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Development of high-strength, low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content
Progress in Natural Science: Materials International (xxxx) xxxx–xxxx
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Original Research
Development of high-strength, low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content☆ Fusheng. Pana,b,c, Jianjun Maoa,b, Gen Zhanga,b, Aitao Tanga,b, Jia Shea,d,
⁎
a
College of Materials Science and Engineering Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China Chongqing Research Center for Advanced Materials, Chongqing Academy of Science and Technology, Chongqing 401123, China d Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610213, China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Magnesium alloy Mg–Zn alloy Microstructure Ultra-fine grains Second phase Mechanical properties
Mg–Zn–Mn-based alloys have received considerable attention because of their high creep resistance, strength, and good corrosion resistance. The alloying element Mn in Mg–Zn-based alloys is commonly less than 1 wt%. In the present study, the effect of high Mn content (1 wt% and 2 wt%) on the microstructures and mechanical properties of Mg–2Zn–0.3Sr extruded alloy was investigated. The results revealed that the high Mn content significantly increased the ultimate tensile strength, tensile yield strength, compress yield strength, and yield asymmetry of the alloy without affecting its ductility. The dynamically recrystallized (DRXed) grains of Mg– 2Zn–0.3Sr were remarkably refined because of the large amount of fine Mn precipitates in the homogenized alloy. The improved strengths were mainly attributed to the fine DRXed grains according to the Hall–Petch effect and to the large amount of spherical and < 0001 > Mn precipitates through the precipitation and dispersion strengthening. The fine DRXed grains and numerous Mn precipitates effectively suppressed the extension twining, substantially enhanced the compress yield strength, and resulted in improved anisotropy.
1. Introduction
5.25Zn–0.6Ca (wt%) alloys significantly increases by ~50 MPa with the addition of 0.3 Mn [10]. The presence of fine precipitates before extrusion is known to effectively restrict the growth of dynamically recrystallized (DRXed) grains and facilitate the formation of fine DRXed grains during extrusion [11,12]. Qi et al. investigated the effect of Sn or RE (Y, Nd) on the microstructure and mechanical properties of Mg–xZn–1Mn (wt%) alloys [13–16]. As cited in existing reports, the alloying element Mn in Mg–Zn-based alloys is commonly present at a concentration of less than 1 wt%, and it exists as fine Mn precipitates. As a result of the low solubility of Mn in Mg matrices (less than 0.1% at a temperature of 300 °C [17]), the addition of relatively high amounts of Mn in Mg–Zn-based extruded alloys is likely to generate many Mn precipitates and fine DRXed grains. According to other reports, Sr can weaken deformation texture, refine DRXed grains, and reduce the yield anisotropy of Mg–Mn or Mg–Zn alloys [18–20]. The present study aims to investigate the effects of Mn on the microstructure, mechanical properties, and anisotropy of Mg–2Zn–0.3Sr extruded alloys with the goal of developing novel Mg–Zn–Mn-based alloys. The addition of 0.3Sr is intended to weaken the texture and refine DRXed grains.
Magnesium and its alloys are attractive structural components that are used in many applications, including consumer electronics, automobiles, and aerospace because of their light weight, high specific strength, and high specific stiffness [1,2]. The tensile yield strength (TYS) of commercial high-strength wrought Mg alloys is approximately 240 MPa, which is much lower than that of medium strength Al (Al– Mg–Si) alloys (TYS of ~300 MPa) [3]. The effort to identify alternatives to aluminum alloys has increased the interest in developing wrought Mg alloys with a TYS of over 300 MPa [4,5]. Mg–Zn–RE (rare earth) wrought alloys with long-period stacking order structures are known to exhibit an extremely high TYS of over 300 MPa [6]. In addition to Mg– Zn–RE alloys, Mg alloy with TYS of over 300 MPa is rare [3]. Therefore, RE-free high-strength wrought Mg alloys should be developed. Mg–Zn–(Mn)-based alloys have garnered increasing attention because of their high corrosion resistance, ductility, and strength [7,8]. Xu et al. reported that as-extruded Mg–5.99Zn–1.76Ca– 0.35Mn (wt%) alloys exhibit a high TYS of 289 MPa with a compression/tension yield ratio of 0.89 [9]. The TYS of as-extruded Mg–
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (J. She). http://dx.doi.org/10.1016/j.pnsc.2016.11.016 Received 13 March 2016; Accepted 30 May 2016 1002-0071/ © 2016 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Pan, F., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.11.016
