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Microstructure, texture and mechanical properties of as-extruded Mg–Zn–Er alloys containing W-phase
Journal of Alloys and Compounds 602 (2014) 32–39
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Microstructure, texture and mechanical properties of as-extruded Mg–Zn–Er alloys containing W-phase Qingfeng Wang, Ke Liu, Zhaohui Wang, Shubo Li, Wenbo Du ⇑ College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China
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Article history: Received 5 November 2013 Received in revised form 6 February 2014 Accepted 6 February 2014 Available online 15 February 2014 Keywords: Mg–Zn–Er alloy W-phase Microstructure Texture Mechanical properties Particle-stimulated nucleation
a b s t r a c t In the present work, the microstructure, texture and mechanical properties of as-extruded Mg–xZn–xEr (x = 2, 4 and 6) alloys were investigated. The results showed that the W-phase in the as-cast alloys was destroyed during hot extrusion process. The distribution of the W-phase is more uniform when the extrusion temperature is 400 °C, and the texture has also been a certain degree of weakening. With increase in the volume fraction of the primary W-phase, the particle-stimulated nucleation (PSN) of recrystallization was activated. It was suggested that the recrystallization via PSN should lead to weak texture. Good matching between strength and elongation is achieved in the Mg–6Zn–6Er alloy extruded at 400 °C, of which the ultimate tensile strength (rb) and the yield tensile strength (r0.2) were 328 (±2.5) MPa and 283 (±2.2) MPa, respectively, companying with an elongation of 19.7% (±1.2). Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, Mg–Zn–RE (–Zr) system alloys attracted significant interest, because they have high strength at both room and elevated temperatures [1–5]. Generally, three kinds of ternary equilibrium phase in Mg–Zn–RE and Mg–Zn–RE–Zr system alloys has been found, i.e. W-phase (Mg3Zn3RE2, cubic structure), I-phase (Mg3Zn6RE, icosahedra quasicrystal structure, quasiperiodically ordered) and Mg12ZnRE long-period stacking order (LPSO) structures (including 6H, 14H and 18R.) [6–8]. As reported, the I-phase and the LPSO structure are more effective strengthening phase, especially for the I-phase [2,5]. The I-phase has many unique properties such as conspicuous hardness, high thermal stability, high corrosion resistance and low surface energy [9]. Furthermore, the I-phase is stable and against the microstructure coarsening at elevated temperature due to the low interfacial energy and it has a great effect on improving the mechanical properties of the wrought magnesium alloys [1,10]. It is reported that the yield tensile strength (YTS) of the Mg–Zn–Y–Zr alloys can reach from 150 MPa to 450 MPa at room temperature depending on the content of the I-phase [11]. Singh et al. [2,10] and Yu et al. [3] have introduced the hot extrusion process to prepare Mg–Zn–Y alloys and Mg–Zn–Gd alloys strengthened by the I-phase, and these alloys exhibited excellent mechanical properties. ⇑ Corresponding author. Tel./fax: +86 10 67392917. E-mail address: [email protected] (W. Du). http://dx.doi.org/10.1016/j.jallcom.2014.02.027 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
However, the precipitation of the I-phase is generally companying with the W-phase, which is harmful to the mechanical properties [12]. It is reported that the W-phase with face-centered cubic structure has weak bonding with Mg matrix [13]. Besides, the Wphase was easily cracked during the tensile process in the as-cast Mg–Zn–Y–Zr alloys, which degraded the mechanical properties [14]. So, the W-phase is generally not considered as a very effective strengthening phase [13,14]. However, the hot extrusion can break the W-phase and make it distributed uniformly, which results in significant improvement (comparing with those of the alloy in as-cast state) of the mechanical properties of the Mg–Zn–RE system alloys via dispersion strengthening. For example, the Mg6Zn4Y alloy containing W-phase after hot extrusion displayed the high strength, and the yield strength and ultimate tensile strength are (350 ± 5) MPa and (371 ± 10) MPa, respectively [4].Yang et al. [15] reported that the uniform distribution of the W-phase with a size of less than 1 lm reduced the possibility of cavitations at the particles, thereby enhancing superplastic elongation. Thus, adoption the reasonable thermomechanical working process (for example, extrusion) and control the volume fraction of W-phase can prepare W-phase strengthening magnesium alloy. We have used extrusion to obtain a fine microstructure in Mg–Zn– Er alloys with different volume fraction of I-phase for higher mechanical strength [16]. During the extrusion process, the primary I-phase was destroyed and lots of nano-scale I-phase particles were precipitated within the matrix. The uniform
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distribution of the I-phase and the nano-scale I-phase particles improved the strength of Mg–Zn–Er alloys. The primary I-phase worked as nucleation sites for the dynamic recrystallization (DRX) via PSN, which make the texture weak, so as to improve the elongation. However, few reports investigated the effects of extrusion on the Mg–Zn–RE alloys containing W-phase and the effects of the volume fraction of W-phase and extrusion parameters on mechanical properties of extruded alloys. In this study, the microstructure and mechanical properties of the as-extruded Mg–xZn–xEr alloys (x = 2, 4 and 6, identified as alloy A, B and C, respectively) were investigated. The effect of the different volume fraction of W-phase and different extrusion temperature on the mechanical properties of as-extruded Mg–Zn–Er alloys was discussed. 2. Experimental procedures The as-cast alloys were prepared from the pure Mg, pure Zn and Mg–Er master alloys in a graphite crucible in an electric resistance furnace under an anti-oxidizing flux. The melt was poured into a steel mold. The as-cast samples were annealed at 400 °C for 10 h, cooled down in 70 °C water, and then machined into ingots with a diameter of 35 mm. Then, the ingots were hot extruded into rods with a ratio of 10 at 300 °C and 400 °C, and the extrusion speed was 3 mm/s. The chemical compositions of alloys have been analyzed by X-ray fluorescence (XRF) analyzer, as shown in Table 1. The phase analysis was performed by X-ray diffraction (XRD) with Cu Ka radiation. The microstructure observations were carried out by optical microscope (OM, Zeiss-Imager.A2m), scanning electron microscope (SEM, HITACHI S-450) and transmission electron microscope (TEM, JEM-2000FX, JEOL). The texture analyses by EBSD were conducted using a JEOL JSM-6500F scanning electron microscope (TFE-SEM) operating at 30 kV equipped with TSL-OIM Analysis 5 software. The average grain size of the alloys was measured via the linear intercept method. The samples for OM and SEM were mechanically polished and etched in a solution of 4 mL nitric acid and 96 mL ethanol. Specimens for TEM were prepared by electro-polishing and ion beam milling at an angle of incidence less than 10°. Samples for texture analysis were prepared by electro-polishing with a solution of 60% methanol, 30% glycerol and 10% nitric acid at 25 °C (voltage of 11) for 10–40 s. Tensile test was carried out by using a DNS-20 universal testing machine under a constant speed of 1.0 mm/min at room temperature. Specimens for the tensile test were made into dog-bone with a size of 5 mm gauge diameter and 25 mm gauge length. Three specimens were tested for each sample. All the tensile samples were sectioned in parallel to the extrusion direction.
