Acta Metall. Sin. (Engl. Lett.), 2014, 27(2), 295–305
DOI 10.1007/s40195-014-0045-3
Physical, Mechanical, and Tribological Attributes of Stir-Cast
AZ91/SiCp Composite
K. K. Ajith Kumar • Abhilash Viswanath • T. P. D. Rajan • U. T. S. Pillai • B. C. Pai
Received: 11 September 2013 / Revised: 16 January 2014 / Published online: 16 April 2014
Ó The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014
Abstract In the present investigation, composites with silicon carbide particle (SiCp) as reinforcement and AZ91
magnesium alloy as matrix have been synthesized using liquid metal stir-casting technique with optimized processing
conditions. The composites with good particle distribution in the matrix, and better grain refinement and good interfacial
bonding between the matrix and reinforcement have been obtained. The effect of SiCp content on the physical, mechanical,
and tribological properties of Mg-based metal matrix composite (MMC) is studied with respect to particle distribution,
grain refinement, and particle/matrix interfacial reactions. The electrical conductivity, coefficient of thermal expansion,
micro- as well as macro-hardness, tensile and compressive properties, and the fracture behavior of the composites along
with dry sliding wear of the composites have been evaluated and compared with the base alloy.
KEY WORDS:
Composite; AZ91D alloy; Casting alloy; Mechanical properties; Wear
1 Introduction
The applications of Mg in various sectors have progressively grown in a span of last few decades. The rapid
strides and impact in a spectrum of automotive and aerospace products are because of lightness of Mg, i.e., 30%
lighter than aluminum, 75% lighter than zinc, and 80%
lighter than steel. Besides, magnesium has the highest
strength-to-weight ratio of commonly used structural metals [1, 2]. Other important advantages of Mg and its alloys
are good castability, high die casting capability, good
machinability, and good ductility. In spite of these properties, Mg alloys have limited creep and wear resistances.
Researchers have used various methods such as elemental
Available online at http://link.springer.com/journal/40195
K. K. Ajith Kumar A. Viswanath T. P. D. Rajan U. T. S. Pillai (&) B. C. Pai
Material Science and Technology Division, CSIR-National
Institute for Interdisciplinary Science and Technology,
Thiruvananthapuram 695019, India
e-mail: [email protected]
addition [3–5] and second-phase particle/fiber additions to
Mg alloys to circumvent these limitations. Among them,
the second-phase particle/fiber addition to Mg alloys is
momentous.
Most of the research works on particle-reinforced
MMCs have been carried out on Al alloys [6, 7] when
compared with Mg alloys. The fabrication methodology
plays a pivotal role in the development of Mg MMCs with
better properties. The main challenges faced during fabrication of Mg composites are oxidation, wettability, and
distribution of reinforcement particles in the matrix. Several methods such as stir casting, squeeze casting, powder
metallurgy, infiltration, gas injection, spray forming, etc.,
have been adopted to overcome these problems [1].
Majority of the reported studies in Mg-MMCs involve
infiltration technique as a prime fabrication method. Kevorkijan [8] has successfully fabricated AZ80/SiCp system
by pressure-less, low pressure (about 0.3 MPa), and moderate pressure (about 0.8 MPa) infiltrations techniques and
reported that the composite fabricated through moderate
pressure infiltration is void free, having homogeneous
microstructure with superior mechanical properties in
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comparison with the base alloy. Vacuum stir-casting
method has been employed by Gui et al. [9] to fabricate
15 wt% SiCp-reinforced Mg–9Al–1Zn and Mg–5Zn–1Zr
systems and found that this process eliminates entrapment
of external gas into the melt and oxidation of magnesium
during stirring. Wang et al. [10] have followed ultrasonic
method for introducing SiC nanoparticles in Mg matrix and
reported that the grains of the composites are refined and
exhibited improved mechanical properties. Gertsberg et al.
[11] have used high-pressure die-casting method for making AZ91D/SiC composite components after fabricating
the composite with stir casting and reported that the composite yielded superior high-temperature properties with
improved wear resistance.
Among the liquid-state processes, stir-casting technology is considered to be the most potential method for
engineering applications because it is easy and economical.
In addition, the composites fabricated by this process can
be formed in near net shape using conventional foundry
techniques. This empowers stir casting as the most popular
commercial method of producing Al-based MMCs [1]. In
recent years, research on Mg-based MMCs are more
demanding for engineering applications and very little
work has been reported on the fabrication of the composite
by stir-casting method. The present study aims to synthesize SiCp-reinforced AZ91 alloy composite using stircasting technique and characterize their physical,
mechanical, and tribological properties.