Progress in Natural Science: Materials International (xxxx) xxxx–xxxx
F. Pan et al.
phases (see inset in Fig. 1(c)). ZMJ220 shows the most Mn particles among the as-cast samples. Fig. 2 shows the cross-sectional (perpendicular to the ED) OM images of the alloys. ZJ20 is fully recrystallized (Fig. 2(a)), and its average grain size is ~17.8 µm. Fig. 2(b) and (c) indicate that ZMJ210 and ZMJ220 consist of a partially recrystallized (DRXed) microstructure with coarse-grained regions comprising volume proportions of ~12% and ~23%, respectively, as well as fine DRXed grains with sizes of ~3.1 µm and ~1.5 µm, respectively. Fig. 3 exhibits the SEM images of the as-extruded ZJ20, ZMJ210, and ZMJ220. Coarse Mg–Zn–Sr phases are observed at the grain boundaries. The fine DRXed grains form around the Mg–Zn–Sr phases (red rings in Fig. 3(a)–(c)). The coarse Mg–Zn–Sr phases are absent in the un-DRXed regions. According to previous studies [21], large particles ( > 1 µm) serve as nucleation sites for DRX during hot extrusion, hence the name particle-simulated nucleation (PSN). In the current study, the coarse Mg–Zn–Sr phases act as heterogeneous nucleation sites of recrystallization during hot extrusion, thereby resulting in the fine DRXed grains. The high magnification images of the DRXed grains of the alloys are illustrated in Fig. 3(d)–(f). The average grain size of the DRXed grains is gradually reduced with the increase in the Mn content, and the same is revealed by the OM results. The textures of the alloys determined via the XRD analyses of the cross sections perpendicular to the ED are illustrated in Fig. 4. All samples comprise a basal plane that is parallel to the ED. As typically observed for as-extruded Mg alloy bars, a ring-fiber texture is believed to exist [22]. The (0001) texture of the alloys is gradually strengthened with the increase in Mn mainly because of the formation of un-DRXed grain regions, which commonly show strong (0001) textures [9]. Generally, with the addition of a high amount of Mn in the ZJ20 alloy, the DRXed grains are refined remarkably, un-DRXed grain regions appear, and the (0001) texture is strengthened. Bright-field TEM images of ZJ20 are presented in Fig. 5(a). Coarse Mg–Zn–Sr phases are distributed at the grain boundaries, and the formation of ternary Mg–Zn–Sr phases exhausts the alloying element Zn. Therefore, Mg–Zn (β′) precipitates are difficult to find in the alloy. Bright-field TEM images of ZMJ220 observed with the incident beam along the [0001] and [112 0] zone axes of the Mg matrix are illustrated in Fig. 5(b) and (c), respectively. Large amounts of fine precipitates are clearly distributed uniformly in the Mg matrix. The morphologies of the precipitates of ZMJ220 exhibit two shapes, namely, spheres and rods lying along the < 0001 > direction. EDS was employed to identify these precipitates. The spherical precipitates and the < 0001 > rod-shaped precipitates are Mn phases. The rod precipitates are 12 ± 5 nm long and 3 ± 1 nm thick; the diameter of the spherical precipitates is 4– 10 nm. The orientation relationships between Mg and the spherical and rod-shaped Mn precipitates are expressed as [0001]Mg||[111]Mn and (0110)Mg||(110)Mn and [112 0]Mg||[001]Mn and (0001)Mg||(110)Mn, respectively [23].
2. Experiments Mg–2Zn–0.3Sr, Mg–2Zn–0.3Sr–1Mn, and Mg–2Zn–0.3Sr–2Mn alloys (the numbers indicate wt%; Mg and Zn, 99.90%; Sr (Mg–33.3Sr in wt% master alloy), Mn (Mg–4Mn in wt% master alloy) were melted in a crucible placed in a resistance furnace and protected by a gas mixture of CO2 and SF6 (100:1). The melt was maintained at 740 °C for 10 min and then homogenized via mechanical stirring at 300 rpm. After mixing, the melt was held at 740 °C for another 20 min and then poured into a permanent mold (diameter of 80 mm, height of 200 mm) that had been preheated to 300 °C to obtain a casting billet. Hereafter, Mg–2Zn–0.3Sr, Mg–2Zn–0.3Sr–1Mn, and Mg–2Zn–0.3Sr–2Mn are denoted as ZJ20, ZMJ210, and ZMJ220, respectively. The synthesized alloys were homogenized at 400 °C for 24 h followed by quenching with water to induce supersaturated solid solutions. The ingot measuring 80 mm in diameter was preheated at 300 °C for 1 h. The extrusion was conducted with a direct extrusion die at 300 °C with an extrusion ratio of 25:1 and speed of 1 m/min. The extrusion machine was an XJ-500 horizontal extrusion machine with 500 t and φ85×500 mm container (WuxiYuanchang Machine Manufacture Co. Ltd., China). The extruded bar measured 16 mm in diameter. The specimens with a gage length of 25 mm and a gage diameter of 5 mm were used for the tension tests, whereas the samples with a gage length of 15 mm and a gage diameter of 10 mm were used for the compression tests. Tensile and compression properties were tested using a Sansi UTM5000 instrument at an initial tensile strain rate of 1.0×10−2 s−1 along the stress direction parallel to the extrusion direction (ED). The secondary phases in the alloys were characterized via transmission electron microscopy (TEM; Zeiss Libra 200 FE) at an accelerating voltage of 200 kV. The samples were polished and etched with a mixture containing 1.5 g picric acid, 25 mL ethanol, 5 mL acetic acid, and 10 mL water prior to analyzing their microstructures via optical microscopy (OM) and scanning electron microscopy (SEM; Tescan Vega2) equipped with energy-dispersive X-ray spectroscopy (EDS; Inca Energy 350). Texture characterization was performed via Xray diffraction (XRD). All of the microstructure observation for the specimens were taken from the middle of the extrusion bar. 3. Results and discussion 3.1. Microstructure Fig. 1(a)–(c) show the SEM images of the as-homogenized ZJ20, ZMJ210, and ZMJ220, respectively. Many Mg–Zn–Sr phases (1–5 µm) are observed along the grain boundaries and within grains (green arrows), as indicated by the EDS analysis. The Mg–Zn–Sr phases mainly comprised 50 wt% Mg, 38 wt% Zn, and 12 wt% Sr. The crystal structure is undetermined and requires further investigation. Compared with ZJ20, the alloys containing Mn clearly present a greater number of relatively small particles within grains or near grain boundaries (less than 200 nm, red arrows), which are identified as Mn
Fig. 1. SEM images of as-homogenized alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220.
