The microstructural evolution during long-term aging at 1000℃ and 1130℃ and the stress rupture property at 1100℃/140 MPa of a fourth-generation nickel-based single crystal superalloy with 6Re/3Ru were investigated. The microstructure and dislocation configuration of the alloy were observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) in terms of its microstructural stability and stress rupture property. The results showed that the alloy displayed excellent microstructural stability. During long-term aging at 1000℃, γ′ phase grew up gradually, but γ′ phase still remained high cubic degree, however, the TCP phases did not precipitate after aging for 1000 h. After aging at 1130℃ for 500 h, the γ′ phase connected each other and became rafting ed. The TCP phase started to precipitate after long-term aging at 1130℃ for 200 h, however, the content of TCP phase was only 0.04% after aging for 1000 h in the alloy. After aging at 1130℃ for 500 h, dislocation networks formed at γ/γ′ interfaces which became more regular and denser with the increase of aging time. The stress rupture life of the alloy at 1100℃/140 MPa was 676.5 h, which reached the level of the fourth-generation nickel-based single crystal superalloy. The γ′ phase underwent N-type rafting, the needle-like TCP phase (μ phase) precipitated, and dislocation pile-up was found near TCP phase after fracture. During stress rupture test, the formation of γ/γ′ interfacial dislocation networks and a<010> super-dislocations in the alloy was beneficial to the stress rupture property.
Composites 15%B4C/Al-6.5Zn-2.8Mg-1.7Cu with various size (7 μm, 14 μm, 20 μm) of reinforced paricales B4C were prepared by powder metallurgy vacuum hot pressing method. B4C particles can be uniformly distributed in the three composite materials, and the B4C-Al interface reaction is relatively slight, and no obvious interfacial reaction products are observed. In the matrix of the three composites, the size of the precipitates is basically the same, all of which are about 5.5 nm. When the B4C particle size is 7 μm, the composite has the best performance, i.e., yield strength of 648 MPa, tensile strength of 713 MPa and elongation of 3.3%. With the increase of particle size, the strength and elongation of the composites decreased. The strengthening mechanism and fracture mechanism of the three composites were analyzed, and the results showed that the composite reinforced with smaller B4C particles had higher strength. The particles are not easy to break during deformation, so they have better plasticity.
Gass-ceramics can be well used for making energy storage capacitors because of their unique structure of crystalline particles uniformly distributed in the glass matrix. Compared with conventional silicon glass-based glass-ceramics, phosphate glass-ceramics have significant advantage of less energy consumption. In this study, glass-ceramics of P2O5-Nb2O5-BaO-Na2O-CeO2 were prepared by melting and sintering respectively. The structure of the prepared phosphate glass-ceramics was characterized through XRD and SEM. The effect of preparation method on the structure and properties of the phosphate glass-ceramics were systematically studied in terms of the bulk density, microhardness, dielectric properties, polarization performance and energy storage etc. The results showed that the more compact glass-ceramics could be obtained by the melting method, which could improve the bulk density and micro-hardness of the glass-ceramics, the corresponding dielectric loss was reduced. In addition, when the addition of CeO2 was increased to 1% (mole fraction), the phosphate glass-ceramics obtained by the two methods were also achieved good densification. The addition of CeO2 played a role in promoting crystallization, which enhanced the dielectric constant and polarization performance. The released energy density of 13.5 mJ/cm3 and energy storage efficiency of 50.1% were obtained for the glass-ceramic with addition of 1% CeO2 (mole fraction) prepared by melting method.
The work aims to study the effect of doped Cu content on the structure, electrical properties and carrier transport behavior of amorphous carbon (a-C) films. The Cu doped a-C (a-C:Cu) films were deposited by a homemade High Power Impulse Magnetron Sputtering set with the Cu-C composite target as sputtering source. A series of a-C: Cu films with Cu content less than 10% (atomic fraction) were deposited by adjusting the position of substrates. The results demonstrated that increasing the doped Cu content led to the enhancement of the content and cluster size of sp2-C in films. Particularly, as the Cu content increased from 2.77% to 7.28%, the sp2-C content increased from 48% to 54%. Accordingly, this decreased the bandgap width from 3.87 eV to 2.93 eV, which corresponds to the reduction of electrical resistivity and transmittance in a-C: Cu films. For a-C: Cu films with Cu content in the range of 2.77%~7.28%, the voltage was positively linear correlated with the excitation in the I-V test, suggesting the dominated ohmic behavior. The resistance of all the a-C:Cu films decreased monotonically with the increase of temperature, demonstrating the typical semiconductor behavior. Specifically, when the Cu content varied in the range of 2.77%~3.88%, the electrical transport of a-C: Cu films was ascribed to the three-dimensional Mott-type variable range hopping conduction in lower temperature from 150 K to 250 K and the thermal activation transport within higher temperature range of 250~350 K, respectively. However, for a-C: Cu films with Cu content of 5.4%~7.28%, only Mott-type variable range hopping conduction played the key role for the carrier transport in temperature of 150~350 K. The results showed that the optical and electrical properties of amorphous carbon films could be significantly controlled by doping Cu, which brought forward the promising potential to develop the carbon-based photoelectric devices with high-performance.
