Abstract:Various heat treatments were conducted on Inconel 718 superalloy, and the resultant microstructures and properties were investigated to analyze the mechanisms of heat treatments. Results show that the type and quantity of precipitate phases and the grain size have different effects on the properties of Inconel 718 superalloy after various heat treatments. The γ" and γ' phases as well as grain size mainly influence the strength, and the δ phase mainly influences the plasticity. Besides, the precipitation of γ" and γ' strengthening phases can improve the yield strength. The alloy strength is inversely proportional to mean grain size when the γ" and γ' phases have similar contents. The plasticity is susceptible to the content and shape of δ phase. A proper amount of δ phase is beneficial to the plasticity, but excessive δ phase degrades plasticity.

Abstract:The influence of the low voltage pulsed magnetic field (LVPMF) on the microstructure transition of K4169 superalloy was investigated. The gradient microstructure of K4169 superalloy composed of columnar grains, coarse grains, and fine grains was prepared through the combined method of LVPMF with directional solidification, which provided a new approach for the preparation of superalloy with gradient microstructure. The distribution of the Lorentz force and flow field under LVPMF effect was simulated, and therefore the microstructure transition mechanism was revealed. Results show that the microstructure transition should be attributed to the coupling effects of the Lorentz force and forced convection.

Abstract:IN706 superalloy is particularly sensitive to the parameters of hot working process. The flow stress of the IN706 superalloy was investigated during reduction deformation of 30%, 45%, and 60% under the isothermal compression conditions of temperature at 1143–1393 K and strain rate at 0.01, 0.1, 0.5, and 1 s^-1. The exponent-type Zener-Hollomon equation was used to describe the impact of strain and temperature on the thermal deformation. Meanwhile, the strain effect of various material constants, such as α, n, Q, and lnA, was considered in the constitutive equation considering the strain compensation, and the correlation coefficient R and the average absolute relative error were verified. On the basis of constitutive equation construction, the hot processing map of IN706 superalloy was drawn, and the instability region was obtained based on the Murty criterion. Results show that the reasonable thermal working process parameter window is strain rate of 0.1 s^-1 and temperature of 1313–1353 K.

Abstract:In this paper, NiCoCrAlYHf (HY5) coating was deposited on the surface of the second-generation nickel-based DD6 single-crystal high-temperature alloy by vacuum arc plating process, and the surface coating was removed by physical blowing sand method and chemical solution immersion method, respectively, for a comparative study of the effect of removal of the coating by the two and the degree of influence on the matrix alloy. The results show that: the thickness of the coating is about 20 μm, which is mainly composed of γ" and β phases; no coating remains on the surface of the substrate after treatment by the physical method or chemical method, and the original coatings are completely removed. Under the cutting action of the sand blowing particles in the physical method, the surface of the matrix alloy presents a concave-convex morphology on the macro level, and the deformation layer is generated by the deformation along the <110> orientation on the micro level; the physical method destroys the integrity of the surface of the matrix alloy, which is more damaging to the matrix. The chemical treatment only produces a γ" phase damage layer in the local area, which is less damaging to the substrate.

Abstract:The heating process before deformation has significant influence on the hot workability of superalloys. In this paper, the evolution behaviors of γ′ characteristic and grain structure with the heating temperature and holding time in hard-deformed superalloy U720Li were studied by isothermal heating experiments, which can provide theoretical guidance for improving its hot working performance. The results show that the area fraction and size of primary γ′ decrease with the increase of heating temperature and holding time. The dissolution of fine secondary γ′ is easier than the coarse primary γ′. The former can be completely dissolved after holding at 1100 ℃ for 30 min, while the latter can be completely dissolved after holding at 1180 ℃ for 30 min. The continuous dissolution of primary γ′ weakens its pinning effect on grain boundaries, causing the average grain size to gradually increase with the increase of heating temperature and holding time. Moreover, as the temperature increases to 1180 ℃ (higher than γ′ dissolution temperature), the grains undergo abnormal growth in a short space of time, forming a mixed grain structure. Finally, according to the grain growth behaviors under different heating conditions, a grain growth kinetic model for U720Li alloy has been established.

