Effects of Ca and Sr additions on microstructure, mechanical properties, and ignition temperature of hot-rolled Mg–Zn alloy

Materials Science & Engineering A 769 (2020) 138474

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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea

Effects of Ca and Sr additions on microstructure, mechanical properties, and ignition temperature of hot-rolled Mg–Zn alloy Mengran Zhou a, Xinsheng Huang b, *, Yoshiaki Morisada a, Hidetoshi Fujii a, Yasumasa Chino b a b

Joining and Welding Research Institute (JWRI), Osaka University, Osaka, Japan Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Japan

A R T I C L E I N F O

A B S T R A C T

Keywords: Mg alloy Rolling Microstructure Mechanical properties Ignition temperature

In this study, the microstructures, mechanical properties, and ignition temperatures of hot-rolled Mg-1.5Zn-X (X: Ca and/or Sr) alloys were investigated. The Mg-1.5Zn, Mg-1.5Zn–1Ca and Mg-1.5Zn–1Sr alloys had the mean grain sizes of 56.1 μm, 8.7 μm, and 18.5 μm, respectively. This indicates that Ca addition exerts a stronger effect on grain refinement than Sr addition. Both Ca and Sr additions significantly weakened the basal texture to approximately one-quarter in intensity for the annealed sheets. Ca-containing alloys exhibited texture splitting in the transverse direction, while Sr-containing alloys tended to maintain it in the rolling direction after annealing. The Ca and Sr additions improved the mechanical strength considerably due to grain refinement, which over­ compensated for the influence of texture softening. Meanwhile, benefiting from the weakened textures, the 1% Ca and Sr additions remarkably increased the Erichsen values from 3.8 to 8.0 and 7.1, respectively. However, further addition resulted in the deterioration of both tensile ductility and stretch formability due to the cracking of secondary-phase particles. Ca addition had a better effect on enhancing the flame-retardant property than Sr addition, and 2% Ca and Sr additions increased the ignition temperature from 629 � C to 800 � C and 720 � C, respectively.

1. Introduction With the aim of a lower carbon dioxide emission society, lightweight strategies are effective in numerous industrial fields especially in manufacturing automotive, railway and aerospace equipment. Magne­ sium (Mg) alloys have many attractive properties such as low density and high specific strength, which make it possible to replace the con­ ventional materials in manufacturing transport vehicles to reduce their weight [1]. However, relatively low ignition point and poor formability restrict the industrial applications of Mg alloys. For expanding their industrial applications, improvements in flame-retardant property and formability are required. Compared to the grain size, the basal texture intensity which indicates the degree of crystalline orientation of the basal plane, plays a more vital role in determining the stretch form­ ability of Mg alloys [2–5]. It has been reported that minor addition of specific elements such as calcium (Ca), yttrium (Y) and cerium (Ce) significantly improves the stretch formability due to the randomization of the crystal orientation and the tilting of the basal pole in the trans­ verse direction (TD) of the sheet [6–8]. The reasons for the crystal orientation randomization have been explained both theoretically and

experimentally, e.g., enhanced activities of non-basal slips [9,10], and the restriction of preferential growth of basal-oriented grains due to solute segregation at grain boundaries [11]. Ca and Sr elements belong to the same group in the periodic table. The improvement in the flame-retardant property has been achieved by adding chemically active elements such as Ca or Sr to conventional Mg alloys [12–16]. As the ignition of Mg may occur between Mg vapor and oxygen, the vapor­ ization rate of Mg is an important influencing factor. The improved flame-retardant property due to Ca and Sr additions originates from the formation of a dense oxidation layer (CaO or SrO) at the surface of the molten Mg alloy, which prevents the inward diffusion of oxygen from the surface into the interior as well as the evaporation of Mg [13]. However, a comparison of the addition effects between Ca and Sr into Mg alloys is still rarely reported. Moreover, little attention has been paid to the mutual effects of Ca and Sr additions on the microstructure, me­ chanical properties, and ignition temperature of the Mg alloy. In the present study, systematic investigations were conducted on the effects of Ca and Sr additions on the microstructure, mechanical prop­ erties, and ignition temperature of the hot-rolled Mg-1.5Zn alloy, aiming at the achievement of Mg alloy sheets with the combination of flame-

* Corresponding author. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.msea.2019.138474 Received 28 February 2019; Received in revised form 9 September 2019; Accepted 27 September 2019 Available online 30 September 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.

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retardant property and superior formability.

micro-hardness test machine (Mitutoyo HM-200) was used for the micro-hardness test with 4.9 � 10 3 N load and 10 s hold time on these intermetallic compound particles in the Mg-1.5Zn–2Ca and Mg1.5Zn–2Sr sheets. The Vickers microhardnesses of the intermetallic compounds were calculated by averaging 15 measured points for each sample. An Erichsen value test machine (Universal Sheet and Strip Metal Testing Machine Model 142, ERICHSEN GmbH & Co.) was used for investigating the index Erichsen (IE) value. A schematic illustration of the Erichsen test is shown in Supplementary Fig. S1. A hemispherical punch with a diameter of 20 mm was used for Erichsen tests. The punch speed and the force of the holder were 5 mm/min and 10 kN, respec­ tively. Graphite grease was used as the lubricant. The blanks used for the Erichsen tests had a diameter of 60 mm. The ignition temperature was measured using chips machined from the Mg-1.5Zn, Mg-1.5Zn-xCa (x ¼ 1 or 2), Mg-1.5Zn-xSr (x ¼ 1 or 2) and Mg-1.5Zn–1Ca–1Sr sheets. The chips from each sample were prepared using a milling machine. A typical morphology of the machined chips is shown in Supplementary Fig. S2. Supplementary Fig. S3 shows a sche­ matic illustration of the ignition test used in this study. A ceramic boat was used as a container of 0.1 g chips during the ignition tests. A ceramic boat containing the chips was pushed into a ceramic pipe pre-heated to 1000 � C. A type-R thermocouple was inserted into the chips, and their temperature change during the heating was recorded. A digital video recorder was also used to confirm the ignition in the ceramic boat as the synchronous data of the thermocouple.

