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Effect of Mg content on the minimum grain size of Al–Mg alloys obtained by friction stir processing
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Scripta Materialia 64 (2011) 355–358 www.elsevier.com/locate/scriptamat
Effect of Mg content on the minimum grain size of Al–Mg alloys obtained by friction stir processing Taiki Morishige,a,⇑ Tomotake Hirata,b Tokuteru Uesugi,a Yorinobu Takigawa,a Masato Tsujikawaa and Kenji Higashia a
Department of Materials Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan b Metals and Machinery Department, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Received 16 August 2010; revised 15 October 2010; accepted 25 October 2010 Available online 30 October 2010
Friction stir processing (FSP) is one of the severe plastic deformation processes that have been developed to improve the mechanical properties of both metals and alloys by producing an ultrafine-grained structure. In this study, it was used to realize the minimum grain size in Al–Mg alloys. The results indicate that the grain size in Al–Mg alloy decreases with increasing Mg content because of the influence of stacking fault energy in the Al–Mg alloy. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir processing (FSP); Aluminum alloy; Severe plastic deformation (SPD); Minimum grain size; Stacking fault energy
In recent years, severe plastic deformation (SPD) processes, such as equal-channel angular extrusion (ECAE) [1], accumulative roll-bonding (ARB) [2], highpressure torsion (HPT) [3] and friction stir processing (FSP) [4], have been developed to obtain ultrafine-grained microstructures. SPD-processed Al alloys have a submicron-grained microstructure. The deformation conditions, such as the strain rate ð_eÞ and temperature (T), during the SPD processes can be compared using the Zener–Hollomon parameter (Z): Q ; ð1Þ Z ¼ e_ exp RT where Q represents the activation energy for self-diffusion (142 kJ mol1 in aluminum [5]) and R denotes the gas constant. In general, the grain size decreases with increasing Z value. Recently, Morishige et al. [6] reported that SPD-processed Al metals with various purities attained an almost constant grain size at Z P 1016 s1. On the other hand, Iwahashi et al. [7] reported that the grain size in ECAE-processed Al alloys decreased with increasing Mg content in spite of the same processing conditions and Z value. They attributed the effect of the Mg addition
⇑ Corresponding
author. Tel.: +81 72 254 9313; fax: +81 72 254 9912; e-mail: [email protected]
to reductions in the dislocation mobility and recovery rate in the solid-solution Al alloys. However, it is important to quantitatively understand the influence of the alloying element and the solubility on the grain size in obtaining an ultrafine-grained microstructure. Therefore, the aim of this study was to investigate the effect of Mg content on the minimum grain size. Al–Mg alloys were prepared by FSP under common processing conditions. We evaluated the minimum grain size of aluminum alloys of varying compositions. The microstructures in friction stir processed Al–Mg alloys were evaluated via comparisons with the results from previous studies, in which aluminum alloys were prepared by other SPD processing methods. Furthermore, the effect of the Mg content on the minimum grain size in aluminum alloys was discussed in terms of the difference of the stacking fault energy. The base materials used for this study included 5052 (Al–2.5 Mg) and 5083 (Al–4.6 Mg–0.7Mn) aluminum alloys. The thickness of the sample plates was 3 mm. FSP was performed with an 80 mm length of these plates. The shoulder and probe diameters of the cylindrical tool were 12 and 4 mm, respectively, while the probe length was 2.9 mm. A tool rotational speed of 300 min1 and a traversal speed of 200 mm min1 were applied to acquire the minimum grain size, i.e. FSP was conducted at Z P 1016 s1. In order to estimate the Zener–Hollomon
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.10.033
356
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
Figure 1. The TEM microstructures of friction stir processed Al–Mg alloys: (a) Al–2.5 Mg (AA5052) and (b) Al–4.6 Mg–0.7Mn (AA5083).
