Special Issue: “Processing and Treatment of Hexagonal Metals”

There is currently an increasing demand for metals with a hexagonal close-packed structure (HCP) [...]

Evoking ordered vacancies in metallic nanostructures toward a vacated Barlow packing for high-performance hydrogen evolution

Metallic nanostructures are commonly densely packed into a few packing variants with slightly different atomic packing factors. The structural aspects and physicochemical properties related with the vacancies in such nanostructures are rarely explored because of lack of an effective way to control the introduction of vacancy sites. Highly voided metallic nanostructures with ordered vacancies are however energetically high lying and very difficult to synthesize. Here, we report a chemical method for synthesis of hierarchical Rh nanostructures (Rh NSs) composed of ultrathin nanosheets, composed of hexagonal close-packed structure embedded with nanodomains that adopt a vacated Barlow packing with ordered vacancies. The obtained Rh NSs exhibit remarkably enhanced electrocatalytic activity and stability toward the hydrogen evolution reaction (HER) in alkaline media. Theoretical calculations reveal that the exceptional electrocatalytic performance of Rh NSs originates from their unique vacancy structures, which facilitate the adsorption and dissociation of H2O in the HER.

Amorphization in extreme deformation of the CrMnFeCoNi high-entropy alloy

Ever-harsher service conditions in the future will call for materials with increasing ability to undergo deformation without sustaining damage while retaining high strength. Prime candidates for these conditions are certain high-entropy alloys (HEAs), which have extraordinary work-hardening ability and toughness. By subjecting the equiatomic CrMnFeCoNi HEA to severe plastic deformation through swaging followed by either quasi-static compression or dynamic deformation in shear, we observe a dense structure comprising stacking faults, twins, transformation from the face-centered cubic to the hexagonal close-packed structure, and, of particular note, amorphization. The coordinated propagation of stacking faults and twins along {111} planes generates high-deformation regions, which can reorganize into hexagonal packets; when the defect density in these regions reaches a critical level, they generate islands of amorphous material. These regions can have outstanding mechanical properties, which provide additional strengthening and/or toughening mechanisms to enhance the capability of these alloys to withstand extreme loading conditions.

Constitutive Analysis of the Anisotropic Flow Behavior of Commercially Pure Titanium

Plastic anisotropy is an important issue for metals possessing a hexagonal close-packed structure. This study investigated the anisotropic deformation characteristics of commercially pure titanium with basal texture. A quasi-static uniaxial compression gave rise to clear differences in flow curves and strain-hardening rates depending on the loading direction. This study employed a constitutive approach to quantify the contribution of (i) dynamic Hall–Petch strengthening, (ii) dislocation pile-up, and (iii) texture hardening with respect to the total flow stress. Such an approach calculated a flow stress comparable to the measured value, providing logical validity. The microstructural and mechanical differences depending on the loading direction (i.e., anisotropy) were successfully interpreted based on this approach.

Towards Self-Organized Anodization of Aluminum in Malic Acid Solutions—New Aspects of Anodization in the Organic Acid

In this work, aluminum (Al) anodization in malic acid electrolytes of different concentrations (0.15 M, 0.25 M, and 0.5 M) was studied. The close-packed hexagonal pore structure was obtained for the first time in this organic acid in a 0.5 M solution, at 250 V and temperature of 5 °C. Moreover, the process was investigated as a function of the number of cycles carried out in the same electrolyte. A repetition of anodization under seemingly the same external electrochemical parameters (applied voltage, temperature, etc.) induced serious changes in the electrolyte. The changes were reflected in the current density vs. time curves and were most evident in the higher concentrated electrolytes. This phenomenon was tentatively explained by a massive incorporation of malate anions into anodic alumina (AAO) framework. The impoverishment of the electrolyte of the malate anions changed internal electrochemical conditions making easier the attraction of the anions to the Al anode and thus the AAO formation. The electrolyte modification was advantageous in terms of pore organization: In a 0.25 M solution, already after the second anodization, the pore arrangement transformed from irregular towards regular, hexagonal close-packed structure. To the best of our knowledge, this is the first observation of this kind.

Microstructure Simulation and Constitutive Modelling of Magnetorheological Fluids Based on the Hexagonal Close-packed Structure

This paper presents a new constitutive model of high particles concentrated magnetorheological fluids (MRFs) that is based on the hexagonal close-packed structure, which can reflect the micro-structures of the particles under the magnetic field. Firstly, the particle dynamic simulations for the forces sustained by carbonyl iron powder (CIP) particles of MRFs are performed in order to investigate the particles chain-forming process at different time nodes. Subsequently, according to the force analyses, a hexagonal close-packed structure, which differs from the existing single-chain structure and body-cantered cubic structure, is adopted to formulate a constitutive model of MRFs with high concentration of the magnetic-responsive particles. Several experiments are performed while considering crucial factors that influence on the chain-forming mechanism and, hence, change the field-dependent shear yield stress in order to validate the proposed model. These factors include the magnetic induction intensity, volume fraction and radius of CIP particles, and surfactant coating thickness. It is shown that the proposed modeling approach can predict the field-dependent shear yield stress much better than the single-chain model. In addition, it is identified that the shear yield stress is increased as the particle volume fraction increases and surfactant coating thickness decreases. It is believed that the proposed constitutive model can be effectively used to estimate the field-dependent shear yield stress of MRFs with a high concentration of iron particles.