Journal of Materials Science 【-逻*辑*与-】amp; Technology, 2020, 49(0): 117-125 doi: 10.1016/j.jmst.2019.04.048

Research Article

## A simultaneous improvement of both strength and ductility by Sn addition in as-extruded Mg-6Al-4Zn alloy

Xiao-Yuan Wanga,b, Yu-Fei Wanga,b, Cheng Wang,a,b,c,*, Shun Xua,b, Jian Ronga,b, Zhi-Zheng Yanga,b, Jin-Guo Wang,a,b,*, Hui-Yuan Wanga,b,c

a State Key Laboratory of Super Hard Materials, Jilin University, Changchun, 130012, China

b Key Laboratory of Automobile Materials of Ministry of Education & School of Materials Science and Engineering, Nanling Campus, Jilin University, No.5988 Renmin Street, Changchun, 130025, China

cInternational Center of Future Science, Jilin University, Changchun, 130012, China

Corresponding authors: * State Key Laboratory of Super Hard Materials, JilinUniversity, Changchun, 130012, China.E-mail addresses:chengwang@jlu.edu.cn(C. Wang),

Received: 2019-01-21   Revised: 2019-04-6   Accepted: 2019-04-8   Online: 2020-07-15

Abstract

Commercial wrought Mg alloys normally contain low alloying contents to ensure good formability. In the present work, high-alloyed Mg-6Al-4Zn-xSn (x = 1, 2 and 3 wt.%, respectively) alloys were fabricated by extrusion. Hereinto, Sn was proven to play an effective contribution to simultaneous improvement in strength and ductility that are traditional trade-off features of synthetic materials. It was found that the average grain size of those alloys decreases significantly from ~11 to ~4 μm as a function of Sn contents increasing from 0 to 3 wt.%, while the amounts of Mg2Sn and Mg17Al12 particles continuously increase. More importantly, the addition of Sn leads to the transformation of dominated deformation modes from {10$\bar{1}$2} extension twinning (1 wt.%) to pyramidal <c+a> slip (3 wt.%) during tensile tests along the extrusion direction at room temperature. The advantageous combination of ultimate tensile strength (~366 MPa) and elongation (~19 %) in Mg-6Al-4Zn-3Sn alloy is mainly attributed to the strong strain hardening ability induced by the enhanced activity of non-basal <c+a> slip. This work could provide new opportunities for the development of high-alloyed wrought Mg alloys with promising mechanical properties.

Keywords： Magnesium alloys ; Microstructure ; Deformation modes ; Ductility

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Xiao-Yuan Wang, Yu-Fei Wang, Cheng Wang, Shun Xu, Jian Rong, Zhi-Zheng Yang, Jin-Guo Wang, Hui-Yuan Wang. A simultaneous improvement of both strength and ductility by Sn addition in as-extruded Mg-6Al-4Zn alloy. Journal of Materials Science & Technology[J], 2020, 49(0): 117-125 doi:10.1016/j.jmst.2019.04.048

## 1. Introduction

As the lightest structural metals, magnesium (Mg) alloys make a significant contribution in diverse fields [[1], [2], [3], [4]]. However, the poor deformation ability of Mg alloys at room temperature limits their application, especially for Mg alloys with high alloying contents. For example, the extrusion process of Mg-9Al-1Zn (Mg alloy AZ91) is much tougher than that of Mg alloys containing a lower quantity of alloying elements [5,6]. Accordingly, commercial Mg alloy AZ91 is mostly produced through casting. Besides, the low-alloyed and mediate-alloyed Mg alloys produced via extrusion are accompanied with low strength, which is usually attributed to the absence of strengthening phases [7].

