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AZ91D nanocomposites with high ductility at room temperature
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Plastic deformation mechanism of MWCNTs/AZ91D nanocomposites with high ductility at room temperature ⁎
⁎⁎
Zhirui Lia, Congyang Zhanga,b, , Yongsheng Yea, , Youlu Yuana, Haihua Wua, Wenzhen Lic,
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a
Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, PR China China Energy Conservation DADI Environmental Remediation Co. Ltd., Beijing 100082, PR China c School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium matrix nanocomposites High ductility High strain zones Twining Slip
1.0 wt% MWCNTs/AZ91D magnesium matrix nanocomposites were prepared by metal mold gravity casting process assisted by high-intensity ultrasonic dispersion, and the ductility was further improved by T4 solid solution treatment. The tensile test results at room temperature revealed that the tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. The ductility of magnesium matrix nanocomposites was significantly improved by the addition of Multiwalled carbon nanotubes (MWCNTs) and T4 solid solution treatment. SEM and TEM were used to observe the microstructure of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation at room temperature. A considerable number of slip bands and twins were found in the microstructure of the nanocomposites. High strain zones were formed around MWCNTs during the tensile process at room temperature. A large number of dislocations and stacking faults were generated in the high strain zones, which could be used as the origin of slip bands and twins, so MWCNTs can promote slip and twining. MWCNTs can further activate non-basal slip and cross slip, and improve the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites.
1. Introduction As a kind of structural metal, magnesium alloys have the advantages of light weight, high specific strength, high specific stiffness and good damping performance, etc., so magnesium alloys have a wide application prospect in aerospace, automobile, 3C products, sports equipment and other fields. However, it has poor plastic deformation capacity at room temperature due to its HCP structure, which limits its practical application. In recent years, in order to improve the ductility and mechanical properties of magnesium alloys, the preparation of magnesium matrix nanocomposites with better mechanical properties has become a hot topic. The common reinforcements of magnesium matrix nanocomposites include SiC nanoparticles, carbon nanotubes, graphene nanoplatelets, nano-diamond, etc. Carbon nanotubes (CNTs) are characterized by low density and excellent mechanical properties. Although their density is only one fifth of that of steel, their strength can reach 100 times that of steel and they have good thermal stability [1]. Therefore, CNTs are
considered as an ideal reinforcement for preparing magnesium matrix nanocomposites. A lot of scholars at home and abroad have investigated magnesium matrix nanocomposites reinforced by CNTs. They found that the mechanical properties of magnesium matrix nanocomposites reinforced by CNTs were significantly better than magnesium alloys, especially the ductility could be greatly improved [2–7]. G.Q. Han et al. [6] suggested that CNTs addition could promote tensile twinning in Mg. However, there was no specific explanation for how CNTs promote twinning. C.S. Goh et al. [7] deduced that cross slip in non-basal slip planes may be activated by the presence of CNTs which is responsible for the increased ductility observed, but further verifications using TEM are required to confirm this deduction. Therefore, the detailed mechanisms pertaining to the improved ductility need to be further investigated. In this paper, MWCNTs/AZ91D magnesium matrix nanocomposites reinforced by MWCNTs were prepared by high-intensity ultrasonic dispersion method and metal mold gravity casting process, and the ductility was further improved by T4 solid solution treatment. Magnesium matrix nanocomposites with high ductility at room
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Correspondence to: C. Zhang, Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, PR China. ⁎⁎ Corresponding authors. E-mail addresses: [email protected] (C. Zhang), [email protected] (Y. Ye), [email protected] (W. Li). https://doi.org/10.1016/j.matchar.2019.110020 Received 6 August 2019; Received in revised form 21 October 2019; Accepted 14 November 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Zhirui Li, et al., Materials Characterization, https://doi.org/10.1016/j.matchar.2019.110020
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temperature were obtained. The microscopic mechanism of high ductility at room temperature was studied in detail, and the influence of MWCNTs on the ductility of nanocomposites was revealed, providing reference for the preparation and development of cast magnesium matrix nanocomposites with high ductility. 2. Materials and methods 2.1. Preparation A commercial AZ91D alloy ingot was selected as the matrix of MWCNTs/AZ91D magnesium matrix nanocomposites, and its chemical composition (wt%) is 8.5%–9.5%Al, 0.45%–0.9 0%Zn, 0.17%–0.40% Mn, and balance Mg. S-MWCNT-2040 multi-walled carbon nanotubes were used as the reinforcements. The external diameter of MWCNTs was 20–40 nm and the length of MWCNTs was less than 5 μm. Firstly, about 2 kg AZ91D alloy was placed into a heat-resistant metal crucible, and the alloy was heated until it melted completely by a magnesium alloy resistance melting furnace. Then the alloy became a semisolid slurry by cooling to 595 °C, and the semisolid magnesium alloy slurry was mechanically stirred. MWCNTs with mass fraction of 1.0% were mixed into the slurry after vortices were generated during the stirring process. The whole stirring time should not be too long and should be controlled within 5 min. Then, the slurry was rapidly heated to 620–630 °C unit it was completely liquid, and high-intensity ultrasonic dispersion processing was started. In order to disperse better MWCNTs within the matrix, a high-intensity ultrasonic wave with a 20 kHz, a maximum 1.4 kW power output and a titanium alloy waveguide of 40 mm in diameter was used for processing MWCNTs/AZ91D melt. After 15 min of ultrasonic processing, the furnace temperature was raised to 700 °C for metal mold gravity casting. The metal mold was preheated to 300 °C, and 1.0 wt% MWCNTs/AZ91D magnesium matrix nanocomposites were obtained by cooling after casting. In order to avoid oxidation and burning of the magnesium alloy, the magnesium melt was protected by CO2 + 0.2%SF6 (volume fraction) during the whole melting and casting process, and the ratio of mixed gas and gas flow could change according to specific operations in the experiment. For comparison, AZ91D magnesium alloy samples without MWCNTs were prepared by the same method. The 1.0 wt% MWCNTs/AZ91D and AZ91D samples (100 × 6 × 4 mm) were subjected to T4 solid solution treatment at 415 °C for 24 h followed by water quenching at 75–85 °C. Heat treatment should also be carried out under the protection of CO2 + SF6.
