Effect of texture symmetry on mechanical performance and corrosion resistance of magnesium alloy sheet

Journal of Alloys and Compounds 723 (2017) 213e224

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Effect of texture symmetry on mechanical performance and corrosion resistance of magnesium alloy sheet Junjie He a, Bin Jiang a, b, *, Jun Xu a, Jianyue Zhang c, Xiaowen Yu a, Bo Liu d, Fusheng Pan a, b 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 Department of Mechanical Engineering Technology, Purdue University, West Lafayette, IN 47906, USA d Chongqing Chang-an Automobile Co., Ltd, Chongqing 400023, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2017 Received in revised form 30 May 2017 Accepted 24 June 2017 Available online 27 June 2017

The strong asymmetric c-axis//transverse direction (TD) preferred orientation of the Mg-3Li-3Al-1Zn alloy sheet could induce different deformation mechanisms when the sheet was loaded along different directions. Consequently the related poor planar isotropy in mechanical properties may further deteriorate the sheet formability. More unfortunately, this type of texture component would give rise to poor corrosion resistance owing to a large amount of f1010g/f1120g crystallographic planes with high surface energy were exposing to the sheet surface. Appropriate pre-deformation was carried out on the sheet to tailor the initial asymmetric distribution of basal poles to be symmetric. The results indicated that the introduced symmetric weak basal texture significantly improved the planar isotropy of the sheet as well as enhanced its comprehensive mechanical performance. The absolute strength of the modified sample was enhanced and the Erichsen value increased up to ~80% compared with the as-extruded sheet. Besides, the immersion tests and electrochemical polarization tests suggested that the re-orientation of the prismatic planes through pre-deformation could effectively enhance the corrosion resistance of the sheet simultaneously. The evolution of the microstructure, texture, mechanical properties and corrosion resistance during this study were investigated in detail. © 2017 Elsevier B.V. All rights reserved.

Keywords: Texture symmetry Twin Sheet formability Corrosion resistance Magnesium alloy sheet

1. Introduction Magnesium alloys, owing to a number of advantages they offer, have received much attention in the automotive and electronics industries [1e3]. In recent decades, magnesium alloys have been studied intensely from a variety of perspectives. However, there are still two main problems restricting their further applications. One is the poor formability at room temperature, which is mainly due to the insufficient operable slip systems [4,5]. Especially for the wrought magnesium alloys, the existing strong texture induced during primary processing can further deteriorate the comprehensive mechanical properties and devalue the materials [6e8]. The other is its unsatisfied corrosion resistance. The high corrosion susceptibility of magnesium alloy, even in natural environment, provides a strong barrier on their commercial use [9e11]. * Corresponding author. Sha Zheng Street 174#, Sha Ping Ba District, Chongqing, China. E-mail address: [email protected] (B. Jiang). http://dx.doi.org/10.1016/j.jallcom.2017.06.269 0925-8388/© 2017 Elsevier B.V. All rights reserved.

Massive studies have been made to solve these above two issues [11e14]. Nevertheless, few achievements have been reported based on the enhancement of both the mechanical properties and corrosion resistance concurrently. Currently, microstructural modification by the equal channel angular extrusion (ECAE) seems to open up the possibility [15e17]. The microstructural refinement of magnesium alloy during ECAE can lead to efficient improvement in mechanical performance due to a more evenly deformation behavior [18]. Besides, the acquired ultra-fine grain size and the break-up of second phase particles with more uniform distribution achieved through ECAE were examined lead to increase in corrosion resistance [16,19]. However, ECAE process was limited in industry due to unmanageable and costly, as well as not suitable to fabricate wide Mg alloy sheets. In addition to the microstructure, the crystallographic orientation, which is usually regarded as an important factor to control the mechanical performances, can remarkably influence the corrosion, dissolution and oxidation responses of Mg alloys as well [20e23]. The relationship between crystallographic orientation and corrosion response is believed to

