Effect of electroceramic particles on damping behaviour of aluminium hybrid
composites produced by ultrasonic cavitation and mechanical stirring
C. Montalba a,⁎, D.G. Eskin b, A. Miranda b, D. Rojas a, K. Ramam a
a Departamento de Ingeniería de Materiales, Universidad de Concepción, Edmundo Larenas 270,
Concepción 4070409, Chile
b Brunel Center for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge,
Middlesex UB8 3PH, UK
In this study, electroceramics PBN and PLZT along with SiC were included in Al–3.96 wt.% Mg
(A514.0) master alloy. Ultrasonic cavitation (UST) and mechanical stirring (MS) were
employed to improve wettability and dis- persion during casting. Two composite systems
were produced: PBN system (5 wt.% PBN + 1 wt.% SiC and 15 wt.% PBN + 1 wt.% SiC) and
the PLZT system (follows the same designation). The influence of fabrication method on the
microstructures, particle distribution and wettability as well as electroceramic impact on
dynamo-mechanical properties of prepared composites were investigated. Optical microscope
(OM) and scan- ning electron microscope (SEM) results indicate that the processing technique
was effective as it promoted wet- tability and homogeneous dispersion of particles throughout
the Al matrix. Dynamic mechanical analysis (DMA) study of the composites demonstrated that
the addition of the functional particles to the Al alloy matrix improved damping capacity (Tan
δ) at 200 °C. The composites exhibited an increase in Tan δ of 24.3 ± 0.3% and 91.4 ± 0.2% for 5
and 15 wt.% PBN + 1 wt.% SiC and an increase of 19.7 ± 0.5% and 42.5 ± 0.3% for 5 and 15
wt.% PLZT + 1 wt.% SiC, respectively, when compared to the aluminium alloy matrix.
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1. Introduction
The interest in hybrid materials research has increased exponentially in the later years due to
superior and diverse functionality of this class of materials [1–3]. The definition of hybrid materials
is somewhat ambiguous as some different descriptions can be found in literature [4,5]. Ashby
and Bréchet [6] defined hybrid material “as a combination of two or more materials in a
predetermined geometry and scale, optimally serving a specific engineering purpose”.
From research and development point of view, metal matrix com- posites (MMCs) are
categorized as structural or functional materials in terms of their applicability, where functional
composite materials have found engineering applications with increased research interest [7,8].
The combination of these two characteristics is currently recognized as multifunctional composite
materials evolving as second generation of uni-functional composites. Metal matrix composites
have been investigated for decades with the purpose of increasing mechanical resistance and
upgrading thermal behaviour, being aluminium (Al) the most extensively studied and
developed metal matrix, and designated as aluminium matrix composites (AMCs) [9,10].
Lately, AMCs have been benefiting from “material functionalism” where the synergy of distinct
fillers is explored in order to materialize a hybrid material with both characteristics, functional
and structural. From carbon based materials such as carbon nanotubes [11,12], to shape
memory elements [13,14] passing through piezoelectric com- pounds [15–17] scientific
encouragement has created a footpath where multifunctional hybrid composites have found a
successful route to emerge.
According to Qin and Peng [18], in order to design a multifunctional hybrid composite from the
abstract prototype to the final product, two basic concepts must be followed: (1) a functional
filler is essential to achieve multifunctionality with a relatively simple composite architecture,
and (2) homogenous dispersion of fillers is the priority for integrity and implementation.
In this study, the functional fillers selected were pyroelectric lead barium niobate (PBN)
[PbxBa(1 − x)Nb2O6] and piezoelectric lead lanthanum zirconate titanate (PLZT) [Pb(1 −
x)Lax(ZrzTi(1 − z)(1 − x)/4)O3] electroceramic systems. Both electroceramics possess
well-proven high dielectric, ferroelectric and piezoelectric properties as also good constructive
vibration damping capacity [19,20].
Materials with elevated capacity to dissipate energy when exposed to mechanical vibration
(damping) are relevant to prevent failures due to vibration or noise during service [21].