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Fig. 2. OM images of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220.
Fig. 3. SEM images of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220; high magnification SEM of as-extruded alloys (d) ZJ20, (e) ZMJ210 and (f) ZMJ220.
3.2. Mechanical properties
sion of the alloys at ambient temperature are presented in Fig. 6(a) and (b), respectively, and the values obtained from these curves are presented in Table 1. The ultimate tensile strength (UTS), TYS,
Typical nominal stress–strain curves for the tension and compres-
Fig. 4. (0002) pole figures of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220. The samples are taken from a cross-section perpendicular to the extrusion direction.
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Fig. 5. (a) Bright-TEM of as-extruded ZJ20, (b) Bright-TEM of as-extruded ZMJ220 taken from [0001] zone axis, (c) Bright-TEM of as-extruded ZMJ220 taken from [112 0] zone axis of the Mg matrix and (d) High-resolution Bright-TEM of as-extruded ZMJ220.
Fig. 6. Strain-stress curves of (a) tension and (b) compression.
addition of 2 wt% Mn. The TYS, UTS, CYS, and σCYS/σTYS of ZMJ220 are 312 MPa, 333 MPa, 240 MPa, and 0.77, respectively. No difference is noted between ZJ20 and ZMJ220 in terms of TF elongation. In general, the addition of a large amount of Mn can significantly improve the mechanical properties and anisotropy of ZJ20. The TYS of the ZMJ220 extruded alloy (312 MPa) is much higher than that of a commercial high-strength ZK60 extruded alloy (255 MPa), the newly developed high-strength Mg–6Zn–0.4Ag–0.2Ca–0.6Zr extruded alloy (288 MPa) [11,24], Mg–9.8Sn–1.2Zn–1.0Al extruded alloy (308 MPa) [4], and Mg–4.7Zn–0.5Ca extruded alloy (291 MPa) [25]; the TYS of ZMJ220 reaches the level of the TYS of T6-treated medium-strength aluminum alloys [26].
Table 1 Mechanical properties of the alloys. Sample
UTS (MPa)
TYS (MPa)
CYS (MPa)
εTF (%)
σCYS/σTYS
ZJ20 ZMJ210 ZMJ220 ZK60
258 298 333 354
185 257 312 255
126 196 240 167
17.5 21.1 17 14
0.68 0.76 0.77 0.65
compressive yield strength (CYS), and yield asymmetry (σCYS/σTYS) are observed to significantly increase because of the addition of Mn. TYS, UTS, CYS, tensile fracture (TF) elongation, and σCYS/σTYS of ZJ20 increase by 72 MPa, 40 MPa, 70 MPa, 3.6%, and 0.08, respectively, with the addition of 1 wt% Mn. The TYS, UTS, CYS, TF elongation, and σCYS/σTYS of ZMJ210 are 257 MPa, 298 MPa, 196 MPa, 21.1%, and 0.76, respectively. The TYS, UTS, CYS, and σCYS/σTYS of ZJ20 increase by 127 MPa, 75 MPa, 114 MPa, and 0.09, respectively, with the
4. Discussion Many relative large particles (Mg–Zn–Sr) and numerous small Mn particles are observed in the as-homogenized ZMJ210 and ZMJ220 specimens, as shown in Fig. 1. The presence of a hard Mg–Zn–Sr phase 4
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Fig. 7. The typical Bright-TEM of as-extruded ZMJ220 (a) low magnification image and (b) high magnification image, Mn precipitates pinning grain boundaries growth during the hot extrusion.
is considered as the key deformation mode for the alloys. The Schmid factor for a basal plane can be defined as M=cosθ.cosλ [31], where θ and λ are the angles between the tensile direction and the normal direction of the basal plane and the angle between the tensile direction and the [112 0] slip direction, respectively; the Schmid factor is denoted as M. Obviously, M is decreased by increasing θ. The YS of the alloys could be increased with the reduction of M. From the texture evolution, the basal texture is gradually strengthened by increasing the Mn content, thereby indicating that the basal plane of the grains is nearly parallel to ED (that is, θ is gradually increased). Therefore, a strong basal texture also contributes to strength improvement. (c) Fine precipitates. As mentioned, basal slip is the key deformation mode for the alloys. The effect of precipitate on basal slip can be assumed to depend on the Orowan stress required to bow dislocations around the particles [32]. The Orowan equation can Dp Gb 1 be written as [33] ∆τ = ( 2π 1 − ν )( λ )ln ( r ), where ∆τ is the critical 0 resolved shear stress (CRSS); G, b, v, Dp, λ, and r0 are the shear modulus, Burgers vector of gliding dislocations, Poisson ratio, planar diameter of the particles on the basal plane, effective planar inter-obstacle spacing, and dislocation core radius, respectively. The presence of a large number of fine basal and < 0001 > Mn precipitates effectively hinders the dislocation and leads to its accumulation. The increment of Mn precipitates results in a reduction in λ and an enhancement in CRSS (∆τ ). Accordingly, the high TYS of ZMJ220 can be attributed to the ultra-fine DRXed grains based on the Hall–Petch effect, the large number of spherical Mn and < 0001 > Mn precipitates through precipitation and dispersion strengthening, and the strong basal textures of the un-DRXed grains.
