- Email: [email protected]
multi-Al nanocomposite
Vacuum 122 (2015) 1e5
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Unstable stacking faults in submicron/micron Al grammins in multi-SiCp/multi-Al nanocomposite Wenshu Yang a, *, Ronghua Dong a, Longtao Jiang a, Gaohui Wu a, Murid Hussain b a
Department of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Department of Chemical Engineering, COMSATS Institute of Information Technology, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 7 September 2015
It is difficult to obtain planar defects in aluminum due to its high stacking fault energy, in particular in submicron/micron Al grains. In this work we provide evidence for planar defects in submicron/micron Al grain of composites with multi nano-particles by transmission electron microscope observations. NanoSiC particles (<100 nm) were found within micron-Al grains (>2 mm), while submicron SiC particles (200 e500 nm) were present at the boundary of ultrafine Al grains (100e500 nm). Zigzag defects and linear defects were observed in both the micron-Al grains and ultrafine Al grains. These defects are made up of distortion areas, edge dislocations, stacking faults which contain Frank partial dislocations and twinning. Therefore, these defects are in a state of extreme instability, which would “disappear” under the electron beam irradiation in a few seconds. These results highlight that the increase of interface could lead to the formation of stacking faults, even in the micron Al grains. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Metal-matrix composites (MMCs) Nano-structures Microstructures Liquid metal infiltration
1. Introduction Nano-metallic materials usually demonstrate significant different microstructure and performance to the traditional metal alloys [1,2]. As grain sizes are reduced to the nanometer scale and the percentage of grain boundary atoms increases correspondingly, dislocation sources and pileup are hardly to exist and it is considered that deformation is mainly carried out mostly by the grain boundaries [3,4]. The deformation twins rather than dislocations have been observed in nanocrystalline aluminum at a grain size of 10e35 nm [5], which has been supported by molecular dynamics simulations [6,7]. Therefore, it raises a question that whether the Al matrix nanocomposites also have different microstructure and properties than the traditional Al matrix composites, and this question has not been fully understood yet. Several research efforts have been carried out to investigate the microstructure and performance of nano-reinforced Al matrix composite. It has been widely established that the mechanical properties of Al matrix composites could be significantly improved
* Corresponding author. P. O. 3023 Science Park, No. 2 Yikuang Street, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.vacuum.2015.09.002 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
by the addition of low content (usually < 10 wt.%) nanoparticles [8e10], nanotubes [11,12], nanowires [13] and graphene nanoplates [14], implying that the nano-reinforcements demonstrate more distinguished strengthening effect than the micron-size reinforcements. H.R. Ezatpour et al. [15] prepared 6061 Al matrix composites reinforced by nano-Al2O3 particles and the 0.2% yield strength of 1.5wt.% Al2O3/6061Al composite was about two times of the Al matrix. Currently, the microstructure characters of Al matrix (for instance grain size and form of defects) in nanocomposites have been slightly affected by the nano-reinforcements due to their low content. It is suggested that high content nano-reinforcements might be introduced to stimulate the nano-effect in Al matrix. However, to the knowledge of the authors, microstructure of high content Al matrix nanocomposites (>25 vol.%) have not been reported yet. Therefore, in the present work, 30 vol.% multi-SiC particles (average size of 30 and 220 nm, respectively) were introduced into pure Al by pressure infiltration method. It is very interesting to state that the grain size of Al grain also demonstrated quasi-multi distribution and stacking faults rather than dislocations were found in Al matrix. The microstructure of Al matrix and the stacking faults of multi-SiCp/multi-Al nanocomposite have been explored in depth.
2
W. Yang et al. / Vacuum 122 (2015) 1e5
2. Materials and methods SiC particles with average size of 30 and 220 nm (supplied by Harbin Xin Ceramics Co., Ltd. China) respectively were used in the present work. The two kinds of SiC particles were ball-milled for 48 under Ar flow, while the weight ratio of 30 nm SiC/220 nm SiC particles was 1/80. The microstructure and corresponding size distribution of the mixed SiC powders is shown in Fig. 1a and b, respectively. It is obvious that the mixed SiC powders demonstrated a multi-size distribution (30 and 220 nm), implying that the balling process has minor effect on the size of SiC particles. Afterward, the mixed SiC particles were adopted to increase the amount of SiC content in the final composites. The mixed SiC particles were then put into a steel mold and further pressed to the set volume content (30 vol.%). Meanwhile, high pure Al (supplied by Northeast Light Alloy Co., Ltd. China) was molten at 800 C under Ar. The preheating temperatures for the preform and pressure infiltration dies were 500 and 760 C, respectively. During the infiltration process, a pressure of 10 MPa was applied and maintained for 10 min, followed by the solidification of the composites in air. In order to release the residual thermal stress, annealing treatment was performed before microstructure observation and hardness tests. Microstructure of mixed SiC particles was observed by FEI Sirion Quanta 200 scanning electron microscope (SEM). Further observation of 30 vol.% multi-SiCp/multi-Al nanocomposite was performed on JEM-2010F high resolution transmission electron microscopy (HRTEM). 3. Results and discussion
Fig. 2. TEM micrograph of nanocomposites with multi particles.