Damping properties in piezoelectric composites are attributable to an inelastic strain response
of ferroelastic domains to externally applied stress, affecting the domain structure and orientation
followed by a portion of the applied stress energy dissipation as it is used for domain
reorientation [22,23].
On the other hand, silicon carbide (SiC) particles were selected as structural fillers providing
strengthening and thermal stability [24].
In this study, the main goal was to develop an adequate composite processing route able to
achieve homogeneous dispersion of the reinforcements throughout the matrix, by attaining
adequate wettability between the matrix and the reinforcement, thus processing multifunctional
composites. Difficulties regarding incorporation and dispersion of reinforcing particles within
liquid aluminium alloys are mainly due to the poor wettability that leads to inhomogeneity of
particle distribution and the presence of detrimental gases that instigates porosity [25]. In order to
overcome such problems, ultrasonic cavitation treatment (UST) assisted by mechanical stirring
(MS) was implemented, since these methods facilitate melt degassing, wetting, deagglomeration and good dispersion of the particles [26,27].
The present work introduces the development of novel multifunctional hybrid metal matrix
composites (HMMCs) (with two different ceramic reinforcements) with high damping capacity
and elevated stiffness for elevated temperature applications. Thus, structural and functional
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(electroceramic) reinforcements have been selected and dispersed in the Al–3.96 wt.% Mg (A514.0)
matrix (see Table 1). The weight percentage ratios of electroceramics PLZT and PBN with SiC were
5:1 and 15:1, respectively.
Particle phase evaluation of the functional electroceramics was performed by the X-ray
diffraction (XRD) technique. Composite micro- structure was analysed using optical and scanning
electron microscopy in order to investigate the distribution of reinforcements in the matrix and
the reinforcement/matrix interaction. Storage modulus (E′) and damping capacity (Tan δ) of
the matrix alloy and the composites were studied using temperature dependent functionality of
dynamic mechanical analysis (DMA).
2. Experimental procedure
The following sections give a detailed description on the development of hybrid composites
produced.
2.1. Functional reinforcement preparation
Two types of electroceramic reinforcements, pyroelectric lead barium niobate (PBN) [PbxBa(1 −
x) Nb2O6] and piezoelectric lead lanthanum zirconate titanate (PLZT) [Pb(1 − x)Lax(ZrzTi(1 − z)(1 −
x)/4)O3] were prepared for composite processing.
Analytical reagent grade powders (Sigma-Aldrich, USA, purity 99.99%) of PbO, BaCO3, Nb2O5 to
produce PBN [Pb0.63Ba0.38Nb2O6] and PbO, La2O3, ZrO2, and TiO2 for PLZT
[Pb0.988La0.012(Zr0.53Ti0.47)0.997O3] were used as raw materials to obtain the respective
compounds and prepared via the solid-state reaction method. An excess of 5 wt.% of PbO was
added to the stoichiometric batch systems to compensate the lead volatilization during the
sintering process. The weighed starting reagents with appropriate stoichiometric ratios were
mixed for each compound in an agate mortar using ethanol as mixing media to obtain a
homogenous mixture. Powders were sintered at 1200 °C (PBN) and
1240 °C (PLZT) for 3 h in a high purity alumina crucible in air. The sintered powders were
manually ground in the agate mortar to crush agglomerates and reduce the particle size.
2.2. Structural reinforcement preparation
The structural reinforcement consisted of silicon carbide (SiC) particles, i.e. α-phase, 99.8% metal
basis with 1–2 μm particle size (Alfa Aesar Chemicals, USA). The as-received SiC powders were
heat-treated at 900 °C for 1 h to remove humidity and facilitate wetting by creating a SiO2 layer,
which reduces the surface tension between SiC particle and molten Al [28,29]. Finally, the
powder was left to cool down and stored in a desiccator to avoid humidity and atmospheric
contamination.
2.3. Composite processing
An aluminium–magnesium (Al–Mg) based alloy was selected as the matrix, since the addition of
Mg reduces the surface energy of aluminium, decreasing the contact angle between the molten Al
and the ceramic particles, thus facilitating wettability [30]. The chemical composition of the Al–
3.96 wt.% Mg (A514.0) alloy produced in this study is given in Table 1.
