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Author’s Accepted Manuscript
Fabrication, microstructure refinement and
strengthening mechanisms of nanosized SiCP/Al
composites assisted ultrasonic vibration
Qiang Li, Feng Qiu, Bai–Xin Dong, Run Geng,
Ming–ming Lv, Qing–Long Zhao, Qi–Chuan Jiang
www.elsevier.com/locate/msea
PII:
DOI:
Reference:
S0921-5093(18)31136-5
https://doi.org/10.1016/j.msea.2018.08.060
MSA36831
To appear in: Materials Science & Engineering A
Received date: 24 May 2018
Revised date: 16 August 2018
Accepted date: 19 August 2018
Cite this article as: Qiang Li, Feng Qiu, Bai–Xin Dong, Run Geng, Ming–ming
Lv, Qing–Long Zhao and Qi–Chuan Jiang, Fabrication, microstructure
refinement and strengthening mechanisms of nanosized SiCP/Al composites
assisted ultrasonic vibration, Materials Science & Engineering A,
https://doi.org/10.1016/j.msea.2018.08.060
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Fabrication, microstructure refinement and strengthening mechanisms of
nanosized SiCP/Al composites assisted ultrasonic vibration
Qiang Lia,b, Feng Qiua,b,c*, Bai–Xin Donga,b, Run Genga,b, Ming–ming Lva,b, Qing–Long
Zhaoa,b, Qi–Chuan Jianga,b
a
State Key Laboratory of Automotive Simulation and Control, Jilin University, P.R. China
b
Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials
Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun, Jilin Province,
130025, P.R. China
c
Qingdao Automotive Research Institute of Jilin University, No. 1, Loushan Road, Qingdao
266000, China
*
Corresponding author. Tel./fax:+86 431 85094699. qiufeng@jlu.edu.cn
Abstract
The performances of particulate–reinforced aluminum matrix composites are strongly
dependent on alloying elements, precipitates and added particulates. To reveal the
sole influence mechanisms of high volume fraction of nanosized particulates on the
solidification behavior, microstructure and mechanical properties of aluminum alloys,
nanosized SiCP (60 nm) was incorporated into commercial pure Al at different volume
fractions (i.e., 0, 1, 3, 5, 7 and 9 vol.%) by stir–casting assisted ultrasonic vibration.
The results reveal that a fairly uniform dispersion of nanosized SiCP throughout the
matrix was achieved at a volume fraction as high as 7 vol.%. Average α–Al dendritic
sizes were significantly refined from 270 μm for the matrix to 90 μm in the solidified
microstructure of nanocomposites. Thermal analysis during solidification indicates
that the presence of nanosized SiCP increased the nucleation temperature of α–Al,
whilst recalescence during solidification process disappeared. Additionally, the yield
and ultimate tensile strength of the nanosized SiCP/Al composites at both ambient
1
temperature and 453 K were remarkably improved, whilst remaining suitable fracture
strain. Theoretical analysis suggests that the significant strength increments induced
by nanosized SiCP at ambient temperature could be attributed to thermal mismatch
strengthening, Orowan strengthening and grain refinement strengthening, while the
pinning effect of nanosized SiCP could predominantly account for the strengthening
effect at 453 K.
Keywords
Nanosized SiC particulates; Nanosized SiCP/Al composites; Grain refinement;
Mechanical properties; Strengthening mechanisms;
1. Introduction
In recent years, particulate–reinforced aluminum matrix composites have been
considered as potential materials to meet the urgent requirements of industry
applications in the aerospace and automobile industries due to their unique superior
comprehensive properties compared to monolithic matrices [1–4]. Among the
developed reinforcing particles, SiCP has been applied successfully in a series of
aluminum alloys and is preferred for its favorable properties, such as low cost, good
thermodynamic stability, low coefficient of thermal expansion (CTE) and high
hardness [5].
Over the past decades, extensive studies have been devoted to investigating the
influences of particulates on the microstructures and mechanical properties of
2
aluminum alloys. Most researchers have focused on micron–sized particulate
reinforcements for obtaining significantly improved strengths, whereas this
improvement is always inevitably accompanied by the sacrifice of ductility [6–8].
