Abstract:With the high-speed development of manufacturing, the brazing technology is becoming widely used and turning toward a greenization, high efficiency, automation and high reliability direction. The geometrical morphology of the brazing filler metal plays an important role in guaranteeing the automation and high reliability of the brazing. At present, there are numerous researches on brazing filler metal, many of them are based on the material composition, properties, manufacturability and usage range, etc. Systematic research about the geometrical morphology of the brazing filler metal is rarely reported. Choosing the perfect geometrical morphology of the brazing filler metal reasonably can optimize the process and improve the brazing quality. In this paper, the geometrical morphology of the brazing material is the main line, and the characteristics, application scope, representative systems, preparation methods and current situation of the filiform/strip, the stick, bulk, foil tape, powder brazing filler metals, the paste solder, the flux-cored wire, the amorphous and the preformed brazing filler metals are systematically summarized. The powder, flux-cored and amorphous brazing filler metals and suitable filler metal geometries for some common brazing methods are introduced in details. Research suggests that the developing direction of the morphology of the future brazing filler metal is greenization, high-efficiency, low-cost and adapted to the requirement of new materials, automation, digitization and intelligence.

Abstract:Kinetics of hydrogen absorption in TC4 alloy was studied by hydrogenation experiments at different temperatures and initial hydrogen pressures, and the effects of hydrogenation on the microstructure and phase composition of TC4 alloy at room temperature were studied. The results show that the reaction rate constant of TC4 alloy increases with the rise of hydrogenation temperature, and the time required for the reaction to reach equilibrium is gradually shortened. The apparent activation energy of hydrogen absorption reaction of TC4 alloy is 79.42 kJ/mol by solving the Arrhenius equation. δ hydride and α" martensite appear in the microstructure of hydrogenated TC4 alloy.

Abstract:The 6005A-T5 aluminum alloy welded joints are prepared by use of friction stir welding (FSW) at different welding speeds. The microstructure and mechanical properties of these joints have been investigated. The relationship between evolution of precipitates in different regions and the mechanical properties of welded joints is established. The β″ phases completely dissolve back into the aluminum matrix due to enough welding heat input in nugget zone (NZ) during the welding processing. GP zones are formed during the subsequent natural aging, which results in the hardness recovery of NZ. Incomplete recrystallization occurs in the ther-mo-mechanically affected zone (TMAZ), and the grains are elongated with high dislocation density. The heat affected zone (HAZ) contains Q" and β″ phases. With the decrease of welding speed, β″ phase gradually disappears and Q" phase is formed, which leads to a decrease in the strength of welded joints. The average value of longitudinal residual stress is higher than that of transverse residual stress. With the increase of welding speed, the peak longitudinal residual tensile stress increases, but the effect on the transverse residual tensile stress is negligible.

Abstract:Analysis the contribution of constituent phases to mechanical properties is critical to design the microstructure of titanium alloys. For this reason, the microscopic stress and strain in soft primary α (α<sub>p</sub>) and hard transformed β matrix (β<sub>t</sub>) are quantitatively analyzed by a microstructure-based finite element model to determine their contributions to the strength and ductility of Ti-6Al-2Zr-1Mo-1V alloy. The results show that microscopic stress in both α<sub>p</sub> and β<sub>t</sub> shows a normal distribution. The peak stress and stress peak height of β<sub>t</sub> are much larger than that of α<sub>p</sub>. While, α<sub>p</sub> has large peak strain and β<sub>t</sub> has large strain peak height. As α<sub>p</sub> volume fraction decreases from 49% to 12%, the contribution of β<sub>t</sub> to ultimate tensile strength (UTS) and failure strain (FS) of the alloy increases from 59% to 91% and from 36% to 75%, respectively. However, as the contribution of β<sub>t</sub> increases, UTS of the alloy increases 17% and FS decreases 21%. This finding quantitatively reveals the contribution of constituent phases to strength and ductility and provides a basis to design the microstructure of titanium alloys.

Abstract:A β titanium alloy Ti-6Mo-5V-3Al-2Fe(wt.%) was designed in terms of d-electron alloy design method. Aging treatment was performed at various temperatures ranging from 450℃ to 600℃ for 4h to study the effect of aging temperature on microstructure evolution and tensile properties. The results show that the secondary α phase with smaller size and inter-particle spacing formed under ω-assisted nucleation mechanism at the aging temperature of 500℃. The highest ultimate tensile strength of 1510MPa is obtained due to the strengthening of fine acicular secondary α phase within β grain, while poor elongation of 4.6% is found as a result of the inevitable precipitation of α phase at grain boundary and the formation of precipitate free zone near grain boundaries. Fine secondary α precipitates tends to coarsen with the increasing aging temperature. Coarse α precipitates can bring about broad inter-particle spacing and can result in less α/β interfaces that act as effective dislocation barriers. The increase of aging temperature leads to the variation of tensile properties, i.e. the strength decreased while ductility changed in opposite way. A considerable improvement of elongation to 12.2% is achieved by increasing aging temperature to 600℃, in association with the formation of parallel secondary α laths near β grain boundaries and broad inter-particle spacing of secondary α phase within β grains.

