Pérez-Bustamante, R., González-Ibarra, M. J., González-Cantú, J., EstradaGuel, I., Herrera-Ramírez, J. M., Miki-Yoshida, M., & Martínez-Sánchez, R. (2012).
AA2024–CNTs composites by milling process after T6-temper condition. Journal of
Alloys and Compounds, 536, S17-S20.
http://dx.doi.org/10.1016/j.jallcom.2011.12.001
Acknowledgements
This research was supported by CONACYT (106658). Thanks to Redes
Temáticas de Nanotecnología y Nanociencias, Reg. 0124623. Thanks to O. SolisCanto, C. Leyva-Porras, P. Pizá-Ruíz and C. Ornelas-Gutiérrez for their technical
assistance.
Abstract
The 2024 aluminum alloy produced from elemental powders and reinforced
by the addition of carbon nanotubes were microstructural and mechanically
characterized. Composites were synthesized through milling process followed by
cold consolidation, sintering and hot extrusion. The effect of the nanotubes on the
microstructure and mechanical properties were analyzed in the composites after T6
thermal treatment. Al4C3 formation was due to the reaction of carbon nanotubes
and Al matrix. Precipitated Al–Cu phase coexists and presents interaction with
crystallized carbide and dispersed nanotubes. There is a direct relationship of
carbon nanotubes content and hardness answer.
1. Introduction
The continuous development of materials with improved mechanical
performance for aerospace, aeronautic and automotive industries has been a
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constant interest by many research groups. In this respect, aluminum has been
widely investigated and is presented as key material in order to face these
challenges. When alloyed with other metals, its mechanical properties are vastly
improved making it suitable for innumerable structural applications [1–3]. Aluminum
alloys have been extensively investigated in order to improve their mechanical
performance. Even though aluminum is alloyed with several metals through foundry
routes, and susceptible to strengthening by the precipitation due to heat treatments
[4,5], notable results have been achieved by the dispersion of strengthening
particles by using powder metallurgy technique [6,7]. However, the dispersion
process can be improved by the use of mechanical alloying (MA) [8–10], which is a
technique that allows the introduction of the reinforcement phase in the matrix by the
repeated cycle of welding–fracture–welding of the particles involved in the process.
This process allows a homogeneous dispersion of the strengthening phase and the
formation of super-saturated solid solutions [11].
Several strengthening phases have been dispersed by MA into aluminum
alloys achieving composites with noticeable mechani-al properties. The use of
carbon nanotubes (CNTs) as one of them has attracted the attention of several
research groups [9,10,12]. By MA it is achieved the formation of a solid solution,
microstructure refining and, at the same time, the homogeneous CNTs dispersion
into aluminum matrix. Nanocomposite formed during milling pro-cess will present
better mechanical properties than unreinforced alloy produced by the same route
[8,10,13–17].
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The focus of this work is to produce the AA2024 aluminum alloy from
elemental powders and the dispersion of CNTs by a milling process. The effect of
heat treatment of precipitation on consolidated samples is evaluated from
mechanical and microstructural point of view.
2. Experimental procedure
Al (99.5% pure, −200 mesh), Cu (99.3% pure, −325 mesh), Mg (99.8%
pure,−325 mesh), Mn (99.3% pure, −325 mesh), Ti (99.3% pure, −325 mesh) and Zn
(99.9%pure, −100 mesh) powders were used to fabricate AA2024-based
composites. CNTs synthesized by chemical vapor deposition (CVD) were added in
amounts of 0.0, 0.5, 2.0 and 5.0 wt.%. Each mixture (80 g) was milled during 5 h in a
high energy mill (Simoloyer). Ar as inert atmosphere was used for all runs. Due that
the production of the AA2024 is from elemental powders and the highly ductile
nature of the aluminum, pure methanol (0.5 ml/∼10 drops) was used as process
control agent (PCA) with the aim of avoiding excessive agglomeration of aluminum
particles during the milling process. Milled powders were compacted under 60 tons
during 2 min using a die with an inlet diameter of 25 mm [14]. Compacted samples
were sintered during 2 h at 773 K under vacuum with a heating and cooling rate of 5
K/min. Sintered products were held for 0.5 h at 773 K and hot extruded in rods of 10
mm in diameter by using indirect extrusion and an extrusion ratio of 16. Extruded
rods were solution heat treated at 768 K during 3 h and then water quenched
followed by artificial aging at 464 K during 8 h (T6 condition). CNTs and heat treated
products were characterized by means of Raman spectroscopy with a Micro Raman
LabRAM VIS-633 with an He–Ne laser light (632.8 nm), X-ray diffraction (XRD) in a
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Panalytical X’Pert PRO with X’Celerator detector. Samples for transmission electron
microscopy (TEM) were prepared by focused ion beam (FIB) in a JEOL model JEM9320FIB. TEM samples were imaged at 200 keV by Z-contrast and bright field
micrographs in STEM mode on the JEOL JEM 2200FS. The mechanical behavior of
the samples was measured by means of the hardness test in a Wilson Rockwell
hardness tester.