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relate to the binding energy of the surface atoms of Mg alloy. For the magnesium alloy sheets, the surface energy of (0002), f1010g and f1120g are quite different owing to varying degrees of atom closearrangement, further give rise to different corrosion rates at different crystalline planes [20,24]. Thus, it is wondered an appropriate texture optimization could be another possible way to optimize the mechanical properties and the corrosion resistance of Mg alloys simultaneously. In last several years, the investigation about the role of Lithium (Li) element in Mg alloys' performances is always a hot topic [25e27]. Li element can be considered as a special texture modifier for Mg, which can alter the texture feature markedly owing to its effective modification on the c/a ratio [28]. In our team, Li et al. found that with 3 wt.% Li addition in Mg-3Al-1Zn (AZ31) alloy, the extruded texture was transformed from typical basal texture into a prismatic texture and the texture intensity decreased ~50% compared with the Li-free AZ31 alloy [27]. The texture weakening is beneficial to improve the uniaxial mechanical performances, while the multiaxial sheet formability such as the stamping formability is still limited and not improved that much, which is believed mainly due to the asymmetric distribution of the basal poles caused by the asymmetric texture [29]. Last but not the least, it should be noted that the poor corrosion resistance of the Mg-Li alloy is another difficult issue. Since the Li element is more active than Mg element, thus Li addition can further deteriorate the corrosion resistance of the Mg alloy rapidly [30,31]. What makes it even worse is that the f1010g and f1120g prismatic planes with high surface energy were nearly parallel to the sheet surface, which would further conduce to deteriorate the corrosion resistance of the extruded sheet. Since such a special asymmetric c-axis//TD texture rarely appeared in the conventional AZ series alloys, and there hardly any effective process has been reported to deal with it related unsatisfied mechanical properties and corrosion resistance. This study proposes the “texture symmetry” concept to try to optimize the mechanical properties and the corrosion resistance. We provide a simple but effective method to improve both these two performances though special grain re-orientation by employing the prestraining process. The introduction of symmetric weak basal texture is in favor of improving both of the ductility, strength and sheet formability. Besides, the re-orientation of the f1010g/f1120g crystalline planes significantly enhanced the corrosion resistance of the Mg-3Li-3Al-1Zn alloy sheet. 2. Experimental The as-extruded Mg-3Li-3Al-1Zn (Mg-2.93 wt.% Li-2.61 wt.% Al0.78 wt.% Zn-0.32 wt.% Mn, denoted as LAZ331) alloy sheet with 120 mm in width along the transverse direction (TD), 2000 mm length along extrusion direction (ED) and 1.8 mm in thickness, was cut into 120 mm  50 mm (TDED) rectangular specimens for prestraining process. Uniaxial pre-stretching process was carried out on a CMT6305-300 KN electronic universal testing machine along the length of the rectangular specimens. The specimens were then pre-stretched by 5% along TD. The principle of making samples please refer to [29]. Subsequently, some of the pre-strained samples were annealed at 180  C for 5 h to remove the dislocations while retain the deformation texture, the others were annealed at 300  C for 1 h to obtain a fully recrystallized microstructure. For the sake of brevity, the former can be defined as PRS (pre-stretched followed by stress relieving annealing) sample and the latter as PA (prestretched followed by recrystallization annealing) sample. Dog bone tensile samples 10 mm in gage length, 3 mm in gage width were machined from the as-extruded LAZ331 sheet, the PRS specimen and the PA specimen along three different directions tilting of

0 , 45 and 90 to ED to evaluate the mechanical properties. The speed of the tensile test was set as 1.5 mm min1. In tensile test, three samples were tested for each condition and the average value was then obtained. Meanwhile, square samples with 50 mm  50 mm were machined from the various specimens for Erichsen tests. The Erichsen tests were carried out using a hemispherical punch with a diameter of 20 mm to examine the sheet formability of the various samples at room temperature. The X-ray phase and macro texture analysis was carried out by Rigaku D/Max 2500. The micro grain orientation of the various samples was revealed by EBSD analysis using a HKL Channel 5 system (Oxford system equipped in a FEI Nova 400 FEG-SEM). Square samples with dimensions of 15 mm  10 mm (length  width) were cut from the as-extruded sheet, PRS specimen and PA specimen to examine the corrosion response by employing a hydrogen evolution method in 3.5 wt% NaCl solution at 25  C. The square specimens were cold-mounted in an araldite disc with one side of the specimen surface exposed. The exposed surface area for the following corrosion testing was set as 1.5 cm2 (15 mm  10 mm). Prior to corrosion tests, the exposed surface of the various samples was polished gingerly on 400e2000 sandpapers to reduce the effect of surface roughness on corrosion response. Corrosion rate was estimated by evaluating the hydrogen evolution. Furthermore, Corrosion products were then removed using a cleaning solution containing 200 g l1 CrO3, 10 g l1 AgNO3 and 20 g l1 Ba(NO3)2 at room temperature, and the surface morphologies with corrosion products removed were then characterised by scanning electron microscopy (SEM, TESCAN VEGA) to study evolution of the corrosion attack. Electrochemical measurements were performed on a Parstat 2273 electrochemical workstation. Potentiodynamic polarisation curves were recorded at a scan rate of 1 mV s1 after 10 min of stabilization at open-circuit potential (OCP). Corrosion current density (Icorr) and corrosion potential (Ecorr) were estimated by Tafel fitting. During the corrosion tests and the electrochemical measurements, at least three replicates were performed for each test to ensure reproducibility of the results. 3. Results 3.1. Microstructure and texture analysis of the as-extruded LAZ331 sheet Fig. 1 shows the microstructure and grain orientation distribution of the as-extruded LAZ331 sheet. The average grain size is measured to be ~35.0 mm. It is worth noting that the 3 wt.% Liadded AZ31 sheet exhibits an unconventional non-basal texture with their basal poles deflecting to TD completely. Such a texture feature is rarely reported in the AZ series alloys while it is universal in the wrought titanium alloys [32]. Furthermore, in addition to the asymmetric distribution of basal poles, there also exists a near < 1010 > //ND preferred orientation of prismatic planes, according to the f1010g pole figure. Thus, the extruded LAZ331 sheet shows an extremely asymmetric texture distribution. 3.2. EBSD results of the as-extruded sheet, PRS sample and PA sample The EBSD orientation maps of the three samples are shown in Fig. 2. Unlike the traditional extruded AZ31 alloy sheet (grains mainly in red color, represents the basal planes nearly parallel to extrusion direction), the grains in the as-extruded LAZ331 sheet are mainly in blue or near blue color, which indicates that the c-axis of the majority grains are parallel to TD and the basal planes are perpendicular to the extrusion direction, as shown in Fig. 2 (a). This