3. Experimental results
whether the extrusion temperature is 300 °C or 400 °C, as shown in Fig. 3(a) and (b). There are still some unDRXed regions in the alloy B which was extruded at 300 °C, whereas a fine microstructure with uniform grain size distribution is produced in alloy B extruded at 400 °C. The alloy C reveals a completely DRX organization. It is indicated that a DRX have taken place in these alloys. The DRX happens more complete with the increase volume fraction of the W-phase and the enhancement of extrusion temperature. The values of the grain size obtained from the alloys A, B and C extruded different temperature is listed in Table 2. It reveals that there is a decrease trend in the grain size with the increase of volume fraction of the W-phase. The grain size of the alloy extruded at 300 °C is finer than that of alloy extruded at 400 °C. Fig. 4 shows the SEM images of the as-extruded alloys along the extrusion direction (ED). The W-phase observed in the cast alloy is sharply broken and then distributed along the extrusion direction. Besides, the distribution of the broken W-phase is more uniform in all three alloys extruded at 400 °C, especially in the alloy C, as shown in Fig. 4(f). Fig. 5 shows the high magnification SEM images of alloys A and C as-extruded at 300 °C and corresponding EDS result of the alloy C. The EDS suggests that the composition of the broken W-phase is Mg50.36Zn32.72 Er16.92, which is closed to the Mg3Zn3Er2 phase. Most of the broken W-phase particles had a size of about 1 lm, a small number of large W particles with a size of 2–5 lm were also observed. Furthermore, many nanoscale particles appear in matrix. To further observe the microstructure of the as-extruded alloy, TEM observation has been conducted for the alloy C extruded at both 300 °C and 400 °C. Fig. 6(a) shows the TEM image and the corresponding selected area electron diffraction (SAED) patterns of the W-phase. According to the SAED pattern, the large particle is the broken W-phase. No cavitation/crack is observed at the
Table 1 Chemical compositions of the as-cast Mg–Zn–Er alloys. Nominal alloy
Alloy A Alloy B Alloy C
Composition (wt.%)
Zn/Er
Zn
Er
Mg
2.0 3.7 5.5
2.3 4.0 6.2
Bal. Bal. Bal.
3.1. Microstructure of the as-cast alloys The XRD pattern of the as-cast alloy is shown in Fig. 1. It reveals that the three alloys mainly consist of two kinds of phase. One is the a-Mg solid solution; the other is the W-phase. Meanwhile, with the increase of Zn and Y content, the diffraction peak of Wphase will be gradually intensified. The W-phase in the as-cast Mg–Zn–Er alloy has previously been reported by Li et al. [8]. No other phase is detected within the sensitivity limit of XRD. Fig. 2 shows the OM and SEM images of as-cast alloys. It indicates that all of the as-cast alloys are composed of the interdendritic microstructure. The W-phases with different morphologies can be found at both matrix and interdendritic boundaries. In addition, the W-phase at interdendritic boundaries becomes larger and more continuous and its volume fraction increases with increase in the content of alloying elements. Even some places appear W-phase eutectic organization. The volume fraction of the W-phase in alloy A, B and C was measured by an image analysis method, the values of which is 3.2%, 9.2% and 14.8%, respectively. 3.2. Microstructure of the as-extruded alloys Fig. 3 shows the OM images of the as-extruded alloys along the extrusion direction (ED). The alloy A shows a bimodal grain structure consisting of fine grains and unDRXed region, no matter
Fig. 1. XRD pattern of the as-cast alloys A, B and C.
0.87 0.96 0.89
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Fig. 2. OM and SEM images of the as-cast alloys: (a) OM image and (b) SEM image obtained from alloy A; (c) OM image and (d) SEM image obtained from alloy B; (e) OM image and (f) SEM image obtained from alloy C.