To obtain excellent performance and broaden applications of wrought Mg alloys, it is of great necessity to develop Mg alloys with high alloying contents. According to literature, some AZ series alloys emerged by adjusting Al and Zn contents and exhibited combination of improved strength and ductility [8]. It was considered that AZ series Mg alloys with mediate Al and Zn contents (3-6 wt.%) possess moderate fluidity and castability [[9], [10], [11]], while little attention has been paid to the deformation ability of these alloys. Rong et al. [10] recently reported a new type Mg-7Al-5Zn (Mg alloy AZ75) processed by novel hard-plate rolling (HPR) route, which exhibits superior mechanical properties with high strength and ductility at room temperature as well as high superplasticity at elevated temperatures. This work further verifies the deformation potential of AZ series alloys with mediate Al and Zn contents. However, there is little research about applying normal processing method (e.g. extrusion) to develop Mg alloys with both high strength and ductility, which needs further optimization of the composition of wrought Mg alloys.

Recently, Sn is found to be an effective alloying additive to improve the mechanical properties of AZ series alloys [12,13]. For instance, Jiang et al. [12] found that as-extruded Mg-8Al-1Zn alloy with 0.5 wt.% Sn exhibits outstanding tensile strength of 386 MPa due to fine grain size and a large number of second phases. Lee et al. [14] considered that Mg17Al12 and Mg2Sn resulted in enhanced precipitation strengthening in as-forged Mg-8Al-4Sn-2Zn (wt.%) alloy. The contribution of Sn element to Mg alloys can be summarized as: solid-soluted Sn is conducive to reducing the stacking fault energies of Mg alloys, which could impede cross slip or climb of dislocations, and suppress dynamic recovery during deformation [15,16]. Moreover, Mg2Sn particles with a high melting point are beneficial to improving precipitation strengthening and high thermal stability [17]. Therefore, we focus on developing a new kind of extruded Mg-Al-Zn-Sn alloy with comparable high strength and ductility, aiming at deepening the fundamental theory of alloy design and extending the application of wrought Mg alloys.

In this work, the Mg-6Al-4Zn (AZ64) alloy possessing a lower Al and a higher Zn content than Mg alloy AZ91, was selected as the based system for achieving the combination of excellent strengthening and toughening potential [10]. Mg-6Al-4Zn-xSn (x = 0, 1, 2 and 3 wt.%) alloys were processed by extrusion to systematically investigate the effect of Sn addition on the microstructure evolution and mechanical properties. The Sn content adopted here is no more than 3 wt.%, since excessive Sn (i.e. exceeding the maximum solubility of 4.75 wt.% at 468 °C) was reported to result in the formation of coarse undissolved Mg2Sn particles [18], which might act as nuclei of microcracks and induce crack initiation easily. Besides, the extrusion processing was chosen in this work in view of the great diversity of product shape relative to rolling [19].

## 2. Experimental procedures

### 2.1. Materials preparation

Mg-6Al-4Zn (Mg alloy AZ64) based alloys with different Sn contents were prepared from commercial pure Mg (99.85 wt.%), Al (99.90 wt.%), Zn (99.90 wt.%) and Sn (99.90 wt.%). These components were completely melted in an electric resistance furnace at ~700 °C under a gas mixture of CO2 and SF6, and then poured into a steel mould with a diameter of 94 mm. Table 1 summarizes the notations and chemical compositions of the samples measured by an optical spectrum analyzer (ARL 4460, Switzerland) in wt.%. The casted ingots were machined into billets with 90 mm in diameter followed by solid-solution treatment at 430 °C for 3 h in a resistance furnace. Then, the ingots were hot extruded into plates with cross section of 40 mm × 5 mm at 390 °C via an extruding machine (630 ton) with an extrusion ratio of 35:1. For convenience, the as-extruded AZ64-xSn (x = 0, 1, 2 and 3 wt.%) alloys were designated as AZ64, AZT641, AZT642 and AZT643, respectively.

Table 1   Nominal and measured compositions of the studied alloys.

NotationNominal compositionMeasured composition (wt.%)
AlZnSnMg
AZ64Mg-6Al-4Zn5.804.13-Bal.
AZT641Mg-6Al-4Zn-1Sn5.984.270.99Bal.
AZT642Mg-6Al-4Zn-2Sn6.114.631.89Bal.
AZT643Mg-6Al-4Zn-3Sn6.184.522.95Bal.