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Fig. 1. Microstructure and grain orientation distribution obtained from ED-TD plane of the as-extruded LAZ331 sheet.

EBSD result is consistent with the macro-texture measurement. After pre-stretching along TD by 5%, however, the grains in blue color almost disappear, which are replaced by many grains in red

color and a large number of twins, as illustrated in Fig. 2 (b). The type of the twins introduced during pre-straining are exclusively the f1012g extension twins, for which are confirmed of satisfying

Fig. 2. EBSD results of the three samples with different conditions: (a) as-extruded sheet; (b) pre-strained sample; (c) PA sample.

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the characteristic misorientation of 86.3 < 1120 > relationship. The number fraction of the twin boundaries is ~0.336. After stress relieving annealing at 180  C for 5 h, the twin structure and grain orientation distribution of the pre-stretched sample are fully retained for which only the dislocation density can be remitted in such an annealing process, thus the EBSD map of the PRS sample is omitted. When the pre-strained sample is annealed at 300  C for 1 h, a homogenous recrystallized microstructure is acquired (see in Fig. 2 (c)). The average grain size of the PA sample is measured to be ~17.2 mm, which is smaller than the average grain size of the asextruded sheet. 3.3. Mechanical property evolution of the various samples Fig. 3 shows the true stress-true strain curves of the as-extruded sheet, the PRS sample and the PA sample. It can be seen the asextruded LAZ331 sheet shows a strong planar anisotropy in mechanical properties, such as the yield strength and ductility. While this anisotropic degree in properties decreases in the PRS sample and subsequently almost disappear in the PA sample. Moreover, the tensile yield strength (YS), ultimate tensile strength (UTS), tensile elongation (EL), work hardening exponent (n-value), YS/UTS ratio and Lankford value (r-value) of the various samples are compared and listed in Table 1. It can be seen that there exists a huge variation in tensile YS (about 60 MPa) along three different directions of the as-extruded sheet. Besides, the El and the n-value of the EDstretched sample are much lower than that of the other directions, indicating poor deformation ability during tension along ED. This strong planar anisotropy in mechanical properties will extremely restrict the usability of the sheet. For the as-extruded

LAZ331 sheet, the low YS along TD can influence the overall loadbear ability of the material. As another disadvantage, the great difference in plastic deformation ability along different directions will also restrict the sheet's secondary forming ability, especially for the stamping formability. After pre-stretching 5% along TD followed by stress relieving annealing, however, the planar anisotropy decreases effectively. Compared with the as-extruded sheet, the YS of the ED, 45 and TD sample increase about 13 MPa, 52 MPa and 60 MPa, while the EL only sacrifice about 1.7%, 2.9% and 3.5% respectively, and meanwhile the EL remain an acceptable level. In addition, the increase of the average UTS of the three directions exceeds ~30 MPa, suggesting an effective strengthening of mechanical performance by means of the PRS process. Furthermore, the planar anisotropy further reduces after the pre-strained sample experienced recrystallization annealing. The maximum deviation in YS reduces to less than 10 MPa, and the ductility also become similar along three directions. Furthermore, the n-value of the ED sample increases effectively, and n-values at different directions tend to be uniform. These results reveal that the plastic deformation ability of the sheet is enhanced and the mechanical properties become more symmetric along different directions. Erichsen tests are employed to further examine the formability of the various samples, the measured Erichsen values (IE) are shown in Fig. 4. During Erichsen tests, at least five samples were tested in each condition and the most three balanced values were selected to weigh the formability. It can be seen that the asextruded LAZ331 sheet shows a moderate average IE of 3.2 mm. It is interesting that the macroscopical cracks formed in the asextruded LAZ331 sheet during Erichsen test are almost all perpendicular to ED (not showed here). For the PRS sample, the IE

Fig. 3. True stress-true strain tensile curves obtained from ED, 45 and TD of the various samples: (a) as-extruded sheet; (b) PRS sample; (c) PA sample.