Abstract:An important parameter in the TLP welding process is the time required for complete isothermal solidification. In this study, KCo10 intermediate layer was mainly used for the tests, which were carried out at temperatures of 1030 ℃, 1050 ℃ and 1080 ℃ (1303 K, 1323 K and 1353 K) with different holding times. The average ASZ size was measured using the solid solution on both sides of the eutectic liquid phase as the boundary. By measuring the size of the ASZ with the square root of the variation of the welding time, data fitting was used to derive a linear relationship between the ASZ size and the bonding time, and then a mathematical model of the isothermal solidification process was established based on Fick"s second law of diffusion equation with time, and the isothermal solidification time tIS was simulated and solved so as to predict the approximate time required for isothermal solidification, and was experimentally verified The accuracy of the mathematical model in predicting the isothermal solidification time was verified experimentally.

Abstract:Cold-forged Co-Cr-W-Ni-Mo alloy was aged at 400 ~ 900 °C. The carbide, substructure, hcp phase and stacking faults evolution of Co-Cr-W-Ni-Mo alloy during aging were studied by means of scanning electron microscopy, transmission electron microscopy, XRD and chemical phase analysis, and the influence of their changes on strength and toughness were analyzed. The results show that the alloy aged at 600 °C for 2 h has high strength and hardness, while maintaining good ductility and toughness. M23C6 and M6C carbides are precipitated after aging. The precipitation temperature range of M23C6 is 600 ~ 800 °C, and the precipitation temperature of M6C is 700 °C. The precipitation of carbides leads to the increase of strength and hardness, but the precipitation of a large number of network M6C along the grain boundaries deformation bands at 900 °C will reduce the strength and ductility at the same time. When aged at 600 °C, the fcc phase reduces the energy stability by forming stacking faults, which increases the density of stacking faults and improves the strength and ductility. The hcp phase reverses to fcc phase after aging at 800 °C, which makes the strengthening effect of strain-induced martensitic transformation disappear. At the same time, recrystallization occurs at this temperature, which leads to the disappearance of cold deformation structure and further decrease of strength. The recrystallization nucleation mechanism of the alloy is a combination of three nucleation mechanisms : protrusion mechanism, subgrain boundary migration and subgrain merging.

Abstract:Forging is the key forming process for producing advanced aero-engine powder turbine disks. The microstructure of as-extruded alloy mostly depends on the chemical composition of alloy and extrusion processing parameters. Average grain size number is greater than 13 for the novel powder nickel-based superalloy FGH4113A after hot extrusion. As-extruded alloy has two phases microstructure of γ+γ". Fine homogenization two phases microstructure makes it possible to broaden isothermal forging processing window. In this study, Gleeble hot compression experiments for as-extruded FGH4113A alloy with ultrafine grains were carried out at temperatures of 1000, 1040, 1080, 1120℃ and strain rates of 0.001, 0.01, 0.1s-1. Height reduction is 60% for samples under all experiment conditions. The results show that no crack was found on the surface of alloy samples under all experiment conditions, suggesting as-extruded FGH4113A alloy with ultrafine grains has a wide isothermal forging process window. The average activation energy for the alloy is 515.375kJ/mol. True stress-true strain curves show steady flow characteristic for samples with slow strain rate (0.001s-1) and the change from work hardening to recrystallization softening for samples with fast strain rate (0.1s-1). γ" is sensitive to temperature. Recrystallization grains grow obviously at high deformation temperature due to the large amount of dissolution of γ". Furthermore, the study shows that the main recrystallization way is discontinuous dynamic recrystallization for as-extruded FGH4113A alloy with ultrafine grains. This study offers experiment evidence for designing forging processing of the as-extruded FGH4113A alloy with ultrafine grains.

Abstract:Laves phase NbCr2/Nb two-phase alloy has attracted extensive research attention due to its potential as high temperature structural materials. Based on the isothermal constant strain rate compression experimental data of the alloy at temperatures ranging from 1273 to 1473 K and strain rates ranging from 0.001 to 0.1 s^-1, the flow stress behavior of the alloy was analyzed, the thermal deformation activation energy Q, power dissipation efficiency η and instability factor were calculated, response surface models with the deformation process parameters as input variables and Q, η and as response targets was established, and the appropriate window conditions of deformation process parameters were obtained by multi-objective optimization. The results show that Laves phase NbCr2/Nb two-phase alloy is a positive strain rate and negative temperature-sensitive material. The values of Q, η and values fluctuate within the range of 157.0-659.3 J/mol, 0.01-0.81 and -0.6229-0.6359, respectively, which indicates that the plastic deformation capacity of the alloy is sensitive to the change of process parameters. The established response surface models for Q, η and have high prediction accuracy with the determination coefficients R² reaching 0.992, 0.999 and 0.953, respectively, and the average absolute relative errors AARE are 1.29%, 0.63% and 11.5%, respectively. The interaction order of deformation process parameters on Q (from large to small) is as follows: deformation temperature/strain rate>strain rate/true strain>deformation temperature/true strain, while the interaction order of deformation process parameters on η and is basically the same, that is, deformation temperature/strain rate>deformation temperature/true strain>strain rate/true strain. Based on the multi-objective optimization of low Q, high η and , the appropriate deformation process window conditions are 1440-1473K and 0.001-0.05s^-1, and the optimal deformation process window condition is around 1473K and 0.001s-¹. Microstructural verification under the optimal deformation process conditions confirms the correctness of the deformation process window conditions obtained through multi-objective optimization.