W-phase/a-Mg interface in the extruded alloy, in spite of the strength and plasticity of the W-phase is different from that of the matrix. In addition, some nano-scale precipitates appear in the as-extruded alloys. The nano-scale precipitates with morphology of rod mainly exist in the alloy C extruded at 300 °C. In addition to the rod nano-scale precipitates, some granular nano-scale precipitates appear in the alloy C extruded at 400 °C. The volume fraction of the nano-sized precipitates in the alloy extruded at 400 °C is higher than that in the alloy extruded at 300 °C. It may indicate that the high extrusion temperature can activate the precipitation of the precipitates. Detailed research on nano-scale precipitates will be discussed in other places. 3.3. Texture Fig. 7 shows the inverse pole figure (IPF) maps of the alloys A, B and C extruded at 300 °C and 400 °C taken parallel to the extrusion direction obtained by using EBSD. It is obvious that the three kinds of the as-extruded alloys are mainly composed of the grains exhibiting their (0 0 0 2) basal plane. Moreover, it shows that some unDRXed region appears in the alloy A extruded at 300 °C and 400 °C and in the alloy B extruded at 300 °C. Previous study [17] showed that the sample will have stronger texture because the elongated deformed grains have strong basal texture component. The alloy C shows a nearly uniform grain structure which can be considered
as a reasonably equiaxed grain regardless of the extrusion temperature at 300 °C or 400 °C. Therefore, it can be concluded that the orientation of the grains becomes more dispersed as the content of the W-phase increases and the extrusion temperature increase. Fig. 8 displays the pole figures of the extruded alloys A, B and C. It indicates that the three alloys exhibit strong (0 0 0 2) basal texture. It means that the (0 0 0 2) basal plane of the most grains tends to be parallel to the extrusion direction (ED) [18]. The 0i fiber texture which was found in the as-extruded (0 0 0 2) h1 0 1 Mg–Zn–RE (–Zr) alloys is universal [16,19]. In addition, it can be found that the maximum value of the texture intensity decreases as both the content of the W-phase and the extrusion temperature increase. The maximum values of the texture intensity in the alloys A, B and C extruded at 300 °C are 15.5, 13.1 and 8.7, respectively. In addition, the values of the alloys A, B and C extruded at 400 °C are 12.9, 7.8 and 8.1, respectively. 3.4. Mechanical properties The stress–strain curves and the tensile properties of the extruded alloys at room temperature are shown in Fig. 9 and listed in Table 3, respectively. It can be seen that the ultimate tensile strength (UTS) displays a tendency to increase with increase in the volume fraction of the W-phase regardless of the extrusion temperature at 300 °C or 400 °C. The values of the yield tensile
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Fig. 3. OM images of the as-extruded alloys: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
strength (YTS) of these alloys extruded at 300 °C are converse on the whole. The YTS of the alloy A is higher than that of the alloys B and C. However, the YTS of these alloys extruded at 400 °C increase with increase in the volume fraction of the W-phase as well as the UTS. With the volume fraction of the W-phase and the extrusion temperature increase, the values of the elongation of the alloys A, B and C are improved on a whole. Usually, the strength of wrought magnesium alloys is closely related with the grain size, texture and precipitation [20,21]. Comparing with the results of grain size, texture and precipitation in the extruded alloys A, B and C, it is not difficult to understand the performance of tensile properties. The UTS is on the rise, which is mainly because the volume fraction of the W-phase is enhanced. The YTS of the alloy A extruded at 300 °C is higher than that of the
Table 2 The average grain size (lm) of the as-extruded alloys. Alloys
Alloy A Alloy B Alloy C
Temperature 300 °C
400 °C
3.3 3.2 2.6
5.0 4.1 3.8
alloys B and C mainly due to the strongest basal texture of the alloy A. The maximum value of the texture intensity in the alloy A extruded at 300 °C is 15.5, while the maximum values of the texture intensity of alloys B and C are 13.1 and 8.7, respectively. It is well known that a typical fiber texture parallel to the extrusion direction limited the movement of the basal slip because of the small value of the Schmid factor, resulting in obvious improvement in YTS [22]. The maximum values of the texture intensity of the alloys decrease when the extrusion temperature rises to 400 °C. Therefore, the precipitate and grain size played an important role in improvement of tensile properties. As a result, the YTS of these alloys extruded at 400 °C shows a trend of increase with the volume fraction of the W-phase increase and the refinement of the grain size. The elongation is affected by the texture intensity, grain size and the distribution of the second phase in this study. No matter what extrusion temperatures (300 °C/400 °C) is, the texture intensity still decreases with the increase in content of the W-phase after hot extrusion, which results in the increase in the elongation. In addition, the decrease in the grain size also helps to the elongation increase. It is worth noting that the uniform distribution of the second phase also plays a significant role in improvement of the elongation. Comparative Fig. 4(e) and (f), it can be seen that the distribution of W-phase is more uniform when the alloy C extruded at 400 °C. Thereby, the improvement of the extrusion
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Fig. 4. SEM images of the as-extruded alloys: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
temperature is beneficial to the uniform distribution of the W-phase, which makes the elongation improved from 16.8% to 19.7% in the extruded alloy C. In the alloy A, due to the low volume fraction of the W-phase, the effect of the distribution of the W-phase on the advancement of the elongation become low and the grain size plays a great part in improving elongation. Therefore, the extruded alloy A extruded at 300 °C shows a higher elongation.
4. Discussion In the present study, the W-phases with different morphologies in the as-cast alloys can be found at both matrix and interdendritic boundaries. The W-phase at interdendritic boundaries becomes larger and more continuous with increase in the content of alloying elements. As reported in the Ref. [14], the morphology of the W-phase will transform into coarse net-like microstructure at the grain boundaries when the volume fraction of the W-phase exceeds 17.5%, resulting in a great decrease in mechanical properties. Hot extrusion is an effective way to break the coarse eutectic structure [23]. In the as-extruded alloys, the W-phase was sharply broken and then distributed along the extrusion direction. Most of the broken W-phase has a size of about 1 lm, and a small number of large W-phase particles about 2–5 lm in size were also observed. It reduced the possibility of cavitation at the interface of the W-phase/a-Mg and micro cracks at the W-phase particles [15], thereby enhancing mechanical properties. Besides, the
distribution of the broken W-phase is more uniform in the alloys extruded at 400 °C, especially in the alloy C. In the as-extruded SiCp/AZ91 composites, the distribution of the particles was improved with the extrusion temperature and ratio increased [24], which played an important role in mechanical properties of extruded composites. Thereby, more uniform distribution of the broken W-phase in the alloys would improve the mechanical properties, especially for the elongation. Besides, the DRX took place in all the three alloys during extrusion processing. With the volume fraction of the W-phase and the extrusion temperature increasing, the DRX happens more completely. The average grain size of alloys shows a decrease trend as the volume fraction of the W-phase increases and the extrusion temperature decreases. It indicated that the DRX is closely related to the extrusion temperature and second phase volume fraction. The increase in the extrusion temperature can make the dislocation motion and DRX nucleation rate increase, which can promote the occurrence of DRX. The increase volume fraction of W-phase may be via the particle-stimulated nucleation (PSN) mechanism to promote the DRX [16]. At the same time, some nano-scale precipitates appear in the as-extruded alloys, which would have the precipitation strengthening effect to a certain extent. In addition, all the three alloys exhibit strong (0 0 0 2) basal texture after hot extrusion, which has been found in the as-extruded Mg–Zn–RE (–Zr) alloys universally [16,19]. The maximum value of the texture intensity decreases as the volume fraction of W-phase increases and the extrusion temperature increases. It is indicated that the
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Fig. 6. TEM bright field image of the as-extruded alloy C: (a) extruded at 300 °C and the corresponding SAED patterns; (b) extruded at 400 °C.