Recently, it has been reported that composites reinforced by nanosized particulates
could achieve an optimum combination of higher strength and ductility compared to
their counterparts reinforced by micron-sized particulates [8–11]. In addition, as
heterogeneous nuclei of α–Al, nanosized particulates are more potent and effective
than micron–sized particulates because two to three orders of magnitude more
nanosized particulates will be introduced compared with micron–sized particulates at
the same addition levels. In addition, it has been claimed that nanosized particulates
pushed to the advancing solid/liquid (S/L) interface could inhibit the growth of grains
during solidification, which is beyond the function of micron–sized particulates [12].
Furthermore, nanosized particulates could exert a stronger “Zener pinning” effect on
grain boundaries than micron–sized particulates, allowing for higher elevated–
temperature strengths [13]. Among various methods for fabricating aluminum matrix
nanocomposites, stir–casting is the simplest, most cost–effective and easiest route for
mass production. Nevertheless, it is extremely challenging to obtain a uniform
distribution of nanosized particulates in aluminum melts because of their serious
agglomeration inclination [14–17]. Recently, high–intensity ultrasound has been
found to be an effective method to disperse nanosized particulates into melts [18–21].
In addition, it is supposed that nanosized particulate addition by master alloys is
beneficial for promoting particulate dispersion because the agglomerations will be
3
disrupted and separated in advance [22]. K. Amouri et al. [23] prepared 1.5 wt.%
nanosized SiCP particulate–reinforced A356 via the stir–casting technique and found
that the yield and ultimate tensile strength of this composite were improved
considerably by 19.2% and 19.7%, respectively. Lei Wang et al. [22] noted that the
tensile strength of the prepared Al–Cu alloy reinforced by 2 wt.% nanosized SiC
particulates increased remarkably at 298 K, 453 K and 493 K while exhibiting high
plasticity. However, these previous efforts were focused primarily on the aluminum
alloy matrices containing various alloying elements. It is commonly recognized that
complicated interactions usually occur between precipitates and reinforcement
particulates. On the one hand, these elements could affect the refinement potency and
strengthening effect of the particulates via the solute suppression nucleation effect,
constitutional supercooling, and the poisoning effect [24–27]. On the other hand, the
reinforcement particulates could accelerate the precipitation kinetics of the
composites due to the mismatch of thermal expansion coefficients between the
particulates and matrix during cooling, especially for heat–treated alloys [28–30]. As
a result, the strengthening effects of particulates on the matrix are always confounded
by precipitates and solution atoms. Furthermore, the addition levels of nanosized
particulates in composites were very low in most researches. Nevertheless, in many
fields, more particulates are usually required to be incorporated into the matrix to alter
certain properties of composites for structural and functional applications, such as
higher strength, hardess, thermal stability, elastic modulus and lower thermal
expansion coefficients. Therefore, the fabrication techniques should be optimized for
4
the composites with high addition level of nanosized particulates. Conclusively, it is
of great significance and necessity to elucidate the underlying influences of high
addition level of nanosized particulates alone on the aluminum alloys without
interference of the alloying elements.
In this present work, in order to eliminate the interference of solutes and
precipitates, commercial pure Al as the matrix was deliberately employed. High
volume fraction of nanosized SiCP (60 nm) as reinforcements, in the form of Al/SiCP
mixture compacts, were incorporated into pure Al melt using the stir–casting method
assisted ultrasonic vibration. The uniform dispersion of nanosized SiCP in the matrix
was achieved, whilst the interfacial reaction between SiCP and molten pure Al was
successfully avoided. The sole influences of nanosized SiCP on the solidification
behavior, microstructure evolution and mechanical properties of aluminum alloys
were investigated. The underlying grain refinement mechanism and ambient and
elevated–temperature
strengthening
mechanisms
of
the
composites
were
systematically discussed.
It is supposed that this work is of theoretical and engineering significance for
further studying the improving mechanisms of nanosized particulates on the
performance of aluminum alloys and provide fundamental insight into the widespread
applications of aluminum matrix composites reinforced by high volume fraction of
nanosized particulates.
