Roberto B. Figueiredo’s research while affiliated with Federal University of Minas Gerais and other places

The present work investigates the range of critical cooling rates required to form icosahedral quasicrystals during the solidification of an Al90Cu4Fe2Cr4 (%at) alloy. The previous studies on this alloy have shown that icosahedral quasicrystals can form under rapid solidification conditions. Therefore, determining the critical cooling rate necessary for obtaining these phases is essential for selecting appropriate processing techniques to achieve their formation. In this work, over-sprayed particles of an Al90Cu4Fe2Cr4 (%at) alloy produced by spray-forming were characterized by x-ray diffraction, scanning, and transmission electron microscopy. Based on the particles’ diameters, cooling rates were calculated. A large fraction volume of icosahedral quasicrystal (i-QC) with quasi-spherical shape was obtained for particles with diameters inferior to 25 μm (cooling rates higher than 2.1 × 105 K/s). Microstructural changes, including the suppressing of i-QC phase and the emergence of new phases (Al2Cu, Al13Cr2 and Al13Fe4), were observed due to the decrease in particles’ sizes and consequent increase in the cooling rates. The disappearance of the i-QC characteristic peak in 43.6° (XRD analysis) between 45 μm < d < 80 μm and 80 μm < d < 100 μm, associated with SEM images, establishes 2.1 × 104 K/s as the critical cooling rate, below which the formation of i-QC does not occur in the Al90Cu4Fe2Cr4 (%at) alloy.

Al-Mg alloy disks were produced from Mg sandwiched between Al through 100 turns of high-pressure torsion (HPT) at 6.0 GPa at room temperature, resulting in high microhardness of Hv 300–350 in regions experiencing a nominal shear strain > ~ 390. While compositional mapping using scanning electron microscopy energy-dispersive spectroscopy (EDS) showed a uniform distribution of Mg through the disk thickness at 1.5 mm and 3.0 mm from the disk center, transmission electron microscopy EDS showed a heterogeneous distribution of Mg remained on the nanoscale. Although HPT induces enough mixing to result in face-center-cubic Al with supersaturations of Mg of up to ~ 20 at.% near the disk surfaces, β-Al3Mg2, γ-Al12Mg17 and Al2Mg intermetallic phases were identified by electron diffraction throughout the disk thickness even in regions experiencing high shear strain. This study visually captures detailed compositional heterogeneity throughout the sample thickness following intense mechanical alloying, nanoscale re-structuring and phase transformations.

Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life.

It is known that the grain size plays a major role in the mechanical properties of magnesium. The aim of the present study is to evaluate its role in long‐term corrosion rate. Samples of pure magnesium with grain sizes in the range of 0.9–82 μm are produced through severe plastic deformation and annealing treatments. The mechanical properties are evaluated using tensile tests and the corrosion behavior is evaluated using immersion tests in Hank's solution. A maximum yield stress of ≈150 MPa is observed in the sample with 1.8 μm of grain size and an elongation larger than 25% is observed in the ultrafine‐grained sample. Ultrafine‐ and fine‐grained magnesium display uniform corrosion with a decreasing corrosion rate while coarse‐grained magnesium displays localized corrosion with an accelerated corrosion rate. A corrosion rate of ≈0.2 mm year⁻¹ is observed in the ultrafine‐ and fine‐grained magnesium. The corrosion product layer of the fine‐grained magnesium contains elements absorbed from the media. An analysis of the data in the literature suggests that grain refinement changes the corrosion type from localized to uniform corrosion. The exact relationship between grain size and the corrosion rate remains elusive.

Solid‐state welding of Al 1043 sheets is achieved via high‐pressure torsion (HPT) processing to produce bulk nanostructured Al disks. A homogeneous nanostructure without segregation is observed, with grain sizes of ≈430–470 nm. Miniature tensile testing, coupled with the digital image correlation (DIC) technique, is employed to determine the room‐temperature tensile deformation behavior, particularly the nonuniform behavior with necking, of the HPT‐bonded ultrafine‐grained (UFG) aluminum, comparing it with annealed coarse‐grained counterpart. The HPT‐bonded UFG Al exhibits a large fraction of post‐necking strain, which is supported by the estimated high strain rate sensitivity value of m = 0.085, suggesting the delay of local necking leading to tensile fracture. Detailed DIC analysis reveals prolonged diffuse necking, thus delaying local necking, in the HPT‐bonded UFG Al, while the annealed samples show high fractions of local necking during the nonuniform deformation. Moreover, the DIC data illustrate that local necking predominantly occurred at a limited neck zone, maintaining a plateau strain distribution at the out‐of‐neck zone throughout necking deformation toward tensile failure for both annealed and UFG aluminum. The DIC method offers an alternative means to demonstrate the transition in necking behaviors of materials by estimating the plastic lateral contraction exponent.