parameter, the strain rate during FSP was calculated by the following equation [8]: e_ ¼
Rm 2pre ; Le
ð2Þ
where Rm denotes the average material flow rate, which is roughly half of the tool rotational speed. re and Le denote the effective radius and depth, respectively, of the dynamically recrystallized zone. The strain rate during FSP under the present processing condition was estimated to be about 10 s1. During FSP, the temperature of these plates was measured using K-type thermocouples. All FSP specimens were quenched by liquid nitrogen just after the passage of the rotational tool. The microstructures of these samples were observed for grain size determination using a JEOL JEM-2000FX transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. During FSP, the temperature of all specimens did not exceed 473 K. Using this value and the strain rate from Eq. (2), the Zener–Hollomon parameter achieved during FSP was over 1016 s1 under the present processing conditions. As a result, the microstructures of each alloy had the minimum grain size. Figure 1 shows the TEM microstructures of friction stir processed specimens. The friction stir processed Al–Mg system alloys have recrystallized grains with a size of less than 0.5 lm. The grain size in the friction stir processed Al–4.6 Mg alloy is smaller than that in Al–2.5 Mg. The difference between the Al–Mg alloys, i.e. the effect of Mg solubility on the minimum grain size, was correlated with the stacking fault energy (c) of the alloy. The metals and alloys with lower stacking fault energy delay the rate of recovery due to the difficulty of the cross slip. As a result, the severely deformed grains retained their fine grain size. According to Mohamed [9], in the case of various metals processed by ball milling, the measured minimum
grain sizes (dmin) are related to the stacking fault energy through the following expression: c q d min ¼A ; ð3Þ Gb b where b denotes the Burgers vector and G represents the shear modulus for each metal. According to the experimental data, the value of the exponent of the normalized stacking fault energy is q = 0.65 and A is a dimensionless constant. This result signifies that grain refinement could be achieved by decreasing the stacking fault energy. In fact, Zhao et al. [10] reported that the reduction in the stacking fault energy by Zn solute atoms caused refinement in the minimum grain size in Cu alloys obtained by HPT. However, the effect of solubility on the minimum grain size in terms of the variation in the stacking fault energy has not been reported. Table 1 lists the reported values of stacking fault energies, determined by several methods, in pure Al [11–14] and some Al–Mg alloys [15,16]. Gallagher [17] reported that the effect of thesolubility of alloying elements on the stacking fault energy in alloys could be expressed through the following relationship: 2 c c ; ð4Þ ¼ kc ln c0 1þc where c0 indicates the stacking fault energy of pure metal, c specifies the alloying concentration ratio (c = x/x*, x denoting the alloying concentration and x* the solubility limit at high temperature) and kc is a dimensionless constant. The curve fitting the data on the stacking fault energy in Al–Mg alloys [15,16] (see Table 1) can be represented by the following equation: 8 !2 9 < = xMg =xMg ; ð5Þ cAlMg ¼ c0 exp k c : 1 þ ðxMg =xMg Þ ;
Table 1. Values of stacking fault energy in pure aluminum and Al–Mg alloys. Alloy
Stacking fault energy, c (mJ m–2)
Method of measurement
Ref.
Pure Al
(99.999% purity) (99.99% purity) –
210 135 ± 20 103124
Grain-corner twin Dislocation loop First-principles calculation
[11] [12] [13,14]
Al–0.5 Mg Al–0.65 Mg Al–1.1 Mg Al–3.25 Mg
(Al–0.55 (Al–0.72 (Al–1.22 (Al–3.59
102 110 ± 20 87 54
Creep Dislocation loop Creep Creep
[15] [16] [15] [15]
at.% at.% at.% at.%
Mg) Mg) Mg) Mg)
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
357
Table 2. Experimental values for various SPD-processed Al–Mg alloys.
*
Alloy
Mg content (at.%)
Stacking fault energy*,c (mJ m2)
Processing
Deformation temperature, T (K)
Zener–Hollomon parameter, Z (s1)
Grain size, d (lm)
Ref.