area. However, only one set of Al diffraction patterns was found, implying that these highly oriented SiC particles were distributed in a large Al grain, whose size was larger than 1.4 mm. Therefore, it could be concluded that the submicron-SiC particles were found at the boundaries of submicron Al grains while the nano-SiC particles were distributed within a micron Al grains. 3.2. Unstable stacking faults in Al matrix of multi-SiCp/multi-Al nanocomposite
3.1. Microstructure of multi-SiCp/multi-Al nanocomposite Representative microstructure of the studied or under study SiCp/Al composite is shown in Fig. 2. SiC particles were uniformly distributed in the Al matrix, and no cluster of SiC particles was found. Further observation of Al matrix surrounding submicron-SiC (>200 nm) and nano-SiC (<100 nm) indicated that the grain size of Al also demonstrated multi-distribution, which has been shown in Fig. 3a and c, respectively. Submicron SiC particles were usually surrounded by several submicron Al grains (Fig. 3b), with average size of about 400 nm. However, no clear grain boundaries were observed in the Al matrix surrounding nano-SiC particles (Fig. 3c). Selected area electron diffraction (SAED) of Fig. 3d shows that quasi-polycrystalline rings of SiC were detected, indicating that the direction of SiC particles were significantly different within this
Two kinds of defects (linear and zigzag) were found in Al matrix, as shown in Fig. 4a and b. These defects were found both in submicron and micron Al matrix, and were only composed of Al element according to EDS analysis results (Fig. 4c). Therefore, these defects were not the segregation of the alloying elements. Moreover, it should be noted that linear or tangled dislocations in Al matrix, which are usually found near sharp corners of SiC particles due to thermal mismatch between SiC particles and Al matrix, have not been found in the present work. The typical linear defect and its Fast Fourier Transform (FFT) patterns are shown in Fig. 5a and b, respectively. The observed linear defects were usually perpendicular to the [011] zone axes of Al matrix. Frank partial dislocations (marked by the blue box in Fig. 5c) found in the Fourier-filtered image of the selected region in Fig. 5c, implied the existence of stacking faults in the submicron
Fig. 1. (a) SEM micrograph of multi SiC particles. (b) Particle size distributions. The size of SiC particles is 20e500 nm, the two peaks are 30 and 220 nm respectively, and the average size is 106 nm.
W. Yang et al. / Vacuum 122 (2015) 1e5
3
Fig. 3. TEM micrographs of nanocomposites. (a) TEM micrograph of nanocomposites with submicron SiC particles. (b) The schematic diagram of composites. (c) and (d) Bright field image and dark field image of nanocomposites with nano SiC particles, respectively. (e) Selected area electron diffraction of composites in (c) and (d).
Fig. 4. HRTEM micrograph of the matrix of composites. (a) Two kinds of defects (linear and zigzag) were found in Al matrix. (b) Linear defects were found in the interface of nano SiC particles and Al matrix. (c) EDS results of Al matrix.
Fig. 5. HRTEM micrographs of linear defect. (a) The typical linear defect. (b) Fast Fourier Transform patterns of the defect. (c) The Fourier-filtered image of the defect.
and micron Al matrix. It showed that the distortion caused by the deviation of atom from equilibrium position and edge dislocations were also observed in the linear defects. Detailed analysis of the zigzag defect is shown in Fig. 6. The FFT
patterns (Fig. 6b) indicate that the zigzag defects were also perpendicular to the [011] zone axes of Al matrix. The Fourierfiltered images of the three selected regions in the Fig. 6a are shown in Fig. 6cee, respectively. Similar to the linear defects, the
4
W. Yang et al. / Vacuum 122 (2015) 1e5
Fig. 6. HRTEM micrographs of zigzag defect. (a) The typical zigzag defect. (b) Fast Fourier transform patterns of the defect. (c), (d) and (e) The Fourier-filtered images of the three selected regions.
