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Relationship between formability and microstructure of Al alloy sheet locally modified by friction stir processing
Scripta Materialia 57 (2007) 17–20 www.elsevier.com/locate/scriptamat
Relationship between formability and microstructure of Al alloy sheet locally modified by friction stir processing Suk Hoon Kang,a Hee-Suk Chung,a Heung Nam Han,a,* Kyu Hwan Oh,a Chang Gil Leeb and Sung-Joon Kimb a
Department of Material Science and Engineering, Seoul National University, San 56-1 Shillim-dong, Gwanak-gu, Seoul 151-744, Republic of Korea b Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-city, Gyeongnam 641-010, Republic of Korea Received 15 November 2006; revised 2 March 2007; accepted 13 March 2007 Available online 16 April 2007
The effect of friction stir processing (FSP) without the stirring pin on the formability and microstructure of Al5052-H32 sheets was investigated. The FSP produced a microstructural modification in local regions of the thin sheets and resulted in the improvement of their formability. Both electron backscattered diffraction (EBSD) and transmission electron microscopy observations were carried out to evaluate the relationship between the microstructure and the mechanical properties. The pattern quality, which is one of the representative EBSD results, was used to estimate the amount of work hardening in the FSPed region. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir welding; Friction stir processing; Aluminum alloy; EBSD; Formability
Friction stir welding (FSW) is a well-known solidstate joining process, which uses a rotating and traversing tool consisting of a stirring pin and a tool shoulder [1–6]. The pin is driven into the joint line between two sheets which are butted together. Surface friction welding (SFW) [7,8] utilizes only the surface friction between the tool shoulder and the parent material, without any stirring by the threaded pin. This process can only be applied to thin sheets due to the absence of the tool pin. It is known that after both FSW and SFW, the severe shear deformation and evolved heat due to the friction and plastic work may cause dynamic recrystallization in the weld region and result in localized grain size refinements in the specimen. Recently, friction stir processing (FSP) has been developed based on the fundamental principles of FSW and has proved to be useful for bringing about the microstructural modification of materials [9,10]. In this study, FSP without the stirring pin, which is similar to the method of SFW, was used to modify the microstructure of thin sheets. As-received Al5052-H32 sheets have a plastically deformed microstructure, which is caused by the rolling * Corresponding author. Tel.: +82 2 880 9240; fax: +82 2 885 9671; e-mail: [email protected]
process used to endow them with a predetermined strength. The formability and elongation of this Al alloy are somewhat inferior to those of other heat-treated Al alloys. When the FSP without the stirring pin is applied to local regions on the Al5052-H32 alloy sheet, an increase of the latter’s formability can be expected, due to the microstructural modification in the FSPed region. In this study, Al5052-H32 alloy sheets were subjected to one-pass FSP. The microstructural change in the FSPed region was observed by both electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The formability of the specimen was measured by the limiting dome height (LDH) test. The pattern quality (PQ), which is one of the representative EBSD results, was used to estimate the amount of work hardening of the FSPed region. Finally, the relationship between the microstructural change in the FSPed region and the formability of the Al sheet was discussed on the basis of the TEM and EBSD observations. The 1.5 mm thick Al5052-H32 sheets were subjected to one-pass FSP at a tool rotation speed of 1600 rpm and a welding speed of 100 mm min 1. Figure 1a and b shows a schematic view of the FSP procedure and the tool design without the stirring pin, respectively. Tensile test specimens with a gauge length of 25 mm
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.021
18
S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20
(a) 1
3
(b)
2
1: Base material (Al 5052-H32 sheet) 2: Tool 3: FSPed region
Figure 1. (a) Schematic drawing of friction stir processing and (b) tool design without stirring pin.
were prepared for both the FSPed region and the base material. The tensile direction of the FSPed region was parallel to the advancing direction of the tool. The crosshead speed used for the tensile test was 2.0 mm min 1. The formability was evaluated by the conventional LDH test. A schematic diagram of the procedure used for the LDH test is shown in Figure 2. The blank size and punch-rate were 210 mm · 120 mm and 2 mm min 1, respectively. The blank holding force was 60 kN and no lubricant was used. The tool diameters
(a)
160.0
Unit : mm 105.6
5.8R
101.6
(b)
No FSP
5mm tool
8mm tool
10mm tool
Figure 2. (a) Schematic diagram of LDH test and (b) LDH samples tested after one-pass FSP with 5, 8 and 10 mm diameter tools.
