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Effect of the Zn content on the microstructure and mechanical properties of indirect-extruded Mg–5Sn–xZn alloys
Materials and Design 32 (2011) 3537–3543
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of the Zn content on the microstructure and mechanical properties of indirect-extruded Mg–5Sn–xZn alloys W.N. Tang a,b,⇑, S.S. Park a, B.S. You a a b
Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 November 2010 Accepted 3 February 2011 Available online 26 February 2011
a b s t r a c t Mg–5Sn–xZn alloys with varying Zn contents were subjected to indirect extrusion and the effects of the Zn content on the microstructure and mechanical properties of the as-extruded alloys were investigated. It was found that, the grain size and the basal texture are basically similar, however, the amount of fine particles consisting of Mg2Sn and MgZn phases increases markedly as the Zn content increases. A higher number of these particles would be responsible for the better comprehensive mechanical properties as well as a lower degree of yield asymmetry. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction As the demand for weight reductions in areas related to transportation and electronic devices has increased, more and more attention has been paid to lightweight Mg alloys. Compared to Mg cast products, however, there is a lack of competitive Mg wrought products, particularly in extruded form, which are greatly needed for numerous weight-sensitive applications such as automobile components. The Mg–Sn alloy system is a promising alloy as a heat-resistant one since the secondary phase, Mg2Sn, has a high melting temperature. So far, despite there existence of numerous investigation on the as-cast Mg–Sn based alloys [1–3], few works have been reported on extruded Mg–Sn based alloys, even though Sasaki [4] achieved a higher strength in the extruded Mg–Sn alloy at a very low ram speed of 0.1 mm/s (equal to a rod speed of about 0.12 m/min) with an extrusion ratio of 20. As we all know, it is generally regarded that the main obstacle to overcome in Mg extrusions is the low extrusion speed of only 0.5–2.5 m/min at rod speeds such as for AZ80 and ZK60 alloys, leading to a high production cost [5]. Recently, our previous work showed that Mg–Sn based alloys have a great potential for use in high-speed extrusion processes for the higher incipient melting temperatures of their secondary phases compared to conventional AZ and ZK series alloys. However, it indicated that the mechanical properties, especially the ductility, of these developed Mg–Sn based alloys are still not high enough to be more competitive on the engineering application [6]. ⇑ Corresponding author at: Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China. Tel.: +86 24 23998482; fax: +86 24 23894149. E-mail address: [email protected] (W.N. Tang). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.02.012
It was suggested that the addition of Zn could enhance the ductility and precipitation strengthening by the refinement and homogeneous dispersion of secondary phases in as-cast Mg–Sn based alloys [7,8]. From this point, zinc element is attractive to be considered for further improving the mechanical properties of extruded Mg–Sn based alloys. Therefore, in the present study, a set of Mg–5Sn–xZn alloys with varying Zn contents were subjected to an indirect extrusion process and the effect of Zn on the microstructure and mechanical properties of indirect-extruded alloys were investigated. 2. Experimental The compositions of Mg–5Sn–xZn (TZ5x, x = 1, 2, 4) alloys used in this study are listed in Table 1. To prepare the billets for extrusion, the alloys were melted under an inert atmosphere containing a mixture of CO2 and SF6 and were then stabilized at 700 °C and poured into a steel mould pre-heated to 200 °C. For homogenization, the cast billets were stabilized at 330 °C for 2 h, heated up to 420 °C at 1 °C/min, heat-treated at 420 °C for 24 h, and finally water-quenched. The dimensions of the billet were 80 mm in diameter and 200 mm in length. Prior to indirect extrusion, the billets were pre-heated in a resistance furnace set at 250 °C for 1 h. Extrusion experiments were implemented at an initial billet temperature of 250 °C and a ram speed of 1.3 mm/s (equal to the rod speed of about 2 m/min). This temperature is somewhat lower compared to those usually used in the conventional direct extrusion of Mg alloys. A rod profile was extruded with a fixed extrusion ratio of 25. Microstructural examinations and texture measurement were conducted on the midsections parallel to the extrusion direction (ED). The longitudinal tensile and compressive properties in the as-extruded condition were measured at room temperature and at an initial strain rate of 1.0 103 s1, by using round tensile
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Table 1 Chemical compositions (wt.%) of Mg–5Sn–xZn alloys with different Zn contents. Alloy
Sn
Zn
Mg
TZ51 TZ52 TZ54
4.95 4.98 5.01
1.02 1.95 3.96
Bal. Bal. Bal.