2. Experimental procedures In this study, pure Mg (>99.9%), Zn (>99.99%), and Sr (>99%) metals and Mg–5Ca (wt%) alloy were used for preparing Mg-1.5Zn-X (X: Ca and/or Sr) cast alloys with different Ca and Sr contents. Casting of Mg-1.5Zn and Mg-1.5Zn-xCa (x ¼ 1 or 2) alloys was conducted at 740 � C under an argon protective atmosphere. For the Mg-1.5Zn-xSr (x ¼ 1 or 2), Mg-1.5Zn–1Ca–1Sr, Mg-1.5Zn–1Ca-0.1Sr and Mg-1.5Zn–1Sr-0.1Ca alloys, casting was performed under an argon protective atmosphere at 820 � C. The Mg-1.5Zn and Mg-1.5Zn-X (X: Ca and/or Sr) alloys were hotextruded to plates of thickness 5 mm at 400 � C and 350 � C, respec­ tively. The extrusion ratio was 6:1 and the ram speed was maintained at 5 mm/min. The solution treatment was subsequently conducted on all plates at 450 � C for 8 h. All plates were rolled down to 1 mm by seven passes at 350 � C with a thickness reduction of 21% per pass. After roll­ ing, a heat treatment at 350 � C for 1 h was conducted on all specimens to obtain a recrystallized structure. The chemical compositions of the rol­ led alloys were analyzed using inductively coupled plasma (ICP) spec­ troscopy, as shown in Table 1. The (0002) pole figures were measured at the mid-layers of rolled sheets by X-ray texture analyses using the Schulz reflection method operated at 40 kV and 40 mA, and the X-ray diffraction (XRD) patterns were measured within the range of 20� –80� (Rigaku RINT Ultima III, Cu target, λ ¼ 1.5418 Å). Regarding the Schultz reflection method for evaluating the (0002) pole figure, the Bragg angle was fixed corre­ sponding to the (0002) diffraction peak, and the sample was rotated about the sheet normal and tilted at α-angles between 15� and 90� with an interval of 2.5� . An optical microscope (Nikon ECLIPSE MA100) was used for observing the intermetallic compound particles. The mean particle size along with variance, size distribution and area fraction of these particles were evaluated using the ImageJ and OriginPro software. Electron backscatter diffraction (EBSD) analyses were also conducted on all annealed sheets by using a scanning electron microscope (SEM, JSM5910) at 20 kV equipped with a data collector (EDAX 1500M-GE) and an analysis software (TSL OIM ver.7). Before the EBSD analyses, all speci­ mens were mechanically polished by 50-nm alumina suspension and subsequently cleaned using an argon ion beam shower. The step size for the EBSD analyses was 1 μm. The Schmid factor for basal slip was evaluated by averaging the Schmid factors of individual grains from the EBSD data. An electron probe micro analyzer (EPMA, JEOL JXA-8900L) was used for analyzing the chemical composition distribution at 15 kV. The tensile properties were investigated using a universal tensile test machine (INSTRON 5565Q6662) with a conductive mechanical exten­ someter. The parallel length, width, and thickness of the tensile speci­ mens were 12 mm, 4 mm and 1 mm, respectively. The speed of the crosshead was set at 2 mm/min, corresponding to a strain rate of 3 � 10 3 s 1. The tensile tests were performed along the rolling direction (RD), at 45� from the RD, and along the TD. The Lankford value (r-value) which is the plastic strain ratio of width to thickness, was measured on the tensile specimen deformed at a permanent strain of ~9%. After fracture, energy-dispersive X-ray spectrometry (EDS, JEOL7100F) analyses were applied on the regions with a distance of ~100 μm from the fracture surfaces for the Mg-1.5Zn–1Ca, Mg1.5Zn–1Sr, and Mg-1.5Zn–1Ca–1Sr alloys to reveal the influence of the intermetallic compound particles during the tensile deformation. A

3. Results and discussion 3.1. Rollability Fig. 1 shows the macroscopic morphology of the hot-rolled sheets with different Ca and Sr contents. Mg-1.5Zn alloy exhibited preferable rollability without edge cracking. After the addition of 1% Ca (Mg1.5Zn–1Ca alloy), obvious cracking occurred at both edges. Cracks became serious with increasing the Ca content to 2% (Mg-1.5Zn–2Ca alloy). This may be related to the increased fraction of the Ca-containing intermetallic compound particles with increasing Ca content. In case of addition of 1% Sr (Mg-1.5Zn–1Sr alloy), a good appearance without cracks at the edge was achieved. When even increasing the Sr content to 2% (Mg-1.5Zn–2Sr alloy), the length of the cracks was much smaller than that of the Mg-1.5Zn–1Ca alloy. In addition, almost no difference in the crack size and appearance was observed when adding 0.1% Sr into the Mg-1.5Zn–1Ca alloy. However, an obvious edge cracking occurred when adding only 0.1% Ca into the Mg-1.5Zn–1Sr alloy. In case of the combined addition of 1% Ca and 1% Sr, the crack length was smaller than that of Mg-1.5Zn–2Ca alloy but larger than that of Mg-1.5Zn–2Sr alloy. Thus, we can conclude that both Ca and Sr additions deteriorate the rollability, but it is more serious for the former case. 3.2. Microstructure and texture 3.2.1. Intermetallic compound Fig. 2 illustrates the typical morphology and distribution of the intermetallic compound particles. Few intermetallic compound particles can be confirmed in the Mg-1.5Zn alloy. After adding 1% Ca, spherical intermetallic compound particles with a light brown color were observed. By increasing the Ca content from 1% to 2%, the number of

Table 1 Compositions (in wt%) of Mg alloys analyzed by ICP spectroscopy. Zn Ca Sr

Mg-1.5Zn–1Ca

Mg-1.5Zn–2Ca

Mg-1.5Zn–1Sr

Mg-1.5Zn–2Sr

Mg-1.5Zn–1Ca-0.1Sr

Mg-1.5Zn–1Sr-0.1Ca

Mg-1.5Zn–1Ca–1Sr

1.33% 1.00% –

1.35% 2.00% –

1.31% – 0.81%

1.28% – 1.71%

1.35% 0.98% 0.16%

1.22% 0.10% 0.85%

1.32% 0.99% 0.83%

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Fig. 1. Macroscopic morphology of hot-rolled sheets with different Ca and Sr additions: (a) Mg-1.5Zn, (b) Mg-1.5Zn–1Ca, (c) Mg-1.5Zn–2Ca, (d) Mg-1.5Zn–1Sr, (e) Mg-1.5Zn–2Sr, (f) Mg-1.5Zn–1Ca-0.1Sr, (g) Mg-1.5Zn–1Sr-0.1Ca, (h) Mg-1.5Zn–1Ca–1Sr.