Abstract:The effect of graphene doping on the microstructure and superconducting properties of MgB<sub>2</sub> bulks has been examined in comparison with the case of un-doped MgB<sub>2</sub>. The correlations among annealing temperatures, microstructures and superconducting properties in graphene doped MgB<sub>2</sub> bulks were investigated. The phase, microstructure and superconductivity of MgB<sub>2</sub> were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and superconducting quantum interference device (SQUID). It is found that the grapheme doping results in an obviously improvement of the critical current density. The highest critical current density reaches 1.8×10<sub>5</sub>A/cm<sub>2</sub> at 20 K and 1 T.

Abstract:The sufficient and stable evaporation of metal precursors is crucial to obtain reproducible YBa₂Cu₃O_7-δ (YBCO) film with high quality in MOCVD technique, and a practicalSSwaySis needed to evaluate the thermal stability of metal precursors in evaporation. In this work, the thermal properties and decomposition kinetics of three precursors with different evaporation characteristics (Y(DPM)₃, Cu(DPM)₂, Ba(DPM)₂) were comparatively investigated by non-isothermal thermogravimetric analysis, and the apparent activation energy (E_a) of evaporation process was evaluated by the Ozawa, Kissinger and Friedman methods. The results show that the decomposition reaction of Ba(DPM)₂ is indeed concomitant with its evaporation process and highly sensitive to the change of temperature. In addition, the consistent results from TGA-DSC and the kinetic analysis can also confirm that evaporation kinetics study is suitable for investigating the thermostability of metal-organic compound in MOCVD process.

Abstract:The influence of heat-treatment schedule including sealing temperature and holding time on glass-to-metal sealing behavior in terms of average pore diameter and porosity have been examined in detail by optical microscope. With increasing sealing temperature, the average value of pore diameter firstly increased, then decreased, at last increased. However, the porosity increases at first and appears constant above 980 ℃. Moreover, as the holding time prolonged, the average pore diameter increased gradually at first, and then decreased rapidly. While the porosity increased greatly, then increased slowly and decreased rapidly with increasing holding time. Additionally, increases of both sealing temperature and holding time resulted in the increase of the thickness of bubble-free layer. A spatially varying bubble structure has been presented relating profiling to processing conditions.

Abstract:Due to high thermal stability and purely oxide ionic conductivity, yttria-stabilized zirconia (YSZ) is the most commonly used electrolyte material for solid oxide fuel cell (SOFC). Standard electrolyte preparation techniques for planar SOFC comprise wet ceramic techniques like tape-casting or screen printing, requiring sintering steps at temperatures above 1300 ℃. Plasma spray-physical vapor deposition (PS-PVD) as a novel technique can provide a more rapid and cost efficient method to produce nano-structured electrolyte layer without sintering. High-temperature sintering requires long processing time and can lead to oxidation of metal alloys used as mechanical supports, or to detrimental inter-reactions between the electrolyte and adjacent electrode layers. Besides, the PS-PVD is different from traditional technique atmospheric plasma spray (APS) for various deposition mechanisms. Through this novel method, dense thin 7YSZ electrolyte layers with nano-structure are fabricated based on vapor deposition in PS-PVD processing. The 7YSZ electrolyte layer with a thickness of 8.7～12.3 μm was prepared, and its gas permeability can achieve 2.24～2.29 10-8 cm4gf-1s-1.

Abstract:Tailored production of titanium dioxide thin films (TTFs) are widely used in green energy field due to their outstanding advantages. In this work, the TTFs wereSprepared by the electron beam evaporation system and annealed in air and H₂ respectively. With excellent stability, samples annealed in H₂ maintain their pale yellow in the air for nearly a year. The structure, morphology, spectral and photocatalytic properties of TTFs have been observed and discussed detailedly. Titanium oxide films annealed in H₂ have better catalytic performance than those annealed in air. The results show that Ti₃₊ can improve photocatalytic efficiency since the electronic structures of TTFs annealed in H₂ can be finely tuned by the doped Ti₃₊. The existence of a Ti₃₊ induced an electronic band below the conduction band and narrows the band gap.