Fig. 5. The high magnification SEM images of the as-extruded alloys: (a) alloy A extruded at 300 °C; (b) alloy C extruded at 300 °C; (c) EDS result of the position ‘‘A’’.
texture can be changed by adjustment of both the volume fraction of the second phase and the extrusion temperature. Much work has been done on the modification of texture for magnesium alloys, such as the equal channel angular process (ECAP) [25] and alloying with RE [26,27]. It is suggested that the DRX via PSN is related with the second phase particles [28]. During deformation of the particle-containing alloy, the enforced strain gradient in the vicinity of a non-deforming particle creates a region of high dislocation density and large orientation gradient, thus both high dislocation density and large orientation promote nucleation of recrystallization [29]. The random texture may also be ascribed to the DRX via PSN. As reported [30], the Al–Sr particles in the Mg–3Al–1Zn–xSr (x = 0.4–0.8, wt.%) alloys increased the potential nucleation of PSN, resulting in the weakening of the texture. In the present investigation, the presence of the primary W-phase particles acted as nucleation sites for PSN and made the texture to be weak as the volume fraction of the W-phase increase. As a result, the maximum intensity values of the texture in the as-extruded alloys decreased with increase in the volume fraction of the W-phase particles, which contributed to the plasticity of the as-extruded alloys. Besides, texture intensity is reduced with the increase of extrusion temperature. The same phenomenon has been reported in AZ31 [31]. It may be caused by difference of DRX mechanism. Based on the above discussion, it is easy to understand the mechanical properties of the as-extruded alloys. The diffusive distribution of the W-phase in the Mg matrix after extrusion
process can pin the dislocation and contribute some dispersionstrengthening to the alloys. Therefore, the value of the UTS is on the rise with the volume fraction of the W-phase increases. The YTS was mainly affected by the texture and grain size. Due to the strongest basal texture, the YTS of the alloy A extruded at 300 °C is higher than that of the alloys B and C. When the texture gradually weakens, both the W-phase and the grain size played an important role in advancing tensile properties. Therefore, with the volume fraction of the W-phase increasing and the refinement of the grain size, the YTS of these alloys extruded at 400 °C shows a trend to increase. The elongation of the alloys is affected by the texture intensity, grain size and the distribution of the second phase in the present investigation. The elongation is on the rise with the texture gradually weakened and grain refinement. Besides, the uniform distribution of W-phase also helps the elongation increase. Therefore, the alloy C extruded at 400 °C shows a best elongation with 19.7%. 5. Conclusions The effects of the W-phase on the microstructure, texture and mechanical properties of as-extruded Mg–Zn–Er alloys were investigated. Some conclusions can be summarized as below: 1. The W-phase could be broken and distributed along the extrusion direction during hot extrusion. Most of the broken W-phase has a size of about 1 lm. Besides, the higher extrusion temperature can make the broken W-phase distribution more uniform. The diffusive distribution of the W-phase can contribute dispersion-strengthening to the alloys, which make the UTS is on the rise with the volume fraction of the W-phase increases. 2. The primary W-phase particles worked as nucleation sites for the DRX via PSN, which contributes to the refined microstructure and weaken texture. As a result, the Mg–6Zn–6Er alloy
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Fig. 7. Inverse pole figures map of the as-extruded alloys: (a) and (d) alloy A extruded at 300 °C and 400 °C; (b) and (e) alloy B extruded at 300 °C and 400 °C; (c) and (f) alloy C extruded at 300 °C and 400 °C.
Fig. 8. Pole figures of the as-extruded alloys, the RD standing for the ED: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
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Technology Commission (Z131100003213019), Beijing Municipal Commission Education (KM201310005001, KM201410005014) and Beijing Natural Science Foundation (2142005, 2144043). References
Fig. 9. Stress–strain curves of the as-extruded alloys A, B and C tested under a constant speed of 1.0 mm/min at room temperature.
Table 3 Tensile properties of the as-extruded alloys A, B and C at room temperature. Alloy
Extrusion temperature (°C)
UTS (MPa)
YTS (MPa)
E (%)
Alloy A
300 400
320 (7.9) 279 (6.8)
310 (6.5) 247 (6.2)
12.8 (1.2) 12.1 (1.6)
Alloy B
300 400
330 (3.0) 319 (3.1)
295 (2.8) 278 (2.9)
13.7 (2.1) 17.6 (2.0)
Alloy C
300 400
343 (7.0) 328 (2.5)
299 (6.3) 283 (2.2)
16.8 (1.2) 19.7 (1.2)
Note: This table shows the average value. The in-bracket values are that of standard deviation.
extruded at 400 °C exhibited better comprehensive mechanical properties. The UTS (rb) and the YTS (r0.2) of the as-extruded alloy were 328 MPa and 283 MPa, respectively, with an elongation of 19.7%.
