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Microstructure and deformation mechanism of 0 0 0 1 magnesium single crystal subjected to quasistatic and high-strain-rate compressiveloadings
Materials Science & Engineering A 568 (2013) 96–101
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and deformation mechanism of 0 0 0 1 magnesium single crystal subjected to quasistatic and high-strain-rate compressive loadings Qizhen Li n Chemical and Materials Engineering, University of Nevada, 1664 N. Virginia St, MS388, Reno, NV 89557, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2012 Received in revised form 12 January 2013 Accepted 16 January 2013 Available online 21 January 2013
0 0 0 1 magnesium single crystal samples were mechanically tested under compressive loadings at a quasistatic strain rate (0.001 s 1) and a dynamic strain rate (1000 s 1), respectively. The tested samples were then investigated through various microstructural characterization techniques. Dynamic loading led to much higher maximum strength and larger strain hardening rate than quasistatic loading for the tested material. The microstructure features were clearly different from each other for the two loading conditions, which indicated that the deformation mechanisms for the quasistatic testing were different from those for the dynamic testing. The microstructure analysis showed that (a) prismatic and secondary pyramidal dislocation operations happened in the samples under quasistatic loading and dynamic loading, while tension twinning and tension–compression double twinning also happened in the sample under quasistatic loading; (b) the sample under high-strain-rate dynamic loading remained as single crystal, while that under quasistatic loading was refined to become polycrystalline; and (c) twinning operations under quasistatic loading led to grain refinement. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnesium single crystal Twinning Dislocation Grain refinement Deformation mechanism
1. Introduction Magnesium and its alloys are becoming increasingly attractive candidates for constructing various structural components due to their low densities and high specific strengths. Materials often experience different loading conditions such as quasistatic and high strain rate dynamic/shocking loading conditions, when being used for different automobile and aircraft components. The appealing potential applications motivate extensive research interest and efforts to explore the processing methods to improve their mechanical properties, and uncover their mechanical behaviors and microscopic deformation mechanisms under different loading strain rates (e.g., [1–7]). The mechanical behavior of materials under dynamic loading is generally significantly different from that under quasistatic loading. Thus, it is often required to study both quasistatic and dynamic mechanical behavior and the corresponding microscopic deformation mechanisms of magnesium and its alloys. It will be a good start to investigate materials with a simple microstructure such as single crystals to uncover the mechanical behaviors and the fundamental microscopic deformation mechanisms. To date, extensive research results were reported for single crystals with FCC and BCC crystal structures such as Cu, Ni, In, Cu–Co, Cu–Fe [8–18]. For example, copper-based single
n
Tel.: þ1 775 784 4507; fax: þ1 775 327 5059. E-mail address: [email protected]
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.01.028
crystals were broadly studied to disclose their recrystallization texture, plastic deformation, recovery of deformation, and deformation behavior under cyclic loading and various testing strain rates. A few research results were reported for magnesium single crystals with HCP crystal structure [19–25]. Among these reported papers, only Refs. [21–23] were about magnesium single crystals under dynamic loading and they mainly focused on mechanical behaviors with little work on studying microstructures and deformation mechanisms of the materials. This work aimed to uncover the underlying quasistatic and dynamic microscopic deformation mechanisms of magnesium single crystal subjected to quasistatic and dynamic loadings, respectively, using optical microscopy (OM), scanning electron microscopy (SEM), and electron back-scattered diffraction (EBSD) techniques.
2. Experimental procedure Mechanical testing: Cylindrical magnesium single crystal samples were tested under ambient compressive loadings at a dynamic strain rate of 1000 s 1 and also at a quasistatic strain rate of 0.001 s 1, respectively. Quasistatic testing was performed using a universal testing machine, while dynamic testing was performed using a Split Hopkinson Pressure Bar system. The loading direction was along the axial direction of a cylindrical sample. The displacement and load data were recorded for both tests.
Q. Li / Materials Science & Engineering A 568 (2013) 96–101
Microstructure characterization: The tested samples were cut along a plane parallel to the loading direction. The cutting surfaces were ground and polished using polishing suspensions all the way to a 0.05 mm alumina slurry, and etched for microstructural observations. The microstructures of the samples were characterized using OM, SEM, and EBSD techniques in the study.
