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Effects of phase composition and content on the microstructures and mechanical properties of high

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Materials and Design 88 (2015) 915–923

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Materials and Design
journal homepage: www.elsevier.com/locate/jmad

Effects of phase composition and content on the microstructures and mechanical properties of high strength Mg–Y–Zn–Zr alloys
Zhiqiang Zhang ?, Xuan Liu, Zhankun Wang, Qichi Le, Wenyi Hu, Lei Bao, Jianzhong Cui
Key Lab of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, 314 Mailbox, Shenyang 110819, People's Republic of China

a r t i c l e

i n f o

a b s t r a c t
In this work, the microstructures and mechanical properties of nine different Mg–Y–Zn–Zr alloys have been investigated. The investigated Mg–Y–Zn alloys mainly contain four types: W-phase, W ? + X-phase and Xphase + Mg24Y5 alloys, which should be determined by both the Y/Zn mole ratios and the total alloying content. The phase compositions and content play a fatal role on the mechanical properties of the as-extruded Mg–Y–Zn– Zr alloys. However, the Mg–Y–Zn–Zr alloys received almost no aging hardening response, except for the alloy containing X-phase and Mg24Y5. This is because a supersaturated matrix was not available to these alloys containing only the stable ternary X- or W-phase. Nevertheless, the as-extruded Mg–12Y–5Zn–0.6Zr alloy could still own extraordinary high strength, whose ultimate tensile strength and yielding tensile strength were 429 MPa and 351 MPa, respectively. It should be attributed to the X-phase strengthening effects. After peak-aging, the Mg–12Y–3Zn–0.6Zr alloy established an ultimate tensile strength and yielding tensile strength of 440 MPa and 350 MPa, respectively. The effects of phase compositions and content on mechanical properties have been discussed in detail. ? 2015 Elsevier Ltd. All rights reserved.

Article history: Received 27 May 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Available online 18 September 2015 Keywords: Mg–Y–Zn–Zr alloys Phase compositions Phase content Microstructures Mechanical properties

1. Introduction During the past decade, the development of high strength and lightweight magnesium alloys has drawn much attention in a wide range of ?elds such as the automobile industry [1–3]. However, there are limited magnesium alloys for designers to select from for speci?c applications, and within these limited choices, the most cost-effective magnesium alloys have inadequate properties such as yield strength, creepresistance, formability, and corrosion resistance [4]. Many researches have been developed for the approach to improving the strength of Mg alloys. Magnesium alloys with RE (rare earth) addition, were known for the excellent performance at both the elevated temperature and the room temperature. For instance, it was demonstrated recently that an appreciably high 0.2% proof strength of 445 MPa could be achieved in a Mg–14Gd–0.5Zr (wt.%) alloy produced by the combined processes of hot extrusion, cold work and aging [5]. However, the present magnesium industry is not ready to manufacture these expensive alloys due to the overmuch RE additions. Fortunately, the recent Mg–Y–Zn wrought alloys have drawn much attention due to their excellent performance and relative low-cost. Based on many reports about Mg–Y–Zn and Mg–Y–Zn–Zr system alloys [6–10], it was thought that the secondary phases mainly included icosahedral quasicrystal I-phase (Mg3Zn6Y), long period stacking ordered X-phase (Mg12YZn) and cubic W-phase (Mg3Y2Zn3). So far,
? Corresponding author. E-mail address: zqzhang@mail.neu.edu.cn (Z. Zhang).