The composite processing involve ultrasonic cavitation treatment (UST) and mechanical stirring
(MS), both are recognized techniques concerning superior dispersion and notable wettability
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[31].
UST generates strong non-linear effects in the liquid melt such as transient cavitation and
acoustic streaming. During ultrasonic cavitation, particle clusters are loosely packed together in
the melt and various gases like air, inert gas, or metal vapour can be trapped inside voids
within the clusters and act as nuclei for bubble generation. These bubbles grow and collapse
reaching localized extremely high temperatures and pressures up to 5000 °C and 500 atm,
respectively [32]. This phenomenon provides enough energy to break away particle clusters and
clean its surface (remove surface gases) promoting wetting with molten metal. Additionally, the
oscillating and collapsing cavities produce a dispersive effect facilitating the homogenization of
the composite microstructure [26].
In this investigation, two different composite systems were used, la- belled as A514/1 wt.% SiC/X
wt.% PBN or Y wt.% PLZT, with respective compositions shown in Table 2. Based on literature in
the same research field [17,22] reinforcement weight percentages were selected presuming
linear results in both aluminium hybrid metal matrix composites.
Inside A6 Salamander crucibles, 0.5 kg of matrix alloy (A514.0) was placed in, and melted in an
electric resistance furnace up till the alloy reached the UST processing temperature, 750 °C.
Ultrasonic equipment used features a 5 kW generator and a 5 kW water-cooled magnetostrictive
transducer (Reltec, Russia) with a niobium tip (sonotrode). When the master alloy reach the
temperature, the conical Nb sonotrode was submerged 20 mm deep into the melt and a 4 kW
output power was used to generate ultrasonic cavitation and acoustic waves into the melt for
a continuous period (~ 7 min for PBN5 and PLZT5 and ~ 20 min for PBN15 and PLZT15). The
reinforcing particles (functional and structural) wrapped in an aluminium foil were fed into the
melt cavitation zone at a rate of 4 g·min-1 as illustrated in Fig. 1.
After reinforcement incorporation, the melt was mechanically stirred at 600 rpm with a four
blade stainless steel impeller coated with boron nitride paint. The composite melt was stirred
from 700 °C to 620 °C (until semi-solid state), and then re-heated to 750 °C. Again the sonotrode
was submerged into the melt and cavitation treatment performed for another 5 min to ensure
particle homogeneity and total degassing of the melt. Finally, the liquid composite was
poured in metallic moulds and left to solidify and cool down in air.
Table 1
Chemical composition of the aluminium alloy matrix (wt.%).
Al–Mg
Aluminium Silicon
Iron
A514
matrix
95.8
0.08
0.08
Copper
0.01
Manganese Magnesium Titanium
0.05
3.96
Table 2
Composite designation and ratios of reinforcement content in wt.%.
Sample nomenclature
Matrix
PBN5
PBN15
PLZT5
PLZT15
Composite (wt.%)
A514.0
A514/1 wt.% SiC/5 wt.% PBN
A514/1 wt.% SiC/15 wt.% PBN
A514/1 wt.% SiC/5 wt.% PLZT
A514/1 wt.% SiC/15 wt.% PLZT
0.01
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2.4. Functional particle characterization methods
The average size of the functional particles was measured using a laser diffraction particle size
analyser Mastersizer 2000 (Malvern, UK) device. Phase evaluation of the particles was
performed using a D4 ENDEAVOR X-ray diffractometer (Bruker, Germany) in the range of 2θ =
20°–60° with a step size of 0.02°s− 1 and Cu-Kα radiation at 0.1542 nm, 40 kV and 25 mA.
Diffraction patterns of PBN and PLZT were indexed using ICDD standard powder diffraction file
numbers 73–196 and 29–776, respectively.
2.5. Composite characterization methods
The composite samples produced were prepared by standard grinding and polishing procedures
and subjected to a comprehensive characterization. An optical microscope (Olympus GX51) and
a scanning electron microscope (JEOL JSM 6380LV) were used for microstructural analysis.