for the FSP were 5, 8 and 10 mm. The major axis of the LDH test was parallel to the advancing direction of the tool. The FSPed region was placed in the middle of the punch for the LDH test. The microstructures of the local region FSPed by a tool with a diameter of 10 mm and the base material were observed before and after the LDH test using a transmission electron microscope (JEM 200CX). Electrolytic polishing was carried out for the preparation of the TEM sample. An EBSD system (SEM: JSM 6500F and EBSD: Oxford Inca System) was used for the quantitative analysis of work hardening of the FSPed region. In order to ensure that all of the specimens had the same surface condition, they were finished by electrolytic polishing under the same conditions. Figure 3a shows the cross-sectional macroscopic view of the FSPed Al5052-H32 sheet. The elliptical shaped weld nugget which is typically observed in the FSWed area is not observed in the FSPed sheet, due to the absence of the stirring pin. It was observed that the microstructure-modified region exists directly under the trace of the tool movement and extends to the opposite end of the sheet. Figure 3b and c represents the microstructures of the base material (as-received Al5052-H32 alloy sheet) and FSPed region, respectively, in the form of the channeling contrast images obtained by an Ga+ ion beam. It is seen that small homogeneous grains developed in the FSPed region. It can be understood that these fine grains were caused by the dynamic recrystallization due to the severe shear deformation and the evolved heat. The average grain size in the FSPed region was about five times smaller than that of the base material. Figure 4 shows the tensile behaviors of the base material and the FSPed region. The elongation of the FSPed region is about twice that of the base material. In general, it is well known that the refinement of the grain size can lead to an increase in the strength of the material [11]. However, it is noted that the flow stress of the FSPed region, whose grain size is about five times smaller than that of the base material, is lower than that of the base material, as shown in Figure 3. Figure 5 shows the variation of the measured LDH value with the tool diameter. It is seen that the formability of the Al sheet is increasingly improved as the tool diameter is increased. This result indicates that the microstructural
Figure 3. (a) Cross-sectional macrographs of an FSPed Al5052-H32 sheet of 1.5 mm thickness, and typical microstructure of (b) base material and (c) FSPed region observed in the form of a channeling contrast image obtained by a Ga+ ion beam.
S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20 300
True stress, MPa
250 200 150 100 base material FSPed region
50 0 0.00
0.05
0.10
0.15
0.20
True strain
Figure 4. Stress–strain curves of the base material and the FSPed region.
26
LDH, mm
24 22 20 18
No FSP
5mm tool
8mm tool
10mm tool
16
Figure 5. LDH values of FSPed sheets as a function of tool diameter.
modification induced by FSP results in an improvement of the formability of the Al5052-H32 sheet and this improvement of the formability is closely related to the size of the FSPed region. In order to understand precisely the effect of the microstructural modification including the grain size refinement in the FSPed region on the mechanical properties, the accumulated work hardening in the specimen, which is related to the dislocation density, should be quantified. In this study, the details of the microstructures in the FSPed region and base material were observed by TEM. Figure 6 shows the transmission electron micrographs of the base material and the FSPed region before and after the LDH test. It can be
Figure 6. TEM bright field images of the base material and the FSPed region. (a) Base material before the LDH test, (b) FSPed region before the LDH test, (c) base material after the LDH test and (d) FSPed region after the LDH test.