5
2. Experimental procedure
2.1 Preparation of nanosized SiCP/Al composites
Fig. 1 Schematic of processing of nanosized SiCP/Al composites, (a) mixing of SiC
and Al powders by ball milling, (b) cold pressing the blend into an Al/SiCP mixture
compact, (c) introducing nanosized SiC particulates into the pure Al melt through
Al/SiCP mixture compacts and dispersing them assisted ultrasonic vibration, and (d)
pouring the molten melt into the preheated steel mold.
In the present work, commercial pure Al ingots with a purity of 99.0% were used
as the matrix. Al powders (13 μm) and SiC powders (60 nm) were employed as raw
materials to fabricate Al/SiCP mixture compacts with a volume fraction ratio of 4:1.
Fig. 1 shows a nanoprocessing schematic of nanosized SiCP/Al composites. The
preparation process of Al/SiCP mixture compacts included two steps, as shown in Fig.
1(a, b): First, the Al powders and SiC powders were uniformly mixed by high energy
ball milling at 60 rpm for 30 h. Subsequently, the blends were condensed coldly into
cylindrical compacts with a size of ϕ30 mm×(40–50) mm. Prior to the addition of the
Al/SiCP mixture compacts, pure Al was melted and homogenized at 1073 K for one
hour inside an electric resistance furnace. Then, the molten aluminum was first stirred
mechanically by a graphite stirrer for approximately 5 min to disperse the SiCP, and
then, ultrasonic vibration was applied immediately to further disperse the nanosized
6
particulates, as shown in Fig. 1(c). Subsequently, the melt was degassed and poured
into a steel mold preheated to 373 K with a size of 200 mm×150 mm×12 mm at a
temperature of 1023 K while, temperature variations with time during solidification
were carried out, as shown in Fig. 1(d). In our experiments, composites with nominal
SiCP addition levels of 1, 3, 5, 7 and 9 vol.% were prepared successfully, hereafter
denoted as CP–Al–1, CP–Al–3, CP–Al–5, CP–Al–7 and CP–Al–9, respectively. For
comparison, a commercial pure Al ingot was melted and cast with the same process,
denoted as CP–Al–0.
2.2 Microstructural characterization
The metallographic samples were ground, polished and etched by Keller’s reagent.
The microstructures of the pure Al and composites were characterized by optical
microscopy (OM, Olympus PMG3, Japan), scanning electron microscopy (SEM,
Tescan vega3 XM, Czech Republic) and high resolution transmission electron
microscopy (HRTEM, JEM–2100F, Japan). The dendritic sizes of α–Al were
measured by the linear intercept method. The morphologies and distribution of
nanosized SiC particulates in the composites were characterized by field emission
scanning electron microscopy (FESEM, JSM–6700F, Japan).
2.3 Ambient and elevated–temperature mechanical behaviors
The hardness of the prepared nanosized SiCP/Al composites was measured by a
Brinell hardness tester with a ball 5mm in diameter under a 125 kgf load. Tensile tests
were conducted at ambient temperature and 453 K with a tensile rate of 1×10–4 using a
servo–hydraulic material testing system (MTS, MTS 810, USA). For the tensile tests
7
at ambient and 453 K, the samples were cut into dogbone–shaped specimens with a
gauge cross–section of 4.0×2.5 mm2 and a gauge length of 10.0 mm.
3. Results and discussion
3.1 Microstructures of nanosized SiCP/Al composites
Fig. 2 Optical micrographs of the pure Al and nanosized SiCP/Al composites with
varying SiCP contents: (a) CP–Al–0, (b) CP–Al–1, (c) CP–Al–3, (d) CP–Al–5, (e)
CP–Al–7 and (f) CP–Al–9.