the atomistic structures of the I-phase, W-phase and X-phase (LPSO) have been widely examined and reported in overwhelming kinds of ternary Mg–Y–Zn alloys [7,11–14]. Meanwhile, it has been widely examined that the phase composition should depend on the Y/Zn mole ratios [15]. It was the unique second phase that insured the excellent performance of Mg–Y–Zn alloys. Depending on the volume fraction of the I-phase, Mg– Y–Zn–Zr alloys could have a yield stress ranging from 150 to 450 MPa at room temperature [16]. Similarly, the X-phase was also bene?cial to toughening the alloy [17,18]. However, the cubic W-phase would degrade the mechanical properties of the Mg–Y–Zn–Zr alloys [19,20]. Thus, these three ternary phases established different effects on the mechanical properties of the alloys. It is necessary to clarify the possible effects of these Mg–Y–Zn phases and their content on the strength of the wrought Mg–Y–Zn alloys. It has been well demonstrated for the corporate effects of I- and W-phase on the microstructures and mechanical properties [8]. However, it is still not widely reported for the corporate effects of W- and X-phase on the microstructures and mechanical properties of Mg–Y–Zn–Zr alloys, especially in both high Zn and Y alloying additions. This work has designed a series of Mg–Y–Zn–Zr alloys with both high Zn and Y alloying additions, in order to investigate the effects of phase compositions and content on the microstructures and mechanical properties of the Mg–Y–Zn–Zr alloys. 2. Experimental The nine Mg–Y–Zn–Zr alloys were prepared by high-purity Mg (99.9%), pure Zn (99.9%), Mg–50Y and Mg–33Zr (wt.%) master alloys

http://dx.doi.org/10.1016/j.matdes.2015.09.087 0264-1275/? 2015 Elsevier Ltd. All rights reserved.

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by direct electrical resistance melting in a mild steel crucible at 710 °C. The melt were homogenized by mechanical stirring at 200 rpm for 3 min before casting. Under the cover gas of 0.5%SF6 + CO2, the melt were casting into a preheated steel mold (200 ± 10 °C) after 10 min of stewing. The chemical compositions of the prepared alloys are listed in Table 1. The casting ingots were 60 mm in diameter and 120 mm in height. After casting process, the ingots were homogenized in air and at 510 °C for 16 h. Then the ingots were machined into Φ47 mm cylinders with a length of 100 mm, and were indirectly extruded into Φ12 mm rods with a speed of 5.6 cm s? 1 in a Φ50 mm extruding container at 400 °C. Meanwhile, the extrusion ratio was 15:1, and the container and mold were preheated to 350 °C. The rods were machined into tensile specimens of 6 mm gauge diameter and 25 mm gauge length according to the ASTM standard B557M-10. The tensile direction was parallel to the extrusion direction. Tensile test was performed in triplicate at room temperature by using a Shimadzu AG-X (10 kN) machine (the speed of tensile was 1 mm/min). The as-cast and as-extruded samples were mechanically polished and etched with a 4%nital and an ethanol solution of picric acid and glacial acetic acid (2.0 g picric acid, 5 ml glacial acetic acid, 5 ml water and 25 ml ethanol), respectively. Microstructures of the alloys were observed by optical microscope (OM), transmission electron microscope (TEM) and scanning electron microscope (SEM) coupled with an energy dispersive X-ray analyzer (EDS). The phase analyses were performed with an X-ray diffractometer (XRD), and the scanning angle were from 15 to 75° with a speed of 8°/min. The XRD patterns were indexed using PDF standard card (2004). Volume fraction of second phase was estimated by image analysis technique using at least ten areas for each alloy. Vickers hardness test was carried out by 3 kg load for 15 s. Seven indentations have been made for the average hardness values. 3. Results and discussion 3.1. Microstructure observations Fig. 1 shows the XRD patterns of as-cast and as-extruded alloys. The nine investigated alloys could be mainly composed of α-Mg solid solution, X-phase (Mg12YZn), W-phase (Mg3Y2Zn3) and even minor Mg24Y5 phase. Meanwhile, the phase compositions do not change greatly after the alloys were hot extruded at 400 °C. It manifests the thermalstability of these ternary Mg–Y–Zn phases. Moreover, the phase compositions are deeply dependent on the Y/Zn mole ratios. It could be seen that the second phase constituent trend to transform from W-phase to X-phase as the Y/Zn mole ratio increases. The W-phase comes to disappear once the Y/Zn ratio is over 1.41. Table 2 list the Y/Zn mole ratios of the investigated alloys and the theoretic phase compositions according to the Ref. [6].The XRD results show a primary agreement with the previous work besides some exceptions. One is that the WZ067 alloy should contain α-Mg and single W-phase, as shown in Fig. 1a and b. The other one should be that Mg24Y5 phase comes to precipitate in the as-cast alloy when the Y/Zn mole ratio is about 3. These contradictions will be discussed later.
Table 1 The chemical compositions of the investigated alloys. Alloy Nominal composition Actual composition (wt.%) Y WZ127 WZ097 WZ067 WZ125 WZ095 WZ065 WZ123 WZ093 WZ063 Mg–12Y–7Zn–0.6Zr Mg–9Y–7Zn–0.6Zr Mg–6Y–7Zn–0.6Zr Mg–12Y–5Zn–0.6Zr Mg–9Y–5Zn–0.6Zr Mg–6Y–5Zn–0.6Zr Mg–12Y–3Zn–0.6Zr Mg–9Y–3Zn–0.6Zr Mg–6Y–3Zn–0.6Zr 12.1 8.91 5.73 12.7 8.99 5.68 11.8 8.62 5.74 Zn 7.04 6.39 6.89 5.70 5.10 4.77 2.81 3.12 3.02 Zr 0.355 0.459 0.585 0.378 0.498 0.566 0.453 0.523 0.558 Mg Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.