zigzag defects also contain twinning (Fig. 6c), distortion area (the black circles in Fig. 6d) and Frank partial dislocations (the blue box in Fig. 6e). The classic schematic of Frank partial dislocations and the re-constructed schematic of partial dislocations according to FFT results in the present work are shown in Fig. 7a and b, respectively. It is clear that they are equivalent to each other. Therefore, it could be concluded that stacking faults were found in the Al matrix of 30 vol.% multi-SiCp/multi-Al nanocomposite. Usually, due to its high stacking fault energy, it is difficult to obtain stacking faults in submicron and micron grains in Al alloys [16]. In coarse-grained Al metals, plastic deformation is mainly achieved by dislocations [17]. Dislocations can move through the crystal grains and can interact with each other. However, as grain sizes are reduced to the nanometer scale and the percentage of grain boundary atoms increases correspondingly, this traditional view of dislocation-driven plasticity in Al metals needs to be reconsidered. V. Yamakov et al. [6] have demonstrated that, in contrast to coarsegrained Al, mechanical twinning may play an important role in the deformation behavior of nanocrystalline Al by molecular dynamics simulation. M. Chen et al. have provided evidence for deformation twinning in nanocrystalline aluminum by transmission electron microscope observations. Grain boundary plays a vital role in this transformation. For instance, more than 10% atoms are in grain boundary when the grain size is less than 20 nm [3]. In coarse-grained Al metals, grain boundaries often hinder their transmission, creating a dislocation pile-up at the boundary and thereby making the material more difficult to deform. As the grain sizes are reduced to nanoscale, several partial or imperfect dislocations nucleate in different
regions of the grain boundary [18], in contrast to the emission of full or perfect dislocations known from coarse grained metals [19]. In our present work, the grain size of Al matrix was in submicron and micron scale. However, different to common Al metals, high density SiCeAl interface existed within submicron/micron Al matrix. Moreover, different to that in Al grain boundary, the Al atoms surrounding SiCeAl interface could be easily moved since SiC could be considered as a rigid phase than Al. Therefore, the thickness of the area that affected by the SiCeAl interface might be more than 5 nm, while more than 30% Al atoms are in the SiCeAl interface affected area in 30 vol.% multi-SiCp/multi-Al nanocomposite. Therefore, stacking faults were found in the present work even Al grain size was in submicron or micron scale. Hence, not only the increase of grain boundary, but also increase of interface could lead to the formation of stacking faults. It is very interesting that the linear and zigzag defects were very unstable, and would disappear during the HRTEM observation under 200 kV electron beam irradiation, as shown in Video 1 and Fig. 8. During recording, the observed two defects were disappeared after 38 and 102 s, respectively, indicating that these defects were in unstable state. The linear and zigzag defects have not been observed in the common Al alloy, and they could be observed in the present work might be due to the presence of unstable stacking faults [4]. During HRTEM observation, these linear and zigzag defects were under radiation of high energy electrons, and the Frank partial dislocations could be transformed to become perfect dislocations, leading to the disappearance behavior of these defects. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.vacuum.2015.09.002.
4. Conclusions The high volume fraction nanocomposites with multi-particles show a unique microstructure. Due to the multi sized particles, the grain size in the matrix showed two different states. The submicron grains were existed around the bigger particles and the micron grains swallowed the smaller particles. Frank partial dislocations and planar defects formed instead of perfect dislocations in submicron/micron Al grains. Moreover, the defects were very unstable, and would disappear during the HRTEM observation under 300 kV electron beam irradiation. The stacking faults in our present work are different to the planar defects investigated in nanocrystalline Al. The results demonstrate not only the increase of grain boundary, but also the increase of interface could lead to the formation of stacking faults, while, the stacking faults could exist
Fig. 7. The schematic of Frank partial dislocations. (a) The classic schematic of Frank partial dislocations. (b) the re-constructed schematic of partial dislocations according to FFT results in the present work.