Fig. 2. Optical micrographs showing the presence of intermetallic compounds in hot-rolled sheets with different Ca and Sr additions: (a) Mg-1.5Zn, (b) Mg1.5Zn–1Ca, (c) Mg-1.5Zn–2Ca, (d) Mg-1.5Zn–1Sr, (e) Mg-1.5Zn–2Sr, (f) Mg-1.5Zn–1Ca-0.1Sr, (g) Mg-1.5Zn–1Sr-0.1Ca, (h) Mg-1.5Zn–1Ca–1Sr.

intermetallic compound particles obviously increased without a notable change in size. The 1% Sr addition introduced another type of inter­ metallic compound particles with a light gray color. The 2% Sr addition

increased the number of these particles. The ImageJ and OriginPro software were used for numerically un­ derstanding the influences of Ca and Sr additions based on the size

Table 2 Results of analyses on intermetallic compound particles in the annealed sheets. The Mg-1.5Zn–1Ca, Mg-1.5Zn–2Ca, Mg-1.5Zn–1Sr, Mg-1.5Zn–2Sr, Mg-1.5Zn–1Ca0.1Sr, Mg-1.5Zn–1Sr-0.1Ca, and Mg-1.5Zn–1Ca–1Sr alloys are denoted as 1Ca, 2Ca, 1Sr, 2Sr, 1Ca0.1Sr, 1Sr0.1Ca, and 1Ca1Sr, respectively. The Ca and Sr-containing intermetallic compound particles in the Mg-1.5Zn–1Ca–1Sr alloy are denoted as 1Ca1Sr(Ca) and 1Ca1Sr(Sr), respectively. Mean (μm) Variance Area (μm2) Area (%)

1Ca

2Ca

1Sr

2Sr

1Ca0.1Sr

1Sr0.1Ca

1Ca1Sr

1Ca1Sr(Ca)

1Ca1Sr(Sr)

1.91 0.76 441.69 4.33%

1.75 0.91 743.70 7.29%

1.99 0.85 352.44 3.46%

2.02 0.97 862.66 8.46%

1.77 0.83 311.40 3.05%

1.84 0.83 420.09 4.12%

1.81 0.64 603.00 5.91%

1.69 0.59 297.46 2.91%

1.94 0.71 306.01 3.00%

3

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distribution of the intermetallic compound particles. A selected area of 10200 μm2 was used for the calculation of each sample. The mean particle size, variance, total particle area, and area fraction of the par­ ticles are shown in Table 2 (Supplementary Fig. S4 illustrates the results of normal distribution fitting). For the 1% Ca-added alloy (i.e., Mg1.5Zn–1Ca), the mean size and area fraction of the particles were 1.91 μm and 4.33%, respectively. With increasing Ca content to 2%, the mean particle size slightly decreased from 1.91 μm to 1.75 μm, while the area fraction of the particles significantly increased from 4.33% to 7.29%. As confirmed in Fig. 2b,c and Supplementary Figs. S4(a and b), the number of particles with sizes smaller than 1 μm increased, and thus led to a slight decrease in the mean particle size with increasing Ca additive amount. In addition, a tiny difference in the mean particle size can be found between the 1% and 2% Sr additions. Regarding the Mg1.5Zn–1Ca-0.1Sr and Mg-1.5Zn–1Sr-0.1Ca alloys, a considerable in­ crease in the number of particles smaller than 1 μm was confirmed (compare Supplementary Figs. S4(a and c) with Figs. S4(e and f)), resulting in a slight decrease in the mean particle size. In addition, based on the difference in color, the mean sizes of the particles containing Ca and Sr were analyzed separately for the Mg-1.5Zn–1Ca–1Sr alloy. The Sr-containing intermetallic compound particles had a larger mean size compared to the Ca-containing intermetallic compound particles (1.94 μm vs 1.69 μm). The XRD analyses were conducted for confirming the type of inter­ metallic compounds. The obtained diffraction patterns are shown in Fig. 3. The Ca addition resulted in the formation of two types of inter­ metallic compound particles; i.e. Mg2Ca and Ca2Mg6Zn3 (see Fig. 3b and c). Mg2Ca exhibited multiple diffraction peaks with relatively higher intensities, while Ca2Mg6Zn3 only displayed one weak peak. This sug­ gests that Ca atoms mainly existed in the form of the former compound. Increasing the Ca addition from 1% to 2% remarkably strengthened the Mg2Ca peaks, but had no influence on the Ca2Mg6Zn3 peak. This in­ dicates that the increase in Ca from 1% to 2% only increased the number of the Mg2Ca particles. On the other hand, only Mg17Sr2 was confirmed in the case of the Sr addition (see Fig. 3d and e). In addition, no new type of compound, besides Mg2Ca and Mg17Sr2, could be confirmed in the Mg-1.5Zn–1Ca–1Sr alloy. It is interesting to note that the Ca2Mg6Zn3 peak cannot be detected in the Mg-1.5Zn–1Ca–1Sr alloy. This infers that even though no compound was formed between Zn and Sr, the Sr addition consumed a certain amount of Zn in another way, and in turn, suppressed the Ca2Mg6Zn3 formation. The results of EPMA element distribution mapping are shown in Fig. 4. The Zn element mapping in­ dicates that Zn rarely dissolved into the Mg2Ca particles in the Mg1.5Zn–2Ca alloy, but easily dissolved into the Mg17Sr2 particles in the Mg-1.5Zn–2Sr alloy. Therefore, in case of the combined addition of Ca and Sr, a preferential solid solution of Zn into Mg17Sr2 may occur for the Mg-1.5Zn–1Ca–1Sr alloy. This preferential solid solution may be the reason for the absence of Ca2Mg6Zn3 in the Mg-1.5Zn–1Ca–1Sr alloy. 3.2.2. Texture and grain structure It is well known that the (0001) crystal orientation distribution plays a vital role in determining the mechanical properties of Mg alloys because deformation is generally dominated by the basal slip due to the crystalline anisotropy of the hexagonal closed pack (hcp) structure [11,17–19]. Fig. 5 shows the (0002) pole figures of the alloys with different compositions. Both as-rolled and subsequently annealed (at 350 � C for 1 h) sheets were investigated to reveal the effects of Ca and Sr additions on the crystal orientation before and after static recrystallization. For the as-rolled sheets, the Mg-1.5Zn sheet showed a high (0002) texture intensity of 7.18 and a double-peak texture, which had the in­ tensity maxima at approximately �10� in the RD (RD-split texture) together with an orientation spread along the RD. The 1% Ca addition remarkably decreased the (0002) texture intensity from 7.18 to 3.86. In addition, the RD-split texture increased the tilting angle of the basal pole to approximately �20� and the orientation spread changed its direction

