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Improving mechanical properties of ZM61 magnesium alloy by aging before extrusion
Journal of Alloys and Compounds 690 (2017) 553e560
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Improving mechanical properties of ZM61 magnesium alloy by aging before extrusion Daliang Yu a, b, Dingfei Zhang a, b, *, Jing Sun a, b, Yuanxin Luo c, Junyao Xu a, b, Hongju Zhang a, b, Fusheng Pan b, d a
College of Materials Science and Engineering, Chongqing University, Chongqing, 400045, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing, 400044, China College of Mechanical Engineering, Chongqing University, Chongqing, 400030, China d Chongqing Academy of Science and Technology, Chongqing, 401123, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2016 Received in revised form 8 August 2016 Accepted 14 August 2016 Available online 17 August 2016
The effect of aging treatment before extrusion on microstructure and mechanical properties of Mg-6Zn1Mn alloy was investigated. Results showed that the existence of dispersive fine MgZn2 particles before extrusion promoted dynamic recrystallization in subsequent extrusion process and leaded to fine microstructures (grain size 1~2 mm). The rod-shaped and plate-shaped precipitates in aged alloys transformed to fine and dispersed spherical precipitates after extrusion process. With aging and extrusion treatment, ZM61 alloy showed an outstanding tensile strength and elongation balance (YS 281 MPa, UTS 378 MPa and elongation 19.3%); compressive yield strength increased from 148 MPa to 259 MPa. The tensionecompression yield asymmetry was reduced at the same time. The strengthening effect of aging treatment before extrusion in ZM61 alloy proved in this paper provided a new method for the design of relatively inexpensive high-performance magnesium alloys. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Extrusion Aging Mechanical properties
1. Introduction As the lightest structure material, magnesium (Mg) alloys have excellent properties such as high specific strength, high specific stiffness, good castability and machinability, high damping capacity and good electromagnetic shielding [1e3]. With the increasing of fuel efficiency and decreasing of CO2 emissions, these advantages make Mg alloys very attractive as structural materials in a wide variety of applications, especially in automobile industry. However, due to their poor mechanical properties, applications of Mg alloys have been limited to some extent. Although great efforts have been done, the mechanical properties of Mg alloys still can't satisfy the needs [4]. For the applications of Mg alloys, their mechanical properties should be further improved. Conventional thermo-mechanical treatments, such as extrusion, rolling and forging, are effective techniques for improving the mechanical properties of Mg alloys through refining grains, eliminating cast defects and homogenizing microstructure [5].
* Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400045, China. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.08.128 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Nonetheless, with the help of conventional thermo-mechanical equipment and processing, the mechanical properties of Mg alloys are still not satisfactory. Aging hardening is widely used in aluminium (Al) alloys and Mg alloys. However, aging treatment, the final strengthen step, improves strength at the expense of ductility which confines its application. Some works have been done to reveal the influence of particles on the recrystallization behavior and microstructure evolution in Mg and Al alloys [5e10]. Robson et al. [7] reported that in MgeMn alloys precipitates did not represent the principal sites for nucleation of new grains during hot deformation, and the orientation gradient in the vicinity of coarse particles resulted in the occurrence of new high-angle grain boundaries (HAGBs) during deformation. They also found direct evidence that particle-stimulated nucleation (PSN) around large particles and small particles had a minor effect on dynamic recrystallization (DRX) [7,8]. Tong et al. suggested that the DRX could primarily take place in the region close to the coarse phases and then gradually enlarged into the interior of original Mg grains with further straining during extrusion [5]. Yu et al. found that in Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr the plate-like Mg5RE precipitates that introduced in pre-ageing treatment could cause grain refinement through grain boundary
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pinning [9]. Previous studies by T. Al-Samman have reported the particles in the modified alloy resulted in new orientated grains, and the new orientations were different from the deformed orientation [10]. All of these precursory studies reveal that particles have an influence on DRX behavior and microstructure of Mg alloys. Mg-6Zn-1Mn (ZM61) alloy is a cost-effective, high-strength and age hardenable wrought Mg alloy. Due to high zinc content in ZM61 alloy, it shows noticeable age-hardening response [11]. The agehardening response of ZM61 alloy results from Mg-Zn interme0 tallic compounds. The metastable rod-shaped b1 precipitates are the key age-hardening precipitates, while extensive precipitation of
0
stable plate-shaped b2 phase causes overaging [11e14]. Based on our previous work, after double aging treatment the yield strength and ultimate tensile strength of ZM61 ally can be 336 MPa and 366 MPa, respectively [15,16]. However, the elongation decreased sharply due to the aging treatment. According to aforesaid work, the pre-existence of precipitates has an influence on microstructure in the subsequent thermomechanical process. Precipitates can either promote or suppress recrystallization depending on their size, spacing and fraction [7]. For aged ZM61 alloy, it contains high density nano-sized Mg-Zn precipitates. If these high density precipitates can improve
Fig. 1. Typical microstructures before extrusion: SEM micrographs of (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy, (d) 48h-aged alloy and (e) bright field TEM image of 48h-aged alloy.
D. Yu et al. / Journal of Alloys and Compounds 690 (2017) 553e560
microstructure of ZM61 alloy, it may open up a new way to enhance Mg alloys. Based on this idea, the influence of Mg-Zn precipitates that separate out before extrusion on extruded microstructure and mechanical properties was revealed by using thermo-mechanical treatments of aging treatment and subsequent extrusion. 2. Experiments The material used in this study was Mg-6Zn-1Mn (wt.%) alloy (ZM61). The alloy was prepared by melting in an electrical resistance furnace under a SO2þCO2 protective gas and then cast into a steel mould. The actual composition of Mg-Zn-Mn alloy was 5.81 wt% Zn, 0.80 wt% Mn, and the balance Mg, measured by the Xray fluorescence spectrometer (Shimadzu XRF1800). The ingots were homogenized at 420 C for 12 h and followed by hot water (~90 C) quenching to remove macrosegregation and form supersaturated solid solution. In order to form different number densities of precipitates, the homogenized ingots were aged at 230 C for 6 h, 24 h, 48 h, respectively. An ingot without aging treatment before extrusion was also used in these experiments for comparison. The comparison alloy and aged alloys were hot extruded at 300 C with a reduction ratio of 25 and a relatively slow extrusion speed of 1 mm s1 to avoid precipitates dissolving into matrix by the plastic working (hereafter, denoted as 0h-aged alloy, 6h-aged alloy, 24haged alloy and 48h-aged alloy). Before the extrusion, these billets were carefully held in extrusion container at 300 C for about 40 min.