Fig. 1. Schematic pictures showing the geometric sizes of specimens (unit: mm) used by the testing for mechanical properties: (a) tensile specimen, (b) compressive specimen.
specimens with a gage diameter of 5 mm as well as a gage length of 25 mm and cylindrical compressive specimens with 8 mm in diameter and 12 mm in height, respectively, as schematically shown in Fig. 1.
3. Results and discussion The as-cast microstructures of TZ5x alloys investigated here are shown in Fig. 2. They show that coarse second-phase particles are present at a-Mg grain boundaries, which become larger as the Zn content increases. Analyses by energy dispersive spectroscopy (EDS) and X-ray diffraction indicate that the particles in the TZ51 and TZ52 alloys are mostly Mg2Sn in despite of some MgZn phase also exists in the matrix, while those in TZ54 mainly consist of both Mg2Sn and MgZn phases. It was also noted that the size of the Mg2Sn particles becomes coarser as the Zn content increased, implying that the solubility of Sn in Mg varies with the Zn content in Mg–5Sn alloy. In addition, average grain sizes of the as-cast microstructures obtained from the OM micrographs shown in Fig. 2 decrease monotonically from about 170 lm in TZ51 (Fig. 2b) to about 120 lm in TZ54 (Fig. 2f) with the increasing Zn. After a homogenization heat-treatment, the fraction of the secondary phase particles becomes much smaller than that in the ascast condition, as shown in Fig. 3. However, small Mg2Sn particles remain at the grain boundaries, indicating that they were not completely dissolved during the heat-treatment. Fig. 4 shows micrographs of the as-extruded alloys exhibiting much refined microstructures compared to the as-cast alloys shown in Fig. 2. This was undoubtedly a consequence of dynamic recrystallization during the extrusion process. The grain sizes in
Fig. 2. SEM and OM micrographs of as-cast Mg–5Sn–xZn alloys with different Zn contents: (a, b) TZ51, (c, d) TZ52, and (e, f) TZ54.
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Fig. 3. SEM micrographs of homogenized Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
Fig. 4. Optical micrographs of extruded Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
the extruded condition are basically similar, in despite of decreasing slightly with the Zn content increased, as listed in Table 2. Similarly, there are no remarkable differences in the textures among the as-extruded alloys, although the distribution of (0 0 0 1) pole figures has a little change and their intensity became a little stronger with Zn increasing, as shown in Fig. 5. It is clearly shown that, the coarse Mg2Sn particles present in the homogenized condition are aligned along the ED (as shown in Fig. 6a and c by arrows), at the same time, it seems that no substantial difference appears
between these elongated coarse particles according to our observation. However, most of all, it is clear that numerous small particles with sizes of less than 1 lm, which were not present prior to extrusion, are distributed in the a-Mg matrix, as revealed by SEM with higher magnification in Fig. 6b and d. The volume fraction of these small particles obviously tends to increase as the Zn content increases from 1% (Fig. 6b) to 4% (Fig. 6d). Furthermore, TEM result in Fig. 7 also demonstrated that, a number of smaller particles with a size of about
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Table 2 Grain sizes and mechanical properties of Mg–5Sn–Zn alloys with different Zn contents: (T: tensile, C: compressive, and R: asymmetry ratio of yield point). Alloy
TZ51 TZ52 TZ54
Grain size (lm)
Yield strength (MPa)
Ultimate strength (MPa)
Elongation (%)
Extruded
T
C
T
C
T
C
7.4 7.3 6.9
170 172 173
125 135 149
250 265 287
322 332 358