19
seen that numerous dislocations formed after the LDH test in both the base material and the FSPed region. It is noted that, before the LDH test, the dislocation density in the base material is higher than that in the FSPed region. It is very difficult to quantify the accumulated work hardening in materials by only TEM observation. In this study, the PQ, which is one of the representative EBSD results, was used to estimate the amount of work hardening in the specimen. Since the difference of the dislocation density in the grains is reflected in the quality of the Kikuchi pattern on the viewing plate, the PQ has generally been adopted as a means of defining the degree of accumulated work hardening in materials [12]. However, the criterion based on the PQ seems to be insufficient to allow the complete quantification of the work hardening, because the PQ strongly depends on the microscope, the imaging system and the sample preparation procedure [12–14]. In order to ensure that the surface conditions of the specimens were the same, in this study all of the EBSD samples were subjected to a finishing procedure using electrolytic polishing under the same conditions. Figure 7 shows the EBSD PQ maps of the base material and the FSPed region, including the grain and subgrain boundary maps. Here, the subgrain boundary was decided as the grain boundary with a low angle of less than 15. In Figure 7, the thick black and relatively thin lines represent the grain and subgrain boundaries, respectively. The PQ map was constructed by using those pixels whose PQ values are between 0 and 40, which represent the relatively lower PQ values. It was confirmed that the distribution of these pixels with lower PQ values corresponds to the grain and subgrain boundary maps. Therefore, it can be asserted that the EBSD PQ is closely related to the quantity of defects, such as dislocations, in the material. Figure 8a and b shows the PQ histograms of the base material and the FSPed region before the LDH test, respectively. The average value of PQ was increased by about 20 after the FSP. This can be understood from the fact that the dynamic recrystallization due to the severe shear deformation and evolved heat during the FSP
Figure 7. EBSD grain boundary and PQ maps of the base material and the FSPed region. (a) Base material before the LDH test, (b) FSPed region before the LDH test, (c) base material after the LDH test and (d) FSPed region after the LDH test.
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S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20
in the FSPed region. It was confirmed that the dynamic recrystallization due to the severe shear deformation and evolved heat caused by the friction and plastic work during the FSP reduces the dislocation density in the FSPed region of the Al5052-H32 sheet. This research was supported by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Commerce, Industry and Energy (MOCIE), Republic of Korea.
Figure 8. EBSD PQ histograms of (a) base material and (b) FSPed region before the LDH test.
reduces the dislocation density in the FSPed region in the Al5052-H32 sheet. This annealing effect of FSP for the H temper base material increases the capacity of the work hardening. Therefore, it can be concluded that FSP can be effectively applied to improve the formability of Al5052-H32 sheets. In this study, the FSP without the stirring pin was employed to improve the formability of Al5052-H32 sheets. The formability of the Al sheets was increasingly improved as the tool diameter increased. It was found that fine grains developed in the FSPed region. Both EBSD and TEM observations were performed to evaluate the relationship between the microstructure and mechanical properties. EBSD PQ was able to be used to estimate the amount of accumulated work hardening
[1] H.G. Salem, A.P. Reynolds, J.S. Lyons, Scripta Mater. 46 (2002) 337. [2] C. Zhou, X. Yang, G. Luan, Scripta Mater. 53 (2005) 1187. [3] S. Lim, S. Kim, C.G. Lee, S.-J. Kim, Metall. Mater. Trans. A 35A (2004) 2837. [4] B. Heinz, B. Skrotzki, Metall. Mater. Trans. B 33B (2002) 489. [5] J.-H. Cho, D.E. Boyce, P.R. Dawson, Mater. Sci. Eng. A 398 (2005) 146. [6] S. Hong, S. Kim, C.G. Lee, S.-J. Kim, Scripta Mater. 55 (2006) 1007. [7] H.N. Han, C.G. Lee, S.-J. Kim, Korea Patent No. 0543160 (2006), USA Patent No. 20050072832 (2005), UK Patent 200326324 (2003). [8] C.G. Lee, S.-J. Kim, H.N. Han, J. Korean Inst. Met. Mater. 43 (2005) 718. [9] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scripta Mater. 42 (2000) 163. [10] Z.Y. Ma, R.S. Mishra, Scripta Mater. 53 (2005) 75. [11] J.C.M. Li, Y.T. Chou, Metall. Trans. 1 (1970) 1145. [12] F.J. Humphreys, J. Microsc. 195 (1999) 170. [13] J.H. Driver, D.J. Jensen, N. Hansen, Acta Metall. 42 (1994) 3105. [14] S.R. Claves, A. Deal, Microsc. Microanal. 11 (2005) 514.
