High mechanical performance alumina-reinforced aluminum nanocomposite metal foam produced by powder metallurgy: fabrication, microstructure characterization, and mechanical properties

In this research, alumina-reinforced aluminum nanocomposite foams with high compressive performance were produced using the powder compact melting (PCM) process and CaCO3 foaming agent. The content of Al2O3 nanoparticles, with sizes smaller than 80 nm, varied between 0–15 wt%. Aluminum, Al2O3, and CaCO3 powders were mixed and compacted to dense precursors with 318 MPa stress. Finally, foam samples were fabricated by heating up the precursors at 1000 °C for 10 min. A range of porosity values was achieved between 34% to 43% for different combinations of aluminum matrix and alumina reinforcement phase. The crystallite size of the alumina reinforcement phase in Al-3 wt% alumina and Al-15 wt% alumina composite foam was estimated as 23.51 nm and 23.24 nm while the crystallite size of the aluminum matrix was determined as 15.72 nm and 15.44 nm for the same samples. The Field Emission Scanning Electron Microscope (FESEM) images indicated micro-size pores, which were connected via channels. The composite foam with 3 wt% of Al2O3 nanoparticles had a more uniform microstructure and more channels than other samples. The Vickers hardness values of the composites foam increased with an increase in wt% of alumina particles. This amounts to an approximately 93% increase in hardness when 15 wt% alumina particles are added to the aluminum foam. The sample with 3 wt% Al2O3 nanoparticles showed enhanced compressive strain up to 50% with high compressive strength up to 45 MPa and a uniform microstructure.


Introduction
Porous materials have attracted a lot of interest both in academia and industry because of their exceptional mechanical and physical properties [1][2][3][4]. In the recent decades, much attention has been paid to aluminum foams in different industries, especially in the automobile, aerospace and building industries, mainly due to their unique properties such as low density, high strength-to-weight ratio, high energy absorption at compression, sound absorption and heat resistance [5][6][7][8][9]. In general, pure aluminum foams demonstrate poor mechanical properties, depending on their production method, composition, microstructure, and density [10].
The effects of ceramic particles, especially SiC and Al 2 O 3 , on liquid foam stability, cell microstructure and mechanical properties of aluminum foams have been extensively studied in powder metallurgy and casting routs [15][16][17][18][19][20][21][22][23]. M Haesche et al [18] showed that adding 1%-5% wt% SiC and Al 2 O 3 particles improve the maximum expansion, collapse behavior and cell microstructure of aluminum foams. They also reported that smaller ceramic particles resulted in higher efficiencies.
A R Kennedy et al [21] demonstrated that the addition of 3 vol% of Al 2 O 3 , SiC and TiB 2 particles to aluminum foams resulted in an increase in foam expansion by reducing the critical cell wall thickness before the foam ruptures. A study by W Deqing et al [22] showed that the cell size and a wall thickness of aluminum foams can be increased by raising the amounts and particle size of SiC and Al 2 O 3 . Mahmutyazicioglu et al [23] investigated the effects of Al 2 O 3 content on hardenability, microstructure and compressive behavior of 6061 aluminum foams. According to their results, the cell structure and drain reduction were improved by the addition of no more than 5 vol% of Al 2 O 3 particles [23].
The nano-size alumina particles compare to micro-size ones provide a higher surface-area-to-volume ratio, reduce the brittleness of foams and, consequently, smaller volume fractions are required to improve the desired properties [33,34]. However, the reports concerning the effects of ceramic nanoparticles on the properties of aluminum foams are very scarce [34]. B Prabhu et al [34] showed that the addition of SiC nanoparticles to aluminum foams decreased the cell sizes by 50.4% and improved the cell structure, yield stress, and energy absorption capacity.
In order to reinforce the aluminum foams, Al 2 O 3 nanoparticles can be suitable candidates due to their unique characteristics, including (a) high capability for strengthening the cell walls because of their hardness and strength; (b) the absence of chemical interfacial reactions between molten aluminum and alumina particles, which results in the loss of structural integrity. Herein, the aim of our study is to investigate the physical, microstructural and compressive properties of aluminum nanocomposite foams reinforced with Al 2 O 3 nanoparticles with sizes smaller than 80 nm, produced via a powder metallurgy method and low-cost CaCO 3 foaming agent.
The morphology and particle size of the aluminum powder and alumina nanoparticles are shown in the FESEM images of figures 1(a), (b). The high magnification image in figure 1(a) exhibits the homogeneous distribution of semi-spherical aluminum powder particles with an average size of 150 nm. The α-alumina nanoparticles with the size of 80 nm and spherical morphology can be clearly seen in figure 1(b).

