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Journal of Alloys and Compounds 769 (2018) 848e857
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Variation of microstructure and mechanical properties with nano-SiCp
levels in the nano-SiCp/AlCuMnTi composites
Jianyu Li, Shulin Lü, Shusen Wu*, Wei Guo, Fei Li
State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2018
Received in revised form
5 August 2018
Accepted 7 August 2018
Available online 10 August 2018
Nano-sized SiC particles (SiCnp for short) reinforced Al-5Cu-0.5Mn-0.15Ti composites have been successfully fabricated by a process consisting of high energy ball milling, mechanical stirring and ultrasonic
treatment for composite slurry and squeeze casting. The variation of microstructure and mechanical
properties with SiCnp levels in xSiCnp/AlCuMnTi (x ¼ 0.5, 1, 1.5, 2 wt%) composites is investigated for the
first time. The SiCnp are uniformly distributed in the SiCnp/Al-5Cu composites made with the process.
With the increase of SiCnp content from 0.5 to 2.0 wt%, the primary a-Al and Al2Cu phases are refined
significantly up to 1.5 wt% but then become coarser at 2.0 wt%. The optimal mechanical properties are
obtained in 1.5 wt% SiCnp/AlCuMnTi composites, which exhibit 298 MPa in ultimate tensile strength
(UTS), 178 MPa in yield strength (YS) and 12.9% in elongation. These properties are increased by 18.7%,
11.3% and 25.3%, respectively, compared with the AlCuMnTi matrix alloy. The enhancement of strength is
attributed to four strengthening mechanisms, among which the DsCTE and DsOrowan are the most
important contributors. It is noteworthy that with more SiCnp in the present composites, the strength
increases while elongation increases as well.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Aluminum matrix composites
Nano-sized SiC particles
Ultrasonic treatment
SiCnp content
Mechanical properties
1. Introduction
Aluminum matrix composites (AMCs) have recently attracted
much attention due to their high specific strength, high elastic
modulus and good wear resistance, etc. [1e4]. In general, AMCs are
reinforced by various ceramic particles such as SiC, Al2O3 and TiC,
among which SiC particle is regarded as a suitable reinforcement in
aluminum matrix for its unique physical and mechanical properties
[1e5]. Recent studies reveal that the nano-sized ceramic particle is
more favorable than micron-sized ceramic particle and it is
becoming a research hotspot in the metal matrix composites [6,7].
However, the SiCnp tend to agglomerate in the molten Al alloys due
to their poor wettability, attractive Van Der Waals interactions and
large surface-to-volume ratio [8], which can reduce the mechanical
properties of composites. Therefore, it is essential to develop new
preparation processes to improve the distribution of nano-sized
reinforcements.
Up to now, AMCs have been fabricated using a variety of conventional fabrication methods including solid-state processing and
* Corresponding author.
E-mail address: ssw636@hust.edu.cn (S. Wu).
https://doi.org/10.1016/j.jallcom.2018.08.066
0925-8388/© 2018 Elsevier B.V. All rights reserved.
liquid-state solidification processing, such as powder metallurgy
and stir casting [8,9]. In molten-metal processes, ultrasonic treatment (UT) is a promising technology to prepare the SiCnp reinforced
AMCs, for the ultrasonic vibration can give rise to great effects on
the melt by the cavitation and acoustic streaming [10e12]. In order
to facilitate more homogenous dispersion of SiCnp in the SiCnp/
AlCuMnTi composites, other processes including high energy ball
milling (HEBM) and squeeze casting are also needed. However, it
has not been publically reported about the preparation of SiCnp/
AlCuMnTi composites using the similar processes.
In general, SiCnp content also determines the mechanical properties of composites [8,13]. Wang et al. found that increasing SiCnp
content in aluminum alloy led to an increase in UTS and YS of
composites fabricated by semisolid stirring assisted with hot
extrusion, but the elongation was decreased by 54.4% [8]. Yao et al.
found that increasing the volume fraction of SiCnp in AA6063 alloy
caused an increase in UTS and YS of composites fabricated by
powder metallurgy, but the elongation to fracture decreased from
10.0% to 2.3% [13]. These researches indicated that more SiCnp led to
higher UTS and YS, while lower elongation was obtained. Up to
now, few researches have been carried out on the variation of
microstructure and mechanical properties with SiCnp levels in
SiCnp/AlCuMnTi composites fabricated by the similar processes.
