghorban, Mater. Sci. Eng. A 527 (2009) 141–145. B. Mani, M. Jahedi, M.H. Paydar, Mater. Sci. Eng. A 528 (2011) 4159–4165. L.S. Tóth, M. Arzaghi, J.J. Fundenberger, B. Beausir, O. Bouaziz, R. ArruffatMassion, Scr. Mater. 60 (2009) 175–177. L.L. Chang, Y.N. Wang, X. Zhao, J.C. Huang, Microstructure and mechanical properties in an AZ31 magnesium alloy sheet fabricated by asymmetric hot extrusion, Mater. Sci. Eng. A 496 (2008) 512–516. Q.S. Yang, B. Jiang, J.J. He, B. Song, W.J. Liu, H.W. Dong, F.S. Pan, Tailoring texture and refining grain of magnesium alloy by differential speed extrusion process, Mater. Sci. Eng. A 612 (2014) 187–191. Q.S. Yang, B. Jiang, G.Y. Zhou, J.J. He, F.S. Pan, Enhancing strength and ductility of AZ31 magnesium alloy sheets by the trapezoid extrusion, Mater. Sci. Technol. 30 (2014) 227–230. Q.S. Yang, B. Jiang, G.Y. Zhou, J.H. Dai, F.S. Pan, Influence of an asymmetric shear deformation on microstructure evolution and mechanical behavior of AZ31 magnesium alloy sheet, Mater. Sci. Eng. A 590 (2014) 440–447. J. Victoria-Hernandez, S. Yi, D. Letzig, D. Hernandez-Silva, J. Bohlen, Microstructure and texture development in hydrostatically extruded Mg–Al–Zn alloys during tensile testing at intermediate temperatures, Acta Mater. 61 (2013) 2179–2193. S.E. Ion, F.J. Humphreys, S.H. White, Dynamic recrystallization and the development of microstructure during the high temperature deformation of magnesium, Acta Metall. 30 (1982) 1909–1919. Q. Ma, B. Li, W.R. Whittington, A.L. Oppedal, P.T. Wang, M.F. Horstemeyer, Texture evolution during dynamic recrystallization in a magnesium alloy at 450 ℃, Acta Mater. 67 (2014) 102–115.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Ameliorating the mechanical properties of magnesium alloy: Role of texture a
Qinghang Wang , Bin Jiang Bo Liud, Fusheng Pana,b
a,b,⁎
a,⁎⁎
, Aitao Tang
a
c
MARK
a
, Shaoxing Ma , Zhongtao Jiang , Yanfu Chai ,
a
State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Chongqing Academy of Science and Technology, Chongqing 401123, China Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China d Chongqing Chang-an Automobile Co., Ltd., Chongqing 400023, China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Differential speed extrusion Asymmetric shear deformation Grain refinement Texture weakening Formability
AZ31 magnesium alloy sheet was fabricated by two different approaches, namely conventional extrusion (CE) and differential speed extrusion (DSE), under the same process conditions and extrusion ratio. The microstructures, textures and mechanical properties of AZ31é magnesium alloy sheets processed by the CE and DSE processes were investigated. It was found that with the DSE process more uniform and finer microstructures were obtained. The DSE sheet exhibited the lower yield strength and yield ratio, but the larger n value combined with the lower r value, which made the DSE sheet achieve an excellent formability. Those results were mainly attributed to a tilted weak texture resulted from the differential flow speed of alloy at the severe deformation zone.
