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Internal microstructure observation of enhanced grain-boundary sliding at room temperature in AZ31 magnesium alloy
Materials Science & Engineering A 666 (2016) 94–99
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Internal microstructure observation of enhanced grain-boundary sliding at room temperature in AZ31 magnesium alloy D. Ando n, Y. Sutou, J. Koike Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2015 Received in revised form 14 March 2016 Accepted 8 April 2016 Available online 11 April 2016
The origin of grain boundary sliding (GBS) is known to be slip-induced due to plastic incompatibility near the grain boundary at room temperature. In this study, the relationship between GBS and crystal orientation was investigated in AZ31 Mg alloy rolled sheets at room temperature. The GBS tendency was determined as related to basal dislocation slip where the GBS boundaries were generally located between the grains with respectively high and low or high and high Schmid factors for basal slip. The results indicate that GBS is attributed to the plastic incompatibility caused by anisotropic basal and prismatic slip. Furthermore, GBS was located in regions with localized deformation near grain boundaries. Crosssectional focused ion beam/transmission electron microscopy (FIB/TEM) observations of these regions revealed seriately arranged subgrains adjacent to a grain boundary. Therefore, we propose that RT-GBS in AZ31 can be caused by localized crystal rotation due to dynamic recover and recrystallization by stress concentration near the grain boundary but not ordinary GBS. & 2016 Elsevier B.V. All rights reserved.
Keywords: AZ31 Magnesium alloy Grain boundary sliding Grain boundary shear zone Crystal orientation Subgrain
1. Introduction The occurrence of room-temperature grain boundary sliding (RT-GBS) in Mg has been long debated. The possibility of GBS in Mg alloys below ambient temperature was reported by Hauser et al. [1,2], where GBS was termed grain boundary shearing, and the possibility of its operation was examined in pure Mg at room temperature and 78 K [2]. The displacement of scribed lines at grain boundaries was considered to be evidence for the occurrence of grain boundary shearing. However, they reported that grain boundary shearing was often accompanied by grain boundary fracture, and thus failed to clearly demonstrate whether or not GBS occurred at room temperature and 78 K. Gifkins and Langdon carefully examined the possibility of RTGBS in pure Mg and Mg alloys with grain sizes of 100–300 mm [3]. Optical microscopy observations revealed the offset of scribed lines and of interference fringes after deformation. In most cases, the scribed lines were continuously bent near grain boundaries without any offset. They concluded that RT-GBS did not occur in Mg and Mg alloys, and the step formation was due to localized shear deformation of different magnitudes between adjacent grains. This phenomenon is referred to as zone sliding and is distinct from GBS. However, other authors have reported that RT-GBS can occur in n
Corresponding author. E-mail address: [email protected] (D. Ando).
http://dx.doi.org/10.1016/j.msea.2016.04.030 0921-5093/& 2016 Elsevier B.V. All rights reserved.
nanocrystalline Mg [4,5]. Koike et al. recently showed experimental evidence of RT-GBS in AZ31 rolled sheets deformed to 10% under tension at a strain rate of 1 10 3 s 1 [6,7]. They also showed that the contribution of RT-GBS strain to the total strain was approximately 8% by measuring the surface step height at a grain boundary location. Based on the temperature dependence of the GBS strain, they considered GBS above 423 K to be diffusioninduced GBS, whereas that below 373 K was slip-induced GBS. The slip-induced GBS that occurred was considered to accommodate the incompatibility of plastic strain at grain boundaries in the presence of anisotropic dislocation slip. Similar results for RT-GBS have been reported not only in Mg, but also in other HCP metals [8–14]. The relationship between plastic incompatibility and RTGBS has been reported only in bicrystalline Zn [15], while no experimental evidence for this has been reported in polycrystalline Mg alloys. Therefore, in this study, we attempt to correlate RT-GBS with the tendency of crystal orientation and plastic deformation in polycrystalline Mg alloys. These experiments were performed using the same sample as in our previous report [16].