Abstract:Mixed grain micstructures of GH4720Li superalloy with different coarse and fine grain ratios were prepared by forging deformation. The high-temperature tensile properties of different mixed grain micstructures at 650oC were tested. The quantitative relationship between mixed grain micstructures and high-temperature tensile strength was established, and the mechanism of influence of mixed grain structures on high-temperature tensile properties was revealed. The results show that the primary γ" phase distribution directly affects the evolution of the mixed grain structures during the high-temperature forging process of coarse crystal GH4720Li alloy. The more uneven the primary γ" phase distribution is, the easier it is to form the mixed grain micstructure after the deformation of the coarse grain structures. During the 650oC tensile process, the coarse grain size and volume fraction in the mixed grain micstructures significantly affect the tensile properties. With the increase of the coarse grain size and volume fraction, the high-temperature tensile strength of the alloy decreases slowly and then rapidly, while the plasticity decreases rapidly. The tensile strength shows a Hall-Petch quantization relationship with the equivalent grain size. But the effect of coarse grain size on the equivalent grain size is higher than that of fine grain size. The RD//<111> oriented grains are formed in the microstructure during high-temperature tensile deformation. The smaller the difference in the ratio of coarse and fine grains in the mixed grain micstructure, the fewer RD//<111> oriented grains are formed, the greater the dislocation slip obstruction, and the worse the high-temperature tensile property of the mixed grain micstructure.

Abstract:Nickel-based single crystal superalloys are composed of various elements, each of which has a unique strengthening effect. It is difficult to realize the rapid development of new alloys by traditional alloying design methods such as "trial and error" and phase diagram calculation. The relationship between composition, organization, technology and properties can be quickly obtained by "material genome engineering", which greatly improves the research and development rate of materials and reduces the production cost. The development trend of alloying design of Nickel-based single crystal superalloys is briefly described, including the application of new research technologies such as high throughput preparation and characterization technology and machine learning in alloying design. The ability of atom probe tomography (APT) to quantitatively study the content and distribution of elements in the microstructure of alloys is also expounded. It is expected that this paper can provide ideas for alloying design of Nickel-based single crystal superalloys.

Abstract:Ti-25Ta alloy samples were fabricated by selective laser melting, and the relative density, microstructure, microhardness and tensile properties of the as-built and hot isostatic pressing (HIP)-prepared samples were characterized. Results show that the track width and penetration depth are increased with the increase in laser power, and the surface morphology is improved. The maximum relative density improves from 95.31% to 98.01% after HIP process. Moreover, the microstructure is refined into the lath martensite and cellular grains with the increase in input power. After densification treatment, the subgrain coalescence occurs and high angle grain boundaries are formed. In addition, HIP process stabilizes the microhardness and enhances the tensile strength and elongation.

Abstract:The electrochemical corrosion, wear, and tribocorrosion behavior of the novel Ti-19Zr-10Nb-1Fe alloy were investigated. The electrochemical corrosion analysis results show that the corrosion resistance of the Ti-19Zr-10Nb-1Fe alloy is better than that of the Ti-6Al-4V alloy under the test conditions in this research. Compared with the static electrochemical corrosion, the corrosion resistance of Ti-19Zr-10Nb-1Fe alloy during tribocorrosion decreases significantly, because the wear accelerates corrosion. The wear volume of Ti-19Zr-10Nb-1Fe alloy is increased with the increase in applied load whether the electrochemical corrosion occurs or not. Due to the acceleration effect of electrochemical corrosion, the wear volume caused by electrochemical corrosion is larger than that without electrochemical corrosion. The results of W_a/W_c are much greater than 10, indicating that during the tribocorrosion process, the material loss caused by mechanical wear is much larger than that caused by electrochemical corrosion. Through SEM observation of the wear morphologies of Ti-19Zr-10Nb-1Fe alloy after tribocorrosion, it can be inferred that the micro-abrasion is the main wear mechanism. The above results show that during the tribocorrosion process, the corrosion accelerates wear, and the wear accelerates corrosion.