20.1 24.9 24.8
12.7 12.0 15.0
R
0.74 0.78 0.86
Fig. 5. Pole figures of (0 0 0 2) plane showing the textures of the as-extruded Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
100 nm precipitated homogenously in the magnesium matrix, as well as a larger quantity in the alloy with a higher Zn content. EDS analyses in SEM suggest that these particles are the mixture of Mg2Sn and MgZn phases (mostly Mg2Sn and a few MgZn phases), as shown in Fig. 8 for the TZ54 alloy. By EDS in TEM, it is also confirmed that they are a mixture of Mg2Sn and MgZn. Regarding the formation of fine particles in Mg alloys during extrusion, similar results have been reported elsewhere [4,9], which are confirmed to be associated with dynamic precipitation during extrusion. The tensile and compressive stress–strain curves of TZ5x alloys are given in Fig. 9, and also summarized in Table 2. Under tension, the yield and ultimate strengths increases as the Zn content increases, although the increase in the yield strength is not as marked as that of the ultimate strength, indicating that the amount of work-hardening increases with the Zn addition. To evaluate the work-hardening capacity, the inverse of the yield ratio (yield strength/ ultimate strength) was measured for the TZ5x alloys [10,11]. These values are 1.49, 1.54, and 1.66 for the TZ51, TZ52, and TZ54 alloys, respectively. The TZ52 and TZ54 alloys with a relatively high Zn content in fact show better ductility compared to the TZ51 alloy. Similarly to the observed results with the tension, the yield and ultimate strengths promote with the increasing Zn content under compression. As compared to the tensile yield strength (TYS), however, the compressive yield strength (CYS) was found to depend largely on the
Zn content, leading to an increase in the yield point asymmetry ratio R (CYS/TYS) with the increase in the Zn content. The present study shows that increasing the Zn content results in better combinations of mechanical properties in as-extruded TZ5x alloys. As mentioned above, the amount of fine particles consisting of Mg2Sn and MgZn phases increases as the Zn content increases, while neither the grain size nor the texture are influenced significantly by the Zn content. Generally, presence of fine intermetallic particles in the microstructure is quite beneficial to the mechanical properties. It has been shown that the refinement of Al–Mn particles in Al alloys results in an improvement in both the strength and ductility, as fine particles retard the motion of dislocations to increase the strength; simultaneously, the blocked dislocations change the slip systems as a result of crossslip to maintain good ductility [12]. Likewise, here, it is considered that the larger amount of work-hardening in tension and hence the higher ultimate tensile strength in the TZ54 alloy compared to that in TZ51 is mainly due to the presence of numerous fine particles, as shown in Figs. 6–8. Moreover, it is believed that the workhardening behavior is related to the ductility of TZ5x alloys to some extent. The work-hardening exponents obtained from the Hollomon equation, r ¼ K en , are 0.19, 0.21, and 0.23 for the TZ51, TZ52, and TZ54 alloys, respectively. It is generally known that a large work-hardening exponent leads to low sensitivity to strain localization, resulting in a greater elongation [11]. This indicates that the TZ54 alloy has the lowest sensitivity to strain localization
W.N. Tang et al. / Materials and Design 32 (2011) 3537–3543
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Fig. 6. SEM micrographs of extruded Mg–5Sn–xZn alloys with different Zn contents: (a, b) TZ51 and (c, d) TZ54.
Fig. 7. TEM micrographs for showing finer particles in the as-extruded alloys: (a) TZ51 and (b) TZ54.