Al–0.13 Mg Al–1 Mg Al–1.5 Mg Al–2.5 Mg(AA5052)
0.12 0.90 1.35 2.86
102 97.5 91.6 65.3
ECAE ECAE SHPB ECAE
Al–3 Mg
2.80 3.3
66.5 57.6
Al–4.4 Mg–0.7Mn (AA5083)
4.96
33.1
Al–4.6 Mg–0.7Mn Al–4.8 Mg (AA5056)
5.13 5.30
31.1 29.2
FSP ECAE ECAE ARB ECAE FSP ECAE
298 298 473 323 373 466 298 298 473 298 450 413
2.40 1024 2.00 1025 9.78 1018 1.89 1023 1.57 1020 8.54 1016 2.00 1025 2.80 1025 2.25 1017 2.00 1025 3.14 1017 >1.00 1018
0.55 0.45 0.35 0.26 0.26 0.35 ± 0.15 0.27 0.28 0.28 0.25 0.27 ± 0.10 0.22
[20] [7] [21] [22] [22] This work [7] [19] [23] [24] This work [25]
The value of stacking fault energy in each alloy was calculated by Eq. (5).
with kc = 25.6 and xMg* = 18.6 at.% signifies the solubility limit of Mg in the Al matrix at 723 K [18]. The value of c0 = 103 mJ m2 in Eq. (5) is a reasonable value of the stacking fault energy in pure aluminum as learnt from comparisons with the reported value for pure Al [14]. The stacking fault energy in all Al–Mg alloys with various Mg contents was calculated by Eq. (5), and the experimental results of several SPD-processed Al–Mg alloys were listed in Table 2. Figure 2 shows the double-logarithmic plot of dmin/b against c/Gb obtained for the present alloys and for reported SPD-processed Al–Mg alloys with various Mg contents [7,19–25]. There is a linear relationship in double-logarithmic scale between the stacking fault energy and the minimum grain size. This relationship can be expressed as follows: c 0:65 d min ¼ 2:4 104 ð6Þ Gb b
The exponent of the normalized stacking fault energy is q = 0.65. This value is in good agreement with previous experimental results of various ball-milled metals reported by Mohamed [9]. Moreover, it is noteworthy that the experimental results of HPT + cold-rolled Cu (c = 78 mJ m2) and Cu–10 wt.% Zn alloy (c = 35 mJ m2) reported by Zhao et al. [10] can be represented by this relationship. This implies that the relationship between the minimum grain sizes obtained by SPD and the stacking fault energy in Al–Mg alloys with various Mg determined by Eq. (5) is reasonable. The addition of an alloying element, which decreases the stacking fault energy, can decrease the minimum grain size in Al alloys obtained by SPD processings, including FSP. In SPD-processed Al–Mg solid-solution alloys, the minimum grain size was realized by FSP. The grain sizes of the Al–2.5 Mg and Al–4.6 Mg alloys were decreased to 0.35 and 0.27 lm, respectively. The solute effect of
4
10
Al–0.13Mg ECAE [20] Al–1Mg ECAE [7] Al–1.5Mg SHPB [21] Al–2.5Mg FSP (this work) Al–2.5Mg ECAE [22] Al–3Mg ECAE [7] [19] Al–4.4Mg ARB [23] Al–4.4Mg ECAE [24] Al–4.6Mg FSP (this work) Al–4.8Mg ECAE [25] Pure Cu HPT + cold rolling [10] Cu–10Zn HPT + cold rolling [10]
Z ≥10 s–1
Normalized minimum grain size, dmin /b
16
3
10
0.65 1
2
10
10
-3
-2
10 Normalized stacking fault energy, γ /Gb
-1
10
Figure 2. Relationship between the normalized stacking fault energy and the normalized minimum grain size plotted on a double-logarithmic scale.
