查看更多>>摘要：? 2021 The AuthorsA new methodology has been developed to study the fine details of GP zones in age-hardenable aluminium alloys. It is complementary to atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, and combines scanning precession electron diffraction with diffraction simulations. To evaluate the method, data was collected from an Al-Zn-Mg alloy in a condition with a dense distribution of GPI zones. Diffraction patterns were recorded in the 〈001〉Al orientation, capturing GPI zones in three projections: along the unique [001]GPI axis, and along the two other mutually orthogonal orientations. The GPI zones viewed along [001]GPI revealed how the truncated octahedron units of the GPI zones were connected in multi-unit GP zones, while the two orientations normal to [001]GPI highlight the internal structure. The stability of the atomic models developed based on the experimental results was verified by density functional theory calculations.
查看更多>>摘要：? 2021The study of the influence of phase transformations on residual stresses in heterophase materials is very important for predicting the service life of machines and constructions. Welded joints made of dissimilar materials are of particular interest. The chemical and phase compositions, microstructure, microhardness, strength, and fracture surface characterization of dissimilar AISI 321/Cu/Ti laser joints have been studied. Distinctive features of the laser welding conditions are the displacement of the focal spot to the steel/Cu interface and the deepening of the focus by 3 mm. The chosen mode of laser welding made it possible to obtain the following chemical composition of the melting zone (MZ): 55.3 wt% Cu, 32.2 wt% Fe, 8.5 wt% Cr, 4.0 wt% Ni, and 1.0 wt% Ti. After crystallization in MZ, two supersaturated solid solutions are formed: one based on copper and the other based on iron. During post-welding cooling, (Fe,Cr)2Ti intermetallic particles sized 10 nm homogeneously precipitate in the Cu-based solid solution, and 10-nm TiCu4 particles precipitate in the Fe-based solid solution. Fe-based regions become additionally hardened (to 540 HV 0.025) due to the formation of martensite crystals in the austenite matrix. Alloying of titanium with copper causes βTi stabilization at the interface Ti/MZ. During contact melting of titanium, Ti2Cu intermetallic particles of three types emerged: from 2 to 5 μm eutectic, from 0.1 to 0.5 μm eutectoid, and those homogeneously precipitated in βTi during cooling, sized 10 nm. After quickly cooling, the compressive residual stresses formed in the welded joint. When specimens with welded joints were tensile tested, they fractured in the MZ according to a mixed type. Ductile fracture occurred in Cu- and Fe-based microconstituents by the MZ zone; brittle fracture occurred in the zone with (βTi + Ti2Cu) structure near Ti. The ultimate tensile strength of the resulting welded joints is 470–515 MPa, and this significantly exceeds the available data.
查看更多>>摘要：? 2022 Elsevier Inc.The effect of hot rolling on the evolution of Fe phase and the properties of Cu-6.5Fe-0.3 Mg alloy is explored. The results show that the optimal performance (i.e. ultimate tensile strength (818 MPa), electrical conductivity (61.7%IACS), and electromagnetic shielding (85–100 dB)) was obtained by hot rolling + cold rolling + heat treatment. It was found that the tadpole-like Fe phase in the Cu-6.5Fe-0.3 Mg alloy after primary hot rolling was smaller and denser than those after primary cold rolling, which is attributed to the hot grooving and splitting behavior of the Fe phase during hot rolling. The depth of thermal slotting of the Fe phase increases with the increasing of the heat treatment temperature. After large plastic deformation, the tadpole-like Fe phases transforms into Fe fibers and its density is higher. The strengthening method of Cu-6.5Fe-0.3 Mg alloy is fiber reinforcement and its electromagnetic shielding performance increases with the density of Fe fibers. The splitting of the Fe phase during hot rolling significantly increases the density of Fe fiber, thereby improving its tensile strength and electromagnetic shielding performance. This work will provide a new guideline for the preparation of high-performance Cu-Fe-Mg alloys.