1. Introduction Magnesium (Mg) alloys possessing qualities of low density, high specific strength and specific stiffness have, over the past decade, created considerable interest in automotive application and aircraft industries to improve fuel efficiency and reduce CO2 emissions [1]. Mg alloy products mainly focus on strips, bars and profiles fabricated by primary processing, such as hot-extrusion or rolling. However, these products tend to present a strong basal texture whose basal plane is parallel to extrusion or rolling direction, resulting in a poor formability during second processing at room temperature [2,3]. Therefore, tailoring basal texture is considered as an effective way to ameliorate the formability of Mg alloys. Recently, the severe plastic deformation technologies aiming to refine grains and modify a strong basal texture of Mg alloys have received great attentions. They have been developed rapidly and widely used in preparing the high-performance Mg alloy. These technologies include twist extrusion (TE) [4], simple shear extrusion (SSE) [5] as well as dual equal channel lateral extrusion (DECLE) [6], torsionalequal channel angular pressing (T-ECAP) [7] and high-pressure tube twisting (HPTT) [8]. However, they are only applicable for bulk products limiting the promising of the thin sheets. Given that the mentioned disadvantage, Chang et al. [9] and Yang et al. [10–12] reported some new extrusion methods to manufacture
⁎
high-performance thin Mg alloy sheets. Those approaches were collectively entitled as “Asymmetric Extrusion”, such as differential speed extrusion (DSE) [10], trapezoid extrusion (TE) [11] and asymmetric extrusion (ASE) [12]. Inspiration of differential speed extrusion (DSE) derived from differential speed rolling (DSR) which could effectively alter the texture and enhance mechanical properties of the rolled sheets. This change was mainly attributed to an asymmetric shear deformation in the whole thickness of the sheets giving rise to the difference of velocity between the top and bottom surface. The principle of DSE process was that altering a flow passage forms asymmetric shear deformation in the whole thickness of the DSE sheet, causing a grain refinement, a tilted weak basal texture and the improvement of mechanical properties. Yang et al. [10] only focused on the effect of the microstructure and texture characteristics on the final mechanical properties of the DSE sheet, but there was no systematic investigation into the microstructure and texture evolution of a workpiece during the whole extrusion process. Therefore, the present work was concentrated on the influence of microstructures and texture evolutions of the workpieces during the CE and DSE processes on the final mechanical properties of Mg alloy sheet. Additionally, the formability of both sheets was performed by the Erichsen cupping test.
Corresponding author at: State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (A. Tang).
⁎⁎
http://dx.doi.org/10.1016/j.msea.2017.02.067 Received 12 October 2016; Received in revised form 16 January 2017; Accepted 17 February 2017 Available online 20 February 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic sectional view of the extrusion die: (a) the conventional extrusion (CE) die and (c) the differential speed extrusion (DSE) die. Illustration of the flow passage for both dies: (b) the CE die and (d) the DSE die in details [10].
diameter of 20 mm at a punch speed of 5 mm/min at room temperature. Each sample of Erichsen cupping test was machined into a block with 60 mm in length, 60 mm in width and 1 mm in thickness. The tests were repeated three times for each sheet. DEFORM-3D simulation was employed to analyze the extrusion process in the present study. In order to clearly demonstrate the metal flow during the extrusion process, 1/2 symmetric geometry model was established. The geometry models of the workpieces (AZ31 alloy), the CE and DSE dies made by Unigraphics NX software were imported to DEFORM-3D system. In this simulation, the workpieces (AZ31 alloy) were set as plastic body, while the dies were defined as the rigid ones. The friction coefficient between workpiece (AZ31 alloy) and die was set as 0.25. The temperature and speed of each extrusion process were set as 430 ℃ and 10 mm/s, respectively.
