查看更多>>摘要：The extraction replica technique is a useful sample preparation method to fabricate transmission electron microscopy samples for subsequent studies of small particles, such as precipitates, in different types of alloys. With other sample preparation methods, such as thin foil fabrication, the surrounding matrix might infuence compositional or crystallographic analysis in the TEM. The extraction replica technique omits that problem, as the precipitates are extracted onto a thin, amorphous flm (most commonly carbon) and the infuence of the matrix is thus eliminated. The replica can be produced in a direct manner, by a two stage-method or in certain cases by the oxide replica method. A variety of precipitates from common engineering alloys including carbides, nitrides, oxides and various intermetallic phases in steels, nickel-, zirconium-, aluminium-, copper-, and magnesium-based alloys, can be extracted and the techniques are summarized in this work. The choice of etchant is crucial for the end result, as is also showcased experimentally in this work. Furthermore, the direct replica and two-stage replica method are directly compared with each other by new experiments on a low alloyed steel and a zirconium alloy. There are indications that the direct method has a greater particle extraction effciency of smaller particles compared to the two-stage method. Moreover, the direct method also facilitates the analysis of other features, such as grain boundaries, in special cases.
查看更多>>摘要：Duplex stainless steels (DSS) are a family of stainless steel alloys that beneft from the presence of two relatively equally proportioned phases, ferrite and austenite. The alloys are designed to have an enhanced resistance to corrosion and superior strength properties in comparison to more common stainless steel alloys such as 316 L. Design engineers are now exploring the introduction of additively manufactured (AM) DSS into industrial components, to beneft from these enhanced capabilities provided by the alloy and the greater fexibility in design offered by AM. This research focuses on the mechanical and microstructural characterisation of DSS 2205, manufactured by the AM process laser powder bed fusion (LPBF). Results have been generated through both uniaxial tensile testing and small punch (SP) testing on as built and heat-treated conditions, across a range of temperatures up to 750 ℃. Microstructural assessments have been conducted using advanced microscopy to determine relevant phase distributions and texture morphologies present in the materials, to understand how this infuences mechanical performance.
查看更多>>摘要：? 2022 Elsevier Inc.In this work, the hollow porous cauliflower shaped cobalt spheres/graphite nanosheets (hp-Co/GNs) composites with brilliant absorbing performance have been successfully fabricated through a simple solvothermal method and a subsequent hydrothermal reduction process. When the filling amount of absorbing agent is 30 wt%, the composite exhibits excellent absorbing properties in the range of 4–7.92 GHz in S band and 16.24–18 GHz in Ku band. The optimum reflection loss value (RLmin) can be up to ?52.1 dB at a thickness of 4.5 mm. By changing the time of the second step hydrothermal reduction reaction, the morphology of the composite changes and its microwave absorption properties are regulated. Its excellent microwave absorption performance benefits from the special hollow porous cauliflower shaped structure and the synergism of magnetic loss and dielectric loss. This study demonstrated that cauliflower shaped hp-Co/GNs composites can be applied as a better electromagnetic wave absorbing material.
查看更多>>摘要：? 2021One of the most important goals in the development of Cu-based alloys for electrical engineering applications is the realization of materials with both high mechanical strength and electrical conductivity. Cu-Ti alloys are promising alternatives to Cu-Be alloys, which are expensive and can be hazardous to health. Herein, an age-hardenable binary Cu96Ti4 alloy and its Mg-doped analog were compared by advanced electron microscopy techniques. The mechanical properties of the Mg-doped alloy are superior to those of the binary alloy. The effects of Mg-doping on the microstructure are investigated by atomic-scale scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. The Cu96Ti4 (at.%) solid-solution alloy developed lamellar aggregates when subjected to prolonged aging treatment at 450 °C for 100 h owing to discontinuous precipitation (DP) along the grain boundaries. The DP consisted of a face-centered cubic (FCC) Cu solid-solution phase (denoted as α-Cuss) and a stable Cu4Ti intermetallic phase (β-Cu4Ti). Conversely, while the Cu94Ti4Mg2 counterpart had similar lamellar aggregates, the amount of these aggregates was reduced, and no β-Cu4Ti phase was observed. The lamellar in the Cu94Ti4Mg2 counterpart consisted of two distinct FCC Cuss phases with different Ti-solute contents. Accordingly, the possible effects of Mg doping on the local structures of the DP in the Cu-Ti alloy are explained, and the structural characteristics of the DP that may result in improved mechanical properties is discussed.
查看更多>>摘要：? 2022 Elsevier Inc.Generally, in metallic alloys, attaining ultra-fine sub-micron grain sizes by recrystallisation requires severe plastic deformation (SPD), however SPD techniques are difficult to apply to large quantities of material. In this work, we explore a strategy to reach an ultra-fine grained microstructure in a metastable-β titanium Ti-20Nb-6Zr (at.%) by controlled recrystallisation after conventional rolling and annealing. The thermally stable sub-micronic dual phase microstructures are associated with the formation of strain-induced martensite during rolling and its subsequent reversion during annealing. Some specimens were cold-rolled to change most of the β microstructure to stress-induced martensite (SIM); others were “warm rolled” just above the critical temperature for formation of SIM (453 K). The role of SIM in grain refinement was isolated by comparing the microstructural evolution of the two sets of specimens during annealing treatments. The cold rolled specimens produced an effective ultrafine grain recrystallisation, which was identified to start from 723 K, and a final equiaxed alpha-beta microstructure of ~250 nm. In warm rolled specimens without SIM, no new beta grains were observed, only an array of intragranular acicular alpha precipitates of approximately 50 nm thickness.