Samples fabrication
Following the 'powder compact melting' method, aluminum, Al 2 O 3 (0-15 wt%) and CaCO 3 (15 wt%) powders were mixed together for 30 min to obtain homogeneous mixtures. The mixed powders were then compacted using uniaxial cold pressing with 318 MPa pressure to form dense cylindrical components called 'foamable precursors' with a diameter of 20 mm, the height of 19.5 mm and relative density of 77%-80% (figure 2). In order to form the primary connections between aluminum and Al 2 O 3 particles, precursors were preheated at 450°C for 30 min under air atmosphere. In the next step, the precursors were placed in a cylindrical steel mold (20 mm in inner diameter, 100 mm in height) which was open only at the top. Foaming was done in a preheated furnace at 1000°C for 10 min under air atmosphere. Since the precursors and the mold had similar diameters, expansions occurred only in the height direction. After the foaming process, the samples were removed from the furnace and foams were solidified through natural cooling in the air.

Characterization
The foam's density (ρ s ) was calculated from their mass and geometry (ASTM-B962-17 standard test methods for the density of compacted or sintered (PM) products). Furthermore, the relative density of the samples (ρ * ) was calculated by dividing their density by the theoretical density of bulk aluminum (2.7 g cm −3 ) as follows: ASTM E92 standard test methods for Vickers hardness and knoop hardness of metallic materials were utilized for produced composite foams. The hardness distribution in the foams was examined by the microindentation technique. To this end, a micro-hardness machine, Qualitest QV-1000DM, equipped with a tetrahedral diamond indenter with tip angle 68°was used. Micro-Vickers hardness was measured under a 25 g load and a 15 s dwell time. At least five micro-hardness measurements were performed on each sample.
In order to investigate the compressive behavior of the materials under the ASTM C365-05-compression test standard, the samples were cut into cylindrical parts, 20 mm in diameter with a height-to-diameter ratio between 1.5 and 2. Therefore, the minimum dimension of the specimen was at least seven times the cell size to avoid the size effects [6]. The compression tests were performed at room temperature using a Zwick/Roell Z250 testing machine, at a constant ram speed of 6 mm min −1 . Furthermore, samples were sectioned by a wirecutting machine to investigate the microstructure and elemental analysis using Field Emission Scanning Electron Microscope (FESEM), TESCAN Mira 3-XMU) with 15.0 kV accelerating voltage.

Measurement of relative density
The relative density of the nanocomposite foams as a function of Al 2 O 3 content is plotted in figure 3. A range of porosity values was achieved between 34% to 43% for different combinations of aluminum matrix and alumina reinforcement phase. The highest value of relative density (∼65%) was obtained using 3 wt% of Al 2 O 3 nanoparticles. In fact, the uniform distribution of Al 2 O 3 nanoparticles inhibited the growth of cells throughout the precursor by affecting the viscosity and surface tension between CO 2 bubbles and melted aluminum particles [19]. Moreover, the addition of 15 wt% of Al 2 O 3 nanoparticles increased the density due to the agglomeration of CaCO 3 and Al 2 O 3 particles and diminished the foaming agent in the aluminum matrix.

