Dev elopment and Characterization o f Mg-SiC Nan oco mpo site Po w de rs
S y nthesized by M echan i cal Milling
Daniela Penther 1 , a * , Claudi a Fleck 1 , Alireza Gha s emi 1, c ,
Ralf Riedel 2 an d Sepide h Kamrani 1, b
1 Department o f Materials Sci ence and Engineerin g, TU Berlin , 10623 Berli n, Germany
2 Department o f Materials and Ea rth Sci ences, TU Darmstadt , 64287 Dar mstadt, Germany
a penther@tu-berli n. de, b sepideh.kamrani@tu-be rlin.de, c ghasemi@ t u-berli n. de
Key w ords: M agnesium, Si C nanoparticle, pow der, nanocomposite, MM C, ball millin g.
Abstract. Magnesium powder in micron scale and various volume fractions of SiC particles with an
average diameter of 50 nm were co-milled b y a high energ y planetar y b all mill for up to 25 h to
produce Mg-SiC nanocomposite powders. The milled M g -SiC nanocomposite powders were
character ized b y scanning electron microscopy (SEM) and laser particle siz e analysis (PS A) t o
study morphological evolutions. Fur thermore, XRD, TEM, EDAX and SEM analy s es were
performe d to investigate the microstructur e of the magne si um matrix and distribution of SiC-
reinforcement. I t w as shown that with addition of and increase in SiC nanoparticle content, fine r
particles with n arrower siz e distribution are obtained aft er mechanical mil ling. The morpholo gy of
these particles also became more equiaxed at shorter milling times. The mi crostruc tural observation
revea led that the m illing process ensured uniform distribution of SiC nanoparticles in t he
magnesium matrix even with a high volume frac ti on, up to 10 vol%.
Introduction
Magnesium is the lightest eng ine ering metal wit h a densit y of 1.74 g/cm³. Further advantages are it s
high dimensional stabilit y and superior dampin g characteristics why it is widel y used in the
automobile and aviation industries [1, 2] . Nevertheless, m ag nesium has some limit ations such as
low strength and poor ductilit y [3]. Thi s is due to its hexagonal closed packed (hcp) structure whic h
provides a limited number of independent slip s ystems. A way t o improve t he mec hanical properties
is t o reinforce the ma gnesium matrix with strong er and stif fer p articles li ke ceramics to create a
metal matrix composite (MMC ) [4]. Thus, t he cha racteristic properties o f metals and ceramics are
combined and le ad to superior spe cific properties of the MMCs such as s trength, elastic m odulus
and creep resistance at a low densit y [5–7]. However , the reinforcement wi th micron sized ceramic
particles usuall y deteriorates the ductilit y [8]. Rece ntly, it has been demonstrated that the addition
of nanosized reinf orcements such as cera mic ox ides, SiC or carbon nanotubes can lead to a
simultaneous i ncre ase in strength and ductilit y of m ag n esium [ 9–11]. Nano si zed reinforcements can
withstand deformation w ithout fracturing which o verall results in a si gnifica nt increase i n s trength
and ductilit y of the composite [ 6, 12–15]. Howe ver, the function of nanoparticles in a metallic
matrix is related to t heir distribut ion in the m atrix, which can stron gly affect th e me chanical
properties o f the compo site. Nevertheless, fabrication of n anocomposites is difficult b eca use the
uniform dispersion of nanoparticles throughout the metal matrix is still a challenging task. One of
the common procedur es to fabricate nano composites is mechanica l milli n g. This method results in
an a cce ptable distribution of the reinforcing p articles without the t ypical segregation of casted
composites [12] . I n additi on, milling is one of the most eff ective methods for mechanicall y reducing
grain size and producing nanocrystalline powders.
I n the present study , high-ene rgy mechanica l milli ng was us ed to incorporate 1 vol%, 3 vol % and
10 vol% S iC nanopar t icles homoge neously i n a pure m ag nesium matrix . The effect of the
nanoparticle reinforce ment on the morphology and mi crostruc ture of mil led nanocomposite
powders was investigate d, as a function of the volume fra ction of the reinforceme nt.
Key Engineering Materials Submitted: 2017-02-07
ISSN: 1662-9795, Vol. 742, pp 165-172 Revised: 2017-03-29
doi:10.4028/www.scientific.net/KEM.742.165 Accepted: 2017-03-30
© 2017 Trans Tech Publications Ltd, Switzerland Online: 2017-07-03
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications Ltd, www.scientific.net. (#537965573, TU Berlin, Berlin, Germany-24/03/20,07:45:51)