Acknowledgments This project was supported by the National Natural Science Fund of China (51071004, 51301006), Beijing Municipal Science and
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure, texture and mechanical properties of as-extruded Mg–Zn–Er alloys containing W-phase Qingfeng Wang, Ke Liu, Zhaohui Wang, Shubo Li, Wenbo Du ⇑ College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China
a r t i c l e
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Article history: Received 5 November 2013 Received in revised form 6 February 2014 Accepted 6 February 2014 Available online 15 February 2014 Keywords: Mg–Zn–Er alloy W-phase Microstructure Texture Mechanical properties Particle-stimulated nucleation
a b s t r a c t In the present work, the microstructure, texture and mechanical properties of as-extruded Mg–xZn–xEr (x = 2, 4 and 6) alloys were investigated. The results showed that the W-phase in the as-cast alloys was destroyed during hot extrusion process. The distribution of the W-phase is more uniform when the extrusion temperature is 400 °C, and the texture has also been a certain degree of weakening. With increase in the volume fraction of the primary W-phase, the particle-stimulated nucleation (PSN) of recrystallization was activated. It was suggested that the recrystallization via PSN should lead to weak texture. Good matching between strength and elongation is achieved in the Mg–6Zn–6Er alloy extruded at 400 °C, of which the ultimate tensile strength (rb) and the yield tensile strength (r0.2) were 328 (±2.5) MPa and 283 (±2.2) MPa, respectively, companying with an elongation of 19.7% (±1.2). Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, Mg–Zn–RE (–Zr) system alloys attracted significant interest, because they have high strength at both room and elevated temperatures [1–5]. Generally, three kinds of ternary equilibrium phase in Mg–Zn–RE and Mg–Zn–RE–Zr system alloys has been found, i.e. W-phase (Mg3Zn3RE2, cubic structure), I-phase (Mg3Zn6RE, icosahedra quasicrystal structure, quasiperiodically ordered) and Mg12ZnRE long-period stacking order (LPSO) structures (including 6H, 14H and 18R.) [6–8]. As reported, the I-phase and the LPSO structure are more effective strengthening phase, especially for the I-phase [2,5]. The I-phase has many unique properties such as conspicuous hardness, high thermal stability, high corrosion resistance and low surface energy [9]. Furthermore, the I-phase is stable and against the microstructure coarsening at elevated temperature due to the low interfacial energy and it has a great effect on improving the mechanical properties of the wrought magnesium alloys [1,10]. It is reported that the yield tensile strength (YTS) of the Mg–Zn–Y–Zr alloys can reach from 150 MPa to 450 MPa at room temperature depending on the content of the I-phase [11]. Singh et al. [2,10] and Yu et al. [3] have introduced the hot extrusion process to prepare Mg–Zn–Y alloys and Mg–Zn–Gd alloys strengthened by the I-phase, and these alloys exhibited excellent mechanical properties. ⇑ Corresponding author. Tel./fax: +86 10 67392917. E-mail address: [email protected] (W. Du). http://dx.doi.org/10.1016/j.jallcom.2014.02.027 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
However, the precipitation of the I-phase is generally companying with the W-phase, which is harmful to the mechanical properties [12]. It is reported that the W-phase with face-centered cubic structure has weak bonding with Mg matrix [13]. Besides, the Wphase was easily cracked during the tensile process in the as-cast Mg–Zn–Y–Zr alloys, which degraded the mechanical properties [14]. So, the W-phase is generally not considered as a very effective strengthening phase [13,14]. However, the hot extrusion can break the W-phase and make it distributed uniformly, which results in significant improvement (comparing with those of the alloy in as-cast state) of the mechanical properties of the Mg–Zn–RE system alloys via dispersion strengthening. For example, the Mg6Zn4Y alloy containing W-phase after hot extrusion displayed the high strength, and the yield strength and ultimate tensile strength are (350 ± 5) MPa and (371 ± 10) MPa, respectively [4].Yang et al. [15] reported that the uniform distribution of the W-phase with a size of less than 1 lm reduced the possibility of cavitations at the particles, thereby enhancing superplastic elongation. Thus, adoption the reasonable thermomechanical working process (for example, extrusion) and control the volume fraction of W-phase can prepare W-phase strengthening magnesium alloy. We have used extrusion to obtain a fine microstructure in Mg–Zn– Er alloys with different volume fraction of I-phase for higher mechanical strength [16]. During the extrusion process, the primary I-phase was destroyed and lots of nano-scale I-phase particles were precipitated within the matrix. The uniform
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distribution of the I-phase and the nano-scale I-phase particles improved the strength of Mg–Zn–Er alloys. The primary I-phase worked as nucleation sites for the dynamic recrystallization (DRX) via PSN, which make the texture weak, so as to improve the elongation. However, few reports investigated the effects of extrusion on the Mg–Zn–RE alloys containing W-phase and the effects of the volume fraction of W-phase and extrusion parameters on mechanical properties of extruded alloys. In this study, the microstructure and mechanical properties of the as-extruded Mg–xZn–xEr alloys (x = 2, 4 and 6, identified as alloy A, B and C, respectively) were investigated. The effect of the different volume fraction of W-phase and different extrusion temperature on the mechanical properties of as-extruded Mg–Zn–Er alloys was discussed. 2. Experimental procedures The as-cast alloys were prepared from the pure Mg, pure Zn and Mg–Er master alloys in a graphite crucible in an electric resistance furnace under an anti-oxidizing flux. The melt was poured into a steel mold. The as-cast samples were annealed at 400 °C for 10 h, cooled down in 70 °C water, and then machined into ingots with a diameter of 35 mm. Then, the ingots were hot extruded into rods with a ratio of 10 at 300 °C and 400 °C, and the extrusion speed was 3 mm/s. The chemical compositions of alloys have been analyzed by X-ray fluorescence (XRF) analyzer, as shown in Table 1. The phase analysis was performed by X-ray diffraction (XRD) with Cu Ka radiation. The microstructure observations were carried out by optical microscope (OM, Zeiss-Imager.A2m), scanning electron microscope (SEM, HITACHI S-450) and transmission electron microscope (TEM, JEM-2000FX, JEOL). The texture analyses by EBSD were conducted using a JEOL JSM-6500F scanning electron microscope (TFE-SEM) operating at 30 kV equipped with TSL-OIM Analysis 5 software. The average grain size of the alloys was measured via the linear intercept method. The samples for OM and SEM were mechanically polished and etched in a solution of 4 mL nitric acid and 96 mL ethanol. Specimens for TEM were prepared by electro-polishing and ion beam milling at an angle of incidence less than 10°. Samples for texture analysis were prepared by electro-polishing with a solution of 60% methanol, 30% glycerol and 10% nitric acid at 25 °C (voltage of 11) for 10–40 s. Tensile test was carried out by using a DNS-20 universal testing machine under a constant speed of 1.0 mm/min at room temperature. Specimens for the tensile test were made into dog-bone with a size of 5 mm gauge diameter and 25 mm gauge length. Three specimens were tested for each sample. All the tensile samples were sectioned in parallel to the extrusion direction.