A ZrO2/Cr composite coating on Zr-2 alloy was prepared by micro-arc oxidation (MAO) and filtered cathodic vacuum arc deposition (FCVAD) treatments. The oxidation resistance of bare and ZrO2/Cr-coated Zr alloys was estimated in 900~1100°C steam environment using a thermogravimetric analyzer (TGA) and their cross-sectional structures, phase constituents and composition depth profiles before and after steam oxidation were analyzed. The results showed that the mass gain per unit area of ZrO2/Cr-coated Zr alloy was about 3/8, 1/4 and 2/5 of that of bare Zr alloy after 3600 s steam oxidation at 900, 1000 and 1100℃, respectively. In high temperature water vapor, the dense Cr2O3 film formed on the surface of ZrO2/Cr-coated one suppressed the inward oxygen diffusion, which improved the steam oxidation resistance of Zr-2 alloy and inhibited the occurrence of breakaway oxidation at 1000°C. Before the Cr layer of ZrO2/Cr composite coating was completely oxidized into Cr2O3, the oxidation of Cr layer was principally controlled by the outward chromium diffusion rather than inward oxygen diffusion. The MAO interlayer restrained hydrogen permeation into Zr alloy substrate during the steam oxidation.
The solidification behavior of a Ni-based Ni-Cr-Al-W-Ta single crystal superalloy in normal gravity (1g) and microgravity (μg) conditions were comparatively investigated by using a 50 metre-high drop tube. The solidification microstructure of the alloy was observed using optical metalloscopy (OM), and the primary and secondary dendrite spacing of the samples were measured and counted by using an image analysis software. Scanning electron microscope (SEM-EDS) was used to determine the chemical compositions of dendrite trunk and interdendrite at different locations, and then the microsegregation coefficient was calculated. The results show that the dendrite characteristics and microsegregation are significantly different in 1g and μg conditions respectively. The primary and secondary dendrite spacing tested in 1g sample are larger than those in μg sample, and the difference of primary dendrite spacing between 1g and μg sample gradually increases with the increase of solidification distance, while the difference of secondary dendrite spacing does not change much. With the process of solidification, the contents of Ta, Cr and Al between dendrites tested in μg sample show a trend of increasing obviously at first and then decreasing slightly, while the W content has a trend of decreasing gradually, and the interdendritic liquid phase density shows a trend of decreasing slightly. The distribution of the Ta, Cr and Al content between dendrites tested in 1g sample are basically similar to those in μg sample, while the distribution of W is significantly different, showing an upward trend in most solidification stages, resulting in the increase of the interdendritic liquid phase density just along the opposite direction of gravity. These results indicate that the convection effect caused by the solute density difference at the front of solidification interface was weak in normal gravity condition, which was not the main reason for the increase of dendrite spacing. It is believed that the main reason should be related to the reduction of temperature gradient caused by thermal convection at the front of solidification interface.
Nitrate as one of the water pollutants is one of the major environmental problems. Photocatalytic reduction of nitrate nitrogen has attracted a lot of attention because of its high efficiency and environmental friendliness. The g-C3N4/Ag/BiOBr composite photocatalyst was prepared by high temperature calcination, reaction synthesis and photoreduction. The photocatalysts were characterized by SEM, XRD, EPMA、FT-IR, XPS and UV-vis, and the reduction effect of the composite on nitrate nitrogen (50 mg/L) under the irradiation of metal halide lamp was studied. The results showed that when 1g/L g-C3N4/Ag/BiOBr catalyst was used, the nitrate concentration was 2.4 mg/L, and the removal rate was 95.2% after 180 min photoreaction. Compared with g-C3N4, BiOBr and g-C3N4/BiOBr photocatalysts, the removal rates increased by 38.8%, 34.6% and 13.1%, respectively. Nitrogen was the main product in the photocatalytic conversion of nitrate nitrogen. The proportion of N2 in the main products of nitrate nitrogen photocatalyzed by g-C3N4/Ag/BiOBr was the highest (88.0%), and the selectivity of nitrogen was 92.4%. Ag can be used as an electron trapping agent to effectively reduce the recombination of electron-hole pairs in photocatalytic materials. Under the action of silver, the photogenerated electrons of BiOBr are transferred to the valence band of g-C3N4 by silver elemental material, forming a Z-type composite photocatalytic structure. Nitrate nitrogen can be directly oxidized by the composite photocatalyst, and the hole scavenger formic acid can be converted into a strong oxidizing substance (COO.-) under the action of the composite hole, which can further reduce nitrate nitrogen.
Porous graphene /Ti(Gr/Ti) composites were fabricated by selective laser melting (SLM) technique with the mixture of Ti powder and graphene as raw material, and the effect of graphene (Gr) addition on the microstructure, mechanical properties and corrosion resistance of porous Gr/Ti composites were investigated. The results show that the macrostructure of the porous Gr/Ti composite is not significantly different from the designed structure, but the porosity is lower than the designed structure. The porous pure titanium prepared by SLM consists of small equiaxed grains, and the grain size of which is further reduced after the addition of graphene, while the graphene uniformly distributed in the Ti-matrix. A small portion of graphene reacts with Ti-matrix leading to the formation of TiC, as a result, the in-situ generated TiC as second phase particles can reinforce the base metal through dispersion strengthening. The compression stress-strain curves of the porous Gr/Ti composite display elastic deformation stage, stress plateau stage and densification stage. The hardness, compressive strength and compression ratio of the porous Gr/Ti composite were 503HV, 317.38 MPa and 42%, respectively. Its corrosion potential and the corrosion current density were -0.325 V and 3.28×10-7 A·cm-2 respectively, indicating a better corrosion resistance in the comparison to the one of pure Ti.