J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
Therefore, it is significant to study the contributions of SiCnp content and various strengthening mechanisms to the increase of
strength and elongation of SiCnp/AlCuMnTi composites.
In this study, the microstructure and mechanical properties of
SiCnp/AlCuMnTi (Hereafter, Al-5Cu for short of the AlCuMnTi alloy)
composites were investigated, which were successfully fabricated
by a process consisting of HEBM, UT for composite slurry and
squeeze casting. The SiCnp content various from 0.5 wt% to 2 wt% in
order to evaluate the effects of it. Significantly, it is found that with
more SiCnp in the present composites, the strength increases while
elongation increases as well. Various strengthening mechanisms
are discussed to analyze this behavior in detail.
2. Experimental procedure
2.1. Materials
The preparation process for (0.5, 1, 1.5, 2) wt.% SiCnp/Al-5Cu
composites is as following developed by the authors. Firstly, preoxidation treatment was used for SiCnp at 850 C for 2 h to form a
SiO2 coating layer about 3.6 nm in thickness, which would prevent
the reaction between Al and SiCnp. The details of pre-oxidation
treatment could be found in our previous study [14]. After preoxidation of SiCnp, the SiCnp/Al compound granules containing
SiCnp were prepared by HEBM according to our previous work [10].
The mass fractions of SiCnp was 6 wt% in compound granules with
the size of 1~2 mm, as shown in Fig. 1(a). Furthermore, the SiCnp
were uniformly distributed in compound granules, which would be
beneficial for the dispersion of SiCnp in the composites, as shown in
Fig. 1(b).
The chemical compositions of the Al-5Cu alloys used as matrix
alloy were 5 wt% Cu, 0.5 wt% Mn, 0.15 wt% Ti, 0.3 wt% Mg, 0.15 wt%
Fe, and balance Al. The raw materials including pure Al, pure Mg,
pure Cu, Al-5%Ti-B master alloy and Al-10%Mn master alloy were
melted in a graphite crucible at 750 C. Then the molten metal was
degassed with pure argon gas for 10 min. After that, the SiCnp/Al
compound granules were added to melts with mechanical stirring
at 120 rmp for 10 min, which was beneficial to accelerate the
melting of SiCnp/Al compound granules and promote the dispersion
of SiCnp in melt. The whole process was protected by argon gas. The
SiCnp content in the composites was controlled to 0, 0.5, 1, 1.5, and
2 wt%, respectively.
Then, the melt was treated by UT to assist the homogenous
dispersion of SiCnp. The details of UT system could be found in our
previous study [10]. The ultrasonic horn preheated for 5 min at
720 C was inserted into the melt below surface at 10e15 mm, and
the UT time was set at 5 min. The whole process of UT was
849
protected by argon gas. Finally, the composite ingots with diameter
of 30 mm and height of 100 mm were fabricated by squeeze casting.
The squeeze pressure was set at 50 MPa.
2.2. Characterization
Specimens for metallographic observation were cut from the top
of casting ingots. The microstructure characterization was performed by using a XRD-7000S X-ray diffraction (XRD), a DMM480C optical microscopy (OM), a JSM-7600F scanning electron
microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) and a Tecnai G2-F30 telecom electron micrograph (TEM).
The grain size of the primary a-Al phase was calculated by using a
self-developed software system (Solidvf 3.0) with Heyn's linear
intercept method [10]. Then the room temperature tensile properties were measured by a SHIMADZU AG-IC machine under a
constant rate of 1 mm/min. The average UTS, YS and elongation
values were obtained from three samples for each specified condition. The size of sample was shown in Fig. 2.
