Acta Materialia最新文献

Magnesium orthovanadate Mg₃V₂O₈ provides an excellent model for studying the transport of magnesium cations in a three-dimensional matrix with two types of Mg(1)O₆ and Mg(2)O₆ octahedra connected by edges and chains of VO₄ tetrahedra isolated from each other. In the present paper, the magnesium cations have been found to be the ionic charge carriers in this system using the Tubandt method. The electrical conductivity (σ) has been studied by the impedance spectroscopy in the range 770–1270 K. The activation energy of σ (E_σ) has been demonstrated to be 1.25 eV in the range 923–1023 K. For the first time, the ⁵¹V NMR spectra have been obtained and analysed in the range 295‒900 K. The activation energy (E_NMR) for the diffusive jumps of Mg²⁺ cations has been identified to be 1.03 eV by analyzing the temperature dependence of the ⁵¹V spin-lattice relaxation. The smaller E_NMR value is due to the rapid movement of magnesium along the chains of Mg(2)O₆ octahedra. Large E_σ value and low ionic conductivity indicate that the limiting step in Mg₃V₂O₈ is the cation hopping between the chains of Mg(2)O₆ octahedra through the intermediate Mg(1) positions.

An emerging motor type, the variable flux permanent magnet (VFPM) motor, has created a demand for permanent magnets with a new set of properties. These magnets must exhibit easily tunable magnetization, demanding for moderate coercivity values, low magnetizing fields, and first-order reversal curves (FORCs) with high flatness. In this work, we report that the doping of Sm and La in Nd-Fe-B-based hot-deformed magnet, followed by the Nd-Cu diffusion process, resulted in desirable magnetic properties, such as, a moderate coercivity (µ₀H_c) of 0.55 T, a low magnetizing field (µ₀H_mag) of 0.94 T and a high FORC flatness of 0.96, while maintaining a high remanence (µ₀M_r) of 1.3 T. The FORC flatness achieved in this work is the highest and the comprehensive properties are superior to the previously reported Nd-Fe-B magnets for VFPM motors. Microscopic investigations revealed that the high flatness achieved in this hot-deformed magnet is attributed to the formation of Fe-lean thick intergranular phases realized by Nd-Cu grain boundary diffusion. The overall combination of the magnetic properties for the diffusion processed (Nd,Sm,La)-Fe-B magnet is excellent, in comparison with a commercially available Sm₂Co₁₇-type magnet, showing the promise of the former for use in VFPM motors.

It is well known that long time natural aging (NA) after quenching from solution treatment will significantly reduce the precipitation hardening kinetics and peak hardness of most 6xxx aluminum alloys during later artificial aging (AA). Here we demonstrate an effective strategy to accelerate precipitation hardening, taking advantage of NA. It is found that by a short time pre-aging (PA) at AA temperature, NA for up to 1 year can reduce the time to peak strength in a 6082 alloy during later AA. A simultaneous increase in yield strength and uniform elongation at peak-aged condition can be achieved as a result of finer and denser age-hardening precipitates than those formed by direct AA treatment. Quantitative characterization of the precipitate microstructure by annular dark field scanning transmission electron microscopy (ADF-STEM) and atom probe tomography (APT) reveals that PA generates a small fraction of fine β″ needle precipitates composed of 6–9 β″-eyes and a substantially high density of GP-zones composed of 3–5 β″-eyes, which are stable at room temperature and can grow easily into β″ upon AA. During NA after PA, more GP-zones with at least 2 β″-eyes form while the larger GP-zones inherited from PA grow further, both of which can act as the precursors of β″ precipitates during later AA.

Recent research has found some intermetallic compound particles with even stronger hydrogen trapping capacity (e.g., Al₇Cu₂Fe) than the age-hardening precipitates that are reported to be the origin of hydrogen embrittlement in aluminium. Such intermetallic compound particles can reduce hydrogen concentration at the interface between the precipitates and aluminium by absorbing hydrogen in their interiors, thus preventing the hydrogen embrittlement of aluminium. However, this cannot be achieved if the particles, which have absorbed large amounts of hydrogen, are damaged due to hydrogen embrittlement. In this study, the hydrogen embrittlement of aluminium was observed in situ by X-ray CT, and the damage behaviour was analysed of all the particles that were located in the gauge section of a single tensile specimen. After exhaustive quantification of the size, shape, and spatial distribution of the particles, coarsening processes identified highly correlated design variables. Subsequently, particle damage behaviour was analysed utilizing a surrogate model using a support vector machine. The damage to Al₇Cu₂Fe particles could be described only by design variables representing size and shape, while those representing spatial distribution were removed through the coarsening processes. No change was observed in the damage behaviour of Al₇Cu₂Fe particles with increasing hydrogen concentrations, and it was concluded that the dispersion of Al₇Cu₂Fe particles is effective in preventing hydrogen embrittlement of aluminium. The contribution of damaged particles to the formation of fracture surfaces and the damage behaviour of Mg₂Si particles, where damage is accelerated by hydrogen, were also analysed.

