•Pure aluminum was reinforced with graphene-platelets by using mechanical milling.•The composites were studied after sintering condition.•Milling time and graphene-platelet enhance the mechanical ...behavior of the composites.
Graphene can be considered as an ideal reinforcement for the production of composites due to its outstanding mechanical properties. These characteristics offer an increased opportunity for their study in the production of metal matrix composites (MMCs). In this research, the studied composites were produced by mechanical alloying (MA). The employed milling times were of 1, 3 and 5h. GNPs were added in 0.25, 0.50 and 1.0wt% into an aluminum powder matrix. Milled powders were cold consolidated and subsequently sintered. Composites were microstructurally characterized with Raman spectroscopy and electron microscopy and X-ray diffraction. The hardness behavior in composites was evaluated with a Vickers micro-hardness test. A homogeneous dispersion of graphene during MA and the proper selection of sintering conditions were considered to produce optimized composites. The obtained results with electron microscopy indicate a homogeneous dispersion of GNPs into the aluminum matrix. Analyses showed GNPs edges where the structure of the graphene layers conserved after MA is observed.
Novel Al-based nanocomposites reinforced with multi-walled carbon nanotubes were produced by mechanical milling. Next, pressure-less sintering at 823
K under vacuum and hot extrusion at 773
K were ...carried out. The interface between Al matrix and the multi-walled carbon nanotubes was examined using transmission electron microscopy. The values of yield strength (
σ
y), maximum strength (
σ
max) and microhardness Vickers (HVN) of the composites were evaluated and reported as a function of carbon nanotubes content. The concentration of multi-walled carbon nanotubes has an important effect on the mechanical properties of the nanocomposite. Formation of aluminum carbide in the nanocomposites was observed. Possible strengthening mechanisms are presented and discussed.
AlxCoCrFeMnNi high-entropy alloys with different aluminum concentrations (x = 0.5, 1, and 1.5 at%) were synthesized by mechanical alloying followed by consolidation using two different sintering ...methods, conventional (CS) and high-frequency induction heat + conventional (HFIHS + CS). The results show the presence of FCC, BCC, and B2ordered phases in all systems, regardless of the sintering method. The BCC phase exhibits morphological changes (cuboidal-type and plate-like) associated with the two sintering methods involving different diffusion rates and affecting the hardness values. The M23C6 carbide is identified in systems sintered by the CS method; meanwhile, the M7C3 carbide is identified in the HFIHS + CS method. Finally, the HFIHS + CS method results in a higher level of densification (~95%) than the CS method (~80%).
•AlxCoCrFeMnNi (x = 0.5, 1 and 1.5 at%) HEAs present B2ordered and FCC phases•BCC phase precipitates from B2ordered phase in Alx (x = 1 and 1.5 at%)•Precipitates (BCC) with cuboidal and plate-like morphology are observed•The coarsening kinetics of the BCC phase is dependent on the sintering method•The HFIHS + CS achieves higher densification than the CS and HFIHS alone
The plastic deformation effect on activation energy, transformed volume fraction, and Avrami exponent for the phase precipitation in a 2024 aluminum alloy was evaluated in this study. Differential ...scanning calorimetry characterization was carried out in deformed and non-deformed samples, focusing on the study of S phase precipitation (Al2CuMg precipitates with orthorhombic structure, space group Cmcm, and lattice parameters a = 4.03 Å, b = 9.3 Å, c = 7.08 Å). The results show that the solid-state reactions were activated, kinetically controlled, and modified by the plastic deformation. The enthalpy changes, activation energy, and peak temperature shifting confirm that the applied deformation alters the reaction sequence. Two different Avrami exponent values were found, one close to 1.7, which indicates that nucleation occurs in preexisting nucleation sites, and the other near 1.3, which corresponds to phase transformation driven by diffusion mechanisms.
•The S-phase precipitation was thermally activated and kinetically controlled.•%ε modify the ΔH, Tp, and Q but does not affect the precipitation sequence.•Avrami exponent ~1.7 indicated that nucleation occurs in preexisting sites.•Avrami exponent ~1.3 showed that phase transformation is driven by diffusion.
•The vapor pressure, polar or non-polar nature, and type of milling affect the PSD.•The O and C are incorporated as interstitial elements into the crystalline lattice.•In wet milling, the formation ...of the BCC structure is related to Al content.•In dry milling, the formation of the BCC structure is related to collision energy.•FCC-BCC region is broader in the Al1.5 system than those values calculated by VEC.