Dynamic mechanical behaviour of the composites was evaluated by DMA Q800 (TA Instruments) in
the range of temperatures from 40° to 200 °C at 1 Hz frequency. Storage modulus (E′) and
dissipation factor (Tan δ) were determined for the matrix and the composites with the gauge
dimension of 1 mm × 3 mm × 22 mm.
3. Results and discussion
3.1. Piezoelectric particle analysis
In MMC processing, the reinforcement selection plays a major role as it will dictate the optimization
of a wide range of parameters, from the selected processing route to the final composite
application. Particle selection criteria include density, particle size and shape, elastic modulus,
melting temperature and cost, to name a few [33].
Fig. 2(a–b) shows the particle size distribution of PBN and PLZT ceramic powders, respectively. In
the case of PBN, the particle size distribution ranged from 1.5 μm to 100 μm with an average value
of 10 μm (~ 62% of the volume). On the other hand, PLZT shows a distribution size from 2 μm to
105 μm with an average particle size of 12 μm (52% of the volume).
The size and shape of particles have an important role in strengthening metal matrix composites.
In terms of size, the particles can be categorized as coarse (d > 100 μm), fine (d < 10 μm) and
ultra-fine (sub-micron, d < 0.1 μm), with each size category contributing differently to the
composite final mechanical properties [34]. Therefore, functional particles, PBN (10 μm) and PLZT
(12 μm), are categorized between coarse and fine ranges, yielding the composites' direct strength,
i.e. an applied load transferred from the matrix to the reinforcing particle as well indi- rect
strength caused by the incorporation of dislocations due to thermal mismatch between the
particles and the matrix [35].
Regarding the properties of the functional particles, different studies have demonstrated a direct
relationship between piezoelectric particle size and functional (dielectric and piezoelectric)
performance. Randall et al. [36] noted a lattice distortion along the tetragonal c-axis for the
PZT compound as the grain size decreased. The spontaneous tetragonality unit-cell distortion
modifies the intrinsic polarization and also increases the internal stress reducing functional
properties of piezoelectric ceramic particles. Piezoelectric effect in ceramics is based solely on the
non-symmetrical structure, intrinsic from this class of materials and any distortion could
diminish or eliminate piezo, pyro or ferroelectric phenomena [37]. In order to verify the crystal
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structure of PBN (10 μm) and PLZT (12 μm) functional particles, an X-ray diffrac- tion study was
performed.
Fig. 3 presents the X-ray diffraction spectrum of sintered PBN and PLZT functional ceramic
particles. PBN ceramics belong to Tungsten Bronze (TB) family, and these crystals are used as
pyroelectric detectors and piezoelectric transducers [38]. PBN [PbxBa(1−x) Nb2O6] with compositions between 0.53 b x b 0.63 shows coexistence of tetragonal and orthorhombic crystal
symmetry that results in enhancement in dielectric and piezoelectric properties among others
[39]. X-ray diffraction patterns (Fig. 3(a)) have demonstrated complete coexistence of the
tetragonal and orthorhombic (m2m) PBN phases [40].
The XRD pattern of PZT doped with La3+ is presented in Fig. 3(b). PLZT compositions closed to
the morphotropic phase boundary on PLZT phase diagram (with atomic ratios between Zr/Ti ≈
53/47) present outstanding ferroelectric and piezoelectric properties [41].
XRD patterns of PLZT (1.2/53/47) clearly distinguish the phase transition from “coexisting
ferroelectric rhombohedral and ferroelectric tetragonal phases” to “intensified tetragonal phase”
[42]. The diffraction spectrum of sintered PLZT powders show characteristic tetragonal peak
splitting of (200) and (002) planes near 2θ = 43.2° and 43.6°.
According to the literature, XRD results of PBN and PLZT have shown neither intermediate phases
nor un-reacted phase formation, confirming complete sintering process by forming pure end
members, and inhibition of any lattice distortion due to particle size. From the reinforcement
selection point of view, according to the quality and particle size study of PBN and PLZT
functional ceramic particles used, good conditions are met to produce composites with
desirable mechanical properties maintaining intrinsic functional properties.