3. Results and discussion
3.1. XRD analysis of SiCnp/Al-5Cu composites
The XRD patterns of xSiCnp/Al-5Cu (x ¼ 0.5, 1, 1.5, 2 wt%) composites are shown in Fig. 3, which exhibits the peaks for SiCnp, a-Al
and Al2Cu phase. It indicates that the SiCnp have been successfully
introduced into the melt by the novel process of UT combined with
HEBM and squeeze casting. No peak for Al4C3 can be found, which
suggests that the pre-oxidation can efficiently prevent the reaction
between SiCnp and Al-melt. In addition, the intensity of SiCnp peak
is very low because of the low content (2 wt% for maximum).
Fig. 2. The draft of the tensile test sample.
Fig. 1. Composite granules after HEBM: (a) morphology, (b) SiCnp distribution in compound granules.
850
J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
Fig. 5. The average grain size of a-Al.
Fig. 3. XRD spectrums of SiCnp/Al-5Cu composites: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%,
(d) 2.0 wt%.
3.2. Effects of SiCnp content on microstructure of SiCnp/Al-5Cu
composites
Fig. 4 shows the optical microstructures evolution of composites
with different SiCnp content. As can be seen, the SiCnp content plays
a significant role in controlling the grain size of primary a-Al. The
quantitative analysis result of grain size is shown in Fig. 5.
Obviously, the grain size decreases with the increase of SiCnp content from 0 to 1.5 wt%. However, when the SiCnp content increases
to 2.0 wt %, the grain size is no longer further reduced. That may be
caused by the agglomeration of SiCnp, which can weaken the effect
of SiCnp on hindering the growth of primary a-Al grains [10,15].
Although the UT can homogenize the distribution of SiCnp in the
composites, the effects of ultrasonic vibration are weakened due to
the high viscosity of the composite melt with 2 wt% SiCnp. Therefore, the optimal SiCnp content is found to be around 1.5 wt%, in
which the grain size is decreased by 55% compared with the Al-5Cu
Fig. 4. Optical microstructures of the SiCnp/Al-5Cu composites with different SiCnp content: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, (d) 2 wt%.
J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
matrix alloy. The finer grains generate more grain boundaries
leading to the refinement of phases in boundary, which will be
beneficial to improve the mechanical properties of composites [10].
In this regard, the refinement of a-Al grains in the composites can
be attributed to the following reasons: (i) the pinning effect of SiCnp
located at the grain boundaries, hindering the growth of grains; (ii)
the cavitation and acoustic streaming of UT, creating nuclei and
enhancing refinement of grains [10].
Fig. 6 represents the SEM microstructures of SiCnp/Al-5Cu
composites with different SiCnp content. As shown in Fig. 6(a),
coarse light-gray phases can be found at the grain boundaries in the
composites containing 0.5 wt% SiCnp, and they are Al2Cu according
to the EDS results shown in Fig. 7(e). These Al2Cu phases have two
shapes including dot and bar, which are marked with orange
rectangles and circles, respectively. The Al2Cu phases are mainly in
the form of bar and the amount of dot shape is small. With the
increase of SiCnp content, the Al2Cu phases are significantly refined.
With addition of 1 wt% SiCnp, the Al2Cu phases with small size are
obtained and they are mainly in the form of dot, as shown in
Fig. 6(b). The amount of dot phases increases significantly, but there
are still a small amount of bar phases in the composites. When the
SiCnp content increases to 1.5 wt%, the Al2Cu phases are further
refined, as shown in Fig. 6(c). All the bar phases are refined to dot
phases and the Al2Cu phases are uniformly distributed in the
composites. However, increasing SiCnp to more than 1.5 wt% has no
obvious effect on the refinement of Al2Cu phases. The further
increment in SiCnp content even leads to an increase in the size of
Al2Cu phases. This increase might be attributed to SiCnp agglomeration existing in the composites at higher SiCnp content (beyond
1.5 wt%), as shown in Fig. 6(d). For the refinement of Al2Cu phases,
there are three main reasons: (i) The refinement of a-Al phases
leads to the thinning of Al2Cu phases in grain boundary. (ii) With
851
the application of UT, there are effects of the cavitation and the
acoustic streaming left in the composites, leading to the refinement
of Al2Cu phases. (iii) The existence of SiCnp can hinder the growth of
Al2Cu phases because Al2Cu phases precipitate at grain boundaries
at the last stage.