2 Experimental
2.2 Physical Attributes
The volume fraction of the reinforcements in the matrix
was determined using the following equation:
Vr ¼
Wr =qr
;
Wr =qr þ Wm =qm
where Vr is the volume fraction of reinforcement, Wr is the
mass fraction of reinforcement, Wm = 1 - Wr is the mass
fraction of matrix, and qr = 3.2 g/cm3 and qm = 1.8 g/cm3
are the densities of reinforcements and the matrix,
respectively. Archimedes’ principle was used to obtain
measured density (qmc). The density measurements
involved weighing of samples when in air and when
immersed in distilled water. The theoretical density (qth)
was obtained using rule of mixture as mentioned below:
qth ¼ qm Vm þ qr Vr ;
The materials chosen for this investigation were AZ91
(Mg–9.3Al–0.8Zn–0.18Mn) alloy as matrix and SiCp
having an average size of *23 lm as reinforcement. The
Mg composites were synthesized with different fractions of
SiCp (5, 10, 15, 20, and 25 wt%) using stir-casting technique. The synthesis involved melting of the Mg alloy
under argon atmosphere in a steel crucible using a resistance heating furnace. Once the metal temperature reached
750 °C, the melt was stirred using a steel impeller with a
rotation speed of 750 r/min. During stirring, the SiC particles (preheated to 600 °C to remove volatile impurities
and other oxide layers) were added into the melt. The
stirring time was increased with increase in amount of
SiCp. After complete addition of SiCp, the stirring was
continued for 10 min. Subsequently, the melt was poured
into a 300 °C preheated steel plate mold with 10 mm
thickness. During pouring, sulfur was dusted to avoid
oxidation of the melt surface.
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ð2Þ
where Vm = 1 - Vr is the volume fraction of the matrix.
Density measurement values were compared with the
theoretical density values. These values are correlated
using the following relations to obtain porosity (P) of the
composites:
P ¼ 1 qmc =qth :
ð3Þ
The coefficient of thermal expansion (CTE) of the AZ91
alloy and AZ91/SiCp composites (using 6-mm-diameter
and 20-mm-length samples) were determined using
Netzsch DIL402PC Dilatometer, by measuring the linear
displacement of the samples as a function of temperature in
the temperature range of 30–150 °C. Theoretical CTE
values (a) were formulated using the following equation:
ath ¼ ðEr ar Vr þ Em am Vm Þ=ðEr Vr þ Em Vm Þ;
2.1 Composite Casting
ð1Þ
ð4Þ
where Er = 450 GPa and Em = 45 GPa are the Young’s
moduli of SiCp and AZ91, ar = 6.6 9 10-6/K and
am = 26 9 10-6/K are the CTE values of reinforcement
and the matrix, respectively.
Electrical conductivity measurements were also carried
out on the polished samples using TECHNOFOUR conductivity meter type 901 Eddy current testing machine.
2.3 Phase Identification and Microstructural
Characteristics
XRD technique was used to evaluate the composition of
AZ91 alloy and AZ91/SiCp composites using Philips PW
1710 Powder Diffractometer with CuKa (k = 0.15418 nm)
radiation. This analysis was carried out on fine powders of
the composite within the diffraction range of 20°–80° at a
scanning speed of 2°/min.
The microstructure of the specimens was examined
using an optical microscope (Leica DMRX optical
K. K. Ajith Kumar et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(2), 295–305
microscopy). The specimens for microscopy were prepared
by standard polishing methods using SiC abrasive papers
followed by selvyte cloth polishing with diamond paste. To
understand the variation in grain size, specimens were
etched with acetic picral etchant (5 mL acetic acid, 6 g
picric acid, 10 mL H2O, and 100 mL ethanol). The
microstructural images were taken from the same area of
the castings to have comparable cases. Detailed microstructural observations requiring magnification were carried out using scanning electron microscope (SEM) (JEOL,
JSM 35C) with an accelerating voltage of 15–30 keV.
