Microstructure and Compressive Behavior of Al–Y 2 O 3 Nanocomposites Prepared by Microwave-Assisted Mechanical Alloying

: In this study, Al–Y 2 O 3 nanocomposites were synthesized via mechanical alloying and microwave-assisted sintering. The e ﬀ ect of di ﬀ erent levels of yttrium oxide on the microstructural and mechanical properties of the Al–Y 2 O 3 nanocomposites were investigated. The density of the Al–Y 2 O 3 nanocomposites increased with increasing Y 2 O 3 volume fraction in the aluminum matrix, while the porosity decreased. Scanning electron microscopy analysis of the nanocomposites showed the homogeneous distribution of the Y 2 O 3 nanoparticles in the aluminum matrix. X-ray di ﬀ raction analysis revealed the presence of yttria particles in the Al matrix. The mechanical properties of the Al–Y 2 O 3 nanocomposites increased as the addition of yttria reached to 1.5 vol. % and thereafter decreased. The microhardness ﬁrst increased from 38 Hv to 81 Hv, and then decreased to 74 ± 4 Hv for 1.5 vol. % yttria. The Al–1.5 vol. % Y 2 O 3 nanocomposite exhibited the best ultimate compressive strength and yielded a strength of 359 ± 7 and 111 ± 5 MPa, respectively. The Al–Y 2 O 3 nanocomposites showed higher hardness, yield strength, and compressive strength than the microwave-assisted mechanically alloyed pure Al. microhardness Microhardness analysis carried out to investigate the e ﬀ ect of yttria on the of the Al–Y 2 3 nanocomposite, carrying the load of 25 gf and a dwell time of 10 s, for each sample with an average of ﬁve successive indentations. Compressive strength analysis was performed at room temperature using a universal testing machine (Lloyd), under an engineering strain rate of 10 − 4 / s.


Introduction
Metal matrix composites (MMCs) find noteworthy applications in many engineering sectors due to their superior properties such as high strength, high-temperature capability, specific modulus, and good wear resistance compared to monolithic base materials. The mechanical performances of MMCs often show greater improvement than can be achieved by conventional strengthening methods in monolithic alloys [1][2][3][4].
Aluminum (Al)-based metal matrix composites (AMMCs) are an excellent choice for automotive, aerospace, defense, and nuclear power sectors because of their lightweight and favorable mechanical, thermal, and physical properties. Aluminum (Al)-based metal matrix composites are capable of achieving high strength, high-fatigue resistance, high-wear and corrosion resistance, and good compatibility with various manufacturing processes [5][6][7][8].
At present, ceramic particle-reinforced Al-matrix nanocomposites have been prepared primarily by mechanical alloying, forging, and casting routes [9][10][11]. Among these methods, mechanical alloying (MA) has been widely used to fabricate Al-matrix nanocomposites due its cost-effectiveness, simplicity, and its ability to improve the properties vis-a-vis those of the unreinforced matrix [12,13]. There are many sintering techniques such as conventional, spark plasma, vacuum, and microwave sintering processes [14][15][16][17]. Among these techniques, the microwave sintering process is a heating method that offers the ability to balance the radiant and microwave heating effects. In this process, heat is generated within the sample by rapid oscillation of dipoles at microwave frequencies. Microwave sintering provides efficient internal heating, and energy is supplied directly to the material. Therefore, this process avoids the significant temperature gradient between the surface and interior. Microwave sintering is a high-technology heating process that can save both energy and time [18].
In AMMCs, the most common types of reinforcement that can be used are SiC, Si 3 N 4 , Y 2 O 3 , TiC, and Al 2 O 3 [19][20][21][22][23]. Among these ceramics, Y 2 O 3 was selected as the reinforcement to be used in this study due to its high strength, hardness, melting point, and thermal conductivity [24][25][26]. Yttria is an air-stable particle, white in color and solid in substance. By adding the yttria to the aluminum, the strength, corrosion resistance, and wear properties are improved [27]. Yttria is well sintered to a high density and low coefficient of thermal expansion, and has excellent strength properties [28,29]. According to the authors' knowledge, there are no reports in the literature on Al-Y 2 O 3 nanocomposites processed by mechanical alloying and microwave sintering.
Therefore, in this current research, Al-Y 2 O 3 nanocomposites were prepared by mechanical alloying and microwave heating, and the effect of Y 2 O 3 addition on the microstructure and mechanical performance of Al-Y 2 O 3 nanocomposites were investigated.