2. Materials and methods Rolled sheets of AZ31 (Mg-3Al-1Zn, mass%, 1.2 mm thick, Nippon Kinzoku Co., Ltd.) were used as samples. The as-received sheets were annealed at 400 °C for 8 h. The average grain size of the annealed samples was determined by a linear interception method and was approximately 80 mm. No abnormal grain growth
D. Ando et al. / Materials Science & Engineering A 666 (2016) 94–99
was observed and the grain size distribution was uniform. Tensile test samples were prepared by spark discharge machining of the annealed sheets to a gauge length of 15 mm and a gauge width of 4 mm. The sample surfaces were mechanically polished using abrasive paper (#1200 to #4000), followed by chemical polishing to a mirror finish with a solution of ethanol (50 mL) and nitric acid (8 mL) for 20 s. Grids were then scribed on the top surface of the sample to assist visualization of the local deformation in each grain. These grids were scribed by local sputtering of a samples region using a focus ion beam (FIB) system. The grids consisted of 400 squares, each square grid with an area of 100 mm2. The targeted dimension of the grids was 100 nm wide and 100 nm deep. Prior to tensile testing, the crystallographic orientation distribution was investigated using an electron backscatter diffraction (EBSD) apparatus attached to a scanning electron microscope (SEM) with step size of 0.8 mm. Following the EBSD measurement, room temperature tensile tests were performed along the rolling direction to an engineering strain of 10% and a fracture strain
95
ranging from 24% at a constant crosshead speed that corresponds to an initial strain rate of 1.0 10 3 s 1. After the tensile test, all samples were observed using FIB to examine the displacement of the scribed lines at grain boundaries. Optical microscopy was used to measure the magnitude of deformation. The experimental methods were exactly the same as those used in our previous work [16]. The EBSD data provided the Euler angles for all the measured grains and the Schmid factor of a given slip system. In addition, the grain boundary angle was measured with respect to the tensile axis, and the Schmid factor for GBS was calculated under the assumption that the grain boundary planes were arranged perpendicular to the surface plane. Finally, the GBS regions were cut using FIB to prepare thin foils for cross-sectional TEM observations of the internal microstructure. Selected area diffraction (SAED) patterns were acquired with the incident electron beam parallel to the ¯ > direction with the aid of Kikuchi diffraction patterns. <1210
Fig. 1. (a) Sample surface with scribed grids after deformation to 10% nominal strain. The change of grid shape indicates the actual strain distribution. White arrows show the grain boundary with GBS. (b) Typical magnified image of GBS. (c) Magnified image of discontinuous grid line shifts by internal slip line trace indicated by the black arrow.
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3. Results Fig. 1 shows an optical microscope image of the scribed grids after deformation to 10%. The initial grid shape was a 100 mm2 square and the crystal orientation was a typical basal texture obtained by rolling, which has been reported in detail elsewhere [16]. After deformation, the grids were heavily distorted and the top surface became rough. Some GBS was observed at locations where the scribed lines were shifted discontinuously, as indicated by the white arrows in Fig. 1(a). Fig. 1(b) shows a magnified view of typical GBS, which confirmed the occurrence of RT-GBS. In addition, similar shifts were observed along some traces of dislocation slip lines. Fig. 1(c) shows that the grid lines were shifted at internal grains by the slipping line traces. These shifts were distinguished from the GBS. Koike et al. suggested that the main mechanism for RT-GBS is not grain boundary diffusion, but is deformation induced at low temperature [5]. Therefore, the possibility of deformation-induced GBS were considered in this work. The observed grains and grain boundaries were counted; 42 grains and 63 grain boundaries were identified from Fig. 1(a). Using the crystallographic information from EBSD measurements, the Euler angle of each grain was obtained and the Schmid factors for active slip systems were calculated for each grain under the applied stress conditions. These Schmid factors are classified by the indicated color gradient shown in Fig. 2. From the 63 observed grain boundaries, 18 grain boundaries have GBS, as shown by the white lines in Fig. 2. The difference in the Schmid factors between neighboring grains could be used as a measure for the tendency of slip-induced GBS. For non-basal slip, the Schmid factor was almost the same and very high over all the samples, as shown in Fig. 2. On the other hand, in the case of basal slip, the Schmid factor was distributed over a wide range. At low temperature, these non-basal slip systems have high critical resolved shear stress. Basal slip is considered to be a dominant factor that causes strain incompatibility at grain boundaries. Furthermore, the parameters of latent hardening for non-basal slip decrease after basal slip in single crystal [17,18]. Therefore, the strain incompatibility at the grain boundary will be further increased by non-basal slip. Fig. 2(a) shows that the Schmid factors for basal slip are notably different between adjacent grains joined by grain boundaries that underwent GBS. The GBS boundaries are generally located between grains with respectively high and low Schmid factors. 15 grain boundaries out of