among the TZ5x alloys investigated here, leading to a better ductility. In addition, an increase in the Zn content leads to a reduction in the yield asymmetry; in other words, it leads to an increase in the R value. It is widely accepted that the yield asymmetry between tension and compression in the as-extruded Mg alloys is derived from 2g < 1 0 1 1 > tensile twins in compresthe formation of the f1 0 1 sion at an early stage of deformation, which lowers the compressive yield strength [13]. In the one hand, tensile twinning is largely affected by the fiber texture that forms in extruded Mg alloys, as twinning can occur easier within grains having low Schmid factors for basal slip [13]. It is therefore true that, the number of grains unfavorable for basal slip, as well as the fraction of twins occurring as a result, would decrease under compression as the fiber texture becomes weaker in the as-extruded alloys, which will result in a higher R value. In the other hand, a decrease in the grain size has an effect on the suppression of twinning activity [14], leading to an increase in the R value. However, in the present study, results show that there are considerable increases in the R value with Zn content increasing, which cannot be originated from its fiber texture strengthening that will decrease the R value, or its grain size refinement that will not result in great variation in the R value because of a very slight difference existed in these three different Zn-content alloys. Recently, it was reported in a Mg–5Zn
alloy that, the fine particles included in the magnesium matrix increased the number of formed twins by promoting twin nucleation, but reduced the size and total volume fraction of twins through suppressing the twin growth [15,16]. Consequently, it is believed that, more dispersive fine particles in the present alloys may have a positive effect on the weakening of yield anisotropy, as the suggestion mentioned in the TAZ811 magnesium alloys [6]. The presence of the numerous fine particles would be an important factor as regards the lower yield asymmetry in the TZ54 alloy. 4. Conclusions The effect of Zn content on the microstructural characteristics and mechanical properties of the indirect-extruded Mg–5Sn–xZn alloys with varying Zn contents was investigated in this study. The results can be summarized as follows: (1) The grain sizes and the intensity of (0 0 0 1) texture in the extruded TZ5x alloys are basically similar in despite of that the former becomes a little smaller and the latter a little stronger, while the amount of fine particles consisting of Mg2Sn and MgZn phases increases markedly as the Zn content increases.
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Fig. 8. (a) SEM micrograph obtained under high magnification and (b and c) EDS spectra of TZ54 alloy.
(3) The yield point asymmetry ratio increases from 0.74 to 0.86 with increasing zinc content in TZ5x alloys, which should be ascribed to the presence of more fine particles of Mg2Sn and MgZn phases due to the increasing Zn addition.
Acknowledgements This work was supported by a Grant from the World Premier Materials (WPM) Program funded by the Ministry of Knowledge Economy, Republic of Korea. References
Fig. 9. Tensile (T) and compressive (C) engineering stress–strain curves of extruded Mg–5Sn–xZn alloys with different Zn contents.
(2) Either under tension or under compression in the TZ5x alloys with higher zinc content, the better combination of yield strength, ultimate strength, as well as the ductility is mainly due to the larger amount of fine Mg2Sn and MgZn particles precipitated during extrusion.
[1] Kang DH, Park SS, Oh YS, Kim NJ. Effect of nano-particles on the creep resistance of Mg–Sn based alloys. Mater Sci Eng A 2007;449–451:318–21. [2] Kang DH, Park SS, Kim NJ. Development of creep resistant die cast Mg–Sn–Al– Si alloy. Mater Sci Eng A 2005;413–414:555–60. [3] Harosh S, Miller L, Levi G, Bamberger M. Microstructure and properties of Mg– 5.6%Sn–4.4%Zn–2.1%Al alloy. J Mater Sci 2007;42:9983–9. [4] Sasaki TT, Yamamoto K, Honma T, Kamado S, Hono K. A high-strength Mg–Sn– Zn–Al alloy extruded at low temperature. Scripta Mater 2008;59:1111–4. [5] Bettles CJ, Gibson MA. Current wrought magnesium alloys: strengths and weakness. JOM 2005;57:46–9. [6] Park SS, Tang WN, You BS. Microstructure and mechanical properties of an indirect-extruded Mg–8Sn–1Al–1Zn alloy. Mater Lett 2010;64:31–4.
