Journal of Materials Science & Technology  2020 , 42 (0): 175-189 https://doi.org/10.1016/j.jmst.2019.10.010

Orginal Article

The texture and its optimization in magnesium alloy

Jialin Wu*, Li Jin, Jie Dong, Fenghua Wang, Shuai Dong

National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China

Corresponding authors:   *Corresponding author. E-mail address: jjinli@sjtu.edu.cn (L. Jin).

Received: 2019-09-1

Revised:  2019-10-22

Accepted:  2019-10-28

Online:  2020-04-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

he crystallographic orientations are generally non-random in wrought Mg alloy, which will lead to their macroscopic physical properties to be anisotropic. Understanding the texture evolution in processing of Mg alloy billets and its effect on mechanical properties is therefore an important project for all scientists and engineers in material area. This paper is concerned with the description of texture, with the mechanisms of texture evolution and with the interrelationships between texture and mechanical properties in Mg alloy. It is a full review of understanding of the basic mechanism on texture evolution, of texture altering by alloying or processing, and of the mechanism of texture weakening. Moreover, it provides theories necessary and available techniques to develop high-performance Mg wrought with optimized texture in the field.

Keywords: agnesium alloy ; Texture ; Rare earth (RE) ; Anisotropic ; Mechanical property

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Jialin Wu, Li Jin, Jie Dong, Fenghua Wang, Shuai Dong. The texture and its optimization in magnesium alloy[J]. Journal of Materials Science & Technology, 2020, 42(0): 175-189 https://doi.org/10.1016/j.jmst.2019.10.010

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

2. Typical texture and its effect on mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . 176

2.1. Typical texture in magnesium alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

2.1.1. Fiber texture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

2.1.2. Rolling texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

2.2. Effect of texture on mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

3. Texture optimization of magnesium alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

3.1. Alloying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

3.2. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

4. Mechanism of texture weakening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

4.1. Modification of texture by addition of RE elements . . . . . . . . . . . . . . . . . . . . . . . . . . 185

4.2. Modification of texture by changing deformation path .. . . . . . . . . . . . . . . . . . . . . . . 188

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

Acknowledgements . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 188

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 188

1. Introduction

Magnesium and its alloys have attracted much attention due to their low density, high specific strength and good castability. However, poor ductility of traditional wrought Mg alloys at room temperatures has restricted their possible application. The lower formability of wrought Mg alloys at room temperature originates from their hexagonal crystal (HCP) structures that provide limited slip systems [1]. In addition, a strong texture always exists in the wrought Mg alloy after plastic deformation, which will result in the anisotropic mechanical properties. Therefore, understanding the texture evolution in processing of Mg alloy and its effect on mechanical properties is very important for all researchers in material area. Appreciable work has been done related to the texture of Mg alloy in the past few decades. Research results on texture in Mg alloy have been published in a large number of reputed journals in the past 20 years, especially concerned with the texture weakening by alloying and processing.

However, literature lack to provide a review dealing with the determination of texture and texture-induced behavior of Mg alloy. The motivation here is to provide a broad review on proposed topic to help other researchers to enter this field easily.

2. Typical texture and its effect on mechanical properties

2.1. Typical texture in magnesium alloys

It has long been understood that the strong texture of Mg alloy at room temperature is attributed to the limited deformation modes. In comparison to cubic structures HCP is less symmetric, leading to a range of different slip systems a) involving < a > dislocations on the basal and prism planes b) and < c+a > on the pyramidal planes [2]. However, the critical resolved shear stress (CRSS) required for various slip modes to be activated is significantly different. According to reported data, the CRSS for basal dislocations at room temperature is much lower than non-basal slip systems on the prismatic and pyramidal planes [3]. Apart from the slip mechanism, it is well known that twin modes also play an important role during the deformation of Mg alloys. Extension and contraction twins are {10-12} <-1011 > , and {10-11} <-1012> respectively are the predominantly observed modes in Mg, which can provide extension or contraction along the c-axis of sample [4]. Extension twins has the lowest CRSS among all twin modes. Therefore, plastic deformation in the form of both the extension twin and basal slip for Mg alloys appears simultaneously. The limited number of active deformation systems in wrought Mg alloys at room temperature results in the formation of a strong texture upon mechanical processing [3]. It is important to mention here the two representative textures of Mg alloy a) fiber texture b) rolling texture.

