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Strength and ductility of novel Mg–8Sn–1Al–1Zn alloys extruded at different speeds
Materials Letters 65 (2011) 1525–1527
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Strength and ductility of novel Mg–8Sn–1Al–1Zn alloys extruded at different speeds W.L. Cheng a,b, H.S. Kim b, B.S. You b, B.H. Koo a, S.S. Park c,⁎ a b c
Changwon National University, Changwon 641-773, Republic of Korea Korea Institute of Materials Science, Changwon 642-831, Republic of Korea Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 February 2011 Accepted 2 March 2011 Available online 8 March 2011 Keywords: Metals and alloys Magnesium Extrusion
a b s t r a c t A novel Mg–8Sn–1Al–1Zn alloy developed for high-speed extrusion was successfully extruded at speeds in a range of 2–10 m/min at a temperature of 250 °C. The effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test. © 2011 Elsevier B.V. All rights reserved.
1. Introduction It is generally thought that the main obstacle to overcome in Mg extrusions is their low extrusion speed, which leads to a decrease in productivity. Normally, high-strength Mg alloys such as AZ80 and ZK60 are extrudable only at speeds of 0.5–2.5 m/min, well below the speeds attainable with Al alloys [1]. This is mainly due to an increase in the susceptibility to hot shortness during extrusion with increases in alloying elements such as Al and Zn, which is caused by the incipient melting of second-phase particles. In this respect, it is believed that Mg–Sn based alloys have great potential for use in high-speed extrusion processes, as they usually have higher incipient melting temperatures than conventional AZ and ZK series Mg alloys [2]. Recently, it has been reported that Mg–Sn based alloys are readily extrudable at low temperature and they exhibit excellent mechanical properties after extrusion [3–5]. However, the extrusion speeds tried so far have only been in a range of 0.12–2 m/min, which means that the potential of these alloys for high-speed extrusion processes have not been fully explored. In the present study, therefore, an experimental Mg–8Sn–1Al–1Zn (TAZ811) alloy was subjected to extrusion preformed at different speeds and the microstructure and tensile properties of extruded alloys were comparatively investigated.
2. Experimental The analyzed composition of the TAZ811 alloy used here was Mg–7.92Sn–0.98Al–0.91Zn (wt.%). Details of the billet casting and
⁎ Corresponding author. Tel.: +82 52 217 2328; fax: +82 52 217 2309. E-mail address: [email protected] (S.S. Park). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.010
homogenization procedures are described elsewhere [4,5]. The dimensions of the billet were 80 mm in diameter and 200 mm in length. Indirect extrusion experiments were implemented at an initial billet temperature of 250 °C, at an extrusion ratio of 25 and at three different ram speeds of 1.3, 3.0 and 6.7 mm/s, respectively. The corresponding extrusion exit speeds (Ve) are 2, 4.5 and 10 m/min, respectively. Texture measurements were taken via X-ray diffraction in the back reflection mode with Cu-Kα radiation. The electron backscatter diffraction (EBSD) was also used for microstructural and textural investigations. Longitudinal tensile properties were measured by using round tensile specimens with dimensions of a 25 mm gage length and a 6 mm gage diameter at an initial strain rate of 1.0 × 10−3 s−1 at room temperature. 3. Results and discussion Representative optical micrographs of the TAZ811 alloys extruded at different speeds are presented in Fig. 1. It can be clearly seen that grain size strongly depends on the extrusion speed. The average grain sizes are 3.8, 8.0 and 10.5 μm for the extrusion speeds of 2, 4.5 and 10 m/min, respectively. A number of studies have shown that grain size is a function of the Zener-Hollomon parameter (Z) [6,7], which is defined as ε˙ ⋅ expðQ = RT Þ where ε˙ is the average strain rate, Q is the activation energy for diffusion (135 kJ/mol for Mg), R is the gas constant and T is the absolute temperature. The average strain rate during the extrusion process was calculated following the equation proposed by Feltham [8]. Here, the exit temperature (Te) was used instead of the initial billet temperature in order to calculate Z since the former was considered to be closer to the actual temperature of alloys experienced during the extrusion process. According to the information provided in Table 1, ln Z values are 27.8, 26.9 and 26.6 for the extrusion speeds of 2, 4.5 and 10 m/min, respectively. It should be
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W.L. Cheng et al. / Materials Letters 65 (2011) 1525–1527
Fig. 1. Optical micrographs and inverse pole figures of the TAZ811 alloys extruded at different speeds: (a) 2, (b) 4.5 and (c) 10 m/min.