3. Experimental results
whether the extrusion temperature is 300 °C or 400 °C, as shown in Fig. 3(a) and (b). There are still some unDRXed regions in the alloy B which was extruded at 300 °C, whereas a fine microstructure with uniform grain size distribution is produced in alloy B extruded at 400 °C. The alloy C reveals a completely DRX organization. It is indicated that a DRX have taken place in these alloys. The DRX happens more complete with the increase volume fraction of the W-phase and the enhancement of extrusion temperature. The values of the grain size obtained from the alloys A, B and C extruded different temperature is listed in Table 2. It reveals that there is a decrease trend in the grain size with the increase of volume fraction of the W-phase. The grain size of the alloy extruded at 300 °C is finer than that of alloy extruded at 400 °C. Fig. 4 shows the SEM images of the as-extruded alloys along the extrusion direction (ED). The W-phase observed in the cast alloy is sharply broken and then distributed along the extrusion direction. Besides, the distribution of the broken W-phase is more uniform in all three alloys extruded at 400 °C, especially in the alloy C, as shown in Fig. 4(f). Fig. 5 shows the high magnification SEM images of alloys A and C as-extruded at 300 °C and corresponding EDS result of the alloy C. The EDS suggests that the composition of the broken W-phase is Mg50.36Zn32.72 Er16.92, which is closed to the Mg3Zn3Er2 phase. Most of the broken W-phase particles had a size of about 1 lm, a small number of large W particles with a size of 2–5 lm were also observed. Furthermore, many nanoscale particles appear in matrix. To further observe the microstructure of the as-extruded alloy, TEM observation has been conducted for the alloy C extruded at both 300 °C and 400 °C. Fig. 6(a) shows the TEM image and the corresponding selected area electron diffraction (SAED) patterns of the W-phase. According to the SAED pattern, the large particle is the broken W-phase. No cavitation/crack is observed at the
Table 1 Chemical compositions of the as-cast Mg–Zn–Er alloys. Nominal alloy
Alloy A Alloy B Alloy C
Composition (wt.%)
Zn/Er
Zn
Er
Mg
2.0 3.7 5.5
2.3 4.0 6.2
Bal. Bal. Bal.
3.1. Microstructure of the as-cast alloys The XRD pattern of the as-cast alloy is shown in Fig. 1. It reveals that the three alloys mainly consist of two kinds of phase. One is the a-Mg solid solution; the other is the W-phase. Meanwhile, with the increase of Zn and Y content, the diffraction peak of Wphase will be gradually intensified. The W-phase in the as-cast Mg–Zn–Er alloy has previously been reported by Li et al. [8]. No other phase is detected within the sensitivity limit of XRD. Fig. 2 shows the OM and SEM images of as-cast alloys. It indicates that all of the as-cast alloys are composed of the interdendritic microstructure. The W-phases with different morphologies can be found at both matrix and interdendritic boundaries. In addition, the W-phase at interdendritic boundaries becomes larger and more continuous and its volume fraction increases with increase in the content of alloying elements. Even some places appear W-phase eutectic organization. The volume fraction of the W-phase in alloy A, B and C was measured by an image analysis method, the values of which is 3.2%, 9.2% and 14.8%, respectively. 3.2. Microstructure of the as-extruded alloys Fig. 3 shows the OM images of the as-extruded alloys along the extrusion direction (ED). The alloy A shows a bimodal grain structure consisting of fine grains and unDRXed region, no matter
Fig. 1. XRD pattern of the as-cast alloys A, B and C.
0.87 0.96 0.89
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Fig. 2. OM and SEM images of the as-cast alloys: (a) OM image and (b) SEM image obtained from alloy A; (c) OM image and (d) SEM image obtained from alloy B; (e) OM image and (f) SEM image obtained from alloy C.