Fig. 2 shows the OM morphologies of pure Al and nanosized SiCP/Al composites
with different contents of nanosized SiC particulates. As observed in Fig. 2(a), the
typical microstructure of pure Al exhibits coarse α–Al grains of asymmetrical sizes of
approximately 270 μm. A significant reduction in the grain size of α–Al dendrites
accompanying a morphology transition was observed upon the addition of nanosized
SiC particulates. Apparently, the columnar–to–equiaxed microstructure transition
occurred progressively with increasing SiCP addition from 0 vol.% to 9 vol.%, which
is clearly indicated in Fig. 2(a–f). The average dendritic size variations as a function
8
of SiCP content are plotted in Fig. 3 to quantify the refinement efficiency of nanosized
SiC particulates. With minor addition of 1 vol.% SiCP, the coarse columnar α–Al
dendrites were replaced by refined dendrites, and the average dendritic size decreased
sharply from 270 to 122 μm. When the addition level further reached 5 vol.%, much
smaller dendritic sizes of approximately 100 μm were obtained. However, as the
addition of SiCP continued to increase, the refinement efficiency of nanosized SiC
particulates tended to saturate, and the dendritic sizes decreased slightly in response to
the increase in addition levels. Specifically, the finest dendritic size of approximately
90 μm was achieved with the addition of 7 vol.% SiCP, a reduction of 66.7%
compared to the unreinforced pure Al. Nevertheless, further addition of 9 vol.% SiCP
deteriorated the refinement efficacy and the corresponding composites exhibited a
somewhat larger dendritic size distribution compared to the 7 vol.% SiCP addition.
Fig. 3 The average dendritic sizes of α–Al in the pure Al and nanosized SiCP/Al
composites with varying SiCP contents.
9
Fig. 4 FESEM analysis of the distribution of the nanosized SiC particulates for (a) the
uniform distribution of nanosized SiC particulates in CP–Al–7 and (b) aggregation
regions in CP–Al–9, (c) TEM micrograph of nanosized SiCP particulate in CP–Al–7,
and (d) HRTEM image of the interface between SiCP and the α–Al matrix.
The distribution of nanosized SiC particulates in CP–Al–7 is shown in Fig. 4(a).
Obviously, uniform dispersion of nanosized SiC particulates throughout the matrix
was achieved at a volume fraction as high as 7 vol.%, assisted by ultrasonic
processing. However, clear micro–clusters of nanosized SiC particulates were
detected in CP–Al–9, as marked by the white dotted lines in Fig. 4(b). TEM
micrographs of CP–Al–7 are presented in Fig. 4(c, d). It is evidently revealed that a
nanosized SiC particulate was incorporated into the pure Al matrix. A clear and
perfectly bonded interface between the α–Al and SiCP was also found without
undesirable intermetallics and contaminations, which could be attributed to the short–
10
time residence of nanosized SiC particulates in the melt during stirring and
solidification. Moreover, ultrasonic vibration could facilitate the surface activation of
nanosized SiC particulates [31]. Therefore, a strong interface strength was established,
which could effectively transfer the load from the soft α–Al to the hard SiCP during
tensile deformation. The disregistry δ between the SiCP and pure Al matrix could be
calculated by the Turnbull–Vonnegut equation [32]:
δ=∣as-ac∣/ ac×100%
(1)
where as and ac are the interplanar spacings of the SiCP and matrix, respectively.
Accordingly, the calculation revealed that a small disregistry of approximately 10.3%
(<15%, the maximum value in terms of effective heterogeneous nuclei) was present at
the interface.
Fig. 5 (a) Comparison of the cooling curves of pure Al and nanosized SiCP/Al
composite with 1 vol.% nanosized SiC particulates, (b) magnified image of the dotted
circle in (a), and (c) cooling rate curves of pure Al and (d) the nanosized SiCP/Al
composite with 1 vol.% nanosized SiC particulates.
11
To determine the effect of nanosized SiC particulates on the microstructure of
pure Al, the solidification behaviors of pure Al and the representative composite with
1 vol.% SiCP addition were investigated by thermal analysis and are compared in Fig.
5(a). The crystallization intervals, including nucleation and subsequent grain growth,
of the two alloys are particularly magnified and exhibited in Fig. 5(b). In addition, the
characteristic parameters of thermal histories were obtained, as shown in Fig. 5(c,d),
by combining the cooling curves and the corresponding derivative curves for each
case. As observed, the nucleation of pure Al was initiated at 663.5 ℃. From the
cooling curves in Fig. 5(b), an evident thermal arrest of 662.3 ℃was clearly detected
in the solidification process of pure Al melt, which means that the latent heat released
by the growing nuclei exactly compensated for the heat extraction. Thus, no
undercooling existed, and further nucleation will be stopped as the temperature
decreases to 662.3 ℃. Subsequently, only growth of the α–Al crystal will continue.