Table 1 Extrusion and tensile properties of the TAZ811 alloys extruded at different speeds. Ve (m/min)
Extrusion properties Te (°C)
ε˙ (s−1)
lnZ
YS (MPa)
UTS (MPa)
El (%)
2 4.5 10
288 323 350
0.32 0.73 1.63
27.8 26.9 26.6
244 231 199
312 311 286
17.5 15.2 14.8
Tensile properties
noted here that the Z value decreases as the extrusion speed increases, in order words, as the average strain rate increases. This discrepancy results because Te also increases due to frictional and deformational heating depending on the extrusion speed. Similar to other reports [6,7], the average grain size (d) is a function of Z, which can be formulated in the typical form as ln d = A + B ⋅ ln Z, with A and B values of 23.9 and − 0.81, respectively. In Fig. 1a and b, some grains appear to be elongated along the extrusion direction (ED), indicating that dynamic recrystallization (DRX) remains incomplete for such grains even when extrusion processing is complete. The average area fractions of the elongated grains (aspect ratio N 10) are 5.7 and 3.3% for the alloys extruded at 2
and 4.5 m/min, respectively, while there were almost no elongated grains in the alloy extruded at 10 m/min, indicating that the extrusion condition greatly influences the DRX fraction as it does grain size. The inverse pole figures referring to the ED are also provided in Fig. 1. They all reveal a type of fiber texture in which basal poles are preferentially to the ED and the maximum intensity is h perpendicular i
centered at 1010 , which is typical of extruded Mg alloys. However, texture appears to become weaker as the extrusion speed increases. Such textural weakening can be understood by considering the DRX fraction mentioned above. It has been shown that recrystallized grains have texture different from deformed grains in Mg and Mg alloys, demonstrating that the texture of the recrystallized grains is much more randomized than that of the deformed grains [9]. Similarly, the present EBSD result from the TAZ811 alloy extruded at 2 m/min reveals that the whole grains composed of both recrystallized and elongated grains (Fig. 2a) have a stronger texture than the grains consisting of only recrystallized grains (Fig. 2b), suggesting that the textural weakening with increased extrusion speed is associated with the decreased fraction of elongated grains retaining strong fiber texture. Tensile properties of extruded alloys are summarized in Table 1. As the extrusion speed increases, both strength and ductility deteriorate,
Fig. 2. EBSD orientation maps and inverse pole figures of the TAZ811 alloy extruded at 2 m/min (a) with and (b) without elongated grains.
W.L. Cheng et al. / Materials Letters 65 (2011) 1525–1527
1527
Fig. 3. Optical micrographs from gage sections of fractured tensile samples: extruded at (a) 2 and (b) 10 m/min.
which is different from the typical relationship between strength and ductility [1]. The decrease in strength with increasing extrusion speed is mainly due to reduced grain boundary strengthening, following the Hall–Petch relationship. As can be seen in Fig. 3a and b, after the tensile test a number of twins are found in the alloy extruded at 10 m/min, whereas twins are rarely observed in the alloy extruded at 2 m/min except in the areas of coarse elongated grains. Cracks are found to occur along the twinning lamellar, suggesting that the tensile ductility of the extruded alloys is closely related to twinning. EBSD analyses indicate that the twins have misorientation n o n angles o inna range o ofn23–30°, o which
or 1013 − 1012 double n o twins but quite smaller than those of 1012 b1011 N tension twins n o n o (~86°), 1011 b1210 N contraction twins (~56°) or 1013 b1210 N are close to those of
1011 − 1012
contraction twins (~64°) [10]. It has been well known that double twinning accelerates cracking, which is induced by dislocation pile-ups at the twin-matrix interface [11]. Double twinning is expected to more easily occur during tensile deformation along the ED as fiber texture becomes stronger and grain size becomes larger [11]. As shown in Fig. 3a, the portion of the sample with coarse elongated grain and strong fiber texture is the place where double twinning can readily occur during the tensile test. However, experimental results also indicate that double twinning actively occurs at locations of coarse recrystallized grains with relatively weak fiber texture as shown in Fig. 3b, suggesting that in the extruded TAZ811 alloys investigated here grain size has greater influence on the occurrence of double twinning during tensile deformation than does texture.