Abstract:Ti6Al4V alloy manufactured by electron powder bed fusion (EPBF) was separately heat-treated by stress-relief annealing at 600 °C, annealing at 800 °C, and solid solution at 920 °C for 1 h. Then, the friction and wear tests were conducted on the samples before and after heat treatment to analyze the properties and mechanism of friction and wear behavior. Results show that the sample annealed at 600 °C for 1 h has the optimal wear resistance, and the wear mass loss reduces by 44%. The sample annealed at 800 °C for 1 h possesses the optimal anti-friction performance, and the coefficient of friction reduces by 14%. This research provides a simple heat treatment method to improve the friction and wear resistance of Ti6Al4V alloy manufactured by EPBF.

Abstract:Near-β titanium alloys for marine engineering applications may be suffered from stress corrosion cracking. The phase composition has a significant impact on stress corrosion behavior. Different phase compositions of a near-β titanium alloy, Ti-3Al-5Mo-4Nb-4Cr-2Zr, were obtained by undergoing various heat treatments. The microstructures were characterized by scanning electron microscope and X-ray diffraction. The electrochemical measurements and slow strain rate testing were carried out to investigate the effect of phase composition on stress corrosion behavior. The results indicate that the single β phase is obtained by solution treatment at β phase field (Pβ). After aged at 500℃ for 6h, fine secondary α phase with narrow spacing precipitate within β matrix (Pβ+fα). After aged at 650℃ for 6h, coarsened secondary α phase with wide spacing precipitate within β matrix (Pβ+cα). For the Pβ+cα, the corrosion potential and elevation angle of capacitive impedance arc as well as impedance modulus at 0.01Hz are relatively minimum, and the corrosion current density is relatively maximum. The Pβ+cα exhibits relatively poor corrosion resistance. The relatively better order is the Pβ+fα and Pβ. The stress corrosion susceptibility index order from high to low is Pβ+cα、Pβ+fα、Pβ. The fracture morphology exhibits a mixed characteristic with shallow dimples and micro-cracks as well as tear ridges and flat facets. The combination of Absorption Induced Dislocation Emission (AIDE) and Hydrogen Enhanced Localized Plasticity (HELP) is the main mechanism for stress corrosion cracking. The decrease of secondary α phase spacing has a beneficial effect on reducing the stress corrosion susceptibility.

Abstract:Annealing treatments in the temperature range of 800℃ to 900℃ was applied to the electron beam weldments of powder metallurgy Ti2AlNb alloys to investigate its influence on microstructure and tensile properties at room temperature and 650℃. The results show that there are coarse columnar and equiaxed B2 grains in the fusion zone (FZ) in the as-welded weldments. Transformation from B2 phase to O+B2 phase occurs in the FZ during the post-weld annealing treatments. The morphology of the O phase is characterized as the basket-weave structure and the size of the O phase plate increases with the increase of the annealing temperature. The microhardness in the FZ of the as-welded weldments are higher than that in the base metal (BM). After the annealing treatments, the microhardness in the FZ increases significantly, which gradually decreases with the increase of annealing temperature. At room temperature, all tensile speciems are fractured in the FZ. The tensile strength of the as-welded weldments is close to that of the BM and local large plastic deformation occurs in the FZ. However, the strength and ductility of the weldments decrease slightly after the post-weld annealing treatment. At 650℃, most of the tensile speciems are fractured in the FZ. The as-welded weldments are failured in the FZ, which are characterized as brittle intergranular fracture. After the annealing treatments, there is an apparent improvment of strength and ductility in the FZ of the weldments. With the annealing temperature increasing up to 900 ℃, the failured locations are shifted from the FZ to the BM.