Relationship between formability and microstructure of Al alloy sheet locally modified by friction stir processing Suk Hoon Kang,a Hee-Suk Chung,a Heung Nam Han,a,* Kyu Hwan Oh,a Chang Gil Leeb and Sung-Joon Kimb a
Department of Material Science and Engineering, Seoul National University, San 56-1 Shillim-dong, Gwanak-gu, Seoul 151-744, Republic of Korea b Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-city, Gyeongnam 641-010, Republic of Korea Received 15 November 2006; revised 2 March 2007; accepted 13 March 2007 Available online 16 April 2007
The effect of friction stir processing (FSP) without the stirring pin on the formability and microstructure of Al5052-H32 sheets was investigated. The FSP produced a microstructural modification in local regions of the thin sheets and resulted in the improvement of their formability. Both electron backscattered diffraction (EBSD) and transmission electron microscopy observations were carried out to evaluate the relationship between the microstructure and the mechanical properties. The pattern quality, which is one of the representative EBSD results, was used to estimate the amount of work hardening in the FSPed region. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir welding; Friction stir processing; Aluminum alloy; EBSD; Formability
Friction stir welding (FSW) is a well-known solidstate joining process, which uses a rotating and traversing tool consisting of a stirring pin and a tool shoulder [1–6]. The pin is driven into the joint line between two sheets which are butted together. Surface friction welding (SFW) [7,8] utilizes only the surface friction between the tool shoulder and the parent material, without any stirring by the threaded pin. This process can only be applied to thin sheets due to the absence of the tool pin. It is known that after both FSW and SFW, the severe shear deformation and evolved heat due to the friction and plastic work may cause dynamic recrystallization in the weld region and result in localized grain size refinements in the specimen. Recently, friction stir processing (FSP) has been developed based on the fundamental principles of FSW and has proved to be useful for bringing about the microstructural modification of materials [9,10]. In this study, FSP without the stirring pin, which is similar to the method of SFW, was used to modify the microstructure of thin sheets. As-received Al5052-H32 sheets have a plastically deformed microstructure, which is caused by the rolling * Corresponding author. Tel.: +82 2 880 9240; fax: +82 2 885 9671; e-mail: [email protected]
process used to endow them with a predetermined strength. The formability and elongation of this Al alloy are somewhat inferior to those of other heat-treated Al alloys. When the FSP without the stirring pin is applied to local regions on the Al5052-H32 alloy sheet, an increase of the latter’s formability can be expected, due to the microstructural modification in the FSPed region. In this study, Al5052-H32 alloy sheets were subjected to one-pass FSP. The microstructural change in the FSPed region was observed by both electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The formability of the specimen was measured by the limiting dome height (LDH) test. The pattern quality (PQ), which is one of the representative EBSD results, was used to estimate the amount of work hardening of the FSPed region. Finally, the relationship between the microstructural change in the FSPed region and the formability of the Al sheet was discussed on the basis of the TEM and EBSD observations. The 1.5 mm thick Al5052-H32 sheets were subjected to one-pass FSP at a tool rotation speed of 1600 rpm and a welding speed of 100 mm min 1. Figure 1a and b shows a schematic view of the FSP procedure and the tool design without the stirring pin, respectively. Tensile test specimens with a gauge length of 25 mm
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.021
18
S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20
(a) 1
3
(b)
2
1: Base material (Al 5052-H32 sheet) 2: Tool 3: FSPed region
Figure 1. (a) Schematic drawing of friction stir processing and (b) tool design without stirring pin.
were prepared for both the FSPed region and the base material. The tensile direction of the FSPed region was parallel to the advancing direction of the tool. The crosshead speed used for the tensile test was 2.0 mm min 1. The formability was evaluated by the conventional LDH test. A schematic diagram of the procedure used for the LDH test is shown in Figure 2. The blank size and punch-rate were 210 mm · 120 mm and 2 mm min 1, respectively. The blank holding force was 60 kN and no lubricant was used. The tool diameters
(a)
160.0
Unit : mm 105.6
5.8R
101.6
(b)
No FSP
5mm tool
8mm tool
10mm tool
Figure 2. (a) Schematic diagram of LDH test and (b) LDH samples tested after one-pass FSP with 5, 8 and 10 mm diameter tools.