X-ray diffraction characterization
In order to identify the phase of the materials, x-ray diffraction (XRD) characterization was carried out on the aluminum powder, alumina nanoparticles and composite foams with 3 wt% and 5 wt% of Al 2 O 3 nanoparticles.     5(a)) a uniform distribution of cells, connected via channels, was observed. Interestingly, adjusting the critical fabrication parameters in this study resulted in semi open-cell aluminum foams with highly distributed pores while all the previous reports on aluminum foam fabrication using this method showed the closed-cell structure [35][36][37][38][39][40][41][42][43]. This uniform microstructure was due to CaCO 3 decomposition product (i.e. CaO), the oxide layer on the surface of aluminum particles and further oxidation of aluminum during foaming, which stabilized the foam by enhancing the viscosity [35,[44][45][46][47]. The cells were irregularly shaped and smaller than 100 microns, due to very fine aluminum particles [48].
The small pores between primary aluminum particles, which is prominent in the inset image of figure 5(a) was due to the powder compact melting process. The formation of these pores was a consequence of the presence of an oxide layer on the surface of aluminum particles, which can be deteriorated the sufficient diffusion and complete bonding of particles. PM pores are likely the sites for crack initiation in mechanical tests [49].
As evidenced in figure 5(b), the sample with 3 wt% of Al 2 O 3 nanoparticles has a more uniform microstructure and more channels than other samples. The Al 2 O 3 nanoparticles can help the stabilization of aluminum foam by affecting the viscosity and the surface tension of CO 2 gas/aluminum melt. However, increasing the Al 2 O 3 content to 6 wt% decreased the uniformity of the microstructure ( figure 5(c)). Moreover, the nanocomposite foams with a high content of Al 2 O 3 nanoparticles showed pore coarsening, unusually large pores and non-uniform microstructure ( figure 5(d)). These undesirable effects were caused by the  agglomeration of the particles (white dots in figure 6(a)). Considering the white color of these regions, the agglomerated particles would be Al 2 O 3 and/or CaCO 3 . Considering the geometry of the foaming mold, which was open only at the top, the foaming process began from the top of the precursors. Consequently, the bottom of the precursors remained relatively dense. Furthermore, higher stresses in the upper and lower zones of the precursor during compaction resulted in the formation of shear bands and large sectional cracks in the samples (figure 6(a)) [50].
The FESEM image and related energy dispersive x-ray (EDX) analysis of the agglomerated regions is shown in figures 6(b) and (c), respectively. The Aluminum, oxygen, calcium, and carbon were detected by EDX analysis (figure 6(c)), which were assigned to the CaCO 3 and Al 2 O 3 particles. It was found that Al 2 O 3 nanoparticles are homogeneously distributed inside the aluminum matrix.

Micro-hardness of composite foams
The micro-indentation technique was used to evaluate the distribution of alumina nanoparticles in the composite foams since it was impossible to detect and distinguish Al 2 O 3 nanoparticles in the aluminum matrix by EDX analysis. Figure 7 and table 1 show the micro-hardness values for the foams containing different levels of Al 2 O 3 nanoparticles.
The inset image of figure 7 shows a representative optical image of the micro-indentation test. The similar indentation diameter demonstrated the mechanical isotropy of the aluminum matrix. It can be expected that the measured values of micro-hardness were less than the actual hardness values of bulk aluminum which are arrised from PM pores in the foams matrix.
Moreover, the micro-indentation data gives additional information on the uniformity of the alumina particles distribution. The micro-hardness data of the reinforced foams with 3 wt% of Al 2 O 3 displayed a narrow range of hardness values, suggesting that the alumina nanoparticles were uniformly distributed in this sample. However, a wide range of micro-hardness values was observed for samples with larger amounts of Al 2 O 3 nanoparticles ( 3 wt%). This shows a nonuniform distribution of Al 2 O 3 nanoparticles in the foams, which can be confirmed by the agglomerated regions in nanocomposite foam with 15 wt% Al 2 O 3 ( figure 6(a)).  It was clearly observed that compared to the unreinforced aluminum foam, the Vickers hardness values of the composites foam reinforced with alumina nanoparticles, increased with an increase in wt% of alumina nanoparticles. This amounts to an approximately 93% increase in hardness when 15 wt% alumina nanoparticles are added to the aluminum matrix. Increasing hardness is due to the higher volume fraction of alumina nanoparticles in the aluminum matrix. Figure 8 and table 1 show the experimental stress-strain data obtained by the compression test of aluminum composite foams with 0-15 wt% Al 2 O 3 nanoparticles. All of the compressive curves in figure 8 can be divided into three stages: (1) elastic deformation, (2) plateau region and (3) close contact of cell walls and collapsing [6,49,51,52]. In all samples, the stress increased considerably with the strain in the plateau region. This response is due to various possible sites for the crack initiation, including PM pores [49,53], undecomposed CaCO 3 , CaO (product of CaCO 3 decomposition), the broken part of the oxide layer on the surface of aluminum particles [54] and the Al 2 O 3 nanoparticles as a reinforcing agent. The interfaces between these brittle solid particles, which have low plasticity, and the aluminum matrix are the appropriate sites for the crack nucleation.