3. Results and discussions Fig. 1 reported the stress–strain curves for the mechanically tested samples. The untested magnesium single crystal had its [0 0 0 1] direction about 101 away from the sample’s axial direction Z as shown in the inset in Fig. 1. The quasistatic stress–strain curve had a distinct transition from elastic to plastic deformation as shown in Fig. 1. The dynamic stress– strain curve did not show a clear transition point from elastic to plastic deformation. The dynamic maximum strength and strain hardening rate were about 30% and 20% higher than the corresponding quasistatic values [23]. Microstructure analysis was conducted for the dynamic loading case. The sample was cut along the loading direction. The cutting plane was parallel to the loading direction and was polished to be observed using EBSD. The EBSD results from a representative location on the polished surface were reported in Fig. 2 to illustrate the microstructure of the tested material. The EBSD scanning was performed in the red rectangular region in Fig. 2(a). The rectangular region was chosen because it had both the matrix and the residual deformation feature shown as a thick line. The EBSD data were analyzed below to determine if the line was a dislocation or a twin. The inverse pole figure (IPF) mapping in Fig. 2(b) showed that the whole region had one orientation. Based on the color-coded orientation legend in Fig. 2(b), it can be determined that the polished surface was along f1 0 1 0g . The grain boundary misorientation distribution in Fig. 2(c) confirmed that there was no grain boundary in the studied region. Thus, the thick line in the red rectangle in Fig. 2(a) was a part of a dislocation. The pole figures in Fig. 2(d) showed a nearly ideal f1 0 1 0g/0 0 0 1S orientation of the sample. This indicated that the cutting plane was f1 0 1 0g. The Kikuchi patterns were obtained for the five spots labeled by orange hexagons in Fig. 2(a). These five points
380MPa 0.001/s 481MPa
transition
1000/s
Fig. 1. Stress–strain curves of ambient compressive testing at the quasistatic strain rate of 0.001 s 1 and the dynamic strain rate of 1000 s 1, respectively. The inset is a schematic illustration of the orientation relation between the crystal [0 0 0 1] direction and the loading direction along the cylindrical axial direction Z.
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covered the matrix region and the dislocation region. These five points had the same Kikuchi pattern as shown in Fig. 2(e), which indicated that the different points investigated had the same orientation and confirmed that the line was a dislocation. Fig. 3 showed that the microstructure features of magnesium single crystal under dynamic compressive loading at a strain rate of 1000 s 1. Fig. 3(b) reported three groups (A–C) of parallel dislocation arrays as indicated by the arrows. The stress–strain curves in Fig. 1 indicated that basal slip did not exist and deformation should be accommodated by non-basal slip such as prismatic ({1 1 0 0}/1 1 2 0S) and secondary pyramidal ({1 1 2 2}/1 1 2 3S) dislocation slip. The angle between {1 1 0 0} planes is 601, and the angle between the traces of {1 1 0 0} and {1 1 2 2} is 301. Fig. 3 showed that the angle between A and B was about 601, while the angles between C and A and between C and B were about 301. A and B were two groups of {1 1 0 0}/1 1 2 0S slip systems, while C was a group of {1 1 2 2}/ 1 1 2 3S slip system. Fig. 3(a) showed A and C dislocation arrays. The upper right corner of Fig. 3(a) was close to the sample outer surface. It was shown that the dislocation array was much denser in the region near the surface than that away from the surface. Inside the sample, dislocation arrays were more uniformly distributed as shown in Fig. 3(b), and the distance between two neighboring dislocations was about 30 mm. Microstructure analysis was conducted for the quasistatic loading case. The sample was cut along the loading direction. The cutting plane was parallel to the loading direction and was polished to be observed using OM and EBSD. The dominating deformation mechanism was dislocation motion ({1 1 0 0} /1 1 2 0S slip systems and {1 1 2 2}/1 1 2 3S slip systems) as shown in Fig. 4(a). However, there were some twins appearing in the sample as shown in the SEM image in Fig. 4(b). The two rectangle regions (labeled as 1 and 2) in Fig. 4(b) were studied using EBSD to examine the sample’s microstructure. Fig. 5 reported the IPF mapping, grain boundary mapping, grain boundary misorientation distribution, and pole figures for the region 1 in Fig. 4(b). The color-coded orientation legend in Fig. 2(b) was applicable to Fig. 5(a). The IPF mapping in Fig. 5(a) showed that the matrix was along a {1 0 1 0} plane and there were twins with the orientation close to {0 0 0 1}. These twins were