W.N. Tang et al. / Materials and Design 32 (2011) 3537–3543 [7] Pekguleryuz MO, Avedesian MM. Magnesium alloying, some potentials for alloy development. J Jpn Inst Light Met 1992;42:679–86. [8] Sasaki TT, Oh-ishi K, Ohkubo T, Hono K. Enhanced age hardening response by the addition of Zn in Mg–Sn alloys. Scripta Mater 2006;55:251–4. [9] Mendis CL, Ohishi K, Kawamura Y, Honma T, Kamado S, Hono K. Precipitationhardenable Mg–2.4Zn–0.1Ag–0.1Ca–0.16Zr (at.%) wrought magnesium alloy. Acta Mater 2009;57:749–60. [10] Kim SK, Kim YM, Lim YJ, Kim NJ. Relationship between yield ratio and the mechanical constants of the swift equation. Met Mater Int 2006;12:131–5. [11] Kang DH, Kim DW, Kim S, Bae GT, Kim KH, Kim NJ. Relationship between stretch formability and work-hardening capacity of twin-roll cast Mg alloys at room temperature. Scripta Mater 2009;61:768–71.
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[12] Kim KC, Nam SW. Effects of Mn-dispersoids on the fatigue mechanism in an Al–Zn–Mg alloy. Mater Sci Eng A 1998;244:257–62. [13] Wang YN, Huang JC. The role of twinning and untwinning in yielding behavior in hot-extruded Mg–Al–Zn alloy. Acta Mater 2007;55:897–905. [14] Barnett MR. A rationale for the strong dependence of mechanical twinning on grain size. Scripta Mater 2008;59:696–8. [15] Stanford N, Barnett MR. Effect of particles on the formation of deformation twins in a magnesium-based alloy. Mater Sci Eng A 2009;516:226–34. [16] Robson JD, Stanford N, Barnett MR. Effect of particles in promoting twin nucleation in a Mg–5 wt.% Zn alloy. Scripta Mater 2010;63:823–6.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of the Zn content on the microstructure and mechanical properties of indirect-extruded Mg–5Sn–xZn alloys W.N. Tang a,b,⇑, S.S. Park a, B.S. You a a b
Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 November 2010 Accepted 3 February 2011 Available online 26 February 2011
a b s t r a c t Mg–5Sn–xZn alloys with varying Zn contents were subjected to indirect extrusion and the effects of the Zn content on the microstructure and mechanical properties of the as-extruded alloys were investigated. It was found that, the grain size and the basal texture are basically similar, however, the amount of fine particles consisting of Mg2Sn and MgZn phases increases markedly as the Zn content increases. A higher number of these particles would be responsible for the better comprehensive mechanical properties as well as a lower degree of yield asymmetry. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction As the demand for weight reductions in areas related to transportation and electronic devices has increased, more and more attention has been paid to lightweight Mg alloys. Compared to Mg cast products, however, there is a lack of competitive Mg wrought products, particularly in extruded form, which are greatly needed for numerous weight-sensitive applications such as automobile components. The Mg–Sn alloy system is a promising alloy as a heat-resistant one since the secondary phase, Mg2Sn, has a high melting temperature. So far, despite there existence of numerous investigation on the as-cast Mg–Sn based alloys [1–3], few works have been reported on extruded Mg–Sn based alloys, even though Sasaki [4] achieved a higher strength in the extruded Mg–Sn alloy at a very low ram speed of 0.1 mm/s (equal to a rod speed of about 0.12 m/min) with an extrusion ratio of 20. As we all know, it is generally regarded that the main obstacle to overcome in Mg extrusions is the low extrusion speed of only 0.5–2.5 m/min at rod speeds such as for AZ80 and ZK60 alloys, leading to a high production cost [5]. Recently, our previous work showed that Mg–Sn based alloys have a great potential for use in high-speed extrusion processes for the higher incipient melting temperatures of their secondary phases compared to conventional AZ and ZK series alloys. However, it indicated that the mechanical properties, especially the ductility, of these developed Mg–Sn based alloys are still not high enough to be more competitive on the engineering application [6]. ⇑ Corresponding author at: Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China. Tel.: +86 24 23998482; fax: +86 24 23894149. E-mail address: [email protected] (W.N. Tang). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.02.012