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Table 1 Mechanical property evolution of the as-extruded sheet, the PRS sample and the PA sample evaluated at three different directions. sample

Angle

YS (MPa)

UTS (MPa)

EL (%)

n-value

YS/UTS

r-value

As-extruded

ED 45 TD ED 45 TD ED 45 TD

168.6 123.5 109.4 180.0 175.6 169.6 151.0 148.3 141.1

293.6 291.7 305.4 329.1 332.1 327.9 299.6 301.0 304.1

16.9 24.8 21.1 15.2 21.9 17.6 22.7 24.0 22.1

0.22 0.42 0.54 0.23 0.30 0.39 0.36 0.35 0.38

0.59 0.42 0.36 0.56 0.53 0.52 0.50 0.49 0.47

2.82 1.38 0.36 1.78 1.48 1.54 1.26 1.06 1.15

PRS sample

PA sample

has a small increment of ~20%. While for the PA sample, the average IE reaches to 5.8 mm, which increases about 80% compared with the as-extruded sheet. This result reveals PA process can effectively enhance the formability of the LAZ331 sheet at room temperature. 3.4. The corrosion resistance of the various samples Hydrogen evolution of the as-extruded LAZ331 alloy sheet, the PRS sample and the PA sample as a function of immersion time in 3.5 wt.% NaCl solution are plotted in Fig. 5 (a). The total hydrogen volumes at a given time obtained from the three samples always decrease in the order: as-extruded sample >PRS sample >PA sample. This means the PA sample which possesses of symmetric weak basal texture exhibits the best corrosion resistance, while the as-extruded LAZ331 sheet with basal poles bias to TD shows the worst corrosion resistance. Based on the above measured data of emitted hydrogen gas, the corrosion rate r of the various samples is calculated by using the following formula: r ¼ (VtnVtn-1)/{(TnTn1)*S}, where Vtn represents the total volume of emitted hydrogen gas during the 0-Tn time and S represents the acreage of the exposed surface in the solution. Thus, the corrosion rate r can represent an instantaneous rate during the corrosion process within a specified time period in per unit area. The corrosion rates of the three samples are all unstable during the earlier test and gradually tend to relative stable. It can be seen the as-extruded sample always shows the highest corrosion rate, while the corrosion response is effectively restricted in case of the PA sample. The measured stable corrosion rates by means of linear fitting (as can be seen in Fig. 5 (b)) decrease in the order: as-extruded sample >PRS sample >PA sample with a ratio around 2.6:2.0:1.0.

Immersion test is an intuitive method to evaluate the corrosion resistance of the materials. Fig. 6 shows the weight loss measurement of the three samples after immersion in 3.5% NaCl solution for 7 days. The weight loss experienced by the samples during constant immersion is proportional to the corrosion resistance. In this study, the weight loss for the as-extruded sample, PRS sample and PA sample are measured to be 73.4 ± 10.0 mg, 58.4 ± 7.8 mg and 25.5 ± 6.4 mg, respectively. The results indicate that PA sample shows the lowest weight loss and its value is only about one-third of the weight loss for the as-extruded sample, suggesting a significant enhancement of corrosion resistance after PA process. Fig. 7 presents the potentiodynamic polarization curves of the three samples immersion in 3.5% NaCl solution at 25  C. It can be seen that the corrosion potential (Ecorr) of the as-extruded sample, 5% PRS sample and 5% PA sample are 1.55 V, 1.51 V and 1.46 V, respectively. Through Tafel fitting, corrosion current density (Icorr) for the as-extruded sample, PRS sample and the PA sample are calculated as 126.1 mA/cm2, 96.6 mA/cm2 and 32.0 mA/cm2, respectively. The comparisons of the Ecorr and Icorr of the three samples with different condition indicate that PA sample and PRS sample have much better corrosion resistance than that of the as-extruded sheet. The electrochemical polarization results show good consistency with the hydrogen evolution and the weight loss measurement results. In order to further inspect the corrosion responses, the various samples were immersed in the 3.5 wt% NaCl solution for 7 days. The SEM micrographs of the surface morphology with removed corrosion products are exhibited in Fig. 8. It is obvious that there exist significant different degrees of corrosion cases among the three samples. The pit morphology as observed on the surface of the various samples clearly indicates that higher corrosion attack with larger and deeper pits in as-extruded sample as compared to PA sample showing shallower pits, as can be seen in the macro images in Fig. 8. Especially for the PA sample, there still remains some original matrix (denoted by red arrows) after 7 days immersion test. Besides, the PRS sample shows a scaly morphology and the degree of surface damage is between the as-extruded sample and the PA sample. This result is also in agreement with the hydrogen evolution experiment and the weight loss measurement. The corrosion resistance of the PRS sample and the PA sample are found to be much better as compared to the initial as-extruded LAZ331 sheet.