2. Materials and methods The as-received AZ31 (Mg-2.78 Al-0.63 Zn-0.21 Mn in wt%) cast billets with 85 mm in diameter were cut into ingots with Φ 80 mm×60 mm. The billets then were homogenized at 400 ℃ for 12 h and cooled in air to obtain supersaturated solution. The AZ31 Mg alloy billets were extruded into the CE and DSE sheets with 1 mm in thickness and 60 mm in width at 430 ℃ with an extrusion ratio of 101:1. The extruded sheets were allowed to cool naturally in air after exiting the dies. The schematic section of flow passage in CE and DSE dies was shown in Fig. 1 [10]. The DSE die was equipped with a different parallel flow passage length (L=4 mm) [10]. Specimens remaining in the CE and DSE dies were split after extrusion processes. The longitudinal sections of specimens were etched or electro-polished, and observed using Optical Microscopy (OM) and electro backscatter diffraction (EBSD). The sample preparation for EBSD consisted of grinding on SiC papers of grit size 400, 600, 800, 1000, 1200, 2000 and electro-polishing at a voltage of 20 V for 120 s at a temperature of −10 ℃ with a special electrolyte named as AC2. The analyses of EBSD were done in a FEI Nova 400 FEG-SEM equipped with an EBSD detector. EBSD was performed at the operating voltage of 20 kV, 15 mm working distance and a 70° tilt. Scan steps for the sample CE and DSE were set as 1 µm. The EBSD data were evaluated by orientation imaging microscopy (OIM, HKL-channel 5) software. To support the result of EBSD data, the macro-textures of the CE and DSE sheets from the extrusion direction (ED)-the transverse direction (TD) plane were carried out by X-ray Diffraction (XRD, Rigaku D/Max 2500) using Cu Kαradiation at a wavelength of 0.15406 nm. To investigate the anisotropy of the mechanical properties, tensile specimens with 10 mm in initial gauge length, 6 mm in gauge width and 1 mm in gauge thickness were machined from the CE and DSE sheets in the tensile directions of 0, 45 and 90°. Tensile tests were carried out by a CMT6305–300 kN universal testing machine with a strain rate of 10−3 S−1 at room temperature. Tensile tests in the different tensile directions were repeated three times to ensure repeatability and to confirm the consistency of the results. However, a representative curve was shown for each test. The Lankford coefficient (r value) was examined at a permanent strain of 10% in each tensile direction. The strain hardening exponent value (n value) was obtained within a uniform strain using power law regression. The fractured tensile samples of the CE and DSE sheets were observed by the OM in the tensile directions of 0, 45 and 90°. Erichsen cupping tests were carried out to determine the formability of the CE and DSE sheets using a hemispherical punch with a
3. Results 3.1. Microstructures and textures EBSD inverse pole figure (IPF) maps and (0002) pole figures of typical locations of the specimen CE and DSE are demonstrated in Fig. 2. Low angle grain boundaries (LAGBs) are defined as boundaries with an angle θ such that 2°＜ θ ＜15° and are indicated in the figures by white lines, and high angle grain boundaries (HAGBs) are defined as boundaries such that 15° ≤θ and are indicated by black lines. It is apparent that from positionⅠto position Ⅱ of the specimen CE, coarse and elongated grains gradually are refined and grain boundaries become smoother in Fig. 2a (Ⅱ). When an AZ31 alloy is extruded from position Ⅱ to position Ⅲ, grains are further refined into fine dynamic recrystallized ones in Fig. 2a (Ⅲ). This microstructural feature is one of the typical characteristics of Mg alloys that underwent complete dynamic recrystallization [13]. This is ascribed to continuous production and absorption of dislocation in low angle grain boundaries, as well as progressive transformation to high angle grain boundaries [14]. As regards to the specimen DSE, the microstructure evolution shows a similar process (Fig. 2b). Compared with the specimen CE, however, the specimen DSE exhibits relative microstructural homogeneity and fine grain size at the corresponding positions. The average grain size of each position for the specimens CE and DSE are summarized in Table 1. This finding indicates that the DSE process is in favor of refining the microstructure of extruded sheet. Besides the microstructure evolution of extrusion process, the texture evolution is also a significant factor on the mechanical property of Mg alloy sheet. The (0002) pole figures reveal that both the specimens CE and DSE have a typical basal texture feature, despite