Compressive properties
Comparing the curves in figure 8 and table 1 clearly demonstrated that the addition of 3 wt% of Al 2 O 3 nanoparticles enhanced the densification strain of the foams (from ∼45% to ∼50%) as a result of the improved microstructure. This would be mainly because of the small pores formed between primary aluminum particles during the compacting, the higher cell walls strength, as well as uniformity of cell microstructures improved by alumina nanoparticles.
Moreover, increasing the amount of alumina reinforcement agent to more than 3 wt% reduced the plateau length and compressive strength of the samples due to the pore coarsening and unusual pores (figures 5(c) and (d)). The advantages of the nanocomposite foam with 3 wt% of Al 2 O 3 nanoparticles will be best identified when compared with the results of non-reinforced aluminum foams and those reinforced with micro-size ceramic particles.
Despite the higher relative density (∼66% compare with <30%), the fabricated nanocomposite foams showed comparable strain densification (∼50%) and high compressive strength (∼45 MPa compare with <50 MPa) than aluminum foams reported by other researchers [23,35,51,55]. However, the compressive response of the fabricated nanocomposite foams was similar to those aluminum foams reinforced with SiC nanoparticles [34]. This confirms the significant role of ceramic nanoparticles in enhancing the compressive behavior of aluminum foams.

Conclusion
Here we report for the first time, that adjusting the parameters of the 'powder compact melting' process using alumina reinforcement nanoparticles and CaCO 3 foaming agent resulted in semi open-cell aluminum nanocomposite foams. To obtain desired physical and mechanical properties like high hardness, high strength, and high stiffness, Al foam was reinforced with different content of alumina nanoparticles. The results showed an increase in relative density (∼66%) and uniform microstructure by the addition of 3 wt% of Al 2 O 3 nanoparticles. Adjusting the fabrication parameters in this study resulted in semi open-cell aluminum foams with highly distributed pores. This uniform microstructure was due to CaCO 3 decomposition product (i.e. CaO), the oxide layer on the surface of aluminum particles and further oxidation of aluminum during foaming, which stabilized the foam by enhancing the viscosity. The cells were irregularly shaped and smaller than 100 microns. The sample with 3 wt% of Al 2 O 3 nanoparticles had a more uniform microstructure and more channels than other samples. Increasing the Al 2 O 3 content to 6 wt% decreased the uniformity of the microstructure. Moreover, the nanocomposite foams with a high content of Al 2 O 3 nanoparticles showed pore coarsening, unusually large pores and non-uniform microstructure. The addition of 3 wt% alumina nanoparticles to the aluminum matrix resulted in a compressive strain of ∼50% and high compressive strength up to 45 MPa. These are due to the effective roles of Al 2 O 3 nanoparticles on foam viscosity and surface tension between CO 2 bubbles and molten aluminum. However, agglomeration of Al 2 O 3 and CaCO 3 particles was observed for large amounts of Al 2 O 3 nanoparticles (>3 wt%). The pore coarsening, non-uniformity of the microstructure and deteriorated compressive properties were observed due to decomposition products of CaCO 3 and Al 2 O 3 nanoparticles in the aluminum matrix. Due to a semi open-cell microstructure and high compressive strain and stress of aluminumalumina nanocomposite foams, these materials can be considered as promising candidates for both structural and functional applications in the near future.