查看更多>>摘要：? 2021In this study, NiAl bronze (NAB) alloys are processed using an ultrasonic vibration-assisted milling (UVAM) surface engineering technique. The strengthening mechanisms at different cutting speeds are analysed through well-established models and experiments. The increase in cutting speed produced decrements of 29.5%, 26.6%, and 5.6% in the friction torques, accumulated dissipated energy, and worn surface area, respectively. The microhardness gradually increased by 11%, which helped improve the fretting wear performance. Grain refinement, precipitation, and dislocation strengthening were the main mechanisms. Grain refinement (?σHP) had the greatest contribution to performance, followed by precipitation (?σor) and dislocation (?σd). Nanotwin strengthening was observed at a cutting speed of 150 m/min. Martensite nanotwins may nucleate at the grain boundary and grow toward the grain interior.
查看更多>>摘要：? 2022 Elsevier Inc.Medium-entropy alloys (MEAs) have attracted considerable research attention because of their excellent mechanical properties. In these, the γ’ phase has been utilized as a strengthening phase. However, the η phase has not been widely investigated. Herein, we explain the precipitation behavior of the η phase in a non-equimolar CoCrNiTi MEA. The MEA adopts a face-centered cubic (FCC) matrix upon Ni3Ti η phase precipitation. Various characterization techniques were used to analyze the microstructure, phase structure, and phase composition of the as-prepared MEA. The grain size and volume fraction of the η phase were ~ 70 nm and ~ 35%, respectively. The FCC-η interface exhibited semi-coherence, adopting an interplanar angle of 23°. Upon precipitation strengthening, the MEA exhibited a yield strength of ~1.7 GPa, an ultimate tensile strength of ~2 GPa, and an elongation of ~2.87%. This work facilitates a novel approach to ultrahigh-strength alloy production.
查看更多>>摘要：? 2022 Elsevier Inc.Microstructure and behavior of parts fabricated via additive manufacturing (AM) usually differ from their conventionally manufactured counterparts. In an effort to further study such differences, this paper is concerned with comparing weldability characteristics of AM and wrought materials, for integration of replacement AM parts into legacy systems. A Gleeble 1500 was used to simulate the heat affected zone (HAZ) generated during fusion welding. Weldability of the as-built AM samples of 17-4 stainless steel (SS) was compared with that of wrought (Condition A) 17-4 SS samples. Furthermore, the AM samples were manufactured from 17-4 SS powders atomized in nitrogen gas and argon gas to evaluate whether differences in the initial powder feedstock would affect weldability. X-ray diffraction, optical microscopy, and electron backscattered diffraction (EBSD) were employed to characterize microstructures. Results show that both the wrought and the AM samples made with argon atomized powder after HAZ simulations contain large concentrations of undesirable delta-ferrite phase, which is known to deteriorate the toughness of the material. In contrast, the AM samples made with nitrogen atomized powder contain no delta-ferrite. The absence of delta-ferrite is a consequence of the additional nitrogen picked up during the atomization process. Nitrogen promotes the growth of austenite and suppresses delta-ferrite at high temperatures. These findings reveal that the nitrogen atomized 17-4 SS powder may be a better choice than the argon atomized 17–4 SS powder if the part is intended to be installed or repaired via welding.
查看更多>>摘要：? 2022 Elsevier Inc.It is a long-term requirement to reduce the arc ablation rate and the W content of W–Cu composites. In this work, laser surface alloying was employed to prepare a novel W–Cu/Cu composite that contained a high W content on the surface but a low W content for the whole composite. The new W–Cu electrode exhibited a lower arc ablation rate than the commercial one. The underlying mechanism was attributed to the higher thermal conductivity of the new W–Cu/Cu composite with the low W consumption as a whole. This study opens a door to overcome the trade-off of the ablation resistance and W consumption of the W–Cu electrodes.