Fig. 3. XRD patterns of hot-rolled sheets with different Ca and Sr additions: (a) Mg-1.5Zn, (b) Mg-1.5Zn–1Ca, (c) Mg-1.5Zn–2Ca, (d) Mg-1.5Zn–1Sr, (e) Mg1.5Zn–2Sr, (f) Mg-1.5Zn–1Ca-0.1Sr, (g) Mg-1.5Zn–1Sr-0.1Ca, (h) Mg-1.5Zn–1Ca–1Sr.

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3.70. The (0002) pole figures of the as-rolled Mg-1.5Zn–1Ca-0.1Sr and Mg-1.5Zn–1Sr-0.1Ca alloys indicate that a minor addition of 0.1% Sr or 0.1% Ca exerts little influence on both the basal texture intensity and the orientation distribution after the hot rolling process. In addition, the asrolled Mg-1.5Zn–1Ca–1Sr sheet exhibited a similar orientation distri­ bution with the Ca or Sr-containing alloys, but had the lowest basal texture intensity of 3.27. After annealing at 350 � C for 1 h, different effects of the Ca and Sr additions on the tilting angle of the basal pole were observed. For Mg1.5Zn alloy, the strong basal texture further increased from 7.18 to 10.69 in intensity after annealing and the double-peak texture changed to the single-peak texture. Similar results have been reported, and this phenomenon may be related to dense basal dislocations in the sub­ structure before annealing introduced by the basal slip activation and the preferential growth of the recrystallized grains with basal ori­ entations [6–9,11]. In contrast, the weakening of the basal texture and the large inclination of the basal poles toward the TD can be observed for the annealed Mg-1.5Zn–1Ca and Mg-1.5Zn–2Ca alloys, which have basal texture intensities lower than 2.5. It has been reported that texture weakening may be induced by nonbasal slips [20], discontinuous static recrystallization (SRX) at a pre-existing grain boundary [21], twin-related recrystallization [22,23], particle-stimulated nucleation (PSN) [24], and dynamic recrystalliza­ tion (DRX) [25,26]. Transmission electron microscopy (TEM) observa­ tion revealed that both prismatic and slips occurred in the 1% tensile deformed Mg-1.5Zn-0.1Ca alloy [6]. In addition, a first-principles calculation revealed that the additions of Ca and Sr into the Mg–Zn alloy increased the ratio of the unstable stacking fault energies between the basal and prismatic slips to be close to 1, thus leading to a lowered plastic anisotropy [9,10]. It has been suggested that the accumulated non-basal dislocations in substructure contribute to the randomization of crystalline orientation for SRXed grains and the enhanced prismatic slip promotes the formation of the TD-split texture [6,23]. The Sr addition (see the Mg-1.5Zn–1Sr and Mg-1.5Zn–2Sr alloys in Fig. 3) has a similar effect on the weakening of the basal texture and can result in a low basal texture intensity of ~2.5 after annealing. However, the change in the location of the basal pole is quite different between the cases of the Ca and Sr additions. After annealing, the Ca-containing alloys exhibited a change in the tilting direction of the basal pole from the RD to the TD, while the RD-split texture persisted with an increased tilting angle from ~15� to ~25� for the Sr-containing alloys. The inverse pole figure (IPF) and the kernel average misorientation (KAM) maps of Mg-1.5Zn, Mg-1.5Zn–2Ca, and Mg-1.5Zn–2Sr alloys in the as-rolled condition which were investigated by EBSD, are presented in Supplementary Fig. S5. All alloys exhibited a deformed microstruc­ ture containing some deformation twins, as shown in the IPF maps. The KAM maps also suggested the existence of highly accumulated disloca­ tions. Therefore, it can be considered that SRX may occur due to the high stored energy and exert a strong influence on texture change during annealing. In addition, it is known that texture evolution during annealing is affected by grain growth after the occurrence of SRX [24]. Further investigations on the SRX and grain growth behaviors are needed for elucidating the reason for the different texture formations between the Ca and Sr additions into the Mg–Zn alloy. In case of a minor addition of 0.1% Sr, there was no notable differ­ ence in the texture change between the Mg-1.5Zn–1Ca-0.1Sr and Mg1.5Zn–1Ca alloy after annealing. In contrast, a minor addition of 0.1% Ca into the Mg-1.5Zn–1Sr alloy induced the formation of the TD-split texture. This suggests that solute Ca atoms have a major effect on the texture evolution during annealing. The combined addition of 1% Ca and 1% Sr (i.e. Mg-1.5Zn–1Ca–1Sr alloy) exhibited the lowest basal texture intensity of 2.04. In addition, the annealing texture exhibited a ring-like distribution of intensity maxima, which is a different feature from the other alloys. As mentioned above, the Ca addition exhibited the effect of tilting the basal pole to the TD after annealing, while the Sr addition has the effect in maintaining the tilted basal pole in the RD.

Fig. 4. EPMA element mapping of Mg, Ca, Sr, and Zn in Mg-1.5Zn–2Ca, Mg1.5Zn–2Sr and Mg-1.5Zn–1Ca–1Sr alloys.