2.1.1. Fiber texture

Fig. 1 shows that wrought Mg alloys produce a strong fiber texture during hot extrusion, i.e. basal planes parallel to the extrusion direction (ED). Several types of fiber textures have been reported by the literature [[5], [6], [7], [8]]. Fig. 2 shows the inverse pole figures (IPF) in the extrusion direction of Mg alloys. Fig. 2a shows the texture of AZ31 extruded at 703 K, a most commonly observed <10-10 > fiber texture with the basal plane, and <10-10 > direction of the crystallites parallel to the ED [7]. Besides, <10-10 > -<11-20> and <11-20> fiber textures are also the most commonly observed in other Mg alloys samples (Fig. 2b and c) [6,7]. Furthermore, Fig. 3 shows the pole figures of extruded Mg alloys with different cross sections. The textures will be different when the cross section of the sample changes [5,9,10]. It is suggested, that the texture variation is closely related to the processing parameters, initial microstructure and composition [5].

Fig. 1.   Schematic diagram for orientation of grains after extrusion of Mg alloy [14].

Fig. 2.   Inverse pole figures of (a) AZ31B alloy extruded at 703 K; (b) AZ31B extruded at 673 K [7]; (c) As-extruded Mg-2.5Y [6].

Fig. 3.   Pole figures of extruded (a) AZ31 tube [9]; (b)AZ31 bar [5]; (c)AZ31 sheet [10].

2.1.2. Rolling texture

In case of magnesium with a c/a = 1.622, approximately equal to the ideal value of 1.633, tend to form [0001] texture with crystallographic c-axis parallel to the normal direction of the sheet during rolling [[11], [12], [13], [14], [15]], as shown in Fig. 4. And the rolling texture is related to the processing parameters such as rolling temperature [[16], [17], [18], [19]] and thermal treatment [12,13]. For (0002) pole figures in Fig. 5 the sheet rolled at high temperature exhibits a weaker basal texture intensity which depict that texture intensity depends on the rolling temperature [16]. Simultaneously annealing the sample AZ31B for 1 min and 7 days resulted in different XRD pole figures as shown in Fig. 6. It is interesting to mention that the rolling texture type tend to shift from {0001}<10-10 > to {0001}<11-20> [12,13]. In addition, other investigations showed that not only the processing parameters were important, but the choice of the initial microstructure such as sheet thickness has a significant role [[20], [21], [22]]. As indicated in Fig. 7, by the increasing sheet thickness, a significant reduction in intensity and a spread in the basal pole towards the transverse direction (TD) were observed [21].

Fig. 4.   (a) Schematic diagram showing orientation of grains after rolling Mg alloy [14];(b) Pole figures of as-rolled AZ31 sheet [13].

Fig. 5.   (0002) pole figures of the sheet rolled at temperatures of (a) 498 K, (b) 823 K [16].

Fig. 6.   XRD pole figures of a rolled AZ31B sample after annealing at 400℃ for 1 min and 7 days [12].

Fig. 7.   (0002) pole figures of AZ61 after different rolling: (a) 10 mm thick; (b) 4 mm thick; (c) 1 mm thick [21].

2.2. Effect of texture on mechanical properties

Wrought Mg alloys with strong texture always have shown obvious anisotropy at room temperature [[23], [24], [25]]. The two deformation mechanism i.e. slip and twinning are predominantly impacted by the texture and hence their mechanical properties. The relationship between applied stress and slip system can be illustrated by the Schmid factor (SF), as indicated in Fig. 8, that the dislocation can be easily activated for higher values of SF [14]. The higher SF values leads to ease of magnesium formability.

Fig. 8.   Schematic diagram of Schmid factor [14].