However, the recalescence phenomenon disappeared in the presence of 1 vol.%
nanosized SiC particulates, and the corresponding cooling rate was slightly higher
than that of the pure Al melt. Moreover, the nucleation temperature of the composite
increased to 675.4 ℃, as shown in Fig. 5(d). These thermal transformations imply that
nanosized SiC particulates are potent in stimulating the nucleation of α–Al. The
underlying refinement mechanism is schematically illustrated by the schematic
diagram in Fig. 6. As reported [33], a small fraction of nanosized SiC particulates
with larger sizes could serve as effective nucleation substrates for α–Al nuclei at the
beginning of solidification because of their great superiority in terms of undercooling.
12
Afterwards, the newly formed nuclei further grow and simultaneously release large
amounts of latent heat into the melt. As a result, the undercooling of the liquid is
lowered and other potent nucleating particulates may be stifled. Nevertheless, with
advancement of the solid/liquid interface, most nanosized particulates are pushed into
the liquid phase along the solid/liquid interface. They could directly inhibit the growth
of α–Al crystals by impeding the diffusion of the Al atoms on the growing interface.
As the thermal conductivity of SiCP is lower than that of the molten melt [34], the
release of latent heat from the growing α–Al crystals into the surrounding melt is
restricted. As a result, undercooling away from the solid/liquid interface will be
ensured, and the time for nucleation will be prolonged. In this regard, nanosized SiC
particulates could refine the final dendrites of pure Al by both stimulating
heterogeneous nucleation and inhibiting the growth of α–Al crystals. It is easily
conjectured that the inhibition effect will be intensified with more SiCP added.
Nevertheless, it is worth noting that too much addition of nanosized SiC particulates,
as such as in CP–Al–9, will lead to more SiCP being pushed to the grain boundaries
and interdendritic regions, which will accumulate into agglomerations, thus seriously
deteriorating the refinement efficacy.
13
Fig. 6 Schematic diagram of the effect of nanosized SiC particulates on the nucleation
and growth of an equiaxed α–Al dendrite in composites during the solidification
process. (a) A new nuclei formed on a nanosized SiCP; (b) the majority of nanosized
SiC particulates are pushed to the solid/liquid interface, and only a few can be
captured in the interior of the matrix during the growth process; (c) the Al atoms are
blocked by the SiCP pushed to the solid/liquid interface; and (d) the final solidified
microstructure.
3.2 Mechanical properties of nanosized SiCP/Al composites
To investigate the influence of nanosized SiC particulates on the mechanical
properties of composites, Brinell hardness and tensile testing at both ambient
temperature and 453 K of the prepared composites and pure Al were carried out.
3.2.1 Mechanical properties at ambient temperature
The mechanical properties, including Brinell hardness and tensile properties, of
pure Al and nanosized SiCP/Al composites at ambient temperature are presented in
Fig. 7(a). The detailed statistical data are summarized in Table 1. It can be seen more
clearly in Fig. 7(b) that the hardness of the nanosized SiCP/Al composites was
significantly enhanced compared with pure Al, and improved with the increase of
the SiCP addition level from 1 vol.% to 9 vol.%. Specifically, the highest BH (43.8
HB) was reached with the addition of 9 vol.% SiCP, 140.7% higher than that of pure
Al (18.2 HB). Moreover, the yield strength (YS), ultimate tensile strength (UTS) and
14
fracture strain (FS) of pure Al were 37.0 MPa, 66.3 MPa and 39.7%, respectively. As
exhibited in Fig. 7(b) and Table 1, with the addition of 1 vol.% SiCP, the YS and UTS
improved to 51.6 MPa and 83.2 MPa, while the corresponding FS slightly decreased
from 39.7% to 36.2%. As the SiCP addition level increased to 7 vol.% SiCP, the YS
and UTS of the composites were further enhanced to 101.9 MPa and 169.3 MPa,
175.4% and 155.4% higher than those of pure Al, whereas the FS was decreased to
28.6%. However, in the case of 9 vol.% SiCP addition, the YS and UTS of composites
were marginally improved, while the FS dropped sharply to 18.0%, which could be
ascribed to the serious aggregation of nanosized SiC particulates, as observed in Fig.
