•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.
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.
The effect of graphite, CeO2 and nanohybrid CeO2-graphite on an Al–Mg–Si system (6063 alloys) was studied in this paper. Such alloy was reinforced using mechanical milling, followed by a conventional ...sintering process. Studies for structure, microstructure, and mechanical properties were carried out through X-ray diffraction, electron microscopy, and compression and Vickers microhardness testing. Results showed that the milling process enables homogeneous dispersion of the different reinforcement materials in the Al–Mg–Si matrix. The best performance, concerning reinforcement capacity and microhardness, was obtained with samples reinforced with graphite. The mechanisms involved in enhancing the mechanical properties were active and latent. The first includes grain-size refinement and strengthening by dispersion of a second phase, and the second is related to thermal mismatch.
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•The Gr, CeO2, and nanohybrid CeO2-Gr reinforcements were homogeneously dispersed.•The reinforcements containing Gr presented the formation of Al4C3 during sintering.•The CeO2 presented a partial reduction into Ce7O12 during sintering.•The reinforcements containing Gr showed the best mechanical response.•The active and latent strengthening mechanisms enhanced the mechanical properties.
•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.
•Equiatomic AlCoNi alloys with two added carbon allotropes were fabricated through powder metallurgy.•Graphite addition increased the hardness of the AlCoNi alloy 11%.•Different composition, ...morphology, and growth rates were observed on the oxide scale based on carbon-type addition.•CNTs benefits the formation of a protective aluminum-oxide scale instead of Ni-Co oxide.
Equiatomic AlCoNi alloys with 0.1 wt% of carbon addition in different forms such as graphite and carbon nanotubes were fabricated by powder metallurgy. The effect of carbon addition on the microstructure and oxide scale formation was investigated as a function of carbon type. Evidence showed that both carbon forms addition reduced the porosity of the sintered AlCoNi alloy. A mixture of Al and Ni-Co oxide scale was formed in all samples after a heat treatment at 1000 °C for 2 h in air. After subsequent oxidation at 900 °C for 100 h, graphite induced the Ni-Co oxide growth; meanwhile, carbon nanotubes favored the Al oxide growth. The dispersion of carbon nanotubes produced the lowest growth rate of the oxide scale.
•Multi-component systems of AlCoFeMoNiTi were produced by mechanical alloying.•Consolidated samples were fabricated by two different processing routes, sintering and arc melting.•Effect of routes of ...consolidation on microstructural evolution and microhardness is reported.•High hardness values are found in consolidated samples.•Alloying elements, grain size, and precipitates have a high effect on microhardness.
A nanostructured AlCoFeMoNiTi high entropy alloy was synthesized through the mechanical alloying process. Bulk samples were obtained by two different routes to compare the microstructural evolution and hardness behavior: sintering and arc melting. Through electron microscopy analyses the formation of Mo-rich and Ti-rich phases were identified in the melted sample, while Ti-rich nano-precipitates were observed in the sintered sample. A higher microhardness value was achieved on the sintered sample than for the melted sample. The disadvantage of porosity in the sintered sample in comparison to the melted one was overcome by the hardening effect produced by the mechanical alloying.
•Multi-component systems were produced by mechanical alloying and sintering.•Effect of Cr, Mo and Ti on microstructural evolution and microhardness is reported.•Alloying elements, grain size, and ...precipitates have a high effect on microhardness.
This study reports the synthesis of multi-component systems by mechanical alloying, their subsequent consolidation by conventional sintering followed by mechanical and microstructural characterization. Three multi-component systems were selected and the effect of Cr, Mo and Ti is reported. Mechanical alloying leads to formation of nanocrystalline powder alloys of about 5–25nm after 10h of milling. XRD patterns indicate the presence of several solid solution phases with BCC and FCC structures in both the as-milled and the sintered products. After the sintering process, new equilibrium phases are crystallized from milled powder. Crystallized phases present simple structures (BCC and FCC) but with some differences in composition, besides tetragonal and/or rhombohedral structures in alloys with Cr and Mo. High hardness values are found in consolidated samples. Alloying elements, grain size, proportion and nano-size of dispersed phases have an important effect on mechanical properties.
The effects of the addition <2wt.% of Nickel and solid solution treatment times on microstructure and hardness of Al-Si-Cu alloy (A319) are reported. The results obtained show that Ni additions and ...heat treatments influence changes in microstructure, hardness and precipitation kinetics. The dendrites size and number density of Al-Cu-Ni and Al-Ni intermetallic compounds in as-cast condition are affected with Ni additions; however, occurs a reduction of Al-Cu-Ni phases at longer solution times (7h). The hardness values in alloyed samples with Ni are higher than those of reference samples, the maximum hardness peak are reached at shorter aging times; nonetheless, the decrease was slower during overaging step. The hardness values increase with Ni content ~4% HV where an increase of maximum hardness ~6% HV is reached with aging. After 10h of aging treatment at 170°C, the alloyed samples and not alloyed present different type of precipitates. In reference sample θ′-Al2Cu precipitates were identified, and in the alloyed samples, two types of coherent precipitates with different thickness and size were observed, in addition those precipitates present smaller size and higher number density than those observed in the reference alloy.
•Ni additions increase number density of Al-Cu-Ni, Al-Fe-Ni and Al-Ni intermetallic.•Ni additions increase the hardness of A319 alloy in as-cast and after T6-treatments.•Ni additions change the precipitation sequence of A319 Alloys during T6-treatments.•During overaging step, the growth and coarsening of precipitates are delaying in A319 alloy with Ni additions.