for the FSP were 5, 8 and 10 mm. The major axis of the LDH test was parallel to the advancing direction of the tool. The FSPed region was placed in the middle of the punch for the LDH test. The microstructures of the local region FSPed by a tool with a diameter of 10 mm and the base material were observed before and after the LDH test using a transmission electron microscope (JEM 200CX). Electrolytic polishing was carried out for the preparation of the TEM sample. An EBSD system (SEM: JSM 6500F and EBSD: Oxford Inca System) was used for the quantitative analysis of work hardening of the FSPed region. In order to ensure that all of the specimens had the same surface condition, they were finished by electrolytic polishing under the same conditions. Figure 3a shows the cross-sectional macroscopic view of the FSPed Al5052-H32 sheet. The elliptical shaped weld nugget which is typically observed in the FSWed area is not observed in the FSPed sheet, due to the absence of the stirring pin. It was observed that the microstructure-modified region exists directly under the trace of the tool movement and extends to the opposite end of the sheet. Figure 3b and c represents the microstructures of the base material (as-received Al5052-H32 alloy sheet) and FSPed region, respectively, in the form of the channeling contrast images obtained by an Ga+ ion beam. It is seen that small homogeneous grains developed in the FSPed region. It can be understood that these fine grains were caused by the dynamic recrystallization due to the severe shear deformation and the evolved heat. The average grain size in the FSPed region was about five times smaller than that of the base material. Figure 4 shows the tensile behaviors of the base material and the FSPed region. The elongation of the FSPed region is about twice that of the base material. In general, it is well known that the refinement of the grain size can lead to an increase in the strength of the material [11]. However, it is noted that the flow stress of the FSPed region, whose grain size is about five times smaller than that of the base material, is lower than that of the base material, as shown in Figure 3. Figure 5 shows the variation of the measured LDH value with the tool diameter. It is seen that the formability of the Al sheet is increasingly improved as the tool diameter is increased. This result indicates that the microstructural
Figure 3. (a) Cross-sectional macrographs of an FSPed Al5052-H32 sheet of 1.5 mm thickness, and typical microstructure of (b) base material and (c) FSPed region observed in the form of a channeling contrast image obtained by a Ga+ ion beam.
S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20 300
True stress, MPa
250 200 150 100 base material FSPed region
50 0 0.00
0.05
0.10
0.15
0.20
True strain
Figure 4. Stress–strain curves of the base material and the FSPed region.
26
LDH, mm
24 22 20 18
No FSP
5mm tool
8mm tool
10mm tool
16
Figure 5. LDH values of FSPed sheets as a function of tool diameter.
modification induced by FSP results in an improvement of the formability of the Al5052-H32 sheet and this improvement of the formability is closely related to the size of the FSPed region. In order to understand precisely the effect of the microstructural modification including the grain size refinement in the FSPed region on the mechanical properties, the accumulated work hardening in the specimen, which is related to the dislocation density, should be quantified. In this study, the details of the microstructures in the FSPed region and base material were observed by TEM. Figure 6 shows the transmission electron micrographs of the base material and the FSPed region before and after the LDH test. It can be
Figure 6. TEM bright field images of the base material and the FSPed region. (a) Base material before the LDH test, (b) FSPed region before the LDH test, (c) base material after the LDH test and (d) FSPed region after the LDH test.