W. Yang et al. / Vacuum 122 (2015) 1e5
5
Fig. 8. HRTEM observation of defects under 300 kV electron beam irradiation.
even in the submicron or micron Al grains. Acknowledgments This work has been supported by “the Fundamental Research Funds for the Central Universities” (Grant No. HIT. NSRIF. 20161) and “the National Natural Science Foundation of China” (51501047). References [1] Y. Yue, P. Liu, Z. Zhang, X. Han, E. Ma, Approaching the theoretical elastic strain limit in copper nanowires, Nano Lett. 11 (2011) 3151e3155. mez, E. Restrepo-Parra, P.J. Arango, X-ray [2] D. Escobar, R. Ospina, A.G. Go microstructural analysis of nanocrystalline TiZrN thin films by diffraction pattern modeling, Mater. Charact. 88 (2014) 119e126. [3] H. Van Swygenhoven, Polycrystalline materials. Grain boundaries and dislocations, Science 296 (2002) 66e67. [4] J. Schiotz, K.W. Jacobsen, A maximum in the strength of nanocrystalline copper, Science 301 (2003) 1357e1359. [5] M. Chen, E. Ma, K.J. Hemker, H. Sheng, Y. Wang, X. Cheng, Deformation twinning in nanocrystalline aluminum, Science 300 (2003) 1275e1277. [6] V. Yamakov, D. Wolf, S.R. Phillpot, A.K. Mukherjee, H. Gleiter, Dislocation processes in the deformation of nanocrystalline aluminium by moleculardynamics simulation, Nat. Mater. 1 (2002) 45e48. [7] V. Yamakov, D. Wolf, S.R. Phillpot, H. Gleiter, Deformation twinning in nanocrystalline Al by molecular dynamics simulation, Acta Mater. 50 (2002) 5005e5020. [8] H.R. Ezatpour, S.A. Sajjadi, M.H. Sabzevar, Y.Z. Huang, An investigation of the tensile and compressive properties of Al6061 and its nanocomposites in ascast state and in extruded condition, Mater. Sci. Eng. A 607 (2014) 589e595.
[9] Z.B. Lei, K. Zhao, Y.G. Wang, L.N. An, Thermal expansion of Al matrix composites reinforced with hybrid micro-/nano-sized Al2O3 particles, J. Mater. Sci. Technol. 30 (2014) 61e64. [10] D.S. Zhou, F. Qiu, Q.C. Jiang, Simultaneously increasing the strength and ductility of nano-sized TiN particle reinforced AleCu matrix composites, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 596 (2014) 98e102. [11] X.D. Yang, E.Z. Liu, C.S. Shi, C.N. He, J.J. Li, N.Q. Zhao, et al., Fabrication of carbon nanotube reinforced Al composites with well-balanced strength and ductility, J. Alloys Compd. 563 (2013) 216e220. [12] K. Kondoh, H. Fukuda, J. Umeda, H. Imai, B. Fugetsu, Microstructural and mechanical behavior of multi-walled carbon nanotubes reinforced AleMgeSi alloy composites in aging treatment, Carbon 72 (2014) 15e21. [13] Y.B. Tang, Y.Q. Liu, C.H. Sun, H.T. Cong, AlN nanowires for Al-based composites with high strength and low thermal expansion, J. Mater. Res. 22 (2007) 2711e2718. [14] J.W. Yang, X.R. Chen, B. Song, Interaction of graphene-on-Al(111) composite with D-Glucopyranose and its application in biodetection, J. Phys. Chem. C 117 (2013) 8475e8480. [15] H.R. Ezatpour, S.A. Sajjadi, M.H. Sabzevar, Y.Z. Huang, An investigation of the tensile and compressive properties of Al6061 and its nanocomposites in ascast state and in extruded condition, Mater. Sci. Eng. A 607 (2014) 589e595. [16] Y.H. Zhao, X.Z. Liao, S. Cheng, E. Ma, Y.T. Zhu, Simultaneously increasing the ductility and strength of nanostructured alloys, Adv. Mater. 18 (2006) 2280e2283. [17] B.Z. Cai, Y. Long, C. Wen, Y.L. Gong, C.J. Li, J.M. Tao, et al., Role of stacking fault energy and strain rate in strengthening of Cu and CueAl alloys, J. Mater. Res. 29 (2014) 1747e1754. [18] J. Schiotz, F.D. Di Tolla, K.W. Jacobsen, Softening of nanocrystalline metals at very small grain sizes, Nature 391 (1998) 561e563. [19] P.V. Liddicoat, X.Z. Liao, Y. Zhao, Y. Zhu, M.Y. Murashkin, E.J. Lavernia, et al., Nanostructural hierarchy increases the strength of aluminium alloys, Nat. Commun. 1 (2010) 63.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Unstable stacking faults in submicron/micron Al grammins in multi-SiCp/multi-Al nanocomposite Wenshu Yang a, *, Ronghua Dong a, Longtao Jiang a, Gaohui Wu a, Murid Hussain b a
Department of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Department of Chemical Engineering, COMSATS Institute of Information Technology, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 7 September 2015
It is difficult to obtain planar defects in aluminum due to its high stacking fault energy, in particular in submicron/micron Al grains. In this work we provide evidence for planar defects in submicron/micron Al grain of composites with multi nano-particles by transmission electron microscope observations. NanoSiC particles (<100 nm) were found within micron-Al grains (>2 mm), while submicron SiC particles (200 e500 nm) were present at the boundary of ultrafine Al grains (100e500 nm). Zigzag defects and linear defects were observed in both the micron-Al grains and ultrafine Al grains. These defects are made up of distortion areas, edge dislocations, stacking faults which contain Frank partial dislocations and twinning. Therefore, these defects are in a state of extreme instability, which would “disappear” under the electron beam irradiation in a few seconds. These results highlight that the increase of interface could lead to the formation of stacking faults, even in the micron Al grains. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Metal-matrix composites (MMCs) Nano-structures Microstructures Liquid metal infiltration