Heterogeneous alloy designs have come to the forefront of material science due to their potential in achieving a superior combination of strength and ductility. To harness this potential, we proposed a structural strategy for the fabrication of a novel heterogeneous multi-gradient α-TiAl alloy through in-situ modulation of aluminium concentration during the additive manufacturing process. Compared with homogeneous Ti (with yield strength (σ_y) of 440 MPa and elongation to fracture (ε_f) of 37.6 %) and homogeneous Ti-10Al [at%] (σ_y ∼910 MPa, ε_f ∼6.1 %) fabricated using the same methodology, this heterogeneous multi-gradient α-TiAl alloy achieved a significant improvement in yield strength (σ_y ∼760 MPa) but with only a minor reduction in ductility (ε_f ∼33.4 %). Comprehensive experimental characterizations were carried out to probe the underlying mechanisms. The findings elucidate that the diffusion of aluminium in different printed layers promoted the formation of an innovative heterogeneous multi-gradient structure, engendering a synergy of multi-gradient strains that contribute to an exceptional combination of strength and ductility. These findings not only furnish an efficacious avenue for substantially augmenting the mechanical properties of α-Ti alloys but also applicable broadly in other alloy systems. The novel implementation of heterostructrure design could potentially overcome the enduring challenge of reconciling the trade-off between strength and ductility.

Skyrmion, a local bubble-like topological magnetization structure, can collectively emergent in magnets in a lattice form skyrmion crystal (SkX). SkX has great application potential in functional devices because it can manipulate material properties via coupling with atomic lattices. The lattice defects such as vacancy widely exist in the SkX as well, and they have rich dynamic behaviors and have great implications for the host material. However, although the nature of ideal SkX is well studied, the characteristics of SkX defects are relatively underdeveloped. Here, we deeply studied the structural properties of a vacancy defect in the SkX by a thermodynamic phase-field simulation. We found that the higher external magnetic and temperature fields favor rigid skyrmions (crystal), in which the SkX vacancy is less deformed, while the lower fields favor softer skyrmions where the SkX vacancy structure is considerably deformed. Such unique deformation and stability of the SkX vacancy are mainly the results of the competition between free energies in the view of thermodynamics. Our study demonstrated that the external field-controlled static properties of SkX vacancy highly depend on the quasiparticle nature of skyrmions. This indicates the properties of SkX defects can be controlled by the SkX features under different fields, which should open an avenue for the study and design of smart materials and advanced devices by engineering skyrmion crystals.

Crystalline defects, such as dislocations, disclinations, twins and grain boundaries, play critical roles in determining the mechanical properties of metals and alloys. In particular, with multiple competitive deformation modes activated, the mechanical behaviors of hexagonal close-packed metals are strongly influenced by the interactions and reactions of various types of defects. Despite extensive studies on the elastic interactions of defects, a theoretical framework capturing crystallographic reactions, especially reaction products and associated local stress concentration, is still unavailable. Here we suggest a disclination-based method to quantify defect reactions. By using a combination of crystallographic calculations and phase field modeling/simulations, twin-twin and twin-grain boundary reactions in hexagonal close-packed metals have been quantitatively analyzed. It has been found that partial disclinations, accompanied with other defects (e.g., $\left\{11\overline{2}6\right\}$ and $\left\{11\overline{2}2\right\}$ high-index twins), can be generated by defect reactions as typical byproducts. The orientation change and stress fields caused by disclination formation have been systematically calculated, which offers a rigorous mathematical foundation to explore twin-twin, twin-grain boundary reactions. By quantitatively determining defect reactions and local stress fields, our work provides new insights into the deformation mechanism and microstructure-property relationship in metallic materials.

Modifying the energy band structure in heterostructured photocatalysts enhances charge separation efficiency and improves photoelectrochemical (PEC) performance by decreasing the charge transfer barrier. In this study, cobalt doping into WO₃/TiO₂ core/shell heterojunction nanorod arrays introduces versatile valence states of cobalt altering the oxygen coordination environment around W atoms in WO₃, resulting in an increase in W⁵⁺ ions and oxygen vacancy defects in WO₃ lattice, facilitating the water splitting reaction. Photogenerated electrons transfer easily from the WO₃:Co shell to the TiO₂ core due to the lower conduction band minimum (CBM) of WO₃:Co shell. Moreover, photogenerated holes transfer from the TiO₂ core to the WO₃:Co shell efficiently due to the higher valence band maximum (VBM) of TiO₂ core. The heterostructure has a high photogenerated carrier density (9.89 × 10¹⁸ cm^-3), improving photoconversion efficiency (2.55 mA cm^-2 at 1.23 V vs. RHE) and reducing charge recombination rates (6.51 × 10^–4 s^-1). Co doping increases the -OH bond on WO₃/TiO₂ surface, improves its hydrophilicity, and is more conducive to the reaction in aqueous electrolyte. Additionally, the nanorod array structure facilitates PEC reaction kinetics by providing open spaces for mass exchange. This work proposes a feasible strategy for improving photogenerated charge transport and enhancing PEC by combining regulation of the band structure of WO₃/TiO₂ heterostructures with morphology design.