4. Discussion 4.1. Texture evolution and the enhanced comprehensive mechanical performances

Fig. 4. Sheet formability of the various samples during Erichsen tests.

For the as-extruded LAZ331 sheet, the strong planar anisotropy of mechanical properties is mainly caused by the extremely asymmetric texture. As shown by the grain orientation model in

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Fig. 5. Hydrogen evolution (a) and corrosion rate curves (b) of the as-extruded sample, the PRS sample and the PA sample.

Fig. 9 (a), the typical basal texture obtained in AZ series alloy sheets mainly shows a relatively symmetric distribution of basal planes in the ED-TD plane. This type of texture component usually gives rise to a spot of differences in Schmid factors (SF) for a given operable slip/twinning system when the sheet is loaded along different directions. Generally speaking, basal texture would only lead to small or moderate planar anisotropy in the mechanical properties although the strong basal texture shows adverse impact on the comprehensive mechanical performances. Whereas for the LAZ331 sheet, the strong planar anisotropy of mechanical performance caused by the asymmetric distribution of basal planes will extremely restrict its commercial application although its uniaxial mechanical performances are more excellent than the AZ31 sheet, such as the satisfactory ductility, high n-value and YS/UTS ratio along 45 and TD. According to the macro-texture analysis in Fig. 1, the c-axis of the majority grains is partial to the TD completely, as illustrated in Fig. 9 (b). This strong asymmetric distribution of grain orientations around sheet normal will lead to different deformation mechanisms when the sheet is subjecting loads along different directions. When the sheet is stretched along ED, the basal slip can't be activated due to a very small SF (close to zero), only the prismatic slip system with high critical shear stress (CRSS) can coordinate the deformation. Besides, the nearly < 1010 > //ND preferred orientation of a-axis would further decrease the SF of the prismatic

slip system, leading to a high tensile YS and low ductility, as can be seen in Table 1. When the sheet is stretched along TD, however, the tensile load is parallel to the c-axis and the f1012g extension twin mechanism now obtain a large SF. With low CRSS, the f1012g extension twin can be activated easily, leading to a low tensile YS and high ductility along TD. In general, the strong planar anisotropy of mechanical performances is mainly attributed to the asymmetric deformation mechanisms induced by the strong asymmetric texture distribution. After pre-stretched 5% along TD, numerous f1012g extension twins and many grains in red color are introduced, as illustrated in Fig. 2 (b). According to the IPF scale, the grains in red color should represents the basal textured grains with basal planes parallel to the sheet surface. It can be confirmed that the grains in red color are also the f1012g twin products, because the SF for the basal slip during tension along TD is too small (according to the macrotexture analysis of the as-extruded sheet). It should be noted that there is a preferred orientation of prismatic planes titled about 10 away from the ND (see in Fig. 10), thus the plane I is more likely to act as the potential twinning plane because a larger driving force would effect on this twinning plane during in-plane tension along TD. The preferentially introduced f1012g twins with basal textured feature would grow up easily and subsequently merged each other, resulting in introducing basal textured grains. This phenomenon

Fig. 6. Weight loss of the samples after immersion in 3.5% NaCl solution for 7 days.

Fig. 7. Potentiodynamic polarization curves of the three samples immersion in 3.5% NaCl solution at 25  C.

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Fig. 8. SEM micrographs of surface morphology of the various samples after immersed in 3.5 wt.% NaCl for 7 days with corrosion products removed: (a) as-extruded sample; (b) PRS sample; (c) PA sample.

Fig. 9. Schematics of the grain orientation distributions for the samples: (a) typical wrought AZ series sheet; (b) the as-extruded LAZ331 sheet.

can be explained by the twin dynamic recrystallization (TDRX) [33,34]. Soon afterwards, the adjacent potential twinning plane II is activated and the c-axis of the introduced twins are mainly titled away from ND to ED in a certain angle. Previous studies reported that the misorientation angle between the twin variants introduced by the neighbouring twinning planes was about 60 [35]. The angle between peak A and peak B in the Fig. 10 is exactly near 60 , further indicating the introduction of abundant twin variants during the pre-straining process. As a result, the initial grains with soft orientation for extension twinning are consumed largely, the f1012g twin mechanism is no longer able to be activated easily when the sample is further stretched along TD, thus the tensile YS of the PRS sample along TD increases about 60 MPa (from 109.4 MPa to 169.6 MPa) and the ductility decreases to some extent due to alteration on deformation mechanisms. Besides, the