Fig. 7 shows the high-magnification SEM images of each composite to further confirm the distribution of SiCnp. The EDS results
shown in Fig. 7(f) indicate that the bright particles are SiCnp.
Obviously, SiCnp show a tendency of distributing along grain
boundaries in all SiCnp/Al-5Cu composites, as shown in Fig. 7(aed).
Since the lattice misfit between SiCnp and a-Al is over 5%, SiCnp
cannot be captured by a-Al during solidification and are subsequently pushed to the solid/liquid interface or grain boundaries
[16,17]. The SiCnp distributed at the grain boundaries are mixed
with Al2Cu phases since intermetallic Al2Cu phases is formed at the
last stage of solidification [18]. Meanwhile, a few SiCnp can be
observed inside the a-Al, which may be attributed to the semi
coherent relationship between them [19,20]. Fig. 8 shows the TEM
images of 1.5 wt% SiCnp/Al-5Cu composites. Very clean interface
between SiCnp and matrix can be observed and the interface has no
any reaction products, which indicates that these dispersed SiCnp
can act as suitable reinforcements for Orowan strengthening
[10,15].
In addition, UT has been applied in this study to homogenize the
distribution of SiCnp. Fig. 9 shows the illustration of how UT to affect
the distribution of SiCnp in the composites. The application of ultrasonic vibration can greatly affect the melt by cavitation and
acoustic streaming. A lot of cavitation bubbles, which are generated
in the melt under cyclic high-intensity ultrasonic waves, will undergo the process of formation, growth and collapse, repeatedly.
The collapse of bubbles forms transient high pressure of about
1000 atm and micro-flows or injection of liquid (faster than 100 m/
Fig. 6. Low-times SEM image of the SiCnp/Al-5Cu composites with different SiCnp content: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, (d) 2 wt%.
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J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
Fig. 7. High-times SEM image of the SiCnp/Al-5Cu composites with different SiCnp content: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, (d) 2 wt%, (e) Spectrum of the gray Al2Cu phases, (f)
Spectrum of the bright SiCp phases.
Fig. 8. TEM images of 1.5 wt% SiCnp/Al-5Cu composites: (a) a higher magnification of SiCnp, (b) a HRFEM image of the interface between SiCnp and a-Al matrix, with the corresponding SAED pattern obtained from black particle in (a).
J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
853
Fig. 9. Sketch of effects of ultrasonic cavitation and acoustic streaming on particles distribution.
s) [21e23]. Meanwhile, the turbulent whirlpools are generated in
the melt with the higher speed than that of heat convection, which
can strongly stir the melt to accelerate the transfer of solute and
SiCnp [18]. Therefore, the cavitation and acoustic streaming can
cooperatively promote the uniform distribution of SiCnp in the
composites. Additionally, the refinement of a-Al grains is also
enhanced in the composites with UT owing to the effects of the
cavitation and acoustic streaming [24,25].
3.3. Density analysis of SiCnp/Al-5Cu composites
Table 1 shows the theoretical and measured density of the
matrix alloy and xSiCnp/Al-5Cu (x ¼ 0.5, 1, 1.5, 2 wt%) composites.
The density of specimens was measured by Archimedes' method
[26]. As shown in Table 1, when SiCnp content increases to 2.0 wt%,
the relative density of composites decreases due to the SiCnp
agglomeration existing in the composites at higher SiCnp content.
The entrapped air inside the SiCnp agglomeration can prevent metal
flowing into them and contribute to the reduction of measured
density [10]. However, the relative densities of both matrix and
SiCnp/Al-5Cu composites with UT are above 99% due to the ultrasonic degassing effect [27]. Therefore, it indicates that an ideal
degassing of composites can be obtained by the processing of UT
combined with Ar-gas bubbling.
3.4. Mechanical properties of SiCnp/Al-5Cu composites
Fig. 10 shows the variation of mechanical properties of SiCnp/Al5Cu composites with SiCnp content. For comparison, the mechanical
properties of unreinforced matrix alloy are also included in Fig. 10.