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The microstructures of the alloys were observed by optical microscopy (OM; OLYMPUS OLS4000). Scanning electron microscopy (SEM; FEI Nova 400) and transmission electron microscopy (TEM; FEI TECNAI G2 F20) were used to characterize the precipitates in these alloys with different heat treatments states. The microstructures of extruded specimens were also investigated by EBSD. EBSD was operated at 20 kV acceleration voltage with an emission current of 3.0 nA. Oxford Instruments Naordlys Nano EBSD system with Channel 5 data acquisition and analysis software was used for data acquisition and analysis. The macrotexture was measured by X-ray diffraction (XRD, Rigaku D/MAX-2500PC) with Cu-Ka radiation at 40 kV and 30 mA and a scan rate of 0.03 deg s1 in a 2q range of 10e90 . The tensile properties were measured using smooth dog bone-shaped tensile specimens with a gauge size of 5 mm in diameter and 35 mm in length. The compression specimens were cylindrical with a diameter of 8 mm and a height of 12 mm. All the specimens had an axis along the extrusion direction. The tensile tests were performed at a constant strain rate of 2 103 s1 with the load direction parallel to the specimen axis, the yield stress was determined as the 0.2% offset. 3. Results and discussion 3.1. Microstructures before extrusion The typical microstructures of ZM61 alloys before extrusion are shown in Fig. 1. Fig. 1a, b 1c and 1d show SEM micrographs of 0h-
Fig. 2. Typical optical microstructures after extrusion: (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy and (d) 48h-aged alloy.
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aged alloy, 6h-aged alloy, 24h-aged and 48h-aged specimen, respectively. As shown in Fig. 1, dispersed precipitates were detected in all the specimens. In addition, precipitates in 24h-aged and 48h-aged specimens were much coarser than that in 0h-aged and 6h-aged specimens. The precipitates in 0h-aged specimen mainly separated out in preheat process before extrusion. It should be noticed that 6h-aged specimen shows high density precipitates and these precipitates are much finer than those in 24h-aged and 48h-aged specimens. The specific orientation relationship between two kinds of precipitates could be found in 24haged and 48h-aged specimens. However, 48h-aged specimen had more precipitates than 24h-aged specimen. According to related studies [12e14,17], the precipitation sequence of Mg-Zn based 0 alloy is supersaturated solid solution (SSSS) / GP zones / b1 0 (rod-shaped) / b2 (plate-shaped) / b (MgZn or Mg2Zn3), and 0 the growth of b2 occurred with a substantial reduction in the 0 number density of b1 precipitates. It could be concluded that 6h0 aged alloy had more b1 than 48h-aged alloy. With aging process, 0 0 the number of b1 gradually decreased and b2 coarsened at the same time. Based on our previous work, there are three kinds of precipitates in ZM61 alloy subjected to aging: spherical Mn phase particles with 0
the size of 40e60 nm [15,18,19], rod-shaped b1 and plate-shaped 0
0
b2 phases. Rob-shaped b1 is high coherency with matrix and plate0
shaped b2 retains an incoherent lattice relationship with matrix. 0
Furthermore, rob-shaped b1 is formed along the-axis and it has been reported that the aspect ratio of the length and width was about 20e100 [12,20]. The orientation relationships for these two
precipitate phases are ½0001b0 ==½1120a andð1120Þb0 ==ð0001Þa , 1 1 ½1120b0 ==½1010a and ð0001Þb0 ==ð0001Þa , respectively [12,14,21, 2
2
22]. These two orientation relationships with matrix could be found in Fig. 1c, d and e. The thin and long precipitates in Fig. 1d are 0
0
rod-shaped b1 and the thick precipitates are plate-shaped b2 . The orientation relationships for these two precipitate phases can be distinguished. Fig. 1e showed bright field TEM image of 48h-aged 0
alloy and this image was taken along the ½2110Mg direction. b1 and 0
b2 were marked by the black arrows. After aging treated for 48 h, 0 0 the number of b1 has been reduced and the aspect ratio of b1 also has been decreased to about 10. 3.2. Optical microstructures after extrusion Fig. 2 shows optical micrographs obtained on the cross section perpendicular to the extrusion direction. The extruded bars were subjected to different aging treatment before extrusion. Owing to the occurrence of DRX during hot extrusion, the optical microstructures of all the alloys were refined remarkably. It's interesting to find that the grain size decreased with prolonging the aging time before extrusion. The microstructure of 0h-aged alloy showed a typical duplex grain structure with the grain size ranging from 3 mm to 50 mm and fine DRXed grains cluster distributed around the coarse grains. The grain size of 6h-aged alloy was more uniform in comparison with 0h-aged alloy as shown in Fig. 2b, but many coarse grains could still be found. In 24h-aged alloy, a few coarse grains with the size of ~15 mm could
Fig. 3. SEM images of ZM61 alloy after extrusion: (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy and (d) 48h-aged alloy.
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Fig. 4. Typical TEM images of extruded alloy: (a) 0h-aged specimen and (b) 48h-aged specimen.
be observed. The 48h-aged specimen showed the most heterogeneous and finest microstructure among these alloys. The grain size of 48h-aged alloy was almost the same with the size of ~1 mm. In comparison with optical micrographs, it's no doubt that the nano-sized Mg-Zn precipitates have a remarkable influence on promoting grain refinement during hot extrusion in ZM61 alloy.
extrusion, Mg-Zn particles were hardly to be deformed plastically. As a consequence, Mg-Zn particles were torn up by extrusion and a deformation zone generated in the vicinity of Mg-Zn particles which were preferred sites for nucleation of dynamic recrystallization [20,23].