### 2.2. Microstructure characterization

Microstructure characterization was performed by using an optical microscope (OM; Carl Zeiss-Axio Imager A2m, Germany), a scanning electron microscope (SEM; VEGA 3 XMU, TESCAN, Czech) equipped with an energy dispersive spectrometer (EDS) analyzer, and a transmission electron microscope (TEM; JEM-2100 F, Japan). The metallographic samples for observations were taken from the planes perpendicular to the normal direction (ND). A SEM equipped with an Oxford Instruments NordlysNano electron back scattered diffraction (EBSD) detector was used for collecting and analyzing data by the AZtec and Channel 5.0 software. The EBSD characterization was performed at 20 kV, 15 mm working distance, 70° tilt, and 0.4-0.6 μm scanning steps. X-ray diffraction (XRD, D/Max 2500PC, Rigaku, Japan) was employed to analyze the phase constituents at 50 kV and 200 mA by Cu Kα radiation with a scanning speed of 4°/min and acquisition step of 0.02° (2θ). The pole figure analysis was conducted at 40 kV and 150 mA. Samples for TEM observation were mechanically grinded followed by ion milling at an ion accelerating voltage of 5.0 kV. The grain size of each sample was measured using the software Nano Measurer System 1.2, and at least six images were selected to ensure the accuracy.

### 2.3. Tensile tests

Standard tensile samples (a gauge size of 10 mm × 4 mm × 1.2 mm) were machined along the extrusion direction (ED) from the as-extruded plates. Tensile tests were performed under a strain rate of 1.0 × 10-3 s-1 at room temperature using INSTRON 1121 testing machine (USA). Besides, all tensile tests were repeated at least three times for each condition.

## 3. Results and discussion

### 3.1. Mechanical properties

Fig. 1 shows the engineering stress-strain curves of as-extruded samples tested along ED at room temperature (RT). Yield strength (YS), ultimate tensile strength (UTS), elongation and fracture strain are summarized in Table 2 accordingly. With an increase in Sn content, both strength and ductility increase, which overcomes the traditional trade-off between strength and ductility in metallic materials. To be specific, an increment in Sn content from 0 to 3 wt.% leads to an apparent increase in YS from ~163 MPa to ~207 MPa and UTS from ~283 MPa to ~366 MPa at RT, without an expense of ductility (elongation increases from ~11 % to ~19 %).

### Fig. 1.

Fig. 1.   Engineering stress-strain curves of as-extruded AZ64-xSn alloys under tension along extrusion direction at RT.

Table 2   Tensile properties of as-extruded AZ64-xSn alloys with different Sn contents tested along extrusion direction at RT.

Nominal compositionYield strength σ0.2 (MPa)Ultimate tensile strength σb (MPa)Elongation ε (%)Fracture strain εb (%)
AZ64$163_{-1}^{+2}$$283_{-2}^{+3}$$11.9_{-0.2}^{+0.8}$$12.1_{-0.2}^{+0.8} AZT641171_{-6}^{+5}$$328_{-7}^{+6}$$14.8_{-1.7}^{+1.4}$$16.3_{-1.7}^{+1.4}$
AZT642$197_{-1}^{+2}$$349_{-2}^{+3}$$17.8_{-0.4}^{+0.9}$$17.9_{-1.2}^{+1.9} AZT643207_{-3}^{+3}$$366_{-1}^{+2}$$19.1_{-0.6}^{+0.9}$$20.3_{-0.6}^{+0.9}$

The elongation and UTS of various extruded Mg alloys with an extrusion ratio from 25:1 to 40:1 reported in literatures [12,14,[20], [21], [22], [23], [24], [25], [26], [27], [28]] are given in Fig. 2, to make a comparison with this work. It can be noticed that although high UTS can be achieved in commercial AZ91 and some of Mg-Al-Zn-Sn alloys with high alloying contents [12,14,23], these alloys always exhibit poor ductility and may also have bad extrudability. On the contrary, Mg-Sn-Zn-Mn alloys generally exhibit further enhancement of ductility but at the expense of strength [20], which originates from the lack of Al constituent. By comparison, the addition of increasing Sn in this work leads to a simultaneous improvement of ductility and strength in AZ64-xSn alloys. Hereinto, the AZT643 sample possesses a combination of superior strength and ductility, which can be considered as a promising wrought Mg alloy that meet potential demands in industries.