effectively hinders dislocations and results in high stress concentration during hot extrusion. This high stress concentration can lead to local cracks around coarse particles. As shown in Fig. 3, these particles are partially broken during extrusion, and their size remains in the range of > 1 µm after extrusion. With further deformation, the coarse Mg– Zn–Sr particles become subject to dislocation accumulation and serve as nucleation sites for DRX. As indicated by the red circle in Fig. 3, the fine DRXed grains near the Mg–Zn–Sr particles are induced by particle simulation nucleation. The fine Mn precipitates effectively prevent the growth of newly recrystallized grains and refine the DRXed grains during extrusion [12]. After extrusion, Mn precipitates are observed around the fine DRXed grain boundaries (red regions in Fig. 7(a), bright-field TEM images of as-extruded ZMJ220). Moreover, many relatively coarse Mn precipitates with diameters of 15–30 nm are clearly observed at the grain boundaries (red arrows in Fig. 7(b)). Jung et al. [12] reported that fine Mg17Al12 precipitates formed by aging prior to extrusion effectively reduce the average size of DRXed grains in the Mg–7.6Al–0.4Zn extruded alloy. Mg(Zn,Zr) precipitates formed before extrusion also result in fine DRXed grains in ZK60 alloys [11,24]. Pan et al. [27] also indicated that the formation of fine DRXed grains in extruded Mg–Ca alloys can be attributed to the effective pinning of DRXed grain boundaries by pre-formed precipitates. Coarse Mg–Zn–Sr particles can be considered to act as heterogeneous nucleation sites of recrystallization, and fine Mn precipitates formed before extrusion can effectively prevent the growth of newly recrystallized grains. Thus, coarse Mg–Zn–Sr particles and high Mn content are the main factors that lead to the formation of fine DRXed grains in Mn-based alloys. In the present study, the strengths of ZJ20 are gradually improved with the increased Mn content. Several factors are considered to result in the increased tensile strength:
PSN is known to modify the textures and improve the anisotropy of extruded alloys [34]. A mass of Mg–Zn–Sr phases are observed in the alloys in the present study. Thus, the σCYS/σTYS of the Mg–Zn–Sr-based alloy is higher than that of ZK60, which was fabricated under the same conditions. Extension twinning is regarded as an important factor in the reduction of CYS in Mg alloys [29]. Numerous spherical Mn precipitates can effectively prevent dislocation motion and increase dislocation pile-ups, thereby overcoming the back stress of twinning (when twinning occurs through shear) and suppressing extension twinning when the sample is subjected to compressive loads along the ED [35]. Although the coarse unDRXed grain regions tend to produce tension twinning, the width of the regions is less than that of the DRXed grains of the ZJ20 alloy, and the high fraction of fine DRXed grains significantly prevents tension twinning. Overall, these developments improve the CYS and yield anisotropy of the alloys. The addition of high amounts of Mn to ZJ20 effectively refines the
(a) Fine grain size. The Hall–Petch coefficient for Mg alloys is ~0.7 MPa m−1/2[3], which is much higher than that of Al alloys (~0.04 MPa m−1/2) [28]. The YS of AZ31 increases from 160 MPa to 280 MPa because of grain refinement through the alternate biaxial reverse corrugation method [29]. In the present study, increasing Mn results in the effective refinement of the size of the DRXed grains from 17.8 µm to ~3.1 µm and ~1.5 µm. Moreover, the increased Mn increases the volume fraction of the unDRXed grain regions, the widths of which are less than those of the DRXed grains of the ZJ20 alloy. Thus, grain refinement significantly contributes to the improvement of Mg alloy strength. (b) Strong basal texture. Mg and its alloys are known to have several deformation modes, i.e., basal slip, prismatic slip, and second pyramidal slip [30]. Generally, deforming crystals is more difficult along the c-axis than along any other crystal axis. Thus, basal slip 5
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DRXed grains and substantially enhances the TYS and CYS without causing a reduction in ductility. ZMJ220 is a potential candidate for application as a low-cost, high-strength, and high-plastic wrought magnesium alloy. 5. Conclusion In summary, the addition of large amounts of Mn to the ZJ20 extruded alloy effectively enhances the strength and improves the anisotropy of the alloy without altering its ductility. The mechanical properties of ZJ20 are gradually improved with the increased Mn content. With the addition of 2 wt% Mn, respective increases in TYS, UTS, CYS, and σCYS/σTYS of 127 MPa, 75 MPa, 114 MPa, and 0.09 are observed. The TYS, UTS, CYS, TF, and σCYS/σTYS of ZMJ220 are 312 MPa, 333 MPa, 240 MPa, 17%, and 0.77, respectively. The presence of ultrafine DRXed grains and a large number of spherical Mn and < 0001 > Mn precipitates are the main factors that explain the high strength and low yield anisotropy. Acknowledgments The present work was supported by the National Natural Science Foundation of China (Project 51474043), the Ministry of Science and Technology of China (2014DFG52810), the Ministry of Education of China (SRFDR 20130191110018 and CDJZR13130086) and Chongqing Municipal Government (CSTC2013JCYJC60001, Two River Scholar Project and The Chief Scientist Studio Project). Natural Science Foundation project of Chongqing Science and Technology Commission (CSTC2012ggB50003 and CSTC2013jjB50006). References [1] [2] [3] [4]