4. Conclusion An experimental TAZ811 alloy was successfully extruded at different speeds of 2, 4.5 and 10 m/min and the effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Properties of grain size, recrystallization fraction and texture were found to be significantly influenced by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The decrease in tensile strength and ductility results from reduced grain boundary strengthening and the occurrence of double twins during tensile deformation. Acknowledgement This work was supported by a grant from the World Premier Materials Program funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Bettles CJ, Gibson MA. JOM 2005;57(5):46–9. Kang DH, Park SS, Kim NJ. Mater Sci Eng A 2005;413–4:555–60. Sasaki TT, Yamamoto K, Honma T, Kamado S, Hono K. Scr Mater 2008;59:1111–4. Park SS, Tang WN, You BS. Mater Lett 2010;64:31–4. Park SS, Kim YJ, Cheng WL, Kim YM, You BS. Phil Mag Lett 2011;91:37–44. Galiyev A, Kaibyshev R, Gottstein G. Acta Mater 2001;49:1199–207. Kim SI, Yoo YC. Mater Sci Eng A 2001;311:108–13. Feltham PE. Met Treat 1956;23:440–4. Stanford N, Barnett MR. Mater Sci Eng A 2008;496:399–408. Cizek P, Barnett MR. Scr Mater 2008;59:959–62. Barnett MR. Mater Sci Eng A 2007;464:8–16.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Strength and ductility of novel Mg–8Sn–1Al–1Zn alloys extruded at different speeds W.L. Cheng a,b, H.S. Kim b, B.S. You b, B.H. Koo a, S.S. Park c,⁎ a b c
Changwon National University, Changwon 641-773, Republic of Korea Korea Institute of Materials Science, Changwon 642-831, Republic of Korea Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 February 2011 Accepted 2 March 2011 Available online 8 March 2011 Keywords: Metals and alloys Magnesium Extrusion
a b s t r a c t A novel Mg–8Sn–1Al–1Zn alloy developed for high-speed extrusion was successfully extruded at speeds in a range of 2–10 m/min at a temperature of 250 °C. The effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test. © 2011 Elsevier B.V. All rights reserved.
1. Introduction It is generally thought that the main obstacle to overcome in Mg extrusions is their low extrusion speed, which leads to a decrease in productivity. Normally, high-strength Mg alloys such as AZ80 and ZK60 are extrudable only at speeds of 0.5–2.5 m/min, well below the speeds attainable with Al alloys [1]. This is mainly due to an increase in the susceptibility to hot shortness during extrusion with increases in alloying elements such as Al and Zn, which is caused by the incipient melting of second-phase particles. In this respect, it is believed that Mg–Sn based alloys have great potential for use in high-speed extrusion processes, as they usually have higher incipient melting temperatures than conventional AZ and ZK series Mg alloys [2]. Recently, it has been reported that Mg–Sn based alloys are readily extrudable at low temperature and they exhibit excellent mechanical properties after extrusion [3–5]. However, the extrusion speeds tried so far have only been in a range of 0.12–2 m/min, which means that the potential of these alloys for high-speed extrusion processes have not been fully explored. In the present study, therefore, an experimental Mg–8Sn–1Al–1Zn (TAZ811) alloy was subjected to extrusion preformed at different speeds and the microstructure and tensile properties of extruded alloys were comparatively investigated.