primarily produced by {1 0 1 2} tension twinning based on the information in the grain boundary (GB) mapping (Fig. 5(b)). The GB mapping showed that the orientation difference was about 86o between the matrix and the twins, and these twins were formed due to the tension twinning. Grain boundary misorientation distribution in Fig. 5(c) showed that the misorientation of less than 21 had the high intensity, the misorientation of around 301 had the low intensity, and the misorientation of around 861 had the intermediate intensity. The twins with the 30o misorientation angle were formed due to {1 0 1 2} tension—{1 0 1 1} compression double twinning. Similarly, Fig. 6 reported the IPF mapping, grain boundary mapping, grain boundary misorientation distribution, and pole figures for the region 2 in Fig. 4(b). The color-coded orientation legend in Fig. 2(b) was applicable to Fig. 6(a). Fig. 6(a) had four parallel twins, and they were produced by {1 0 1 2} tension twining and had a misorientation angle of about 861 as shown in Fig. 6(c). Twinning operations resulted in large angle grain boundaries and the sample became polycrystalline after being compressed. The pole figures in Figs. 5(d) and 6(d) showed a nearly ideal {1 0 1 0}/0 0 0 1S orientation of the sample, which indicated that the cutting plane was {1 0 1 0}. Fig. 7(a) was a SEM image of the quasistatically tested sample. Multiple points in the matrix (shown as diamonds) and the twins (shown as triangles) were analyzed. The diamond points (i.e., the matrix region) had one Kikuchi pattern, while the triangle points (i.e., the twins) had
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Q. Li / Materials Science & Engineering A 568 (2013) 96–101
Fig. 2. Microstructural analysis of magnesium single crystal sample under dynamic loading at a strain rate of 1000 s 1. (a) SEM image; (b) IPF mapping of the region in the rectangle in (a); (c) grain boundary misorientation distribution of the region in the rectangle in (a); (d) pole figures; and (e) Kikuchi pattern. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Micrographs of magnesium single crystal under dynamic compressive loading at a strain rate of 1000 s 1. (a) SEM micrograph; (b) optical micrograph (two groups of prismatic slip systems and one group of secondary pyramidal slip system are identified using arrows).
another Kikuchi pattern. The Kikuchi patterns for the diamonds and triangles were reported in Fig. 7(b). Comparing the two patterns, the misorientation was about 901, which indicated that the
regions labeled with triangles corresponded to tension twins. Twinning changed the orientation of a part of a grain, which produced a high angle misorientation and refined the grain
Q. Li / Materials Science & Engineering A 568 (2013) 96–101
99
Fig. 4. Microstructure observations of magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) OM image; and (b) SEM image. (Note, the two rectangles in (b) showed the regions studied using EBSD.)
Fig. 5. Microstructural analysis of the rectangle region 1 in Fig. 4(b) in magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) IPF mapping; (b) grain boundary mapping; (c) grain boundary misorientation distribution, and (d) pole figures.
size. Because of twinning, the sample that experienced the quasistatic compressive loading was refined and became polycrystalline. Table 1 presented a summary of the deformation mechanisms for the samples tested under quasistatic loading and dynamic loading. Both mechanical testing and microstructure characterization showed that magnesium single crystal behaved differently under quasistatic loading and dynamic loading. A similar phenomenon about the strain rate effect had also been reported for other materials such as aluminum single crystals [26]. Resembling the reasoning in Ref. [26], the difference between the quasistatic and dynamic mechanical behavior may be resulted from the increase in the critical resolved shear stresses for dislocation slip and deformation twinning, with the increase of testing strain rate for magnesium single crystal. This critical resolved shear stress increase can explain the increase of the maximum strength and the strain hardening rate under dynamic loading. A possible reason for the disappearance of twinning operation under dynamic loading may be the faster increase of the critical resolved shear stress for twinning than that for dislocation slip. Thus, dislocation slip was the main deformation mechanism for the dynamic testing, and both twinning and
dislocation slip happened for the quasistatic testing. Dislocation slip did not change the orientation of material and the sample tested under dynamic loading remained as a single crystal. Deformation twinning resulted in a different crystal orientation inside the twins from that in the matrix, and realized grain refinement.
