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Effect of electroceramic particles on damping behaviour of aluminium hybrid composites produced by ultrasonic cavitation and mechanical stirring

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.

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, de-agglomeration 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 (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 [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 tempera