358
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
Mg on the minimum grain size was quantitatively estimated from the stacking fault energy. An increase in Mg solubility led to a decrease in the minimum grain size because the stacking fault energy in aluminum decreased with increasing Mg solubility. The doublelogarithmic plot of the normalized minimum grain size, dmin/b, vs. the normalized stacking fault energy, c/Gb, is found to be linear. The value of the exponent of the normalized stacking fault energy is in good agreement with those published in previous reports. This suggested that the submicron-grained Al alloys produced by SPD processing could be obtained by adding an alloying element, which effectively lowers the stacking fault energy. This work was supported in part by a Grant-inAid for Young Scientists (B) from The Ministry of Education, Culture, Sports, Science and Technology Japan (Grant No. 20760502). [1] V.M. Segal, Mater. Sci. Eng., A 197 (1995) 509. [2] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Mater. 47 (1999) 579. [3] G. Sakai, Z. Horita, T.G. Langdon, Mater. Sci. Eng., A 393 (2005) 344. [4] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng., R 50 (2005) 1. [5] T.S. Lundy, J.F. Murdock, Appl. Phys. 33 (1962) 1671. [6] T. Morishige, T. Hirata, M. Tsujikawa, K. Higashi, Mater. Lett. 64 (2010) 1905. [7] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29A (1998) 2503.
[8] C.I. Chang, C.K. Lee, J.C. Huang, Scripta Mater. 51 (2004) 509. [9] F.A. Mohamed, Acta Mater. 51 (2003) 4107. [10] Y.H. Zhao, Y.T. Zhu, X.Z. Liao, Z. Horita, T.G. Langdon, Mater. Sci. Eng., A 463 (2007) 22. [11] L.E. Murr, Acta Metall. 21 (1973) 791. [12] P.S. Dobson, Philos. Mag. 16 (1967) 9. [13] S. Crampin, K. Hampel, D.D. Vvedensky, J.M. MacLaren, J. Mater. Res. 5 (1990) 2107. [14] T.C. Schulthess, P.E.A. Turchi, A. Gonis, T.G. Nieh, Acta Mater. 46 (1998) 2215. [15] M.S. Soliman, J. Mater. Sci. 28 (1993) 4483. [16] S. Kritzinger, P.S. Dobson, R.E. Smallman, Philos. Mag. 16 (1967) 217. [17] P.C.J. Gallagher, Metall. Trans. 1 (1970) 2429. [18] J.L. Murray. In: T.B. Massalski (ed.), Binary Alloy Phase Diagrams, second ed., Metals Park, OH ASM International, 1, 1990, pp. 169–171. [19] A. Yamashita, D. Yamaguchi, Z. Horita, T.G. Langdon, Mater. Sci. Eng., A 287 (2000) 100. [20] Y. Huang, P.B. Prangnell, Acta Mater. 56 (2008) 1619. [21] R. Kapoor, J.B. Singh, J.K. Chakravartty, Mater. Sci. Eng., A 496 (2008) 308. [22] Y.C. Chen, Y.Y. Huang, C.P. Chang, P.W. Kao, Acta Mater. 51 (2003) 2005. [23] N. Tsuji, K. Shiotsuki, Y. Saito, Mater. Trans. JIM 40 (1999) 765. [24] V.V. Stolyarov, R. Lapovok, J. Alloys Compd. 378 (2004) 233. [25] T. Mukai, M. Kawazoe, K. Higashi, Nanostruct. Mater. 10 (1998) 755.