查看更多>>摘要：? 2022 Elsevier Inc.To develop new titanium alloys for laser-based direct energy deposition, Ti-Zr-V-Nb alloys were designed from a basic Ti60.94Zr39.06 congruent composition alloyed with constant V content and varying Nb contents. The microstructure and properties of the alloys, fabricated by laser-based direct energy deposition, were systematically studied. The results show that all as-deposited alloys consist of near-equiaxed β grains with multiple orientations. The difference is that the lattice parameter of the phase enlarges and its grain size reduces with increasing Nb content, which leads to a continuous increase in hardness, strength, wear and corrosion resistance. But the ductility shows a reverse variation due to the combination of enhanced solid solution strengthening and dislocation strengthening. Owing to the combined effect of fluidity and spreadability of melt, the surface roughness firstly decreases and then increases with the increase of Nb content. The optimized alloy with 3.91 at.% Nb exhibits a good match of mechanical, tribological, and chemical properties compared with Ti60.94Zr39.06 congruent alloy, being a promising candidate used for laser-based direct energy deposition.
查看更多>>摘要：? 2022 Elsevier Inc.This study developed a series of Alx(TiZrHfNb)100-x (x = 0, 3, 5, 7, 12 at. %) refractory high-entropy alloys (RHEAs) with an aim to improve the poor oxidation and wear resistance at elevated temperature of the base alloy TiZrHfNb. These alloys were prepared by arc melting, and the mechanical properties, oxidation and wear resistance at elevated temperatures were studied systematically. The results indicate that all the alloys are single solid solution phase with body-centered cubic (BCC) structure. The addition of Al significantly improves the strength of the RHEAs due to the large solid solution strengthening effect, and the yield strength and Al content obey the liner relation. The oxidation and wear test results indicate that the Al element will preferentially react with O atoms and accumulate on the surface to form a dense oxide layer, which prevents other elements reacting with O atoms and makes the alloy have better oxidation resistance at high temperatures, especially in alloys with 12% Al. The friction coefficient and wear rate of the RHEAs dramatically decrease with the increase of Al, which can be attributed to the combined effect of mechanical properties and oxidation resistance. This study provides an effective method for improving the oxidation resistance and wear resistance of RHEAs in a relatively high temperature range.
查看更多>>摘要：? 2022Effect of scanning strategy (CB, R90, R67) on the microstructure and corrosion behaviors of Ti35Nb2Ta3Zr0.3O (wt%, TNTZO) prepared by selective laser melting (SLM) in 3.5 wt% NaCl solution were systematically investigated. The results of electrochemical tests indicated that the corrosion resistance of vertical planes was better than that of horizontal planes for all three scanning strategies, particularly sample R67 and R90, of which sample R67 performing the highest corrosion resistance. The dependence of corrosion resistance on the scanning strategy only exhibits for the vertical planes. The various electrochemical properties of horizontal and vertical planes are attributed to the density of crystallographic texture more than the grain boundary for the as-built TNTZO alloy.
查看更多>>摘要：? 2022 The AuthorsThe paper is devoted to the investigation of the structure and properties of coatings obtained by micro-arc oxidation (MAO). For the first time, the possibility of using sub-terahertz ellipsometry with an operating frequency f = 0.14 THz for non-destructive non-contact determination of the integral values of the thickness, structure, and composition of such coatings is studied. The frequency was chosen since the corresponding wavelength (λ = 2.14 mm) significantly exceeds the characteristic size of the roughness of the coatings, which makes it possible to use the geometrical optics approximation in the analysis of experimental results. When constructing models of the structure of coatings and determining their parameters, additional independent methods were used, such as optical microscopy, XRD, SEM, terahertz reflectometry, and the theoretical modeling of the electric field distribution during the microplasma oxidation process. The A1 alloy 1100 was used as the material for research. It is shown that ellipsometry is very sensitive to processes on the surface during MAO even at the earliest stages of oxide layer formation. Based on the data of the ellipsometry experiment, several planar models of the surface structure were constructed, and their parameters were determined at different stages of oxidation. The constructed models and their parameters (thickness, phase composition, and optical constants) are in good agreement with the results of independent measurements and do not contradict the physical concepts of the processes occurring during MAO. For the first time, it was proposed to use piecewise models for interpreting ellipsometric measurements of film structures non-uniform over the surface. The proposed model adequately describes the structure of MAO coatings at all stages of the technological process. It makes it possible to use ellipsometry for non-destructive non-contact express control of the state of these coatings during production without resorting to additional more complex analytical methods (XRD, SEM, etc). When analyzing ellipsometric data, estimates for the optical constants of mullite (Al5SiO10?x) in the terahertz range were obtained for the first time. It is also shown that the main results of the work can be used in the analysis of MAO coatings on other metals.
查看更多>>摘要：? 2022 The AuthorsUltrathin metallic films are important functional materials for optical and microelectronic devices. Dedicated characterization with high spatial resolution and sufficient field of view is key to the understanding of the relation between microstructure and optical and electrical properties of such thin films. Here, we have applied on-axis transmission Kikuchi diffraction (TKD) and scanning precession electron diffraction (SPED) to study the microstructure of 10 nm thick polycrystalline gold films. The study compares the results obtained from the same specimen region by the two techniques and provides insights on the limits of each diffraction technique. We compare the physical spatial resolution of on-axis TKD and SPED and discuss challenges due to the larger probe size in scanning electron microscopy (SEM). Moreover, we present an improvement for the physical spatial resolution (PSR) of on-axis TKD through acquisition in immersion mode. We show how this method extends the capabilities of SEM-based microstructure characterization of ultrathin films and achieve PSR comparable to semi-automated SPED.
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