Fig. 1. The representative stress-strain curves of AZ91D magnesium alloys and 1.0 wt% MWCNTs/AZ91D nanocomposites at room temperature.
temperature. The mechanical properties of as-cast AZ91D alloys were very poor. The mechanical properties were significantly improved after T4 solution heat treatment or the addition of MWCNTs, while the mechanical properties of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment were extraordinarily good. As shown in Table 1, according to the average, the tensile strength and elongation of as-cast AZ91D alloys were only 130.7 MPa and 1.7%, the tensile strength and elongation of AZ91D alloys after T4 solid solution treatment were improved to 189.7 MPa and 5.5%, and the tensile strength and elongation of as-cast 1.0 wt% MWCNTs/AZ91D nanocomposites were improved to 191.1 MPa and 3.7%, respectively. The tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. Compared with AZ91D alloys after T4 solid solution treatment, the tensile strength and elongation were improved by 48.5% and 180.0%, respectively. Compared with as-cast 1.0 wt% MWCNTs/ AZ91D nanocomposite, the tensile strength and elongation were improved by 47.4% and 316.2%, respectively. As the brittle phases of βMg17Al12 at grain boundary are often the crack source during the tensile process, T4 solid solution treatment makes the as-cast structure of βMg17Al12 dissolve in the matrix and form a single α-Mg matrix, which reduces the initiation of cracks in the tensile process and improves the tensile strength and elongation [8]. Therefore, T4 solid solution treatment or the addition of MWCNTs can improve the ductility of AZ91D alloys, but their combined action can extraordinarily improve the ductility of AZ91D alloys.
2.2. Characterization The tensile tests were carried out by a SANS 4105 test machine at a strain rate of 0.5 mm/min at room temperature, following GB/T 2282002 of China National Standards, subsize flat tensile specimens (25 mm in gage length, 6 mm in width, and 4 mm in thickness) machined from the gravity casting bars. All of the tensile properties were obtained based on the average of three tests. The morphologies of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile tests were characterized by using a scanning electron microscope (SEM, FEI-Siron200 with a field emission source). The microstructure of MWCNTs in the matrix and the slip bands and twins generated in the matrix of the nanocomposites after the tensile tests were investigated by a transmission electron microscope (TEM, Tecnai G2 F30 with a field emission source).
3.2. Microstructure Fig. 2(a) and (b) are the SEM microstructure of as-cast AZ91D alloy and 1.0 wt% MWCNTs/AZ91D nanocomposites, in which β-Mg17Al12 coarse plates and lamellar were formed along the grain boundaries. However, owing to the addition of MWCNTs, it can be seen that the grain and β-Mg17Al12 phases size of 1.0 wt% MWCNTs/AZ91D Table 1 Average tensile properties of 1.0 wt% MWCNTs/AZ91D magnesium nanocomposites and AZ91D magnesium alloys at room temperature.
3. Results and discussion
Composition
Heat treatment
σb/MPa
3.1. Mechanical property
AZ91D 1.0 wt% MWCNTs/AZ91D AZ91D 1.0 wt% MWCNTs/AZ91D
F F T4 T4
130.7 191.1 189.7 281.7
Fig. 1 is the representative stress-strain curves of AZ91D magnesium alloys and 1.0 wt% MWCNTs/AZ91D nanocomposites at room 2
± ± ± ±
δ/% 4.1 9.6 16.0 2.9
1.7 ± 0.6 3.7 ± 0.8 5.5 ± 0.7 15.4 ± 1.6
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Fig. 2. SEM images (SE mode) of AZ91D alloy and 1.0 wt% MWCNTs/AZ91D nanocomposites. (a) As-cast AZ91D alloy. (b) As-cast 1.0 wt% MWCNTs/AZ91D nanocomposites. (c) AZ91D-T4 after the tensile plastic deformation. (d) 1.0 wt% MWCNTs/AZ91D-T4 after the tensile plastic deformation.