As mentioned above, extruded magnesium alloy exhibits strong fiber texture with basal planes of grain orientations parallel to the extrusion direction (ED). The c-axis of grains will experience compression, if Mg extrusion rod under subsequent loading in tension along the extrusion direction (ED), and will result in minute {10-11} contraction twinning to be activated, and meanwhile, SF of basal dislocation will be zero, which would make its activation difficult. On the other hand, compression along the ED will impose a tensile strain component on the c-axis and will result in the activation of {10-12} extension twinning [20,26]. The CRSS for {10-11} contraction twinning at room temperature is relatively high, compared to {10-12} extension twinning, which make the compression yield stress along the ED significantly lower than the tension yield stress. This contributes to apparent tensile/compressive yield asymmetry of wrought Mg alloys [3,[27], [28], [29]].

Like extruded magnesium, the rolled sheet with strong basal texture faces similar anisotropy, that the yield strength in rolling direction (RD) is lower than the strength in transverse direction (TD) [20,30]. The anisotropy of the rolled sheet can be illustrated by the r- values (Lankford coefficient) that is the measure of formable sheets to resist thinning, while the rolled sheet with the r- value close to one has poor anisotropy and good formability. The plastic anisotropy parameter “r” under uniaxial stress is measured as follows [24]:

$r=\frac{ε_{w}}{ε_{t}}=\frac{ln(w/w_{0})}{ln(t/t_{0})}$

Where εw and εt are the true strains in width and thickness respectively, while w and t are the width and thickness at ∼15 % tensile strain, whereas w0 and t0 are the original width and thickness respectively. Calculating the average plastic strain ratio in different directions, the weighted average value is usually taken as [24]:

$r_{avg}=\frac {1}{4}(r_{RD}+2r_{45}+r_{TD})$

This research investigated the relationship between different texture types and the tension/compression asymmetry (Fig. 9). The magnitude of σt,002c,002 was used as a measure of asymmetry. The wrought Mg alloy with <0001> texture type exhibited high asymmetry, with the value of σt,002c,002 tends to 2.33. The wrought Mg alloys with <4-2-23> texture type and with the <2-1-12> - <90-94> mixed texture type exhibited the lowest asymmetry. Their values of σt,002c,002 were close to 1. It is clearly demonstrated that texture type have a significant effect on the mechanical properties of wrought Mg alloys. Therefore, texture control of wrought Mg alloy has significant influence to improve the plastic formability.

Fig. 9.   Relationship between texture type and tension/compression asymmetry.

3. Texture optimization of magnesium alloy

Magnificent importance of texture on mechanical properties, made the texture optimization so important that its been addressed by so many researchers so far. And various methods have been investigated, among them two are the key importance i.e. alloying and intelligent processing.

3.1. Alloying

Improving the characteristics of various metals compelled the scientists to use the famous idea of alloying and so has been used in case of Mg [31]. Plenty of research has been done over alloying of Mg with various elements, and with focus over texture [23,[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65]]. As shown in Fig. 10, Gd, Ce and Y are all effective texture modifiers and produce rare earth (RE) texture once used for alloying with Mg [39].

Fig. 10.   Texture data for different alloys measured using EBSD [39].

Stanford and Barnett [51] successfully developed Mg-Gd alloy rods and reported the texture strength reduced by 51.7 % compared with pure Mg. Moreover, a shift in the orientation peak from <10-10 > to <11-21> was observed, which was referred as “RE texture component” (Fig. 11). Resultantly, tensile ductility was found to be higher than pure Mg which is attributed to the texture weakening (Fig. 12). Similarly, extrudability of Mg-1.58Zn-0.52Gd and AZ31 was compared by Jiang et al. [49]. In comparison to AZ31, Mg-1.58Zn-0.52Gd was successfully extruded at a high extrusion speed of 60 m/min, indicating that extrudability of Mg-1.58Zn-0.52Gd was better than the previous (Fig. 13). Under subsequent mechanical test, the ductility of Mg-1.58Zn-0.52Gd was twice that of AZ31 (Fig. 14). This was mainly because Mg-1.58Zn-0.52Gd develop an RE texture that favored the operation of both basal slip and {10-12} extension twins [49] (Fig. 15). Both the extrusion and rolling textures could have been weaken by the addition of Gd [37,42,54,66]. As evident in Fig. 16, the pure Mg exhibited a typical sharp rolling texture with an intensity of 16 multiples of random distribution (MRD). In comparison to pure Mg, the rolled Mg-1Gd showed a much weaker texture (6MRD) with a similar basal characteristic [66], along with that, Gd concentration variations also modifies the morphology of the rolling texture. Fig. 17 depicts, that by increasing the quantity of Gd the strong basal texture splited into two lobes in the rolled direction (RD), and the split basal poles tends to merge when the Gd concentration exceeded 4 %. Mechanical testing show that by increasing the amount of Gd will modifies the texture and accordingly more ductile and high strength as shown in Fig. 18 [42].