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and 39.4 N with sliding speed and sliding distance maintained at 1 m/s and 1,800 m, respectively. Since the wear
testing was carried out on a microprocessor-controlled
machine, the height loss and frictional force could be
monitored simultaneously. Wear surfaces and wear debris
of the selected samples were characterized using SEM
(JSM-5800, JEOL)
3 Results and Discussion
3.1 Physical Characteristics
2.4 Mechanical Testing
The macrohardness values of AZ91 base alloy and AZ91/
SiCp composites were measured on the polished surface of
the samples using INTENDEC hardness testing machine
for Brinell hardness measurements with a load of 612.5 N
and a dwelling time of 30 s having the ball indenter
diameter of 2.5 mm. The Vickers’ microhardness measurements were carried out using CLEMEX microhardness
machine and the indentations were made on the samples
with a pyramidal diamond indenter having 245 N indenting
loading and a dwelling time of 15 s. The measurements
were taken from the immediate vicinity of the particle/
matrix interface.
Room-temperature tensile tests were carried out using
computer-controlled INSTRON 8801 universal testing
machine. Samples for tensile testing were prepared
according to ASTM E8 standard as shown in Fig. 1. The
compressive tests were carried out on the cylindrical
specimens as per ASTM E9 standard. The fractured surfaces of the tensile tested specimens were also examined
under SEM.
2.5 Tribological Analysis
The dry sliding wear tests of the alloy and composites were
conducted on a pin-on-disk wear testing machine (TR-20,
DUCOM). The test samples were pin specimens having
6 mm diameter and 25 mm length machined out from the
castings. The contact surfaces of the specimens prepared by
grinding with 600-grit silicon carbide paper were subsequently cleaned with acetone. The counter-face material
was hardened chromium steel disk (Rc 64). The wear tests
were carried out with two different normal loads of 19.6
The density values measured using both Archimedes’
principle (qmc) and the rule of mixtures (qth) for the base
alloy and the composites along with porosity are given in
Table 1. The density values of the composites are found to
increase with the increasing mass fraction of SiCp , which
is due to the presence of higher density SiCp, when compared with AZ91 [11]. The porosity also exhibits a gradual
increase as the amount of reinforcements gets enhanced in
the composite. Similar observations are also reported in
[12] about Mg matrix composites reinforced with SiCp via
stir casting. However, the porosity levels in the present
composites are much lower compared to the literature
owing to the lesser particle size and better processing
parameters.
Both the theoretical and measured CTE values are presented in Table 2. The results reveal that the average CTE
values of AZ91/SiCp composites decrease with increase in
SiCp content. Since the CTE value of the SiCp is
(*6.6 9 10-6/K) much lower than the AZ91 alloy
(*26 9 10-6/K), there exists a thermal expansion mismatch between SiCp and AZ91 matrix during heating.
Thus, the SiCp in the composites constrain the expansion of
the matrix [13]. Consequently, the average CTE of the
composite is reduced as the SiCp increases. The theoretical
CTE values of the AZ91/SiCp composites calculated using
the rule of mixtures are also comparable with the measured
CTE values due to less porosity of the composites.
Table 1 Measured density (qmc), theoretical density (qth), and
porosity (P) of AZ91 alloy and AZ91/SiCp composites and the volume fraction of SiCp (Vr)
Material
qmc (g/cm3)
Vr (vol%)
–
qth (g/cm3)
P (vol%)
AZ91
1.804
1.810 [12]
0.33
AZ91/5SiCp
1.834
2.9
1.841
0.38
AZ91/10SiCp
1.873
5.9
1.883
0.53
AZ91/15SiCp
1.913
9.0
1.926
0.67
AZ91/20SiCp
AZ91/25SiCp
1.955
1.988
12.3
15.8
1.972
2.021
0.86
1.09
Fig. 1 Schematic diagram of tensile specimen (unit: mm)
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Table 2 Physical properties of AZ91 and AZ91/SiCp composites
Conductivity
(%IACS)
Theoretical
conductivity
(%IACS)
CTE
(10-6/K)
Theoretical
CTE
(10-6/K)
AZ91
12.70
12.70
25.69
25.69
AZ91/
5SiC
12.60
12.36
23.24
25.04
AZ91/
10SiC
11.00
12.05
22.38
24.45
AZ91/
15SiC
10.57
11.70
21.68
23.77
AZ91/
20SiC
10.17
11.58
20.26
23.54
AZ91/
25SiC
10.11
11.31
18.25
23.01
The electrical conductivity values of the composites
%IACS (internationally annealed copper standards) are
presented in Table 2. The increase in SiCp content in the
composite has decreased the electrical conductivity which
is due to the lower conductivity of SiCp (1.0%–1.7%
IACS). The theoretical electrical conductivity values of
AZ91/SiCp composites, calculated using the rule of mixtures, are very much comparable with the measured values.