prismatic slip may still dominate the tensile deformation along ED owing to a very small SF for basal slip, and the YS increases slightly due to twin-induce strengthen effect. Finally, the overall strength of the material enhances substantially and the planar anisotropy of the sheet reduces effectively. Thus, pre-straining along appropriate direction followed by stress reliving annealing is suggested as an effective method to enhance the strength of magnesium alloy sheet which possesses of such an asymmetric texture component. When the pre-strained sample experienced the recrystallization annealing, the previous introduced multiple-peak texture components could provide various types of orientation gradients along different directions, and the related diverse driving force would provide random growth directions for the recrystallized new grains during annealing [29]. As expected, a symmetric weak basal texture with intensity of only 7.9 is acquire, as indicated in Fig. 10. This

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Fig. 10. Texture evolution of the samples during PRS and PA processes.

effective texture modification can significantly minimize the planar anisotropy of mechanical properties by balancing the SF of slip/ twinning systems when loading along different directions, as well as can contribute to enhance the comprehensive mechanical performance of the LAZ331 sheet. Furthermore, the average grain size of the PA sample decreases moderately compared with the extruded one (from 35.0 mm to 17.2 mm), which is mainly attributed to the introduction of numerous twin boundaries during predeformation process. The extension twin boundaries, especially the twin intersections, are easily act as the nucleation sites during annealing, further give rise to an effective promotion on nucleation and conduce to refining the microstructure [36]. In general, the mechanical properties of the LAZ331 sheet are improved markedly owing to the effective texture modification and microstructural refinement. In this study, the stamping formability of the various samples is also examined. Previous studies suggested that the stamping formability of the magnesium alloy sheet was sensitive to the texture symmetry and the strain coordination [37,38]. As reported by chino et al., the generation of the cracks of Mg-Zn-Y alloy sheets was always perpendicular to the strong basal textured orientation during punch test [37]. This phenomenon indicated that the direction with weaker deformation ability would be preferentially deformed to failure during multiaxial deformation process, which may subsequently encumber the overall punch performance of the sheet as well as lead to a waste of better deformation ability along other directions. Thus, in addition to the texture weakening, the texture symmetry is confirmed to be another important factor to influence the sheet formability of the Mg alloys. For the LAZ331 sheet in this study, the extremely asymmetric texture would result in strong planar anisotropy in yield strength and ductility. The observed macro cracks of the as-extruded LAZ331 sheet formed during punch tests are almost all perpendicular to the ED. It is owing to the fact that the stretching deformation ability along ED is strongly limited by the initial hard orientations for both basal slip and twinning. This weakness will significantly reduce the formability of the LAZ331 sheet. As a result, the initial LAZ331 sheet shows a moderate IE of only 3.2 mm. Compared with the as-

extruded sample, the IE of the PRS sample increases slightly by ~20%, and the IE of the PA sample with an excellent value of 5.8 mm increases nearly 80%. The improvement of IE is mainly attributed to the reduction of planar anisotropy in mechanical properties as well as the texture weakening. Moreover, in addition to the measured YS, EL and n-value parameters, the strain hardening behaviors are equally important to influence the formability of the magnesium alloy sheets. The work hardening curves of the various samples, which can reflect the actual deformation behavior and deformation ability during deformation, are exhibited in Fig. 11. It can be seen the as-extruded sample shows strong anisotropy in work hardening behaviors at different directions, which is mainly attributed to different initial texture on slip and twinning along different directions during plastic deformation [39]. This huge anisotropy in work hardening behaviors, of cause, would result in poor harmony on multiaxial planar deformation during punch process. As for the PA sample, the strong planar anisotropy of strain hardening behaviors is minimized effectively due to the introduction of the symmetric texture. This texture symmetry effect along with the grain refinement would play positive effect on the secondary forming process. Furthermore, in addition to the planar strain compatibility, the coordination between the planar strain and the thickness strain is equally important to influence the sheet formability of the magnesium alloys during Erichsen test. This type of strain compatibility can be weighed by the Lankford value (r-value) listed in Fig. 12. For the ED-stretched sample of the as-extruded LAZ331 sheet, the prismatic slip dominated the deformation due to initial hard orientation for both basal slip and extension twinning. Under this mechanical model, the width strain could be generated by the prismatic slip, while the thickness strain was hard to be coordinated because the dislocation with Burgers vector along < 1120 > was barely to accommodate the thickness strain along the normal direction [40]. Thus, a high r-value is acquired when the sheet is deformed along ED, which would lead to a poor coordination during in-plane deformation. However, when the sheet is stretched along TD, the extension twins dominated the deformation. The effective re-orientation of the c-axis by almost 90 owing to the abundant twins (especially the twins in red color)