boundaries became clear after the specimens were etched with acetic picric, and MWCNTs in the matrix emerged from the twin boundaries. Although the grain boundaries and twin boundaries often become the crack initiation, there are no obvious cracks at the grain boundaries and twin boundaries due to the bridge effect by MWCNTs that retards the crack propagation [6,12,13], and there are almost no MWCNTs at the crack in Fig. 3(c), so the cracks of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites tend to initiate in the absence of MWCNTs. As shown in Fig. 4(a), MWCNTs are long curved tubes, which bind well with the matrix, and no interfacial cracking behavior is found in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite after the tensile plastic deformation at room temperature. A lot of stacking faults is observed around MWCNTs. The presence of MWCNTs in Fig. 4(b) makes the surrounding matrix uneven. Geometrically necessary dislocations were generated because of the coefficient of thermal expansion mismatch between AZ91D matrix and MWCNTs in the gravity casting and T4 solid solution treatment process [14]. The deformation of MWCNTs and AZ91D matrix was incompatible due to elastic modulus mismatch between them during the following tensile process, resulting in a high strain zone around MWCNTs [15]. The deformation around MWCNTs was serious and dislocation density increased sharply. These dislocations caused disarray of stacking sequence in the crystal structure which was different from the normal arrangement, leading to the extension of dislocations into stacking faults in the matrix [16]. G.Q. Han et al. [6] suggested that CNTs could promote tensile twinning in the plastic deformation process of CNTs/Mg composites. According to the inhomogeneous nucleation theory of twins, some dislocations in the matrix were arranged and decomposed into single or multiple stacking faults during the plastic deformation process, which form twin nuclei [17]. Therefore, these dislocations and stacking faults around MWCNTs in the matrix were inevitably related to the role of MWCNTs in promoting tensile twinning. A lot of dislocations and stacking faults were introduced around MWCNTs due to the coefficient of thermal expansion and elastic modulus mismatch between AZ91D matrix and MWCNTs during the heat treatment and plastic deformation process, which became the origin of the twins. The stress concentration in high
nanocomposites was refined. Refinement of grain and Mg17Al12 phases contributes to the strength and toughness of nanocomposites. Fig. 2(c) and (d) are the SEM microstructure of AZ91D-T4 alloy and 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation, in which β-Mg17Al12 phases were almost completely dissolved into α-Mg matrix. There are a lot of twins in the matrix of the nanocomposites, but no twins are found in the AZ91D-T4 alloy. Thus, 1.0 wt % MWCNTs/AZ91D-T4 nanocomposites are more likely to form twins. As shown in Fig. 3(a), it is clear that these twins are lamellar twins in the matrix of the nanocomposites after the tensile plastic deformation at room temperature. Most twins in a grain are parallel to each other and run through the whole grain. This type of twins belongs to tensile twins [9], but there are a few twins at an angle to them, resulting in twin intersections (Fig. 3(b)). The easily activated basal slip system of magnesium alloys at room temperature is < a > dislocation slip due to the typical HCP structure of magnesium, and the direction of basal slip is parallel to basal plane and perpendicular to c-axis, so the strain of c-axis direction cannot be coordinated. The strain of c-axis could be coordinated by twinning during the process of tensile plastic deformation at room temperature [10]. When the stress concentration was caused by the slip hindered and the orientation of the grains favored twinning, twins were formed in the grains, and the twins of the same twin system were parallel to each other. The adjacent or alternate twinning planes were activated in succession under the action of further tensile strain, resulting in twin intersections. Moreover, the existing twins were prone to secondary or multiple twining, which coordinated crystal orientation and released stress concentration in a certain extent [11]. In Fig. 3(c), twin boundaries and cracks can be clearly seen, and there are obvious white particles or tubes in the twin boundary area. The White particles are dimly observed in the matrix, but there are almost no white particles or tubes at cracks. Fig. 3(d) is a further magnification of the twin boundaries, and EDS analysis confirmed that these particles or tubes are MWCNTs in the matrix (Fig. 3(e)). MWCNTs were dispersed in AZ91D matrix by high-intensity ultrasonic dispersion method, and twins were formed in the matrix during the tensile plastic deformation at room temperature. The grain boundaries and twin 3
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(a)
(b)
(c)
(d)
crack
MWCNTs
(e)
Fig. 3. SEM images (SE mode) of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation. (a) Twins in the matrix. (b) Twin intersections. (c) The twin boundaries and cracks. (d) The further magnification of the twin boundaries. (e) EDS analysis of the white particles or tubes.
(b)
(a) MWCNTs
MWCNTs
stacking faults stacking faults
Fig. 4. HRTEM images of MWCNTs in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites. 4
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(a)
(b) slip bands
dislocations
(c)
(d) stacking faults
slip stacking faults
slip
MWCNTs
1.38nm ˄002˅Al78Mn22 MWCNTs Fig. 5. TEM images of slip and slip bands formed by the tensile plastic deformation of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites. (a) The slip bands consisting of a lot of parallel slip lines. (b) Some local slip bands and slip lines. (c) & (d) The stacking faults and slip lines around MWCNTs.
strain zones around MWCNTs provided power for the nucleation and growth of twins. Many slip bands appeared in the matrix of 1.0 wt% MWCNTs/ AZ91D-T4 nanocomposites after the tensile plastic deformation at room temperature. In Fig. 5(a), there are the slip bands consisting of a lot of parallel slip lines in the matrix. The distance between the slip lines is 20–50 nm, and there are also a high density of dislocations between the slip lines. In Fig. 5(b), there are some local slip bands and slip lines. The local slip bands consist of several slip lines, and some dislocations exist in the slip bands. In Fig. 5(c) and (d), MWCNTs in the matrix are dark rod-shaped, and there were some stacking faults around MWCNTs and some slip lines parallel to the tube walls of MWCNTs after the tensile tests at room temperature. A lot of dislocations and stacking faults were formed in the high strain zones around MWCNTs during the plastic deformation process, and the dislocations moved in the matrix under the action of tensile strain, forming the dislocation slip lines. The slip lines were parallel to the tube walls of MWCNTs, because dislocation slip that was not parallel to the tube walls of MWCNTs was hindered by MWCNTs, and the slip parallel to the tube wall of MWCNTs was easy to activate. The slip lines in Fig. 5(a) and (b) may evolve from the dislocations in the high strain zones around MWCNTs. The formation of the slip lines was generally accompanied by the high density of dislocations because of the high strain zones. The difference in bandwidth and distance between slip lines s in Fig. 5(a) and (b) may be due to the different distribution of MWCNTs and orientation of grains. Basal slip is the most easily activated deformation mechanism of magnesium alloys at room temperature, and its critical shear stress (CRSS) is only 0.5–0.7 MPa [10], which provides strain parallel to basal plane. Therefore, dislocations around MWCNTs were more likely to move in basal planes when the tube walls of MWCNTs were parallel to basal planes, thus forming a lot of basal slip bands. Fig. 6 is a schematic of the high strain zones and induced slip lines around MWCNTs. If the tube
hinder
basal slip bands non-basal slip high strain zone
˄0001˅
Fig. 6. Schematic of the high strain zones and induced slip lines around MWCNTs.