3.2 Phases and Microstructure
The XRD analysis of the base alloy and Mg–SiCp composites with 5, 10, 15, 20, and 25 wt% reinforcements
reveals the peaks representative of Mg, SiC, Mg17Al12,
MgO, and Mg2Si phases as in Fig. 2. Among these, formation of MgO and Mg2Si phases is not considered as
desirable. Normally Mg melt has high tendency toward
formation of oxide layers on its surface due to atmospheric
contact. Stirring of the melt may also allow the impregnation of oxides into the melt. However, in the present
process, use of controlled argon atmosphere has greatly
reduced the oxidation levels and this is evident due to very
small MgO peak in the XRD patterns. The presence of
Mg2Si phase has been reported by Bochenek et al. [14] in
SiCp-reinforced Mg–15Al composite. The disintegration of
the reinforcing particle to form SiO2 layer over SiCp is
proposed as the reason behind the formation of Mg2Si.
On visual examination, the machined surfaces of both
the alloy and composites do not show any major casting
defects. The optical micrographs of AZ91 alloy and the
AZ91/SiCp composites with varying SiCp content are
depicted in Fig. 3. Figure 3a shows that the AZ91 alloy
microstructure has two phases, i.e., a-Mg and b-Mg17Al12
phases along and adjacent to the grain boundaries. Figure 3b–f presents the microstructures of AZ91/SiCp composites, which show uniform distribution of SiCp in the
matrix with very few agglomerations. Generally, with
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Mg
SiC
Mg17 Al12
(f)
Intensity (a.u.)
Material
*
Mg 2Si
MgO
*
(e)
(d)
(c)
(b)
(a)
20
30
40
50
60
70
80
2θ (deg.)
Fig. 2 XRD plots of AZ91 a and AZ91/SiCp composites with 5 wt%
b, 10 wt% c, 15 wt% d, 20 wt% e, 25 wt% f SiCp additions
increasing SiCp content, the rate of agglomeration increases in the composites.
The formation of agglomerates may be due to finer size
of the reinforcement and stirring parameters like speed,
time, temperature, etc. [9]. Particles with smaller size are
more prone to clustering. The growth of the primary a-Mg
grains also pushes the particles toward the grain boundary
during solidification. The high surface tension forces, due
to large area/volume ratio at the interface and the small
mass of the particles, contribute to the agglomeration and
clustering of particles at the grain boundaries [12].
Figure 4 shows the microstructures of AZ91 alloy and
AZ91/10SiCp composite after etching. It can be seen from
the microstructures that the grains are very clear for the
base alloy (Fig. 4a) and the SiC particles are located both
at the grain boundaries and inside the primary a-Mg grains
in the composite (Fig. 4b). It is further noticed that addition
of SiCp significantly decreases the grain size of the matrix
in the composite. With increasing SiCp, the grain sizes get
reduced up to 10 wt% SiCp. Beyond that no significant
reduction in grains is achieved. The grain size is reduced
from 65 to 34 lm for AZ91/5SiCp composite and 22 lm
for AZ91/10SiCp composite and beyond that the grain size
reduction is fewer, i.e., 21 lm for AZ91/25SiCp composite.
The refinement of grains in the matrix due to the presence
of SiCp results from the heterogeneous nucleation of the
primary Mg phase. Certain crystallographic orientation
relationships between the particle and the matrix, and the
restricted growth of Mg grains by SiCp during solidification
further contribute to the reduction in grain size [9, 12].
The SEM microstructure of 10 wt% SiCp composite
shown in Fig. 5a reveals good distribution of the particles.
Figure 5b shows the high-magnification SEM micrograph.