Fig. 1. Schematic representation of the composite processing method.
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Fig. 2. Average size distribution of (a) PBN and (b) PLZT ceramic particles.
3.2. Microstructure analysis
Fig. 4(a–b) shows optical and electron micrographs with energy- dispersive X-ray spectroscopy
(EDS) of the as-cast aluminium alloy ma- trix (A514.0). In Fig. 4(a), the Al matrix (white)
intermetallic phases and the presence of discrete porosity are easily discerned. Secondary electron (SE) SEM images supported with EDS (Fig. 4(b)) reveal an Al–Mg matrix composition, as
expected, and AlxFeyMgz intermetallic phase for- mation in the matrix. Generally, iron presence in
aluminium is common due to the primary process to obtain Al from Bauxite [43]. However, in this
case, most of the Fe incorporated in the Al alloy matrix is related with composite processing.
Although the stainless steel stirrer was coated with boron nitride to avoid contamination,
parameters such as, shear stress applied during mixing of the Al melt with particles, aluminium reactivity, and time/temperature of the process pull out the protective coating resulting in Fe
contamination of the composite matrix. Non- boron or nitrogen contamination was found in the
X-ray characterization performed to hybrid composites.
The presence of Al–Fe based intermetallic phases is due to the iron low solubility in aluminium. In
this particular case, Fe solubility in Al de- creases further in the presence of magnesium that, on the
other hand,decreases Mg solubility in Al [44]. Regarding AlxFeyMgz intermetallic, no ternary
phase is formed, and two binary phases of Al3Fe and Mg5Al8 coexist in an Al solid solution
[45]. For non-equilibrium conditions, division tendency of the eutectic increased, causing large
massive crystals of Al3Fe formation. In addition, Mg5Al8 compound tends to appear, like in
this case, for low amounts of Mg [45]. However, Mg presence reduces the size of Al3Fe primary
crystals since this element enhance the settling of larger crystals [43].
Figs. 5(a–b) and 6(a–b) show optical and electronic micrographs with EDS of the composite
systems A514/15 wt.% PBN/SiC (PBN15) and A514/15 wt.% PLZT/SiC (PLZT15), respectively.
Since, with higher percentage reinforcement incorporation there exists more chances of finding
agglomerations, the microstructural analysis was performed to compounds reinforced with 15
wt.% assuming an extrapolation of results. Figs. 5(a) and 6(a) demonstrate the good distribution
of the par- ticles in the matrix and proves the homogenous dispersion of well- sintered ceramic
particles with lower concentration of agglomerates, which is more noticeable for the samples
reinforced with PLZT (Fig. 6(a)). Composites also reveal porosity probably due to some parti- cle
agglomeration spots.
Fig. 5(b) shows the SE SEM micrograph of the PBN15 composite with the SiC and PBN (arrowed)
reinforcements dispersed in a relevant degree throughout the Al matrix.
Fig. 6(b) displays the SE SEM micrograph of PLZT15 composite. The SiC and PLZT particles present
in the matrix are arrowed to help its identification. The matrix grain size was found to decrease in
both the PBN and the PLZT composite samples, when compared to the original matrix.
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Fig. 3. XRD patterns of the (a) PBN and (b) PLZT functional particles.
Fig. 4. Aluminium–magnesium (a) optical (100 ×) and (b) SEM micrographs with EDS
chemical characterization.
This phenomenon could be explained by the fact that reinforcing ceramic particles may be acting
as grain nucleation sites [46], and this effect being further facilitated by ultrasonic cavitation [47].
Considering the results from Figs. 6(a) and 7(a), the application of UST combined with MS proved
to be effective in breaking and dispersing particle agglomerates, to reduce detrimental gas effects,
increase volume fraction of particles inserted in the melt and drop the contact angle between the
particle and matrix [31].
Regarding the interface between the aluminium matrix and the reinforcements, Fig. 7(a–b)
shows the backscattered electron (BSE) SEM images of the functional reinforcement particles
embedded in the A514 matrix.