Scripta Materialia 64 (2011) 355–358 www.elsevier.com/locate/scriptamat
Effect of Mg content on the minimum grain size of Al–Mg alloys obtained by friction stir processing Taiki Morishige,a,⇑ Tomotake Hirata,b Tokuteru Uesugi,a Yorinobu Takigawa,a Masato Tsujikawaa and Kenji Higashia a
Department of Materials Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan b Metals and Machinery Department, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Received 16 August 2010; revised 15 October 2010; accepted 25 October 2010 Available online 30 October 2010
Friction stir processing (FSP) is one of the severe plastic deformation processes that have been developed to improve the mechanical properties of both metals and alloys by producing an ultrafine-grained structure. In this study, it was used to realize the minimum grain size in Al–Mg alloys. The results indicate that the grain size in Al–Mg alloy decreases with increasing Mg content because of the influence of stacking fault energy in the Al–Mg alloy. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir processing (FSP); Aluminum alloy; Severe plastic deformation (SPD); Minimum grain size; Stacking fault energy
In recent years, severe plastic deformation (SPD) processes, such as equal-channel angular extrusion (ECAE) [1], accumulative roll-bonding (ARB) [2], highpressure torsion (HPT) [3] and friction stir processing (FSP) [4], have been developed to obtain ultrafine-grained microstructures. SPD-processed Al alloys have a submicron-grained microstructure. The deformation conditions, such as the strain rate ð_eÞ and temperature (T), during the SPD processes can be compared using the Zener–Hollomon parameter (Z): Q ; ð1Þ Z ¼ e_ exp RT where Q represents the activation energy for self-diffusion (142 kJ mol1 in aluminum [5]) and R denotes the gas constant. In general, the grain size decreases with increasing Z value. Recently, Morishige et al. [6] reported that SPD-processed Al metals with various purities attained an almost constant grain size at Z P 1016 s1. On the other hand, Iwahashi et al. [7] reported that the grain size in ECAE-processed Al alloys decreased with increasing Mg content in spite of the same processing conditions and Z value. They attributed the effect of the Mg addition
⇑ Corresponding
author. Tel.: +81 72 254 9313; fax: +81 72 254 9912; e-mail: [email protected]
to reductions in the dislocation mobility and recovery rate in the solid-solution Al alloys. However, it is important to quantitatively understand the influence of the alloying element and the solubility on the grain size in obtaining an ultrafine-grained microstructure. Therefore, the aim of this study was to investigate the effect of Mg content on the minimum grain size. Al–Mg alloys were prepared by FSP under common processing conditions. We evaluated the minimum grain size of aluminum alloys of varying compositions. The microstructures in friction stir processed Al–Mg alloys were evaluated via comparisons with the results from previous studies, in which aluminum alloys were prepared by other SPD processing methods. Furthermore, the effect of the Mg content on the minimum grain size in aluminum alloys was discussed in terms of the difference of the stacking fault energy. The base materials used for this study included 5052 (Al–2.5 Mg) and 5083 (Al–4.6 Mg–0.7Mn) aluminum alloys. The thickness of the sample plates was 3 mm. FSP was performed with an 80 mm length of these plates. The shoulder and probe diameters of the cylindrical tool were 12 and 4 mm, respectively, while the probe length was 2.9 mm. A tool rotational speed of 300 min1 and a traversal speed of 200 mm min1 were applied to acquire the minimum grain size, i.e. FSP was conducted at Z P 1016 s1. In order to estimate the Zener–Hollomon
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.10.033
356
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
Figure 1. The TEM microstructures of friction stir processed Al–Mg alloys: (a) Al–2.5 Mg (AA5052) and (b) Al–4.6 Mg–0.7Mn (AA5083).