Fig. 11.   Comparing the texture data for (a) Pure Mg to (b) Mg-Gd [51].

Fig. 12.   Uniaxial tension curves of Pure Mg and Mg-Gd [51].

Fig. 13.   Extruded bar surfaces of ZG and AZ31 alloys at 300 °C and different die-exit speeds [49].

Fig. 14.   Engineering stress-strain curves of the ZG alloy extruded at different die-exit speeds [49].

Fig. 15.   (0001) pole figures and inverse pole figures of the extruded ZG and AZ31 alloys [49].

Fig. 16.   Rolled and annealed optical microstructures and textures for a) Pure Mg and b) Mg-1Gd [66].

Fig. 17.   Pole figures for Mg-Gd binary alloys obtained using X-ray diffraction [42].

Fig. 18.   Typical stress-strain curves for Mg-Gd binary alloys [42].

Ce has the lowest solubility limit in Mg among all other known RE elements, and is also known for its texture modifications characteristics for wrought Mg [4,23,[33], [34], [35], [36], [37], [38],67,28,[68], [69], [70]]. Mishra et al. [40] proposed that randomized texture leads to ductility enhancement, that comes from recrystallization of extruded material in the presence of Ce. Obvious weakening in texture (the texture strength from 2.7 to 1.5) was observed by addition of 0.2 % Ce to pure magnesium as shown in Fig. 19, while elongation of Mg alloy was resulted for same dosage of Ce. However, the improvement in ductility was also accompanied by increase in strength, which was rare in the Mg-Ce binary alloy. Luo et al. [69] studied the effect of Zn by incorporating in different concentrations (2.4 %, 4.9 % and 7.6 %), on the microstructure, strength and plasticity of Mg-Zn-Ce alloy. They reported that by increasing the Zn content leads to increase in strength but reduction in plasticity was also observed. Finally, optimized strength and plasticity were observed when the dosage of Zn was kept as 2.4 % (Fig. 20).

Fig. 19.   EBSD data for (a) Pure Mg and (b) Mg-0.2 %Ce [40].

Fig. 20.   Tensile and compression data for extruded Mg and Mg-0.2 Ce rods [40].

Yttrium, with a high solubility limit of 12.4 wt.% in Mg, and is widely known for its character to modify the strong deformation texture for Mg [39,43,[59], [60], [61],[63], [64], [65],67,71]. Wu et al. [64] studied Mg-Y alloy and reported the behavior of Mg-Y as ductile in comparison to pure Mg. Sandlöbes et al. [61] also researched the binary alloy of Mg-Y from ductility point of view, the authors reported that a fivefold increase in tensile ductility was observed at room temperature. The increase in tensile ductility was attributed to the addition of Y, which modified the microstructure and texture depicted by Fig. 21. As shown in Fig. 22, from the EBSD results, the cold rolling texture of pure Mg was a sharp c-type ((0001) ||ND), while Mg-Y showed a clear r-type texture ((0001), 10°-15° from ND towards RD), a typical feature of RE-alloyed magnesium.

Fig. 21.   Stress-strain curves of tensile tests at room temperature [61].

Fig. 22.   Microstructure and texture of Mg (a) and Mg-Y (b) [61].

Despite the numerous positive aspects of RE elements in Mg alloy, its application is still hindered by the high cost at commercial level for wrought Mg. Therefore, economical alloying elements need to be searched out for similar working features. So far, calcium revealed some characteristics similar to RE-alloys as proposed by the literature [[72], [73], [74], [75], [76]]. For instance, Ding et al. [75] utilized Ca as a replacement of RE elements in his study and reported that adding Ca to pure Mg and Mg-Zn alloys contributed to the texture weakening of extruded sheets similar to RE-alloys as explained in Fig. 23. As shown in Fig. 24 the ductility also increases by the additions of Ca and like the RE-alloys. Furthermore, the rectified in ductility was attributed to texture weakening, hence provided an opportunity for development of economical non-RE Mg-alloys.