4(b). It is known that the nanosized particulates among the aggregation regions are
separated by voids. During plastic deformation, cracks will be initiated and propagate
at these weak places first, followed by fracture. As mentioned above, the nanosized
SiCP/Al composite reinforced by 7 vol.% SiCP possesses the optimum comprehensive
properties.
Fig. 7. (a) Tensile engineering stress–strain curves and (b) mechanical properties of
the pure Al and nanosized SiCP/Al composites with varying SiCP contents at ambient
temperature.
15
Table 1 Ambient tensile property values and Brinell hardness of the pure Al and
nanosized SiCP/Al composites with varying SiCP contents.
Alloys
Hardness (HB)
YS (MPa)
UTS (MPa)
FS (%)
CP–Al–0
CP–Al–1
CP–Al–3
CP–Al–5
CP–Al–7
CP–Al–9
18.2±0.3
28.2±0.7
36.4±1.1
40.6±2.0
42.9±2.3
43.8±2.6
37.0±2.0
51.6±0.5
63.3±2.6
76.4±3.4
101.9±3.0
103.1±2.7
66.3±4.0
83.2±2.8
130.5±4.5
138.7±3.8
169.3±3.1
170.0±6.5
39.7±3.3
36.2±2.1
35.0±4.3
30.9±3.6
28.6±2.3
18.0±2.1
3.2.2 Elevated–temperature tensile properties
Fig. 8 Tensile engineering stress–strain curves of the pure Al and nanosized SiCP/Al
composites with varying SiCP contents at 453 K.
Tensile engineering stress–strain curves at 453 K are illustrated in Fig. 8 and the
detailed results are given in Table 2. Apparently, the tensile properties of nanosized
SiCP/Al composites at 453 K exhibited a similar tendency to those at ambient
temperature. The YS, UTS and FS of pure Al at 453 K were 18.2 MPa, 36.3 MPa and
50.3%, respectively. As shown in Fig. 8 and Table 2, the nanosized SiCP/Al
16
composites exhibited increased YS and UTS as well as decreased FS in the presence
of nanosized SiC particulates. For the composites containing 7 vol.% SiCP, the
highest YS and UTS , 87.9 MPa and 115.1 MPa, were achieved, 383.0% and 217.1%
higher than those of pure Al, whereas the corresponding FS was reduced to 25.7%.
Furthermore, compared with those of 7 vol.% SiCP, the YS, UTS and FS were
adversely affected with SiCP addition as high as 9 vol.% due to the presence of
nanosized particulate agglomerations. Additionally, it is worth noting that nanosized
SiC particulates contribute more obviously to the strength at 453 K than to that at
ambient temperature.
Table 2 The tensile property values of the pure Al and nanosized SiCP/Al composites
with varying SiCP contents at 453K.
Alloys
YS (MPa)
UTS (MPa)
FS (%)
CP–Al–0
CP–Al–1
CP–Al–3
CP–Al–5
CP–Al–7
CP–Al–9
18.2±1.3
39.3±2.1
53.6±2.6
70.1±4.3
87.9±4.6
83.1±6.1
36.3±2.6
56.2±2.3
91.6±4.2
86.2±3.9
115.1±6.1
105.3±6.2
50.3±3.5
35.7±3.6
31.7±2.9
39.0±4.2
25.7±2.9
18.9±4.6
3.2.3 Strengthening mechanisms of the nanosized SiCP/Al composites
It has been demonstrated that the hardness and yield strengths of composites are
closely correlated to the distribution and volume fraction of the reinforcing
particulates, concentrations of grain boundaries and dislocation densities. The
obstruction of dislocation motion by reinforcements and grain boundaries will
17
improve the hardness and strength of the matrix. Fig. 9 presents a schematic of the
reinforcing effects of pure Al and composites in the present work. Generally, there
will be much fewer dislocations generated in the pure Al matrix during the cooling
process compared to the composites. Moreover, without the pinning effect of
precipitates, nanosized particulates and solute atoms, the dislocations in the pure Al
can travel across the matrix easily. Therefore, the hardness and strengths of pure Al
are very limited and depend on the strengthening effect of grain boundaries.