walls of MWCNTs are parallel to basal planes, basal slip will be activated around MWCNTs during the plastic deformation process, forming a lot of basal slip bands. If the tube walls of MWCNTs are not parallel to basal planes, basal slip cannot be activated and serious stress concentration will be caused around MWCNTs. Dislocations not only moved on basal planes, but also slipped on prismatic and pyramidal planes when strain was high enough [18], which could effectively activate non-basal slip and produced non-basal slip lines parallel to the tube walls of MWCNTs. According to Mises rule [19], each grain of polycrystalline materials with good ductility needs 5 independent slip systems (or twining systems) to sustain random deformation without fracture. The activated basal slip of AZ91D magnesium alloys at room temperature can only provide 2 independent slip systems, which is the 5
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(a)
(b)
twins twins
cross-slip
matrix
dislocations
Fig. 7. TEM images of twins and cross-slip of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation. (a) Parallel twins. (b) Twins and cross-slip.
fundamental reason for the poor plastic deformation capacity of magnesium and magnesium alloys. Non-basal slip activated by MWCNTs improved the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite. Fig. 7 shows twins and high-density dislocations and cross-slip in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite after the tensile plastic deformation at room temperature. In Fig. 7(a), there are three parallel twins of the same type, and a high density of dislocations are formed locally inside the twins. Some curved slip lines exist near the twin boundaries in Fig. 7(b), and there are also a high density of dislocations inside the twins. When basal slip produced geometric hardening and there was no slip plane which was favorable to slip in the grain, twining was activated because of the dislocation caused by stress concentration in the matrix. The addition of MWCNTs made twinning more likely to be activated, and orientation was adjusted by rotation of twins during the twinning process. Under the further tensile strain, dislocations were generated in the twins due to the high strain zones of MWCNTs. When the orientation of twins was favorable to slip, MWCNTs activated further slip, which caused slip and twining to alternate. A lot of slip lines and twins were formed, thus improving the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite. The curved slip lines in Fig. 7(b) are the results of cross-slip [20]. Some non-basal slips were activated around MWCNTs when basal slip and twining were both in hard orientation, and several slip systems were activated in the matrix. The phenomenon of cross-slip occurred when two or more slip plane glided simultaneously or alternately along a common slip direction. Cross-slip was a main carrier of ductility in HCP metals as it allows dynamic recovery during deformation [21]. As shown in Fig. 8, 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites have a high density of dislocations in the matrix after the tensile plastic deformation at room temperature. The dislocation density is highest in the middle. There are also dislocation networks and dislocation pile-up. High strain zones were formed around MWCNTs during the tensile process. With the increase of the degree of plastic deformation, the stress concentration around MWCNTs was more serious, and the dislocation density in the high strain zones also increased. Therefore, the region with the highest dislocation density in Fig. 8 is the high strain zone. Because dislocations in the matrix may encounter MWCNTs during gliding, the movement of dislocations was blocked, and the subsequent dislocations form the same source were piled up, resulting in the dislocation pile-up. According to the above, dislocations with different orientations exist in the matrix, and dislocation tangles occurred when these dislocations with different orientations encountered [22], forming a dislocation network as shown in Fig. 8.
dislocation network
high strain zone
dislocation pile-up
Fig. 8. TEM images of dislocations in the matrix after the high plastic deformation.
investigated, and plastic deformation mechanism of nanocomposites was characterized in detail by using SEM and TEM. Several conclusions can be drawn as follows. (1) The tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. Compared with AZ91D alloys after T4 solid solution treatment, the tensile strength and elongation were improved by 48.5% and 180.0%, respectively. Compared with as-cast 1.0wt% MWCNTs/AZ91D nanocomposite, the tensile strength and elongation were improved by 47.4% and 316.2%, respectively. T4 solid solution treatment or the addition of MWCNTs can improve the ductility of AZ91D alloys, but their combined action can extraordinarily improve the ductility of AZ91D alloys. (2) The cracks of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites tend to initiate in the absence of MWCNTs due to the bridge effect by MWCNTs that retards the crack propagation, rather than at the grain boundaries and twin boundaries. (3) High strain zones were formed around MWCNTs in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 magnesium matrix nanocomposites
4. Conclusions In this work, the tensile behavior of 1.0wt% MWCNTs/AZ91D magnesium matrix nanocomposites with high ductility was 6
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during the process of the tensile tests at room temperature. A lot of dislocations and stacking faults were generated in the high strain zones. These dislocations and stacking faults evolved into slip bands and twins under the action of tensile strain. MWCNTs promoted activation of slip and twinning, which resulted in the high ductility of nanocomposite. (4) The high strain zones around MWCNTs could also activated nonbasal slip and cross slip when the nanocomposites were subjected to large degree of plastic deformation, which was conducive to improving the ductility of magnesium matrix nanocomposites.
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Acknowledgments This work was supported by The National 863 High Technology Research and Development Program of China (No.2013AA031201), the Natural Science Foundation of Hubei Provincial Department of Education (D20171204), the Research Foundation of Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance (2016KJX08), the Applied Basic Research Project of Yichang Science and Technology Bureau(A18-302-a05), and the National Natural Science Foundation of China (51575313). References [1] W.X. Chen, W.L. Chen, Z.D. Xu, et al., Characteristics of carbon nanotubes and highquality composites, Acta Mater. Comp. Sin. 18 (2001) 1–5. [2] J. Hou, W. Du, G. Parande, et al., Significantly enhancing the strength + ductility combination of Mg-9Al alloy using multi-walled carbon nanotubes, J. Alloys Compd. 790 (2019) 974–982. [3] M. Rashad, F. Pana, M. Asif, et al., Enhanced ductility of Mg–3Al–1Zn alloy reinforced with short length multi-walled carbon nanotubes using a powder metallurgy method, Prog. Nat. Sci. : Mater. Inter. 25 (2015) 276–281. [4] M. Paramsothy, X.H. Tan, J. Chan, et al., Carbon nanotube addition to concentrated magnesium alloy AZ81: enhanced ductility with occasional significant increase in strength, Mater. Des. 45 (2013) 15–23.