It is clearly seen that the added SiCnp have a great influence on the
deformation behavior of the composites. Compared with Al-5Cu
matrix alloy, the SiCnp/Al-5Cu composites exhibit certain
improvement in UTS, YS and elongation, respectively. Among these
composites, the 1.5 wt% SiCnp/Al-5Cu composites show the highest
UTS, YS and elongation, which are increased by 18.7%, 11.3% and
25.3%, respectively, compared with Al-5Cu matrix alloy. However,
the further increase of SiCnp content leads to the degradation of
mechanical properties due to the SiCnp agglomeration. Interestingly, with more SiCnp in the present composites, the strength increases while elongation increases as well, until to reach optimal at
Table 1
Theoretical and measured density of the matrix alloy and xSiCnp/Al-5Cu (x ¼ 0.5, 1,
1.5, 2 wt%) composites.
Specimens
Theory density
Measured density
Relative density
Al-5Cu matrix alloy
0.5SiCnp/Al-5Cu
1.0SiCnp/Al-5Cu
1.5SiCnp/Al-5Cu
2.0SiCnp/Al-5Cu
2.812 g
2.814 g
2.815 g
2.817 g
2.819 g
2.798 g
2.804 g
2.808 g
2.812 g
2.801 g
99.50%
99.64%
99.75%
99.82%
99.37%
1.5 wt%.
The increased elongation of SiCnp/Al-5Cu composites is mainly
attributed to ɑ-Al grains refinement, uniform distribution of SiCnp
and Al2Cu refinement [28e36]: (i) In referring to Fig. 4, ɑ-Al grains
refinement of composites increases the number of crystal grains
obviously. Thus the stress scatters in more fine grains during tensile
deformation, which leads to the less stress concentration and more
uniform plastic deformation [28e30]. Besides, the more grain
boundaries associated with finer a-Al grains are beneficial to hinder
crack propagation and result in better ductility [31], which can be
confirmed by the increased dimples in the fracture surface, as
shown in Fig. 12. (ii) As shown in Fig. 8, a clean interface between
the SiCnp and matrix can enhance the bonding strength of interface
and interfacial load transfer between SiCnp and the matrix, which
are helpful to prolong the shear slide deformation to improve the
fracture elongation of the composites [32,33]. Additionally, uniform
distribution of SiCnp is also beneficial to the improvement of
elongation [34,35]. The SiCnp in composites can change the direction of crack growth, leading to the formation of crack bridging,
branching and deflection in composites. During this process, a lot of
energy is absorbed and the resistance of crack propagation increases, which will improve the elongation to fracture of composites [30]. (iii) The Al2Cu refinement is also beneficial to the increase
of elongation. Usually, the coarse phases can act as stress concentrators, which will provide sites for micro-cracking and reduce the
ductility [36]. In combination with Fig. 6, the finer Al2Cu phases
precipitate uniformly in composites, which can even act as reinforcement phases and hinder the dislocations motions during
deformation. In addition, the high relative densities may also have
contributed to the increase of elongation, as shown in Table 1.
For the variation of UTS and YS, it is reported that there are four
main reasons, including load-bearing strengthening (DsLoad), CTE
(Coefficient of Thermal Expansion) mismatch strengthening
(DsCTE), Orowan strengthening (DsOrowan) and gain refinement
strengthening [37e43]. Firstly, according to load-bearing
strengthening mechanism, the dispersed and well-bonded SiCnp
can directly shear the load to strengthen the composites [37,38].
The DsLoad can be described as the following equation:
1
2
DsLoad ¼ Vp sm
(1)
where sm is the YS of Al-5Cu matrix which is 160 MPa; Vp is the
volume fraction of reinforcement, and the volume fraction of x wt%
SiCnp/Al-5Cu (x ¼ 0.5, 1.0, 1.5, 2.0) composites is 0.44%, 0.88%, 1.32%
and 1.77%, respectively.
Secondly, the difference of CTE between SiCnp and the matrix
leads to the generation of geometrically necessary dislocations
during solidification, which can also strengthen the composites.