### Fig. 2.

Fig. 2.   Elongation and ultimate tensile strength of various Mg based as-extruded alloys with extrusion ratio varied from 25:1 to 40:1 reported in literatures [12,14,[20], [21], [22], [23], [24], [25], [26], [27], [28]].

### 3.2. Microstructure characteristics

To further reveal the microstructure evolution with more Sn addition, the optical micrographs taken from the plane parallel to the ED of as-extruded AZ64-xSn alloys are presented in Fig. 3. As shown by the inset of Fig. 3, the average grain size is refined significantly as Sn contents increase from 0 to 3 wt.%, i.e. ~11, ~9, ~6, ~4 μm, respectively. It can be seen that grain refinement is obviously achieved in the alloy with 2 wt.% Sn, and the grain size further decreases slightly as Sn content increases to 3 wt.%.

### Fig. 3.

Fig. 3.   Optical micrographs (inset: corresponding grain size distribution, dave represents the average grain size) of as-extruded AZ64-xSn alloys: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643. (ED and TD stand for extrusion direction and transverse direction, respectively).

The X-ray diffraction patterns are shown in Fig. 4. The major constituents of these alloys are α-Mg and Mg17Al12, while the diffraction peak of Mg2Sn phase is recognized as Sn content reaches 2 wt.%, and it becomes more obvious with the further addition of Sn. The relative intensity of diffraction peak for Mg17Al12 increases steadily with rise in Sn concentration, indicating that the volume fraction of Mg17Al12 phase increases accordingly. Besides, no Zn-containing phase can be detected, which is in consistent with the results of Rong et al. reported in the HPRed AZ75 alloy [10].

### Fig. 4.

Fig. 4.   X-ray diffraction patterns of as-extruded AZ64-xSn alloys: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643.

Fig. 5 shows the back scattered electron (BSE) and corresponding SEM micrographs of the extruded specimens with increasing Sn contents. According to BSE-SEM in Fig. 5(a)-(d), the bulk Mg2Sn particles that shown as bright contrasts (indicated by the yellow arrows) can only be detected in AZT642 and AZT643 samples (Fig. 5(c) and (d)), and amounts of Mg2Sn particles become larger with increasing Sn contents, which fits well with XRD results. Therefore, it can be assumed that Sn mainly exists in the form of solid solution in AZT641, while displays as Mg2Sn phase as Sn content added up to 2 and 3 wt.%. Besides, it can be seen that Mg2Sn particles with relatively coarse size ranging from 2 to 5 μm are unevenly distributed. As reported in Mg-4Zn-1.5Al-xSn alloys [29], fine Mg2Sn particles were mainly formed by direct separation from the large divorced eutectic Mg2Sn during extrusion process, while parts of large particles would still exist if they were not crushed into small particles by extrusion pressure [30].

### Fig. 5.

Fig. 5.   BSE-SEM and corresponding SE-SEM micrographs of as-extruded AZ64-xSn alloys: (a, e and i) AZ64; (b, f and j) AZT641; (c, g and k) AZT642; (d, h and l) AZT643. AfMg2Sn and Af represent the area fraction of Mg2Sn and total precipitates, respectively.