1. Introduction Nano-metallic materials usually demonstrate significant different microstructure and performance to the traditional metal alloys [1,2]. As grain sizes are reduced to the nanometer scale and the percentage of grain boundary atoms increases correspondingly, dislocation sources and pileup are hardly to exist and it is considered that deformation is mainly carried out mostly by the grain boundaries [3,4]. The deformation twins rather than dislocations have been observed in nanocrystalline aluminum at a grain size of 10e35 nm [5], which has been supported by molecular dynamics simulations [6,7]. Therefore, it raises a question that whether the Al matrix nanocomposites also have different microstructure and properties than the traditional Al matrix composites, and this question has not been fully understood yet. Several research efforts have been carried out to investigate the microstructure and performance of nano-reinforced Al matrix composite. It has been widely established that the mechanical properties of Al matrix composites could be significantly improved
* Corresponding author. P. O. 3023 Science Park, No. 2 Yikuang Street, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.vacuum.2015.09.002 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
by the addition of low content (usually < 10 wt.%) nanoparticles [8e10], nanotubes [11,12], nanowires [13] and graphene nanoplates [14], implying that the nano-reinforcements demonstrate more distinguished strengthening effect than the micron-size reinforcements. H.R. Ezatpour et al. [15] prepared 6061 Al matrix composites reinforced by nano-Al2O3 particles and the 0.2% yield strength of 1.5wt.% Al2O3/6061Al composite was about two times of the Al matrix. Currently, the microstructure characters of Al matrix (for instance grain size and form of defects) in nanocomposites have been slightly affected by the nano-reinforcements due to their low content. It is suggested that high content nano-reinforcements might be introduced to stimulate the nano-effect in Al matrix. However, to the knowledge of the authors, microstructure of high content Al matrix nanocomposites (>25 vol.%) have not been reported yet. Therefore, in the present work, 30 vol.% multi-SiC particles (average size of 30 and 220 nm, respectively) were introduced into pure Al by pressure infiltration method. It is very interesting to state that the grain size of Al grain also demonstrated quasi-multi distribution and stacking faults rather than dislocations were found in Al matrix. The microstructure of Al matrix and the stacking faults of multi-SiCp/multi-Al nanocomposite have been explored in depth.
2
W. Yang et al. / Vacuum 122 (2015) 1e5
2. Materials and methods SiC particles with average size of 30 and 220 nm (supplied by Harbin Xin Ceramics Co., Ltd. China) respectively were used in the present work. The two kinds of SiC particles were ball-milled for 48 under Ar flow, while the weight ratio of 30 nm SiC/220 nm SiC particles was 1/80. The microstructure and corresponding size distribution of the mixed SiC powders is shown in Fig. 1a and b, respectively. It is obvious that the mixed SiC powders demonstrated a multi-size distribution (30 and 220 nm), implying that the balling process has minor effect on the size of SiC particles. Afterward, the mixed SiC particles were adopted to increase the amount of SiC content in the final composites. The mixed SiC particles were then put into a steel mold and further pressed to the set volume content (30 vol.%). Meanwhile, high pure Al (supplied by Northeast Light Alloy Co., Ltd. China) was molten at 800 C under Ar. The preheating temperatures for the preform and pressure infiltration dies were 500 and 760 C, respectively. During the infiltration process, a pressure of 10 MPa was applied and maintained for 10 min, followed by the solidification of the composites in air. In order to release the residual thermal stress, annealing treatment was performed before microstructure observation and hardness tests. Microstructure of mixed SiC particles was observed by FEI Sirion Quanta 200 scanning electron microscope (SEM). Further observation of 30 vol.% multi-SiCp/multi-Al nanocomposite was performed on JEM-2010F high resolution transmission electron microscopy (HRTEM). 3. Results and discussion
Fig. 2. TEM micrograph of nanocomposites with multi particles.