Fig. 23.   {0001} Plane texture (TD vertical) (a) Pure Mg; (b) Mg-0.5RE; (c) Mg-0.5Ca [76].

Fig. 24.   Mechanical properties of Mg samples taken from extruded sheets (a) Pure Mg; (b) Mg-0.5Ca [76].

3.2. Processing

In addition to alloying, the texture can also be altered by changing the deformation path [[77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97]]. Many deformation methods have been introduced for magnesium alloys and were excellent in developing weaker or non-basal textures, e.g. equal channel angular pressing (ECAP) [[77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90]], asymmetric rolling (ASR) [[91], [92], [93], [94]].

Using the concept of changing the deformation path Mukai et al. [78] modified the Mg texture and presented the schematic illustration in Fig. 25 developing the basal plane orientation through the conventional direct extrusion and ECAE process. The tensile ductility enhancement in AZ31 alloy after ECAP was twice larger than that for the conventionally extruded counterpart (Fig. 26). Sabat [4] investigated the texture and mechanical properties of Mg-0.5Ce using rolling, multi-axial forging (MAF) and ECAP. The author determined that MAF sample developed a weak texture component {0-1-1-2} [-23-12] in addition to the basal texture and the ECAP sample exhibited a strong non-basal texture component {1213} [2111] shown in Fig. 27. The yield tensile strength, the ultimate strength and the uniform elongation of the rolled samples were determined to be 110 MPa, 250 MPa and 17 % respectively, while for MAF the respective parameters values were found to be 60 MPa, 200 MPa and 30 %. In case of ECAP the forementioned parameters were 55 MPa, 250 MPa and 40 %, respectively as explained in Fig. 28. In addition, many researchers studied the texture evolutions and mechanical properties of the deformed Mg using ECAP procedure following various paths [80,84,85].

Fig. 25.   Schematic illustration developing the basal plane orientation in magnesium through (a) Conventional direct extrusion (b) ECAE process [78].

Fig. 26.   Nominal stress ± strain relations for the annealed AZ31 alloy followed by ECAE, and the same alloy by direct extrusion [78].

Fig. 27.   (0002) pole figures for (a) rolled, (b) MAF and (c) ECAP samples before and after annealing [4].

Fig. 28.   True (stress vs strain) characteristics [4].

However, it is difficult to produce Mg sheet using ECAP due to the limitation of mold structure, and hence, asymmetric rolling (ASR) have been introduced for the texture control in sheets [91,92,[98], [99], [100], [101]]. Kim et al. [99] worked over ASR and determined that basal texture was notably weakened when compared with traditional rolling process as shown in Fig. 29. Fig. 30 presents the yield stresses as high as 300 MPa and a maximum elongation of 35 %. Huang et al. [100] executed the same procedure with different speed rolling (DSR) and the resulted AZ31 sheet sample exhibited a weaker inclined texture when compared with the normal rolled sheet (Fig. 31). The process also resulted in a larger uniform elongation with smaller r- value presented in Fig. 32. Biswas et al. [91] reported in his findings that the asymmetric rolling (Fig. 33) introduces a shear component which shift the texture by approximately 5°-10° from the conventional rolling texture. In Fig. 34 the author discussed that minute compromise on strength was accepted with appreciable increase in ductility. Another study described a method apart from ECAP and ASR, Qin et al. studied the effect of spinning temperature on texture of AZ31B. The specimens after spinning showed a typical basal texture, but the texture strength was compromised.

Fig. 29.   The (0002) pole figures of the AZ31 sheets: (a) symmetrically rolled (b) asymmetrically rolled [100].

Fig. 30.   The engineering stress-engineering strain relations for the AZ31 sheets (top) asymmetrically rolled with a 30 % thickness reduction at 473 K and (bottom) symmetrically rolled with a 30 % thickness reduction at 473 K [100].

Fig. 31.   (0002) pole figures of (a) the normal-rolled sheet and (b) the DSR-processed sheet [101].

