The effect of CeO2 nanoparticles on the microstructure of two different metal matrixes, AA2024 aluminum alloy and pure aluminum, were studied. The aluminum-based composites were synthesized by mechanical milling and subsequently sintering at 550°C under an argon atmosphere. The microstructural evolution of consolidated composites was evaluated by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The experimental results revealed that the addition of CeO2 nanoparticles in the AA2024 alloy produced a dispersed needle-like Ce-Cu-rich phase that reduces the hardness of samples after sintering. On the contrary, the dispersion of CeO2 in pure aluminum shows significant improvement of hardness in comparison with the reinforced and unreinforced AA2024 aluminum alloy. During the sintering, the CeO2 nanoparticles show higher chemical stability in the aluminum matrix in comparison with the AA2024 matrix.
Aluminum and its alloys are attractive alternatives to replace some ferrous materials in many industrial applications because of their intrinsic properties, such as good machinability, surface finish capabilities, damage tolerance, low density, high specific strength, and good thermal and electrical conductivity [
For aluminum reinforcement, several metal oxide particles are excellent options to take into account. Moreover, the addition of oxides at the nanometric scale can improve the mechanical properties of the MMCs by reducing the interparticle spacing. Unfortunately, fine particles present a higher tendency toward heavy agglomeration. The strengthening mechanisms, i.e., nanoparticles addition, have a strong relationship based on their type, size, morphology, volume fraction, and distribution, as well during the composite synthesis.
From the wide variety of synthesis routes to prepare MMCs, the solid routes based on powder metallurgy (PM) provide better results regarding their versatility and ability to produce homogeneous dispersions of secondary phases into the aluminum matrixes. The PM process involves the mixing of reinforcement particles with the metal matrix (in powder form), followed by cold consolidation and sintering steps. The mechanical response of aluminum and its alloys can be enhanced with the use of mechanical alloying (MA) and/or mechanical milling (MM) techniques.
There is an increased interest for the development of advanced materials using some aircraft grade aluminum alloys as the metal matrix of composites preparation. The AA2024 aluminum alloy is extensively used in the aerospace industry due to their high strength, excellent wear resistance, lightweight, and good casting characteristics [
In the present study, two metallic matrixes were selected: pure aluminum and a commercial AA2024 aluminum alloy. The aim of the present work is to study the effect of CeO2 nanoparticles dispersion on the microstructure of pure aluminum and AA2024 aluminum alloy, investigate the stability of these nanoparticles, and observe their interaction with the alloying elements.
The CeO2 nanoparticles were dispersed in two different aluminum-based materials by the mechanical milling route. Pure Al powder and AA2024 burrs (Table
Composition of the AA2024 alloy (wt.%).
Al | Cu | Mg | Mn | Si | Fe |
---|---|---|---|---|---|
Balance | 4.5 | 1.5 | 0.6 | 0.5 | 0.4 |
Figure
(a) TEM micrograph and (b) XRD pattern of CeO2 nanoparticles.
Some dark-field TEM micrographs of the AA2024-CeO2 and the Al-CeO2 are presented in Figure
TEM-DF micrograph, EDS analyses (in wt.%), and elemental mapping of (a) AA2024-CeO2 and (b) Al-CeO2 powder samples after MM.
Figure
SEM images showing the microstructural changes of AA2024-CeO2 composite after sintering. (a) 1 h. (b) 2 h. (c) 3 h. (d) 4 h. (e) 5 h. (f) 7.5 h.
The chemical composition of the bright phase observed in Figure
EDS mapping (a) and hardness (b) of the AA2024-CeO2 composite after sintering.
Copper plays an important role in strengthening aluminum aircraft alloys such as AA2024-T6 due to the precipitation strengthening phenomena occurred during solution and artificial aging. The amount, distribution, and size of the precipitated phase play an important role in the strengthening rate, depending largely on the composition and processing route of the alloys [
The images from Figure
Microstructural characteristics of the Al-CeO2 sintered composite: (a) SEM micrograph, (b) TEM image, (c) close up of Ce-rich nanophase, (d) HRTEM micrograph showing the CeO2/Al interface, (e) corresponding simulated image, and (f) SAED and simulated patterns of the interface.
Some TEM observations were carried out on the above-mentioned agglomerated bright phase. The TEM micrograph displayed in Figure
In order to evaluate the chemical stability of the added nanoparticles and their reinforcement capacity, the sintered samples (AA2024-CeO2 and Al-CeO2) were added to a pure AA2024 melted alloy at 750°C using a concentration of 1% (in wt.). Figure
SEM micrographs showing the microstructure of samples in as-cast condition: (a) A2024, (b) A2024 + (A2024-CeO2), and (c) A2024 + (Al-CeO2).
Some microhardness results of sintered composites in the Vickers scale are displayed in Figure
Mechanical behaviour of (a) sintered at 550°C for 3 h and (b) casted prepared samples.
In Figure
The present study showed the viability for production of two composites using as reinforcement material ceria nanoparticles and two different metal matrixes, pure aluminum and a commercial AA2024 alloy, used frequently in the aeronautical industry. Both composites were synthesized using a solid-state route complemented by mechanical milling. Used nanoceria has a particle size in the range of 10–50 nm, its purity was confirmed by EDS, and the material presents a single-phase with a lattice constant of 5.411 Å and Fm-3 m space group. The SEM and TEM studies revealed that the addition of CeO2 nanoparticles in the AA2024 alloy produced a dispersed needle-like Ce-Cu-rich phase, which changed their volume fraction, size, and morphology from irregular to a needle-like shape after sintering. A noticeable variation on the porosity as a direct function of the sintering time was also observed. The presence of this phase had a negative effect on the mechanical performance of the AA2024 alloy due to the chemical affinity between Cu and Ce, which alter the kinetics of precipitation of the AlCuX solutes. On the contrary, the dispersion of CeO2 in pure aluminum showed a significant improvement of hardness. During the melting, the CeO2 nanoparticles showed a similar chemical stability compared to low-temperature sintering processing. The remarkable microstructural variation of both metal matrixes is related with Ce-Cu affinity in the AA2024 alloy and the high chemical stability of the ceria nanoparticles in pure aluminum.
The data used to support the findings of this study are included within the article
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors gratefully acknowledge K. Campos-Venegas, E. Guerrero-Lestarjette, and C.E. Ornelas-Gutiérrez for their technical assistance. This research was carried out with economical support from the National Council of Science and Technology (CONACYT) of Mexico (project no. 20450).