parameter, the strain rate during FSP was calculated by the following equation [8]: e_ ¼
Rm 2pre ; Le
ð2Þ
where Rm denotes the average material flow rate, which is roughly half of the tool rotational speed. re and Le denote the effective radius and depth, respectively, of the dynamically recrystallized zone. The strain rate during FSP under the present processing condition was estimated to be about 10 s1. During FSP, the temperature of these plates was measured using K-type thermocouples. All FSP specimens were quenched by liquid nitrogen just after the passage of the rotational tool. The microstructures of these samples were observed for grain size determination using a JEOL JEM-2000FX transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. During FSP, the temperature of all specimens did not exceed 473 K. Using this value and the strain rate from Eq. (2), the Zener–Hollomon parameter achieved during FSP was over 1016 s1 under the present processing conditions. As a result, the microstructures of each alloy had the minimum grain size. Figure 1 shows the TEM microstructures of friction stir processed specimens. The friction stir processed Al–Mg system alloys have recrystallized grains with a size of less than 0.5 lm. The grain size in the friction stir processed Al–4.6 Mg alloy is smaller than that in Al–2.5 Mg. The difference between the Al–Mg alloys, i.e. the effect of Mg solubility on the minimum grain size, was correlated with the stacking fault energy (c) of the alloy. The metals and alloys with lower stacking fault energy delay the rate of recovery due to the difficulty of the cross slip. As a result, the severely deformed grains retained their fine grain size. According to Mohamed [9], in the case of various metals processed by ball milling, the measured minimum
grain sizes (dmin) are related to the stacking fault energy through the following expression: c q d min ¼A ; ð3Þ Gb b where b denotes the Burgers vector and G represents the shear modulus for each metal. According to the experimental data, the value of the exponent of the normalized stacking fault energy is q = 0.65 and A is a dimensionless constant. This result signifies that grain refinement could be achieved by decreasing the stacking fault energy. In fact, Zhao et al. [10] reported that the reduction in the stacking fault energy by Zn solute atoms caused refinement in the minimum grain size in Cu alloys obtained by HPT. However, the effect of solubility on the minimum grain size in terms of the variation in the stacking fault energy has not been reported. Table 1 lists the reported values of stacking fault energies, determined by several methods, in pure Al [11–14] and some Al–Mg alloys [15,16]. Gallagher [17] reported that the effect of thesolubility of alloying elements on the stacking fault energy in alloys could be expressed through the following relationship: 2 c c ; ð4Þ ¼ kc ln c0 1þc where c0 indicates the stacking fault energy of pure metal, c specifies the alloying concentration ratio (c = x/x*, x denoting the alloying concentration and x* the solubility limit at high temperature) and kc is a dimensionless constant. The curve fitting the data on the stacking fault energy in Al–Mg alloys [15,16] (see Table 1) can be represented by the following equation: 8 !2 9 < = xMg =xMg ; ð5Þ cAlMg ¼ c0 exp k c : 1 þ ðxMg =xMg Þ ;
Table 1. Values of stacking fault energy in pure aluminum and Al–Mg alloys. Alloy
Stacking fault energy, c (mJ m–2)
Method of measurement
Ref.
Pure Al
(99.999% purity) (99.99% purity) –
210 135 ± 20 103124
Grain-corner twin Dislocation loop First-principles calculation
[11] [12] [13,14]
Al–0.5 Mg Al–0.65 Mg Al–1.1 Mg Al–3.25 Mg
(Al–0.55 (Al–0.72 (Al–1.22 (Al–3.59
102 110 ± 20 87 54
Creep Dislocation loop Creep Creep
[15] [16] [15] [15]
at.% at.% at.% at.%
Mg) Mg) Mg) Mg)
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
357
Table 2. Experimental values for various SPD-processed Al–Mg alloys.
*
Alloy
Mg content (at.%)
Stacking fault energy*,c (mJ m2)
Processing
Deformation temperature, T (K)
Zener–Hollomon parameter, Z (s1)
Grain size, d (lm)
Ref.
Al–0.13 Mg Al–1 Mg Al–1.5 Mg Al–2.5 Mg(AA5052)
0.12 0.90 1.35 2.86
102 97.5 91.6 65.3
ECAE ECAE SHPB ECAE
Al–3 Mg
2.80 3.3
66.5 57.6
Al–4.4 Mg–0.7Mn (AA5083)
4.96
33.1
Al–4.6 Mg–0.7Mn Al–4.8 Mg (AA5056)
5.13 5.30
31.1 29.2
FSP ECAE ECAE ARB ECAE FSP ECAE
298 298 473 323 373 466 298 298 473 298 450 413
2.40 1024 2.00 1025 9.78 1018 1.89 1023 1.57 1020 8.54 1016 2.00 1025 2.80 1025 2.25 1017 2.00 1025 3.14 1017 >1.00 1018
0.55 0.45 0.35 0.26 0.26 0.35 ± 0.15 0.27 0.28 0.28 0.25 0.27 ± 0.10 0.22
[20] [7] [21] [22] [22] This work [7] [19] [23] [24] This work [25]
The value of stacking fault energy in each alloy was calculated by Eq. (5).