L. Guo, F. Fujita, J. Magn. Alloy. 3 (2015) 95–105. A.A. Luo, J. Magn. Alloy. 1 (2013) 2–22. K. Hono, C.L. Mendis, T.T. Sasaki, K. Oh-ishi, Scr. Mater. 63 (2010) 710–715. T.T. Sasaki, K. Yamamoto, T. Honma, S. Kamado, K. Hono, Scr. Mater. 59 (2008)
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Original Research
Development of high-strength, low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content☆ Fusheng. Pana,b,c, Jianjun Maoa,b, Gen Zhanga,b, Aitao Tanga,b, Jia Shea,d,
⁎
a
College of Materials Science and Engineering Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China Chongqing Research Center for Advanced Materials, Chongqing Academy of Science and Technology, Chongqing 401123, China d Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610213, China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Magnesium alloy Mg–Zn alloy Microstructure Ultra-fine grains Second phase Mechanical properties
Mg–Zn–Mn-based alloys have received considerable attention because of their high creep resistance, strength, and good corrosion resistance. The alloying element Mn in Mg–Zn-based alloys is commonly less than 1 wt%. In the present study, the effect of high Mn content (1 wt% and 2 wt%) on the microstructures and mechanical properties of Mg–2Zn–0.3Sr extruded alloy was investigated. The results revealed that the high Mn content significantly increased the ultimate tensile strength, tensile yield strength, compress yield strength, and yield asymmetry of the alloy without affecting its ductility. The dynamically recrystallized (DRXed) grains of Mg– 2Zn–0.3Sr were remarkably refined because of the large amount of fine Mn precipitates in the homogenized alloy. The improved strengths were mainly attributed to the fine DRXed grains according to the Hall–Petch effect and to the large amount of spherical and < 0001 > Mn precipitates through the precipitation and dispersion strengthening. The fine DRXed grains and numerous Mn precipitates effectively suppressed the extension twining, substantially enhanced the compress yield strength, and resulted in improved anisotropy.
1. Introduction
5.25Zn–0.6Ca (wt%) alloys significantly increases by ~50 MPa with the addition of 0.3 Mn [10]. The presence of fine precipitates before extrusion is known to effectively restrict the growth of dynamically recrystallized (DRXed) grains and facilitate the formation of fine DRXed grains during extrusion [11,12]. Qi et al. investigated the effect of Sn or RE (Y, Nd) on the microstructure and mechanical properties of Mg–xZn–1Mn (wt%) alloys [13–16]. As cited in existing reports, the alloying element Mn in Mg–Zn-based alloys is commonly present at a concentration of less than 1 wt%, and it exists as fine Mn precipitates. As a result of the low solubility of Mn in Mg matrices (less than 0.1% at a temperature of 300 °C [17]), the addition of relatively high amounts of Mn in Mg–Zn-based extruded alloys is likely to generate many Mn precipitates and fine DRXed grains. According to other reports, Sr can weaken deformation texture, refine DRXed grains, and reduce the yield anisotropy of Mg–Mn or Mg–Zn alloys [18–20]. The present study aims to investigate the effects of Mn on the microstructure, mechanical properties, and anisotropy of Mg–2Zn–0.3Sr extruded alloys with the goal of developing novel Mg–Zn–Mn-based alloys. The addition of 0.3Sr is intended to weaken the texture and refine DRXed grains.
Magnesium and its alloys are attractive structural components that are used in many applications, including consumer electronics, automobiles, and aerospace because of their light weight, high specific strength, and high specific stiffness [1,2]. The tensile yield strength (TYS) of commercial high-strength wrought Mg alloys is approximately 240 MPa, which is much lower than that of medium strength Al (Al– Mg–Si) alloys (TYS of ~300 MPa) [3]. The effort to identify alternatives to aluminum alloys has increased the interest in developing wrought Mg alloys with a TYS of over 300 MPa [4,5]. Mg–Zn–RE (rare earth) wrought alloys with long-period stacking order structures are known to exhibit an extremely high TYS of over 300 MPa [6]. In addition to Mg– Zn–RE alloys, Mg alloy with TYS of over 300 MPa is rare [3]. Therefore, RE-free high-strength wrought Mg alloys should be developed. Mg–Zn–(Mn)-based alloys have garnered increasing attention because of their high corrosion resistance, ductility, and strength [7,8]. Xu et al. reported that as-extruded Mg–5.99Zn–1.76Ca– 0.35Mn (wt%) alloys exhibit a high TYS of 289 MPa with a compression/tension yield ratio of 0.89 [9]. The TYS of as-extruded Mg–
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (J. She). http://dx.doi.org/10.1016/j.pnsc.2016.11.016 Received 13 March 2016; Accepted 30 May 2016 1002-0071/ © 2016 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Pan, F., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.11.016