3.3. SEM microstructures of extruded specimens
In order to disclose the evolution of particles after extrusion, the microstructures of extruded specimens were examined in detail by TEM. Fig. 4a and b shows TEM bright field micrographs of extruded 0h-aged and 48h-aged specimens, respectively. For comparing the morphology of particles with specimen before extrusion, all the TEM images were taken along the ½2110Mg direction. As shown in Fig. 4a, a few fine spherical particles could be found in 0h-aged specimen. However, in 48h-aged specimen, a lot of particles were randomly distributed and much coarser 0 than those in 0h-aged specimen. The rod-shaped b1 and plate0 shaped b2 particles were replaced by uniformly dispersed of spherical particles. It was believed that the plastic deformation 0 0 tore b1 and b2 apart and resulted in the morphology changes [20]. It had been reported that precipitate morphology had a clear effect on strength, ductility as well as fracture toughness [24,25]. Spherical precipitates were more promising in increasing mechanical properties since they strongly pinned dislocation
Fig. 3 shows the SEM microstructures of extruded. As shown in Fig. 3 all the extruded alloy contained many fine precipitates with the size less than 200 nm. Fine participates are randomly distributed in both grain boundaries and grain interior. It can be seen clearly that the amount of precipitates in extruded alloy increased with the increasing of aging time before extrusion. Since low content of Mn in ZM61 alloy, most of these dispersed precipitates were Mg-Zn particles. It implied that the pre-existent Mg-Zn particles didn't dissolve into matrix and they played an important role in DRX. It's noteworthy that the distribution of precipitates in Fig. 3 was quite different from those in Fig. 1. In Fig. 1 two kinds of precipitates showed a particular orientation relationship with each other (shown in Fig. 1c, d and e), however, the orientation relationship was disappeared and precipitates were distributed randomly in extruded specimens (shown in Fig. 3). During
3.4. TEM images of extruded specimens
Fig. 5. Inverse pole figure maps of extruded microstructure: (a) 0h-aged alloy, (b) 48h-aged alloy.
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motion in plastic deformation [25]. The spherical precipitates in extruded alloy would also result in an enhancement of mechanical properties. 3.5. IPF maps of extruded specimens Fig. 5a and b shows typical EBSD inverse pole figure (IPF) maps of extruded 0h-aged and 48h-aged specimens, respectively. The results of EBSD analysis showed that there was a significantly difference in grain sizes between 0h-aged and 48h-aged specimens. Fig. 5a shows a typical duplex grain structure while the microstructure in Fig. 5b was more homogeneous. The color of the most grains was green and blue, indicating that most of the ð1120Þ planes were parallel to the observed surface. With the aging treatment before extrusion, the extruded microstructures were distinctly refined. No twins were detected in extruded alloys. 3.6. Macrotexture of extruded specimens Fig. 6a and b illustrate the textures of the extruded 0h-aged and 48h-aged specimens in the form of pole figures, respectively. The extruded ZM61 alloys show a fibre-type texture that ð0001Þ basal planes are mainly oriented parallel to the extrusion direction. This type of texture is known as typical texture of Mg alloys that develops during the uni-axial deformation [4]. In the present study, it is interesting to find that the aging treatment before extrusion can slightly sharpen the extrusion texture compared with that of nonaged specimen. However, the type of texture is almost the same in 0h-aged and 48h-aged specimen. Due to the hexagonal closepacked (HCP) lattice of Mg alloys, they have limited slip systems to accommodate imposed plastic deformation, and textures are easy to form during the thermos-
mechanical processing [10,26,27]. Ball and Prangnell first discovered the texture weakening effect in an extruded WE54 Mg alloy and they attributed it to the result of PSN [28]. Ever since, many researchers have found that the particle during extrusion can lead to texture weaken. However, in this study, it is worthy to find that the texture intensity in 48h-aged specimen is higher than that in 0h-aged alloy, which is quite different from the known results. It is recognized that the texture changes in this study is directly related to the presence of precipitates. If particles do not deform compatibly with matrix, plastic relaxation around the particles will result in lattice rotations in the regions adjacent to the particles [4]. During extrusion, nucleation of recrystallization may occur in these regions and the precipitates will obstruct the local slip patterns in the matrix [20]. Though PSN can lead to texture weakening in many RE-containing Mg alloys, however, some studies also show that PSN doesn't play the dominated role in texture evolution because the grains that formed in the vicinity of particles are much less than that formed at other sites [4,7]. The retardation of grain growth can also result in the texture weakening. The particles in this study are nano-sized which are much smaller than those in previous with the size of 1e10 m m [7,8], thus the retardation effect could be much slighter in this study. It can be seen clearly in Fig. 1 that the Mg-Zn precipitates in aged specimens have a strict orientation relationship with matrix and the orientation relationship in non-aged specimens is ambiguous. 0 Wei [12] has reported that the plate-shaped b2 has a hexagonal crystal lattice with the lattice parameters of a ¼ 0.525 nm and c ¼ 0.855 nm that is quite similar with matrix. Thus during extrusion process the Mg-Zn precipitates can promote the preferential growth of the grains with certain orientations and enable grain growth in specific orientation [29]. Consequently, the texture intensity increases with grain growth.
Fig. 6. (0002) and (10-10) pole figures of the as-extruded alloy: (a) 0h-aged alloy, (b) 48h-aged alloy.
D. Yu et al. / Journal of Alloys and Compounds 690 (2017) 553e560
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Fig. 7. Engineering stressestrain curves of all the extruded alloys: (a) tensile, (b) compressive.
3.7. Mechanical properties of extruded alloys Typical tensile and compressive stressestrain curves of extruded alloy with different aging treatment before extrusion are shown in Fig. 7. Tensile strength and ductility increase with prolonging aging time before extrusion. After aging treatment before extrusion, yield strength and tensile strength of extruded alloys increased from 211 MPa and 310 MPa of 0h-aged alloy to 227 MPa and 328 MPa, 275 MPa and 367 MPa, 281 MPa and 371 MPa of 6haged alloy, 24h-aged alloy and 48h-aged alloy, respectively. The elongation also increased from 14.4% of 0h-aged alloy to 16.6%, 18.7% and 19.3% of 6h-aged alloy, 24h-aged alloy and 48h-aged alloy, respectively (see Table 1). In comparison with 0h-aged alloy, the mechanical properties of 24h-aged alloy and 48h-aged alloy have been significantly improved. The tensile strength, compressive strength and the asymmetry ratio (the ratio of the yield strength in compression to the yield strength in tension) were shown in Table 1. Extruded Mg alloys normally exhibit strong fiber type texture with basal planes aligned parallel to the extrusion direction [30,31]. In polycrystalline Mg alloys, because of the low symmetry of HCP structure the deformation mechanisms depends on the orientation of the applied stress and crystallographic texture of the material. When the direction of compressive stress is parallel to the extrusion direction the preferentially developed fiber textures are unfavourable for the activation of basal slip. For extruded bar, f1012g extension twinning is easy to activate under tension perpendicular to extrusion direction or compressive load along extrusion direction. As a result, the sharp texture gives rise to its aggravated tension-compression yield asymmetry so that the application of Mg alloys is restricted to a certain extent. In addition to texture, microstructure also has a great influence on twinning behavior in Mg alloy during deformation.