Interface nature is a fundamental factor to limit the contribution of incorporated reinforcement concerning
composite properties. The principal role of the interface between the matrix and the reinforcement phase in
a MMC is strength, i.e. transfer of the load from the matrix to the reinforcement. The interface may also be
expected to serve as mechanical fuse or as a diffusion reaction barrier. The key features of the interface are
the chemical reactions promoted and the strength of bonding [35].
In both HMMCs, SEM micrographs (Fig. 7(a–b)) confirmed the successful incorporation of ceramic
particles in the metal matrix with optimum wettability between the matrix and the reinforcement, supporting
the optical micrograph findings. Furthermore, no porosity was observed in the boundary between the
particles and the matrix. It can be inferred that a clean and good bonding interface was formed during the
composite processing.
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Main elements reacting in the interfacial layer probably would be Al and Pb. According to literature, Al and
Pb have shown immiscibility [45], thus, in our study, no compound would be formed by these two elements.
On the other hand, Mg addition reduces the liquid miscibility gap of Al–Pb until approximately 25–30%Mg,
on contrary, it disappears at lower concentrations of Mg. Nevertheless, in this study, the matrix has less than
4 wt.% of Mg thus Al–Pb miscibility remains constant.
3.3. Dynamic mechanical analysis (DMA)
Mechanical vibration properties at different temperatures were measured by DMA that reveals mechanical
properties of materials under oscillatory forces as a function of time or temperature.
The load applied (stress) promotes a response (strain) with a phase shift δ, were E* is the complex elastic
modulus, E′ is known as the storage modulus and E″ the loss modulus (Eq. (1)).
E*=E″ + E′ =(stress/strain) cos δ + (stress/strain) sin δ
(1)
and
Tan δ= E″/ E′
(2).
Eq. (2) presents the damping capacity or Tan δ of the material, which represents the ability to dissipate
elastic strain energy during wave propagation or mechanical vibration [48].
Fig. 8 shows Tan δ and E′ for the A514.0 matrix and the composite systems, PBN5, PBN15, PLZT5 and
PLZT15 (Table 2) measured for the range of temperatures between 40 and 200 °C. According to the DMA
results, as expected, the overall damping capacity increased with increasing content (wt.%) of the functional
reinforcement.
Fig. 8(a) shows the comparison between Tan δ and the E′ for the Al alloy matrix and the PBN composite
system with 5 and 15 wt.% of functional particles as a function of the temperature. At 200 °C, the matrix
shows a damping coefficient of ~0.032 ± 0.001 which increases ~0.008 ± 0.001 (~0.041 ± 0.001) when 5
wt.% of PBN is added (values on parenthesis represent empiric results). The higher value of damping
coefficient was observed when 15 wt.% of PBN was incorporated in the matrix, with an increment of ~
0.03 ± 0.001 (~ 0.062 ± 0.001) when compared to the matrix. At the same temperature,
regarding the E′ results, matrix showed a value of ~ 37.5 ± 0.1 GPa with an increase of ~ 8.46 ±
0.01 GPa (~ 46.0 ± 0.1 GPa) and ~ 17.65 ± 0.01 GPa (~ 55.2 ± 0.1 GPa) comparing matrix
with composites with 5 wt.% and 15 wt.% PBN, respectively.
Tan δ and the E′ of the Al matrix and composites reinforced with PLZT are presented in Fig. 8(b).
At 200 °C, the composite with 5 wt.% PLZT shows an increase in Tan δ and E′ of ~ 0.006 ± 0.001
(~ 0.039 ± 0.001) and ~ 11.50 ± 0.01 GPa (~ 49.0 ± 0.1 GPa), respectively and for 15 wt.% PLZT,
Tan δ and E′ an increase of ~ 0.014 ± 0.001 (~ 0.046 ± 0.001) and ~ 16.13 ± 0.01 GPa (~ 53.7 ±
0.1 GPa), respectively.
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Fig. 5. PBN15 composite (a) optical (100 ×) and (b) SEM micrographs with EDS chemical
characterization.
Fig. 6. PLZT15 composite (a) optical (100 ×) and (b) SEM micrographs with EDS chemical
characterization.