Fig. 2 shows the optical microstructures of the as-cast alloys. Broadly speaking, most of the as-cast alloys mainly consist of different volume fractions of the similar second phase except for the WZ067 alloy. The similar gray plate-like second phase form a three-dimensional discontinuous honeycomb-like network at Mg grain boundaries in these alloys, such as WZ063 alloy (as shown in Fig. 2a). It is quite similar to the widely reported X-phase with a LPSO structure [12,21]. The other alloys also mainly contain the X-phase but different volume fractions, which would not be described repeatedly. It could be clearly found that the WZ067 alloy contains a network of black eutectic phase, which should be the W-phase, according to the XRD pattern in Fig. 1. Furthermore, it could not ?nd any plate-like X-phase instead in the WZ067 alloy. Fig. 2h and g also shows that some similar slender phase with dark contrast (as pointed out by arrow 1 in Fig. 2h, and arrow 2 in Fig. 2g) are distributing alternately with these plate-like second phase in the as-cast WZ097 and WZ127 alloys. It should be the W-phase, according to the XRD patterns. However, another kind of slender phase (arrow 3) has also been widely found in WZ123 alloy, as shown in Fig. 2c. Their morphology differs from the minor W-phase found in WZ097 alloy, and these might be the indexed Mg24Y5 phase in Fig. 1d. Fig. 3a shows a clear morphology of W-phase eutectics in as-cast WZ067 without any other second phase. It further testi?es that the WZ067 alloy only contains a network of eutectic W-phase. Fig. 3b shows the SEM images of the as-cast WZ065 alloy in order to clearly reveal the way in which the X- and W-phase distribute. The W-phase eutectics seem to be embedded into the X-phase, since the boundaries of the two phases are almost parallel, as shown in the magnifying image of Fig. 4b. As for the direct cause for the alternative distribution of W-phase eutectics and X-phase plates in these high alloyed Mg–Y– Zn alloys, it should be mainly attributed to the non-equilibrium solidi?cation in a permanent metal mold, and we have given a brief discussion in our published work [17]. What is more, the possible Mg24Y5 phase seems to own a similar distributing manner with the X-phase in the as-cast WZ123 alloy, as shown in Fig. 3c. It could not ?nd the similar W-phase eutectics in WZ123 alloy. The EDS analysis of point A in Fig. 3c is Mg–7.5Y–4.2Zn (at.%), suggesting that this is X-phase. Meanwhile, Point B indicates another kind of second phase, whose chemical composition is Mg–13.4Y–1.5Zn (at.%). The ratio of Mg/Y + Zn is close to 5, thus this phase is no doubt the Mg24Y5 compounds. According to the Gr?bner's calculating work on the Mg-rich part of Mg–Y–Zn isothermal phase diagram (500 °C) [22], the phase composition should be actually Mg, 14H type X-phase and Mg24Y5 when the alloy's Y content is over 10 wt.% and the corresponding Y/Zn mole ratio is far over than 2. However, this has not been widely found in the overwhelmingly reported Mg–Y–Zn alloys. Meanwhile, it could be seen that the WZ093 and WZ125 alloys also seem to contain the similar minor Mg24Y5 phase (arrows 4 and 5), as shown in Fig. 2b and f. However, the XRD patterns have not shown any strong diffraction peaks for the Mg24Y5 phase. Meanwhile, it could almost observe the sole X-phase (Point C, Mg–6.7Y–4.2Zn, at.%) in WZ093 alloy, as shown in Fig. 3d. It is quite reasonable that the minor phase is missing in a selected microarea. Even so, the phase constituents of other alloys in this work also keep in much better agreement with the Gr?bner's calculating work [22,23]. According to the discussion above, the nine investigated alloy could be divided into three groups by their phase compositions: Wphase (WZ067), W ? + X-phase (WZ063, WZ065, WZ095, WZ097 and WZ127), and X-phase + Mg24Y5 (WZ093, WZ123 and WZ125) alloy. The following discussion will be launched on these three groups. Thus, some alloys might be omitted to avoid repeated description and discussion. Fig. 4 shows the volume fraction of the second phase contained in the nine investigated alloys. In general, the volume fraction of second phase increases as the alloying additions rise up. However, this conclusion could be partially drawn besides some exceptions in this work. For