This strengthening mechanism can be described as the following
equation [13,39].
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J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
Fig. 10. The variation of mechanical properties of the SiCnp/Al-5Cu composites with different SiCnp content: (a) UTS and YS strength, (b) elongation.
DsCTE
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
12DaDTVp
¼ bGm b
bdp 1 Vp
(2)
Vp* ¼ εVp
where b is strengthening coefficient equal to 1.25; Gm is shear
modulus of Al matrix equal to 25.8 GPa; b is Burgers vector of Al
matrix which is 0.286 nm; Da is the CTE difference between SiCnp
and matrix which is 19.9 106 K1; DT is the difference between
the pouring temperature and test temperature which is 690 K; dp
and Vp are the mean size and volume fraction of SiCnp, respectively.
Thirdly, the addition of SiCnp refines ɑ-Al grains and generates
more grain boundaries, which can effectively hinder the dislocation
movement during deformation. Moreover, the dispersed SiCnp
themselves are beneficial to improve the UTS and YS of the composites, by pinning the dislocations initiated from the matrix and
forming a large number of dislocation loops around SiCnp. Thus
SiCnp can serve as strong obstacles to the dislocation movement,
leading to strength enhancement [40]. Overall, the strength
enhancement resulting from gain refinement and dispersed SiCnp
can be explained by the Hall-Petch relationship (Dshp ) and Orowan strengthening (DsOrowan ), respectively. These two main
strengthening mechanisms can be calculated by using the
following equations, respectively [41e43].
DsOrowan ¼
dp
0:13Gm b
1 ln
2b
1 3 1
dp 2V
p
Dshp ¼ khp
.pffiffiffiffiffiffi
dm
strengthening mechanisms of composites, namely the modified
method [45].
(6)
where Vp* is the effective SiCnp content and ε is a coefficient related
to microstructural features of the composites.
In order to obtain ε value, the load-bearing strengthening, CTE
strengthening and Orowan strengthening are taken into consideration because they are related with volume fraction of SiCnp. Lloyd
et al. reported that only the grain sizes smaller than 10 mm would
obviously influence the yield strength in AMCs due to a low kh-p
value of Al alloys [46]. However, the average grain size of the matrix
and composites is from ~60 mm to ~30 mm in this work. Therefore,
the effect of gain refinement strengthening can be ignored in the
process of obtaining the ε value [45].
Fig. 11 shows the comparison of calculated YS using the arithmetic summation method and measured YS, in which the calculated values deviate far from the experimental ones. It is because
that the calculated YS by Eq. (5) is based on the simplifying and
ideal hypothesis. For example, the SiCnp are assumed to be uniformly distributed in the whole matrix and the shape of them is
assumed to be a perfect sphere. However, the SiCnp are not uniformly distributed in the matrix and most of SiCnp are distributed at
(3)
(4)
where khp is the Hall-Petch constant and dm is the average grain
diameter, respectively.
The yield strength of the composites is estimated by considering
multiple strengthening mechanisms, namely the arithmetic summation method. Thus, the predicted YS (sc ) of the composites can
be calculated by the following equation according to the arithmetic
summation method [42,44]:
sc ¼ sm þ DsLoad þ DsCTE þ DsOrowan
(5)
However, recent studies have reported that the predicted values
are much higher than the experimental values by using this
method. Since the strengthening effects are related to the nominal
SiCnp content, Wang et al. suggested that a coefficient ε should be
introduced to account for the effective SiCnp content (Vp*) in the
Fig. 11. Calculated YS using the arithmetic summation method and the modified
method (ε ¼ 0.01) without regard for grain refinement strengthening.
J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
855
Fig. 12. SEM images of the tensile fracture surface of SiCnp/Al-5Cu composites with different SiCnp content: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, (d) 2 wt%.
the grain boundaries, which can significantly reduce the
strengthening effects. In order to calculate YS more accurately, we
replace the Vp with the Vp* in Eqs. (1)e(3). The results shown in
Fig. 11 suggest that the calculated yield strength using the modified
method can fit well with the measured data when choosing 0.01 as
the value of ε, which implies the rationality of this modified
method. However, when the content is beyond 1.5 wt%, the
measured YS again deviates from the calculated value, which may
result from the SiCnp agglomeration.