The addition of Sn also results in a significant change in morphology and amounts of Mg17Al12 phase, as shown in Fig. 5(e)-(h). In as-extruded AZ64-xSn samples, numerous fine Mg17Al12 particles are dispersed both along grain boundaries (discontinuous precipitation) and within grains (continuous precipitation), but are more pronounced along the grain boundaries. To observe the characteristics of these precipitates more clearly, highly magnified SEM images are presented in Fig. 5(i)-(l). For AZ64 (Fig. 5(i)) and AZT641 (Fig. 5(j)) samples, most of the continuous Mg17Al12 precipitates have a granulated morphology with size below 1 μm, while some of the discontinuous Mg17Al12 distributing along grain boundaries exhibit elliptical shape. With Sn content increasing, the Mg17Al12 particles gradually distribute more densely and are of coarser size in the as-extruded AZT642 (Fig. 5(k)) and AZT643 (Fig. 5(l)) samples.

By Image Pro Plus software, total area fraction of second phases in the AZ64 sample derived from at least six micrograph images is estimated to be ~6 %, while that is ~8 %, ~11 % and ~15 % in alloys with addition of 1, 2 and 3 wt.% Sn, respectively. Nevertheless, area fraction of Mg2Sn particles in the AZT642 is estimated as ~0.9 %, and it further increases to ~1.2 % in AZT643. The amounts of Mg17Al12 are therefore inferred to increase with Sn increasing, which agrees well with the XRD results. It is reasonable due to the competitive dissolution between Al and Sn solutes in Mg matrix, where the increased solid solution of Sn would reversely result in the decreased dissolution of Al in α-Mg and facilitates the precipitation of coarse Mg17Al12 phases [8].

According to the BSE-SEM micrograph and corresponding EDS line scans taken from the AZT643 sample (Fig. 6(a) and (b)), the distribution of detected elements can be clearly observed. Interestingly, Zn element is detected to substitute for part of Al atoms in Mg17Al12 phase, forming into Mg17(Al, Zn)12 phase because of the similar atomic radius between Al and Zn [31,32]. Meanwhile, parts of Zn solutes also dissolve into α-Mg matrix, as indicated by the spread of Zn intensity. We propose that the dark gray Mg17Al12 phase may precipitate adhere to the bright Mg2Sn phase (as indicated by the arrows). Similar phenomenon has also been observed in literature [33,34]. Although there is no coherent crystallographic orientation relationship been reported between the Mg2Sn and Mg17Al12 phases, Xiao et al. [33] pointed out that the Mg2Sn nucleus is occasionally found in the granular Mg-Al phase of Mg-7Zn-5Al-4Sn (ZAT754) alloy from TEM observation. It is supposed that the increased Mg2Sn particles by Sn addition can provide more nucleation sites for Mg17Al12, and hence the amounts of submicro- Mg17Al12 particles that associated with dynamic precipitation increase accordingly.

### Fig. 6.

Fig. 6.   (a) BSE-SEM micrograph of as-extruded AZT643 sample; (b) EDS line scan for Mg, Al, Zn and Sn of the particle in (a).

The EBSD inverse pole figure (IPF) maps of the as-extruded samples and corresponding statistical results for different types of grains are shown in Fig. 7(a)-(h), wherein the grains with misorientation angle lower than 1° are denoted as recrystallized grains (marked by blue), and those larger than 7.5° refer to deformed grains (marked by red). The substructured grains are defined with the misorientation angle ranging from 1° to 7.5° (marked by yellow). It can be seen that most of the grains have undergone dynamic recrystallization and the size of recrystallized grains decreases significantly with Sn addition. This originates from the increasing presence of coarse Mg2Sn particles induced by Sn addition, which might create more nucleation sites for dynamic recrystallized (DRXed) grains [35]. Actually, similar phenomenon has been widely reported [17,29]. In addition, the increasing submicro- and nano- Mg17Al12 phases, especially those distributing along grain boundaries, can effectively restrict the growth of DRXed grains via grain boundary pinning, resulting in the obvious grain refinement by Sn addition.

### Fig. 7.

Fig. 7.   EBSD inverse pole figure (IPF) maps of as-extruded AZ64-xSn alloys and the different types of grains: (a,e) AZ64; (b,f) AZT641; (c,g) AZT642; (d,h) AZT643: blue-recrystallized, yellow-substructured, red-deformed.