with kc = 25.6 and xMg* = 18.6 at.% signifies the solubility limit of Mg in the Al matrix at 723 K [18]. The value of c0 = 103 mJ m2 in Eq. (5) is a reasonable value of the stacking fault energy in pure aluminum as learnt from comparisons with the reported value for pure Al [14]. The stacking fault energy in all Al–Mg alloys with various Mg contents was calculated by Eq. (5), and the experimental results of several SPD-processed Al–Mg alloys were listed in Table 2. Figure 2 shows the double-logarithmic plot of dmin/b against c/Gb obtained for the present alloys and for reported SPD-processed Al–Mg alloys with various Mg contents [7,19–25]. There is a linear relationship in double-logarithmic scale between the stacking fault energy and the minimum grain size. This relationship can be expressed as follows: c 0:65 d min ¼ 2:4 104 ð6Þ Gb b
The exponent of the normalized stacking fault energy is q = 0.65. This value is in good agreement with previous experimental results of various ball-milled metals reported by Mohamed [9]. Moreover, it is noteworthy that the experimental results of HPT + cold-rolled Cu (c = 78 mJ m2) and Cu–10 wt.% Zn alloy (c = 35 mJ m2) reported by Zhao et al. [10] can be represented by this relationship. This implies that the relationship between the minimum grain sizes obtained by SPD and the stacking fault energy in Al–Mg alloys with various Mg determined by Eq. (5) is reasonable. The addition of an alloying element, which decreases the stacking fault energy, can decrease the minimum grain size in Al alloys obtained by SPD processings, including FSP. In SPD-processed Al–Mg solid-solution alloys, the minimum grain size was realized by FSP. The grain sizes of the Al–2.5 Mg and Al–4.6 Mg alloys were decreased to 0.35 and 0.27 lm, respectively. The solute effect of
4
10
Al–0.13Mg ECAE [20] Al–1Mg ECAE [7] Al–1.5Mg SHPB [21] Al–2.5Mg FSP (this work) Al–2.5Mg ECAE [22] Al–3Mg ECAE [7] [19] Al–4.4Mg ARB [23] Al–4.4Mg ECAE [24] Al–4.6Mg FSP (this work) Al–4.8Mg ECAE [25] Pure Cu HPT + cold rolling [10] Cu–10Zn HPT + cold rolling [10]
Z ≥10 s–1
Normalized minimum grain size, dmin /b
16
3
10
0.65 1
2
10
10
-3
-2
10 Normalized stacking fault energy, γ /Gb
-1
10
Figure 2. Relationship between the normalized stacking fault energy and the normalized minimum grain size plotted on a double-logarithmic scale.
358
T. Morishige et al. / Scripta Materialia 64 (2011) 355–358
Mg on the minimum grain size was quantitatively estimated from the stacking fault energy. An increase in Mg solubility led to a decrease in the minimum grain size because the stacking fault energy in aluminum decreased with increasing Mg solubility. The doublelogarithmic plot of the normalized minimum grain size, dmin/b, vs. the normalized stacking fault energy, c/Gb, is found to be linear. The value of the exponent of the normalized stacking fault energy is in good agreement with those published in previous reports. This suggested that the submicron-grained Al alloys produced by SPD processing could be obtained by adding an alloying element, which effectively lowers the stacking fault energy. This work was supported in part by a Grant-inAid for Young Scientists (B) from The Ministry of Education, Culture, Sports, Science and Technology Japan (Grant No. 20760502). [1] V.M. Segal, Mater. Sci. Eng., A 197 (1995) 509. [2] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Mater. 47 (1999) 579. [3] G. Sakai, Z. Horita, T.G. Langdon, Mater. Sci. Eng., A 393 (2005) 344. [4] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng., R 50 (2005) 1. [5] T.S. Lundy, J.F. Murdock, Appl. Phys. 33 (1962) 1671. [6] T. Morishige, T. Hirata, M. Tsujikawa, K. Higashi, Mater. Lett. 64 (2010) 1905. [7] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29A (1998) 2503.
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