19
seen that numerous dislocations formed after the LDH test in both the base material and the FSPed region. It is noted that, before the LDH test, the dislocation density in the base material is higher than that in the FSPed region. It is very difficult to quantify the accumulated work hardening in materials by only TEM observation. In this study, the PQ, which is one of the representative EBSD results, was used to estimate the amount of work hardening in the specimen. Since the difference of the dislocation density in the grains is reflected in the quality of the Kikuchi pattern on the viewing plate, the PQ has generally been adopted as a means of defining the degree of accumulated work hardening in materials [12]. However, the criterion based on the PQ seems to be insufficient to allow the complete quantification of the work hardening, because the PQ strongly depends on the microscope, the imaging system and the sample preparation procedure [12–14]. In order to ensure that the surface conditions of the specimens were the same, in this study all of the EBSD samples were subjected to a finishing procedure using electrolytic polishing under the same conditions. Figure 7 shows the EBSD PQ maps of the base material and the FSPed region, including the grain and subgrain boundary maps. Here, the subgrain boundary was decided as the grain boundary with a low angle of less than 15. In Figure 7, the thick black and relatively thin lines represent the grain and subgrain boundaries, respectively. The PQ map was constructed by using those pixels whose PQ values are between 0 and 40, which represent the relatively lower PQ values. It was confirmed that the distribution of these pixels with lower PQ values corresponds to the grain and subgrain boundary maps. Therefore, it can be asserted that the EBSD PQ is closely related to the quantity of defects, such as dislocations, in the material. Figure 8a and b shows the PQ histograms of the base material and the FSPed region before the LDH test, respectively. The average value of PQ was increased by about 20 after the FSP. This can be understood from the fact that the dynamic recrystallization due to the severe shear deformation and evolved heat during the FSP
Figure 7. EBSD grain boundary and PQ maps of the base material and the FSPed region. (a) Base material before the LDH test, (b) FSPed region before the LDH test, (c) base material after the LDH test and (d) FSPed region after the LDH test.
20
S. H. Kang et al. / Scripta Materialia 57 (2007) 17–20
in the FSPed region. It was confirmed that the dynamic recrystallization due to the severe shear deformation and evolved heat caused by the friction and plastic work during the FSP reduces the dislocation density in the FSPed region of the Al5052-H32 sheet. This research was supported by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Commerce, Industry and Energy (MOCIE), Republic of Korea.
Figure 8. EBSD PQ histograms of (a) base material and (b) FSPed region before the LDH test.
reduces the dislocation density in the FSPed region in the Al5052-H32 sheet. This annealing effect of FSP for the H temper base material increases the capacity of the work hardening. Therefore, it can be concluded that FSP can be effectively applied to improve the formability of Al5052-H32 sheets. In this study, the FSP without the stirring pin was employed to improve the formability of Al5052-H32 sheets. The formability of the Al sheets was increasingly improved as the tool diameter increased. It was found that fine grains developed in the FSPed region. Both EBSD and TEM observations were performed to evaluate the relationship between the microstructure and mechanical properties. EBSD PQ was able to be used to estimate the amount of accumulated work hardening
[1] H.G. Salem, A.P. Reynolds, J.S. Lyons, Scripta Mater. 46 (2002) 337. [2] C. Zhou, X. Yang, G. Luan, Scripta Mater. 53 (2005) 1187. [3] S. Lim, S. Kim, C.G. Lee, S.-J. Kim, Metall. Mater. Trans. A 35A (2004) 2837. [4] B. Heinz, B. Skrotzki, Metall. Mater. Trans. B 33B (2002) 489. [5] J.-H. Cho, D.E. Boyce, P.R. Dawson, Mater. Sci. Eng. A 398 (2005) 146. [6] S. Hong, S. Kim, C.G. Lee, S.-J. Kim, Scripta Mater. 55 (2006) 1007. [7] H.N. Han, C.G. Lee, S.-J. Kim, Korea Patent No. 0543160 (2006), USA Patent No. 20050072832 (2005), UK Patent 200326324 (2003). [8] C.G. Lee, S.-J. Kim, H.N. Han, J. Korean Inst. Met. Mater. 43 (2005) 718. [9] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scripta Mater. 42 (2000) 163. [10] Z.Y. Ma, R.S. Mishra, Scripta Mater. 53 (2005) 75. [11] J.C.M. Li, Y.T. Chou, Metall. Trans. 1 (1970) 1145. [12] F.J. Humphreys, J. Microsc. 195 (1999) 170. [13] J.H. Driver, D.J. Jensen, N. Hansen, Acta Metall. 42 (1994) 3105. [14] S.R. Claves, A. Deal, Microsc. Microanal. 11 (2005) 514.