Microstructure and Mechanical Properties of Carbon Nanotubes-Reinforced 7055Al Composites Fabricated by High-Energy Ball Milling and Powder Metallurgy Processing
BI Sheng1,2, LI Zechen3, SUN Haixia3, SONG Baoyong3, LIU Zhenyu1(), XIAO Bolv1, MA Zongyi1
1.Shi -Changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 3.Beijing Institute of Aerospace Systems Engineering, Beijing 100076, China
Cite this article:
BI Sheng, LI Zechen, SUN Haixia, SONG Baoyong, LIU Zhenyu, XIAO Bolv, MA Zongyi. Microstructure and Mechanical Properties of Carbon Nanotubes-Reinforced 7055Al Composites Fabricated by High-Energy Ball Milling and Powder Metallurgy Processing. Acta Metall Sin, 2021, 57(1): 71-81.
In the recent years, lightweight and high-strength structural materials have gained much attention in engineering applications. Carbon nanotube (CNT)-reinforced Al (CNT/Al) composites are promising structural materials owing to the good mechanical properties and high reinforcing efficiency of CNTs. Previous studies on these composites mainly focused on fabricating CNT-reinforced low-strength or medium-high-strength Al alloys (such as pure Al, or 2xxx series or 6xxx series Al alloys) composites via various dispersion methods. However, only few studies investigated composites with super-high-strength Al alloys as the matrices. In the present work, CNT/7055Al composites with CNT volume fractions of 0%, 1%, and 3% were prepared by high-energy ball milling combined with powder metallurgy. The CNT distribution, grain structure, interface, and mechanical properties of the CNT/7055Al composite were investigated using OM, SEM, TEM, and tensile tests. The strengthening mechanism and anisotropy of the composite were analyzed. The results indicated that the composite had a bimodal grain structure consisting of CNT-free coarse grain zones and CNT-enriched ultrafine grain zones. CNTs were well dispersed in the ultrafine grain zones of the Al matrix, and the CNT/Al interface was clean. There were only few reaction products at the interface. The tensile strength of the 3%CNT/7055Al composite reached 816 MPa, but the elongation was only 0.5%. Grain refinement and Orowan strengthening were the main strengthening mechanisms of the CNT/7055Al composite. Because of the load transfer efficiency of CNTs and a coarse grain band structure, the composite exhibited stronger anisotropy than the matrix alloy. The tensile properties of the CNT/7055Al composite normal to the extrusion direction were weaker than those in the extrusion direction.
Fund: National Natural Science Foundation of China(51931009);National Key Research and Development Program of China(2017YFB0703104);Key Research Program of Frontier Sciences, CAS(QYZDJ-SSW-JSC015);Youth Innovation Promotion Association CAS(2020197)
Fig.1 SEM images of the as received 7055Al alloy powders (a) and carbon nanotubes (CNTs) (b)
Fig.2 OM (a, b) and back-scattered SEM (c, d) images of T6 treated 1%CNT/7055Al (a, c) and 3%CNT/7055Al (b, d) composites (The longitudinal direction is the extrusion direction)
Fig.3 TEM images of grain structure (a) and CNT distributions (b) in T6 treated 1%CNT/7055Al composites (The black and white arrows denote CNT and extrusion directions, respectively)
Fig.4 TEM images of grain structures in fine grain zones of T6 treated 1%CNT/7055Al (a) and 3%CNT/7055Al (b) composites
Fig.5 TEM images of the precipitates of T6 treated 1%CNT/7055Al composites in a coarse grain (a) and in a fine grain (b) (The arrows denote precipitates)
Fig.6 Normalized Raman spectra of 3%CNT/7055Al powders milled for 6 h, T6 treated composites, and raw CNT (The D band and G band represent the presence of defects in graphite layers and the highly crystalline graphite, respectively; ID and IG represent corresponding peak intensities of D band and G band, respectively)
Fig.7 HRTEM image of interface of CNT and Al in T6 treated 1%CNT/7055Al composites
Volume fraction of CNT
%
Yield strength
MPa
Tensile strength
MPa
Elongation
%
Modulus
GPa
0
657±11
700±8
2.8±0.6
73
l
692±14
760±10
1.8±0.3
79
3
730±2
816±4
0.5±0.1
81
Table 1 Mechanical properties of the T6 treated CNT/7055Al composites
Fig.8 Mechanical properties of CNT/Al composites with different matrices
Sample
Orientation relationship between extrusion direction and tension direction
Yield strength
MPa
Tensile strength
MPa
Elongation
%
7055Al
Parallel
657±11
700±8
2.8±0.6
Perpendicular
607±3
644±8
2.4±0.6
1%CNT/7055Al
Parallel
692±14
760±10
1.8±0.3
Perpendicular
618±4
661±9
1.0±0.5
Table 2 Mechanical properties of T6 treated 7055Al and 1%CNT/7055Al composites with different loading directions
Fig.9 Low (a, c) and high (b, d) magnified SEM images of fractographs of T6 treated 1%CNT/7055Al composites (The arrows in Figs.9b and d denote CNT)
Position
λp / nm
Precipitate length / nm
Precipitate width / nm
r / nm
In coarse grains
26.30
17.90
2.90
5.20
In fine grains
22.00
9.60
0.95
2.65
Mean
24.15
3.93
Table 3 Measurement results of mean spacing (λp)and mean radius (r) of precipitate
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