Materials and Methods
Pure Al (99.5% purity, with an average particle size of 10 µm) and Y 2 O 3 nanoparticles (99.99% purity, with an average particle size of 50-70 nm) were purchased from Alfa Aesar (Tewksbury, MA, USA) and selected as raw materials for the synthesis of Al-Y 2 O 3 nanocomposites.
Aluminum-yttria composites were prepared with 0, 0.5, 1.0, 1.5, and 2.0 vol. % yttria nanoparticle contents. The mixture of powders was blended at room temperature using a Planetary Ball Mill (PM 200) for 2 h, with a rotation speed of 200 rpm. No balls were used during the blending of powders. The mixed powder (~1.0 gm) was compacted into cylindrical pellets by applying a pressure of 50 MPa with a holding time of 1 min. The compacted cylindrical pellets were sintered in a microwave sintering furnace at a temperature of 550 • C with a heating rate of 10 • C/min and providing a dwell time of 30 min. The microwave furnace had an alumina insulation and silicon carbide susceptor. The silicon carbide susceptor was used to increase the heating rate and hybrid heating. Alumina insulation prevents heat loss and is used as well to protect the interior walls of the microwave oven. The compacted pellets were placed at the center of the cavity and sintering was conducted at the multimode cavity [30]. Figure 1 shows the schematic representation of the microwave sintering furnace.
The density of the sintered samples was calculated using Archimedes' principle. The porosity of the samples was calculated by the theoretical and experimental density of the composite samples. The X-ray diffraction (XRD, PANalytical X'pert Pro, PANalytical B.V., Almelo, The Netherlands) analysis was performed to identify the phases present in Al-Y 2 O 3 nanocomposites. The XRD patterns were recorded in the 2θ range of 20-90 • with a step size of 0.02 • and a scanning rate of 1.5 • /min. The microstructural characterization and determination of the distribution of the yttria nanoparticles in the aluminum matrix were carried out using scanning electron microscopy (SEM, JeolNeoscope JSM6000, Tokyo, Japan) and energy dispersive X-ray spectroscopy (EDS, Tokyo, Japan).
The microhardness of the Al-Y 2 O 3 nanocomposites was determined using Vickers microhardness tester (MKV-h21, USA). Microhardness analysis was carried out to investigate the effect of yttria on the hardness of the Al-Y 2 O 3 nanocomposite, carrying the load of 25 gf and a dwell time of 10 s, for each sample with an average of five successive indentations. Compressive strength analysis was performed at room temperature using a universal testing machine (Lloyd), under an engineering strain rate of 10 −4 /s.
The respective data of each sample were obtained by an average of three successive values of test results. From the load-displacement curves, 0.2% offset compressive yield strength (CYS), ultimate compressive strength (UCS), and compressive strain were determined.

Density and Porosity of Al-Y 2 O 3 Nanocomposites
Density and porosity values of the microwave sintered Al-Y 2 O 3 nanocomposites with different contents of yttria in the Al matrix are shown in Table 1. It can be observed that the density of the composite gradually increased with the increase of the yttria content since the density of yttria (5.01 g\cc) is higher than that of Al (2.70 g\cc). Generally, the higher relative density of sintered samples influences the mechanical properties of the composites. The porosity of the composites decreased by increasing the amount of yttria content. The decrease in porosity with increasing yttria content shows that the presence of the hard yttria particles did not impair the densification of the Al powder [31]. Microwave heating was one of the main reasons for the low porosity of the synthesized composites.

XRD Analysis of Al-Y 2 O 3 Nanocomposites
The X-ray diffraction (XRD) patterns of the microwave sintered pure Al and Al-Y 2 O 3 nanocomposites with different amounts of Y 2 O 3 are shown in Figure 2a. Figure 2b shows the enlarged patterns of the Al-1.5 vol. % Y 2 O 3 nanocomposite. The XRD patterns clearly indicate the presence of Y 2 O 3 nanoparticles in the Al composite matrix. Due to the small volume of yttria reinforcement present in these composites, the yttria peaks were very small compared to the aluminum matrix peaks. Also, it can be seen that the intensity of the yttria diffraction peaks increased with the increasing of yttria percentage. The XRD results show that the main elements of Al (higher peak) and Y 2 O 3 (lower peak) are present in Al-Y 2 O 3 nanocomposites.