Apparently, introducing nanosized SiC particulates as a reinforcing phase could
significantly improve the hardness and strengths of the pure Al, which could be
attributed to the synergistic effects of grain refinement strengthening (∆σHP), Orowan
strengthening (∆σOrwan), thermal mismatch strengthening (∆σCET) and load–bearing
strengthening mechanisms [35,36]. As the amount of nanosized particulates
introduced into the pure Al in the present research was small (9 vol.% at most), the
effect of load–bearing strengthening can be neglected. Thus, the corresponding
increments in yield strengths induced by nanosized SiC particulates can be calculated
quantitatively as follows [37]:
∆σ=∆σHP+[(∆σCET)2+(∆σOrwan)2]1/2
(2)
Grain refinement strengthening: the relationship between grain sizes and yield
strength can be described empirically by the Hall–Petch equation [37]:
∆σHP=k(d–1/2–d0–1/2)
(3)
where k=74 MPa μm1/2 [37] represents the Hall–Petch slope for pure Al, and d
and d0 represent the average grain sizes of the composites and pure Al matrix,
18
respectively. Accordingly, the contributions of grain refinement strengthening were
calculated and are listed in Table 4.
Fig. 9 Schematic of the reinforcing effects of (a) pure Al and (b) nanosized SiCP/Al
composites.
Accordingly, the Orowan strengthening mechanism has a pronounced effect on
the enhancement of the yield strength with regard to nanosized particulates as
reinforcements, which can be calculated by the Orowan–Ashby equation [37]:
∆σOrwan=0.13Gb/λ ㏑
λ=D[(2VP)–1/3–1]
D
2b
(4)
(5)
where G and b represent the shear modulus and Burgers vector of the matrix,
while D, λ, and VP represent the average diameter, average inter–particulate spacing
and volume fraction of nanosized SiC particulates, respectively. For pure Al, G=26.2
GPa and b=0.286 nm [37]. Evidently, λ will be reduced with an increase in the amount
of and uniform dispersion of nanosized SiC particulates, therefore, the effects of
19
Orowan strengthening will also be intensified. The contributions of Orowan
strengthening are listed in Table 4.
Thermal mismatch strengthening: the difference in CTEs between the ceramic
particulates and the matrix promotes residual stress around the particulates, and then,
plastic deformation occurs during solidification. Therefore, an enhanced density of
geometrically necessary dislocations at and in the vicinity of the reinforcement–
matrix interfaces will be generated. The contributions of CTE mismatch strengthening
to the yield strength can be expressed as follows [37]:
∆σCTE=ηGbρ1/2
(6)
ρ=12∆α∆TVP/[bD(1–VP)]
(7)
where η is a constant equal to 1.25 [36], and ρ is the enhanced dislocation density.
∆α is the difference in the CTEs of the matrix (23.6×10–6 K–1) [37] and SiCP (4.7×10–6
K–1) [38]. ∆T is the difference in the processing temperature (solidification
temperature of 933 K) and the mold temperature (373 K). The contributions of
thermal mismatch strengthening are listed in Table 4.
Considering the overall strengthening mechanisms mentioned above, the
theoretical yield strengths σT of the nanosized SiCP/Al composites can be estimated
quantitatively by the equation below [37]:
σT=σ0+∆σHP +[(∆σOrwan)2+( ∆σCTE)2]1/2
20
(8)
Table 4 Theoretical calculations of the contributions of each strengthening
mechanism to the yield strengths of nanosized SiCP/Al composites.