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Fig. 11. Strain hardening curves obtained from ED, 45 and TD of the three samples: (a) as-extruded sample; (b) PRS sample; (c) PA sample.

can give a sufficient strain release along the thickness direction, while the twins could provide smaller contribution on the in-plane deformation. Thus, a low r-value far less than 1 was obtained during tension along TD. This poor coordination during deformation along ED or TD would restrict the multiaxial deformation ability of the sheet. Furthermore, the strong anisotropy in r-values at different directions would further reduce the sheet formability in punch

process owing to poor multiaxial stain compatibility. As for the PRS sample, the IE increases mildly although the ductility decreases to some extent, which is mainly attributed to the acquisition of more uniform mechanical properties (such as YS, n-value) as well as the minimized anisotropy of plastic deformation behaviors at different directions. 4.2. The enhanced corrosion resistance

Fig. 12. The r-values of the various samples during tension along different directions.

The hydrogen evolution results in Fig. 5 show that the asextruded sample, PRS sample and the PA sample present distinctively different corrosion rates during immersing in 3.5 wt.% NaCl solution. The volume of emitted hydrogen gas for the as-extruded sample is much higher than that of the others. The calculated stable corrosion rate of the as-extruded sample is ~2.6 times than the corrosion rate of the PA sample. Besides, the weight loss measurements in Fig. 6 along with the electrochemical polarization tests in Fig. 7 also indicate that the corrosion resistance of the LAZ331 sheet is significantly enhanced after PRS process and PA process. In order to clarify the essence for the variation in corrosion resistance, the influence of microstructure and the grain orientation before and after the PRS and PA process are examined. Microstructure alteration could be a factor to influence the corrosion responses among the three samples. Literatures about the effect of microstructural refinement on the corrosion evolution can be found elsewhere and microstructural refinement possibly increases the corrosion resistance of Mg alloy can be based on the acquisition of ultra-fine grains or/and the break-up of second phase particles with more uniform distribution achieved through SPD

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Fig. 13. XRD patterns of the As-extruded sample, the PRS sample and PA sample.

process [19,41]. Fig. 13 shows the XRD phase analysis of the asextruded sample, PRS sample and PA sample. It can be seen that only small amount of Al8Mn5 phases are detected in the three samples in addition to the a-Mg matrix. Meanwhile, backscatter

electron (BSE) images of the three samples showing the particle distribution are exhibited in Fig. 14. The sparely distributed second phase particles are checked as Al8Mn5 phases by energy dispersive analysis (EDS), and no other particles are clearly found in the three samples. The XRD phase analysis in addition to the SEM images indicate that the existence state (including the type, size, distribution) of the second phase particles is barely changed during the pre-straining and annealing processes. Hence, the influence of the second phase particles on the corrosion evolution for the asextruded sample, PRS sample and PA sample should be equivalent. In addition to the second phase particles, the grain size of the PA sample decreases moderately as compared to the as-extruded sheet. However, it has been demonstrated that grain boundaries in AZ31 alloy cannot effectively retard the corrosion development due to the fact that the barrier effect of the second phase and boundary zone cannot applied to the grain boundaries of the low Al containing AZ31 alloy sheet [42]. This mechanism is also applying to the LAZ331 alloy because of the similar alloying condition of the AZ31 alloy and LAZ331 alloy. Furthermore, the grain size variation between the three samples is not that obvious and its influence on the corrosion resistance of PA sample is considered less significant. Therefore, it can be considered that microstructural alteration among the samples on the corrosion variation in this study was not the main factor. In the present study, the effectively enhanced corrosion resistance of the PRS sample and PA sample are believed mainly

Fig. 14. Backscatter electron images of the three samples with different conditions: (a) As-extruded sample; (b) 5% PRS sample; (c) 5% PA sample.

J. He et al. / Journal of Alloys and Compounds 723 (2017) 213e224

Fig. 15. Area fraction of the (0002), f1010g and f1120g plane normal inclined within 15 to the exposed surface normal, which represents the area fraction of the crystallographic planes exposed to the sheet surface. The statistics were obtained from the EBSD results of the as-extruded sheet, PRS sample and the PA sample, respectively.