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phases (see inset in Fig. 1(c)). ZMJ220 shows the most Mn particles among the as-cast samples. Fig. 2 shows the cross-sectional (perpendicular to the ED) OM images of the alloys. ZJ20 is fully recrystallized (Fig. 2(a)), and its average grain size is ~17.8 µm. Fig. 2(b) and (c) indicate that ZMJ210 and ZMJ220 consist of a partially recrystallized (DRXed) microstructure with coarse-grained regions comprising volume proportions of ~12% and ~23%, respectively, as well as fine DRXed grains with sizes of ~3.1 µm and ~1.5 µm, respectively. Fig. 3 exhibits the SEM images of the as-extruded ZJ20, ZMJ210, and ZMJ220. Coarse Mg–Zn–Sr phases are observed at the grain boundaries. The fine DRXed grains form around the Mg–Zn–Sr phases (red rings in Fig. 3(a)–(c)). The coarse Mg–Zn–Sr phases are absent in the un-DRXed regions. According to previous studies [21], large particles ( > 1 µm) serve as nucleation sites for DRX during hot extrusion, hence the name particle-simulated nucleation (PSN). In the current study, the coarse Mg–Zn–Sr phases act as heterogeneous nucleation sites of recrystallization during hot extrusion, thereby resulting in the fine DRXed grains. The high magnification images of the DRXed grains of the alloys are illustrated in Fig. 3(d)–(f). The average grain size of the DRXed grains is gradually reduced with the increase in the Mn content, and the same is revealed by the OM results. The textures of the alloys determined via the XRD analyses of the cross sections perpendicular to the ED are illustrated in Fig. 4. All samples comprise a basal plane that is parallel to the ED. As typically observed for as-extruded Mg alloy bars, a ring-fiber texture is believed to exist [22]. The (0001) texture of the alloys is gradually strengthened with the increase in Mn mainly because of the formation of un-DRXed grain regions, which commonly show strong (0001) textures [9]. Generally, with the addition of a high amount of Mn in the ZJ20 alloy, the DRXed grains are refined remarkably, un-DRXed grain regions appear, and the (0001) texture is strengthened. Bright-field TEM images of ZJ20 are presented in Fig. 5(a). Coarse Mg–Zn–Sr phases are distributed at the grain boundaries, and the formation of ternary Mg–Zn–Sr phases exhausts the alloying element Zn. Therefore, Mg–Zn (β′) precipitates are difficult to find in the alloy. Bright-field TEM images of ZMJ220 observed with the incident beam along the [0001] and [112 0] zone axes of the Mg matrix are illustrated in Fig. 5(b) and (c), respectively. Large amounts of fine precipitates are clearly distributed uniformly in the Mg matrix. The morphologies of the precipitates of ZMJ220 exhibit two shapes, namely, spheres and rods lying along the < 0001 > direction. EDS was employed to identify these precipitates. The spherical precipitates and the < 0001 > rod-shaped precipitates are Mn phases. The rod precipitates are 12 ± 5 nm long and 3 ± 1 nm thick; the diameter of the spherical precipitates is 4– 10 nm. The orientation relationships between Mg and the spherical and rod-shaped Mn precipitates are expressed as [0001]Mg||[111]Mn and (0110)Mg||(110)Mn and [112 0]Mg||[001]Mn and (0001)Mg||(110)Mn, respectively [23].
2. Experiments Mg–2Zn–0.3Sr, Mg–2Zn–0.3Sr–1Mn, and Mg–2Zn–0.3Sr–2Mn alloys (the numbers indicate wt%; Mg and Zn, 99.90%; Sr (Mg–33.3Sr in wt% master alloy), Mn (Mg–4Mn in wt% master alloy) were melted in a crucible placed in a resistance furnace and protected by a gas mixture of CO2 and SF6 (100:1). The melt was maintained at 740 °C for 10 min and then homogenized via mechanical stirring at 300 rpm. After mixing, the melt was held at 740 °C for another 20 min and then poured into a permanent mold (diameter of 80 mm, height of 200 mm) that had been preheated to 300 °C to obtain a casting billet. Hereafter, Mg–2Zn–0.3Sr, Mg–2Zn–0.3Sr–1Mn, and Mg–2Zn–0.3Sr–2Mn are denoted as ZJ20, ZMJ210, and ZMJ220, respectively. The synthesized alloys were homogenized at 400 °C for 24 h followed by quenching with water to induce supersaturated solid solutions. The ingot measuring 80 mm in diameter was preheated at 300 °C for 1 h. The extrusion was conducted with a direct extrusion die at 300 °C with an extrusion ratio of 25:1 and speed of 1 m/min. The extrusion machine was an XJ-500 horizontal extrusion machine with 500 t and φ85×500 mm container (WuxiYuanchang Machine Manufacture Co. Ltd., China). The extruded bar measured 16 mm in diameter. The specimens with a gage length of 25 mm and a gage diameter of 5 mm were used for the tension tests, whereas the samples with a gage length of 15 mm and a gage diameter of 10 mm were used for the compression tests. Tensile and compression properties were tested using a Sansi UTM5000 instrument at an initial tensile strain rate of 1.0×10−2 s−1 along the stress direction parallel to the extrusion direction (ED). The secondary phases in the alloys were characterized via transmission electron microscopy (TEM; Zeiss Libra 200 FE) at an accelerating voltage of 200 kV. The samples were polished and etched with a mixture containing 1.5 g picric acid, 25 mL ethanol, 5 mL acetic acid, and 10 mL water prior to analyzing their microstructures via optical microscopy (OM) and scanning electron microscopy (SEM; Tescan Vega2) equipped with energy-dispersive X-ray spectroscopy (EDS; Inca Energy 350). Texture characterization was performed via Xray diffraction (XRD). All of the microstructure observation for the specimens were taken from the middle of the extrusion bar. 3. Results and discussion 3.1. Microstructure Fig. 1(a)–(c) show the SEM images of the as-homogenized ZJ20, ZMJ210, and ZMJ220, respectively. Many Mg–Zn–Sr phases (1–5 µm) are observed along the grain boundaries and within grains (green arrows), as indicated by the EDS analysis. The Mg–Zn–Sr phases mainly comprised 50 wt% Mg, 38 wt% Zn, and 12 wt% Sr. The crystal structure is undetermined and requires further investigation. Compared with ZJ20, the alloys containing Mn clearly present a greater number of relatively small particles within grains or near grain boundaries (less than 200 nm, red arrows), which are identified as Mn
Fig. 1. SEM images of as-homogenized alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220.