Experimental Procedure
Magnesium powder wit h an avera g e particle siz e of -325 m esh and t wo Si C powders, on e with an
average particle size of 50 nm (SiC n ) and t he other with an av erage particle siz e of 1 µ m (Si C µ )
were used. All powd ers have a purit y of 99.8 % and were supplied b y Alfa Aesar (Ward Hill, M A,
USA). Fig. 1 shows the morphology of the as-received magnesium and SiC nanoparticle s.

Fig . 1 – Morpholog y of as-received powders a) SEM micrograph of m agnesium and b) TEM
micrograph of SiC nanoparticles.
The ma gne sium powder was mix ed with 1 vol%, 3 vol% or 10 vol% of Si C nanoparticle s b y hi gh-
energy mechanical milli ng . Another powder mixture with 10 vol% SiC microparticle s was prepared
for comparison. An ove rview of the powder m ixtures and their naming i s given in Table 1. To
minimize the cold welding effect, 2 wt% ste aric acid was added to t he mix tures as mi lling process
control agent (PCA) [16]. All powder mixtures were blended for 20 min on a rolling bank. The
high-e nerg y m echanical milli ng was performed in a planetar y ball mill (Pulverisette 5, Fritsch ,
Germany) using zirconia balls in a hard PE vessel for different milling times up to 25 h. The ball-to-
powder wei ght ratio ( BPR) was 10:1 and a rotational speed of 250 rpm was used. All the handling,
mixing, and milling steps were performed under a high purit y argon atmosphere in a glove box.
Table 1 – Mechanically mi lled powder mixtures of magne sium with vol ume fractions of 0 vol%,
1 vol%, 3 vol% and 10 vol% SiC particles and corresponding naming.
MM M1Sn M3Sn M10Sn M10S µ
Mg [vol%] 100 99 97 90 90
SiC
n

[vol%] - 1 3 10 -
SiC
µ

[vol% ] - - - - 10
After 25 h of mechanical mi lling , the mill ed powders were anal y z ed by SEM (CamScan Series 2,
Obducat, S weden) in order to study t heir morph ology. Addit ionally, mill ed powder particles were
embedded in epox y resin. The microstructure and the distribution of the SiC nanoparticles was
investigated on ground and poli shed sections b y high resolution SEM (S-2 700, Hitachi Ltd., Japan).
Further , t ransmission electron microscop y (TEM; Tecnai G² 20 S-TW I N, FEI, USA ) was us ed to
investigate the microstru cture and distribution of t he SiC particles. For t his purpose, a lamella w as
cut out from a powder agglomeration and was thinned to about 100 nm us ing the fo cused ion b eam
technique.
A high performance l aser particle si ze anal y ze r ( L A-950, Horiba, J apan) wit h a measuring range
from 10 nm to 3 mm was used to characterize the particle size and size distribution of the powders
after mil ling. The p articles were dispersed in abs olute ethanol and ultr asound was applied for 2 min
in order to eliminate particle agglomerates. Measureme nts were made to det ermine the D 50 (median
particle siz e), D90 (the particle si ze where 90 % of the particles are belo w that size) and D10 (the
particle size where 10 % of the particles are below that size) values of the particles size distribution.
a) b)
166 21st Symposium on Composites

X-ray diffra ction pattern s were recorded with the X`Pert Pro from P ANalytical (Netherlands) to
obtain information about the number and n ature of the phases. The width of the diffraction peaks
was utili zed to determine grain (cr ystallite) size and the amount of microstrain of the magnesium
matrix in t he mecha nically milled nanocomposite powders.
Results and Discuss ion
The morpholog y of Mg–10% SiC nanocomposite powders, M 10Sn, at diffe rent mill ing t imes is
shown in Fig.