Fig. 5. (0002) pole figures for hot-rolled sheets in as-rolled and annealed (at 350 � C for 1 h) conditions. Maximum intensities are shown below the (0002) pole figures.

to be along the TD. When the Ca addition was increased from 1% to 2%, the (0002) texture intensity slightly decreased from 3.86 to 3.57, but the orientation distribution remained unchanged. For the 1% Sr addition in Mg-1.5Zn alloy, the (0002) texture intensity also considerably decreased from 7.18 to 4.01. Further increasing the Sr addition from 1% to 2% only resulted in a slight decrease in the basal texture intensity from 4.01 to 5

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Therefore, at least three types of recrystallization processes can be ex­ pected for the Mg-1.5Zn–1Ca–1Sr alloy during annealing: (1) recrys­ tallized grains tilting toward the TD due to the effect of Ca addition, (2) recrystallized grains with maintained RD-tilted orientations due to the effect of Sr addition, (3) recrystallized grains with random orientations induced by the PSN mechanism due to the high fraction of secondaryphase particles. It is suggested that the combined influences from the Ca and Sr additions resulted in the formation of a ring-like texture with the lowest basal texture intensity in the Mg-1.5Zn–1Ca–1Sr alloy. EBSD analyses were conducted for the annealed samples. The IPF maps of the annealed samples are shown in Fig. 6. As a summary of the EBSD results, the (0001) texture intensity, grain size, Schmid factors for the basal slip parallel to the RD, 45� , and TD are shown in Table 3. Grain coarsening was evident in the Mg-1.5Zn alloy after annealing because the effect of particle pinning on the grain boundary movement could not be expected due to almost no particle (Fig. 2a). The Ca addition remarkably refined the grain size from 56.1 μm to ~9 μm. No noticeable difference in the grain size between the Mg-1.5Zn–1Ca alloy and Mg1.5Zn–2Ca alloy was observed. This suggests that the dominant effect on the grain refinement originates from the soluted Ca atoms rather than the Mg2Ca particles. The Sr addition also had a grain refinement effect and 1% Sr addition could reduce the mean grain size from 56.1 μm to 18.5 μm. As the Sr content was increased from 1% to 2%, the grain size further decreased from 18.5 μm (Mg-1.5Zn–1Sr alloy) to 14.9 μm (Mg1.5Zn–2Sr alloy). However, the degree of grain refinement was still smaller compared to that induced by the Ca addition. A minor addition of 0.1% Sr into the Mg-1.5Zn–1Ca alloys exhibited almost no influence on the grain size. In contrast, a minor addition of 0.1% Ca into the Mg1.5Zn–1Sr alloy resulted in a further decrease in the grain size from 18.5 μm to 15.4 μm, probably due to the effect of the soluted Ca atoms. In case of the combined addition of 1% Ca and 1% Sr, the grain size was similar to those (~9 μm) of the Mg-1.5Zn–1Ca and Mg-1.5Zn–2Ca al­ loys. It can be considered that the following two main reasons resulted in the significant grain refinement effect from Ca addition. First, the Ca and Zn atoms with a positive misfit and a negative misfit when substituting a Mg atom, may co-segregate at grain boundaries and strongly suppress their migration of grain boundaries due to the dragging effect [11]. Second, as summarized in Supplementary Fig. S4, Ca addition can introduce smaller particles with a sub-micron size and thus an increased number of particles compared to the case of Sr addition. Those particles

may also restrict the grain boundary movement by particle pinning during the grain growth stage and thus, result in a smaller grain size. 3.3. Mechanical property 3.3.1. Tensile properties The results of the tensile tests in the RD, 45� , and TD of the annealed sheets are shown in Table. 4 (a graphical summarization of the tensile test results is provided in Supplementary Fig. S6). The yield strength (YS), ultimate tensile strength (UTS), fracture elongation (FE), strengthening hardening rate (n-value) and r-value are presented in this table. The Ca and Sr additions improved the YS along all directions regardless of the much weaker textures. The 1% Ca addition into the Mg1.5Zn alloy increased the YS along RD, 45� , and TD by 64%, 31%, and 8.7%, respectively. This increase is attributed to the grain refinement with a decrease in the grain size from 56.1 μm to 8.7 μm due to the HallPetch relation for Mg alloys [27–30]. No apparent difference in the YS along the RD, 45� , and TD between the Mg-1.5Zn–1Ca and Mg-1.5Zn–2Ca alloys was observed because of their similar grain sizes and textures. For the Sr addition, the YS increased with the Sr content. For the Mg-1.5Zn–2Sr alloy, the YS values along the RD, 45� and TD were improved by 33%, 17% and 2%, respectively, compared to the Mg-1.5Zn alloy. Even though 2% Sr was added into the Mg-1.5Zn alloy, the YS along all directions was still lower than those of the Mg-1.5Zn–1Ca alloy. This can be explained by the larger grain size (14.9 μm) of the Mg-1.5Zn–2Sr alloy compared to that (8.7 μm) of the Mg-1.5Zn–1Ca alloy (Table 3). A minor addition of 0.1% Sr into the Mg-1.5Zn–1Ca alloy did not improve the YS apparently due to no sig­ nificant change in the grain size. The YS of the Mg-1.5Zn–1Ca–1Sr alloy was roughly equivalent to those of the Mg-1.5Zn–1Ca and Mg-1.5Zn–2Ca alloys due to their similar grain sizes. However, the difference among the YS values along the RD, 45� , and TD cannot be explained only by grain refinement based on the HallPetch relation. All specimens showed equiaxed grains after annealing. This implies that the difference in YS among the RD, 45� , and TD did not originate from the grain morphology. Fig. 7a shows the linear fitting between the inverse square root of the grain size and the YS along the RD, 45� , and TD. The three fitting lines represent the Hall-Petch relation, only considering the grain size and the YS. The slope is highest for the

Fig. 6. Inverse pole figure maps of annealed sheets: (a) Mg-1.5Zn, (b) Mg-1.5Zn–1Ca, (c) Mg-1.5Zn–2Ca, (d) Mg-1.5Zn–1Sr, (e) Mg-1.5Zn–2Sr, (f) Mg-1.5Zn–1Ca0.1Sr, (g) Mg-1.5Zn–1Sr-0.1Ca, (h) Mg-1.5Zn–1Ca–1Sr. 6

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Table 3 Summary of the results analyzed by EBSD on the annealed sheets. Mg-1.5Zn Mg-1.5Zn–1Ca Mg-1.5Zn–2Ca Mg-1.5Zn–1Sr Mg-1.5Zn–2Sr Mg-1.5Zn–1Ca-0.1Sr Mg-1.5Zn–1Sr-0.1Ca Mg-1.5Zn–1Sr–1Ca

0001 intensity

Grain size (μm)

Basal slipSchmid factor//RD

Basal slipSchmid factor//45�

Basal slipSchmid factor//TD

12.6 3.3 3.4 3.6 3.7 3.4 3 2.8

56.1 8.7 9.6 18.5 14.9 8.9 15.4 8.9

0.246 0.244 0.251 0.271 0.276 0.244 0.259 0.256

0.215 0.314 0.311 0.298 0.296 0.314 0.309 0.311

0.171 0.311 0.306 0.304 0.294 0.308 0.313 0.301

Fig. 7. Linear fitting (a) between yield strength and inverse square root of grain size, and (b) between critical resolved shear stress and inverse square root of grain size. The standard deviations of linear fitting in (a) are 32.69 (RD), 19.42 (45� ) and 16.11 (TD). The standard deviations of linear fitting in (b) are 5.40 (RD), 6.63 (45� ) and 5.28 (TD).