Table 1 Tensile and compressive strength of ZM61 alloy. Materials
TYS, MPa
UTS, MPa
d, %
CYS, MPa
UCS, MPa
CYS/TYS
0h-aged alloy 6h-aged alloy 24h-aged alloy 48h-aged alloy
211 227 275 281
310 328 367 378
14.4 16.6 18.7 19.3
148 205 245 259
423 464 483 448
0.70 0.90 0.89 0.92
TYS-tensile yield strength; UTS-ultimate tensile strength; d: elongation; CYS: compressive yield strength; UCS: ultimate compressive strength; CYS/TYS: asymmetry ratio.
It has been shown that fine grains help to prevent formation of deformation twins and activate dislocations on non-basal planes, which will lead to the enhancement of strength as well as ductility [32]. It implies that grain refinement can not only enhance tensile and compressive yield strength but also reduce yield asymmetry by inhibiting twining in compression [33]. For the application of Mg alloys, reducing the yield asymmetry is significant because many Mg components are under compression load in service. All the alloys were extruded at 300 C with the same extrusion parameters and the only difference is the aging time before extrusion. It's noteworthy that the microstructure and mechanical properties of extruded alloy which subjected to aging treatment were significantly improved. The alloy that subjected to aging and subsequent hot extrusion treatment shows an excellent strength and ductility balance which is even more outstanding than that of some Mg-RE wrought alloys [18,34]. It's speculated that this thermo-mechanical treatment can be used in other age hardenable Mg alloys such as Mg-Al-Based and Mg-ZnBased alloys. The possibility of aging and subsequent extrusion treatment in Mg alloys proven in this paper opens up a new avenue for the design of relatively low-cost high performance magnesium alloys. 4. Summary The influence of aging treatment and subsequent hot extrusion process on microstructure and mechanical properties of ZM61 alloy were investigated, the following results were obtained. (a) After aging and subsequent hot extrusion treatment, a considerable improvement in strength and ductility is achieved. Due to the combination of fine grains and dispersion strengthening, the yield strength, tensile strength and elongation increased form 211 MPa, 310 MPa and 14.4% of 0h-aged alloy to 281 MPa, 371 MPa and 19.3% of 48h-aged alloy. The yield asymmetry was reduced at the same time. (b) After aging treatment and extrusion, the strict orientation 0 relationship between rod-shaped b1 precipitates and plate0 shaped b2 precipitates was disappeared and dispersed spherical precipitates were obtained. (c) The presence of Mg-Zn precipitates in hot extrusion process promoted grain refinement and leaded to a slight sharpening of texture intensity. The grain size decreased with the increasing of precipitates amount before extrusion.
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Acknowledgements This work was supported by National Great Theoretic Research Project (2013CB632200); National Natural Science Foundation of China (51474043) and Chongqing Municipal Government Project (CSTC2013JCYJC60001); and Sharing fund of Chongqing University's large-scale equipment. References [1] J. Li, R. Chen, Y. Ma, W. Ke, Effect of Zr modification on solidification behavior and mechanical properties of MgeYeRE (WE54) alloy, J. Magnes. Alloy 1 (2013) 346e351. [2] F. Kabirian, R. Mahmudi, Effects of zirconium additions on the microstructure of As-Cast and aged AZ91 magnesium alloy, Adv. Eng. Mater 11 (2009) 189e193. [3] M. Kaseem, B.K. Chung, H.W. Yang, K. Hamad, Y.G. Ko, Effect of deformation temperature on microstructure and mechanical properties of AZ31 Mg Alloy processed by differential-speed rolling, J. Mater. Sci. Technol. 31 (2015) 498e503. [4] J. Bohlen, S. Yi, D. Letzig, K.U. Kainer, Effect of rare earth elements on the microstructure and texture development in magnesiumemanganese alloys during extrusion, Mater. Sci. Eng. A 527 (2010) 7092e7098. [5] L.B. Tong, X. Li, D.P. Zhang, L.R. Cheng, J. Meng, H.J. Zhang, Dynamic recrystallization and texture evolution of MgeYeZn alloy during hot extrusion process, Mater. Charact. 92 (2014) 77e83. [6] C.H. Liu, X.L. Li, S.H. Wang, J.H. Chen, Q. Teng, J. Chen, Y. Gu, A tuning nanoprecipitation approach for achieving enhanced strength and good ductility in Al alloys, Mater. Des. 54 (2014) 144e148 (1980-2015). [7] J.D. Robson, D.T. Henry, B. Davis, Particle effects on recrystallization in magnesiumemanganese alloys: Particle-stimulated nucleation, Acta Mater. 57 (2009) 2739e2747. [8] S. Wang, R. Ma, L. Yang, Y. Wang, Y. Wang, Precipitates effect on microstructure of as-deformed and as-annealed AZ41 magnesium alloys by adding Mn and Ca, J. Mater. Sci. 46 (2011) 3060e3065. [9] Z. Yu, Y. Huang, C.L. Mendis, N. Hort, J. Meng, Microstructural evolution and mechanical properties of Mge11Gde4.5Ye1Nde1.5Zne0.5Zr alloy prepared via pre-ageing and hot extrusion, Mater. Sci. Eng. A 624 (2015) 23e31. [10] T. Al-Samman, Modification of texture and microstructure of magnesium alloy extrusions by particle-stimulated recrystallization, Mater. Sci. Eng. A 560 (2013) 561e566. [11] S. Guoliang, Z. Dingfei, Z. Xiabing, Z. Kui, L. Xinggang, L. Yongjun, M. Minglong, Precipitate evolution in Mg-6 wt%Zn-1 wt%Mn alloy, Rare Metal. Mat. Eng. 42 (2013) 2447e2452. [12] L.Y. Wei, G.L. Dunlop, H. Westengen, Precipitation hardening of Mg-Zn and Mg-Zn-RE alloys, Metall. Mater. Trans. A 26 (1995) 1705e1716. [13] J. Nie, Precipitation and hardening in magnesium alloys, Metall. Mater. Trans. A 43 (2012) 3891e3939. [14] J.B. Clark, Transmission electron microscopy study of age hardening in a Mg-5 wt.% Zn alloy, Acta Metall. 13 (1965) 1281e1289. [15] F.G. Qi, D.F. Zhang, Z.T. Zhu, X.X. Xu, G.L. Shi, Effect of heat treatment on
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Improving mechanical properties of ZM61 magnesium alloy by aging before extrusion Daliang Yu a, b, Dingfei Zhang a, b, *, Jing Sun a, b, Yuanxin Luo c, Junyao Xu a, b, Hongju Zhang a, b, Fusheng Pan b, d a
College of Materials Science and Engineering, Chongqing University, Chongqing, 400045, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing, 400044, China College of Mechanical Engineering, Chongqing University, Chongqing, 400030, China d Chongqing Academy of Science and Technology, Chongqing, 401123, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2016 Received in revised form 8 August 2016 Accepted 14 August 2016 Available online 17 August 2016