area. However, only one set of Al diffraction patterns was found, implying that these highly oriented SiC particles were distributed in a large Al grain, whose size was larger than 1.4 mm. Therefore, it could be concluded that the submicron-SiC particles were found at the boundaries of submicron Al grains while the nano-SiC particles were distributed within a micron Al grains. 3.2. Unstable stacking faults in Al matrix of multi-SiCp/multi-Al nanocomposite
3.1. Microstructure of multi-SiCp/multi-Al nanocomposite Representative microstructure of the studied or under study SiCp/Al composite is shown in Fig. 2. SiC particles were uniformly distributed in the Al matrix, and no cluster of SiC particles was found. Further observation of Al matrix surrounding submicron-SiC (>200 nm) and nano-SiC (<100 nm) indicated that the grain size of Al also demonstrated multi-distribution, which has been shown in Fig. 3a and c, respectively. Submicron SiC particles were usually surrounded by several submicron Al grains (Fig. 3b), with average size of about 400 nm. However, no clear grain boundaries were observed in the Al matrix surrounding nano-SiC particles (Fig. 3c). Selected area electron diffraction (SAED) of Fig. 3d shows that quasi-polycrystalline rings of SiC were detected, indicating that the direction of SiC particles were significantly different within this
Two kinds of defects (linear and zigzag) were found in Al matrix, as shown in Fig. 4a and b. These defects were found both in submicron and micron Al matrix, and were only composed of Al element according to EDS analysis results (Fig. 4c). Therefore, these defects were not the segregation of the alloying elements. Moreover, it should be noted that linear or tangled dislocations in Al matrix, which are usually found near sharp corners of SiC particles due to thermal mismatch between SiC particles and Al matrix, have not been found in the present work. The typical linear defect and its Fast Fourier Transform (FFT) patterns are shown in Fig. 5a and b, respectively. The observed linear defects were usually perpendicular to the [011] zone axes of Al matrix. Frank partial dislocations (marked by the blue box in Fig. 5c) found in the Fourier-filtered image of the selected region in Fig. 5c, implied the existence of stacking faults in the submicron
Fig. 1. (a) SEM micrograph of multi SiC particles. (b) Particle size distributions. The size of SiC particles is 20e500 nm, the two peaks are 30 and 220 nm respectively, and the average size is 106 nm.
W. Yang et al. / Vacuum 122 (2015) 1e5
3
Fig. 3. TEM micrographs of nanocomposites. (a) TEM micrograph of nanocomposites with submicron SiC particles. (b) The schematic diagram of composites. (c) and (d) Bright field image and dark field image of nanocomposites with nano SiC particles, respectively. (e) Selected area electron diffraction of composites in (c) and (d).
Fig. 4. HRTEM micrograph of the matrix of composites. (a) Two kinds of defects (linear and zigzag) were found in Al matrix. (b) Linear defects were found in the interface of nano SiC particles and Al matrix. (c) EDS results of Al matrix.
Fig. 5. HRTEM micrographs of linear defect. (a) The typical linear defect. (b) Fast Fourier Transform patterns of the defect. (c) The Fourier-filtered image of the defect.
and micron Al matrix. It showed that the distortion caused by the deviation of atom from equilibrium position and edge dislocations were also observed in the linear defects. Detailed analysis of the zigzag defect is shown in Fig. 6. The FFT
patterns (Fig. 6b) indicate that the zigzag defects were also perpendicular to the [011] zone axes of Al matrix. The Fourierfiltered images of the three selected regions in the Fig. 6a are shown in Fig. 6cee, respectively. Similar to the linear defects, the
4
W. Yang et al. / Vacuum 122 (2015) 1e5
Fig. 6. HRTEM micrographs of zigzag defect. (a) The typical zigzag defect. (b) Fast Fourier transform patterns of the defect. (c), (d) and (e) The Fourier-filtered images of the three selected regions.