RD and lowest for the TD. It is commonly accepted that the mechanical properties of materials with a hcp structure are closely related to the crystal orientation because the basal slip dominates deformation at room temperature. To take the texture effect into account, the YS was multiplied by the Schmid factors for basal slip parallel to the RD, 45� , and TD, which were obtained by the EBSD analyses, for calculating the critical resolved shear stress (CRSS). Fig. 7b shows the relationship be­ tween the CRSS and the grain size. The fitting lines of the RD, 45� , and TD showed the similar intercepts and slopes regardless of the different chemical compositions and the numbers of intermetallic compound particles. This indicates that the YS of the hot-rolled Mg-1.5Zn-X (X: Ca and/or Sr) alloys with different Ca and Sr contents was mainly deter­ mined by both the grain size and the crystal orientation distribution. The change in the UTS caused by Ca and Sr additions had the same tendency as that in the YS. Nevertheless, the anisotropy in the UTS along the RD, 45� , and TD was smaller than that in YS. For example, regarding the Mg-1.5Zn–1Ca alloy, the UTS along the TD was only 11 MPa lower than that of the RD, while it is 42 MPa lower for the YS along the TD than that along the RD. This was related to the higher n-value (0.348 vs 0.209) along the TD compared to that along the RD. The Ca and Sr additions into the Mg-1.5Zn alloys resulted in a remarkable improvement in the FE along all directions for the hot-rolled sheets. For example, the Mg-1.5Zn–1Ca alloy exhibited 25.9%, 34.1%, and 32.5% improvement for the FE along the RD, 45� , and TD, respec­ tively. These FEs are equivalent to 1.42, 1.74, and 1.72 times compared to that of the Mg-1.5Zn alloy along the RD, 45� , and TD, respectively. This indicates that the texture randomization significantly contributes to the FE improvement during the tensile tests. The improvement of FE induced by the texture randomization is commonly observed in Mg al­ loys, which can be achieved by specific element additions [6–10,18], and specific processes such as high-temperature rolling [4,21,25,26],

differential speed extrusion [31] and double-sided friction stir welding [27,29,32]. However, the FE deteriorated as the additive amount of Ca or Sr increased from 1% to 2%. The FEs of the Mg-1.5Zn–2Ca alloy were only equivalent to 0.84, 0.78, and 0.80 times compared to those of the Mg-1.5Zn–1Ca alloy along RD, 45� , and TD, respectively. The Sr-containing alloys exhibit the same tendency. The FEs of Mg-1.5Zn–2Sr are equivalent to 0.72, 0.91, and 0.80 times compared with those of the Mg-1.5Zn–1Sr alloy along the RD, 45� , and TD, respectively. To reveal the reason for the FE deterioration of, EDS analyses were applied in the regions close to the fracture surfaces of the tensiledeformed specimens. Fig. 8 shows the SEM images and the EDS element mapping of Mg, Zn, Ca, and Sr for the Mg-1.5Zn–2Ca alloy (Fig. 8a) and the Mg-1.5Zn–2Sr alloy (Fig. 8b). The cracking and cavi­ tation of the intermetallic compound particles were observed for both

Fig. 8. SEM images and EDS element mapping of Mg, Zn, Ca and Sr for (a) Mg1.5Zn–2Ca and (b) Mg-1.5Zn–2Sr after tensile tests. 7

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Materials Science & Engineering A 769 (2020) 138474

Mg2Ca in the Mg-1.5Zn–2Ca alloy and Mg17Sr2 in the Mg-1.5Zn–2Sr alloy (indicated by white arrows in Fig. 8). Most plate-like cavities caused by the cracking of the brittle intermetallic compound particles were perpendicular to the tensile direction. The plate-like cavities may increase in width with the progress of tensile deformation. The cracking and cavitation of particles caused a local stress concentration and the deterioration of the FE during the tensile deformation. Moreover, numerous micron-sized small particles acted as obstacles to restrict the movement of dislocations during the tensile deformation, resulting in the deterioration of the FE. Fig. 9 shows the SEM images and the EDS element mapping of Mg, Zn, Ca, and Sr for the Mg-1.5Zn–1Ca–1Sr alloy. The cracking and cavitation of the intermetallic compound particles was more likely to occur in Mg17Sr2. The Vickers micro-hardness of Mg2Ca in the Mg-1.5Zn–2Ca alloy and that of Mg17Sr2 in the Mg-1.5Zn–2Sr alloy was measured to be 274 � 59 H V and 253 � 49 H V, respectively. In addition, the particle size of Mg17Sr2 was larger than that of Mg2Ca in the Mg-1.5Zn–1Ca–1Sr alloy (1.94 μm vs 1.69 μm). The more serious cracking and cavitation in Mg17Sr2 may be due to its lower hardness and larger particle size compared to Mg2Ca. Consequently, it is reasonable that Mg-1.5Zn–2Sr exhibits the lowest FE along all tensile directions among the Ca and/or Sr added alloys because of its largest mean par­ ticles size and the highest particle area fraction (see Table 2). In addi­ tion, the Mg-1.5Zn–2Ca, Mg-1.5Zn–2Sr and Mg-1.5Zn–1Ca–1Sr alloys with high particle area fractions exhibited relatively small n-values (see Table 4). This can be attributed to the more frequently occurred particle cracking, which induces local softening [33].