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Plastic deformation mechanism of MWCNTs/AZ91D nanocomposites with high ductility at room temperature ⁎
⁎⁎
Zhirui Lia, Congyang Zhanga,b, , Yongsheng Yea, , Youlu Yuana, Haihua Wua, Wenzhen Lic,
⁎⁎
a
Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, PR China China Energy Conservation DADI Environmental Remediation Co. Ltd., Beijing 100082, PR China c School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium matrix nanocomposites High ductility High strain zones Twining Slip
1.0 wt% MWCNTs/AZ91D magnesium matrix nanocomposites were prepared by metal mold gravity casting process assisted by high-intensity ultrasonic dispersion, and the ductility was further improved by T4 solid solution treatment. The tensile test results at room temperature revealed that the tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. The ductility of magnesium matrix nanocomposites was significantly improved by the addition of Multiwalled carbon nanotubes (MWCNTs) and T4 solid solution treatment. SEM and TEM were used to observe the microstructure of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation at room temperature. A considerable number of slip bands and twins were found in the microstructure of the nanocomposites. High strain zones were formed around MWCNTs during the tensile process at room temperature. A large number of dislocations and stacking faults were generated in the high strain zones, which could be used as the origin of slip bands and twins, so MWCNTs can promote slip and twining. MWCNTs can further activate non-basal slip and cross slip, and improve the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites.
1. Introduction As a kind of structural metal, magnesium alloys have the advantages of light weight, high specific strength, high specific stiffness and good damping performance, etc., so magnesium alloys have a wide application prospect in aerospace, automobile, 3C products, sports equipment and other fields. However, it has poor plastic deformation capacity at room temperature due to its HCP structure, which limits its practical application. In recent years, in order to improve the ductility and mechanical properties of magnesium alloys, the preparation of magnesium matrix nanocomposites with better mechanical properties has become a hot topic. The common reinforcements of magnesium matrix nanocomposites include SiC nanoparticles, carbon nanotubes, graphene nanoplatelets, nano-diamond, etc. Carbon nanotubes (CNTs) are characterized by low density and excellent mechanical properties. Although their density is only one fifth of that of steel, their strength can reach 100 times that of steel and they have good thermal stability [1]. Therefore, CNTs are
considered as an ideal reinforcement for preparing magnesium matrix nanocomposites. A lot of scholars at home and abroad have investigated magnesium matrix nanocomposites reinforced by CNTs. They found that the mechanical properties of magnesium matrix nanocomposites reinforced by CNTs were significantly better than magnesium alloys, especially the ductility could be greatly improved [2–7]. G.Q. Han et al. [6] suggested that CNTs addition could promote tensile twinning in Mg. However, there was no specific explanation for how CNTs promote twinning. C.S. Goh et al. [7] deduced that cross slip in non-basal slip planes may be activated by the presence of CNTs which is responsible for the increased ductility observed, but further verifications using TEM are required to confirm this deduction. Therefore, the detailed mechanisms pertaining to the improved ductility need to be further investigated. In this paper, MWCNTs/AZ91D magnesium matrix nanocomposites reinforced by MWCNTs were prepared by high-intensity ultrasonic dispersion method and metal mold gravity casting process, and the ductility was further improved by T4 solid solution treatment. Magnesium matrix nanocomposites with high ductility at room
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Correspondence to: C. Zhang, Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, PR China. ⁎⁎ Corresponding authors. E-mail addresses: [email protected] (C. Zhang), [email protected] (Y. Ye), [email protected] (W. Li). https://doi.org/10.1016/j.matchar.2019.110020 Received 6 August 2019; Received in revised form 21 October 2019; Accepted 14 November 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Zhirui Li, et al., Materials Characterization, https://doi.org/10.1016/j.matchar.2019.110020
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temperature were obtained. The microscopic mechanism of high ductility at room temperature was studied in detail, and the influence of MWCNTs on the ductility of nanocomposites was revealed, providing reference for the preparation and development of cast magnesium matrix nanocomposites with high ductility. 2. Materials and methods 2.1. Preparation A commercial AZ91D alloy ingot was selected as the matrix of MWCNTs/AZ91D magnesium matrix nanocomposites, and its chemical composition (wt%) is 8.5%–9.5%Al, 0.45%–0.9 0%Zn, 0.17%–0.40% Mn, and balance Mg. S-MWCNT-2040 multi-walled carbon nanotubes were used as the reinforcements. The external diameter of MWCNTs was 20–40 nm and the length of MWCNTs was less than 5 μm. Firstly, about 2 kg AZ91D alloy was placed into a heat-resistant metal crucible, and the alloy was heated until it melted completely by a magnesium alloy resistance melting furnace. Then the alloy became a semisolid slurry by cooling to 595 °C, and the semisolid magnesium alloy slurry was mechanically stirred. MWCNTs with mass fraction of 1.0% were mixed into the slurry after vortices were generated during the stirring process. The whole stirring time should not be too long and should be controlled within 5 min. Then, the slurry was rapidly heated to 620–630 °C unit it was completely liquid, and high-intensity ultrasonic dispersion processing was started. In order to disperse better MWCNTs within the matrix, a high-intensity ultrasonic wave with a 20 kHz, a maximum 1.4 kW power output and a titanium alloy waveguide of 40 mm in diameter was used for processing MWCNTs/AZ91D melt. After 15 min of ultrasonic processing, the furnace temperature was raised to 700 °C for metal mold gravity casting. The metal mold was preheated to 300 °C, and 1.0 wt% MWCNTs/AZ91D magnesium matrix nanocomposites were obtained by cooling after casting. In order to avoid oxidation and burning of the magnesium alloy, the magnesium melt was protected by CO2 + 0.2%SF6 (volume fraction) during the whole melting and casting process, and the ratio of mixed gas and gas flow could change according to specific operations in the experiment. For comparison, AZ91D magnesium alloy samples without MWCNTs were prepared by the same method. The 1.0 wt% MWCNTs/AZ91D and AZ91D samples (100 × 6 × 4 mm) were subjected to T4 solid solution treatment at 415 °C for 24 h followed by water quenching at 75–85 °C. Heat treatment should also be carried out under the protection of CO2 + SF6.