Therefore, the theoretical contributions of the four strengthening mechanisms to the YS of the composites can be accurately
calculated according to the modified method. The theoretical
contribution values of four strengthening mechanisms to YS of the
composites are shown in Table 2. Obviously, increasing SiCnp leads
to higher theoretical contributions to YS of the composites, which is
consistent with variation of the measured YS, and both of them
increase until to reach optimal at 1.5 wt%. It can be also seen that
the effect of load-bearing strengthening on the YS of composites is
very limited due to the low SiCnp content (2 wt% for maximum).
Among the four strengthening mechanisms, the DsCTE is the most
important contributor due to the large CTE difference between the
SiCnp and matrix. The contribution of DsOrowan is also important.
Fig. 12 shows the SEM images of the tensile fracture surface of
the SiCnp/Al-5Cu composites with different SiCnp content. Obviously, the SEM analysis shows that the composites exhibit ductile
fracture with dimples. As shown in Fig. 12(a), there are a few
dimples on the fracture surface. With the increase of SiCnp content,
the number of dimples increases and the dimples are uniformly
distributed on the fracture surface. However, the number of dimples decreases when the SiCnp content increases to 2.0 wt%. That
may be attributed to the SiCnp agglomeration existing in the composites at higher SiCnp content. As shown in Fig. 12(d), the hole and
SiCnp agglomeration marked with red circles can be observed on
the fracture surface. Commonly the cracks are easier to form in the
pore and agglomeration regions, leading to the fracture failure of
composites. Thus the 2.0 wt% SiCnp/Al-5Cu composites have low
strength and poor elongation, as shown in Fig. 10.
4. Conclusions
(1) The SiCnp are uniformly distributed in the SiCnp/Al-5Cu
composites by the processes of UT combined with mechanical stirring of the melt.
(2) The primary a-Al and Al2Cu phases are refined in the composites due to the addition of SiCnp, compared with Al-5Cu
matrix alloy. The sizes of primary a-Al and Al2Cu phases
Table 2
Theoretical contributions of four strengthening mechanisms for YS of SiCnp/Al-5Cu composites.
Samples
DsLoad (MPa)
DsCTE (MPa)
DsOrowan (MPa)
Dshp (MPa)
sm (MPa)
0.5SiCnp/Al-5Cu
1.0SiCnp/Al-5Cu
1.5SiCnp/Al-5Cu
2.0SiCnp/Al-5Cu
0.004
0.007
0.011
0.014
6.5
9.3
11.4
13.2
5.1
6.3
7.3
8.0
0.4
2.3
2.6
1.5
160
160
160
160
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J. Li et al. / Journal of Alloys and Compounds 769 (2018) 848e857
decrease, when the SiCnp content increases from 0 wt% to
1.5 wt%.
(3) The composites with SiCnp content from 0 to 1.5 wt% possess
high relative densities, but the further increment in SiCnp
content leads to a decline of density due to the SiCnp
agglomeration.
(4) Compared to Al-5Cu matrix alloy, the SiCnp/Al-5Cu composites exhibit significant enhancement in UTS, YS and elongation when the SiCnp content increases from 0 wt% to 2.0 wt%.
Among them, the SiCnp/Al-5Cu composites having 1.5 wt%
SiCnp exhibit the best UTS, YS and elongation, which are
increased by 18.7%, 11.3% and 25.3%, respectively, compared
with Al-5Cu matrix alloy.
(5) A modified method is applied to predict the theoretical YS,
which is in good agreement with experimental value. The
enhancement of strength is attributed to four strengthening
mechanisms, among which the DsCTE and DsOrowan are the
most important contributors. The increase in elongation is
mainly attributed to a-Al grains refinement, Al2Cu refinement and uniform distribution of SiCnp.
Acknowledgments
This work was funded by the Project 51574129 supported by
National Natural Science Foundation of China, and by the project
JCKY 2016209A001. The authors would also express their appreciation to the Analytical and Testing Centre, HUST.
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