2. Experimental The analyzed composition of the TAZ811 alloy used here was Mg–7.92Sn–0.98Al–0.91Zn (wt.%). Details of the billet casting and
⁎ Corresponding author. Tel.: +82 52 217 2328; fax: +82 52 217 2309. E-mail address: [email protected] (S.S. Park). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.010
homogenization procedures are described elsewhere [4,5]. The dimensions of the billet were 80 mm in diameter and 200 mm in length. Indirect extrusion experiments were implemented at an initial billet temperature of 250 °C, at an extrusion ratio of 25 and at three different ram speeds of 1.3, 3.0 and 6.7 mm/s, respectively. The corresponding extrusion exit speeds (Ve) are 2, 4.5 and 10 m/min, respectively. Texture measurements were taken via X-ray diffraction in the back reflection mode with Cu-Kα radiation. The electron backscatter diffraction (EBSD) was also used for microstructural and textural investigations. Longitudinal tensile properties were measured by using round tensile specimens with dimensions of a 25 mm gage length and a 6 mm gage diameter at an initial strain rate of 1.0 × 10−3 s−1 at room temperature. 3. Results and discussion Representative optical micrographs of the TAZ811 alloys extruded at different speeds are presented in Fig. 1. It can be clearly seen that grain size strongly depends on the extrusion speed. The average grain sizes are 3.8, 8.0 and 10.5 μm for the extrusion speeds of 2, 4.5 and 10 m/min, respectively. A number of studies have shown that grain size is a function of the Zener-Hollomon parameter (Z) [6,7], which is defined as ε˙ ⋅ expðQ = RT Þ where ε˙ is the average strain rate, Q is the activation energy for diffusion (135 kJ/mol for Mg), R is the gas constant and T is the absolute temperature. The average strain rate during the extrusion process was calculated following the equation proposed by Feltham [8]. Here, the exit temperature (Te) was used instead of the initial billet temperature in order to calculate Z since the former was considered to be closer to the actual temperature of alloys experienced during the extrusion process. According to the information provided in Table 1, ln Z values are 27.8, 26.9 and 26.6 for the extrusion speeds of 2, 4.5 and 10 m/min, respectively. It should be
1526
W.L. Cheng et al. / Materials Letters 65 (2011) 1525–1527
Fig. 1. Optical micrographs and inverse pole figures of the TAZ811 alloys extruded at different speeds: (a) 2, (b) 4.5 and (c) 10 m/min.
Table 1 Extrusion and tensile properties of the TAZ811 alloys extruded at different speeds. Ve (m/min)
Extrusion properties Te (°C)
ε˙ (s−1)
lnZ
YS (MPa)
UTS (MPa)
El (%)
2 4.5 10
288 323 350
0.32 0.73 1.63
27.8 26.9 26.6
244 231 199
312 311 286
17.5 15.2 14.8
Tensile properties
noted here that the Z value decreases as the extrusion speed increases, in order words, as the average strain rate increases. This discrepancy results because Te also increases due to frictional and deformational heating depending on the extrusion speed. Similar to other reports [6,7], the average grain size (d) is a function of Z, which can be formulated in the typical form as ln d = A + B ⋅ ln Z, with A and B values of 23.9 and − 0.81, respectively. In Fig. 1a and b, some grains appear to be elongated along the extrusion direction (ED), indicating that dynamic recrystallization (DRX) remains incomplete for such grains even when extrusion processing is complete. The average area fractions of the elongated grains (aspect ratio N 10) are 5.7 and 3.3% for the alloys extruded at 2
and 4.5 m/min, respectively, while there were almost no elongated grains in the alloy extruded at 10 m/min, indicating that the extrusion condition greatly influences the DRX fraction as it does grain size. The inverse pole figures referring to the ED are also provided in Fig. 1. They all reveal a type of fiber texture in which basal poles are preferentially to the ED and the maximum intensity is h perpendicular i
centered at 1010 , which is typical of extruded Mg alloys. However, texture appears to become weaker as the extrusion speed increases. Such textural weakening can be understood by considering the DRX fraction mentioned above. It has been shown that recrystallized grains have texture different from deformed grains in Mg and Mg alloys, demonstrating that the texture of the recrystallized grains is much more randomized than that of the deformed grains [9]. Similarly, the present EBSD result from the TAZ811 alloy extruded at 2 m/min reveals that the whole grains composed of both recrystallized and elongated grains (Fig. 2a) have a stronger texture than the grains consisting of only recrystallized grains (Fig. 2b), suggesting that the textural weakening with increased extrusion speed is associated with the decreased fraction of elongated grains retaining strong fiber texture. Tensile properties of extruded alloys are summarized in Table 1. As the extrusion speed increases, both strength and ductility deteriorate,
Fig. 2. EBSD orientation maps and inverse pole figures of the TAZ811 alloy extruded at 2 m/min (a) with and (b) without elongated grains.