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suggests that dynamic recrystallization can effectively weaken the basal texture during extrusion process, which is in agreement with the result of Ma et al. [15]. The specimen DSE, however, exhibits much weaker scattered basal texture than the specimen CE. Besides, from position Ⅱ to Ⅲ, the specimen DSE still keeps a relative weak tilted basal texture, but with regard to the specimen CE a rotated texture gradually alters into one, whose basal plane is preferentially aligned parallel to the ED. The micro-textures measured by EBSD are supported by macrotexture measurements from XRD. The macro-textures of both sheets are shown in Fig. 3. A distinct texture characteristic is found. It is apparent that, for the CE sheet, c-axis is aligned along the ND and the texture component (Imax=16.586) almost focuses on the center of the (0002) pole figure. The DSE sheet is characterized by the inclination (~12°) of c-axis of grains from the ND to the negative ED, and Imax, which is 10.014, is found to be less than the one for the CE sheet. This result is in agreement with the observation of micro-texture from FEI measurement. Fig. 4 demonstrates EBSD analysis of Schmid factors for basal slip system (mbasal) of the CE and DSE sheets under a tension in the ED and the TD at room temperature. The average mbasal of the CE and DSE sheets are shown in Table 2, when applied the virtual tension along the ED and the TD. In Table 2, the mbasal of CE sheet is obviously low along the ED (0.221) and the TD (0.161). On the contrary, the DSE sheet exhibits the high mbasal along the ED and the TD. It is remarkably evident that the DSE process has a considerable effect on the activation of basal slip along the ED and the TD. This is ascribed to the increased fraction of grains with the mbasal of 0.4–0.5 along the ED and TD (Fig. 4). 3.2. Mechanical properties Tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° are shown in Fig. 5 and their corresponding tensile properties are listed in Table 3. The DSE sheet exhibits a lower yield strength (YS) and ultimate tensile strength (UTS) in the tensile directions of 0, 45 and 90° compared with the CE sheet, especially in the tensile direction of 0°. This result indicates that the larger mbasal of the DSE sheet, when applied the tension on the ED, is beneficial to the decrease of the YS in the tensile direction of 0°. In addition, the yield ratios (YS/UTS) of the DSE sheet are lower than that of the CE sheet in three tensile directions, especially, a maximum decrease of 0.06 is obtained in the tensile direction of 0°. Besides, the fracture elongation (εf) of the DSE sheet in the tensile direction of 0° is larger than that of the CE sheet. However, The εf for the DSE sheet are found to be less than that of the CE sheet in the tensile directions of 45° and 90°. A possible reason is described in “discussion” section. Additionally, in comparison with the CE sheet, the DSE sheet presents the lower r value and higher n value in the tensile directions of 0, 45 and 90°. The largest n and the lowest r value are measured in the tensile direction of 0° for the DSE sheet. Fig. 6 displays Erichsen values (IE) of the CE and DSE sheets from the Erichsen cupping tests at room temperature. The average IEs are 2.73 and 3.14 mm for the CE and DSE sheets, respectively. In contrast with the CE sheet, the average IE increased by ~15% for the DSE sheet. Besides, it is explicitly observed that the lengths of cracks for both Erichsen samples, which are formed by Erichsen cupping tests, are obviously different along the ED. The length of crack for the DSE sheet (~8.5 mm) is smaller than that of crack for the CE sheet (~12 mm). These results mean that the formability of Mg sheets can be effectively ameliorated by the DSE process.
Fig. 2. The EBSD inverse pole figure (IPF) maps and (0002) pole figures of typical positions of the specimens: (a) the specimen CE, (b) the specimen DSE.
Table 1. The average grain size of each position for the specimens CE and DSE. Specimen
Ave (Position Ⅰ)/μm
Ave (Position Ⅱ)/μm
Ave (Position Ⅲ)/μm
CE DSE
7.9 6.1
7.1 5.6
6.2 4.8
4. Discussion the basal plane rotated about a certain degree. In Fig. 2, for both the specimens CE and DSE, there is a trend of decreasing basal texture intensity with development of the extrusion process. This change
The metal flows during the CE and DSE processes shown in Fig. 7 can be divided into two distinct zones based on the metallographic change that occurs within them: a deformation zone and a shear strain 397
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Fig. 3. The (0002) pole figures of the sheets from XRD measurements (a) the CE sheet (b) the DSE sheet.