Abstract:The corrosion properties of pure zirconium (Zr) with different grain sizes in acid, alkali, and salt environments were studied. The microstructures of pure Zr were observed by optical microscope, X-ray diffractometer, and electron backscattered diffraction probe. The corrosion resistance of pure Zr was analyzed by electrochemical corrosion test and immersion test. Results show that pure Zr with grain size of 4–32 μm can be obtained after annealing at 800 °C for different durations, and the relationship between grain size and annealing duration is D³-D₀³=3.35t. The electrochemical corrosion and immersion corrosion test results show that the pure Zr with grain size of about 24 μm (annealing at 800 °C for 20 h) possesses the optimal corrosion resistance.

Abstract:(Fe₇₃Ga_27-xAl_x)_99.8Tb_0.2 (x=0, 1, 2, 3, 4, 5) alloys were prepared by vacuum arc furnace to investigate the effect of Al addition on the microstructure, magnetic properties, and mechanical properties of alloys. Results show that the phase structure of the alloys is still A2 phase and Tb₂Fe₁₇ phase, and the metallographic structure is composed of cellular crystal and columnar dendrite. The decrease in lattice constant, the intensification of (100) orientation, and the generation of Tb₂Fe₁₇ phase at the grain boundary exert significant effect on the magnetostrictive properties. The fracture morphology of the alloys is intergranular brittle fracture and cleavage fracture, and the causes of fracture occurrence include the segregation of Tb and Al elements. The parallel magnetostrictive strain (λ_∥) of (Fe₇₃Ga₂₄Al₃)_99.8Tb_0.2 alloy peaks at 1.04×10^-4. It is worth noting that compared with those of (Fe₇₃Ga₂₇)_99.8Tb_0.2 alloy, the λ_∥ and elongation of (Fe₇₃Ga₂₆Al₁)_99.8Tb_0.2 alloy increase by 18.3% and 53.4%, respectively. Besides, (Fe₇₃Ga₂₆Al₁)_99.8Tb_0.2 alloy possesses the characteristics of high saturation magnetization (M_s), low remanent magnetization (M_r), and coercivity (H_c), which is beneficial to reduce the cost in actual production, but its tensile strength and Vickers hardness decrease to a certain extent. Therefore, the investigation on the micro-mechanism of Al addition on Fe-Ga alloys is of great significance for the development of Fe-Ga alloy devices.

Abstract:The service water environment of high temperature and high pressure was simulated for the steam generator heat transfer tube of pressurized water reactor. 690 TT alloy tube and 405 SS plate were used to form the friction pair for impact slip dual-axis fretting corrosion experiments. The microstructure evolution of 690 TT alloy tube during dual-axis impact slip fretting corrosion was investigated. White light interferometer, scanning electron microscope, transmission electron microscope, and Raman spectrum were used to investigate the microstructure and abrasive products of the abraded surface and near-surface. Results indicate that within 10⁵ cycles, the wear mechanism of 690 TT alloy tube is mainly adhesive wear accompanied by material transfer. With the increase in cycles from 5×10⁵ to 2×10⁶, the wear mechanism of 690 TT alloy tube is mainly crack initiation, propagation, and delamination. In terms of microstructure evolution, mixed layer exists under the three body layer in the cross-section microstructure of samples after 10⁵ cycles. The microstructures of samples after 5×10⁵ and 2×10⁶ cycles show slight difference and present a tribological transfer structure layer with thickness of about 500 nm. Additionally, the microstructure evolution enters the stable stage.

Abstract:The effect of different surface treatments on the bonding strength of composite plates was investigated under the conditions of 400 °C and reduction ratio of 45%. Results show that the wire brush grinding treatment can only eliminate the oxide film on the plate surface, but it can hardly produce a hard layer on the plate surface. The bonding effect depends on the element diffusion promoted by the close contact between the metals on both sides of the interface. After anodic oxidation, there is a hard layer on the metal surface, and the hard layer broken during the rolling process forms a mechanical occlusion at the bonding interface. However, the hard layer cannot form an effective combination with the metal at the interface, and the bonding can only occur in the fresh metal bonding area at the crack of the hard layer. The acid-alkali washing treatment can completely remove the hard layer on the surface of both alloys without increasing the surface roughness of the plate, and the metal on both sides of the interface is more closely bonded during the rolling process. The optimal bonding strength can be obtained by surface treatment of acid-alkali washing for the aluminum-magnesium hot-rolled bonding.