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Fig. 2. OM images of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220.
Fig. 3. SEM images of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220; high magnification SEM of as-extruded alloys (d) ZJ20, (e) ZMJ210 and (f) ZMJ220.
3.2. Mechanical properties
sion of the alloys at ambient temperature are presented in Fig. 6(a) and (b), respectively, and the values obtained from these curves are presented in Table 1. The ultimate tensile strength (UTS), TYS,
Typical nominal stress–strain curves for the tension and compres-
Fig. 4. (0002) pole figures of as-extruded alloys (a) ZJ20, (b) ZMJ210 and (c) ZMJ220. The samples are taken from a cross-section perpendicular to the extrusion direction.
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Fig. 5. (a) Bright-TEM of as-extruded ZJ20, (b) Bright-TEM of as-extruded ZMJ220 taken from [0001] zone axis, (c) Bright-TEM of as-extruded ZMJ220 taken from [112 0] zone axis of the Mg matrix and (d) High-resolution Bright-TEM of as-extruded ZMJ220.
Fig. 6. Strain-stress curves of (a) tension and (b) compression.
addition of 2 wt% Mn. The TYS, UTS, CYS, and σCYS/σTYS of ZMJ220 are 312 MPa, 333 MPa, 240 MPa, and 0.77, respectively. No difference is noted between ZJ20 and ZMJ220 in terms of TF elongation. In general, the addition of a large amount of Mn can significantly improve the mechanical properties and anisotropy of ZJ20. The TYS of the ZMJ220 extruded alloy (312 MPa) is much higher than that of a commercial high-strength ZK60 extruded alloy (255 MPa), the newly developed high-strength Mg–6Zn–0.4Ag–0.2Ca–0.6Zr extruded alloy (288 MPa) [11,24], Mg–9.8Sn–1.2Zn–1.0Al extruded alloy (308 MPa) [4], and Mg–4.7Zn–0.5Ca extruded alloy (291 MPa) [25]; the TYS of ZMJ220 reaches the level of the TYS of T6-treated medium-strength aluminum alloys [26].
Table 1 Mechanical properties of the alloys. Sample
UTS (MPa)
TYS (MPa)
CYS (MPa)
εTF (%)
σCYS/σTYS
ZJ20 ZMJ210 ZMJ220 ZK60
258 298 333 354
185 257 312 255
126 196 240 167
17.5 21.1 17 14
0.68 0.76 0.77 0.65
compressive yield strength (CYS), and yield asymmetry (σCYS/σTYS) are observed to significantly increase because of the addition of Mn. TYS, UTS, CYS, tensile fracture (TF) elongation, and σCYS/σTYS of ZJ20 increase by 72 MPa, 40 MPa, 70 MPa, 3.6%, and 0.08, respectively, with the addition of 1 wt% Mn. The TYS, UTS, CYS, TF elongation, and σCYS/σTYS of ZMJ210 are 257 MPa, 298 MPa, 196 MPa, 21.1%, and 0.76, respectively. The TYS, UTS, CYS, and σCYS/σTYS of ZJ20 increase by 127 MPa, 75 MPa, 114 MPa, and 0.09, respectively, with the
4. Discussion Many relative large particles (Mg–Zn–Sr) and numerous small Mn particles are observed in the as-homogenized ZMJ210 and ZMJ220 specimens, as shown in Fig. 1. The presence of a hard Mg–Zn–Sr phase 4
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Fig. 7. The typical Bright-TEM of as-extruded ZMJ220 (a) low magnification image and (b) high magnification image, Mn precipitates pinning grain boundaries growth during the hot extrusion.
is considered as the key deformation mode for the alloys. The Schmid factor for a basal plane can be defined as M=cosθ.cosλ [31], where θ and λ are the angles between the tensile direction and the normal direction of the basal plane and the angle between the tensile direction and the [112 0] slip direction, respectively; the Schmid factor is denoted as M. Obviously, M is decreased by increasing θ. The YS of the alloys could be increased with the reduction of M. From the texture evolution, the basal texture is gradually strengthened by increasing the Mn content, thereby indicating that the basal plane of the grains is nearly parallel to ED (that is, θ is gradually increased). Therefore, a strong basal texture also contributes to strength improvement. (c) Fine precipitates. As mentioned, basal slip is the key deformation mode for the alloys. The effect of precipitate on basal slip can be assumed to depend on the Orowan stress required to bow dislocations around the particles [32]. The Orowan equation can Dp Gb 1 be written as [33] ∆τ = ( 2π 1 − ν )( λ )ln ( r ), where ∆τ is the critical 0 resolved shear stress (CRSS); G, b, v, Dp, λ, and r0 are the shear modulus, Burgers vector of gliding dislocations, Poisson ratio, planar diameter of the particles on the basal plane, effective planar inter-obstacle spacing, and dislocation core radius, respectively. The presence of a large number of fine basal and < 0001 > Mn precipitates effectively hinders the dislocation and leads to its accumulation. The increment of Mn precipitates results in a reduction in λ and an enhancement in CRSS (∆τ ). Accordingly, the high TYS of ZMJ220 can be attributed to the ultra-fine DRXed grains based on the Hall–Petch effect, the large number of spherical Mn and < 0001 > Mn precipitates through precipitation and dispersion strengthening, and the strong basal textures of the un-DRXed grains.