Fig. 1. The representative stress-strain curves of AZ91D magnesium alloys and 1.0 wt% MWCNTs/AZ91D nanocomposites at room temperature.
temperature. The mechanical properties of as-cast AZ91D alloys were very poor. The mechanical properties were significantly improved after T4 solution heat treatment or the addition of MWCNTs, while the mechanical properties of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment were extraordinarily good. As shown in Table 1, according to the average, the tensile strength and elongation of as-cast AZ91D alloys were only 130.7 MPa and 1.7%, the tensile strength and elongation of AZ91D alloys after T4 solid solution treatment were improved to 189.7 MPa and 5.5%, and the tensile strength and elongation of as-cast 1.0 wt% MWCNTs/AZ91D nanocomposites were improved to 191.1 MPa and 3.7%, respectively. The tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. Compared with AZ91D alloys after T4 solid solution treatment, the tensile strength and elongation were improved by 48.5% and 180.0%, respectively. Compared with as-cast 1.0 wt% MWCNTs/ AZ91D nanocomposite, the tensile strength and elongation were improved by 47.4% and 316.2%, respectively. As the brittle phases of βMg17Al12 at grain boundary are often the crack source during the tensile process, T4 solid solution treatment makes the as-cast structure of βMg17Al12 dissolve in the matrix and form a single α-Mg matrix, which reduces the initiation of cracks in the tensile process and improves the tensile strength and elongation [8]. Therefore, T4 solid solution treatment or the addition of MWCNTs can improve the ductility of AZ91D alloys, but their combined action can extraordinarily improve the ductility of AZ91D alloys.
2.2. Characterization The tensile tests were carried out by a SANS 4105 test machine at a strain rate of 0.5 mm/min at room temperature, following GB/T 2282002 of China National Standards, subsize flat tensile specimens (25 mm in gage length, 6 mm in width, and 4 mm in thickness) machined from the gravity casting bars. All of the tensile properties were obtained based on the average of three tests. The morphologies of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile tests were characterized by using a scanning electron microscope (SEM, FEI-Siron200 with a field emission source). The microstructure of MWCNTs in the matrix and the slip bands and twins generated in the matrix of the nanocomposites after the tensile tests were investigated by a transmission electron microscope (TEM, Tecnai G2 F30 with a field emission source).
3.2. Microstructure Fig. 2(a) and (b) are the SEM microstructure of as-cast AZ91D alloy and 1.0 wt% MWCNTs/AZ91D nanocomposites, in which β-Mg17Al12 coarse plates and lamellar were formed along the grain boundaries. However, owing to the addition of MWCNTs, it can be seen that the grain and β-Mg17Al12 phases size of 1.0 wt% MWCNTs/AZ91D Table 1 Average tensile properties of 1.0 wt% MWCNTs/AZ91D magnesium nanocomposites and AZ91D magnesium alloys at room temperature.
3. Results and discussion
Composition
Heat treatment
σb/MPa
3.1. Mechanical property
AZ91D 1.0 wt% MWCNTs/AZ91D AZ91D 1.0 wt% MWCNTs/AZ91D
F F T4 T4
130.7 191.1 189.7 281.7
Fig. 1 is the representative stress-strain curves of AZ91D magnesium alloys and 1.0 wt% MWCNTs/AZ91D nanocomposites at room 2
± ± ± ±
δ/% 4.1 9.6 16.0 2.9
1.7 ± 0.6 3.7 ± 0.8 5.5 ± 0.7 15.4 ± 1.6
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Fig. 2. SEM images (SE mode) of AZ91D alloy and 1.0 wt% MWCNTs/AZ91D nanocomposites. (a) As-cast AZ91D alloy. (b) As-cast 1.0 wt% MWCNTs/AZ91D nanocomposites. (c) AZ91D-T4 after the tensile plastic deformation. (d) 1.0 wt% MWCNTs/AZ91D-T4 after the tensile plastic deformation.
boundaries became clear after the specimens were etched with acetic picric, and MWCNTs in the matrix emerged from the twin boundaries. Although the grain boundaries and twin boundaries often become the crack initiation, there are no obvious cracks at the grain boundaries and twin boundaries due to the bridge effect by MWCNTs that retards the crack propagation [6,12,13], and there are almost no MWCNTs at the crack in Fig. 3(c), so the cracks of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites tend to initiate in the absence of MWCNTs. As shown in Fig. 4(a), MWCNTs are long curved tubes, which bind well with the matrix, and no interfacial cracking behavior is found in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite after the tensile plastic deformation at room temperature. A lot of stacking faults is observed around MWCNTs. The presence of MWCNTs in Fig. 4(b) makes the surrounding matrix uneven. Geometrically necessary dislocations were generated because of the coefficient of thermal expansion mismatch between AZ91D matrix and MWCNTs in the gravity casting and T4 solid solution treatment process [14]. The deformation of MWCNTs and AZ91D matrix was incompatible due to elastic modulus mismatch between them during the following tensile process, resulting in a high strain zone around MWCNTs [15]. The deformation around MWCNTs was serious and dislocation density increased sharply. These dislocations caused disarray of stacking sequence in the crystal structure which was different from the normal arrangement, leading to the extension of dislocations into stacking faults in the matrix [16]. G.Q. Han et al. [6] suggested that CNTs could promote tensile twinning in the plastic deformation process of CNTs/Mg composites. According to the inhomogeneous nucleation theory of twins, some dislocations in the matrix were arranged and decomposed into single or multiple stacking faults during the plastic deformation process, which form twin nuclei [17]. Therefore, these dislocations and stacking faults around MWCNTs in the matrix were inevitably related to the role of MWCNTs in promoting tensile twinning. A lot of dislocations and stacking faults were introduced around MWCNTs due to the coefficient of thermal expansion and elastic modulus mismatch between AZ91D matrix and MWCNTs during the heat treatment and plastic deformation process, which became the origin of the twins. The stress concentration in high