4. Conclusions Magnesium single crystal samples were mechanically tested at room temperature under dynamic compressive loading at a strain rate of 1000 s 1 and quasistatic compressive loading at a strain rate of 0.001 s 1 , respectively. The untested single crystal had its [0 0 0 1] direction about 101 away from the sample axial direction. The tested samples were characterized using OM, SEM, and EBSD techniques to understand the microscopic deformation mechanisms. The results showed that (a) the dominating deformation mechanism was dislocation motion ({1 1 0 0}/1 1 2 0S prismatic slip systems and {1 1 2 2}/1 1 2 3S secondary pyramidal slip systems) for dynamic loading; (b) both dislocation motion and twinning were observed in the sample under quasistatic loading, and
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Fig. 6. Microstructural analysis of the rectangle region 2 in Fig. 4(b) in magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) IPF mapping; (b) grain boundary mapping; (c) grain boundary misorientation distribution, and (d) pole figures.
Fig. 7. Microstructure observations of magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) SEM image; and (b) Kikuchi patterns for the points in the matrix (regions labeled by diamonds) and the twins (regions labeled by triangles).
Table 1 Summary of deformation mechanisms for the samples tested under quasistatic loading and dynamic loading. Loading (rate)
Deformation mechanisms
Quasistatic (0.001 s 1) Dynamic (1000 s 1)
Prismatic slip, secondary pyramidal slip, tension twinning, tension–compression double twinning Prismatic slip, secondary pyramidal slip
the sample became polycrystalline through {1 0 1 1}–{1 0 1 2} double twinning and {1 0 1 2} tension twinning; and (c) no twinning was observed for the sample subjected to dynamic
loading, and a possible reason was the faster increase of the critical resolved shear stress for twinning than that for dislocation slip.
Q. Li / Materials Science & Engineering A 568 (2013) 96–101
Acknowledgements The support for the research from the US Department of Energy, Office of Basic Energy Sciences under Grant No. DESC0002144 is greatly appreciated. The author also would like to thank Professor Julia R. Weertman at Northwestern University for her comments and remarks. References [1] [2] [3] [4] [5] [6] [7] [8]
S. Kim, B. You, C.D. Yim, Y. Seo, Mater. Lett. 59 (2005) 3876. Q.Z. Li, B. Tian, Mater. Lett. 67 (2012) 81. Y. Chino, J. Lee, K. Sassa, Mater. Lett. 60 (2006) 173. Q.Z. Li, Q. Yu, J. Zhang, Y.Y. Jiang, Scr. Mater. 64 (2011) 233. X. Huang, K. Suzuki, N. Saito, Mater. Sci. Eng., A 508 (2009) 226. Q.Z. Li, Q. Yu, J. Zhang, Y.Y. Jiang, Scr. Mater. 62 (2010) 778. Q.Z. Li, Mater. Sci. Eng., A 540 (2012) 130. M.A. Shehadeh, H.M. Zbib, T. Diaz De La Rubia, Philos. Mag. 85 (2005) 1667.