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Fig. 1. XRD patterns of the as-cast and as-extruded Mg–Y–Zn–Zr alloys.

instance, when the Zn content is ?xed as low as 3 wt.%, the volume fraction of second phase would not increase largely by increasing Y addition independently, as shown in Fig. 4a. Nevertheless, considerable content growth of the second phase would accompany when raising Y additions in the high Zn contained alloys (such as 7 wt.%), as shown in Fig. 2g-i. On the other hand, the volume fraction of second phase would even decrease with the increasing Zn addition, when the Y content is ?xed as low as 6 wt.%, as shown in Fig. 4b. This is because Y/Zn mole ratio varies strongly across the critical value in this case. Consequently, phase type transformation (from X- to W-phase, as shown in Fig. 2) occurs among these alloys. From the chemical formula of the X- and Wphase, it could be concluded that the W-phase (Mg3Y2Zn3) should consume more Y and Zn atoms than the X-phase (Mg12YZn) do. Thus,
Table 2 Theoretic phase composition of the investigated alloys. Alloy WZ127 WZ097 WZ067 WZ125 WZ095 Y/Zn mole ratio 1.26 1.02 0.61 1.64 1.29 Phase composition [6] α-Mg + W + X α-Mg + W + X α-Mg + W + X α-Mg + X α-Mg + W + X This work (XRD) α-Mg + W + X α-Mg + W + X α-Mg ± W α-Mg + X α-Mg + W + X

it makes sense that the volume fraction would decrease when it transforms from the X-phase to W-phase. On the contrary, the volume fraction of second phase would increase rapidly with the increasing Zn additions, among those alloys whose Y content is much higher than Zn content. The proper reason is that the Y/Zn mole ratios of these alloys are far from the critical value for second phase type transformation. What is more, the second phase content also increases rapidly as increasing the Y and Zn addition synchronously. It should be also noted that the amount of second phase increase in a way of that the X-phase grows from a discontinuous network into thick and compact plates almost covering the matrix. However, the volume fraction of this plate-like phase would no longer increase greatly once the Y or Zn content is high enough.

Alloy WZ065 WZ123 WZ093 WZ063

Y/Zn mole ratio 0.88 3.03 2.02 1.41

Phase composition [6] α-Mg + W + X α-Mg + X α-Mg + X α-Mg + X

This work (XRD) α-Mg + W + X α-Mg ± X ± Mg24Y5 α-Mg + X α-Mg ± W ± X

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Fig. 2. Optical microstructures of the as-cast alloys. (a) WZ063; (b) WZ093; (c) WZ123; (d) WZ065; (e) WZ095; (f) WZ125; (g) WZ067; (h) WZ097; (i) WZ127.