17 that underwent GBS showed a Schmid factor average difference of 0.230, which is range from 0.112 to 0.403. The Schmid factor average difference is 0.143 for all grain boundaries. These results suggest that RT-GBS can occur to accommodate concentrated stresses at grain boundaries caused mainly by anisotropic basal dislocation. However, 2 grain boundaries out of 17 that underwent GBS showed a Schmid factor difference of 0.015 and 0.020 which were located between high and high Schmid factor. Therefore, slip-induced GBS may be influenced not only by anisotropic plasticity between two neighboring grains, but rather the plastic deformation may be influenced by the deformation of all adjacent grains. Martin et al. also reported that alternative deformation mechanisms, such as GBS and deformation kinking, may play an important role in Mg-1Zn-0.5Nd alloy in the case of very strong near grain boundary strain intensification [19]. Fig. 3 shows a magnified FIB image of the intersection of slip line traces with a GBS boundary. The white dotted lines indicate the GBS boundaries. GBS gave rise to a shift of the grid lines in the bottom of image. The slip line traces were observed in the middle of image. The grid line was shifted approximately 0.8 mm by the slip trace, which corresponds to a shear strain of 8% under the nominal tensile strain of 10%. The grain orientation was determined as Euler angles (32.9°, 7.1°, 45.2°) by EBSD analysis. The crystallographic arrangement of the hexagonal lattice is also shown in Fig. 3. The c-axis was tilted by 7.1° with respect to the normal direction (ND) of sample. The slip trace was determined to be parallel to the hexagonal edges. Cross slipping regions were also composed of segments parallel to the hexagonal edges. Fig. 4 shows a cross-sectional TEM image from the slip trace. A surface step of approximately 100 nm in height was observed. If the slip trace is attributed to prismatic slip, then a large shift of the grid line can be well explained, i.e., the formation of the small step can be explained by the tilted component of the c-axis. The 7.1° angle is correct with the arctangent between the displacements of the slip line trace (0.8 mm) and the surface step (100 nm). Thus, these slip traces were formed by prismatic slip. Furthermore, these prismatic slip lines were frequently observed near GBS. Fig. 5 shows a TEM image of the internal microstructure of a GBS boundary observed with the incident electron beam parallel ¯ > direction. The GBS boundary was observed in the to the <1210 middle of image and there were three subgrain boundaries on the right side of the grain boundary. SAED patterns were measured from these subgrains and a basal plane trace is indicated by a solid
Fig. 2. Schmid factors for each slip system, where the Schmid factors are classified by the indicated color gradient. Grain boundaries with GBS are indicated by white lines.
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Fig. 3. Magnified FIB image of the intersection of slip line traces with a GBS boundary. The white dotted lines indicate the GBS boundaries. GBS gives rise to a shift of the grid lines in the bottom of the image, as indicated by the black arrow. The slip line traces are represented by yellow arrows. The inset shows the crystallographic arrangement of the hexagonal lattice obtained by EBSD analysis.
Fig. 5. Internal microstructure of a GBS boundary obtained with the incident ¯ > direction. The GBS boundary is observed in electron beam parallel to the <1210 the middle of the image, and there are three subgrain boundaries indicated by white arrows on the right side of the grain boundary. The inset shows the SAED patterns measured for all subgrains and the matrix. The SAED patterns in the lower panel were obtained from each of the 1st, 2nd and 3rd subgrains adjacent to the grain boundary that underwent GBS, and the matrix (right side).
The internal microstructure of another severely shifted GBS boundary was observed. Fig. 6(a) shows an FIB image of a sample deformed to fracture. The grid lines were shifted substantially, as indicated by the red arrow. The TEM thin-foiled sample was prepared from the shifted region using FIB processing. Fig. 6(b) shows a TEM image of the internal microstructure from the shifted region, where the grain boundary is indicated by a white dotted line. A large surface step accompanied GBS. Fig. 6(c) shows a magnified image of the grain boundary in Fig. 6(b), where many subgrain boundaries and fine grains are observed near the grain boundary. These results indicate that GBS accompanies severe localized deformation near the grain boundary.
4. Discussion
Fig. 4. Cross-sectional TEM image from the slip trace shown in Fig. 4. The inset shows an SAED pattern obtained with the incident electron beam parallel to the ¯ > direction. The white solid line indicates the (0001) plane. The slip trace is <1210 accompanied by a small surface step.
line. The crystal orientation changed by a few degrees across each subgrain boundary. The inhomogeneous intensity distribution observed in all of the SAED patterns indicates that these subgrain boundaries had a tilted component around the a-axis and a twisted component around the c-axis, which further indicates that the subgrain boundaries are formed by the accumulation of basal and prismatic edge dislocations.