W.L. Cheng et al. / Materials Letters 65 (2011) 1525–1527
1527
Fig. 3. Optical micrographs from gage sections of fractured tensile samples: extruded at (a) 2 and (b) 10 m/min.
which is different from the typical relationship between strength and ductility [1]. The decrease in strength with increasing extrusion speed is mainly due to reduced grain boundary strengthening, following the Hall–Petch relationship. As can be seen in Fig. 3a and b, after the tensile test a number of twins are found in the alloy extruded at 10 m/min, whereas twins are rarely observed in the alloy extruded at 2 m/min except in the areas of coarse elongated grains. Cracks are found to occur along the twinning lamellar, suggesting that the tensile ductility of the extruded alloys is closely related to twinning. EBSD analyses indicate that the twins have misorientation n o n angles o inna range o ofn23–30°, o which
or 1013 − 1012 double n o twins but quite smaller than those of 1012 b1011 N tension twins n o n o (~86°), 1011 b1210 N contraction twins (~56°) or 1013 b1210 N are close to those of
1011 − 1012
contraction twins (~64°) [10]. It has been well known that double twinning accelerates cracking, which is induced by dislocation pile-ups at the twin-matrix interface [11]. Double twinning is expected to more easily occur during tensile deformation along the ED as fiber texture becomes stronger and grain size becomes larger [11]. As shown in Fig. 3a, the portion of the sample with coarse elongated grain and strong fiber texture is the place where double twinning can readily occur during the tensile test. However, experimental results also indicate that double twinning actively occurs at locations of coarse recrystallized grains with relatively weak fiber texture as shown in Fig. 3b, suggesting that in the extruded TAZ811 alloys investigated here grain size has greater influence on the occurrence of double twinning during tensile deformation than does texture.
4. Conclusion An experimental TAZ811 alloy was successfully extruded at different speeds of 2, 4.5 and 10 m/min and the effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Properties of grain size, recrystallization fraction and texture were found to be significantly influenced by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The decrease in tensile strength and ductility results from reduced grain boundary strengthening and the occurrence of double twins during tensile deformation. Acknowledgement This work was supported by a grant from the World Premier Materials Program funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Bettles CJ, Gibson MA. JOM 2005;57(5):46–9. Kang DH, Park SS, Kim NJ. Mater Sci Eng A 2005;413–4:555–60. Sasaki TT, Yamamoto K, Honma T, Kamado S, Hono K. Scr Mater 2008;59:1111–4. Park SS, Tang WN, You BS. Mater Lett 2010;64:31–4. Park SS, Kim YJ, Cheng WL, Kim YM, You BS. Phil Mag Lett 2011;91:37–44. Galiyev A, Kaibyshev R, Gottstein G. Acta Mater 2001;49:1199–207. Kim SI, Yoo YC. Mater Sci Eng A 2001;311:108–13. Feltham PE. Met Treat 1956;23:440–4. Stanford N, Barnett MR. Mater Sci Eng A 2008;496:399–408. Cizek P, Barnett MR. Scr Mater 2008;59:959–62. Barnett MR. Mater Sci Eng A 2007;464:8–16.