Bearing the results and discussion above around the nine investigated alloys in mind, it is interesting that the Y/Zn mole ratios of the alloys containing both W- and X-phase are around 0.88–1.41. However, when the Y/Zn mole ratio is a bit higher (such as 2.02), the alloy mainly contains X-phase and Mg24Y5. The critical value range for alloy containing both X- and W-phase is suggested about from 0.33 to 1.21, even to 1.32 [6]. Nevertheless, the alloys whose Y/Zn mole ratios are over 1.32 could still contain minor W-phase in this work. What the primary difference is, the W-phase is the only second phase in the WZ067 alloy, but whose Y/Zn mole ratio is about 0.61, much over than 0.32. Meanwhile, the similar Mg–6Zn–6Y–0.6Zr alloy (Y/Zn mole ratio is 0.75) also comprised of the single W-phase [19]. Thus, it is not quite accurate to regard

the Y/Zn mole ratio as the only factor determining the phase constituent of Mg–Y–Zn alloys. Considering the authors in Ref. [6] developed the experiment and calculations in a low alloyed Mg–Y–Zn system, it is reasonable for the different phase composition in high alloyed Mg–Y–Zn alloys, although they have similar Y/Zn mole ratios to these low alloyed alloys. The alloying addition should be actually another important factor requiring for further considerations. Thus, the results and discussion above indicate that the phase type of Mg–Y–Zn–Zr alloys are determined by both the Y/Zn mole ratios and the alloying content. When only using Y/Zn mole ratios to predict the phase compositions, the predictions are not well agreed with the experimental results. The alloying content also play an important role in

Fig. 3. The SEM images of the as-cast alloys. (a) WZ067 alloy; (b) WZ065 alloy; (c) WZ123 alloy; (d) WZ093 alloy.

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Fig. 4. The volume fraction of second phase in the as-cast alloys.

determining the phase composition. However, the instinct for the phase type determination is virtually based on the element consumption of the corresponding second phase. The X-phase (Mg12YZn) requires equal Y and Zn atoms, while the W-phase (Mg3Y2Zn3) should consume a bit more Zn atoms than Y atoms. Once Zn atoms are insuf?cient in Mg– Y–Zn alloys, the increasing Y addition or Y/Zn ratio would bring a new Mg24Y5 phase instead of more X-phase, as shown in Fig. 2a-c. As a matter of course, the volume fraction of second phase would increase rapidly when increase the Y and Zn additions simultaneously, as shown in Fig. 2a, e and i. However, the phase content should be deeply affected by the Y/Zn mole ratios when the total alloying addition almost keeps the same, as shown in Fig. 2c, e and g. Meanwhile, the W-phase seems to be denser than X-phase, due to the preponderant atomic weight. That might explain that the volume fraction of W-phase in WZ067 alloy is much lower than that of X-phase in WZ093 alloy which contains similar alloying additions. The following section would attempt to ?nd out whether the denser W-phase would give a better performance to the alloy than the X-phase does. Thus, the phase type and content deeply depend on the speci?c Y/Zn mole ratios and the total alloying additions. Fig. 5 shows the optical microstructures of as-extruded Mg–Y–Zn–Zr alloys. It could be generally seen that the second phases were broken and elongated along the hot extrusion direction of the nine investigated alloys. What is more, many tiny particles can be observed around the broken X-phase in the W ? + X-phase alloys, such as shown in Fig. 5e. However, it is hard to ?nd the similar particles in the Xphase + Mg24Y5 alloys, as shown in Fig. 5c and f. Meanwhile, all of the nine alloys were dynamically recrystallized during the hot extrusion.