It was suggested that the addition of Zn could enhance the ductility and precipitation strengthening by the refinement and homogeneous dispersion of secondary phases in as-cast Mg–Sn based alloys [7,8]. From this point, zinc element is attractive to be considered for further improving the mechanical properties of extruded Mg–Sn based alloys. Therefore, in the present study, a set of Mg–5Sn–xZn alloys with varying Zn contents were subjected to an indirect extrusion process and the effect of Zn on the microstructure and mechanical properties of indirect-extruded alloys were investigated. 2. Experimental The compositions of Mg–5Sn–xZn (TZ5x, x = 1, 2, 4) alloys used in this study are listed in Table 1. To prepare the billets for extrusion, the alloys were melted under an inert atmosphere containing a mixture of CO2 and SF6 and were then stabilized at 700 °C and poured into a steel mould pre-heated to 200 °C. For homogenization, the cast billets were stabilized at 330 °C for 2 h, heated up to 420 °C at 1 °C/min, heat-treated at 420 °C for 24 h, and finally water-quenched. The dimensions of the billet were 80 mm in diameter and 200 mm in length. Prior to indirect extrusion, the billets were pre-heated in a resistance furnace set at 250 °C for 1 h. Extrusion experiments were implemented at an initial billet temperature of 250 °C and a ram speed of 1.3 mm/s (equal to the rod speed of about 2 m/min). This temperature is somewhat lower compared to those usually used in the conventional direct extrusion of Mg alloys. A rod profile was extruded with a fixed extrusion ratio of 25. Microstructural examinations and texture measurement were conducted on the midsections parallel to the extrusion direction (ED). The longitudinal tensile and compressive properties in the as-extruded condition were measured at room temperature and at an initial strain rate of 1.0 103 s1, by using round tensile
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Table 1 Chemical compositions (wt.%) of Mg–5Sn–xZn alloys with different Zn contents. Alloy
Sn
Zn
Mg
TZ51 TZ52 TZ54
4.95 4.98 5.01
1.02 1.95 3.96
Bal. Bal. Bal.
Fig. 1. Schematic pictures showing the geometric sizes of specimens (unit: mm) used by the testing for mechanical properties: (a) tensile specimen, (b) compressive specimen.
specimens with a gage diameter of 5 mm as well as a gage length of 25 mm and cylindrical compressive specimens with 8 mm in diameter and 12 mm in height, respectively, as schematically shown in Fig. 1.
3. Results and discussion The as-cast microstructures of TZ5x alloys investigated here are shown in Fig. 2. They show that coarse second-phase particles are present at a-Mg grain boundaries, which become larger as the Zn content increases. Analyses by energy dispersive spectroscopy (EDS) and X-ray diffraction indicate that the particles in the TZ51 and TZ52 alloys are mostly Mg2Sn in despite of some MgZn phase also exists in the matrix, while those in TZ54 mainly consist of both Mg2Sn and MgZn phases. It was also noted that the size of the Mg2Sn particles becomes coarser as the Zn content increased, implying that the solubility of Sn in Mg varies with the Zn content in Mg–5Sn alloy. In addition, average grain sizes of the as-cast microstructures obtained from the OM micrographs shown in Fig. 2 decrease monotonically from about 170 lm in TZ51 (Fig. 2b) to about 120 lm in TZ54 (Fig. 2f) with the increasing Zn. After a homogenization heat-treatment, the fraction of the secondary phase particles becomes much smaller than that in the ascast condition, as shown in Fig. 3. However, small Mg2Sn particles remain at the grain boundaries, indicating that they were not completely dissolved during the heat-treatment. Fig. 4 shows micrographs of the as-extruded alloys exhibiting much refined microstructures compared to the as-cast alloys shown in Fig. 2. This was undoubtedly a consequence of dynamic recrystallization during the extrusion process. The grain sizes in
Fig. 2. SEM and OM micrographs of as-cast Mg–5Sn–xZn alloys with different Zn contents: (a, b) TZ51, (c, d) TZ52, and (e, f) TZ54.