nanocomposites was refined. Refinement of grain and Mg17Al12 phases contributes to the strength and toughness of nanocomposites. Fig. 2(c) and (d) are the SEM microstructure of AZ91D-T4 alloy and 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation, in which β-Mg17Al12 phases were almost completely dissolved into α-Mg matrix. There are a lot of twins in the matrix of the nanocomposites, but no twins are found in the AZ91D-T4 alloy. Thus, 1.0 wt % MWCNTs/AZ91D-T4 nanocomposites are more likely to form twins. As shown in Fig. 3(a), it is clear that these twins are lamellar twins in the matrix of the nanocomposites after the tensile plastic deformation at room temperature. Most twins in a grain are parallel to each other and run through the whole grain. This type of twins belongs to tensile twins [9], but there are a few twins at an angle to them, resulting in twin intersections (Fig. 3(b)). The easily activated basal slip system of magnesium alloys at room temperature is < a > dislocation slip due to the typical HCP structure of magnesium, and the direction of basal slip is parallel to basal plane and perpendicular to c-axis, so the strain of c-axis direction cannot be coordinated. The strain of c-axis could be coordinated by twinning during the process of tensile plastic deformation at room temperature [10]. When the stress concentration was caused by the slip hindered and the orientation of the grains favored twinning, twins were formed in the grains, and the twins of the same twin system were parallel to each other. The adjacent or alternate twinning planes were activated in succession under the action of further tensile strain, resulting in twin intersections. Moreover, the existing twins were prone to secondary or multiple twining, which coordinated crystal orientation and released stress concentration in a certain extent [11]. In Fig. 3(c), twin boundaries and cracks can be clearly seen, and there are obvious white particles or tubes in the twin boundary area. The White particles are dimly observed in the matrix, but there are almost no white particles or tubes at cracks. Fig. 3(d) is a further magnification of the twin boundaries, and EDS analysis confirmed that these particles or tubes are MWCNTs in the matrix (Fig. 3(e)). MWCNTs were dispersed in AZ91D matrix by high-intensity ultrasonic dispersion method, and twins were formed in the matrix during the tensile plastic deformation at room temperature. The grain boundaries and twin 3
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(a)
(b)
(c)
(d)
crack
MWCNTs
(e)
Fig. 3. SEM images (SE mode) of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation. (a) Twins in the matrix. (b) Twin intersections. (c) The twin boundaries and cracks. (d) The further magnification of the twin boundaries. (e) EDS analysis of the white particles or tubes.
(b)
(a) MWCNTs
MWCNTs
stacking faults stacking faults
Fig. 4. HRTEM images of MWCNTs in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites. 4
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(a)
(b) slip bands
dislocations
(c)
(d) stacking faults
slip stacking faults
slip
MWCNTs
1.38nm ˄002˅Al78Mn22 MWCNTs Fig. 5. TEM images of slip and slip bands formed by the tensile plastic deformation of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites. (a) The slip bands consisting of a lot of parallel slip lines. (b) Some local slip bands and slip lines. (c) & (d) The stacking faults and slip lines around MWCNTs.
strain zones around MWCNTs provided power for the nucleation and growth of twins. Many slip bands appeared in the matrix of 1.0 wt% MWCNTs/ AZ91D-T4 nanocomposites after the tensile plastic deformation at room temperature. In Fig. 5(a), there are the slip bands consisting of a lot of parallel slip lines in the matrix. The distance between the slip lines is 20–50 nm, and there are also a high density of dislocations between the slip lines. In Fig. 5(b), there are some local slip bands and slip lines. The local slip bands consist of several slip lines, and some dislocations exist in the slip bands. In Fig. 5(c) and (d), MWCNTs in the matrix are dark rod-shaped, and there were some stacking faults around MWCNTs and some slip lines parallel to the tube walls of MWCNTs after the tensile tests at room temperature. A lot of dislocations and stacking faults were formed in the high strain zones around MWCNTs during the plastic deformation process, and the dislocations moved in the matrix under the action of tensile strain, forming the dislocation slip lines. The slip lines were parallel to the tube walls of MWCNTs, because dislocation slip that was not parallel to the tube walls of MWCNTs was hindered by MWCNTs, and the slip parallel to the tube wall of MWCNTs was easy to activate. The slip lines in Fig. 5(a) and (b) may evolve from the dislocations in the high strain zones around MWCNTs. The formation of the slip lines was generally accompanied by the high density of dislocations because of the high strain zones. The difference in bandwidth and distance between slip lines s in Fig. 5(a) and (b) may be due to the different distribution of MWCNTs and orientation of grains. Basal slip is the most easily activated deformation mechanism of magnesium alloys at room temperature, and its critical shear stress (CRSS) is only 0.5–0.7 MPa [10], which provides strain parallel to basal plane. Therefore, dislocations around MWCNTs were more likely to move in basal planes when the tube walls of MWCNTs were parallel to basal planes, thus forming a lot of basal slip bands. Fig. 6 is a schematic of the high strain zones and induced slip lines around MWCNTs. If the tube
hinder
basal slip bands non-basal slip high strain zone
˄0001˅
Fig. 6. Schematic of the high strain zones and induced slip lines around MWCNTs.
walls of MWCNTs are parallel to basal planes, basal slip will be activated around MWCNTs during the plastic deformation process, forming a lot of basal slip bands. If the tube walls of MWCNTs are not parallel to basal planes, basal slip cannot be activated and serious stress concentration will be caused around MWCNTs. Dislocations not only moved on basal planes, but also slipped on prismatic and pyramidal planes when strain was high enough [18], which could effectively activate non-basal slip and produced non-basal slip lines parallel to the tube walls of MWCNTs. According to Mises rule [19], each grain of polycrystalline materials with good ductility needs 5 independent slip systems (or twining systems) to sustain random deformation without fracture. The activated basal slip of AZ91D magnesium alloys at room temperature can only provide 2 independent slip systems, which is the 5
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(a)
(b)
twins twins
cross-slip
matrix
dislocations
Fig. 7. TEM images of twins and cross-slip of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites after the tensile plastic deformation. (a) Parallel twins. (b) Twins and cross-slip.