The DRXed (dynamic recrystallized) grain size is inhomogeneous in the as-extruded alloys. The ?ner grains are in favor of distributing in the vicinities of the X-phase, as clearly shown in Fig. 5e. The grain sizes of the ?ner and coarser grains are 2–3 μm (?ne grain zone) and 7–10 μm (coarse grain zone) respectively, as clearly shown in Fig. 5e. With the growing amount of X-phase in the extruded alloys, the Xphase develops from long and thin ?bers (Fig. 5a) into bulk of thick plates throughout the matrix (Fig. 5i). On the other hand, the DRXed grains are much re?ned. However, it is hard to quantify the grain size when the matrix is almost covered by the huge amount of bulk of Xphase. It could just speculate that the grain size behind the X-phase should be much less than 5 μm which are measured from the positions outside the vicinities of X-phase, as shown in Fig. 5h. Thus, the growing amount of X-phase could surely decrease the grain size inhomogeneity. What is worth noted is that the extruded WZ067 alloy comprises of much more uniform DRXed grains with a fewer amount of second phase (W-phase). The DRXed grain re?nement by the second phase is attributed to the high nucleation rate of grains induced by the mechanism known as particle-stimulated nucleation (PSN) [24] at the interfaces between the second phase and the matrix. The PSN mechanism involves a rapid subgrain boundary migration in the deformation zone around large hard particles during extrusion. The accumulation of misorientation by the rapid subgrain boundary migration could generate high-angle grain boundaries. The migration of the subgrain boundary should rely on the absorption of accumulated dislocations. On the other hand, the large amount of broken X-phase could suppress the growth of DRXed grains through retarding the migration of the grain boundaries. Thus, those grains should grow coarse without the inhibition from the second phase, as shown in Fig. 5. For the WZ06X alloys, the increasing Zn addition turns out to change the phase type constituent, then these different types of second phase perform differently during the hot extrusion. In the WZ067 alloys, the broken W-phase was able to unify the microstructures without almost covering the matrix as the X-phase do. Kink band formation is a major feature for the deformation microstructures of the Mg–Y–Zn alloy with X-phase. It should be noted that the kink bands were also formed along the extrusion direction in the investigated alloys, as clearly shown in Fig. 5f. Since some intermetallic compounds are observed around kink bands (Fig. 5i), these intermetallic compounds can be regarded as a factor to the kink formation [25]. In this work, ?ve of the investigated alloys contain a quantity of X-phase and minor W-phase, then these minor W-phase are the potential obstacles giving rise to the kink band of X-phase. Meanwhile, the W-phase becomes associative particles during the homogenization process, as shown in Fig. 6a. Hence, the extruded WZ067 alloy contains a quantity of dissociative W-phase throughout the matrix, as shown in Fig. 6c. Fig. 6b clearly shows that the remnant W-phase still distributes interactively with the stable X-phase. It is easy to speculate that this remnant W-phase must frustrate the X-phase from ?owing along the extrusion direction. Thus, it could ?nd many bright particles around the kink bands of the X-phase, as shown in Fig. 6d. On the other hand, Fig. 6e and f shows the SEM micrographs of the as-extruded Xphase +Mg24Y5 alloys, respectively. It could hardly ?nd the dissociative W-phase in the two alloys. However, kink band formation could still be observed in this type of alloys, as shown in Fig. 5b and c. It might be ascribed to the broken α-Mg by extrusion, which could also retard the basal slip of the X-phase and then contribute to the kink formation. 3.2. Aging hardening behaviors The aging hardening behaviors will be represented and discussed by the divided three groups above. Fig. 7 shows the aging hardening curves as a function of the aging time. The aging hardening behaviors of the three types of alloys are very similar at the primary stage. The hardness increases slowly and almost linearly during the ?rst 24 h. Subsequently, the aging hardness decreases sharply at 30 h. Then, the hardness values

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Fig. 5. The optical microstructures of the longitudinal cross-section of the as-extruded alloys. (a) WZ063; (b) WZ093; (c) WZ123; (d) WZ065; (e) WZ095; (f) WZ125; (g) WZ067; (h) WZ097; (i) WZ127.