The effect of aging treatment before extrusion on microstructure and mechanical properties of Mg-6Zn1Mn alloy was investigated. Results showed that the existence of dispersive fine MgZn2 particles before extrusion promoted dynamic recrystallization in subsequent extrusion process and leaded to fine microstructures (grain size 1~2 mm). The rod-shaped and plate-shaped precipitates in aged alloys transformed to fine and dispersed spherical precipitates after extrusion process. With aging and extrusion treatment, ZM61 alloy showed an outstanding tensile strength and elongation balance (YS 281 MPa, UTS 378 MPa and elongation 19.3%); compressive yield strength increased from 148 MPa to 259 MPa. The tensionecompression yield asymmetry was reduced at the same time. The strengthening effect of aging treatment before extrusion in ZM61 alloy proved in this paper provided a new method for the design of relatively inexpensive high-performance magnesium alloys. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Extrusion Aging Mechanical properties
1. Introduction As the lightest structure material, magnesium (Mg) alloys have excellent properties such as high specific strength, high specific stiffness, good castability and machinability, high damping capacity and good electromagnetic shielding [1e3]. With the increasing of fuel efficiency and decreasing of CO2 emissions, these advantages make Mg alloys very attractive as structural materials in a wide variety of applications, especially in automobile industry. However, due to their poor mechanical properties, applications of Mg alloys have been limited to some extent. Although great efforts have been done, the mechanical properties of Mg alloys still can't satisfy the needs [4]. For the applications of Mg alloys, their mechanical properties should be further improved. Conventional thermo-mechanical treatments, such as extrusion, rolling and forging, are effective techniques for improving the mechanical properties of Mg alloys through refining grains, eliminating cast defects and homogenizing microstructure [5].
* Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400045, China. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.08.128 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Nonetheless, with the help of conventional thermo-mechanical equipment and processing, the mechanical properties of Mg alloys are still not satisfactory. Aging hardening is widely used in aluminium (Al) alloys and Mg alloys. However, aging treatment, the final strengthen step, improves strength at the expense of ductility which confines its application. Some works have been done to reveal the influence of particles on the recrystallization behavior and microstructure evolution in Mg and Al alloys [5e10]. Robson et al. [7] reported that in MgeMn alloys precipitates did not represent the principal sites for nucleation of new grains during hot deformation, and the orientation gradient in the vicinity of coarse particles resulted in the occurrence of new high-angle grain boundaries (HAGBs) during deformation. They also found direct evidence that particle-stimulated nucleation (PSN) around large particles and small particles had a minor effect on dynamic recrystallization (DRX) [7,8]. Tong et al. suggested that the DRX could primarily take place in the region close to the coarse phases and then gradually enlarged into the interior of original Mg grains with further straining during extrusion [5]. Yu et al. found that in Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr the plate-like Mg5RE precipitates that introduced in pre-ageing treatment could cause grain refinement through grain boundary
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pinning [9]. Previous studies by T. Al-Samman have reported the particles in the modified alloy resulted in new orientated grains, and the new orientations were different from the deformed orientation [10]. All of these precursory studies reveal that particles have an influence on DRX behavior and microstructure of Mg alloys. Mg-6Zn-1Mn (ZM61) alloy is a cost-effective, high-strength and age hardenable wrought Mg alloy. Due to high zinc content in ZM61 alloy, it shows noticeable age-hardening response [11]. The agehardening response of ZM61 alloy results from Mg-Zn interme0 tallic compounds. The metastable rod-shaped b1 precipitates are the key age-hardening precipitates, while extensive precipitation of
0
stable plate-shaped b2 phase causes overaging [11e14]. Based on our previous work, after double aging treatment the yield strength and ultimate tensile strength of ZM61 ally can be 336 MPa and 366 MPa, respectively [15,16]. However, the elongation decreased sharply due to the aging treatment. According to aforesaid work, the pre-existence of precipitates has an influence on microstructure in the subsequent thermomechanical process. Precipitates can either promote or suppress recrystallization depending on their size, spacing and fraction [7]. For aged ZM61 alloy, it contains high density nano-sized Mg-Zn precipitates. If these high density precipitates can improve
Fig. 1. Typical microstructures before extrusion: SEM micrographs of (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy, (d) 48h-aged alloy and (e) bright field TEM image of 48h-aged alloy.
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microstructure of ZM61 alloy, it may open up a new way to enhance Mg alloys. Based on this idea, the influence of Mg-Zn precipitates that separate out before extrusion on extruded microstructure and mechanical properties was revealed by using thermo-mechanical treatments of aging treatment and subsequent extrusion. 2. Experiments The material used in this study was Mg-6Zn-1Mn (wt.%) alloy (ZM61). The alloy was prepared by melting in an electrical resistance furnace under a SO2þCO2 protective gas and then cast into a steel mould. The actual composition of Mg-Zn-Mn alloy was 5.81 wt% Zn, 0.80 wt% Mn, and the balance Mg, measured by the Xray fluorescence spectrometer (Shimadzu XRF1800). The ingots were homogenized at 420 C for 12 h and followed by hot water (~90 C) quenching to remove macrosegregation and form supersaturated solid solution. In order to form different number densities of precipitates, the homogenized ingots were aged at 230 C for 6 h, 24 h, 48 h, respectively. An ingot without aging treatment before extrusion was also used in these experiments for comparison. The comparison alloy and aged alloys were hot extruded at 300 C with a reduction ratio of 25 and a relatively slow extrusion speed of 1 mm s1 to avoid precipitates dissolving into matrix by the plastic working (hereafter, denoted as 0h-aged alloy, 6h-aged alloy, 24haged alloy and 48h-aged alloy). Before the extrusion, these billets were carefully held in extrusion container at 300 C for about 40 min.