As evidenced, SiCp are intimately bonded with magnesium
K. K. Ajith Kumar et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(2), 295–305
299
Fig. 3 Optical micrographs of AZ91 alloy a and AZ91/SiCp composites with 5 wt% b, 10 wt% c, 15 wt% d, 20 wt% e, 25 wt% f SiCp additions
Fig. 4 Microstructures of AZ91 alloy showing grain size a, AZ91/10SiCp composite showing reduced grain size b
matrix, and no evidence of significant chemical reaction
between Mg and SiCp occurred at the interfaces. Moreover,
the interfacial integrity of SiCp with the matrix is found to
be good. The sharp and clean (precipitate and reaction free)
interface further indicates (Fig. 5b) that no reaction has
occurred at the interfaces. Similar observation has been
reported by Poddar et al. [12] for SiCp-reinforced Mg
composite. The successful incorporation of SiCp in the
AZ91 matrices in all the investigated experimental conditions indicate good particle/matrix compatibility for the
cast AZ91/SiCp composites. The feasibility of compo-cast
AZ91/SiCp composites has been studied and it is concluded
that Mg wetted well with SiC and Mg is a better host for
particle embedment than Al [15]. The easy incorporation
and fairly good dispersion of SiCp in Mg matrix is due to
better thermodynamic stability of SiCp than Mg.
3.3 Mechanical Properties
Table 3 shows the mechanical properties of the base alloy
and the composites in terms of macrohardness, microhardness (Vickers’ hardness), ultimate tensile strength
(UTS), yield strength (YS), and ultimate compressive
strength (UCS). It is observed from Table 3 that the
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Fig. 5 SEM micrograph of AZ91/10SiCp composite a, the SEM micrograph with high magnification depicting the cleaner interface between the
reinforcement and the matrix b
Table 3 Mechanical properties of AZ91 and AZ91/SiCp composites
Material
Macrohardness (BHN)
Microhardness (HV)
AZ91
63
63
92
187
310
AZ91/5SiC
82
338
112
184
317
AZ91/10SiC
86
346
124
182
334
AZ91/15SiC
90
358
127
182
344
AZ91/20SiC
92
360
136
184
353
AZ91/25SiC
97
364
141
184
364
increase in hardness values of the composites is remarkable
compared to the base alloy. The increase in the hardness
values of the composites are attributed to the following
factors: (1) hardening of the soft matrix by the reinforcing
particle, (2) Obstruction of the dislocation motion in
thematrix by the reinforcements and (3) refinement of the
matrix grains by SiCp [13, 16]. Microhardness measurements carried out on the polished samples of all the composites (within the immediate vicinity of the matrix and
reinforcement) are also reported in Table 3. It is clear from
the table that the increase in microhardness values is very
much significant for the composite compared to the base
alloy. Moreover, increase of SiCp content in the composite
further increases the hardness. The increase in the microhardness value is due to the presence of harder SiC
particle and the higher stress concentration around the
particle in the matrix (i.e., in the neighborhood).
The strength properties such as YS and UTS of the base
alloy and composites are also reported in Table 3. It can be
seen from Table 3 that YS value of the composites increase
with SiCp and they are higher than the base alloy. However, the UTS values of the composites do not increase
with increasing SiCp addition. The lower UTS value of the
composites with SiCp addition can be attributed to the
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YS (MPa)
UTS (MPa)
UCS (MPa)
following reasons: (1) the influence of residual stress produced during the deformation process, (2) failure of
agglomerated sub-micron SiCp, and (3) the presence of
little amount of porosity as well as the oxide layer.
The fracture surfaces of tensile tested samples provide
valuable information pertaining to microstructural effects
on tensile ductility and fracture properties of the Mg-based
MMC. Figure 6 shows the typical SEM micrographs of
fracture surfaces of the base alloy and the AZ91/SiCp
composites. Both dimple-like characteristics and cleavagelike features (Fig. 6a) can be seen in different areas on the
fracture surfaces. It exhibits nominal to acceptable ductile
fracture occurring on a plane normal to the far-field tensile
stress axis. Moreover, the tensile fracture surface of the
composites depicted both ductile and brittle fracture
mechanisms. The fracture surface reveals that both fractured and deciphered SiCp are surrounded by pockets of
ductile regions that are termed as ‘‘tear ridges.’’ The ridges
can be related to ductile failure and so are the pockets of
dimples of varying size and shape in the tensile fracture
surface. The presence of hard, brittle, and essentially
elastically deforming reinforcing SiCp constraints on the
mechanical deformation of the soft, ductile, and plastically
deforming AZ91 alloy matrix; and the resultant
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301
Fig. 6 Typical fracture surfaces of the tensile tested specimens of AZ91 alloy a, AZ91/5SiCp composite b, AZ91/15SiCp composite c, AZ91/
25SiCp composite d
Fig. 7 SEM micrographs of AZ91/SiCp composite polished just below the fracture surface showing cracked particles a, debonding of particles b
development of a triaxial stress state in the matrix aid in
limiting the flow stress of the composite [17]. Moreover, it
is clearly shown that, with increasing in SiCp, brittle
fracture region is high compared to the ductile region. To
understand the failure mechanisms, SEM micrograph of
fracture surface, polished just below the fracture surface is
shown in Fig. 7. The SEM images demonstrate the following interactive influences of cracking of the hard,
intrinsically brittle and elastically deforming SiCp and its
interface; and decohesion at interfaces between the hard
reinforcing SiCp and the ductile AZ91 alloy matrix [18].