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[9] S.X. Li, R.Q. Yang, J.W. Li, Z.F. Zhang, Philos. Mag. 86 (2006) 5769. [10] X.W. Li, Z.G. Wang, S.X. Li, Mater. Sci. Eng., A 265 (1999) 18. [11] X. Zhang, T. Jin, N.R. Zhao, Z.H. Wang, X.F. Sun, H.R. Guan, Z.Q. Hu, Mater. Sci. Eng., A 492 (2008) 364. [12] C. Watanabe, T. Fujii, S. Onaka, M. Kato, Int. J. Fatigue 24 (2002) 795. [13] B. Zhang, X. Lu, D. Liu, C. Tao, Mater. Sci. Eng., A 551 (2012) 149. [14] L.L. Lisiecki, J.R. Weertman, Acta Metall. Mater. 38 (1990) 509. [15] L.L. Lisiecki, J.R. Weertman, J. Met. 37 (1985) A31. [16] L.L. Lisiecki, J.R. Weertman, J. Met. 37 (1985) A6. [17] R. Maab, S.V. Petegem, H. Swygenhoven Van, P.M. Derlet, C.A. Volkert, D. Grolimund, Phys. Rev. Lett. 99 (2007) 145505. [18] S.H. Oh, M. Legros, D. Kiener, G. Dehm, Nat. Mater 8 (2009) 95. [19] E.R. Buchanan, R.E. Reedhill, Trans. Metall. Soc .AIME 245 (1969) 1929. [20] Q.Z. Li, Mater. Sci. Eng., A 556 (2012) 301. [21] C.H. Ma, D.W. Mitchell, R. Daga, J. Appl. Phys. 40 (1969) 3499. [22] H. Yoshinaga, R. Horiuchi, Mater. Trans. JIM 4 (1963) 1. [23] Q.Z. Li, J. Appl. Phys. 109 (2011) 103514. [24] P.B. Hirsch, J.S. Lally, Philos. Mag. 12 (117) (1965) 595. [25] B. Bhattacharya, M. Niewczas, Philos. Mag. 91 (17) (2011) 2227. [26] S. Yoshida, N. Nagata, Mater. Trans. JIM 8 (1967) 26.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and deformation mechanism of 0 0 0 1 magnesium single crystal subjected to quasistatic and high-strain-rate compressive loadings Qizhen Li n Chemical and Materials Engineering, University of Nevada, 1664 N. Virginia St, MS388, Reno, NV 89557, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2012 Received in revised form 12 January 2013 Accepted 16 January 2013 Available online 21 January 2013
0 0 0 1 magnesium single crystal samples were mechanically tested under compressive loadings at a quasistatic strain rate (0.001 s 1) and a dynamic strain rate (1000 s 1), respectively. The tested samples were then investigated through various microstructural characterization techniques. Dynamic loading led to much higher maximum strength and larger strain hardening rate than quasistatic loading for the tested material. The microstructure features were clearly different from each other for the two loading conditions, which indicated that the deformation mechanisms for the quasistatic testing were different from those for the dynamic testing. The microstructure analysis showed that (a) prismatic and secondary pyramidal dislocation operations happened in the samples under quasistatic loading and dynamic loading, while tension twinning and tension–compression double twinning also happened in the sample under quasistatic loading; (b) the sample under high-strain-rate dynamic loading remained as single crystal, while that under quasistatic loading was refined to become polycrystalline; and (c) twinning operations under quasistatic loading led to grain refinement. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnesium single crystal Twinning Dislocation Grain refinement Deformation mechanism
1. Introduction Magnesium and its alloys are becoming increasingly attractive candidates for constructing various structural components due to their low densities and high specific strengths. Materials often experience different loading conditions such as quasistatic and high strain rate dynamic/shocking loading conditions, when being used for different automobile and aircraft components. The appealing potential applications motivate extensive research interest and efforts to explore the processing methods to improve their mechanical properties, and uncover their mechanical behaviors and microscopic deformation mechanisms under different loading strain rates (e.g., [1–7]). The mechanical behavior of materials under dynamic loading is generally significantly different from that under quasistatic loading. Thus, it is often required to study both quasistatic and dynamic mechanical behavior and the corresponding microscopic deformation mechanisms of magnesium and its alloys. It will be a good start to investigate materials with a simple microstructure such as single crystals to uncover the mechanical behaviors and the fundamental microscopic deformation mechanisms. To date, extensive research results were reported for single crystals with FCC and BCC crystal structures such as Cu, Ni, In, Cu–Co, Cu–Fe [8–18]. For example, copper-based single
n
Tel.: þ1 775 784 4507; fax: þ1 775 327 5059. E-mail address: [email protected]
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.01.028
crystals were broadly studied to disclose their recrystallization texture, plastic deformation, recovery of deformation, and deformation behavior under cyclic loading and various testing strain rates. A few research results were reported for magnesium single crystals with HCP crystal structure [19–25]. Among these reported papers, only Refs. [21–23] were about magnesium single crystals under dynamic loading and they mainly focused on mechanical behaviors with little work on studying microstructures and deformation mechanisms of the materials. This work aimed to uncover the underlying quasistatic and dynamic microscopic deformation mechanisms of magnesium single crystal subjected to quasistatic and dynamic loadings, respectively, using optical microscopy (OM), scanning electron microscopy (SEM), and electron back-scattered diffraction (EBSD) techniques.