turn into a period of ?uctuation. However, the aging hardening response of the extruded WZ123 alloy is stronger than the other two alloys. Its peak-aging hardness could be as high as 178 HV at 92 h, while the WZ065 (X ? + W-phase) and WZ067 (W-phase) received weak aging hardening response. The aging hardening response of WZ123 alloy should be mainly attributed to the large number of β′ precipitates throughout the substrate during the peak-aging [26]. In general, the large amount of dense and small precipitates precipitating from the supersaturated matrix are the direct cause to the aging hardening response [27]. Then, this process might not be accessible to the W-phase and X ? +W-phase alloys during the aging process. 3.3. Mechanical properties Fig. 8 shows the mechanical properties of the as-extruded and aged alloys. The ultimate tensile stress (UTS) and yielding tensile stress (YTS) of the as-extruded alloys range from 300 to 420 MPa, and from 200 to 330 MPa, respectively. It could be also seen that the increasing Y addition could remarkably promote the strength of the as-extruded Mg–Y–Zn–Zr alloys, when the Zn content is changeless. For instance, the UTS and YTS of the WZ063 alloy are only 316 MPa and 200 MPa, respectively. While the Y content grows up to 12 wt.%, the UTS and YTS of the WZ123 could be as high as 380 MPa and 310 MPa, respectively. However, the elongation of the alloys would also decrease drastically from 19% to just not over than 5%. While the Y content is ?xed, the increasing Zn addition would not greatly improve the mechanical properties as the increasing Y element does. Meanwhile, the increasing alloying addition would also decrease the elongations. The investigated nine Mg–Y–Zn alloys established different phase types or amounts of second phase, which could absolutely affect the mechanical properties. Fig. 9 shows the relationship among the mechanical properties, Y/Zn mole ratio and phase content of the alloys. It could be ?rstly seen that the W-phase alloy (WZ067) owns the lowest strength but excellent elongations. The poor strength might be

attributed to the limited amount of second phase (W-phase) and its dissociative distribution in the alloy, as shown in Fig. 6c. All the other groups of alloys could be high strength. Thus, it could be concluded that the denser W-phase are less effective to toughen the alloy. However, the W-phase resulted in a uniform DRXed grain structure which grants the alloy a good ductility. Obviously, the second phase content is critical to the mechanical properties of the extruded alloys. Furthermore, it should be deeply related to the alloying additions as well as the Y/Zn mole ratios. For example, the WZ067 and WZ065 alloys have the close alloying additions but different mechanical properties. The direct reason should be the different second phase type determined by the Y/Zn mole ratios, which further determine the amount of second phase. On the other hand, when the phase composition is not changed, the mechanical properties of the alloy should mainly depend on the alloying additions. For instance, except for W-phase alloy, the other alloys mainly consist of different amounts of X-phase and maybe minor W-phase or Mg24Y5 phase. Then, we could see that the alloy who had the largest alloying additions (volume fraction of X-phase) established the highest strength and the poorest ductility, as shown in Fig. 9. That is to say, X-phase is the primary issue to determine the mechanical properties. In this manner, it is not strange that the X-phase is more effective to strengthen or toughen the alloy than W-phase. Because the volume of X-phase is much larger than that of W-phase as the alloying addition is very close. In a word, the phase composition and content play a fatal role on toughening the Mg–Y–Zn alloys. The as-extruded WZ125 alloy established the highest strength in the nine investigated alloys, whose UTS and YTS could be as high as 429 MPa and 351 MPa, respectively. However, its elongation is very limited. The high strength is mainly attributed to the multi-contribution from the large amount of X-phase. Firstly, the major X-phase was broken and elongated throughout the DRXed magnesium matrix, serving as strengthening ?bers to pin the grain boundaries and dislocations. Secondly, the strong PSN mechanism by these second phases gave rise

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Fig. 6. Microstructures of the as-extruded and as-homogenized WZ067 and WZ065 alloys. (a) and (b) as-homogenized WZ067 and WZ065 alloy, respectively; (c) and (d) as-extruded WZ067 and WZ065 alloys, respectively; (e) as-extruded WZ093 alloy; (f) as-extruded WZ123 alloy.

to drastic DRXed grain re?nement, as shown in Fig. 5. Finally, the load would transfer rapidly to these ?ber-like phase, due to their large volume fractions [18]. However, the effective toughening mechanism brings the alloy a poor ductility, which should be bore in mind during the alloying compositional design. Fig. 8 also shows the mechanical properties of the aged alloys. The mechanical properties are generally decreased after aging treatment, except for the WZ123 alloy. After peak-aging, the UTS and YTS of

WZ123 alloy get promoted by about 15%, which could be as high as 440 MPa and 350 MPa, respectively. For the other alloys, the mechanical properties almost remain unchanged or decrease in certain degrees, especially for the alloys containing W-phase. The aged WZ127 alloy has almost the same mechanical properties to the extruded alloys, and the aged WZ067 alloy encounters a strength deterioration, as shown in Fig. 8. Thus, except for the X-phase +Mg24Y5 alloy, the investigated Mg–Y–Zn–Zr alloys are not able to be toughened by heat treatment, because those alloys are full of bulk LPSO X-phase or W-phase particles,

Fig. 7. The aging hardening curves of alloys with different phase constituent.