Fig. 7. SEM BES micrograph of (a) PBN15 and (b) PLZT15.
The damping coefficient (Tan δ) results demonstrate that stability is achieved for both composite
systems in the whole range of temperature. In conclusion, the damping results show that the
incorporation of functional reinforcement improves damping characteristics, probably due to two
main reasons, (i) ferroelastic-based mechanism of piezoelectric particles and (ii) microstructural
changes induced by the ultrasonic cavitation method.
Piezoelectric ceramic particles exhibit elevated constructive vibration damping capacity due to an
inelastic strain response of ferroelastic domains to externally applied stress affecting domain
structure and orientation, as a portion of the applied stress energy is dissipated because it is
utilized for domain reorientation [21,23]. On the other hand, ultra- sonic cavitation treatment
changes the microstructure from dendritic to non-dendritic or globular small grains [47], that
consequently rise the damping nature due to an increase of interfacial reaction between grain
boundary and vibration wave [49].
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Fig. 8. The mechanical vibration properties vs. temperature comparison of: (a) A514.0
matrix, PBN5 and PBN15 and (b) A514.0 matrix, PLZT5 and PLZT15.
Storage modulus (E′) of the Al matrix and the reinforced composites of both systems have shown
to decrease with increasing temperature strictly related with diffusivity and energy required to
produce atomic movement. E′ for the composites seems to be dependent of the volume fraction of
the reinforcement in the aluminium alloy matrix. This phenomenon is directly related with rule
of mixtures, where increasing fraction of ceramic reinforcing particles with higher hardness
will
have an increase of composite strength [48,50]. Storage modulus of reinforced PBN composites in
the entire temperature range remains thermally stable, attributable to silicon carbide which has
low thermal expansion (4 × 10−6/K) and no phase transformation occurs for the range of
temperatures used in this study [37,39]. Likewise, PLZT5 composite shows the same thermal
stability, however, PLZT15 composite showed a decline trend of storage modulus per
temperature increment, probably due to agglomerations detected in Fig. 6(a).
Piezoelectric PBN and PLZT particles have the ability to improve damping capacity of the metal
matrix without sacrificing dynamic mechanical properties [51]. The damping capacity of the
functional reinforcement due to energy dissipation caused by interfacial sliding in the interface
of matrix–reinforcements in the complete range of temperatures makes these composites
suitable for structural, piezoelectric and frequency dependent applications.
4. Conclusion
Hybrid metal matrix composites reinforced with both functional electroceramic and structural
ceramic particles processed by ultrasonic cavitation treatment (UST) assisted by mechanical
stirring (MS) paves the way for novel advanced multifunctional metal matrix composites for
advanced applications. Functional mixed phase orthorhombic-tetragonal PBN and tetragonal
structured piezoelectric PLZT electroceramics mixed with structural SiC ceramic in an Al alloy
matrix have shown promising damping nature when brought together. In this study, the
following features are observed for the produced composites:
(1) The processing route adopted, ultrasonic cavitation treatment (UST) assisted by
mechanical stirring (MS), improved wettability allowing optimum interaction between
the aluminium alloy matrix and the reinforcement (PLZT/PBN and SiC) at dynamomechanical properties tested (E′ and Tan δ). Moreover, proper dispersion of the
particles acquired is vital to have an appropriate structurally and functionally material
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behaviour.
(2) The combination of orthorhombic and tetragonal crystalline PBN and tetragonal crystalline
PLZT fine particles can act as functional additions to aluminium metal matrix composites
improving the damping properties.
(3) The incorporation of functional reinforcing particles into the aluminium A514.0 alloy matrix
improved the damping capability of the HMMCs produced, being this property more pronounced
with the increasing loading of functional reinforcement.
Acknowledgements
The authors CMW and Koduri Ramam acknowledge and are grateful to CONICYT for Doctoral
Research Fellowship and also acknowledge Brunel University, UK for hosting internship to
carry out doctoral research work as Universidad Carlos III, Spain, for characterization support.
The authors Koduri Ramam and CMW greatly acknowledge the Fondecyt Research Project
Number 1110583 for the financial support with the research project and characterization
equipment.
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