2. Experimental procedure Mechanical testing: Cylindrical magnesium single crystal samples were tested under ambient compressive loadings at a dynamic strain rate of 1000 s 1 and also at a quasistatic strain rate of 0.001 s 1, respectively. Quasistatic testing was performed using a universal testing machine, while dynamic testing was performed using a Split Hopkinson Pressure Bar system. The loading direction was along the axial direction of a cylindrical sample. The displacement and load data were recorded for both tests.
Q. Li / Materials Science & Engineering A 568 (2013) 96–101
Microstructure characterization: The tested samples were cut along a plane parallel to the loading direction. The cutting surfaces were ground and polished using polishing suspensions all the way to a 0.05 mm alumina slurry, and etched for microstructural observations. The microstructures of the samples were characterized using OM, SEM, and EBSD techniques in the study.
3. Results and discussions Fig. 1 reported the stress–strain curves for the mechanically tested samples. The untested magnesium single crystal had its [0 0 0 1] direction about 101 away from the sample’s axial direction Z as shown in the inset in Fig. 1. The quasistatic stress–strain curve had a distinct transition from elastic to plastic deformation as shown in Fig. 1. The dynamic stress– strain curve did not show a clear transition point from elastic to plastic deformation. The dynamic maximum strength and strain hardening rate were about 30% and 20% higher than the corresponding quasistatic values [23]. Microstructure analysis was conducted for the dynamic loading case. The sample was cut along the loading direction. The cutting plane was parallel to the loading direction and was polished to be observed using EBSD. The EBSD results from a representative location on the polished surface were reported in Fig. 2 to illustrate the microstructure of the tested material. The EBSD scanning was performed in the red rectangular region in Fig. 2(a). The rectangular region was chosen because it had both the matrix and the residual deformation feature shown as a thick line. The EBSD data were analyzed below to determine if the line was a dislocation or a twin. The inverse pole figure (IPF) mapping in Fig. 2(b) showed that the whole region had one orientation. Based on the color-coded orientation legend in Fig. 2(b), it can be determined that the polished surface was along f1 0 1 0g . The grain boundary misorientation distribution in Fig. 2(c) confirmed that there was no grain boundary in the studied region. Thus, the thick line in the red rectangle in Fig. 2(a) was a part of a dislocation. The pole figures in Fig. 2(d) showed a nearly ideal f1 0 1 0g/0 0 0 1S orientation of the sample. This indicated that the cutting plane was f1 0 1 0g. The Kikuchi patterns were obtained for the five spots labeled by orange hexagons in Fig. 2(a). These five points
380MPa 0.001/s 481MPa
transition
1000/s
Fig. 1. Stress–strain curves of ambient compressive testing at the quasistatic strain rate of 0.001 s 1 and the dynamic strain rate of 1000 s 1, respectively. The inset is a schematic illustration of the orientation relation between the crystal [0 0 0 1] direction and the loading direction along the cylindrical axial direction Z.