Table 3 shows that the area fractions of recrystallized grains decrease gradually after Sn addition, while those of deformed grains increase reversely. It is reasonable since there is an increased amount of precipitates in the alloys with more Sn addition, which could accumulate more concentrated stress and reserve deformed characteristics.

Table 3   Corresponding area fraction of different types of grains in as-extruded AZ64-xSn alloys.

Nominal compositionRecrystallized (%)Substructured (%)Deformed (%)
AZ6498.201.510.28
AZT64195.083.571.35
AZT64294.202.323.47
AZT64389.993.856.15

According to the pole figures of Fig. 8, there exhibits an increase in maximum intensity of basal pole from ~3.7 to ~7.4, which corresponds to the as-extruded AZ64-xSn alloys with Sn contents ranging from 0 to 3 wt.%. This is different from the results reported by Wang et al. [36], where the decrease of the intensity of basal texture with increasing Sn content is attributed to the rise in fractions of DRXs. However, the fractions of deformed grains in AZ64-xSn alloys increase after Sn addition in this work. The additional deformed grains would increase the maximum basal texture intensity, because the orientations of deformed grains are relatively stronger than that of recrystallized grains [37]. This might be the reason that leads to the deviation in texture evolution.

### Fig. 8.

Fig. 8.   Pole figures of as-extruded AZ64-xSn alloys sheets: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643.

### 3.3. Deformation mechanism

From aforementioned results, the YS, UTS and elongation are all increased by Sn addition in as-extruded AZ64-xSn alloys (Fig. 1). The increasing YS of AZ64-xSn alloys with Sn addition could be mainly attributed to three reasons. Firstly, since the average grain size decreases significantly with introduction of more Sn, it is reasonable to state that grain boundary strengthening contributes to the enhanced YS of AZT643 based on the Hall-Petch relationship (Δσgrain=kd-1). Besides, the increasing number of well-dispersed fine particles within grains could remarkably pin the movement of dislocations and retard the dislocation recovery, which is beneficial to improving the YS of AZT643 by precipitation strengthening. Moreover, the more intensive basal texture with increasing Sn contents could also improve the YS of AZT643 by texture strengthening.

To further investigate the underlying mechanism that strength and ductility increase simultaneously by Sn addition, the AZ64 and AZT643 alloys are chosen to analyze their macroscopic work hardening rates under tension along ED (Fig. 9), where the σ-σ0.2 is related to the dislocation contribution to the flow stress [38]. It can be seen that AZT643 alloy exhibits a slower drop in work hardening rate than that in AZ64 alloy, which means the dynamic recovery process of AZT643 alloy is depressed. Owing to the higher work hardening ability in AZT643, it exhibits a simultaneous increase in strength and ductility compared with AZ64.

### Fig. 9.

Fig. 9.   Hardening curves for AZ64 and AZT643 alloys under tension at RT.

The enhanced work hardening is closely related to the activity of different deformation modes, therefore the deformation modes during tension of AZ64 and AZT643 alloys are analyzed by Schmid factors (SFs). The SF is defined as cosφcosλ, where φ is the angle between normal direction of slip plane and stress axis, λ is the angle between slip direction and stress axis. For polycrystals, the average SFs for basal and non-basal slips are calculated from inverse pole figures [39], as summarized in Table 4. Considering the elongation of AZ64 is ~11 %, the average SF values are calculated in non-deformed and 8% deformed (engineering strain) AZ64 and AZT643 alloys. For AZ64 alloy, the average SF of basal <a> slip increases slightly from 0.3 to 0.312 during the initial 8% strain, and that of prismatic <a> slip increases from 0.138 to 0.146. However, there is no obvious change in the average SF values of pyramidal <a> and <c+a> slips. Inversely, the average SF values of basal <a>, prismatic <a> and pyramidal <a> slips for AZT643 alloy decrease continually with tensile strain increasing, while that of pyramidal <c+a> slip increases from 0.369 to 0.442 after 8% strain.

Table 4   Average Schmid factors calculated from the inverse pole figures of AZ64 and AZT643 samples at different tensile stages along ED.

