查看更多>>摘要：The effect of the hydrogen content of 0.48 wt% and 1.38 wt% on the refinement of the microstructure of Ti-6Al-4V alloy produced by wire arc additive manufacturing was studied, and the microstructure evolution of Ti-6Al-4V alloy after hydrogenation, heat treatment and dehydrogenation was revealed in this work. The result shows that the Ti-6Al-4V alloy prepared by wire arc additive manufacturing technology consist of coarse columnar β grains with the lamellar α cluster inside. After hydrogenation treatment, the α clusters inside the β grains become smaller, and hydrides were produced. Then α?, α?? and metastable phase βM were formed in the microstructure during the quenching from the temperature of Tp+ 10 °C. At last, the metastable phase and hydride were decomposed during the subsequent aging and dehydrogenation process, which significantly refines the α grains in the columnar β grains, forming a large area of a fine-grained microstructure. The shape and size of the original and hydrogen-removed grains was observed by electron backscatter diffraction (EBSD). The result shows that the average size of the original lamellar α grains has been refined from 5.78 μm to 0.84 μm, especially, there were no original α grains with size above 4 μm left. With the hydrogen content of 0.48 wt%, the morphology of α grains after hydrogen removal is fine needle-like, while, when the hydrogen content increases to 1.38 wt%, the morphology of α grains is equiaxed.
查看更多>>摘要：The selective laser melting additive manufacturing can realize near-net forming and produce complex high-precision parts. However, β-columnar crystals will be formed in building direction, which leads to the decrease of material reliability, causing stress concentration and accelerating material fracture. Here, we perform hydrogenation treatment, solution treatment, aging treatment, and finally dehydrogenation treatment on Ti-6Al-4V titanium alloy fabricated by selective laser melting. The results show that the β grains of hydrogenated samples are refined after subsequent heat treatment. After the solution treatment, α' and α'' martensite coexist in the microstructure and decomposed during the subsequent aging and dehydrogenation treatment, resulting in the refinement of the structure within the grains, but still retaining the grain morphology of the previous alloy. When compressed at room temperature，the compressive strain and ultimate compressive strength of hydrogenated samples after heat treatment increase with the increase of hydrogen concentration, and the yield strength decreases. After dehydrogenation, the yield strength and ultimate compressive strength are increase.
查看更多>>摘要：In this research, superhydrophilic-underwater superoleophobic NiO and nickel oxalate micro/nano structured thin films on the stainless steel (SS) meshes have been fabricated via chemical bath deposition (CBD) as a facile route. Taguchi L16 experimental design was used for optimization the effects of six important influential parameters affecting the surface wettability including molar ratio of alkaline to metallic precursor salt, alkaline agent type, NiO seed layer solution concentration, seed layer type, Polyvinylpyrrolidone (PVP) concentration and precursor type. Effect of the PVP concentration (as additive) on the oil contact angle was evaluated for the first time. The molar ratio of the alkaline to metallic salt and PVP concentration were the two most important factors having the highest impact on the underwater oil repellency behavior. The sample prepared under the optimum conditions exhibited underwater superoleophobicity with the oil contact angle (OCA) of 156.15 ± 7.5° and oil contact angle hysteresis (OCAH) of 4.3°. The X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FE-SEM), and roughness measurements were applied for characterization of the fabricated samples. Based on the FESEM analysis, the spherical nano- ZnO seeds have diameters of ~30–50 nm. In addition, the abrasion test was accomplished (12 cycles under 4.7kPa). The obtained results showed an excellent mechanical robustness. Moreover, an acceptable chemical durability (after 240 h) in acidic and alkaline environments with pH values of 4, 6 and 12 was observed.