Fig. 8. Mechanical properties of the as-extruded and aged alloys.

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addition is close. The Mg–Y–Zn–Zr alloys receive almost no aging hardening response, except for the alloy containing X-phase and Mg24Y5 phase. It is because a supersaturated matrix is not available to these alloys containing only the ternary X- or W-phase. Nevertheless, the asextruded Mg–12Y–5Zn–0.6Zr alloy could still own extraordinary high strength, whose ultimate tensile strength and yielding tensile strength could be 429 MPa and 351 MPa, respectively. It should be attributed to the strengthening effects by the abundant X-phase in the alloy. After aging treatment, the Mg–12Y–3Zn–0.6Zr alloy established an ultimate tensile strength and yielding tensile strength of 440 MPa and 350 MPa, respectively.

Acknowledgments This research was ? nancially supported by the National Basic Research Program of China (Grant No. 2013CB632203) and the Liaoning Provincial Natural Science Foundation of China (Grant No. 2014028027).

Fig. 9. Relationship among the mechanical properties, Y/Zn mole ratio and phase content of the as-extruded alloys.

which are absolutely capable of high temperature resistance. Thus, it is almost impossible to obtain a supersaturated matrix, which is necessary for the strengthening precipitates. Although it has been found that the strengthening β′ precipitates would precipitate in Mg95.5Y3Zn1.5 during the aging process [26], their coarse size and low density are not capable of providing strong hardening response, as illustrated in the Ref. [26]. As a result, the alloys with Y/Zn mole ratios around 0.61–1.41 almost show no aging hardening response. As for the WZ123 alloy, the phase composition of as-cast alloy is α-Mg, X-phase and a certain amount of Mg24Y5 phase, which is indistinctive compared to the other investigated alloys. Nevertheless, the WZ123 alloy does not have superior quantity of Xphase over than the WZ063 alloys, as shown in Figs. 2c and 9. It suggests that the quantity of X-phase in WZX3 alloys would not increase drastically when increasing Y content. In other word, the WZ123 alloy could not provide suf?cient Zn solute to form more X-phase. Thus, the redundant Y should be partially solid soluble in the matrix and partially take part in the Mg24Y5 precipitating. These Mg24Y5 phase would resolve into the matrix during the long-time homogenization [28]. Then it could provide a supersaturated matrix, which is suf?cient to precipitating dense and ?ne β′ precipitate during the aging process. That should be the reason for the strong aging hardening response of WZ123 alloys. However, the WZ093 and WZ125 are regarded as X-phase + Mg24Y5 alloy, but also with poor aging hardening response. It might be related to their small amount of Mg24Y5. The phase type could also have an impact on the heat-treatable properties of the Mg–Y–Zn–Zr alloys. When the alloys mainly consist of ternary phase (W- and X-phase), they should not be heat-treatable. However, a combination of ternary phase and binary phase (Xphase + Mg24Y5 phase in this work) would make the Mg–Y–Zn–Zr alloy toughened by heat treatment. Perhaps, a similar combination of I-phase and Mg–Zn phase might be a candidate for high strength Mg– Y–Zn alloys. However, the above results indicate that a suf?cient volume fraction of second phase is necessary for the high strength Mg–Y–Zn alloys. 4. Conclusions In this work, the microstructures and mechanical properties of nine different Mg– Y– Zn– Zr alloys have been investigated. The investigated Mg – Y – Zn alloys mainly contain three types: W-phase, W ? + X-phase and X-phase + Mg24Y5 alloys, which should be determined by both the Y/Zn mole ratios and the total alloying content. The increasing phase content could drastically promote the mechanical properties of the as-extruded Mg–Y–Zn alloys. The X-phase could toughen the alloy more effectively than W-phase, when the alloying

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