AZ64AZT643
Non-deformed8% deformedVariation (%)Non-deformed8% deformedVariation (%)
Basal <a>0.30.31240.3390.236-30.4
Prismatic <a>0.1380.1465.80.1610.082-49
Pyramidal <a>0.2760.274-0.70.3010.203-32.6
Pyramidal <c+a>0.3960.386-2.50.3690.44219.8

Obviously, the pyramidal <c+a> slip is greatly activated by Sn addition, as evidenced by the 19.8 % increase of corresponding SF after 8% strain in AZT643. The enhanced activity of non-basal slips by Sn addition can be explained by the first-principles calculation, where Sn is proven to significantly reduce stacking fault energy (SFE) of Mg [15]. By comparison between the AZ64 and AZT643 alloys, there is an obvious transition of dominated slip modes with introduction of Sn, from common basal <a> to pyramidal <c+a> slips. Since the activation of <c+a> slips in Mg alloys can provide 5 independent deformation modes, it is regarded of the largest significance to ductility enhancement. Therefore, the unique combination of strength and ductility in ATZ643 is closely related to the facilitation of pyramidal <c+a> slip.

Fig. 10 shows the BSD band contrast (BC) maps superimposed by specific twin boundaries to explore the evolution of twinning behavior during the tensile deformation along ED. Almost no twins can be observed in as-extruded alloys. With deformation strains increasing, most of twins that are confirmed to be {10$\bar{1}$2} extension twins (as marked by red lines) are observed in both AZ64 and AZT643 alloys, due to its low critical resolved shear stress (CRSS) at RT and preferred loading orientation [40,41]. It is obvious that the percentage of twins grows faster with increasing strains in AZ64 than in AZT643, which is mainly because the AZT643 alloy yields finer initial grains. The growth and expansion of twins in AZT643 alloy was inhibited remarkably because of the large amounts of grain boundaries. Zhao et al. [42] also reported the similar observation. Therefore, it can be concluded that pyramidal <c+a> slip plays a significant role during tension of AZT643 along ED, while basal slip and {10$\bar{1}$2} twinning contribute more to strain compatibility in the case of AZ64.

### Fig. 10.

Fig. 10.   EBSD band contrast maps of (a-c) AZ64 and (d-f) AZT643 samples superimposed by specific twin boundaries at different stages of tensile strain: 10$\bar{1}$2 extension twins identified by red lines reorient the basal planes by 86°, 10$\bar{1}$1 contraction twins marked by blue lines reorient the basal planes by 56° and 10$\bar{1}$1-10$\bar{1}$2 double twins in green lines reorient the basal planes by 38°.

Fig. 11(a)-(c) and (g)-(i) show the Kernel average misorientation (KAM) maps of AZ64 and AZT643 samples at non-deformed, 8% deformed and fractured stages, respectively. With the deformation strains increasing, both the AZ64 and AZT643 alloys accumulate more dislocations and exhibit higher KAM. However, the increase of dislocations during deformation is more rapid in AZT643 alloy than that in AZ64 alloy. Histograms showing the misorientation angle distributions for AZ64 and AZT643 are presented in Fig. 11(d)-(f) and (j)-(l) respectively. With increasing tensile strains in AZ64 alloy, the fraction of misorientation angle at around ~86°, which represents {10$\bar{1}$2} twins, increases gradually, and becomes dominant at the end of plastic deformation. In contrast, the {10$\bar{1}$2} twins in AZT643 is not activated significantly, as indicated by the low proportion of ~86° misorientation. This result is in consistent with the EBSD band contrast maps in Fig. 10. Moreover, the high angle grain boundaries (>15°) decrease significantly with the increase of deformation degree in AZT643 alloy, while the fraction of low angle grain boundaries (<15°, LAGBs) increases from ~6 % to ~37 %, which implies the effective accumulation of substructures during tension.

### Fig. 11.

Fig. 11.   Kernel average misorientation map and misorientation histograms of (a-f) AZ64 and (g-l) AZT643 samples at different stages of tensile strain.