Grain size and texture are significant factors for the mechanical properties of the Mg alloy sheet. How they affect the mechanical properties of the Mg alloy sheet can be illustrated as follows. The relationships between the YS and the average grain size of the CE and DSE sheets in the tensile directions of 0，45 and 90° are shown in Fig. 9. It is noticeable that the YS of the DSE sheet does not follow the standard Hall-Petch relationship in spite of grain refinement. According to the Hall-Petch relationship [16], the grain refinement increases the YS. However, there is a drop of the YS in three tensile directions, especially in the tensile direction of 0°. Kim et al. [17,18] also reported that the ECAP processed AZ31 and AZ61 rods exhibited a decrease in the YS against d−1/2. As shown in a previous report [16], in the case of ECAP processed AZ31 sheets, texture modification plays a more dominant role in the determination of the YS than grain refinement. In our work, when the CE sheet is applied a virtual tension along the ED, the basal planes of most of grains are almost parallel to the tensile direction, which leads to lower mbasal and difficulty of basal slip. Compared with the CE sheet, the DSE sheet exhibits a tilted weak basal texture. When the DSE sheet is applied a virtual tension along the ED, the basal planes of most of grains and the tensile direction are at the certain angles resulting in the higher mbasal. The relationship between the YS of the sheet and the mbasal is expressed as follows [19]:
zone. The highly plastically deformed shear zone is created throughout the region in contact with the die and follows the sub-surface region of the extruded material. It can also be clearly seen that the shear strain zone during the DSE process is non-symmetric along the ED, and its area ratio is higher than that of the shear zone during the CE process, as evidenced by the white arrows in Fig. 7. This is quite significant, as difference between the severely deformed sub-surface region and deformed center region is responsible for creating microstructural inhomogeneity. Red arrows represent coarse and elongated grains in Fig. 7. Clearly, there is a trend of decreasing the number of coarse and elongated grains and improving microstructural homogeneity in the specimen DSE, due to an asymmetric shear strain. Therefore, the DSE process can effectively obtain much more homogenous refined grains by the asymmetric shear strain. As mentioned above, the homogeneous and refined microstructure is achieved by altering the flow passage. Simultaneously, this change also has a significant impact on texture weakening. The DEFORM-3D simulation is employed to analyze the effect of alteration of the flow passage on crystallographic orientations during the extrusion process. Fig. 8 shows the metal flows at different steps during two extrusion processes. Positions of point tracking at the different steps during the metal flow are marked. Point P1 and P2, which are distributed symmetrically along the ED, are set on the workpiece before the extrusion process (as seen in step −1). With the performance of extrusion process, the point P1 and P2 gradually move. The point P1 keeps pace with the point P2 before entering the severe deformation zone during both extrusion processes (as seen in step 33). However, the point P1 is out of step with the point P2 after accessing to the severe deformation zone during the DSE process (as seen in step 106). Obviously, when the alloy flows into the severe deformation zone, the point P1 is prevented by the die, which gives rise to a low velocity of the point P1. Nevertheless, the point P2 moves until the alloy is extruded out the exit of die. With respect to the CE process, the points P1 and P2 still keep pace after entering the severe deformation zone (as seen in step 225). This difference indicates that altering the flow passage can significantly induce a difference of velocity during the metal flow, which further results in a variation of crystallographic orientation. As shown in Fig. 3, the basal plane of the DSE sheet rotated about 12° away from the ND to the negative ED, but the CE sheet is characterized by a strong basal texture. For the CE process, the point P1 and P2 remain relatively static during the metal flow causing the c-axis of grains to be almost parallel the ND of the sheet (Fig. 8c). On the contrary, for the DSE process, a relative motion is occurred between the points P1 and P2 after accessing the deformation zone resulting in crystal rotation away from the ND to the negative ED (Fig. 8d). It is explained why the DSE sheet presents a tilted weak basal texture.
