712
K. Kallip et al. / Journal of Alloys and Compounds 646 (2015) 710e718
Table 1
CNTs used as reinforcement.
Name
Mean outer Ø [nm]
Length [
m
m]
Producer/comments
Baytubes
EPFL raw
EPFL Recrystallized
VGCF
CEA raw
13e16
11
11
110
55
55
1e10
Bayer Material Science GmbH, Germany. Agglomerates of 0.1e1 mm [15]
Ecole Polytechnique F eꢀ d eꢀ rale de Lausanne, Switzerland [16,17].
Heat treated 1700e2000 C [16,17]
ꢂ
20
250e850
250e850
Hodogaya chemical, Japan
Commissariat ꢁa l'Energie Atomique et aux Energies Alternative, France [18].
ꢂ
CEA Recrystallized
Heat treated 1000 C for 2 h [19],
2.3. Characterizations
D G
two bands I /I is between 0.1 and 1.2 (Fig. 2B) and gives a relative
information on the CNT quality, larger ratio would indicate lower
quality. Among the investigated raw CNTs, the Baytubes seem to
have the largest number of defects while the VGCF seem to have the
fewest defects.
The carbon structure in the raw CNTs, the blends and the bulks
were investigated by Raman spectroscopy (Renishaw invia) at
14 nm with 12.4 mW. At least 10 Raman measurements per
sample were conducted. The spectra were deconvoluted, fitted and
the integrated intensity ratio of the D- and G-band of carbon (I /I
was calculated. The microhardness of the powders and of the bulks
was measured with an MHT-4 Vickers microhardness tester using a
load of 0.15 N for 15 s. The powders morphology was characterized
using Zeiss Axiopan optical microscope and CFE-SEM (Hitachi S-
5
D G
For 6 h milled blends, I /I drastically increases and is in the
D
G
)
range from 2.3 to 3.6 indicating an increase of disorder for all the
blends. Nevertheless the Graphite band is still present. Blends
reinforced with CEA raw CNTs and VGCF have the highest ratio
values whereas blend reinforced with EPFL CNT has the lowest
value. However, there is no direct correlation between the ratio
before and after milling.
4
800). The bulks were characterized with SEM as well as with HR-
TEM (Jeol JEM-2200FS TEM/STEM). TEM samples were prepared
from bulks by Allied multiprep polishing system and ion milled
using Fischione Instruments TEM Mill 1050. Vickers macro hard-
ness was measured on the bulks according to EN ISO 6507-1 with a
load of 20 kg for 15 s (220, GNHEM H a€ rteprüfer AG). Each sample
was measured at least 6 times at different positions on the surface.
The crystallite structure was investigated using Synchrotron radi-
ation at a wavelength of 0.66 Å with 18.8 keV at the Paul Scherrer
Institute (PSI, CH). The crystallite size was determined based on the
The powder hardness has been measured and is presented in
Fig. 3A. After milling, all the powders show a micro-hardness
around 200 HV0.15, about 4 times higher than the non-milled
starting alloy (50 HV0.15). The blends made out of the EPFL recrys-
tallized CNTs are in the same range as unreinforced milled AlMg5
alloy particles and about 100 HV less than the other blends. Due to
the size of the powders relative to the indent print, the hardness
values are highly dispersed.
The crystallite size of the powders and bulks is plotted in Fig. 3B.
The raw non-milled alloy has an Al [111] crystallite size around
85 nm. During milling, even without any CNTs, the crystallite size of
the starting alloy decreases to 50 nm. A further decrease of the
crystallite size is observed with the addition of CNTs with sizes
between 30 and 36 nm. These trends are still observed after hot
compaction. Even if the crystallite size increases after the high
temperature process, all the composite materials exhibit a crys-
tallite size between 74 and 91 nm while the non-reinforced alloys
have Al crystallites at 190 and 140 nm for non-milled and milled
conditions respectively.
XRD [111] Al peak measured with a BRUKER Discovery D8 with Cu
ꢂ
K
a
(l
¼ 1.5148 Å, 40 kV and 40 mA) in the 2
q
range 30e120 using a
linear detector. The instrumental broadening was determined with
-Al standard. The crystallite size was calculated using the
Scherrer equation:
a
a
2 3
O
0
:9l
d ¼
(1)
B cos q
where d is the crystallite size and
l
,
q, and B represent the X-ray
wavelength, the Bragg scattering angle and the full width at half-
maximum (FWHM) respectively. Elemental oxygen and carbon
contents were measured using ELTRA ONH2000 infrared cell.
Flat bone-shape tensile specimens with a thickness of 4 mm,
Beside the Al diffraction peaks, in some cases a few very weak
peaks could be observed in conventional XRD patterns. Therefore
measurements with more powerful X-ray radiations have been
4 3
performed at PSI Synchrotron. Thus it appears clearly that the Al C
gauge length of 12 mm and a distance between shoulders of
phase formed only after hot compaction and not still during milling
(Fig. 4).
2
2
6 mm, were machined from flat 55 ꢁ 55 mm square plates. The
tensile specimens were prepared always perpendicularly to the
pressing direction and the tensile tests were carried out at room
temperature using a universal testing machine (Hug Maschi-
3.2. Composite materials with 1 vol% CNT
ꢀ
1
ꢀ3
nenfabrik AG) with the speed of 1 mm min . The density of the
bulk samples was measured 3 times using the Archimedes method
according to ISO 3369:1975.
All the bulks show densities between 2.70 and 2.75 g cm
,
higher than the theoretical density (2.64). Measurements of the
oxygen and carbon contents in the different bulks indicated a
content of 1 ± 0.02 wt% of O
CNT used. During high temperature compaction at 550 C, these
2
and 1 ± 0.01 wt% of C whatever the
ꢂ
3
. Results
elements are reacting with aluminium to form Al
also probably amorphous Al [20].
4 3
C (Fig. 4) and
3.1. High energy milled 1 vol% CNT reinforced composite blends
2 3
O
The hot compacted pure AlMg5 alloy has a hardness around 60
HV20 (Fig. 5) in agreement with values typically referenced for this
alloy. Milling the pure alloy with stearic acid for 6 h still increases
the hardness to 120 HV. The addition of only 1 vol% of CNT further
improves the hardness and values between 170 and 190 HV20 are
measured. This corresponds to a threefold increase compared to the
unmilled pure alloy bulk. For comparison, the highest strength
First all 6 types of CNTs were milled 6 h with AlMg5 matrix
creating blends with 1 vol% CNT. The morphology of all milled
powders is equiaxial and the particle size is in the range 30e50 m.
In Raman spectroscopy, all the raw CNTs show the distinctive G
m
ꢀ1
ꢀ1
(
Graphite) and D (Disordered) bands at 1340 cm and 1570 cm
respectively (Fig. 2A). The integrated intensity ratio between these