Fig. 32.   (a) r-value and n-value of the normal rolled sheet and the DSR processed sheet in the tensile directions of RD, 45◦ and TD. (b) fracture elongation (ef), uniform elongation (eu) and post-uniform elongation (epu) of the normal rolled sheet and the DSR processed sheet [101].

Fig. 33.   (0002) and (10-10) pole figures for the symmetric and asymmetric rolled samples at room temperature [92].

Fig. 34.   Engineering stress-strain curves for the symmetric and asymmetric rolled samples at 350℃ and 200℃ [92].

4. Mechanism of texture weakening

4.1. Modification of texture by addition of RE elements

Texture weakening was usually triggered during recrystallization and grain growth in Mg alloys. Several mechanisms had been proposed to rationalize the phenomenon of texture evolution in Mg alloys, such as particle stimulated nucleation (PSN) [28,[102], [103], [104]], shear band nucleation [47,51], deformation twinning nucleation (DTN) [54], and solute drag [50,53,58].

Ball and Prangnell [28] investigated the texture weakening of Mg-RE, and found a weak texture in WE54 that was attributed to particle stimulated nucleation (PSN) during dynamic recrystallization. Similarly, Humphrey [102] found that adding 10 % SiC by volume to the magnesium alloy significantly reduced the strength of basal texture, with tendency of the grains to be random, which confirmed the weakening effect of particles on the texture. However, some Mg-RE alloys without particles may also have obvious texture weakening effect after extrusion [105], while some Mg-RE alloys with particles do not show obvious texture weakening [106]. From the above discussion it seems that PSN mechanism is deficient to explain all the texture modification of Mg-RE alloys, that opens a loophole for further investigation in future.

Stanford and Barnett [51] worked over shear band nucleation, and reported that by adding 1.55 wt.% dosage of Gd can produce two shear bands in different directions relative to the extrusion direction. As in Fig. 35 it can be concluded that <1-210 > texture produced in shear bands aligned with extrusion direction, while <1-211> texture oriented with an inclination of ∼25°. Basu et al. [47] also reported that recrystallization behavior was predominantly governed by nucleation at shear bands. However, Guan et al. [107] observed the recrystallisation of texture in shear bands using quasi-in-situ electron backscatter diffraction method and determined that the texture was relatively random, with a weakened basal component, unlike a typical RE texture. On contrary, a typical Mg-RE texture formation bears no shear banding events, and it seems that this mechanism is not enough to explain the texture weakening concept. Deformation twinning has also been observed as DRX nucleation sites for randomly oriented grain because the addition of RE promotes contraction and secondary twin formation [54,108]. To come up with a solution to the problem, Guan et al. [109] investigated that recrystallization of WE43 Mg alloy and for the first time provided direct evidence that recrystallized grains originating from double twins can form the RE texture during the process of annealing. Nevertheless, the effectiveness of such nuclei in grain growth and texture modification was limited by the twin boundaries [110], and this phenomenon also existed in alloys containing no RE element [[111], [112], [113]].

Fig. 35.   EBSD map of Mg-Gd extruded at 415℃ (a) Full EBSD map (b) Shear bands aligned with extrusion direction (c) Shear bands at ∼25° to extrusion direction [51].

