查看更多>>摘要：? 2022 Elsevier Inc.To enrich the database of HECC oxidation and to assess the potential of the hard-and-tough (NbTaZrW)C for high temperature application in oxygen-containing environments, in the current study, the oxidation resistance and behavior of this W-incorporated high-entropy carbide ceramic (HECC) was evaluated and investigated. To achieve this end, isothermal oxidation tests were conducted in a temperature range of 800–1000 °C. Natures of the oxidation products were systematically characterized. The results show that, the oxidation of (NbTaZrW)C follows the parabolic rule, suggesting the abeyance of a diffusion-controlled oxidation process at 800–1000 °C. Mechanistically, the oxidation of (NbTaZrW)C is dominated by the inward diffusion of O2 with no sign of outward diffusion of cation-site metallic elements. Varying with temperature, a few compositionally complex oxides, including (Nb0.5Ta0.5)2Zr6O17, ((Nb0.5Ta0.5)2O5)15(WO3)2, (Nb0.5Ta0.5)16W18O94, as well as (NbTaZrW)Ox with unknown structure. During oxidation, laminated structures with inter-laminate cracks tend to form due to the combined effects from residual stress and gaseous products. Our final comparison shows that, (NbTaZrW)C exhibits significantly lower oxidation resistance than a few other HECCs, such as (HfTaZrNb)C and (HfZrTaNbTi)C. The low oxidation resistance, as quantified by the extremely high kp values, of (NbTaZrW)C suggest its unsuitability for application in high temperature oxygen-containing environment.
查看更多>>摘要：? 2022 Elsevier Inc.In this study, TiCp/near-α titanium matrix composite underwent hot compression deformation to investigate the microstructure evolution and silicide precipitation behavior in the α + β phase region. The results show that the peak stress decreases with the increase of temperature or the decrease of strain rate, and the stress-strain curves show a similar trend with the increase of strain under the same other parameters. The silicides with larger size are mainly distributed at the α/β phase boundaries, and a small number of silicides with smaller size can be observed in α phases. In addition, with the increase of deformation temperature, the number and size of silicides first increase and then decrease. With the increase of strain rate, the number of silicides increases while the size first decreases and then increases. Moreover, with the increase of strain, the number of silicides first increases and then decreases while the size decreases. Meanwhile, 925 °C is more conducive to dynamic recrystallization than 885 °C during hot compression.
查看更多>>摘要：? 2022 Elsevier Inc.Oxide dispersion strengthened (ODS) steels containing yttria are candidate materials for fuel cladding tubes in future nuclear reactors. Their performance is determined by the size, distribution and volume fraction of the oxide dispersoids. The addition of yttria leads to a significant loss in ductility of steel limiting the yttria content in most ODS steels to 0.35%. This limitation may be overcome by powder forging of ODS steel powders. Two steels having nominal compositions (in wt%) of Fe-18Cr-2W-0.285Ti-0.5Y2O3 and Fe-18Cr-2W-0.571Ti-1Y2O3 were prepared by mechanical alloying of elemental powders with yttria powder in a Simoloyer attritor. The alloy powders were consolidated by powder forging at 1473 K in a flowing hydrogen atmosphere. The microstructure of powder forged ODS steels was characterized by electron microscopy. Both the steels exhibited a fine-grained recrystallized microstructure after forging. Fine (<10 nm) cuboidal oxide particles were found to be present in a ferrite matrix. A few coarse oxide particles were also observed. These coarse (>10 nm) oxide particles were present mainly along the ferrite grain boundaries. The crystal structure of oxide particles was determined by applying fast fourier transformation (FFT) on the fringes observed by high resolution transmission electron microscopy. The extracted FFTs were indexed to identify the crystal structure. The coarse particles were found to be Y2O3, TiO2 and Cr2O3. The fine particles were identified as Y2TiO5 and Y2Ti2O7. The face centre cubic Y2Ti2O7 oxide particles were semi coherent whereas the orthorhombic Y2TiO5 oxide particles were found to be incoherent with the ferrite matrix. The number fraction, average size and size distribution of these oxide particles was determined for both the alloys. Cr rich shells were observed around most of the particles which may prevent the coarsening of oxide particles at elevated temperatures. Reasons for this are discussed.