2. At the early stages, the magnesium particles are defor med to a flattened shape with
an increase i n average siz e (Fig.

2a, b). Micro-welding betw een the pa rticles and t he onset o f
fracture were observed at prolonged mill ing times (Fig.

2c). Due t o the welding of the flattened
magnesium particles a p article growth started. Th e powder particles start to break when a suf ficient
level of defects w as generated ( Fig.

2d). The fr actured particles repeatedl y weld together and bre ak
again. A stead y state con dition and formation of equiaxed particles are attained when a b alance
between welding and f racturing is reached. For a milling tim e of 25 h, a change in the morpholo gy
is found where the pa rticles acquir e a m ore r egular and equiaxed s hape (Fig.

2e, f). Furth er, the
average siz e of the particles decreased and a more uniform distribution was reached. The
development of the M10 Sn powder represents all stages d escr ibed p reviousl y [12, 14, 17] such as
flattening, welding, fracture and stead y state .

Fig. 2 – Morpholo gy of M g -10% SiC composite powder after m echanical mill ing for a) 1 h, b) 3 h,
c) 5 h, d) 15 h, e) 20 h and f) 25 h.

a) b) c)
d) e) f)
Key Engineering Materials Vol. 742 167

Fig. 3 – Morpholo gy of the m illed powders a) MM, b) M1Sn, c) M3Sn a nd d) M10Sn after 25 h
mechanical milling, showing the decrease in particle siz e with increasing SiC content.
Besides milling tim e, the volume fraction of t he r einforcing p articles show s a significant influence
on the powder particle m orpholog y durin g m echanical mill ing. Fi g .

3 sho ws the morpholog y of the
MM, M 1Sn, M3S n and M10Sn milled powders after 25 h of mi lling. B y increasing the SiC content,
the particle s izes become finer and more equiaxed afte r the same milli n g time. I t appears that SiC
nanoparticles p romote the fractur e of th e magnesium matrix during mech anical milling. In cas e of
nanoparticles, t he hi gh surface to volume ratio i ncre ases the l ocal deform ation and the rate of work
hardening whi le t he f racture toughness decreases. C onsequently , the fracture process is enhanced in
the presen ce of nanop articles which in fact results in the formation of finer particles with a n arr ow
size distribut ion. Fig.

4 shows the morpholo gy of the M10Sn m illed p owder in comparison to
M10Sµ after 25 h o f mi lling. The m ore equiaxed and fine r particles after the s ame m illing tim e
confirms the prominent effect of the SiC nanoparticles to accelerate the mechanical mill ing process.

Fig. 4 – Morpholog y of milled powder s a) M10S n a nd b) M10S µ after 25 hours of mechanical
milling.
Fig. 5 ill ustrates the particle size dis tributions of the MM, M1Sn, M3Sn, M10Sn and M10Sµ milled
powders after 25 h of mechanical m illing. All the milled p owders exhibit a symmetric log-normal
size distribution. The sy m metrical gaussian be ll-shape i ndicates also the equilibrium between
a) b)
c) d)
a) b)
168 21st Symposium on Composites

frac ture and welding, t ypical of the final s tage of mechanical mi lling [18] . Furthermore, the results
show a reduction in particle size with increasing volume fractions of SiC nanoparticles, which is in
consistenc y with the SEM observations of the powder morphologies.

Fig . 5 – Particle siz e distribution of the milled powders MM, M1Sn, M3Sn and M10Sn.
XRD patterns of the MM , M1Sn, M3Sn and M10Sn powders are shown in Fig . 6a. Besides M g and
SiC, no other phas es were detected. Fig. 6b shows a m agnified vie w of th e (002) reflex of
magnesium. With increasing S iC content, peak broa dening is observed while the maximum
intensit y decreases. As peak broadening represents finer grain siz es and lattice distortion [ 19], the
increase in peak widt h of magnesium indicates the formation of i) fine cr ystallite siz es and ii) high
density of d efects in the ma gne s ium powder with increasing volume fraction of the SiC
nanoparticles.