attributed to the re-orientation of the exposed crystallographic planes. For the as-extruded LAZ331 sheet, the c-axis//TD texture designates the f1010g/f1120g planes exposing to the sheet surface, while the PA sample with a symmetric basal texture would make majority of basal planes exposing to the sheet surface. The corrosion rate of Mg alloys relating to crystallographic orientation can be explained by the alteration of surface energy [43,44]. Previous studies have indicated that the activation energy for dissolution of a closely packed surface was higher than a loosely packed one [20]. Due to the fact that the surface energy was inversely with atomic packing density in the surface planes, thus the atoms in a lower surface energy would be dissolved slower. Song et al. suggested that the surface energy for (0001), f1010g and f1120g of Mg was 1.54  104 j/mol, 3.04  104 j/mol and 2.99  104 j/mol, respectively, indicating that the (0001) surface in Mg was possessed of lowest surface energy and the (0001) plane should be dissolved more slowly [44]. In order to more intuitive, the area fraction of the (0002), f1010g and f1120g plane normal within 15 inclined to the exposed surface normal was measured from the above EBSD data and shown in Fig. 15. It can be seen that the percentage of f1010g and f1120g for the as-extruded sample exposed to the solution exceeds 90% owing to the strong c-axis//TD texture. However, this value significantly decreases to only 6% for the PA sample. Besides, the percentage of exposed (0002) planes increases from 2% to 48% (68% for the (0002) plane normal within 30 inclined to the exposed surface), indicating that the majority of the exposed crystallographic surfaces have become the basal planes. Song et al. also quantificationally measured and compared the corrosion rate of the various crystallographic planes of Mg and suggested that the corrosion rate for the surface which mainly composed of f1010g and f1120g prismatic planes was about 8.4 times than that of surface mainly consisted of (0002) basal planes [44]. Furthermore, it has been reported that the film formed on the basal planes is much more protective than the non-basal planes [45]. Thus, the texture modification by re-orientating the prismatic planes should be an efficient way to improve the corrosion resistance of LAZ331 sheet. It should be noted that the PRS sample also shows better corrosion resistance than the initial LAZ331 sheet, although many extension twins are introduced during the pre-straining process.

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Previous study indicated that with increasing the number of the activated atoms in the twinning planes, the dissolution tendency generally increases [42]. Meanwhile, deformation twinning is generally coupled with stress concentration in grain and twin boundaries, thus the twinned areas may preferentially to be eroded [46]. The results reveal an improvement of corrosion resistance of PRS sample in this study seems conflictive to the previous reports. Nevertheless, it should be mentioned that the generation of the f1012g extension twins here are all based on the initial c-axis//TD grains. The twin I induced new matrix with basal texture would have much better corrosion resistance than the initial c-axis//TD parent grains. Besides, because of a close misorientation angle (only about 7 ) of the twin variants twinned by the para-position, the twin I are easily merged each other and introduced many basal textured grains. This effective re-orientation of the c-axis changed the exposed planes from f1010g/f1120g prismatic planes into (0002) basal planes (with increased (0002) planes and decreased f1010g/f1120g prismatic planes exposed to the sheet surface), which would significantly retard the corrosion speed of the sheet. As a result, the percentage of the exposed f1010g/f1120g prismatic planes decreases to only 20%, and the corrosion resistance of the PRS sample is enhanced to a certain degree. Thus, the effect of the f1012g extension twins on the corrosion resistance of the PRS sample can be regarded as the combined results by considering the above two aspects. It is also worth mentioning that the “texture symmetry” is not only applying to improve the mechanical property and corrosion resistance of the LAZ331 sheet, but is also suitable for optimizing the comprehensive performances of any other Mg alloy sheets which accompany with similar asymmetric texture distribution. For example in recent years, the addition of many alloying elements, such as RE, can usually cause a certain number of titled texture components in addition to aiming at randomizing the basal texture. However, these accessary asymmetric texture components may deteriorate the planar isotropy and the corrosion resistance of the alloys. In this case, control the texture symmetry such as by employing the present simple but useful “pre-strainingþannealing” process can effectively solve these issues simultaneously.

5. Conclusions The mechanical properties and corrosion resistance of the Mg3Li-3Al-1Zn (LAZ331) alloy sheet possessed of strong asymmetric c-axis//TD texture were examined. Such an asymmetric texture would lead to strong planar anisotropy in mechanical properties and thus extremely restrict the sheet formability at room temperature. Moreover, the asymmetric c-axis//TD texture lied f1010g/f1120g prismatic planes parallel to the sheet surface completely, which further deteriorated the corrosion resistance of the Mg alloy sheet. Based on texture modification, a symmetric weak basal texture was introduced by pre-straining to solve the above two issues. The related conclusions are drawn as follows: 1. Due to texture symmetry and texture weakening modification, the tensile yield strength of the sheet along ED, 45 and TD were optimized from 168.6 MPa, 123.5 MPa and 109.4 MPa to 151.0 MPa, 148.3 MPa and 141.1 MPa respectively. Besides, the Erichsen value at room temperature was increased up to 80%. 2. Pre-twinning and annealing process significantly decreased the number fraction of high-surface energy prismatic planes exposing to the sheet surface, the modified samples possessed of symmetric weak basal texture showed much stronger corrosion resistance as compared to the as-extruded LAZ331 sheet with strong c-axis//TD asymmetric texture.

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