zigzag defects also contain twinning (Fig. 6c), distortion area (the black circles in Fig. 6d) and Frank partial dislocations (the blue box in Fig. 6e). The classic schematic of Frank partial dislocations and the re-constructed schematic of partial dislocations according to FFT results in the present work are shown in Fig. 7a and b, respectively. It is clear that they are equivalent to each other. Therefore, it could be concluded that stacking faults were found in the Al matrix of 30 vol.% multi-SiCp/multi-Al nanocomposite. Usually, due to its high stacking fault energy, it is difficult to obtain stacking faults in submicron and micron grains in Al alloys [16]. In coarse-grained Al metals, plastic deformation is mainly achieved by dislocations [17]. Dislocations can move through the crystal grains and can interact with each other. However, as grain sizes are reduced to the nanometer scale and the percentage of grain boundary atoms increases correspondingly, this traditional view of dislocation-driven plasticity in Al metals needs to be reconsidered. V. Yamakov et al. [6] have demonstrated that, in contrast to coarsegrained Al, mechanical twinning may play an important role in the deformation behavior of nanocrystalline Al by molecular dynamics simulation. M. Chen et al. have provided evidence for deformation twinning in nanocrystalline aluminum by transmission electron microscope observations. Grain boundary plays a vital role in this transformation. For instance, more than 10% atoms are in grain boundary when the grain size is less than 20 nm [3]. In coarse-grained Al metals, grain boundaries often hinder their transmission, creating a dislocation pile-up at the boundary and thereby making the material more difficult to deform. As the grain sizes are reduced to nanoscale, several partial or imperfect dislocations nucleate in different
regions of the grain boundary [18], in contrast to the emission of full or perfect dislocations known from coarse grained metals [19]. In our present work, the grain size of Al matrix was in submicron and micron scale. However, different to common Al metals, high density SiCeAl interface existed within submicron/micron Al matrix. Moreover, different to that in Al grain boundary, the Al atoms surrounding SiCeAl interface could be easily moved since SiC could be considered as a rigid phase than Al. Therefore, the thickness of the area that affected by the SiCeAl interface might be more than 5 nm, while more than 30% Al atoms are in the SiCeAl interface affected area in 30 vol.% multi-SiCp/multi-Al nanocomposite. Therefore, stacking faults were found in the present work even Al grain size was in submicron or micron scale. Hence, not only the increase of grain boundary, but also increase of interface could lead to the formation of stacking faults. It is very interesting that the linear and zigzag defects were very unstable, and would disappear during the HRTEM observation under 200 kV electron beam irradiation, as shown in Video 1 and Fig. 8. During recording, the observed two defects were disappeared after 38 and 102 s, respectively, indicating that these defects were in unstable state. The linear and zigzag defects have not been observed in the common Al alloy, and they could be observed in the present work might be due to the presence of unstable stacking faults [4]. During HRTEM observation, these linear and zigzag defects were under radiation of high energy electrons, and the Frank partial dislocations could be transformed to become perfect dislocations, leading to the disappearance behavior of these defects. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.vacuum.2015.09.002.
4. Conclusions The high volume fraction nanocomposites with multi-particles show a unique microstructure. Due to the multi sized particles, the grain size in the matrix showed two different states. The submicron grains were existed around the bigger particles and the micron grains swallowed the smaller particles. Frank partial dislocations and planar defects formed instead of perfect dislocations in submicron/micron Al grains. Moreover, the defects were very unstable, and would disappear during the HRTEM observation under 300 kV electron beam irradiation. The stacking faults in our present work are different to the planar defects investigated in nanocrystalline Al. The results demonstrate not only the increase of grain boundary, but also the increase of interface could lead to the formation of stacking faults, while, the stacking faults could exist
Fig. 7. The schematic of Frank partial dislocations. (a) The classic schematic of Frank partial dislocations. (b) the re-constructed schematic of partial dislocations according to FFT results in the present work.