To explain the texture weakening ambiguity researchers brought the idea of solute drag effect [37,42,50,53,58,62,65]. Some of the studies suggested that the interaction of rare earth atoms with dislocations and grain boundaries will seriously affect the recrystallization behavior of magnesium alloys, and thus changing its texture type and strength [42,50,53,58,62]. Stanford [58] assess and determined that solute segregate to grain boundaries of Mg-1.5Gd using atom probe microscopy. Thereafter, he also proposed that solute segregation serves to significantly retard recrystallization, and accordingly weaken the texture. In another study, Hadorn [114] researched texture weakening using different proportion of Y, he found that weaken texture was formed in case of rich Mg-Y alloy (≥0.17 at. pct.) when compared with the alloy have Y concentration as (≤0.03 at. pct.), being considered as dilute. Furthermore, intergranular misorientation analysis (IGMA) analysis showed that geometrically necessary dislocation (GND) content is variable in case of both the rich and dilute. He further added that, prismatic slip was dominant for higher concentration while the dominant slip was basal for dilute concentration. The modified GND content recommend a change in dynamic recrystallization (DRX) mode. The limited grain boundary mobility due to solute drag suppresses conventional discontinuous DRX, results in the texture weakening phenomenon in RE-Mg alloys. The claim was reinforced by the evidence of dynamic strain aging (DSA) effects in RE-contained Mg alloys. The DSA analyses was studied in detail by Jiang et al. [48] accompanied by mechanical testing on Mg-0.5 wt%Ce and Mg-1.5 wt%Gd at a wide range of strain rates and temperature regimes. They stated that dynamic strain aging (DSA) is dependent upon strain rate and temperature. More interestingly the DSA of Mg-RE alloys was attributed to solute drag of the RE. Subsequently, Hadorn et al. [55] and Bugnet et al. [41] have provided direct evidence of Gd solute clustering and GB segregation in dilute Mg-Gd alloys, using aberration-corrected scanning transmission electron microscope (TEM). They reported that Gd solute clusters present in matrix and its segregation within GBs may provide steric hindrance to the motion of dislocations and GBs, which may affect the kinetics of recrystallization and texture development during deformation at high temperature [49].

Recently, Barrett et al. [65] studied the effect of RE on grain boundaries of Mg-Y using EBSD and molecular dynamics (MD). They also described the mechanism of RE segregation to GBs, which could alter the effective GB energy and mobility variations among GB types. In addition, recent experiments have demonstrated that alloying rare earth elements could significantly improve the activity of pyramidal < c+a > dislocations. Sandlobes et al. [61] studied numerous < c+a > dislocations near the stacking fault in Mg-Y alloys using TEM. As per density function theory (DFT), the addition of Y reduced the intrinsic stacking fault I1 energy (I1 SFE), which can enhance nucleation of < c+a > dislocations [60]. Lateron, Yin et al. [115] comes up with the idea, that the effect of Y on stacking fault energies can also be achieved by Al/Zn, while most Mg-Al and Mg-Zn alloys do not exhibit a strong activity of < c+a > dislocations. The above discussion concludes that enhanced < c+a > dislocation activity and reduced SFE are independent parameters. In the studies of molecular dynamics simulation (MD), Kim et al. [116] provides reasoning for activation of < c+a > slip in Mg-Y alloy, mentioning that Y retards the movement of basal < a > dislocation more strongly than < c+a>. This concludes that it reduces the differences in CRSS values for each slip systems. Wang et al. [117,118] measured the CRSS for basal slip, prismatic slip and pyramidal I < a > slip using far-field high energy X-ray diffraction microscopy (FF-HDEM) and observed that Y enhance the CRSS of basal slip and reduced the CRSS ratio of non-basal slip to basal slip. Further studies have shown that adding RE to Mg alloy not only affect the nature of bonding between Mg-RE but also among the individual Mg atoms [69].

4.2. Modification of texture by changing deformation path

The phenomenon of shear deformation, resulted from changing the direction of stress, made the scientists to modify the texture by changing the deformation path for Mg alloy. As mentioned earlier, heterogeneous microstructure of magnesium alloys results due to deformation more specifically twinning and shear bands within the micro matrix that inturn can provide nucleation sites for recrystallization. These bands can make the basal plane of recrystallized grains parallel to the shear plane, resulting in a certain degree of weakening of texture strength.

5. Conclusion

The texture of wrought Mg alloys and its effect on mechanical properties, as well as texture control of wrought Mg alloys have been introduced, and discussed with supporting literature. A strong texture always exists in the wrought Mg alloy after plastic deformation, which will result in the anisotropic mechanical properties. Therefore, understanding the texture evolution in processing of Mg alloys and its effect on mechanical properties is very important. This article summarizes the key information and effective procedures of texture control which improve the mechanical properties of wrought Mg alloy. Furthermore, this paper also identified the two main methods for texture control a) Alloying b) deformation path. Further understanding the mechanism of texture weakening will also promote the development of high-performance Mg alloys with optimized texture.

Acknowledgements

The authors gratefully acknowledge the financial supports of the National Key Research and Development Plan (Grant No. 2016YFB0701201, No. 2016YFB0301103) and the National Natural Science Foundation of China (Grant No. 51771109, No. 51631006).


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