Abstract:The austenite Fe-Cr steels used as the critical in-pile components because of its good high temperature resistance, corrosion resistance and excellent mechanical properties and thermal strength, bear long-period high temperatures and irradiation; the irradiated vacancies aggregate into voids leading to irradiation swelling and hardening, which seriously affects the service safety of the reactor. The phase field method coupling temperature field is employed by solving the phase field equations based the Fourier spectrum method to investigate the voids behavior of austenite Fe-Cr steels upon a central and a one-dimensional temperature field. As the temperature goes down from the center radically in a central temperature field, the vacancies diffuse toward the high-temperature center region driven by the temperature gradient, resulting in the instability of the double voids model, and gradually dissolves to form a new void in the high temperature center region. In the central temperature field, the voids nucleate earlier and grow faster with a sizeable scale in the high temperature than that in the lower temperature region due to the higher vacancy concentration in the central high temperature region. Based on the force-flow relation in the principle of irreversible thermodynamics, the migration behavior of voids upon a one-dimensional temperature gradient is studied by adding advection term to the phase field evolution equation Cahn-Hilliard equation. It has been observed in the experiments that the size of voids in irradiated austenite Fe-Cr steel is nanoscale, and it is generally believed that the nanoscale voids migration is controlled by the bulk diffusion mechanism and surface diffusion mechanism. Therefore, the effects of temperature gradient and initial size of voids on the voids migration upon a one-dimensional temperature gradient in austenite Fe-Cr steel are studied considering both the bulk diffusion mechanism and surface diffusion mechanism respectively. The migration rate governed by the bulk diffusion mechanism positively depends on the temperature gradients but not the initial void"s size. The migration rate governed by the surface diffusion mechanism positively depends on the temperature gradients but is negatively related to the initial void"s size. At the same time, in the process of migration, the shape of the void will also change, and the void will be elongated along the direction of temperature gradient, and the front end is sharper along the direction of temperature gradient, and the back end is wider. The studies inspire the microstructure aging and properties prediction caused by inhomogeneous heat conduct or macroscopic uneven temperature distribution.

Abstract:C/C composites are the important structure materials for aerospace and nuclear applications. Realizing the joining of C/C composite to metal materials can overcome the drawbacks of high cost and hard-to-forming difficulties of C/C composite. In this paper, C/C composite and Invar alloy were brazed with active Cu-Sn-Ti filler alloy. The effects of brazing temperature and holding time on the microstructure, shear strength and fracture behavior of brazed joints were investigated. The results showed that, on the side of C/C composite, Cu-Sn-Ti filler reacted with C/C composite to form a TiC reaction layer. In addition, Cu-Sn-Ti filler would penetrate into the C/C composite, which formed a zigzag interface that strengthened the joint. On the side of Invar alloy, Invar alloy dissolved into Cu-Sn-Ti form a good interface metallurgical bonding. The interfacial microstructure of brazed joint was determined to be Invar alloy/Cu(s,s)+Fe2Ti+Ni3Ti+Ti6Sn5/Fe2Ti+CuTi+Cu3Sn/TiC+Cu(s,s)/C/C composite. The maximum shear strength of the joint of 61 MPa, which is significantly higher than the strength reported in previous studies, was obtained at brazing temperature of 930 ℃ and the holding time was 10 min.

Abstract:At present, developing bifunctional electrocatalysts plays an important role in the field of metal-air batteries and fuel cells. Non-noble transition metal single atoms loaded on the nitrogen-doped graphene sheets (M-N-C) are considered to be the most promising materials to replace noble metal electrocatalysts owing to the metal coordination compounds (MNxO4-x) in M-N-C with high catalytic activity. In this work, oxygen atoms were introduced in Fe-N-C to construct FeNxO4-x (x = 0, 1, 2, 3, 4) for studying the effect of coordination number x on the catalytic performance of ORR/OER. It was found that Fe-N-C showed the best thermodynamic stability and catalytic activity when x = 4. In addition, the effect of transition metal type was studied by introducing different transition metals, such as M = Mn, Fe, Co, Ni, Cu, in MN4. Among them, the thermodynamically stable CoN4 structure was the best choice for M-N-C to reach the highest ORR/OER catalytic activity. It is expected that this paper could provide a theoretical reference for adjusting the coordination environment of transition metal single atoms and designing high efficient bifunctional electrocatalysts.