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covered the matrix region and the dislocation region. These five points had the same Kikuchi pattern as shown in Fig. 2(e), which indicated that the different points investigated had the same orientation and confirmed that the line was a dislocation. Fig. 3 showed that the microstructure features of magnesium single crystal under dynamic compressive loading at a strain rate of 1000 s 1. Fig. 3(b) reported three groups (A–C) of parallel dislocation arrays as indicated by the arrows. The stress–strain curves in Fig. 1 indicated that basal slip did not exist and deformation should be accommodated by non-basal slip such as prismatic ({1 1 0 0}/1 1 2 0S) and secondary pyramidal ({1 1 2 2}/1 1 2 3S) dislocation slip. The angle between {1 1 0 0} planes is 601, and the angle between the traces of {1 1 0 0} and {1 1 2 2} is 301. Fig. 3 showed that the angle between A and B was about 601, while the angles between C and A and between C and B were about 301. A and B were two groups of {1 1 0 0}/1 1 2 0S slip systems, while C was a group of {1 1 2 2}/ 1 1 2 3S slip system. Fig. 3(a) showed A and C dislocation arrays. The upper right corner of Fig. 3(a) was close to the sample outer surface. It was shown that the dislocation array was much denser in the region near the surface than that away from the surface. Inside the sample, dislocation arrays were more uniformly distributed as shown in Fig. 3(b), and the distance between two neighboring dislocations was about 30 mm. Microstructure analysis was conducted for the quasistatic loading case. The sample was cut along the loading direction. The cutting plane was parallel to the loading direction and was polished to be observed using OM and EBSD. The dominating deformation mechanism was dislocation motion ({1 1 0 0} /1 1 2 0S slip systems and {1 1 2 2}/1 1 2 3S slip systems) as shown in Fig. 4(a). However, there were some twins appearing in the sample as shown in the SEM image in Fig. 4(b). The two rectangle regions (labeled as 1 and 2) in Fig. 4(b) were studied using EBSD to examine the sample’s microstructure. Fig. 5 reported the IPF mapping, grain boundary mapping, grain boundary misorientation distribution, and pole figures for the region 1 in Fig. 4(b). The color-coded orientation legend in Fig. 2(b) was applicable to Fig. 5(a). The IPF mapping in Fig. 5(a) showed that the matrix was along a {1 0 1 0} plane and there were twins with the orientation close to {0 0 0 1}. These twins were primarily produced by {1 0 1 2} tension twinning based on the information in the grain boundary (GB) mapping (Fig. 5(b)). The GB mapping showed that the orientation difference was about 86o between the matrix and the twins, and these twins were formed due to the tension twinning. Grain boundary misorientation distribution in Fig. 5(c) showed that the misorientation of less than 21 had the high intensity, the misorientation of around 301 had the low intensity, and the misorientation of around 861 had the intermediate intensity. The twins with the 30o misorientation angle were formed due to {1 0 1 2} tension—{1 0 1 1} compression double twinning. Similarly, Fig. 6 reported the IPF mapping, grain boundary mapping, grain boundary misorientation distribution, and pole figures for the region 2 in Fig. 4(b). The color-coded orientation legend in Fig. 2(b) was applicable to Fig. 6(a). Fig. 6(a) had four parallel twins, and they were produced by {1 0 1 2} tension twining and had a misorientation angle of about 861 as shown in Fig. 6(c). Twinning operations resulted in large angle grain boundaries and the sample became polycrystalline after being compressed. The pole figures in Figs. 5(d) and 6(d) showed a nearly ideal {1 0 1 0}/0 0 0 1S orientation of the sample, which indicated that the cutting plane was {1 0 1 0}. Fig. 7(a) was a SEM image of the quasistatically tested sample. Multiple points in the matrix (shown as diamonds) and the twins (shown as triangles) were analyzed. The diamond points (i.e., the matrix region) had one Kikuchi pattern, while the triangle points (i.e., the twins) had
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Fig. 2. Microstructural analysis of magnesium single crystal sample under dynamic loading at a strain rate of 1000 s 1. (a) SEM image; (b) IPF mapping of the region in the rectangle in (a); (c) grain boundary misorientation distribution of the region in the rectangle in (a); (d) pole figures; and (e) Kikuchi pattern. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Micrographs of magnesium single crystal under dynamic compressive loading at a strain rate of 1000 s 1. (a) SEM micrograph; (b) optical micrograph (two groups of prismatic slip systems and one group of secondary pyramidal slip system are identified using arrows).
another Kikuchi pattern. The Kikuchi patterns for the diamonds and triangles were reported in Fig. 7(b). Comparing the two patterns, the misorientation was about 901, which indicated that the
regions labeled with triangles corresponded to tension twins. Twinning changed the orientation of a part of a grain, which produced a high angle misorientation and refined the grain
Q. Li / Materials Science & Engineering A 568 (2013) 96–101
99
Fig. 4. Microstructure observations of magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) OM image; and (b) SEM image. (Note, the two rectangles in (b) showed the regions studied using EBSD.)