σs = τ / ms
(1)
where σs is yield strength, τ is resolved shear stress, ms is Schmid factor for basal slip. The σs decreases with increasing the ms. Therefore, the DSE sheet with the higher mbasal can exhibits the lower YS. In our study, the texture weakening and grain refinement are acquired simultaneously during the DSE process and the final yield strength of the sheet was decreased, suggesting that the effect of the texture weakening on declining the yield strength is greater than that of the grain refinement on enhancing the yield strength. The UTS of the DSE sheet seems also related to the texture weakening. In spite of the grain refinement, the UTS presents an obviously decreasing trend due to the weak tilted basal texture. This result is in consistent with a report that the AZ31 Mg alloy with weak basal texture simultaneously exhibited the low UTS by continuous extrusion forming process [20]. Such phenomenon was considered as a result of texture softening induced by changing crystal orientation [20]. Much attention is also paid to the change of the εf. As shown in previous study [21], for the composite extruded AZ31 alloy sheet, the εf was improved from 18.5% to 22.5%, which is related to a tilted weak basal texture. Kim et al. [22] also reported that tensile-ductility improvement in the asymmetrically rolled AZ31 Mg alloys could be attributed to a weakening of the basal texture during the DSR. In our work, the DSE sheet exhibits an obvious higher ductility at room
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Fig. 4. The EBSD analysis of Schmid factors for basal slip of the CE and DSE sheets: (a, b) the Schmid factor maps of the CE and DSE sheets, when applied the virtual tension parallel to the ED, (c, d) the Schmid factor maps of the CE and DSE sheets, when applied the virtual tension parallel to the TD, (e) the average Schmid factors along the ED and the TD.
Table 2. The average Schmid factors for basal slip system of the CE and DSE sheets, when applied a virtual tension along the ED and TD. Sheet
mbasal along the ED
mbasal along the TD
CE DSE
0.221 0.271
0.161 0.242
temperature than the CE sheet in the 0° tensile direction due to an EDtilted weak basal texture. However, there is an opposite result for the DSE sheet in the tensile directions of 45 and 90°. We believe that twinning may induce the formation of cracks resulting in the decrease ductility of the DSE sheet in the tensile direction of 45 and 90°. The optional microscopy of the fractures of the tensile samples in the 45 and 90° tensile directions are shown in Fig. 10. 399
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Fig. 5. The tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° at room temperature: (a) the true stress-strain curves, (b) the 0.2% proof yield strength (YS) and the ultimate tensile strength (UTS), (c) the yield ratio, (d) the n value and r value. Table 3. The tensile properties of the CE and DSE sheets in the tensile directions of 0, 45 and 90° at room temperature. Sheet
Tension direction
YS/(MPa)
UTS/(MPa)
εf (%)
Yield ratio
n value
r value
CE
0° 45° 90°
226.8 235.6 245.3
427.8 414.9 387.2
19.3 21.7 19.6
0.53 0.56 0.63
0.24 0.22 0.20
2.01 2.19 2.86
DSE
0° 45° 90°
180.5 206.4 205.6
378.5 376.7 332.9
22.0 20.1 18.8
0.47 0.54 0.61
0.27 0.23 0.19
1.31 1.90 2.71
As shown in Fig. 10, there are a large number of twins occur in the fractures of the DSE sheet compared with that of the CE sheet, despite the average grain size of the DSE sheet is slight lower than that of the CE sheet. It is well known that the twins were easily formed within the grains with larger grain size rather than within the smaller grains. Apparently, this result is not in agreement with our finding. It is indicated that the grain size is a factor to affect the formation of twins, but it may be not a primary factor. In our paper, the DSE sheet exhibits higher mbasal compared with the CE sheet in the 0 and 90° tensile directions. When it is applied a virtual tension in the 45 and 90° tensile directions, there is a tendency for the extension of the c-axis of grains. This trend is beneficial to the formation of twins at the late stage of tension with increasing the strain, in spite of small grain size of the DSE sheet. For the CE sheet, there is not the formation of twins due to a strong basal texture. It is suggested that the formation of twins is mainly attributed to the crystallographic orientation. In Fig. 10, it is found that the micro-cracks initiated at twins formed within grain of the DSE sheet. As we know, the formation of the micro-cracks indicates
Fig. 6. The Erichsen values (IE) and Erichsen samples of the CE and DSE sheets after Erichsen cupping tests.
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Fig. 7. The metal flows of the exit of the extrusion dies during the two processes consist of two zones: shear strain zone (white arrow) and the deformation zone. Additionally, red arrows represent coarse and elongated grains. (a) The CE process (b) the DSE process.