Fig . 6 – XRD patterns of mil led powders MM, M1Sn, M3Sn and M10S n: a) whole pattern,
b) mag nified view of the (002) Mg peak.
In order to illustrate the distribution of the SiC nanoparticles in the magnesium nanocomposite
powders, cross-sections of M10S n and M 10Sµ m illed powders are shown in Fig. 7 where the
magnesium matrix appears in light gre y and the Si C particles appear as whit e dots. Although it i s
known that with higher volume frac t ion of rei nforce ment particle a u niform dist ribution of the
nanoparticles is increasingl y difficult in comp arison to macroparticles, the used mechanical milling
setup results in a homoge neous distribution of even 10 vol% of SiC nanoparticles in the magnesium
matrix [20]. The EDS elemental mapping re ga rding M g and Si elements in Fig. 8 hi ghlights t he
uniform distribution of SiC nanoparticles a s seen by SEM.
0
2
4
6
8
10
12
14
16
18
1 10 100 1000
q (%)l
Diameter (µm)
M1Sn
M3Sn
M10Sn
MM
D50=47 .1 µm
D50=9. 4 µm
D50=22 .5 µm D50=25 .7 µm
a) b)
2Theta (degr ees)
Intensity ( counts)

Key Engineering Materials Vol. 742 169

Fig. 7 – Backscattered electron im age of a) M10S n and b) M10Sµ shows a homo geneous
distribution of Si C partic l es (white) in the magne si um matrix (light grey) .

Fig. 8 – Elem ental mapping of M10Sn nano composite powder: a) S EM mi crogra ph, b) m agnesium
and c) silicon.
Brightfield TEM micrographs of M10Sn milled powder are shown in Fig.

9. The micro graphs not
only show th e uniform dist ribution of the SiC nanoparticles, but th e y furthermore attest to the
development of a nanocr y st alline magnesium matrix during the mechanical m illing p rocess. The
TEM results are thus in good a greeme nt with the XRD peak broadening. Figure 9b shows a
magnified vi ew of on e SiC particle together with the surrounding ma g nesium matrix. No evidence
of the presence of an interfacial product between the S iC nanoparticle and the magne sium matrix
was found, indicating the formation of well-bonde d interfaces. A strong interfacial bondin g between
the S iC nanoparticles an d the nanocrystalline magnesium matrix is promising in terms of superior
mechanical properties of the magnesium nanocom posites.

Fig. 9 – Brightfield TEM m icrog r aphs of M10S n nanocomposite powder: a) surve y , sho wing t he
distribution of SiC particles within the nanocr y st all ine Mg matrix: the co mpositions of the marked
grains were verified b y EDS (results not s hown); the inset shows the acc ompan y in g SAD pattern; b)
magnified view of SiC particle in Mg matrix showing a well-bonded inte rface.
a) b)
a) b) c)
a) b)
170 21st Symposium on Composites

Summarising Conclusions
Magnesium powder and S iC nanoparticles wer e co-milled to produce M g -SiC nanocomposit e
powders. The morphology as well as t he microstructure of the milled powders were studied. W ith
higher m illing time the particle size of the Mg-SiC powder decre ases. In the same duration of
milling process, the higher contents o f Si C res ult in finer particle s izes wit h more equiaxed
morphology. Th e particle siz e di stribution was in good a gree ment wit h the SEM pictures. S EM and
TEM analy ses verified a homogeneous distribut ion of S iC nanoparticles in the magne s ium matrix,
even with a high volume fraction of 10 vol% SiC.
Acknowledgement
The authors gratefully a cknowledge the financial support b y the DFG (Deutsche
Forschungsgemeinschaft) and the membe rs of the group Non-Metallic Inorg anic Materials, I nstitute
of Materials Science a t TU Darmstadt for providing the ball mill equipment . The authors would also
like to thank the Department of Food T echnology an d Food M aterial Science for the p article size
distribution measurements as well as Ulrich Ge rnert and Sören S elve at Z E LMI for the support
during SEM and TEM analyses.
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