effectively hinders dislocations and results in high stress concentration during hot extrusion. This high stress concentration can lead to local cracks around coarse particles. As shown in Fig. 3, these particles are partially broken during extrusion, and their size remains in the range of > 1 µm after extrusion. With further deformation, the coarse Mg– Zn–Sr particles become subject to dislocation accumulation and serve as nucleation sites for DRX. As indicated by the red circle in Fig. 3, the fine DRXed grains near the Mg–Zn–Sr particles are induced by particle simulation nucleation. The fine Mn precipitates effectively prevent the growth of newly recrystallized grains and refine the DRXed grains during extrusion [12]. After extrusion, Mn precipitates are observed around the fine DRXed grain boundaries (red regions in Fig. 7(a), bright-field TEM images of as-extruded ZMJ220). Moreover, many relatively coarse Mn precipitates with diameters of 15–30 nm are clearly observed at the grain boundaries (red arrows in Fig. 7(b)). Jung et al. [12] reported that fine Mg17Al12 precipitates formed by aging prior to extrusion effectively reduce the average size of DRXed grains in the Mg–7.6Al–0.4Zn extruded alloy. Mg(Zn,Zr) precipitates formed before extrusion also result in fine DRXed grains in ZK60 alloys [11,24]. Pan et al. [27] also indicated that the formation of fine DRXed grains in extruded Mg–Ca alloys can be attributed to the effective pinning of DRXed grain boundaries by pre-formed precipitates. Coarse Mg–Zn–Sr particles can be considered to act as heterogeneous nucleation sites of recrystallization, and fine Mn precipitates formed before extrusion can effectively prevent the growth of newly recrystallized grains. Thus, coarse Mg–Zn–Sr particles and high Mn content are the main factors that lead to the formation of fine DRXed grains in Mn-based alloys. In the present study, the strengths of ZJ20 are gradually improved with the increased Mn content. Several factors are considered to result in the increased tensile strength:
PSN is known to modify the textures and improve the anisotropy of extruded alloys [34]. A mass of Mg–Zn–Sr phases are observed in the alloys in the present study. Thus, the σCYS/σTYS of the Mg–Zn–Sr-based alloy is higher than that of ZK60, which was fabricated under the same conditions. Extension twinning is regarded as an important factor in the reduction of CYS in Mg alloys [29]. Numerous spherical Mn precipitates can effectively prevent dislocation motion and increase dislocation pile-ups, thereby overcoming the back stress of twinning (when twinning occurs through shear) and suppressing extension twinning when the sample is subjected to compressive loads along the ED [35]. Although the coarse unDRXed grain regions tend to produce tension twinning, the width of the regions is less than that of the DRXed grains of the ZJ20 alloy, and the high fraction of fine DRXed grains significantly prevents tension twinning. Overall, these developments improve the CYS and yield anisotropy of the alloys. The addition of high amounts of Mn to ZJ20 effectively refines the
(a) Fine grain size. The Hall–Petch coefficient for Mg alloys is ~0.7 MPa m−1/2[3], which is much higher than that of Al alloys (~0.04 MPa m−1/2) [28]. The YS of AZ31 increases from 160 MPa to 280 MPa because of grain refinement through the alternate biaxial reverse corrugation method [29]. In the present study, increasing Mn results in the effective refinement of the size of the DRXed grains from 17.8 µm to ~3.1 µm and ~1.5 µm. Moreover, the increased Mn increases the volume fraction of the unDRXed grain regions, the widths of which are less than those of the DRXed grains of the ZJ20 alloy. Thus, grain refinement significantly contributes to the improvement of Mg alloy strength. (b) Strong basal texture. Mg and its alloys are known to have several deformation modes, i.e., basal slip, prismatic slip, and second pyramidal slip [30]. Generally, deforming crystals is more difficult along the c-axis than along any other crystal axis. Thus, basal slip 5
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DRXed grains and substantially enhances the TYS and CYS without causing a reduction in ductility. ZMJ220 is a potential candidate for application as a low-cost, high-strength, and high-plastic wrought magnesium alloy. 5. Conclusion In summary, the addition of large amounts of Mn to the ZJ20 extruded alloy effectively enhances the strength and improves the anisotropy of the alloy without altering its ductility. The mechanical properties of ZJ20 are gradually improved with the increased Mn content. With the addition of 2 wt% Mn, respective increases in TYS, UTS, CYS, and σCYS/σTYS of 127 MPa, 75 MPa, 114 MPa, and 0.09 are observed. The TYS, UTS, CYS, TF, and σCYS/σTYS of ZMJ220 are 312 MPa, 333 MPa, 240 MPa, 17%, and 0.77, respectively. The presence of ultrafine DRXed grains and a large number of spherical Mn and < 0001 > Mn precipitates are the main factors that explain the high strength and low yield anisotropy. Acknowledgments The present work was supported by the National Natural Science Foundation of China (Project 51474043), the Ministry of Science and Technology of China (2014DFG52810), the Ministry of Education of China (SRFDR 20130191110018 and CDJZR13130086) and Chongqing Municipal Government (CSTC2013JCYJC60001, Two River Scholar Project and The Chief Scientist Studio Project). Natural Science Foundation project of Chongqing Science and Technology Commission (CSTC2012ggB50003 and CSTC2013jjB50006). References [1] [2] [3] [4]
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