Essa et al. [19] have reported that the damage and failure
mechanisms in MMCs are generally associated with the
breaking and decohering of the reinforcement. It is further
suggested that the fracture and decohesion of the reinforcing particles have a detrimental effect on the overall
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(a)
(b)
Fig. 8 Histogram showing wear rate vs. SiCp content a, coefficient of friction vs. SiCp content b
Fig. 9 SEM images of worn surfaces: a AZ91/10SiCp tested at 19.6 N; b AZ91/10SiCp tested at 39.2 N; c AZ91/25SiCp tested at 19.6 N;
d AZ91/25SiCp tested at 39.2 N
load-bearing capacity of the composite. It reduces the flow
stress, strain-hardening exponent, and ductility of the
composite during monotonic tensile loading. Particles
have cracked during the tensile test as a result of load
transfer from the matrix to particles; whereas decohesion
of particles is related to the interface failure. When fracture or decohesion of particle occurs, it liberates its load to
the surrounding matrix, resulting in higher stresses transmission to the unbroken particles. The duration and stability of this process depend on the strain hardening of the
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matrix. It has been demonstrated by Luo [15] that the
significant higher strain-hardening rate of the composite in
the early stage of the plastic deformation can be attributed
to the presence of the hard SiC particles, which increases
the resistance to slip in the magnesium matrix. During
plastic deformation, the slip behavior of each matrix grain
is highly constrained by SiC particles, which are too hard
to be deformed. When the composite is under external
load, a strong internal stress develops between SiCp and
the matrix, which resists the slip in the matrix to increase
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303
Fig. 10 SEM images of the wear debris: a AZ91/10SiCp tested at 19.6 N; b AZ91/10SiCp tested at 39.2 N; c AZ91/25SiCp tested at 19.6 N;
d AZ91/25SiCp tested at 39.2 N
Fig. 11 SEM images of wear debris showing Mg ribbons suggesting abrasion wear a, fine Mg oxides on the wear debris b
the strain-hardening rate and the YS. In the present study,
both SiCp and b-Mg17Al12 phases act as barriers to the slip
lines in the matrix and, therefore, strengthen the composite. It should be pointed out that in the elastic stage,
even without slip phenomenon, the same load transferring
process will develop internal stress between SiCp and the
matrix, which contributes to the improvement in YS of the
composite. The compression test results show that the
UCS increases with increasing SiCp in the composite
(Table 3). The increase in UCS value is attributed to the
presence of hard reinforcement particle, which restricts the
flow of matrix. Moreover, the compressive strength of the
SiC is as high as 3,900 MPa compared to the base alloy
(310 MPa).
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3.4 Tribological Properties
Figure 8 depicts the variations in volumetric wear rates and
coefficient of friction for the SiCp-reinforced AZ91 composite as a function of SiCp content. The wear rate
increases progressively with increase in loading (Fig. 8a).
The effect of strengthening of the soft Mg matrix by harder
SiCp reinforcements is evident from the decrease in wear
rate of the composite with increase in the SiCp content.
Studies have shown that the wear behavior of the composite is affected by various parameters such as type, size,
and volume of reinforcements [20]. Moreover, the specific
wear rate and wear coefficient are found to be decreasing
with increase of SiCp in the matrix irrespective of the
applied loads. However, the coefficient of friction is also
changed noticeably from one test condition to another,
especially for the lower loads as shown in Fig. 8b. The
coefficient of friction decreases with increase in load as it is
inversely proportional to the applied normal load and
increases with SiCp content due to particle strengthening of
the reinforcement.