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This paper reports the effects of different process control agents (PCA) (methanol, n-heptane, and stearic acid) and aluminum concentration (x = 0.5, 1.0, and 1.5 at%) on the particle size distribution, structure, microstructure, stability, and prediction of phases in AlxCoCrFeMnNi high-entropy powder synthesized by mechanical alloying. The results showed that the vapor pressure, the polar nature of the different process control agents used, and type of milling (dry/wet) generating changes in the particle size distribution, microstructure, and crystalline structure since the use of PCA modifies the balance between the cold-welding and fracture processes. Quantitative analysis showed that the PCA chemical decomposition generated residual oxygen and carbon in all systems. X-ray diffraction showed that all systems present a mixture of crystal structures BCC-FCC with nanometric crystallite size, and the peak intensities changed in function of lattice deformation generated by type milling employed. On the other hand, the results also showed that the boundary between BCC-FCC and FCC regions is even broader and extends to higher Al concentration than those reported by the valence electron concentration criterion.
•In the AlCoCrCuFeNi High-Entropy Alloy, copper was replaced by titanium for hardening effects.•Samples were synthesized following a solid-state route (mechanical alloying and vacuum ...sintering).•Hardness and elastic modulus were determined using nanoindentation testing.•Evidence shows a clear correlation between composition/crystalline structure and final mechanical properties.•Prepared samples were composed of three main phases and nanometric precipitates.
The concept of High-Entropy Alloy (HEA) expanded the research field of advanced metallic materials for various applications, like the development of ultra-hardness ballistic protection materials for national security. Although there can be hundreds of compositions, carefully selecting the constituent is mandatory to improve their mechanical behavior, keeping in mind their microstructural array based on chemical composition. In the present study, the AlCoCrCuFeNi HEA was modified, replacing copper with titanium, looking for the formation of a Ti-rich BCC phase for hardening effects. Samples were prepared following the powder metallurgy route, including mechanical alloying favoring the generation of a nanocrystalline microstructure. Studies based on structure, microstructure and nanoindentation testing on each phase of the alloy were performed to determine the correlation between their composition, crystalline structure and mechanical properties. Evidence showed the formation of three main micrometric phases (two BCC and one tetragonal) coexisting with a nanometric dispersion of rounded precipitates. Nanoindentation testing shows that the main hardening effect was related to the tetragonal phase formation by a solid-solution strengthening mechanism. This phase reached the highest hardness (14.9 GPa); meanwhile, the richest Ti phase showed the lowest elastic modulus, titanium favors the material ductility.
High-frequency induction heating is frequently used to consolidate solid pieces of refractory ceramics. However, this valuable technique has not been deeply evaluated for sample preparation in light ...metal-based systems as an economical and feasible alternative for rapid sintering routes such as spark plasma sintering. This work deals with the potential use of induction heating to produce highly densified samples with refined microstructure, enhanced mechanical properties, and lower oxygen contamination. Here we demonstrate that induction-sintering can increase the hardness and yield strength in 70 and 80% respectively, compared to a commercial hardened alloy (AA-1350-H19). Theoretical calculations demonstrate that this behavior can be attributed to two main reinforcement mechanisms: dislocations obstruction and grain refinement. The increased mechanical response can be imputed to the effective sub-micron microstructure retention due to its shorter processing time and lower temperature compared to the conventional sintering process.
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The present study aims to evaluate plastic deformation's effect by cold-rolling on the precipitation sequence of 2024 aluminum alloy. X-ray diffraction, scanning/transmission electron microscopy, and ...Vickers microhardness tests have been used to characterize the microstructure and mechanical behavior of the alloys. It was observed that plastic deformation induces changes in the precipitation sequence, which affects the mechanical properties and delays the overaging stage. In the deformed alloy, two hardening peaks were observed. These peaks occurred at 30 min (248 HV ± 5) and 600 min (230 HV ± 2) and were attributed to the θ' and S' phases, respectively. However, in the non-deformed alloy, only a single hardening peak was observed. This peak arose after 300 min aging (208 HV ± 4) and was attributed to the S' phase formation. Thus, the precipitation sequence in the deformed alloy was the following: αSSS–CuMg clusters → GPB-II zones/θ''/θ'/ → S' → S, and for the non-deformed alloy was αSSS–CuMg clusters → GPB-II zones/S'/ → S.
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