σ0
∆σHP
∆σOrwan
∆σCTE
σT
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
CP–Al–1
37.0
2.2
28.6
81.1
125.2
CP–Al–3
37.0
2.6
49.3
141.7
189.6
CP–Al–5
37.0
3.1
68.4
184.9
237.2
CP–Al–7
37.0
3.3
82.7
221.1
276.3
Alloys
As exhibited in Table 4, the theoretical calculations suggest that Orowan
strengthening and thermal mismatch strengthening contribute most to the increments
of yield strengths in nanosized SiCP/Al composites. A comparison of the theoretical
and experimental yield strengths is illustrated in Fig. 10. Obviously, the theoretical
yield strengths of the nanosized SiCP/Al composites are much larger than the
experimental values. Moreover, the gaps between them are magnified gradually with
increasing SiCP content. As mentioned above, only a small amount of SiCP is engulfed
and readily contained in the interior of the matrix during the solidification process,
and these SiC particulates are predominantly responsible for the significant
increments of yield strengths as reinforcements. However, the majority of the added
nanosized SiC particulates will be pushed to and distributed at the grain boundaries
and interdendritic regions. They are inefficient in improving the strength of the matrix
at ambient temperature and even detrimental to ductility.
It is well known that the diffusion of atoms at grain boundaries will be
21
accelerated under high temperatures, leading to grain boundary sliding when subject
to even a small external stress. The thermally stable nanosized SiC particulates along
grain boundaries could effectively pin the grain boundaries and retard the sliding and
rotation of grain boundaries. Therefore, nanosized SiC particulates will greatly
contribute to the strengths of nanosized SiCP/Al composites at elevated temperatures,
as exhibited in Fig. 9(b). Moreover, dislocation climbing will be initiated at elevated
temperatures. The nanosized SiC particulates can powerfully hinder the dislocation
climbing at high temperatures. As a result, the strengths of the composites are
significantly improved.
Fig. 10 Comparison of the theoretical and experimental yield strength of the pure Al
and nanosized SiCP/Al composites with varying SiCP contents.
22
4. Conclusions
(1) The solidification microstructures of nanosized SiCP/Al composites were found
to be greatly refined with increasing SiCP additions. The average dendrite size was
sharply reduced by 66.7% at most with 7 vol.% SiCP additions. Thermal analysis
indicates that nanosized SiCP could increase the nucleation temperature of α–Al,
whilst recalescence during solidification process disappeared due to the presence of
nanosized SiCP. These thermal transformations imply that nanosized SiCP are potent
in stimulating the nucleation of α–Al crystals. Moreover, the growth of α–Al crystal
was powerfully inhibited by the nanosized SiCP along the solid/fluid interface. With
the increasing in the addition level, the inhibition effect will play the dominant role in
the refinement of composites.
(3) The Brinell hardness, yield strength and ultimate tensile strength of the nanosized
SiCP/Al composites at ambient temperature were significantly improved with
increasing SiCP addition, whereas the relevant FS reduced. Upon 7 vol.% SiCP
addition, the nanosized SiCP/Al composite exhibited the optimum comprehensive
properties. The BH, YS and UTS were 42.9 HB, 101.9 MPa and 169.3 MPa,
improved by 135.7%, 175.4% and 155.4% compared with those of pure Al,
respectively. Thermal mismatch strengthening and Orowan strengthening may be the
dominant mechanisms responsible for the dramatically enhanced strengths, while
grain refinement strengthening contributes little.
(4) The composite with 7 vol.% SiCP addition exhibited the highest strength at 457 K.
23
The YS and UTS were 87.9 MPa and 115.1 MPa, improved by 383.0% and 217.1%
compared to those of pure Al. Nanosized SiC particulates contributed more obviously
to the strengths at 453 K than to those at ambient temperature. These enhancements
primarily result from the pinning effect of nanosized SiC particulates on grain
boundaries and dislocation climbing.
Acknowledgements
This work is supported by the National Key R&D plan (No. 2017YFB0703101),
the National Natural Science Foundation of China (NNSFC, No.51771081 and No.
51571101), the Source Innovation Plan of Qingdao City, China (No. 18-2-2-1-jch),
and the Project 985–High Properties Materials of Jilin University.
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Highlights
1.Nanosized SiCP
refined the microstructure via heterogeneous nucleation and
inhibited growth.
2.SiCP enhanced strengths at 298 K mainly due to thermal mismatch and Orowan
strengthening.
3.Pinning effects of SiCP on grain boundaries and dislocation climbing enhanced
strengths at 453 K.
30
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