SEM Analysis of Al-Y 2 O 3 Nanocomposites
The SEM and EDS images of the microwave sintered Al-Y 2 O 3 nanocomposites with different contents of yttria are shown in Figure 3. The results of microstructural characterization revealed that yttria particulates were present individually and in relatively smaller clusters indicating an improvement in their distribution. The EDS analysis confirms the aluminum and yttria particles present in the Al matrix. The EDS mapping spectrum of all nanocomposites were mainly composed of Al, Y, and O elements, as shown in Figure 3b,d,f. The microcracks were restricted by the presence of hard and homogeneous yttria particles in the Al-matrix and influenced the microstructure and mechanical properties of Al-Y 2 O 3 nanocomposites. The specimen with 2 vol. % of yttria particles shows the decreasing of the interparticle distances as the concentration of the nanoparticles increased.

Microhardness of Al-Y 2 O 3 Nanocomposites
Vickers microhardness was measured on all specimens to study the effect of Y 2 O 3 content on the microhardness. Figure 4 shows the results of the microhardness of the Al-Y 2 O 3 nanocomposites with different content of yttria. From the Table 2, the microhardness of the composite increased as the yttria increased of up to 1.5 vol. % and then decreased at 2.0 vol. % Y 2 O 3 . The considerable increase in hardness could be attributed to the presence of homogeneously distributed hard ceramic nanoparticles and dispersion hardening effect [34]. Al-2.0 vol. % Y 2 O 3 nanocomposites show a decreased microhardness value, which was mainly due to the agglomeration of the yttria and increasing presence of clustering of yttria in the case of the Al matrix [35]. The microhardness of the microwave sintered samples in this study was found to be higher than the vacuum sintering and arc-melting samples [36].
The increment of microhardness in the composite materials was due to the presence of hard ceramic particles.

Compressive Analysis of Al-Y 2 O 3 Nanocomposites
The compressive test was conducted on the microwave sintered pure Al and Al-Y 2 O 3 nanocomposites and strengths were compared. Figure 5a shows the engineering stress-strain curves of the Al-Y 2 O 3 nanocomposites with different content of yttria. Figure 5b shows the corresponding mechanical data of Al-Y 2 O 3 nanocomposites.  [37]. Al-2.0 vol. % Y 2 O 3 nanocomposites show a decreased microhardness value, mainly due to the agglomeration of nanoparticles and grain growth [38]. Reinforcement amounts, density, heating mechanisms factors also govern the variation of the mechanical properties. However, compression properties of the microwave sintered Al-1.5 vol. % Y 2 O 3 nanocomposites are interestingly superior to those of other reinforced AMMCs [39][40][41][42][43].
There are several strengthening mechanisms to enhance materials' mechanical properties like hardness and compressive strength of the composite materials. The strengthening of the composites is not only dependent on unique strengthening mechanisms, but it also depends on several strengthening mechanisms.
In the present study, the strengthening mechanism of the Al-Y 2 O 3 nanocomposites mainly depended on dispersion hardening due to the hard yttria particles present in the aluminum matrix. The increase in strength and hardness may be attributable to Orowan strengthening [44,45].

Conclusions
The Al-Y 2 O 3 nanocomposites were successfully synthesized by mechanical alloying and microwave sintering method. The influence of yttria nanoparticles on the microstructure and mechanical properties of the Al-Y 2 O 3 nanocomposites were investigated in detail. The density of the composites increased with the increasing of yttria content while porosity decreased. The SEM analysis showed the homogeneous distribution of yttria particles in aluminum composites. The Al-Y 2 O 3 nanocomposites exhibited better mechanical properties compared to pure Al. The optimum hardness (81 ± 3 Hv), yield strength (126 ± 5 MPa), and ultimate compression strength (374 ± 6 MPa) and compressive strain (~60%) values were obtained for the Al-1.5 vol. % Y 2 O 3 nanocomposite. This significant enhancement in mechanical properties in Al-1.5 vol. % Y 2 O 3 nanocomposites make them potential candidates for automotive applications.