3. Results and discussion
Fig. 1 displays the Raman spectrum of the raw multi-walled CNTs used in this
work and it is compared with that of the AA2024 composite reinforced with 5.0 wt.%
of nanotubes. The spectrum corresponding to the CNTs displays two dominant
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peaks for the D band around 1325 cm−1 and the G band located at around 1574
cm−1 which are directly related with nanotubes structure. The addition of nanotubes
into the AA2024 matrix by milling process causes a very clear change in intensity
and a slight shift in wave number of the D (1330 cm−1) and G (1601 cm−1) peaks
attained to the nanotubes frequencies in the composite. This indicates a notorious
increase in the number of defects of the CNTs well as amorphization, which is
revealed by the change in broadening of the D and G bands in addition to the slight
shift to higher frequencies of the G band for the composite, as was observed by
Poirier et al. [12]. The increase in the amorphization could be due to the partial
destruction of the of the CNTs outer walls during the milling process and their
correspond-ing transformation in amorphous carbon. The constant interaction
between Al and carbon atoms pulled-out from the outer walls of the nanotubes, due
to the milling process, is displayed by a signal given at around 715 cm−1 in
coincidence with the metal–carbon (Al–C) bond and belonging to the Al4C3whose
broadening is directly related to the milling time [18].
Fig. 2 shows XRD patterns of the extruded rods in the T6 condition for
different CNTs concentrations. The signal attained to the aluminum phase is
presented by 5 peaks. As can be observed, the crystallization of the aluminum
carbide (Al4C3) arises as the reaction during the sintering process between the
nanotubes and the aluminum. The intensity corresponding to the aluminum carbide
signals, clearly visible from 2.0 wt.% of nanotubes concentration, shows an increase
as function of the CNTs content. The formation of this phase has been observed by
other research groups and its crystallization depends of the processing route for the
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production of Al–CNTs composites [14,15]. The precipitation of the Al2Cu phase, as
a result of the T6 condition of the samples, is identified in all cases by small peaks,
which do not display visible changes in their intensity as function as the nanotubes
content increases.
Fig. 3 presents TEM micrographs of the composite reinforced with 5.0 wt.% of
CNTs after aging at 464 K for 8 h. Bright tiny needle-shaped arrays of Al–Cu
precipitates [5] well dispersed into the dark AA2024 matrix are observed in Fig. 4a
(Z-contrast micrograph). The presence of these precipitates is due to the T6
condition and is the main hardening mechanism for the 2XXX series commercial
alloys. A bright field STEM micrograph given in Fig. 3b displays a nanotube
interacting with Al–Cu precipitates. The inset given by a Z contrast micrograph
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indicates the region of the CNTs embedded into the AA2024 matrix. Fig. 3c displays
a bright field STEM micro graph indicating by white arrows the Al4C3 phase in
needle-shaped arrays randomly dispersed. The inset of a Z contrast micrograph in
Fig. 3c shows a close-up of the aluminum carbide needles coarser than the Al–Cu
precipitates. In addition, Fig. 3d gives a magnified view of the lattice fringes of the
aluminum carbide where a direct measurement on the phase show an interplanar
distance of 0.83 nm corresponding with the (0 0 3) plane. This phase was previously detected by XRD and their presence of in composites seems to depend on
the destruction degree of the CNTs during the milling process leading to the partial
loss in the periodicity of the nanotubes outer shells, which have a reaction with the
Al matrix during the sintering process.
Results from hardness test (Rockwell B) are displayed in Fig. 4. An increment
in hardness is observed as a function of CNTs concentration. The addition of 2.0
wt.% of nanotubes produced an increase of ∼17% over the unreinforced alloy.
However the most noticeable result is achieved by the composite with 5.0 wt.% of
CNTs, which presents an increment in hardness of ∼30% by comparing with the
unreinforced AA2024 produced by the same route.
From the hardness results it is observed that nanocomposites AA2024–CNTs
present higher hardness values than the AA2024 alloy after T6 temper. It is well
documented in literature the strengthening effect of Al–Cu phase during a
precipitation treatment [5]. In nanocomposites, the presence of CNTs and their
partial decomposition to Al4C3 implies additional strengthening mechanisms as has
been suggested before by several authors [17,18].
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4. Conclusions
The AA2024 aluminum alloy was successfully reinforced by the dispersion of
CNTs through milling process. Milled powders were cold compacted, sintered,
extruded and finally heat treated by solution and artificial aging. XRD results
indicated the presence of Al4C3 produced during the sintering process, of as well as
Al2Cu precipitated during T6 heat treatment condition. The presence of both phases
was corroborated by TEM. The observation of CNTs interacting with Al–Cu
precipitates was also corroborated. The mechanical behavior of the composites
displayed an increase of 30% for the composite reinforced with 5.0 wt.% of
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nanotubes over the unrein-forced alloy (AA2024) produced by the same route.
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