As discussed above, the dissimilarity of mechanical response between AZ64 and AZT643 alloys are attributed to the differences in predominant deformation mechanisms during tensile deformation along ED. The deformation of AZ64 alloy is dominated by {10$\bar{1}$2} extension twins, however the activity of pyramidal <c+a> slip is greatly enhanced in AZT643 alloy. Although both {10$\bar{1}$2} extension twin and pyramidal <c+a> slip can provide the deformation along c-axis [43], the degree of strain coordination by twins is quite limited, and is inferior to improve ductility compared to <c+a> slip. Note that the activation of <c+a> slip after Sn addition is on one hand because of the decreased SFE, which makes it difficult for the full dislocation to cross slip or climb and thereby inhibits the dynamic recovery of dislocations [16]. Moreover, Sn addition also facilitates the dynamic precipitation of well-dispersed fine precipitates within grains, which could remarkably pin the movement of dislocations and enhance the multiplication of <c+a> dislocations [44]. Therefore, the high activity of non-basal <c+a> slip and depressed dynamic recovery of mobile dislocations contribute to the increase working hardening of AZT643 alloy, which further leads to a simultaneous increase of strength and ductility.

## 4. Conclusions

In summary, Mg-6Al-4Zn-xSn (x = 1, 2 and 3 wt.%) alloys were produced via extrusion to investigate the effect of Sn contents on microstructure characteristics and mechanical properties in this study. The results show that the average grain size of the as-extruded alloys decreases from ~11 to ~4 μm with Sn contents increasing from 0 to 3 wt.%. The existence of Mg2Sn particles is only found when the Sn content reaches 2 wt.%, while the amounts of Mg17Al12 particles are continuously increased with Sn addition. It is supposed that the increased preexisting Mg2Sn particles by Sn addition can provide more nucleation sites for Mg17Al12 and facilitate the formation of submicro- Mg17Al12 phases, which contributes to the grain refinement via grain boundary pinning during hot extrusion. The Mg-6Al-4Zn-3Sn alloy exhibits superior mechanical properties of high strength and ductility at room temperature, i.e. yield strength of ~207 MPa, ultimate tensile strength of ~366 MPa and elongation of ~19 %. The combination of superior ductility and strength in AZT643 is ascribed to its strong strain hardening ability, which is contributed by the facilitation of pyramidal <c+a> slip with Sn addition. Our work shed lights on the design of new wrought Mg alloys with high performance.

## Acknowledgments

Financial supports from the National Key Research and Development Program (No. 2016YFE0115300) and the National Natural Science Foundation of China (Nos. 51625402, 51790483, and 51801069) are greatly acknowledged. Partial financial supports come from the Science and Technology Development program of Jilin Province (Nos. JJKH20180129KJ and 20190103003JH) and The Changjiang Scholars Program (T2017035).

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AbstractNuclear magnetic resonance (NMR) spectroscopy of 27Al was used to study the development of precipitation in aged Mg–6 wt%Al, Mg–9 wt%Al and Mg–9 wt%Al–(x) wt%Zn alloys. The 27Al spectra for the aged alloys consist of two peaks; one from the aluminium in solid solution and the other from aluminium in the precipitate phase. The proportion of aluminium atoms in the matrix and precipitate phases was measured, as a function of time at temperature, using the relative intensities of peaks. The nucleation of the continuous precipitates was found to be highly dependent on the initial supersaturation and it is proposed that it is a homogeneous process. The Austin–Rickett relation successfully models the amount of continuous precipitation with aging time; the kinetics is consistent with one-dimensional and interface-controlled growth. Changes in composition of the matrix and precipitate phases were correlated with the 27Al Knight shift characterising these phases. The Knight shift data from a series of ternary Mg–9 wt%Al–(x)wt%Zn alloys indicates that the Zn segregates to the precipitate phase during precipitation.]]>

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n) does not show any obvious variation with the twin density. These effects were analyzed in terms of the post-deformation microstructures.]]>

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