In this work, we attempted to correlate RT-GBS with the crystal orientation and plastic deformation tendencies in polycrystalline AZ31 Mg alloy. RT-GBS can be observed to preferentially occur at grain boundaries located between the grains with respectively high and low Schmid factors for basal slip, as shown in Fig. 2. The results suggest that GBS is attributed to the incompatibility of plastic deformation caused by anisotropic basal slip. Furthermore, EBSD and TEM observations revealed that grains with such plastic deformation incompatibility have high prismatic slip activity, as shown in Figs. 3 and 4. These results are consistent with previous reports that indicated substantial cross-slip to non-basal planes is induced by plastic compatibility stress associated with grain boundaries [7]. According to the present experimental data, we
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(a)
(b)
(c)
700 nm
1 μm
10 μm
1 μm
Fig. 6. (a) FIB image of a sample deformed to fracture. The grid lines are shifted substantially, as indicated by the red arrow. (b) TEM image of the internal microstructure from the shifted region. The grain boundary is indicated by a white dotted line. The large surface step accompanied GBS. (c) Magnified TEM image of the grain boundary in (b). There are many subgrain boundaries and fine grains near the grain boundaries. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
propose that the high activation of prismatic slip is related to RTGBS. The occurrence of RT-GBS has long been determined by optical microscopy using the micrometer scale shift of the grid lines. In this case, GBS is a phenomenon that causes an abrupt shift of the grid line. In contrast, when the grid line is bent and continuously shifted across the boundary, it is not considered to be GBS, but localized shear deformation near the boundary, referred to as a grain boundary shear zone. The difference between the two is where dislocation glide occurs. In the case of GBS, dislocations can glide along the grain boundary, while in the case of the shear zone, dislocations do not glide along the grain boundary but adjacent to the grain boundary. However, it is difficult to classify these from only observations of the top surface of the sample. Therefore, RT-GBS was investigated in detail here from the internal microstructural observations using the FIB/TEM method. The largest advantage of this method is that the nanometer scale internal microstructure perform one-to-one correspondence to the micrometer scale GBS. At the micrometer scale, the enhanced GBS observed in this work is in the category of ordinary GBS, which shifts or bends an offset line at a grain boundary. Nanometer scale observation of the cross-sectional internal microstructure of a grain boundary that has undergone GBS could provide the origin of the enhanced GBS. Such observation shows that a change in the localized crystal orientation occurs near the grain boundary because subgrain boundaries form a seriate arrangement adjacent to a grain boundary due to rearrangement of the basal and prismatic dislocation, as shown in Fig. 5. It is like a dynamic recover. Furthermore, a localized shear zone is formed near the grain boundary with severe GBS. In these localized shear zones, new fine grains with high angle grain boundary appears like a dynamic recrystallization in Fig. 6. These new fine grains could also be formed due to rearrangement of the basal and prismatic dislocation. According to the present results, we propose that RT-GBS in AZ31 Mg alloy can be classified as localized crystal orientation change due to dynamic recover and recrystallization near the grain boundary but not ordinary RT-GBS. Furthermore, Ordinary RT-GBS is known to easily occur in nano-grains [4,5]. Therefore, a localized shear zone may be formed in these new fine grains by ordinary GBS. 5. Conclusions Rolled sheet samples of polycrystalline AZ31 Mg alloy were deformed at room temperature under tensile stress applied in the
rolling direction. The origin of GBS at room temperature was investigated and the internal structure of GBS was observed using FIB/TEM. The relationship between GBS and the crystal orientation was investigated using scribed grids and EBSD. GBS was identified as located at a region with localized deformation near grain boundaries by prismatic slip. RT-GBS occurs preferentially at grain boundaries located between grains with respectively high and low or high and high Schmid factors for basal slip. These grains have high activity of non-basal slip such as prismatic slip because of latent hardening. The results indicate that GBS is attributed to the plastic incompatibility caused by anisotropic basal and prismatic slip. The internal structure of the GBS was observed using FIB/TEM. Subgrains were seriately arranged adjacent to a grain boundary that underwent GBS. The crystal orientation of the subgrains was rotated three-dimensionally to produce a fine grain region during severe GBS by basal and prismatic slip due to dynamic recover and recrystallization. Therefore, we propose that RT-GBS in AZ31 can be caused by localized crystal rotation due to dynamic recover and recrystallization by stress concentration near the grain boundary but not ordinary RT-GBS. Furthermore, ordinary GBS may form severe shear zone at the nanograin region due to recrystallization. To date, GBS has not been investigated at the nanometer scale as in the present work. Therefore, further detailed investigations may lead to a new understanding of both the enhanced GBS and ordinary GBS. In addition, the experimental methods proposed here are often applicable to other materials and could contribute to a renewed effort in understanding the GBS mechanism.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
F.E. Hauser, C.D. Starr, L. Tietz, J.E. Dorn, Trans. ASM 47 (1955) 102. F.E. Hauser, P.R. Landon, J.E. Dorn, Trans. ASM 48 (1956) 986. R.C. Gifkins, T.G. Langdon, J. Institute Met. 93 (1964&;1965) 347. S. Hwang, C. Nishimura, P.G. McCormick, Scr. Mater. 44 (2001) 1507. R.Z. Valiev, A.V. Korznikov, R.R. Mulyukov, Mater. Sci. Eng. A 168 (1993) 141. J. Koike, R. Ohyama, T. Kobayashi, M. Suzuki, K. Maruyama, Mater. Trans., JIM 44 (4) (2003) 445. J. Koike, T. Kobayashi, T. Mukai, H. Watanabe, M. Suzuki, K. Maruyama, Acta Mater. 51 (2003) 2055. T. Matsunaga, K. Takahashi, T. Kameyama, E. Sato, Mater. Sci. Eng. A 510- 511 (2009) 356. T. Matsunaga, K. Takahashi, T. Kameyama, E. Sato, Mater. Trans. JIM 50 (2009) 2865. T. Matsunaga, K. Takahashi, T. Kameyama, E. Sato, Philos. Mag. 90 (2010) 4041. T. Yamada, K. Kawabata, E. Sato, K. Kuribayashi, I. Jimbo, Mater. Sci. Eng. A 387– 389 (2004) 719.