Abstract:To solve the problem of selecting and controlling the optical characteristics of visible and infrared compatible stealth, a new type of visible light high transmission and infrared low radiation compatible stealth material was proposed, based on the synergistic effect of optical effects such as induced transmission and radiation suppression of TiO2/Ag/TiO2 film structure integration. The research on the mechanism of the influence of structural features on visible light transmittance and infrared reflectance was carried out, the optimization design and preparation of high transmittance infrared stealth film structures was implemented, and its compatibility stealth performance was tested and characterized. The results show that when the thickness of each film layer in the optimized TiO2/Ag/TiO2 structure is 30/18/35 nm, the average visible light transmittance and mid-far infrared reflectance of the quartz substrate sample can reach 81.51% and 90.78%, respectively, possessing high levels of visible light perspective and high-temperature infrared radiation suppression ability. The average visible light transmittance and mid-far infrared reflectance of the flexible PET substrate sample can reach 65.68% and 84.46%, respectively. It has good comprehensive capabilities in perspective, shading, and infrared radiation suppression, and has high flexible surface conformal ability. The results of this study can provide important technical support for the design and application of multi spectral compatible optical stealth materials.

Abstract:The chemical short-range ordered structure in multi-principal element alloys has been regarded as an effective strengthening method. It also can affect the distribution of elements at grain boundaries. In order to study the effect on the properties of Al0.5CoCrFeNi alloy, the effects of chemical short-range order at several typical grain boundaries on the tensile and radiation resistance of Al0.5CocrfenI alloy were studied by molecular dynamics method. The relevant influence mechanism is explained by dislocation analysis and point defect analysis. The results show that the segregation of elements at grain boundaries is more obvious with higher mismatch degree. Chemical short-range ordering can improve the tensile properties and radiation resistance of bicrystals, especially for bicrystals with large mismatched grain boundaries, which can effectively absorb radiation and produce interstitial atoms.

Abstract:In this study, WO2.9-W2N can be obtained by spraying the precursor during the preparation of carbon nanotube films (CNTs), followed by hydrothermal treatment and calcination. The use of W2N can improve the poor conductivity of WO3. In addition, the oxygen vacancies can be caused by WO2.9 that provide more active sites for the deposition of Li2S, and form Li-O with polysulfides (LiPSs), Li-N bond. The results demonstrate the chemical adsorption and anchoringof LiPSs onto the matrix strengthened, and thereby the prepared battery indicates excellent electrochemical performances, after 500 cycles at a rate of 0.2 C, the discharge specific capacity remains 850 mAh g-1, showing excellent electrochemical reaction kinetics and its catalytic effect to the electrochemical reactions. Therefore, the shuttling inhibition of WO2.9-W2N/CNTs is mainly occurred by trapping-anchoring-catalytic conversion.

Abstract:In order to improve the quality of underwater wet laser welding, a self-designed flux were successfully applied to the work of underwater wet welding 5mm thick 0Cr25Ni6Mo3N duplex stainless steel which existed at the 6mm depth in simulated seawater by laser beam. After welding, the microstructure of the top, middle, bottom weld and heat-affected zone of the welded joint was observed by metallographic microscope, and the mechanism of microstructure evolution was deduced. The austenite phase ratio in each area of ??the welded joint was calculated, and we have found that the austenite in most areas is significantly reduced, which deviates from the balance value of the base metal. The reason for this problem is analyzed in combination with the microstructure and morphology characteristics of each area. Positive suggestions are given to promote the growth of austenite and maintain the balance of phases. After the experiment, the failure analysis of the tensile fracture type and friction and wear behavior of the weld was carried out. The average tensile strength of the sample is 810.7MPa, which is 95.3% of the tensile strength of base metal and the average elongation of these samples is 34.4%. The microstructure of the fracture shows obvious cleavage step and some dimples, which belongs to the mixed fracture type. The friction coefficient of the weld is about 0.557, and the friction reduction is better than that of the base metal. The wear failure mechanism is three-body abrasive wear.