查看更多>>摘要：To optimize the overall structure design of the aero-engine turbine disks, the study of pulsed current diffusion bonding on the second generation nickel-based single-crystal superalloy DD5 and the third generation powder superalloy FGH98 was performed, and high-quality joints were obtained after post-welding heat treatment(PWHT). The microstructure and element diffusion were analyzed by using SEM and EDS, and the microhardness and room / high temperature tensile strength were analyzed. It can be seen from the results that with the pulse current, the diffusion bonding produced a high-temperature gradient from the interface to the base metal, which protected the base metal. The precipitation phase of γ′ phase within the connection interface near the FGH98 side within 200 μm exhibits very small granular-large-size unstable petals-larger granular similar to the base metal. The γ′ phase in DD5 grows abnormally at the interface. Large granular γ′ enhancement phases occurred on the FGH98 side after heat treatment with no significant changes on the DD5 side. The microhardness of the joints is not less than 400HV; and from DD5 side to FGH98 side, the hardness decreases first and then increases. The average tensile strength of the joints at room temperature is 1078 MPa, which is 99% of the tensile strength of the base metal with low performance. The average tensile strength of joints at high temperature (650 °C) is 1089 MPa, which is equivalent to the strength of the base metal.
查看更多>>摘要：The nucleation mechanism of ω-assisted α phase and the selection of α variants during isothermal aging of Ti-15Mo alloy were studied. The results show that α phase preferentially nucleates at the ω/β interface. The β→ω→α phase transition is affected by the long-range diffusion of solute atoms, involving lattice reconstruction and Mo diffusion. Due to the adaptive regulation of α variant, (0001)α1,2,3 and (011?0)α1,2,3 variant were observed in the sample aged at 450 °C for 180 h. The orientation relationship between β matrix and α variants is as follows:<111>β∥<21?1?0>α, {110}β∥{0001}α.
查看更多>>摘要：In this work, damage/fracture development of Ti-6Al-4V alloy was analyzed with the help of in situ tensile test by synchrotron X-ray tomography. The result shows that during the whole damage/fracture procedure for Ti-6Al-4V alloy, the value of stress triaxiality (T) seems to maintain as a constant (T = 0.472–0.510) and to be adjacent with the critical value (Tc = 0.49). This can result in the fracture of the alloy in the way of Mix mode (component induced by cleavage component and ductile component). With the help of X-CT scanning, voids evolution were clearly realized and characterized during damage development. On the basis of void tracking results, Rice-Tracey damage model and Huang damage model were adapted to depict and predict the voids growing behavior by calibrating the material parameter. Finally, the micro-mechanism of the mix damage mode was revealed, which can be attributed to local internal shear stress. The significant act of microstructure morphology on the component from mix damage mode was also determined and discussed.
查看更多>>摘要：Cobalt-doped Li4Ti5O12 (LTO) i.e Li4Ti5?xCoxO12 (x = 0, 0.05, 0.1, 0.15, 0.2) powders have been synthesized by solid state reactions and examined for effects of Cobalt (Co) concentration on structural, optical, electrical and electrochemical properties of LTO. X-ray diffraction (XRD), Raman spectroscopy and field-emission scanning electron microscopy (FESEM) evaluate phase composition and morphology of samples. Optical study reveals reduction in bandgap Eg of LTO from 3.4 to 2.7 eV for x = 0.15, with Eg extension into visible region around 515–740 nm due to increased conduction electrons and energy levels from 3d7 orbitals of Co and induced oxygen vacancies. Electronic conductivity of sample with x = 0.15 increases by 104 as compared to LTO due to conversion of Ti4+ to Ti3+ ions and increased Ti4+-Vo-Ti3+ hopping centers. Ionic conductivity and diffusivity increase upto 2.0 × 10?7 Scm?1 and 4.6 × 10?12 cm2s?1 for x = 0.15, owing to increased lattice spacing by substitution of Ti4+ with Co2+. Frequency dependent conductivity suggests hopping of Li+ ions as dominant conduction in LTO. Thermally activated Li+ ions follow different conduction mechanisms in different temperature regimes. Low activation energies Ea of 0.3–0.5 eV indicate conduction of ions through interstitial pathways i.e 8a-16c-8a. High Ea of 0.7–1.15 eV suggest hopping of ions through vacancy/defect mediated channels or other long routes. Electrochemical tests demonstrate unexpected degradation in electrochemical performance for dilute dopant concentration; x = 0.05 and 0.1, as compared to LTO, which is attributed to substitution of Co2+ ions at Li+ tetrahedral (8a) sites. However, for x = 0.15 electrochemical activity gets better indicating substitution of more of Co2+ ions at Ti4+ octahedral sites.