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H. Adenstedt, Met. Prog. 56 (1949) 658. W.R. Kiessel, M.J. Sinnott, J. Met. 5 (1953) 331. D.R. Luster, W.W. Wentlz, D.W. Kaufman, Mater. Methods 37 (1953) 100. R.Z. Vailed, O.A. Kaibyshev, Acta Metall. 31 (1983) 2121.
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D. Ando, J. Koike, Y. Sutou, Mater. Sci. Eng. A 600 (2014) 145. E.W. Kelley, W.F. Hosfords, Trans. Metall. Soc. AIME 242 (1968) 5. E.W. Kelley, W.F. Hosfords, Trans. Metall. Soc. AIME 242 (1968) 654. G. Martin, C.W. Sinclair, J. Schmitt, Scr. Mater. 68 (2013) 695.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Internal microstructure observation of enhanced grain-boundary sliding at room temperature in AZ31 magnesium alloy D. Ando n, Y. Sutou, J. Koike Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2015 Received in revised form 14 March 2016 Accepted 8 April 2016 Available online 11 April 2016
The origin of grain boundary sliding (GBS) is known to be slip-induced due to plastic incompatibility near the grain boundary at room temperature. In this study, the relationship between GBS and crystal orientation was investigated in AZ31 Mg alloy rolled sheets at room temperature. The GBS tendency was determined as related to basal dislocation slip where the GBS boundaries were generally located between the grains with respectively high and low or high and high Schmid factors for basal slip. The results indicate that GBS is attributed to the plastic incompatibility caused by anisotropic basal and prismatic slip. Furthermore, GBS was located in regions with localized deformation near grain boundaries. Crosssectional focused ion beam/transmission electron microscopy (FIB/TEM) observations of these regions revealed seriately arranged subgrains adjacent to a grain boundary. Therefore, we propose that RT-GBS in AZ31 can be caused by localized crystal rotation due to dynamic recover and recrystallization by stress concentration near the grain boundary but not ordinary GBS. & 2016 Elsevier B.V. All rights reserved.
Keywords: AZ31 Magnesium alloy Grain boundary sliding Grain boundary shear zone Crystal orientation Subgrain
1. Introduction The occurrence of room-temperature grain boundary sliding (RT-GBS) in Mg has been long debated. The possibility of GBS in Mg alloys below ambient temperature was reported by Hauser et al. [1,2], where GBS was termed grain boundary shearing, and the possibility of its operation was examined in pure Mg at room temperature and 78 K [2]. The displacement of scribed lines at grain boundaries was considered to be evidence for the occurrence of grain boundary shearing. However, they reported that grain boundary shearing was often accompanied by grain boundary fracture, and thus failed to clearly demonstrate whether or not GBS occurred at room temperature and 78 K. Gifkins and Langdon carefully examined the possibility of RTGBS in pure Mg and Mg alloys with grain sizes of 100–300 mm [3]. Optical microscopy observations revealed the offset of scribed lines and of interference fringes after deformation. In most cases, the scribed lines were continuously bent near grain boundaries without any offset. They concluded that RT-GBS did not occur in Mg and Mg alloys, and the step formation was due to localized shear deformation of different magnitudes between adjacent grains. This phenomenon is referred to as zone sliding and is distinct from GBS. However, other authors have reported that RT-GBS can occur in n
Corresponding author. E-mail address: [email protected] (D. Ando).
http://dx.doi.org/10.1016/j.msea.2016.04.030 0921-5093/& 2016 Elsevier B.V. All rights reserved.
nanocrystalline Mg [4,5]. Koike et al. recently showed experimental evidence of RT-GBS in AZ31 rolled sheets deformed to 10% under tension at a strain rate of 1 10 3 s 1 [6,7]. They also showed that the contribution of RT-GBS strain to the total strain was approximately 8% by measuring the surface step height at a grain boundary location. Based on the temperature dependence of the GBS strain, they considered GBS above 423 K to be diffusioninduced GBS, whereas that below 373 K was slip-induced GBS. The slip-induced GBS that occurred was considered to accommodate the incompatibility of plastic strain at grain boundaries in the presence of anisotropic dislocation slip. Similar results for RT-GBS have been reported not only in Mg, but also in other HCP metals [8–14]. The relationship between plastic incompatibility and RTGBS has been reported only in bicrystalline Zn [15], while no experimental evidence for this has been reported in polycrystalline Mg alloys. Therefore, in this study, we attempt to correlate RT-GBS with the tendency of crystal orientation and plastic deformation in polycrystalline Mg alloys. These experiments were performed using the same sample as in our previous report [16].