W-phase/a-Mg interface in the extruded alloy, in spite of the strength and plasticity of the W-phase is different from that of the matrix. In addition, some nano-scale precipitates appear in the as-extruded alloys. The nano-scale precipitates with morphology of rod mainly exist in the alloy C extruded at 300 °C. In addition to the rod nano-scale precipitates, some granular nano-scale precipitates appear in the alloy C extruded at 400 °C. The volume fraction of the nano-sized precipitates in the alloy extruded at 400 °C is higher than that in the alloy extruded at 300 °C. It may indicate that the high extrusion temperature can activate the precipitation of the precipitates. Detailed research on nano-scale precipitates will be discussed in other places. 3.3. Texture Fig. 7 shows the inverse pole figure (IPF) maps of the alloys A, B and C extruded at 300 °C and 400 °C taken parallel to the extrusion direction obtained by using EBSD. It is obvious that the three kinds of the as-extruded alloys are mainly composed of the grains exhibiting their (0 0 0 2) basal plane. Moreover, it shows that some unDRXed region appears in the alloy A extruded at 300 °C and 400 °C and in the alloy B extruded at 300 °C. Previous study [17] showed that the sample will have stronger texture because the elongated deformed grains have strong basal texture component. The alloy C shows a nearly uniform grain structure which can be considered
as a reasonably equiaxed grain regardless of the extrusion temperature at 300 °C or 400 °C. Therefore, it can be concluded that the orientation of the grains becomes more dispersed as the content of the W-phase increases and the extrusion temperature increase. Fig. 8 displays the pole figures of the extruded alloys A, B and C. It indicates that the three alloys exhibit strong (0 0 0 2) basal texture. It means that the (0 0 0 2) basal plane of the most grains tends to be parallel to the extrusion direction (ED) [18]. The 0i fiber texture which was found in the as-extruded (0 0 0 2) h1 0 1 Mg–Zn–RE (–Zr) alloys is universal [16,19]. In addition, it can be found that the maximum value of the texture intensity decreases as both the content of the W-phase and the extrusion temperature increase. The maximum values of the texture intensity in the alloys A, B and C extruded at 300 °C are 15.5, 13.1 and 8.7, respectively. In addition, the values of the alloys A, B and C extruded at 400 °C are 12.9, 7.8 and 8.1, respectively. 3.4. Mechanical properties The stress–strain curves and the tensile properties of the extruded alloys at room temperature are shown in Fig. 9 and listed in Table 3, respectively. It can be seen that the ultimate tensile strength (UTS) displays a tendency to increase with increase in the volume fraction of the W-phase regardless of the extrusion temperature at 300 °C or 400 °C. The values of the yield tensile
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Fig. 3. OM images of the as-extruded alloys: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
strength (YTS) of these alloys extruded at 300 °C are converse on the whole. The YTS of the alloy A is higher than that of the alloys B and C. However, the YTS of these alloys extruded at 400 °C increase with increase in the volume fraction of the W-phase as well as the UTS. With the volume fraction of the W-phase and the extrusion temperature increase, the values of the elongation of the alloys A, B and C are improved on a whole. Usually, the strength of wrought magnesium alloys is closely related with the grain size, texture and precipitation [20,21]. Comparing with the results of grain size, texture and precipitation in the extruded alloys A, B and C, it is not difficult to understand the performance of tensile properties. The UTS is on the rise, which is mainly because the volume fraction of the W-phase is enhanced. The YTS of the alloy A extruded at 300 °C is higher than that of the
Table 2 The average grain size (lm) of the as-extruded alloys. Alloys
Alloy A Alloy B Alloy C
Temperature 300 °C
400 °C
3.3 3.2 2.6
5.0 4.1 3.8
alloys B and C mainly due to the strongest basal texture of the alloy A. The maximum value of the texture intensity in the alloy A extruded at 300 °C is 15.5, while the maximum values of the texture intensity of alloys B and C are 13.1 and 8.7, respectively. It is well known that a typical fiber texture parallel to the extrusion direction limited the movement of the basal slip because of the small value of the Schmid factor, resulting in obvious improvement in YTS [22]. The maximum values of the texture intensity of the alloys decrease when the extrusion temperature rises to 400 °C. Therefore, the precipitate and grain size played an important role in improvement of tensile properties. As a result, the YTS of these alloys extruded at 400 °C shows a trend of increase with the volume fraction of the W-phase increase and the refinement of the grain size. The elongation is affected by the texture intensity, grain size and the distribution of the second phase in this study. No matter what extrusion temperatures (300 °C/400 °C) is, the texture intensity still decreases with the increase in content of the W-phase after hot extrusion, which results in the increase in the elongation. In addition, the decrease in the grain size also helps to the elongation increase. It is worth noting that the uniform distribution of the second phase also plays a significant role in improvement of the elongation. Comparative Fig. 4(e) and (f), it can be seen that the distribution of W-phase is more uniform when the alloy C extruded at 400 °C. Thereby, the improvement of the extrusion
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Fig. 4. SEM images of the as-extruded alloys: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
temperature is beneficial to the uniform distribution of the W-phase, which makes the elongation improved from 16.8% to 19.7% in the extruded alloy C. In the alloy A, due to the low volume fraction of the W-phase, the effect of the distribution of the W-phase on the advancement of the elongation become low and the grain size plays a great part in improving elongation. Therefore, the extruded alloy A extruded at 300 °C shows a higher elongation.
4. Discussion In the present study, the W-phases with different morphologies in the as-cast alloys can be found at both matrix and interdendritic boundaries. The W-phase at interdendritic boundaries becomes larger and more continuous with increase in the content of alloying elements. As reported in the Ref. [14], the morphology of the W-phase will transform into coarse net-like microstructure at the grain boundaries when the volume fraction of the W-phase exceeds 17.5%, resulting in a great decrease in mechanical properties. Hot extrusion is an effective way to break the coarse eutectic structure [23]. In the as-extruded alloys, the W-phase was sharply broken and then distributed along the extrusion direction. Most of the broken W-phase has a size of about 1 lm, and a small number of large W-phase particles about 2–5 lm in size were also observed. It reduced the possibility of cavitation at the interface of the W-phase/a-Mg and micro cracks at the W-phase particles [15], thereby enhancing mechanical properties. Besides, the
distribution of the broken W-phase is more uniform in the alloys extruded at 400 °C, especially in the alloy C. In the as-extruded SiCp/AZ91 composites, the distribution of the particles was improved with the extrusion temperature and ratio increased [24], which played an important role in mechanical properties of extruded composites. Thereby, more uniform distribution of the broken W-phase in the alloys would improve the mechanical properties, especially for the elongation. Besides, the DRX took place in all the three alloys during extrusion processing. With the volume fraction of the W-phase and the extrusion temperature increasing, the DRX happens more completely. The average grain size of alloys shows a decrease trend as the volume fraction of the W-phase increases and the extrusion temperature decreases. It indicated that the DRX is closely related to the extrusion temperature and second phase volume fraction. The increase in the extrusion temperature can make the dislocation motion and DRX nucleation rate increase, which can promote the occurrence of DRX. The increase volume fraction of W-phase may be via the particle-stimulated nucleation (PSN) mechanism to promote the DRX [16]. At the same time, some nano-scale precipitates appear in the as-extruded alloys, which would have the precipitation strengthening effect to a certain extent. In addition, all the three alloys exhibit strong (0 0 0 2) basal texture after hot extrusion, which has been found in the as-extruded Mg–Zn–RE (–Zr) alloys universally [16,19]. The maximum value of the texture intensity decreases as the volume fraction of W-phase increases and the extrusion temperature increases. It is indicated that the
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Fig. 6. TEM bright field image of the as-extruded alloy C: (a) extruded at 300 °C and the corresponding SAED patterns; (b) extruded at 400 °C.