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The microstructures of the alloys were observed by optical microscopy (OM; OLYMPUS OLS4000). Scanning electron microscopy (SEM; FEI Nova 400) and transmission electron microscopy (TEM; FEI TECNAI G2 F20) were used to characterize the precipitates in these alloys with different heat treatments states. The microstructures of extruded specimens were also investigated by EBSD. EBSD was operated at 20 kV acceleration voltage with an emission current of 3.0 nA. Oxford Instruments Naordlys Nano EBSD system with Channel 5 data acquisition and analysis software was used for data acquisition and analysis. The macrotexture was measured by X-ray diffraction (XRD, Rigaku D/MAX-2500PC) with Cu-Ka radiation at 40 kV and 30 mA and a scan rate of 0.03 deg s1 in a 2q range of 10e90 . The tensile properties were measured using smooth dog bone-shaped tensile specimens with a gauge size of 5 mm in diameter and 35 mm in length. The compression specimens were cylindrical with a diameter of 8 mm and a height of 12 mm. All the specimens had an axis along the extrusion direction. The tensile tests were performed at a constant strain rate of 2 103 s1 with the load direction parallel to the specimen axis, the yield stress was determined as the 0.2% offset. 3. Results and discussion 3.1. Microstructures before extrusion The typical microstructures of ZM61 alloys before extrusion are shown in Fig. 1. Fig. 1a, b 1c and 1d show SEM micrographs of 0h-
Fig. 2. Typical optical microstructures after extrusion: (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy and (d) 48h-aged alloy.
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aged alloy, 6h-aged alloy, 24h-aged and 48h-aged specimen, respectively. As shown in Fig. 1, dispersed precipitates were detected in all the specimens. In addition, precipitates in 24h-aged and 48h-aged specimens were much coarser than that in 0h-aged and 6h-aged specimens. The precipitates in 0h-aged specimen mainly separated out in preheat process before extrusion. It should be noticed that 6h-aged specimen shows high density precipitates and these precipitates are much finer than those in 24h-aged and 48h-aged specimens. The specific orientation relationship between two kinds of precipitates could be found in 24haged and 48h-aged specimens. However, 48h-aged specimen had more precipitates than 24h-aged specimen. According to related studies [12e14,17], the precipitation sequence of Mg-Zn based 0 alloy is supersaturated solid solution (SSSS) / GP zones / b1 0 (rod-shaped) / b2 (plate-shaped) / b (MgZn or Mg2Zn3), and 0 the growth of b2 occurred with a substantial reduction in the 0 number density of b1 precipitates. It could be concluded that 6h0 aged alloy had more b1 than 48h-aged alloy. With aging process, 0 0 the number of b1 gradually decreased and b2 coarsened at the same time. Based on our previous work, there are three kinds of precipitates in ZM61 alloy subjected to aging: spherical Mn phase particles with 0
the size of 40e60 nm [15,18,19], rod-shaped b1 and plate-shaped 0
0
b2 phases. Rob-shaped b1 is high coherency with matrix and plate0
shaped b2 retains an incoherent lattice relationship with matrix. 0
Furthermore, rob-shaped b1 is formed along the
precipitate phases are ½0001b0 ==½1120a andð1120Þb0 ==ð0001Þa , 1 1 ½1120b0 ==½1010a and ð0001Þb0 ==ð0001Þa , respectively [12,14,21, 2
2
22]. These two orientation relationships with matrix could be found in Fig. 1c, d and e. The thin and long precipitates in Fig. 1d are 0
0
rod-shaped b1 and the thick precipitates are plate-shaped b2 . The orientation relationships for these two precipitate phases can be distinguished. Fig. 1e showed bright field TEM image of 48h-aged 0
alloy and this image was taken along the ½2110Mg direction. b1 and 0
b2 were marked by the black arrows. After aging treated for 48 h, 0 0 the number of b1 has been reduced and the aspect ratio of b1 also has been decreased to about 10. 3.2. Optical microstructures after extrusion Fig. 2 shows optical micrographs obtained on the cross section perpendicular to the extrusion direction. The extruded bars were subjected to different aging treatment before extrusion. Owing to the occurrence of DRX during hot extrusion, the optical microstructures of all the alloys were refined remarkably. It's interesting to find that the grain size decreased with prolonging the aging time before extrusion. The microstructure of 0h-aged alloy showed a typical duplex grain structure with the grain size ranging from 3 mm to 50 mm and fine DRXed grains cluster distributed around the coarse grains. The grain size of 6h-aged alloy was more uniform in comparison with 0h-aged alloy as shown in Fig. 2b, but many coarse grains could still be found. In 24h-aged alloy, a few coarse grains with the size of ~15 mm could
Fig. 3. SEM images of ZM61 alloy after extrusion: (a) 0h-aged alloy, (b) 6h-aged alloy, (c) 24h-aged alloy and (d) 48h-aged alloy.
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Fig. 4. Typical TEM images of extruded alloy: (a) 0h-aged specimen and (b) 48h-aged specimen.
be observed. The 48h-aged specimen showed the most heterogeneous and finest microstructure among these alloys. The grain size of 48h-aged alloy was almost the same with the size of ~1 mm. In comparison with optical micrographs, it's no doubt that the nano-sized Mg-Zn precipitates have a remarkable influence on promoting grain refinement during hot extrusion in ZM61 alloy.
extrusion, Mg-Zn particles were hardly to be deformed plastically. As a consequence, Mg-Zn particles were torn up by extrusion and a deformation zone generated in the vicinity of Mg-Zn particles which were preferred sites for nucleation of dynamic recrystallization [20,23].