W.N. Tang et al. / Materials and Design 32 (2011) 3537–3543
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Fig. 3. SEM micrographs of homogenized Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
Fig. 4. Optical micrographs of extruded Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
the extruded condition are basically similar, in despite of decreasing slightly with the Zn content increased, as listed in Table 2. Similarly, there are no remarkable differences in the textures among the as-extruded alloys, although the distribution of (0 0 0 1) pole figures has a little change and their intensity became a little stronger with Zn increasing, as shown in Fig. 5. It is clearly shown that, the coarse Mg2Sn particles present in the homogenized condition are aligned along the ED (as shown in Fig. 6a and c by arrows), at the same time, it seems that no substantial difference appears
between these elongated coarse particles according to our observation. However, most of all, it is clear that numerous small particles with sizes of less than 1 lm, which were not present prior to extrusion, are distributed in the a-Mg matrix, as revealed by SEM with higher magnification in Fig. 6b and d. The volume fraction of these small particles obviously tends to increase as the Zn content increases from 1% (Fig. 6b) to 4% (Fig. 6d). Furthermore, TEM result in Fig. 7 also demonstrated that, a number of smaller particles with a size of about
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Table 2 Grain sizes and mechanical properties of Mg–5Sn–Zn alloys with different Zn contents: (T: tensile, C: compressive, and R: asymmetry ratio of yield point). Alloy
TZ51 TZ52 TZ54
Grain size (lm)
Yield strength (MPa)
Ultimate strength (MPa)
Elongation (%)
Extruded
T
C
T
C
T
C
7.4 7.3 6.9
170 172 173
125 135 149
250 265 287
322 332 358
20.1 24.9 24.8
12.7 12.0 15.0
R
0.74 0.78 0.86
Fig. 5. Pole figures of (0 0 0 2) plane showing the textures of the as-extruded Mg–5Sn–xZn alloys with different Zn contents: (a) TZ51, (b) TZ52, and (c) TZ54.
100 nm precipitated homogenously in the magnesium matrix, as well as a larger quantity in the alloy with a higher Zn content. EDS analyses in SEM suggest that these particles are the mixture of Mg2Sn and MgZn phases (mostly Mg2Sn and a few MgZn phases), as shown in Fig. 8 for the TZ54 alloy. By EDS in TEM, it is also confirmed that they are a mixture of Mg2Sn and MgZn. Regarding the formation of fine particles in Mg alloys during extrusion, similar results have been reported elsewhere [4,9], which are confirmed to be associated with dynamic precipitation during extrusion. The tensile and compressive stress–strain curves of TZ5x alloys are given in Fig. 9, and also summarized in Table 2. Under tension, the yield and ultimate strengths increases as the Zn content increases, although the increase in the yield strength is not as marked as that of the ultimate strength, indicating that the amount of work-hardening increases with the Zn addition. To evaluate the work-hardening capacity, the inverse of the yield ratio (yield strength/ ultimate strength) was measured for the TZ5x alloys [10,11]. These values are 1.49, 1.54, and 1.66 for the TZ51, TZ52, and TZ54 alloys, respectively. The TZ52 and TZ54 alloys with a relatively high Zn content in fact show better ductility compared to the TZ51 alloy. Similarly to the observed results with the tension, the yield and ultimate strengths promote with the increasing Zn content under compression. As compared to the tensile yield strength (TYS), however, the compressive yield strength (CYS) was found to depend largely on the
Zn content, leading to an increase in the yield point asymmetry ratio R (CYS/TYS) with the increase in the Zn content. The present study shows that increasing the Zn content results in better combinations of mechanical properties in as-extruded TZ5x alloys. As mentioned above, the amount of fine particles consisting of Mg2Sn and MgZn phases increases as the Zn content increases, while neither the grain size nor the texture are influenced significantly by the Zn content. Generally, presence of fine intermetallic particles in the microstructure is quite beneficial to the mechanical properties. It has been shown that the refinement of Al–Mn particles in Al alloys results in an improvement in both the strength and ductility, as fine particles retard the motion of dislocations to increase the strength; simultaneously, the blocked dislocations change the slip systems as a result of crossslip to maintain good ductility [12]. Likewise, here, it is considered that the larger amount of work-hardening in tension and hence the higher ultimate tensile strength in the TZ54 alloy compared to that in TZ51 is mainly due to the presence of numerous fine particles, as shown in Figs. 6–8. Moreover, it is believed that the workhardening behavior is related to the ductility of TZ5x alloys to some extent. The work-hardening exponents obtained from the Hollomon equation, r ¼ K en , are 0.19, 0.21, and 0.23 for the TZ51, TZ52, and TZ54 alloys, respectively. It is generally known that a large work-hardening exponent leads to low sensitivity to strain localization, resulting in a greater elongation [11]. This indicates that the TZ54 alloy has the lowest sensitivity to strain localization
W.N. Tang et al. / Materials and Design 32 (2011) 3537–3543
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Fig. 6. SEM micrographs of extruded Mg–5Sn–xZn alloys with different Zn contents: (a, b) TZ51 and (c, d) TZ54.