2. Materials and methods Rolled sheets of AZ31 (Mg-3Al-1Zn, mass%, 1.2 mm thick, Nippon Kinzoku Co., Ltd.) were used as samples. The as-received sheets were annealed at 400 °C for 8 h. The average grain size of the annealed samples was determined by a linear interception method and was approximately 80 mm. No abnormal grain growth
D. Ando et al. / Materials Science & Engineering A 666 (2016) 94–99
was observed and the grain size distribution was uniform. Tensile test samples were prepared by spark discharge machining of the annealed sheets to a gauge length of 15 mm and a gauge width of 4 mm. The sample surfaces were mechanically polished using abrasive paper (#1200 to #4000), followed by chemical polishing to a mirror finish with a solution of ethanol (50 mL) and nitric acid (8 mL) for 20 s. Grids were then scribed on the top surface of the sample to assist visualization of the local deformation in each grain. These grids were scribed by local sputtering of a samples region using a focus ion beam (FIB) system. The grids consisted of 400 squares, each square grid with an area of 100 mm2. The targeted dimension of the grids was 100 nm wide and 100 nm deep. Prior to tensile testing, the crystallographic orientation distribution was investigated using an electron backscatter diffraction (EBSD) apparatus attached to a scanning electron microscope (SEM) with step size of 0.8 mm. Following the EBSD measurement, room temperature tensile tests were performed along the rolling direction to an engineering strain of 10% and a fracture strain
95
ranging from 24% at a constant crosshead speed that corresponds to an initial strain rate of 1.0 10 3 s 1. After the tensile test, all samples were observed using FIB to examine the displacement of the scribed lines at grain boundaries. Optical microscopy was used to measure the magnitude of deformation. The experimental methods were exactly the same as those used in our previous work [16]. The EBSD data provided the Euler angles for all the measured grains and the Schmid factor of a given slip system. In addition, the grain boundary angle was measured with respect to the tensile axis, and the Schmid factor for GBS was calculated under the assumption that the grain boundary planes were arranged perpendicular to the surface plane. Finally, the GBS regions were cut using FIB to prepare thin foils for cross-sectional TEM observations of the internal microstructure. Selected area diffraction (SAED) patterns were acquired with the incident electron beam parallel to the ¯ > direction with the aid of Kikuchi diffraction patterns. <1210
Fig. 1. (a) Sample surface with scribed grids after deformation to 10% nominal strain. The change of grid shape indicates the actual strain distribution. White arrows show the grain boundary with GBS. (b) Typical magnified image of GBS. (c) Magnified image of discontinuous grid line shifts by internal slip line trace indicated by the black arrow.
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3. Results Fig. 1 shows an optical microscope image of the scribed grids after deformation to 10%. The initial grid shape was a 100 mm2 square and the crystal orientation was a typical basal texture obtained by rolling, which has been reported in detail elsewhere [16]. After deformation, the grids were heavily distorted and the top surface became rough. Some GBS was observed at locations where the scribed lines were shifted discontinuously, as indicated by the white arrows in Fig. 1(a). Fig. 1(b) shows a magnified view of typical GBS, which confirmed the occurrence of RT-GBS. In addition, similar shifts were observed along some traces of dislocation slip lines. Fig. 1(c) shows that the grid lines were shifted at internal grains by the slipping line traces. These shifts were distinguished from the GBS. Koike et al. suggested that the main mechanism for RT-GBS is not grain boundary diffusion, but is deformation induced at low temperature [5]. Therefore, the possibility of deformation-induced GBS were considered in this work. The observed grains and grain boundaries were counted; 42 grains and 63 grain boundaries were identified from Fig. 1(a). Using the crystallographic information from EBSD measurements, the Euler angle of each grain was obtained and the Schmid factors for active slip systems were calculated for each grain under the applied stress conditions. These Schmid factors are classified by the indicated color gradient shown in Fig. 2. From the 63 observed grain boundaries, 18 grain boundaries have GBS, as shown by the white lines in Fig. 2. The difference in the Schmid factors between neighboring grains could be used as a measure for the tendency of slip-induced GBS. For non-basal slip, the Schmid factor was almost the same and very high over all the samples, as shown in Fig. 2. On the other hand, in the case of basal slip, the Schmid factor was distributed over a wide range. At low temperature, these non-basal slip systems have high critical resolved shear stress. Basal slip is considered to be a dominant factor that causes strain incompatibility at grain boundaries. Furthermore, the parameters of latent hardening for non-basal slip decrease after basal slip in single crystal [17,18]. Therefore, the strain incompatibility at the grain boundary will be further increased by non-basal slip. Fig. 2(a) shows that the Schmid factors for basal slip are notably different between adjacent grains joined by grain boundaries that underwent GBS. The GBS boundaries are generally located between grains with respectively high and low Schmid factors. 15 grain boundaries out of