查看更多>>摘要：? 2022 Elsevier Inc.The present work evaluates the microstructure stability of modified 9Cr-1Mo steel welds when subjected to high temperature exposure (HTE) at 775 °C for different times (5, 100, 500, and 1000 h). Prior to HTE, a post weld heat treatment (PWHT) at 760 °C for 2 h was performed. Upon HTE, the individual weld zones viz., weld metal (WM), coarse-grain heat affected zone (CGHAZ) and fine-grain heat affected zone (FGHAZ) demonstrate distinct microstructural variations in terms of precipitate size, grain size and substructure (lath/block/packet boundaries) fractions. The least- and the highest-microstructure stability are respectively seen in FGHAZ and WM after HTE. A detailed investigation involving electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HR-TEM) confirm that a pronounced recovery of the substructure in FGHAZ happens due to enhanced diffusion along the higher fraction of prior-austenite grain boundaries (PAGBs), leading to M23C6 precipitate coarsening and substantial grain growth. The microstructure degradation upon HTE for 1000 h had eventually resulted in deterioration of mechanical properties, where the hardness reduced the most for FGHAZ (~34%), while the least for WM (~7%). Similarly, after HTE, the tensile strength and toughness had deteriorated by ~67% and ~57%, respectively, from the PWHT condition. Fractography further confirms the pronounced degradation after HTE as they result in failure from FGHAZ.
查看更多>>摘要：? 2022 Elsevier Inc.The crystallographic heterostructure of additive manufactured metals has complicated effects on mechanical properties due to the unique thermal history. Therefore, the texture configuration and deformation behavior of powder bed fusion-laser beam (PBF-LB) 18Ni300 maraging steel after different building orientations (BOs) were investigated by electron backscattered diffraction (EBSD), tensile tests and crystal plasticity finite element simulation. The results showed that the yield strength of 90° samples was 2013.8 MPa, higher than that of 0° (1940.5 MPa) and 45° (1847.1 MPa). The higher strength was attributed to the strong 〈110〉 ‖ BO textures on the 90° building samples. Meanwhile, the elongation was in the order of 3.5% (90° building) < 5.6% (45° building) < 7.6% (0° building) by strain rate of 2.5 × 10?4 s?1, which was attributed to the dynamic recovery process of PBF-LB 18Ni300 maraging steel in strain hardening stage I for different crystallographic orientations. The single crystallographic orientation couldn't coordinate the plastic deformation of the 90° building specimen, which resulted in fracturing under a small strain. In contrast, the diversified crystal textures contributed to the sufficient coordinated plastic deformation of the specimens fabricated along 0° building and thus increased the overall elongation to fracture.
查看更多>>摘要：? 2022 Elsevier Inc.In the present work, the friction stir processing (FSP) has been conducted on the extruded Al–6.0Mg–1.1Mn–0.19Er–0.1Zr alloy and subsequently the microstructure evolution and mechanical property were systematically investigated after FSP. The results showed that FSP is an effective approach to achieve remarkable refinement and homogeneity of microstructure. The coarse band shape grains are gradually replaced by the fine equiaxial grains with a mean diameter of 2.54 μm after fully dynamic recrystallization, which can be ascribed to severe plastic deformation and considerable heat input from the stir effect. Additionally, both the primary intermetallic compounds and the short rod-like secondary dispersoids are refined and evenly distributed in the matrix during FSP. The tensile tests demonstrated that the elongation of the FSPed alloy can be improved by 105% without sacrificing any strength compared with that of the base material (BM), proving that FSP is a good method for improving alloy with a desirable combination of strength and ductility after FSP. Compared with the BM, the grain boundary strengthening and the dispersion strengthening mainly contribute to the compensation for the decrease of dislocation strengthening in the FSPed alloy. Based on the observation of the fracture surfaces and the side view near the fracture, the ductilization can be attributed to the refinement of the primary intermetallic compounds and the rod-like secondary dispersoids by suppressing the initiation and propagation of microcracks during the tensile deformation.