Fig. 5. Microstructural analysis of the rectangle region 1 in Fig. 4(b) in magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) IPF mapping; (b) grain boundary mapping; (c) grain boundary misorientation distribution, and (d) pole figures.
size. Because of twinning, the sample that experienced the quasistatic compressive loading was refined and became polycrystalline. Table 1 presented a summary of the deformation mechanisms for the samples tested under quasistatic loading and dynamic loading. Both mechanical testing and microstructure characterization showed that magnesium single crystal behaved differently under quasistatic loading and dynamic loading. A similar phenomenon about the strain rate effect had also been reported for other materials such as aluminum single crystals [26]. Resembling the reasoning in Ref. [26], the difference between the quasistatic and dynamic mechanical behavior may be resulted from the increase in the critical resolved shear stresses for dislocation slip and deformation twinning, with the increase of testing strain rate for magnesium single crystal. This critical resolved shear stress increase can explain the increase of the maximum strength and the strain hardening rate under dynamic loading. A possible reason for the disappearance of twinning operation under dynamic loading may be the faster increase of the critical resolved shear stress for twinning than that for dislocation slip. Thus, dislocation slip was the main deformation mechanism for the dynamic testing, and both twinning and
dislocation slip happened for the quasistatic testing. Dislocation slip did not change the orientation of material and the sample tested under dynamic loading remained as a single crystal. Deformation twinning resulted in a different crystal orientation inside the twins from that in the matrix, and realized grain refinement.
4. Conclusions Magnesium single crystal samples were mechanically tested at room temperature under dynamic compressive loading at a strain rate of 1000 s 1 and quasistatic compressive loading at a strain rate of 0.001 s 1 , respectively. The untested single crystal had its [0 0 0 1] direction about 101 away from the sample axial direction. The tested samples were characterized using OM, SEM, and EBSD techniques to understand the microscopic deformation mechanisms. The results showed that (a) the dominating deformation mechanism was dislocation motion ({1 1 0 0}/1 1 2 0S prismatic slip systems and {1 1 2 2}/1 1 2 3S secondary pyramidal slip systems) for dynamic loading; (b) both dislocation motion and twinning were observed in the sample under quasistatic loading, and
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Fig. 6. Microstructural analysis of the rectangle region 2 in Fig. 4(b) in magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) IPF mapping; (b) grain boundary mapping; (c) grain boundary misorientation distribution, and (d) pole figures.
Fig. 7. Microstructure observations of magnesium single crystal sample under quasistatic loading at a strain rate of 0.001 s 1. (a) SEM image; and (b) Kikuchi patterns for the points in the matrix (regions labeled by diamonds) and the twins (regions labeled by triangles).
Table 1 Summary of deformation mechanisms for the samples tested under quasistatic loading and dynamic loading. Loading (rate)
Deformation mechanisms
Quasistatic (0.001 s 1) Dynamic (1000 s 1)
Prismatic slip, secondary pyramidal slip, tension twinning, tension–compression double twinning Prismatic slip, secondary pyramidal slip
the sample became polycrystalline through {1 0 1 1}–{1 0 1 2} double twinning and {1 0 1 2} tension twinning; and (c) no twinning was observed for the sample subjected to dynamic
loading, and a possible reason was the faster increase of the critical resolved shear stress for twinning than that for dislocation slip.
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Acknowledgements The support for the research from the US Department of Energy, Office of Basic Energy Sciences under Grant No. DESC0002144 is greatly appreciated. The author also would like to thank Professor Julia R. Weertman at Northwestern University for her comments and remarks. References [1] [2] [3] [4] [5] [6] [7] [8]
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