17 that underwent GBS showed a Schmid factor average difference of 0.230, which is range from 0.112 to 0.403. The Schmid factor average difference is 0.143 for all grain boundaries. These results suggest that RT-GBS can occur to accommodate concentrated stresses at grain boundaries caused mainly by anisotropic basal dislocation. However, 2 grain boundaries out of 17 that underwent GBS showed a Schmid factor difference of 0.015 and 0.020 which were located between high and high Schmid factor. Therefore, slip-induced GBS may be influenced not only by anisotropic plasticity between two neighboring grains, but rather the plastic deformation may be influenced by the deformation of all adjacent grains. Martin et al. also reported that alternative deformation mechanisms, such as GBS and deformation kinking, may play an important role in Mg-1Zn-0.5Nd alloy in the case of very strong near grain boundary strain intensification [19]. Fig. 3 shows a magnified FIB image of the intersection of slip line traces with a GBS boundary. The white dotted lines indicate the GBS boundaries. GBS gave rise to a shift of the grid lines in the bottom of image. The slip line traces were observed in the middle of image. The grid line was shifted approximately 0.8 mm by the slip trace, which corresponds to a shear strain of 8% under the nominal tensile strain of 10%. The grain orientation was determined as Euler angles (32.9°, 7.1°, 45.2°) by EBSD analysis. The crystallographic arrangement of the hexagonal lattice is also shown in Fig. 3. The c-axis was tilted by 7.1° with respect to the normal direction (ND) of sample. The slip trace was determined to be parallel to the hexagonal edges. Cross slipping regions were also composed of segments parallel to the hexagonal edges. Fig. 4 shows a cross-sectional TEM image from the slip trace. A surface step of approximately 100 nm in height was observed. If the slip trace is attributed to prismatic slip, then a large shift of the grid line can be well explained, i.e., the formation of the small step can be explained by the tilted component of the c-axis. The 7.1° angle is correct with the arctangent between the displacements of the slip line trace (0.8 mm) and the surface step (100 nm). Thus, these slip traces were formed by prismatic slip. Furthermore, these prismatic slip lines were frequently observed near GBS. Fig. 5 shows a TEM image of the internal microstructure of a GBS boundary observed with the incident electron beam parallel ¯ > direction. The GBS boundary was observed in the to the <1210 middle of image and there were three subgrain boundaries on the right side of the grain boundary. SAED patterns were measured from these subgrains and a basal plane trace is indicated by a solid
Fig. 2. Schmid factors for each slip system, where the Schmid factors are classified by the indicated color gradient. Grain boundaries with GBS are indicated by white lines.
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Fig. 3. Magnified FIB image of the intersection of slip line traces with a GBS boundary. The white dotted lines indicate the GBS boundaries. GBS gives rise to a shift of the grid lines in the bottom of the image, as indicated by the black arrow. The slip line traces are represented by yellow arrows. The inset shows the crystallographic arrangement of the hexagonal lattice obtained by EBSD analysis.
Fig. 5. Internal microstructure of a GBS boundary obtained with the incident ¯ > direction. The GBS boundary is observed in electron beam parallel to the <1210 the middle of the image, and there are three subgrain boundaries indicated by white arrows on the right side of the grain boundary. The inset shows the SAED patterns measured for all subgrains and the matrix. The SAED patterns in the lower panel were obtained from each of the 1st, 2nd and 3rd subgrains adjacent to the grain boundary that underwent GBS, and the matrix (right side).
The internal microstructure of another severely shifted GBS boundary was observed. Fig. 6(a) shows an FIB image of a sample deformed to fracture. The grid lines were shifted substantially, as indicated by the red arrow. The TEM thin-foiled sample was prepared from the shifted region using FIB processing. Fig. 6(b) shows a TEM image of the internal microstructure from the shifted region, where the grain boundary is indicated by a white dotted line. A large surface step accompanied GBS. Fig. 6(c) shows a magnified image of the grain boundary in Fig. 6(b), where many subgrain boundaries and fine grains are observed near the grain boundary. These results indicate that GBS accompanies severe localized deformation near the grain boundary.
4. Discussion
Fig. 4. Cross-sectional TEM image from the slip trace shown in Fig. 4. The inset shows an SAED pattern obtained with the incident electron beam parallel to the ¯ > direction. The white solid line indicates the (0001) plane. The slip trace is <1210 accompanied by a small surface step.
line. The crystal orientation changed by a few degrees across each subgrain boundary. The inhomogeneous intensity distribution observed in all of the SAED patterns indicates that these subgrain boundaries had a tilted component around the a-axis and a twisted component around the c-axis, which further indicates that the subgrain boundaries are formed by the accumulation of basal and prismatic edge dislocations.