Figure 9 depicts the scanning electron micrograph of
worn surfaces of AZ91/10SiCp and AZ91/25SiCp samples
tested at two different loading conditions (19.6 and
39.2 N). The wear surface of samples tested at 19.6 N for
AZ91/10SiCp composite (Fig. 9a) reveals delaminated
material pickup. Also, the traces of parallel grooves on the
wear surface are revealed in Fig. 9b–d. These distinct
patterns of grooves and ridges running parallel to one
another are the typical characteristics of delaminated and
sliding wear. The hard asperities of counter face or
detached particles removed from disk or pin when placed
in surface contact are the possible cause of these scratches.
The wear debris of AZ91/10SiCp and AZ91/25SiCp
samples tested at two different loadings is shown as the
SEM images in Fig. 10. Numerous flakes or sheets have
been observed in the wear debris, which are possibly delaminated from the worn surfaces. The delamination
involves subsurface deformation, crack nucleation, and
crack propagation. The dominant wear mechanism is
abrasion which is facilitated by the wear process where
hard reinforcing materials become the load-bearing constituent [21]. However, the decrease in size of debris is
more significant in case of high SiCp-added matrix [22].
The oxidation type of wear can be visualized as white
patches shown in Fig. 10, d.
Moreover, abrasion of the wear surface due to the load
has also generated Mg ribbon as shown in Fig. 11a.
However, the occurrences of such ribbons are very rare.
The abrasion has taken place primarily through plowing,
where the materials get moved on to the either side of the
abrasion groove without getting removed [20]. Another
possibility is through wedge forming, where tiny wedge-
123
shaped particles are worn by only the initial contact with
abrasive particles [23].
The white powders formed over the wear debris shown
in Fig. 11b are indicative of oxidative wear. The heat
generated during sliding results in oxidation of surface
leading to wear occurring through the removal of oxide
fragment. Over a period of time during sliding, oxide wear
debris fills out the valleys on the pin surface and gets
compacted into a protective layer. This prevents metallic
contact and results in reduction in wear rate [24]
4 Conclusions
Analysis of physical, mechanical, and tribological attributes of AZ91 alloy and AZ91/SiCp composites leads to
the following conclusions:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Liquid metal stir-casting process is a successful
fabrication method for synthesis of AZ91 matrix
composites with as high as 25 wt% of SiCp as
reinforcement.
Microstructures of the composites reveal uniform and
homogeneous distribution of SiCp with minimum
defects.
The reinforcing SiCp phase improves the thermal
stability of the Mg matrix composites as the thermal
expansion mismatch between the reinforcing phase
and the matrix constrains the expansion during heating.
Moreover, the thermal stability of the composites
increases with increase in reinforcement content.
The insulating property of the Mg composites gets
augmented by the presence of reinforcing SiC particles as lower conductivity of the reinforcing phase
imparts higher resistance to the current flow. Higher
volume of reinforcing phase improves the affinity of
the composites toward better insulation.
The SiCp reinforcements improve the hardness (both
macro and micro), increase the tensile YS, and also
strengthen the composites against the compressive
loading. The property improvements are proportional
to the amount of reinforcements. However, irrespective of the reinforcement content, UTS values of the
composites remain comparable with the base alloy
and signify the reduction in plastic deformation
region of the composites during tensile loading.
SiCp reinforcements strengthen the soft magnesium
matrix by decreasing the wear rate and increasing the
wear resistance and coefficient of friction of the
composite as a function of reinforcement content.
Abrasion, delamination, and oxidation are the predominant wear mechanisms in the composites under
the prescribed testing conditions.
K. K. Ajith Kumar et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(2), 295–305
(8)
Delamination and sliding are the prominent types of
wear mechanisms in the composites as the SEM
images of the worn surfaces have distinct patterns of
grooves and ridges running parallel to one another.
However, the occurrence of the Mg ribbons in wear
debris is indicative of abrasion type of wear. Moreover, oxidation type of wear is also evidenced by the
presence of white powders formed over the wear
debris as the heat generated during sliding results in
oxidation of surface leading to wear occurring
through the removal of oxide fragment.
Acknowledgments The authors are grateful to the Aeronautical
Research and Development Board (ARDB), New Delhi for the
financial grant for this work. The help received from Mr. K. K. Ravikumar for mechanical testing, Mr. M. R. Chandran for SEM analysis, and Mr. S. Prasanth for casting and characterization is highly
acknowledged. The authors also extend their gratitude toward Mrs.
K. S. Deepa, Mr. J. S Harikrishnan, and K. A. Ajukumar for their
valuable contributions.
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