initiation sites during deformation. To reveal the relation between the stretch formability and the FE, the mean FE (defined as (FERDþ2 � FE45� þFETD)/4), and the Erichsen values in this study together with other reported data of hot-rolled Mg alloy sheets with different chemical compositions [4,6–9,21,25,26, 35–39] are plotted in Fig. 11. Except for the WE43 alloy [39], most of the alloys from the literature contain very few intermetallic compound particles. Based on the data in the literature, there is a quite large scattering from ~4 mm to ~9 mm for the Erichsen value at the FE of 26%–28% and the Erichsen value cannot be described to have a linear relation with the FE. This indicates that the stretch formability is very strongly affected by the texture rather than the FE. This is due to the different stress states between the tensile and Erichsen tests, i.e. a uni­ axial stress and a biaxial stress for the former and the latter, respectively [4]. For the Mg–Zn-X (X: Ca or Sr) alloys with similar weak basal tex­ tures in this study, the Erichsen value roughly exhibited a linear relation with the FE, as indicated by the red line. The deterioration of the stretch formability can be mainly attributed to brittle Mg2Ca and Mg17Sr2 particles, which tend to crack under tensile stress, and in turn, decrease both the Erichsen value and the FE. On the other hand, the Mg-1.5Zn alloy did not match the fitted red line and exhibited a lower Erichsen value, even though almost no intermetallic compound particles existed. This originates from its quite strong basal texture, which significantly lowers the sheet-thinning capability. 3.4. Ignition temperature

3.3.2. Stretch formability The specimens after the Erichsen tests at room temperature for the hot-rolled and subsequently annealed Mg–Zn and Mg–Zn-X (X: Ca and/ or Sr) alloys are shown in Fig. 10. The index Erichsen values (IE) were 3.8 mm for the Mg-1.5Zn alloy, 8.0 mm for the Mg-1.5Zn–1Ca alloy, 5.7 mm for the Mg-1.5Zn–2Ca alloy, 7.1 mm for the Mg-1.5Zn–1Sr alloy, 6.0 mm for the Mg-1.5Zn–2Sr alloy, 7.0 mm for the Mg-1.5Zn–1Ca-0.1Sr alloy, 7.2 mm for the Mg-1.5Zn–1Sr-0.1Ca alloy, and 6.1 mm for the Mg1.5Zn–1Ca–1Sr alloy. The Erichsen value of the hot-rolled Mg-1.5Zn alloy sheet was considerably improved by the Ca and Sr additions. When 1% Ca was added into the Mg-1.5Zn alloy, the Erichsen value signifi­ cantly increased from 3.8 mm to 8.0 mm, which is comparable to those of commercial Al alloys [34]. This resulted from the weakened basal texture and the large splitting of the basal pole in the TD [6]. Benefitting from the significant basal texture weakening, the 1% Sr addition also remarkably increased the Erichsen value of the Mg-1.5Zn alloy from 3.8 mm to 7.0 mm. However, further increasing the additive amount of Ca and Sr to 2% resulted in the decreased Erichsen values of 5.7 mm and 6.0 mm, respectively. In addition, despite of the lowest basal texture intensity, the Mg-1.5Zn–1Ca–1Sr alloy exhibited a relatively low Erichsen value of 6.1 mm. It is well known that the stretch formability is sensitive to the r-value, and a smaller r-value is beneficial for the sheet thinning for Mg alloys [35]. However, there was no distinct difference in the r-value between the alloys with the additive amounts of 1% and 2% because these alloys had similar texture intensities and the r-value was closely related to the texture. Therefore, the deterioration of the stretch formability for alloys with high additive amounts can be mainly attributed to the increased number of particles, which may act as crack

Besides poor formability, another factor restricting the industrial applications of Mg alloys is their low ignition temperatures. The results of the ignition temperature measurements are shown in Fig. 12. Ap­ pendix Videos 1–6 illustrate the synchronized data obtained from the measured time-temperature curves and the recorded videos for the Mg1.5Zn, Mg-1.5Zn–1Ca, Mg-1.5Zn–2Ca, Mg-1.5Zn–1Sr, Mg-1.5Zn–2Sr, and Mg-1.5Zn–1Ca–1Sr alloys. The videos were played five times faster than the recorded ones, and the occurrence of ignition was confirmed by both the time-temperature curve and the sparking in the recorded video. In the case without the Ca or Sr addition, the Mg-1.5Zn alloy exhibited a low ignition temperature of 629 � C. The ignition temperature consid­ erably increased to 758 � C when 1% Ca was added into the Mg-1.5Zn alloy. The ignition temperature further increased to 800 � C with increasing the Ca content to 2%. On the other hand, the 1% Sr addition into the Mg-1.5Zn alloy did not affect the ignition temperature appar­ ently. This suggests that only 1% Sr is insufficient to form a fully pro­ tective oxidized SrO layer on the sample surface. When the Sr content was increased to 2%, the ignition temperature increased to 720 � C. However, this temperature is still lower than that (758 � C) of 1% Ca addition, indicating a better effect on the improvement in the flameretardant property from the Ca addition compared to the Sr addition. In addition, regardless of a higher total additive amount, the Mg1.5Zn–1Ca–1Sr alloy exhibited a slightly lower ignition temperature of 715 � C than that the Mg-1.5Zn–1Ca alloy (758 � C). This implies that the combined addition of 1% Ca and 1% Sr is likely to deteriorate the improvement in the flame-retardant property. The reason for this deterioration from the mutual effect of the Ca and Sr additions needs further investigation.

Fig. 9. SEM images and EDS element mapping of Mg, Zn, Ca and Sr for Mg-1.5Zn–1Ca–1Sr after tensile test. 8

M. Zhou et al.

Materials Science & Engineering A 769 (2020) 138474

Table 4 Results of the tensile tests in the RD, 45� , and TD directions of the annealed sheets. YS (MPa) Mg-1.5Zn Mg-1.5Zn–1Ca Mg-1.5Zn–2Ca Mg-1.5Zn–1Sr Mg-1.5Zn–2Sr Mg-1.5Zn–1Ca-0.1Sr Mg-1.5Zn–1Sr-0.1Ca Mg-1.5Zn–1Ca–1Sr

UTS (MPa)

RD

45

106 190 190 127 144 193 149 196

121 156 163 111 132 156 124 162

�

FE (%)

TD

RD

45

134 148 153 110 131 150 118 157

209 254 255 218 223 252 230 251

209 240 243 213 218 237 221 235

�

n-value

r-value

TD

RD

45

TD

RD

45

217 243 244 215 221 240 221 240

18.0 25.9 21.7 26.9 19.5 21.6 26.3 22.4

18.9 34.1 26.5 24.2 22.1 29.7 29.4 23.0

17.5 32.5 26.0 31.1 25.0 30.8 33.2 26.6

0.219 0.209 0.191 0.231 0.197 0.195 0.207 0.179

0.200 0.287 0.254 0.285 0.226 0.281 0.283 0.246

�

�

TD

RD

45�

TD

0.158 0.348 0.316 0.328 0.285 0.331 0.357 0.303

1.54 0.82 0.90 0.97 0.92 0.92 0.85 0.73

1.79 1.19 1.08 1.04 1.03 1.13 0.92 1.07

2.68 0.72 0.76 0.80 0.83 0.73 0.68 0.67

Fig. 11. Linear fitting between the IE value and FEmean of Mg-1.5Zn-X (X: Ca and/or Sr) alloys in this work and other previously reported Mg alloys.