In this work, we attempted to correlate RT-GBS with the crystal orientation and plastic deformation tendencies in polycrystalline AZ31 Mg alloy. RT-GBS can be observed to preferentially occur at grain boundaries located between the grains with respectively high and low Schmid factors for basal slip, as shown in Fig. 2. The results suggest that GBS is attributed to the incompatibility of plastic deformation caused by anisotropic basal slip. Furthermore, EBSD and TEM observations revealed that grains with such plastic deformation incompatibility have high prismatic slip activity, as shown in Figs. 3 and 4. These results are consistent with previous reports that indicated substantial cross-slip to non-basal planes is induced by plastic compatibility stress associated with grain boundaries [7]. According to the present experimental data, we
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(a)
(b)
(c)
700 nm
1 μm
10 μm
1 μm
Fig. 6. (a) FIB image of a sample deformed to fracture. The grid lines are shifted substantially, as indicated by the red arrow. (b) TEM image of the internal microstructure from the shifted region. The grain boundary is indicated by a white dotted line. The large surface step accompanied GBS. (c) Magnified TEM image of the grain boundary in (b). There are many subgrain boundaries and fine grains near the grain boundaries. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
propose that the high activation of prismatic slip is related to RTGBS. The occurrence of RT-GBS has long been determined by optical microscopy using the micrometer scale shift of the grid lines. In this case, GBS is a phenomenon that causes an abrupt shift of the grid line. In contrast, when the grid line is bent and continuously shifted across the boundary, it is not considered to be GBS, but localized shear deformation near the boundary, referred to as a grain boundary shear zone. The difference between the two is where dislocation glide occurs. In the case of GBS, dislocations can glide along the grain boundary, while in the case of the shear zone, dislocations do not glide along the grain boundary but adjacent to the grain boundary. However, it is difficult to classify these from only observations of the top surface of the sample. Therefore, RT-GBS was investigated in detail here from the internal microstructural observations using the FIB/TEM method. The largest advantage of this method is that the nanometer scale internal microstructure perform one-to-one correspondence to the micrometer scale GBS. At the micrometer scale, the enhanced GBS observed in this work is in the category of ordinary GBS, which shifts or bends an offset line at a grain boundary. Nanometer scale observation of the cross-sectional internal microstructure of a grain boundary that has undergone GBS could provide the origin of the enhanced GBS. Such observation shows that a change in the localized crystal orientation occurs near the grain boundary because subgrain boundaries form a seriate arrangement adjacent to a grain boundary due to rearrangement of the basal and prismatic dislocation, as shown in Fig. 5. It is like a dynamic recover. Furthermore, a localized shear zone is formed near the grain boundary with severe GBS. In these localized shear zones, new fine grains with high angle grain boundary appears like a dynamic recrystallization in Fig. 6. These new fine grains could also be formed due to rearrangement of the basal and prismatic dislocation. According to the present results, we propose that RT-GBS in AZ31 Mg alloy can be classified as localized crystal orientation change due to dynamic recover and recrystallization near the grain boundary but not ordinary RT-GBS. Furthermore, Ordinary RT-GBS is known to easily occur in nano-grains [4,5]. Therefore, a localized shear zone may be formed in these new fine grains by ordinary GBS. 5. Conclusions Rolled sheet samples of polycrystalline AZ31 Mg alloy were deformed at room temperature under tensile stress applied in the
rolling direction. The origin of GBS at room temperature was investigated and the internal structure of GBS was observed using FIB/TEM. The relationship between GBS and the crystal orientation was investigated using scribed grids and EBSD. GBS was identified as located at a region with localized deformation near grain boundaries by prismatic slip. RT-GBS occurs preferentially at grain boundaries located between grains with respectively high and low or high and high Schmid factors for basal slip. These grains have high activity of non-basal slip such as prismatic slip because of latent hardening. The results indicate that GBS is attributed to the plastic incompatibility caused by anisotropic basal and prismatic slip. The internal structure of the GBS was observed using FIB/TEM. Subgrains were seriately arranged adjacent to a grain boundary that underwent GBS. The crystal orientation of the subgrains was rotated three-dimensionally to produce a fine grain region during severe GBS by basal and prismatic slip due to dynamic recover and recrystallization. Therefore, we propose that RT-GBS in AZ31 can be caused by localized crystal rotation due to dynamic recover and recrystallization by stress concentration near the grain boundary but not ordinary RT-GBS. Furthermore, ordinary GBS may form severe shear zone at the nanograin region due to recrystallization. To date, GBS has not been investigated at the nanometer scale as in the present work. Therefore, further detailed investigations may lead to a new understanding of both the enhanced GBS and ordinary GBS. In addition, the experimental methods proposed here are often applicable to other materials and could contribute to a renewed effort in understanding the GBS mechanism.
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