查看更多>>摘要：? 2021Co-Cr alloy fabricated by selective laser melting was used to investigate the recrystallization behavior and grain boundary character evolution from As-SLM to isothermal annealed at 1200 °C for 5 min to 60 min. The As-SLM Co-Cr alloy has a prominent residual strain, high densities of dislocations and stacking faults, and solute segregation along the subgrain boundaries. The driving force for recrystallization of As-SLM Co-Cr alloy is mainly provided by phase transformation rather than crystal defects, which also offer plenty of routeways for both low- and high-angle grain boundary diffusion. In addition, the stacking faults in As-SLM Co-Cr alloy can generate morphologically complex twins by extending and gliding. The obtained optimal Co-Cr alloy has a large fraction (more than 80%) of special boundaries (Σ3, Σ9, and Σ27) after complete recrystallization, which enables high designing freedom and near-net-shape production towards additive manufacturing (AM) compatible grain boundary engineering (GBE) processing strategy.
查看更多>>摘要：? 2022 Elsevier Inc.In this study, CoCrFeNiAlx (x = 0.1, 0.5, 1) high-entropy alloys (HEAs) are prepared using vacuum arc melting. In order to improve the surface properties of CoCrFeNiAlx HEAs, their microstructure is manipulated by laser surface remelting (LSM). The microstructure, the mechanical properties, and the corrosion resistance of the HEAs before and after LSM are studied systematically. The microstructural analysis reveals that the LSM-samples contained either a single FCC phase (CoCrFeNiAl0.1 and CoCrFeNiAl0.5) or a mixture of BCC + B2 phases (CoCrFeNiAl1) with refined grains. Moreover, BCC precipitates and a spherical nano-dispersed phase (B2) are detected in the surface layer of CoCrFeNiAl0.5 and CoCrFeNiAl1, respectively. The mechanical properties of CoCrFeNiAlx alloys are found to be significantly enhanced after LSM, which is mainly ascribed to the generation of the BCC phase, dislocation reinforcement and grain-strengthening as well as precipitation hardening. The electrochemical measurements confirm that all samples (after LSM) have a positive potential and a lower current density in 3.5 wt% NaCl solution compared to the untreated sample. The results indicate that LSM can help homogenize the structure and improve the properties of CoCrFeNiAlx HEAs.
查看更多>>摘要：? 2022 The AuthorsThis study presents the advances in the field of ZnO/Ag catalysts from the synthesis of hierarchical ZnO nanowires (NWs) decorated with Ag nanoparticles, prepared by a facile solvothermal method at 120°C. It evaluates the photocatalytic efficiency from studying the time reaction of Ag/Zn concentration ratio and the presence of cetyltrimethylammonium bromide (CTAB) as an organic dispersant. X-ray diffraction, scanning electron microscopy, and analytical/high-resolution transmission electron microscopy results confirmed the presence of homogeneous cylindrical ZnO nanowires and quasi-spherical Ag crystals. ZnO NWs exhibited hexagonal wurtzite structure and cubic FCC symmetry in Ag nanoparticles (NPS). Two types of nanostructures, including homogeneous cylindrical ZnO NWs in the absence of Ag and simultaneous presence of ZnO NWs and Ag NPs, formed depending on experimental conditions. The photocatalytic activity was evaluated by studying methylene blue (MB) degradation time under UV light excitation. Diffuse reflectance UV–Vis spectrophotometry (UV–Vis DRS) allowed identifying the ZnO absorption band at ~393 nm. Crystal size varied depending on the reaction time and the addition of CTAB. Synthesis time increased bandgap values, getting better photocatalytic performance in samples synthesized in intermediate times (6 h), higher Ag+/Zn2+ molar ratio (0.2/1.0), and CTAB. According to HRTEM observations, the presence of silver nanocrystals with high content of defects (twinning, stacking faults) could play an essential role in the photocatalytic response. In this context, the specific synthesis conditions of Ag/ZnO might be more appropriate for their use in organic dyes degradation in water and the potential use in protective treatments against materials biodeterioration processes.
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