Fig. 8. The metal flow at different stages during the CE process (a) and the DSE process (b), red squares represent local magnification at entering the severe deformation zone; Schematic diagram of crystal rotation after entering the deformation zone during the CE process (c) and the DSE process (d).
the CE sheet. The high n value results in a low sensitivity to strain localization in the form of necking and is responsible for the increase of uniform elongation. Magnesium alloy with a lower basal texture would have a high n value since a large number of grains would be favorably oriented for deformation [25]. Huang et al. [26] suggested that the lower basal texture intensity increased the n-value due to restricted dynamic recovery and activated {10−12} extension twinning, suggesting a better stretch formability. Additionally, it is important to note that the r value related to the texture is the dominant factor to enhance the formability. The width strain can contribute to the total strain for uniaxial tensile extension, while the thickness strain is most necessary at a biaxial tension stress state of stretch forming [27]. A smaller r value may enhance the deformation capacity of sheet thinning, due to a weak texture [26]. In our work, it is confirmed that the DSE process is
a poor ductility for the sheet. Therefore, we believe that the decrease of ductility for the DSE sheet in the 45 and 90° tensile directions is related to the formation of twins. Jiang et al. [23] also reported that the ductility of Mg-1.58 Zn-0.52 Gd alloy without pre-forging was lower than that of this alloy with pre-forging. This result was attributed to the formation of micro-cracks initiated at twins on the fracture surface. Our finding is in agreement with the report of Jiang et al. The formability of Mg alloy sheet is strongly connected with the yield ratio, the r value and the n value. Han et al. [24] reported that a sheet with the low yield ratio could improve its stretch formability. Besides, they indicate that the stretch formability was mainly dependent on r value and n value [24]. Although the lower YS and UTS occurred in the DSE sheet, the yield ratios are lower than that of the CE sheet, indicating that the DSE sheet can present better formability than
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altering the flow passage. 2. Texture evolutions of the CE and DSE specimens were strikingly different. Compared with the CE specimen, the DSE specimen showed the lower texture intensity at the corresponding position. Besides, the tilted weak basal texture occurred in the DSE sheet, but for the CE sheet a strong basal texture was obtained. This result was the asymmetric flow passage, in which the differential flow speed of the alloy was occurred and the crystallographic orientation was changed into the ED-tilted weak basal one. 3. The formability of DSE sheet was effectively enhanced, which was attributed to the low yield strength and yield ratio, but the larger n value together with the lower r value. Those ameliorated properties were mainly owed to the weak basal texture. Acknowledgements The authors are grateful for the financial supports from Chongqing Science and Technology Commission [CSTC2014jcyjjq0041, cstc2015zdcy-ztzx50003], National Natural Science Foundation of China [51474043,51531002], and The National Key Research and Development Program of China (2016YFB0301104), and Education
Fig. 9. The relationships between the YS and the average grain size of the CE and DSE sheets in the tensile directions of 0，45 and 90°.
Fig. 10. The optional microscopy of the fractured tensile samples in the 45 and 90° tensile directions (a, c) the CE sheet along the 45 and 90° tensile directions, respectively (b, d) the DSE sheet along the 45 and 90° tensile directions, respectively (The red arrows indicate the micro-cracks initiated at twins formed within grains.).
an effective one to modify the basal texture and the DSE sheet exhibits superior formability owing to the lower yield strength, the lower yield ratio and the smaller r value together with the larger n value.
Commission of Chongqing Municipality (KJZH14101), and the Fundamental Research Funds the Central Universities (106112016CDJZR138801).
5. Conclusions
Appendix A. Supplementary material
In the present study, the comparison of the microstructure, texture and mechanical properties of the sheet fabricated by the CE and DSE processes were performed. The results were summarized as follows:
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.msea.2017.02.067. References
1. Through observation of microstructure evolution during two extrusion processes, more effective grain refinement was achieved during the DSE process in comparison with the CE process. A critical reason was that extra asymmetric shear deformation was introduced by
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