Fig. 5. The high magnification SEM images of the as-extruded alloys: (a) alloy A extruded at 300 °C; (b) alloy C extruded at 300 °C; (c) EDS result of the position ‘‘A’’.
texture can be changed by adjustment of both the volume fraction of the second phase and the extrusion temperature. Much work has been done on the modification of texture for magnesium alloys, such as the equal channel angular process (ECAP) [25] and alloying with RE [26,27]. It is suggested that the DRX via PSN is related with the second phase particles [28]. During deformation of the particle-containing alloy, the enforced strain gradient in the vicinity of a non-deforming particle creates a region of high dislocation density and large orientation gradient, thus both high dislocation density and large orientation promote nucleation of recrystallization [29]. The random texture may also be ascribed to the DRX via PSN. As reported [30], the Al–Sr particles in the Mg–3Al–1Zn–xSr (x = 0.4–0.8, wt.%) alloys increased the potential nucleation of PSN, resulting in the weakening of the texture. In the present investigation, the presence of the primary W-phase particles acted as nucleation sites for PSN and made the texture to be weak as the volume fraction of the W-phase increase. As a result, the maximum intensity values of the texture in the as-extruded alloys decreased with increase in the volume fraction of the W-phase particles, which contributed to the plasticity of the as-extruded alloys. Besides, texture intensity is reduced with the increase of extrusion temperature. The same phenomenon has been reported in AZ31 [31]. It may be caused by difference of DRX mechanism. Based on the above discussion, it is easy to understand the mechanical properties of the as-extruded alloys. The diffusive distribution of the W-phase in the Mg matrix after extrusion
process can pin the dislocation and contribute some dispersionstrengthening to the alloys. Therefore, the value of the UTS is on the rise with the volume fraction of the W-phase increases. The YTS was mainly affected by the texture and grain size. Due to the strongest basal texture, the YTS of the alloy A extruded at 300 °C is higher than that of the alloys B and C. When the texture gradually weakens, both the W-phase and the grain size played an important role in advancing tensile properties. Therefore, with the volume fraction of the W-phase increasing and the refinement of the grain size, the YTS of these alloys extruded at 400 °C shows a trend to increase. The elongation of the alloys is affected by the texture intensity, grain size and the distribution of the second phase in the present investigation. The elongation is on the rise with the texture gradually weakened and grain refinement. Besides, the uniform distribution of W-phase also helps the elongation increase. Therefore, the alloy C extruded at 400 °C shows a best elongation with 19.7%. 5. Conclusions The effects of the W-phase on the microstructure, texture and mechanical properties of as-extruded Mg–Zn–Er alloys were investigated. Some conclusions can be summarized as below: 1. The W-phase could be broken and distributed along the extrusion direction during hot extrusion. Most of the broken W-phase has a size of about 1 lm. Besides, the higher extrusion temperature can make the broken W-phase distribution more uniform. The diffusive distribution of the W-phase can contribute dispersion-strengthening to the alloys, which make the UTS is on the rise with the volume fraction of the W-phase increases. 2. The primary W-phase particles worked as nucleation sites for the DRX via PSN, which contributes to the refined microstructure and weaken texture. As a result, the Mg–6Zn–6Er alloy
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Fig. 7. Inverse pole figures map of the as-extruded alloys: (a) and (d) alloy A extruded at 300 °C and 400 °C; (b) and (e) alloy B extruded at 300 °C and 400 °C; (c) and (f) alloy C extruded at 300 °C and 400 °C.
Fig. 8. Pole figures of the as-extruded alloys, the RD standing for the ED: (a) and (b) alloy A extruded at 300 °C and 400 °C; (c) and (d) alloy B extruded at 300 °C and 400 °C; (e) and (f) alloy C extruded at 300 °C and 400 °C.
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Technology Commission (Z131100003213019), Beijing Municipal Commission Education (KM201310005001, KM201410005014) and Beijing Natural Science Foundation (2142005, 2144043). References
Fig. 9. Stress–strain curves of the as-extruded alloys A, B and C tested under a constant speed of 1.0 mm/min at room temperature.
Table 3 Tensile properties of the as-extruded alloys A, B and C at room temperature. Alloy
Extrusion temperature (°C)
UTS (MPa)
YTS (MPa)
E (%)
Alloy A
300 400
320 (7.9) 279 (6.8)
310 (6.5) 247 (6.2)
12.8 (1.2) 12.1 (1.6)
Alloy B
300 400
330 (3.0) 319 (3.1)
295 (2.8) 278 (2.9)
13.7 (2.1) 17.6 (2.0)
Alloy C
300 400
343 (7.0) 328 (2.5)
299 (6.3) 283 (2.2)
16.8 (1.2) 19.7 (1.2)
Note: This table shows the average value. The in-bracket values are that of standard deviation.
extruded at 400 °C exhibited better comprehensive mechanical properties. The UTS (rb) and the YTS (r0.2) of the as-extruded alloy were 328 MPa and 283 MPa, respectively, with an elongation of 19.7%.
Acknowledgments This project was supported by the National Natural Science Fund of China (51071004, 51301006), Beijing Municipal Science and
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