查看更多>>摘要：A study on the microstructure and composition, micro-hardness and strain-rate-dependent compressive behaviors, and the associated failure mechanisms of an ultra-light-weight Mg-Li-Al alloy were conducted. X-ray diffraction and X-ray photoelectron spectroscopy showed a multi-phase material with ~35 wt% Li and ~20 wt% Al, and a dendritic “fishbone” microstructure resulted from the high percentage of both Li and Al. Micro-indentation measurements showed a superior hardness (1.63 ± 0.08 GPa) that is> 1.5x higher than other Mg-Li-Al alloys reported in the literature, with a low density (~1.68 g/cm3) comparable to Mg alloys. Strain-rate-dependent uniaxial compression experiments demonstrated no strain-rate-sensitivity in the peak strength (699.4 ± 74.0 MPa) at strain rates between 10?5 and 103 s?1. High-speed imaging revealed a shear-mode brittle fracture under both quasi-static and dynamic conditions, with an additional splitting crack mechanism observed under dynamic loading. Crack propagation speeds demonstrated a positive correlation with strain rate from ~480 m/s at ~100 s?1 to ~1000 m/s at ~2000 s?1. Post-mortem analysis showed that the “fishbone” structure with a peeling fracture mechanism appears to be the dominant site promoting shear failure across all strain rates.
查看更多>>摘要：In this work, the effects of doped nanoparticles (NPs) and nanocomposites (NCs) such as SnO2/TiO2 NCs, Ni-doped TiO2 NPs, Ag-doped TiO2 NPs and Ag-doped SnO2/TiO2 NCs as photoanode on the performance of Dye synthesized Solar Cells (DSSCs) were studied. The compounds were successfully synthesized using hydrothermal and sol-gel methods. XRD was used to examine the crystalline structures and the effects of dopants and nanocomposites on the crystal form of TiO2. Moreover, FE-SEM and TEM analysis showed that the produced NPs and NCs were porous and their shape, size and dispersion degree were different. Furthermore, by using FTIR spectra, the chemical species of the grown crystals were identified. Besides, variations band-gap were observed through the analysis of optical absorption spectra. In addition, EIS was used to analyze the effect of dopants and nanocomposites TiO2 on charge transfer resistance in the DSSCs. These studies indicated that the DSSCs photovoltaic properties largely depend on the compounds and structure of photoanodes. To this end, the current-voltage characteristics and Incident Photon-to-electron Conversion Efficiency (IPCE) measurements were used to evaluate the performance of DSSCs. As seen, while the conversion efficiency of the DSSC samples varies from 1.2 to ~6.93%, the DSSC-based on Ag-doped SnO2/TiO2 exhibit the highest conversion efficiency, i.e., 6.93%. It is owing to its lower charge transfer resistance, recombination and higher short-circuit photocurrent.
查看更多>>摘要：Creep experiments were performed on miniature K480 nickel-based superalloy specimens at 835 °C and 400 MPa. These specimens were cut from turbine blades processed with different casting processes and pouring temperatures. The microstructure analysis, fracture morphology analysis, and grain size analysis were carried out by scanning electron microscopy (SEM), indicating that the influence of the moulding process and pouring temperature on the microstructure of the alloy is mainly reflected in the difference in the size of crystal grains, grain boundary precipitates and casting defects, which in turn affects the high temperature creep properties of the alloy. Based on crystal plasticity theory, a polycrystalline alloy viscoplastic constitutive equation considering the moulding process and pouring temperature parameters is established, meanwhile, the geometric models of the K480 alloy were established based on the observations of its microscopic structure under SEM. The study revealed the relationship between the casting process of the blade, the microstructure of the material, and the creep property of the specimen. The established plastic constitutive model can predict blade stress rupture life under different casting processes.
NSTL
Elsevier