Fig. 10. Appearances and Erichsen values (IE, mm) of annealed sheets after the Erichsen tests: (a) Mg-1.5Zn, (b) Mg-1.5Zn–1Ca, (c) Mg-1.5Zn–2Ca, (d) Mg1.5Zn–1Sr, (e) Mg-1.5Zn–2Sr, (f) Mg-1.5Zn–1Ca-0.1Sr, (g) Mg-1.5Zn–1Sr0.1Ca, (h) Mg-1.5Zn–1Ca–1Sr.

Supplementary video related to this article can be found at htt ps://doi.org/10.1016/j.msea.2019.138474. 4. Conclusions The effects of Ca and Sr additions on the microstructure, mechanical properties and ignition temperature of the hot-rolled Mg-1.5Zn alloy sheet were investigated. The following conclusions were obtained: (1) The Ca and Sr additions resulted in the formation of Mg2Ca and Mg17Sr2 intermetallic compounds, which exhibited the Vickers micro-hardnesses of 274 H V and 253 H V, respectively. Increasing the additive amount from 1% to 2% caused an increase in the number of intermetallic compound particles rather than the particle size. (2) Even though both Ca and Sr additions could remarkably refine the grain structure, the Ca addition exerted a stronger effect on grain refinement than the Sr addition.

Fig. 12. Time-temperature curves during ignition tests for hot-rolled sheets with different Ca and Sr additions: Mg-1.5Zn, Mg-1.5Zn–1Ca, Mg-1.5Zn–2Ca, Mg-1.5Zn–1Sr, Mg-1.5Zn–2Sr, and Mg-1.5Zn–1Ca–1Sr. Ignition temperatures are labelled near the respective curves.

(3) Both Ca and Sr additions could significantly weakened the basal texture for the hot-rolled and annealed sheets. After annealing, the change from the RD-split texture to the TD-split texture occurred for the Ca-containing alloys, while the RD-split texture tended to be maintained for the Sr-containing alloys. 9

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Materials Science & Engineering A 769 (2020) 138474

(4) Regardless of the influence of texture softening, the Ca and Sr additions improved the mechanical strength considerably due to the refined grain structure. Moreover, benefiting from the weakened textures, 1% Ca and Sr additions remarkably increased the Erichsen value from 3.8 to 8.0 and 7.1, respectively. How­ ever, a further addition to 2% deteriorated both the tensile ductility and the stretch formability due to the cracking of secondary-phase particles. (5) The 2% Ca and Sr additions increased the ignition temperature from 629 � C to 800 � C and 720 � C, respectively, indicating a better effect on enhancing the flame-retardant property by the Ca addition compared to the Sr addition. The Mg-1.5Zn–1Ca alloy exhibited a combination of a flame-retardant property (ignition temperature: 756 � C) and superior stretch formability (Erichsen value: 8.0).

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Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Acknowledgments This study was partially supported by JSPS KAKENHI Grant Number JP15K06518. Mengran Zhou would like to appreciate the Interactive Materials Science Cadet Program from Osaka University and the Min­ istry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.msea.2019.138474. References [1] B.L. Mordike, E. Tü, Magnesium: properties—applications—potential, Mater. Sci. Eng. A 302 (2001) 37–45. [2] T. Al-Samman, G. Gottstein, Room temperature formability of a magnesium AZ31 alloy: examining the role of texture on the deformation mechanisms, Mater. Sci. Eng. A 488 (2008) 406–414. [3] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Enhanced formability at elevated temperature of a cross-rolled magnesium alloy sheet, Mater. Sci. Eng. A 441 (2006) 349–356. [4] X.S. Huang, K. Suzuki, N. Saito, Textures and stretch formability of Mg–6Al–1Zn magnesium alloy sheets rolled at high temperatures up to 793 K, Scr. Mater. 60 (2009) 651–654. [5] C.E. Dreyer, W.V. Chiu, R.H. Wagoner, S.R. Agnew, Formability of a more randomly textured magnesium alloy sheet: application of an improved warm sheet formability test, J. Mater. Process. Technol. 210 (2010) 37–47. [6] Y. Chino, T. Ueda, Y. Otomatsu, K. Sassa, X.S. Huang, K. Suzuki, M. Mabuchi, Effects of Ca on tensile properties and stretch formability at room temperature in Mg-Zn and Mg-Al alloys, Mater. Trans. 52 (2011) 1477–1482. [7] Y. Chino, M. Kado, M. Mabuchi, Enhancement of tensile ductility and stretch formability of magnesium by addition of 0.2 wt%(0.035 at%) Ce, Mater. Sci. Eng. A 494 (2008) 343–349. [8] Y. Chino, K. Sassa, M. Mabuchi, Texture and stretch formability of a rolled Mg–Zn alloy containing dilute content of Y, Mater. Sci. Eng. A 513 (2009) 394–400. [9] M. Yuasa, N. Miyazawa, M. Hayashi, M. Mabuchi, Y. Chino, Effects of group II elements on the cold stretch formability of Mg–Zn alloys, Acta Mater. 83 (2015) 294–303. [10] M. Yuasa, M. Hayashi, M. Mabuchi, Y. Chino, Improved plastic anisotropy of Mg–Zn–Ca alloys exhibiting high-stretch formability: a first-principles study, Acta Mater. 65 (2014) 207–214. [11] Z.R. Zeng, Y.M. Zhu, S.W. Xu, M.Z. Bian, C.H.J. Davies, N. Birbilis, J.F. Nie, Texture evolution during static recrystallization of cold-rolled magnesium alloys, Acta Mater. 105 (2016) 479–494.

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