W. Yang et al. / Vacuum 122 (2015) 1e5
5
Fig. 8. HRTEM observation of defects under 300 kV electron beam irradiation.
even in the submicron or micron Al grains. Acknowledgments This work has been supported by “the Fundamental Research Funds for the Central Universities” (Grant No. HIT. NSRIF. 20161) and “the National Natural Science Foundation of China” (51501047). References [1] Y. Yue, P. Liu, Z. Zhang, X. Han, E. Ma, Approaching the theoretical elastic strain limit in copper nanowires, Nano Lett. 11 (2011) 3151e3155. mez, E. Restrepo-Parra, P.J. Arango, X-ray [2] D. Escobar, R. Ospina, A.G. Go microstructural analysis of nanocrystalline TiZrN thin films by diffraction pattern modeling, Mater. Charact. 88 (2014) 119e126. [3] H. Van Swygenhoven, Polycrystalline materials. Grain boundaries and dislocations, Science 296 (2002) 66e67. [4] J. Schiotz, K.W. Jacobsen, A maximum in the strength of nanocrystalline copper, Science 301 (2003) 1357e1359. [5] M. Chen, E. Ma, K.J. Hemker, H. Sheng, Y. Wang, X. Cheng, Deformation twinning in nanocrystalline aluminum, Science 300 (2003) 1275e1277. [6] V. Yamakov, D. Wolf, S.R. Phillpot, A.K. Mukherjee, H. Gleiter, Dislocation processes in the deformation of nanocrystalline aluminium by moleculardynamics simulation, Nat. Mater. 1 (2002) 45e48. [7] V. Yamakov, D. Wolf, S.R. Phillpot, H. Gleiter, Deformation twinning in nanocrystalline Al by molecular dynamics simulation, Acta Mater. 50 (2002) 5005e5020. [8] H.R. Ezatpour, S.A. Sajjadi, M.H. Sabzevar, Y.Z. Huang, An investigation of the tensile and compressive properties of Al6061 and its nanocomposites in ascast state and in extruded condition, Mater. Sci. Eng. A 607 (2014) 589e595.
[9] Z.B. Lei, K. Zhao, Y.G. Wang, L.N. An, Thermal expansion of Al matrix composites reinforced with hybrid micro-/nano-sized Al2O3 particles, J. Mater. Sci. Technol. 30 (2014) 61e64. [10] D.S. Zhou, F. Qiu, Q.C. Jiang, Simultaneously increasing the strength and ductility of nano-sized TiN particle reinforced AleCu matrix composites, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 596 (2014) 98e102. [11] X.D. Yang, E.Z. Liu, C.S. Shi, C.N. He, J.J. Li, N.Q. Zhao, et al., Fabrication of carbon nanotube reinforced Al composites with well-balanced strength and ductility, J. Alloys Compd. 563 (2013) 216e220. [12] K. Kondoh, H. Fukuda, J. Umeda, H. Imai, B. Fugetsu, Microstructural and mechanical behavior of multi-walled carbon nanotubes reinforced AleMgeSi alloy composites in aging treatment, Carbon 72 (2014) 15e21. [13] Y.B. Tang, Y.Q. Liu, C.H. Sun, H.T. Cong, AlN nanowires for Al-based composites with high strength and low thermal expansion, J. Mater. Res. 22 (2007) 2711e2718. [14] J.W. Yang, X.R. Chen, B. Song, Interaction of graphene-on-Al(111) composite with D-Glucopyranose and its application in biodetection, J. Phys. Chem. C 117 (2013) 8475e8480. [15] H.R. Ezatpour, S.A. Sajjadi, M.H. Sabzevar, Y.Z. Huang, An investigation of the tensile and compressive properties of Al6061 and its nanocomposites in ascast state and in extruded condition, Mater. Sci. Eng. A 607 (2014) 589e595. [16] Y.H. Zhao, X.Z. Liao, S. Cheng, E. Ma, Y.T. Zhu, Simultaneously increasing the ductility and strength of nanostructured alloys, Adv. Mater. 18 (2006) 2280e2283. [17] B.Z. Cai, Y. Long, C. Wen, Y.L. Gong, C.J. Li, J.M. Tao, et al., Role of stacking fault energy and strain rate in strengthening of Cu and CueAl alloys, J. Mater. Res. 29 (2014) 1747e1754. [18] J. Schiotz, F.D. Di Tolla, K.W. Jacobsen, Softening of nanocrystalline metals at very small grain sizes, Nature 391 (1998) 561e563. [19] P.V. Liddicoat, X.Z. Liao, Y. Zhao, Y. Zhu, M.Y. Murashkin, E.J. Lavernia, et al., Nanostructural hierarchy increases the strength of aluminium alloys, Nat. Commun. 1 (2010) 63.