Abstract:The higher requirements are proposed for the thermal stability and impact toughness of drill bits due to the increasingly complex environment of modern oil and coal mining. Polycrystalline diamond composite sheet (PDC) is the core parts of the drill and has a direct impact on the service life of the drill. The PDC always fail in actual working conditions owing to poor thermal stability and fracture. Carbon nanotubes have excellent mechanical properties, chemical stability and thermal stability. Carbon nanotubes are introduced into PCD layer can solve the problems of poor impact toughness and poor thermal stability in PDC. In this study, carbon nanotubes were used as the reinforcing phase and added into PDC. PDC composites were prepared under high pressure and high temperature (5.5 GPa and 1300℃), and the sintering time was 90 s. The effects of carbon nanotubes on mechanical properties and thermal stability of PDC composites were analyzed, the microstructure, element distribution and phase composition of PCD composites were characterized. The results show that the impact toughness of PDC with 1.4 wt% carbon nanotubes increased from 400 J to 550 J, which is 37.5% higher than that of original PDC. The increase of PCD composites impact toughness is attributed to the removal and bridging of carbon nanotubes during crack deflection. The thermal stability of PDC with 1.6 wt% carbon nanotubes increased from 704℃ to 813℃, the friction coefficient decreased from 0.064 to 0.048. The wear ratio reached 159.69×104, which was 59.64×104 higher than that of the original PDC.

Abstract:In this paper, FexCr5Al (x=13, 15, 17, 18, wt%) alloys were prepared and exposed to 1000 and 1200 ℃ steam for 2 h. The high temperature oxidation behaviors of the alloys were studied by a simultaneous thermal analyzer. The microstructure and composition of the oxide film after oxidation were analyzed using XRD, FIB, EDS, and TEM. It was found that the oxidation kinetics of the alloys at two temperatures followed a parabolic growth law. The parabolic rate constants of the alloys were decreased with an increase in Cr content, and the high temperature steam oxidation resistance of the alloys were improved. The ridged oxide films mainly composed of a-Al2O3 were observed in the four alloys. In comparison with the low Cr alloy, the pores of the oxide film were reduced, and the bonding force of the oxide/matrix interface was improved in the high Cr alloy. Consequently, the compactness and adhesion of the oxide film were improved by increasing the Cr content, which promote the formation of a dense Al oxide film and enhance the high temperature steam oxidation resistance of the alloy.

Abstract:TiC/AlMgSc aluminum matrix composites were fabricated by selective laser melting. The phase composition, microstructure, recrystallization distribution and mechanical properties of aluminum matrix composites were systematically analyzed by X-ray diffraction, scanning electron microscope and universal testing tensile machine. The increase of dislocation density and TiC strength relative to alloy strength was calculated theoretically. The results show that the dislocation density of TiC/AlMgSc composites increases by 105% to 0.72×1014 m-2 with the addition of TiC reinforced particles, the proportion of <1 μm grains and sub-grain boundaries increased significantly, and the degree of recrystallization decreases. The tensile strength, yield strength and elongation reached 611 MPa, 589 MPa and 11.0%.Compared with AlMgSc alloy, the yield strength increased by 70 MPa and the elongation decreased by 1.1%.

Abstract:Cemented carbide is a kind of hard phase (WC) and soft bonded phase (Fe, Co, Ni, HEA, etc.) of cermet, its combination of friction resistance, high hardness, good hot hardness and other excellent characteristics make cemented carbide widely used in mines, tunnels, drilling and other geological engineering. Due to the complex actual service environment, cemented carbide in geological engineering applications often face extremely harsh working conditions, and there are many failure mechanisms, such as friction, corrosion and thermal shock, etc. and even a combination will cause the failure of cemented carbide materials. Therefore, understanding the failure mechanism of cemented carbide in geological engineering applications is of great significance for the selection and improvement of cemented carbide materials in different environments. In this paper, the friction and corrosion behavior of cemented carbide for mining are reviewed, focusing on the influence of environmental and thermal stress on cemented carbide failure, in addition, due to the important influence of composition on the microstructure and mechanical properties of cemented carbide, the influence of binder phase and additives on the friction and corrosion behavior of cemented carbide is also reviewed. It aims to provide a reference for the selection, improvement and development of new cemented carbide in the future.

Abstract:In recent years, medical magnesium alloy, as a "third-generation biomedical material", has attracted the attention of many scholars due to its excellent biocompatibility and degradability, and has demonstrated its unique potential in the context of the difficulty of degradation of traditional medical implants. However, due to its poor corrosion resistance, it is difficult to meet the needs of clinical applications of medical implants, so the study of the corrosion resistance of medical magnesium alloys has a pivotal role. In this paper, starting from the corrosion resistance of three typical medical magnesium alloys, the latest research progress of the corrosion resistance of medical magnesium alloys with different elemental content in clinical applications and the corrosion protection technologies such as surface coating and alloying are reviewed, and the results of the application of computer simulation technology in the corrosion study of medical magnesium alloys are introduced, according to which, the future development trend of the medical magnesium alloys materials is prospected.