3.3. SEM microstructures of extruded specimens
In order to disclose the evolution of particles after extrusion, the microstructures of extruded specimens were examined in detail by TEM. Fig. 4a and b shows TEM bright field micrographs of extruded 0h-aged and 48h-aged specimens, respectively. For comparing the morphology of particles with specimen before extrusion, all the TEM images were taken along the ½2110Mg direction. As shown in Fig. 4a, a few fine spherical particles could be found in 0h-aged specimen. However, in 48h-aged specimen, a lot of particles were randomly distributed and much coarser 0 than those in 0h-aged specimen. The rod-shaped b1 and plate0 shaped b2 particles were replaced by uniformly dispersed of spherical particles. It was believed that the plastic deformation 0 0 tore b1 and b2 apart and resulted in the morphology changes [20]. It had been reported that precipitate morphology had a clear effect on strength, ductility as well as fracture toughness [24,25]. Spherical precipitates were more promising in increasing mechanical properties since they strongly pinned dislocation
Fig. 3 shows the SEM microstructures of extruded. As shown in Fig. 3 all the extruded alloy contained many fine precipitates with the size less than 200 nm. Fine participates are randomly distributed in both grain boundaries and grain interior. It can be seen clearly that the amount of precipitates in extruded alloy increased with the increasing of aging time before extrusion. Since low content of Mn in ZM61 alloy, most of these dispersed precipitates were Mg-Zn particles. It implied that the pre-existent Mg-Zn particles didn't dissolve into matrix and they played an important role in DRX. It's noteworthy that the distribution of precipitates in Fig. 3 was quite different from those in Fig. 1. In Fig. 1 two kinds of precipitates showed a particular orientation relationship with each other (shown in Fig. 1c, d and e), however, the orientation relationship was disappeared and precipitates were distributed randomly in extruded specimens (shown in Fig. 3). During
3.4. TEM images of extruded specimens
Fig. 5. Inverse pole figure maps of extruded microstructure: (a) 0h-aged alloy, (b) 48h-aged alloy.
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motion in plastic deformation [25]. The spherical precipitates in extruded alloy would also result in an enhancement of mechanical properties. 3.5. IPF maps of extruded specimens Fig. 5a and b shows typical EBSD inverse pole figure (IPF) maps of extruded 0h-aged and 48h-aged specimens, respectively. The results of EBSD analysis showed that there was a significantly difference in grain sizes between 0h-aged and 48h-aged specimens. Fig. 5a shows a typical duplex grain structure while the microstructure in Fig. 5b was more homogeneous. The color of the most grains was green and blue, indicating that most of the ð1120Þ planes were parallel to the observed surface. With the aging treatment before extrusion, the extruded microstructures were distinctly refined. No twins were detected in extruded alloys. 3.6. Macrotexture of extruded specimens Fig. 6a and b illustrate the textures of the extruded 0h-aged and 48h-aged specimens in the form of pole figures, respectively. The extruded ZM61 alloys show a fibre-type texture that ð0001Þ basal planes are mainly oriented parallel to the extrusion direction. This type of texture is known as typical texture of Mg alloys that develops during the uni-axial deformation [4]. In the present study, it is interesting to find that the aging treatment before extrusion can slightly sharpen the extrusion texture compared with that of nonaged specimen. However, the type of texture is almost the same in 0h-aged and 48h-aged specimen. Due to the hexagonal closepacked (HCP) lattice of Mg alloys, they have limited slip systems to accommodate imposed plastic deformation, and textures are easy to form during the thermos-
mechanical processing [10,26,27]. Ball and Prangnell first discovered the texture weakening effect in an extruded WE54 Mg alloy and they attributed it to the result of PSN [28]. Ever since, many researchers have found that the particle during extrusion can lead to texture weaken. However, in this study, it is worthy to find that the texture intensity in 48h-aged specimen is higher than that in 0h-aged alloy, which is quite different from the known results. It is recognized that the texture changes in this study is directly related to the presence of precipitates. If particles do not deform compatibly with matrix, plastic relaxation around the particles will result in lattice rotations in the regions adjacent to the particles [4]. During extrusion, nucleation of recrystallization may occur in these regions and the precipitates will obstruct the local slip patterns in the matrix [20]. Though PSN can lead to texture weakening in many RE-containing Mg alloys, however, some studies also show that PSN doesn't play the dominated role in texture evolution because the grains that formed in the vicinity of particles are much less than that formed at other sites [4,7]. The retardation of grain growth can also result in the texture weakening. The particles in this study are nano-sized which are much smaller than those in previous with the size of 1e10 m m [7,8], thus the retardation effect could be much slighter in this study. It can be seen clearly in Fig. 1 that the Mg-Zn precipitates in aged specimens have a strict orientation relationship with matrix and the orientation relationship in non-aged specimens is ambiguous. 0 Wei [12] has reported that the plate-shaped b2 has a hexagonal crystal lattice with the lattice parameters of a ¼ 0.525 nm and c ¼ 0.855 nm that is quite similar with matrix. Thus during extrusion process the Mg-Zn precipitates can promote the preferential growth of the grains with certain orientations and enable grain growth in specific orientation [29]. Consequently, the texture intensity increases with grain growth.
Fig. 6. (0002) and (10-10) pole figures of the as-extruded alloy: (a) 0h-aged alloy, (b) 48h-aged alloy.
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Fig. 7. Engineering stressestrain curves of all the extruded alloys: (a) tensile, (b) compressive.
3.7. Mechanical properties of extruded alloys Typical tensile and compressive stressestrain curves of extruded alloy with different aging treatment before extrusion are shown in Fig. 7. Tensile strength and ductility increase with prolonging aging time before extrusion. After aging treatment before extrusion, yield strength and tensile strength of extruded alloys increased from 211 MPa and 310 MPa of 0h-aged alloy to 227 MPa and 328 MPa, 275 MPa and 367 MPa, 281 MPa and 371 MPa of 6haged alloy, 24h-aged alloy and 48h-aged alloy, respectively. The elongation also increased from 14.4% of 0h-aged alloy to 16.6%, 18.7% and 19.3% of 6h-aged alloy, 24h-aged alloy and 48h-aged alloy, respectively (see Table 1). In comparison with 0h-aged alloy, the mechanical properties of 24h-aged alloy and 48h-aged alloy have been significantly improved. The tensile strength, compressive strength and the asymmetry ratio (the ratio of the yield strength in compression to the yield strength in tension) were shown in Table 1. Extruded Mg alloys normally exhibit strong fiber type texture with basal planes aligned parallel to the extrusion direction [30,31]. In polycrystalline Mg alloys, because of the low symmetry of HCP structure the deformation mechanisms depends on the orientation of the applied stress and crystallographic texture of the material. When the direction of compressive stress is parallel to the extrusion direction the preferentially developed fiber textures are unfavourable for the activation of basal slip. For extruded bar, f1012g extension twinning is easy to activate under tension perpendicular to extrusion direction or compressive load along extrusion direction. As a result, the sharp texture gives rise to its aggravated tension-compression yield asymmetry so that the application of Mg alloys is restricted to a certain extent. In addition to texture, microstructure also has a great influence on twinning behavior in Mg alloy during deformation.