Fig. 7. TEM micrographs for showing finer particles in the as-extruded alloys: (a) TZ51 and (b) TZ54.
among the TZ5x alloys investigated here, leading to a better ductility. In addition, an increase in the Zn content leads to a reduction in the yield asymmetry; in other words, it leads to an increase in the R value. It is widely accepted that the yield asymmetry between tension and compression in the as-extruded Mg alloys is derived from 2g < 1 0 1 1 > tensile twins in compresthe formation of the f1 0 1 sion at an early stage of deformation, which lowers the compressive yield strength [13]. In the one hand, tensile twinning is largely affected by the fiber texture that forms in extruded Mg alloys, as twinning can occur easier within grains having low Schmid factors for basal slip [13]. It is therefore true that, the number of grains unfavorable for basal slip, as well as the fraction of twins occurring as a result, would decrease under compression as the fiber texture becomes weaker in the as-extruded alloys, which will result in a higher R value. In the other hand, a decrease in the grain size has an effect on the suppression of twinning activity [14], leading to an increase in the R value. However, in the present study, results show that there are considerable increases in the R value with Zn content increasing, which cannot be originated from its fiber texture strengthening that will decrease the R value, or its grain size refinement that will not result in great variation in the R value because of a very slight difference existed in these three different Zn-content alloys. Recently, it was reported in a Mg–5Zn
alloy that, the fine particles included in the magnesium matrix increased the number of formed twins by promoting twin nucleation, but reduced the size and total volume fraction of twins through suppressing the twin growth [15,16]. Consequently, it is believed that, more dispersive fine particles in the present alloys may have a positive effect on the weakening of yield anisotropy, as the suggestion mentioned in the TAZ811 magnesium alloys [6]. The presence of the numerous fine particles would be an important factor as regards the lower yield asymmetry in the TZ54 alloy. 4. Conclusions The effect of Zn content on the microstructural characteristics and mechanical properties of the indirect-extruded Mg–5Sn–xZn alloys with varying Zn contents was investigated in this study. The results can be summarized as follows: (1) The grain sizes and the intensity of (0 0 0 1) texture in the extruded TZ5x alloys are basically similar in despite of that the former becomes a little smaller and the latter a little stronger, while the amount of fine particles consisting of Mg2Sn and MgZn phases increases markedly as the Zn content increases.
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Fig. 8. (a) SEM micrograph obtained under high magnification and (b and c) EDS spectra of TZ54 alloy.
(3) The yield point asymmetry ratio increases from 0.74 to 0.86 with increasing zinc content in TZ5x alloys, which should be ascribed to the presence of more fine particles of Mg2Sn and MgZn phases due to the increasing Zn addition.
Acknowledgements This work was supported by a Grant from the World Premier Materials (WPM) Program funded by the Ministry of Knowledge Economy, Republic of Korea. References
Fig. 9. Tensile (T) and compressive (C) engineering stress–strain curves of extruded Mg–5Sn–xZn alloys with different Zn contents.
(2) Either under tension or under compression in the TZ5x alloys with higher zinc content, the better combination of yield strength, ultimate strength, as well as the ductility is mainly due to the larger amount of fine Mg2Sn and MgZn particles precipitated during extrusion.
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