查看更多>>摘要：? 2022 Elsevier Inc.Strain localization in commercially pure titanium under uniaxial tension is investigated with in situ digital image correlation (DIC) at different length scales. Speckles in three different sizes are prepared on the surface of titanium specimens, referred to as microscale, mesoscale and macroscale speckles. The microscale and mesoscale speckles for scanning electron microscopy DIC analysis are fabricated by different electro-polishing procedures, and spray painting is used to prepare the macroscale speckles for the optical DIC measurement. Electron back scattering diffraction (EBSD) characterization is conducted for the microscale speckle case. The strain fields obtained at different length scales combined with the microscale EBSD examination establish the relationship between microscale strain localization and macroscale strain concentration bands. Strain localization tends to form at grain boundaries and inside deformation twins, especially at the large-angle grain boundaries, and the amplitude and rate of strain accumulation at grain boundaries are higher than inside deformation twins. The microscale strain-localized regions dispersed at grain boundaries and within deformation twins present themselves as the strain concentration bands observed with the macroscale speckles, and both the strain localization regions and strain concentration bands are at ±45° with the tensile direction.
查看更多>>摘要：? 2022 Elsevier Inc.Compared with the traditional anodization of aged aluminum alloy, a novel process was carried out by anodic oxidation of solid solution aluminum alloy, followed by aging treatment to optimize the interface's composition and microstructure, under which the thickness, hardness and modulus of the oxidation film wouldn't be influenced and the strength of base materials kept the same. As β” precipitates cannot be formed in the subsequent aging process due to the consumption of Mg on the film-base interface, line defects caused by lattice distortion were avoided. Therefore, the stress concentration point in the fatigue process was reduced, and the fatigue property of the anodized material was improved from 89 MPa to 125 MPa. The anodizing of solid solution aluminum alloy helps the dispersion of Mg in the oxide film, which increases the self-corrosion potential of the oxide film from ?0.97 V to ?0.32 V and delays the occurrence of pitting corrosion during long-term immersion in 5 wt% NaCl solution.
查看更多>>摘要：? 2022 Elsevier Inc.The occurrence of dynamic recrystallization in warm rolling of plain extra-low carbon steel was examined by applying 75% thickness reduction in a single pass. The investigated temperature range was ambient to 700 °C. Results show apparently dynamic recrystallization remains absent in symmetric rolling below 600 °C. Nevertheless, dynamic recrystallization not only just occurs but accelerates as the condition of warm rolling changed from symmetric to asymmetric condition. The analysis of grain internal disorientations and geometrically necessary dislocation densities by High-angular Resolution on-axis Transmission Kikuchi Diffraction (HR-TKD) allowed to identify the type of dynamic recrystallization and its evolution through the warm temperature range. New grains formed by either continuous or discontinuous dynamic recrystallization are preferentially orientated according a typical shear texture up to 600 °C in the case of asymmetric rolling. Above 600 °C, dynamically recrystallized grains produce restrengthening in the rolling texture as well as in the overall texture.
查看更多>>摘要：? 2022 Elsevier Inc.The present work describes application of a method for site-specific plan-view sample preparation for atomic column resolution scanning transmission and transmission electron microscopy (STEM and TEM) from free-standing thin films of multi-phase Al-alloys after laser irradiation induced rapid solidification (RS). The electron-transparent multilayer thin film samples (amorphous Si3N4 substrate plus metal alloy layer) had a total initial thickness of ≈ 130–200 nm. At such large total sample thickness frequently multiple grains of the same or different phases overlap along the electron beam path in the RS microstructures of the alloys. Consequently, accurate atomic resolution images in TEM and STEM mode, and composition sensitive analytical data of the metastable microstructural features presenting in the RS microstructures cannot be obtained with confidence. Using low-energy concentrated Ar ion beam milling, locally thinned regions with thickness of 20–30 nm with large fields of view ≥ (30 μm)2 suitable for high-resolution TEM/STEM studies have been created with micron-scale site-specificity. As example application, atomic scale resolution TEM/STEM imaging has been performed of the banded grain regions of the RS microstructure established in multi-phase Al–Cu alloy thin films. The sample preparation strategy described here appears to be readily applicable to freestanding thin film specimens in general.
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