Table 1 Tensile and compressive strength of ZM61 alloy. Materials
TYS, MPa
UTS, MPa
d, %
CYS, MPa
UCS, MPa
CYS/TYS
0h-aged alloy 6h-aged alloy 24h-aged alloy 48h-aged alloy
211 227 275 281
310 328 367 378
14.4 16.6 18.7 19.3
148 205 245 259
423 464 483 448
0.70 0.90 0.89 0.92
TYS-tensile yield strength; UTS-ultimate tensile strength; d: elongation; CYS: compressive yield strength; UCS: ultimate compressive strength; CYS/TYS: asymmetry ratio.
It has been shown that fine grains help to prevent formation of deformation twins and activate dislocations on non-basal planes, which will lead to the enhancement of strength as well as ductility [32]. It implies that grain refinement can not only enhance tensile and compressive yield strength but also reduce yield asymmetry by inhibiting twining in compression [33]. For the application of Mg alloys, reducing the yield asymmetry is significant because many Mg components are under compression load in service. All the alloys were extruded at 300 C with the same extrusion parameters and the only difference is the aging time before extrusion. It's noteworthy that the microstructure and mechanical properties of extruded alloy which subjected to aging treatment were significantly improved. The alloy that subjected to aging and subsequent hot extrusion treatment shows an excellent strength and ductility balance which is even more outstanding than that of some Mg-RE wrought alloys [18,34]. It's speculated that this thermo-mechanical treatment can be used in other age hardenable Mg alloys such as Mg-Al-Based and Mg-ZnBased alloys. The possibility of aging and subsequent extrusion treatment in Mg alloys proven in this paper opens up a new avenue for the design of relatively low-cost high performance magnesium alloys. 4. Summary The influence of aging treatment and subsequent hot extrusion process on microstructure and mechanical properties of ZM61 alloy were investigated, the following results were obtained. (a) After aging and subsequent hot extrusion treatment, a considerable improvement in strength and ductility is achieved. Due to the combination of fine grains and dispersion strengthening, the yield strength, tensile strength and elongation increased form 211 MPa, 310 MPa and 14.4% of 0h-aged alloy to 281 MPa, 371 MPa and 19.3% of 48h-aged alloy. The yield asymmetry was reduced at the same time. (b) After aging treatment and extrusion, the strict orientation 0 relationship between rod-shaped b1 precipitates and plate0 shaped b2 precipitates was disappeared and dispersed spherical precipitates were obtained. (c) The presence of Mg-Zn precipitates in hot extrusion process promoted grain refinement and leaded to a slight sharpening of texture intensity. The grain size decreased with the increasing of precipitates amount before extrusion.
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Acknowledgements This work was supported by National Great Theoretic Research Project (2013CB632200); National Natural Science Foundation of China (51474043) and Chongqing Municipal Government Project (CSTC2013JCYJC60001); and Sharing fund of Chongqing University's large-scale equipment. References [1] J. Li, R. Chen, Y. Ma, W. Ke, Effect of Zr modification on solidification behavior and mechanical properties of MgeYeRE (WE54) alloy, J. Magnes. Alloy 1 (2013) 346e351. [2] F. Kabirian, R. Mahmudi, Effects of zirconium additions on the microstructure of As-Cast and aged AZ91 magnesium alloy, Adv. Eng. Mater 11 (2009) 189e193. [3] M. Kaseem, B.K. Chung, H.W. Yang, K. Hamad, Y.G. Ko, Effect of deformation temperature on microstructure and mechanical properties of AZ31 Mg Alloy processed by differential-speed rolling, J. Mater. Sci. Technol. 31 (2015) 498e503. [4] J. Bohlen, S. Yi, D. Letzig, K.U. Kainer, Effect of rare earth elements on the microstructure and texture development in magnesiumemanganese alloys during extrusion, Mater. Sci. Eng. A 527 (2010) 7092e7098. [5] L.B. Tong, X. Li, D.P. Zhang, L.R. Cheng, J. Meng, H.J. Zhang, Dynamic recrystallization and texture evolution of MgeYeZn alloy during hot extrusion process, Mater. Charact. 92 (2014) 77e83. [6] C.H. Liu, X.L. Li, S.H. Wang, J.H. Chen, Q. Teng, J. Chen, Y. Gu, A tuning nanoprecipitation approach for achieving enhanced strength and good ductility in Al alloys, Mater. Des. 54 (2014) 144e148 (1980-2015). [7] J.D. Robson, D.T. Henry, B. Davis, Particle effects on recrystallization in magnesiumemanganese alloys: Particle-stimulated nucleation, Acta Mater. 57 (2009) 2739e2747. [8] S. Wang, R. Ma, L. Yang, Y. Wang, Y. Wang, Precipitates effect on microstructure of as-deformed and as-annealed AZ41 magnesium alloys by adding Mn and Ca, J. Mater. Sci. 46 (2011) 3060e3065. [9] Z. Yu, Y. Huang, C.L. Mendis, N. Hort, J. Meng, Microstructural evolution and mechanical properties of Mge11Gde4.5Ye1Nde1.5Zne0.5Zr alloy prepared via pre-ageing and hot extrusion, Mater. Sci. Eng. A 624 (2015) 23e31. [10] T. Al-Samman, Modification of texture and microstructure of magnesium alloy extrusions by particle-stimulated recrystallization, Mater. Sci. Eng. A 560 (2013) 561e566. [11] S. Guoliang, Z. Dingfei, Z. Xiabing, Z. Kui, L. Xinggang, L. Yongjun, M. Minglong, Precipitate evolution in Mg-6 wt%Zn-1 wt%Mn alloy, Rare Metal. Mat. Eng. 42 (2013) 2447e2452. [12] L.Y. Wei, G.L. Dunlop, H. Westengen, Precipitation hardening of Mg-Zn and Mg-Zn-RE alloys, Metall. Mater. Trans. A 26 (1995) 1705e1716. [13] J. Nie, Precipitation and hardening in magnesium alloys, Metall. Mater. Trans. A 43 (2012) 3891e3939. [14] J.B. Clark, Transmission electron microscopy study of age hardening in a Mg-5 wt.% Zn alloy, Acta Metall. 13 (1965) 1281e1289. [15] F.G. Qi, D.F. Zhang, Z.T. Zhu, X.X. Xu, G.L. Shi, Effect of heat treatment on
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