fundamental reason for the poor plastic deformation capacity of magnesium and magnesium alloys. Non-basal slip activated by MWCNTs improved the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite. Fig. 7 shows twins and high-density dislocations and cross-slip in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite after the tensile plastic deformation at room temperature. In Fig. 7(a), there are three parallel twins of the same type, and a high density of dislocations are formed locally inside the twins. Some curved slip lines exist near the twin boundaries in Fig. 7(b), and there are also a high density of dislocations inside the twins. When basal slip produced geometric hardening and there was no slip plane which was favorable to slip in the grain, twining was activated because of the dislocation caused by stress concentration in the matrix. The addition of MWCNTs made twinning more likely to be activated, and orientation was adjusted by rotation of twins during the twinning process. Under the further tensile strain, dislocations were generated in the twins due to the high strain zones of MWCNTs. When the orientation of twins was favorable to slip, MWCNTs activated further slip, which caused slip and twining to alternate. A lot of slip lines and twins were formed, thus improving the ductility of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposite. The curved slip lines in Fig. 7(b) are the results of cross-slip [20]. Some non-basal slips were activated around MWCNTs when basal slip and twining were both in hard orientation, and several slip systems were activated in the matrix. The phenomenon of cross-slip occurred when two or more slip plane glided simultaneously or alternately along a common slip direction. Cross-slip was a main carrier of ductility in HCP metals as it allows dynamic recovery during deformation [21]. As shown in Fig. 8, 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites have a high density of dislocations in the matrix after the tensile plastic deformation at room temperature. The dislocation density is highest in the middle. There are also dislocation networks and dislocation pile-up. High strain zones were formed around MWCNTs during the tensile process. With the increase of the degree of plastic deformation, the stress concentration around MWCNTs was more serious, and the dislocation density in the high strain zones also increased. Therefore, the region with the highest dislocation density in Fig. 8 is the high strain zone. Because dislocations in the matrix may encounter MWCNTs during gliding, the movement of dislocations was blocked, and the subsequent dislocations form the same source were piled up, resulting in the dislocation pile-up. According to the above, dislocations with different orientations exist in the matrix, and dislocation tangles occurred when these dislocations with different orientations encountered [22], forming a dislocation network as shown in Fig. 8.
dislocation network
high strain zone
dislocation pile-up
Fig. 8. TEM images of dislocations in the matrix after the high plastic deformation.
investigated, and plastic deformation mechanism of nanocomposites was characterized in detail by using SEM and TEM. Several conclusions can be drawn as follows. (1) The tensile strength and elongation of 1.0 wt% MWCNTs/AZ91D nanocomposites after T4 solid solution treatment reached up to 281.7 MPa and 15.4%, respectively. Compared with AZ91D alloys after T4 solid solution treatment, the tensile strength and elongation were improved by 48.5% and 180.0%, respectively. Compared with as-cast 1.0wt% MWCNTs/AZ91D nanocomposite, the tensile strength and elongation were improved by 47.4% and 316.2%, respectively. T4 solid solution treatment or the addition of MWCNTs can improve the ductility of AZ91D alloys, but their combined action can extraordinarily improve the ductility of AZ91D alloys. (2) The cracks of 1.0 wt% MWCNTs/AZ91D-T4 nanocomposites tend to initiate in the absence of MWCNTs due to the bridge effect by MWCNTs that retards the crack propagation, rather than at the grain boundaries and twin boundaries. (3) High strain zones were formed around MWCNTs in the matrix of 1.0 wt% MWCNTs/AZ91D-T4 magnesium matrix nanocomposites
4. Conclusions In this work, the tensile behavior of 1.0wt% MWCNTs/AZ91D magnesium matrix nanocomposites with high ductility was 6
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during the process of the tensile tests at room temperature. A lot of dislocations and stacking faults were generated in the high strain zones. These dislocations and stacking faults evolved into slip bands and twins under the action of tensile strain. MWCNTs promoted activation of slip and twinning, which resulted in the high ductility of nanocomposite. (4) The high strain zones around MWCNTs could also activated nonbasal slip and cross slip when the nanocomposites were subjected to large degree of plastic deformation, which was conducive to improving the ductility of magnesium matrix nanocomposites.
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Acknowledgments This work was supported by The National 863 High Technology Research and Development Program of China (No.2013AA031201), the Natural Science Foundation of Hubei Provincial Department of Education (D20171204), the Research Foundation of Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance (2016KJX08), the Applied Basic Research Project of Yichang Science and Technology Bureau(A18-302-a05), and the National Natural Science Foundation of China (51575313). References [1] W.X. Chen, W.L. Chen, Z.D. Xu, et al., Characteristics of carbon nanotubes and highquality composites, Acta Mater. Comp. Sin. 18 (2001) 1–5. [2] J. Hou, W. Du, G. Parande, et al., Significantly enhancing the strength + ductility combination of Mg-9Al alloy using multi-walled carbon nanotubes, J. Alloys Compd. 790 (2019) 974–982. [3] M. Rashad, F. Pana, M. Asif, et al., Enhanced ductility of Mg–3Al–1Zn alloy reinforced with short length multi-walled carbon nanotubes using a powder metallurgy method, Prog. Nat. Sci. : Mater. Inter. 25 (2015) 276–281. [4] M. Paramsothy, X.H. Tan, J. Chan, et al., Carbon nanotube addition to concentrated magnesium alloy AZ81: enhanced ductility with occasional significant increase in strength, Mater. Des. 45 (2013) 15–23.
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