Abstract:The NbMo solid solution (denoted as (Nb,Mo)ss hereafter) matrix ceramic composites reinforced with 15vol.%, 30vol.%, 45vol.%, and 60vol.% ZrB2 particles were fabricated by hot-pressing sintering at 2400°C. The effect of the ZrB2 content on the isothermal oxidation resistance and the oxide scales microstructure evolution exposed at 800°C, 1000°C, and 1200°C was investigated. The experimental results showed that both temperature and ZrB2 content have an influence on the oxidation behavior. The oxidation resistance of ZrB2-(Nb,Mo)ss composites increased with increasing ZrB2 content and decreased with increasing oxidation temperature from the view point of parabolic rate constant. The oxide scales at 800°C-1000°C contained special film-like Nb2Zr6O17, acting as a barrier for oxygen diffusing inwards and resulting in low parabolic rate constant. However, no Nb2Zr6O17 layer was observed at 1200°C because the volatile MoO3 and the volume effect of ZrO2 destroyed the Nb2Zr6O17 protective layer which resulted in severe spallation and poor oxidation resistance. Possible oxidation mechanisms at different temperatures with varying ZrB2 content were discussed and related to the observed oxide morphologies.

Abstract:Phase transformation kinetics of Ti-1300 alloy after solution treatment were investigated under isothermal conditions in the temperature range of 400 to 700℃ by using dilatometric method. The decomposition kinetic equation of metastable β phase in the alloy was constructed under isothermal conditions by analyzing the experimental data in the theoretical frame of the Johnson-Mehl-Avrami theory. The different values of k and n parameters were obtained in the temperature range of 400 to 420℃ and 500 to 700℃, indicating that the mechanism of the phase transformation was different in different temperature ranges. The decomposition mechanism of metastable β phase is β_m→β′+β→α+β at the temperature of 400 to 420℃, whereas the decomposition mechanism of metastable β phase is βm→α+β at the temperature of 500 to 700℃. When the metastable β phase of Ti-1300 alloy was annealed isothermally at a given temperature, the amount of α phase increased firstly, and then reached a stable value with increasing the holding time. Based on calculation and experimental results, time-temperature-transformation diagrams (TTT diagram) of Ti-1300 alloys were plotted for the metastable β phase decomposition under isothermal conditions from 500 to 700℃. And the nose temperature of the TTT diagram of the alloy is around 600℃ for the Ti-1300 alloys.

Abstract:In order to understand the nanoscale cutting characteristics of single crystal germanium and improve the optical surface quality of the nanoscale germanium, three-dimensional molecular dynamics (MD) simulations were carried out to study the contact and sliding processes between diamond points and surfaces of single crystal germanium. The material deformation, cutting force, chips pile-up, surface morphology size, and the sliding friction process were investigated. The simulation results show that the cutting force, surface morphology size, and chips plie-up, increase during the contact process with increasing vertical force, and there is no obvious correlation with the cutting speed. The fundamental reason for the fluctuation of the cutting force in the cutting process is caused by the generation of dislocation and the energy fluctuation caused by the destruction of the lattice of single crystal germanium. In order to verify the correctness of the simulation results, nanometer cutting experiments on single crystal germanium were carried out using nano-scratch tester. The experimental results are in agreement with the simulation results, which verify the correctness and effectiveness of the MD model.

Abstract:Stress corrosion cracking (SCC) in nickel-based alloys is one of the most important potential safety hazards in primary circuit of nuclear power plants. Considering the randomness of physical parameters and based on the theory of oxide film rupture, the dispersion law of mechanical properties at SCC tip of nickel-bases alloys is studied. To improve the efficiency of numerical analysis with random parameters, combining with the advantages of MATLAB and sub-model technology of ABAQUS, MATLAB is employed in the secondary development for ABAQUS. With the help of finite element numerical simulation and Latin hypercube sampling method, Effect of random parameters such as Young’s modulus, yield strength on the stress and strain of the oxide film region and the base metal region is investigated. Meanwhile, the feasibility of the method is verified. The results show that the influence of randomness should not be ignored, the randomness of yield stress has the greatest influence on the dispersion of stress at SCC tip, and the randomness of Young’s modulus has the most significant effect on the plastic strain dispersion at SCC tip.

Abstract:The study of the manufacture of niobium/316L stainless steel transition joints can lead to a new breakthrough in improving the material for the helium vessel of the superconducting radio frequency cavities. In this study, 316L stainless steel and niobium composite plates were fabricated by explosive welding technique. Microstructure and mechanical properties of the composite plates were investigated both immediately after explosive welding and after heat treatment processes. The investigation of microstructure demonstrated that no brittle intermetallic layer was formed and no diffusion phenomenon was observed after heat treatment processes. Mechanical tests including tensile tests, shear tests and bending tests were conducting both at room temperature and cryogenic temperature. At low temperature, the composite plates held higher strength than at the room temperature. In cryogenic tensile tests, we observed that the Niobium section of the composite plate broke in layers when the sample reached yielding strength. Beside the raw material, transition joints with Nb-SS adapters were designed and fabricated. The leak check showed the joints’ leak rates meet project requirements and indicated explosively bonded Nb-SS is viable way to fabricate stainless steel helium vessel for SRF cavities.

Abstract:In order to study the microstructure evolution of Al2O3 ceramics coatings during cathodic plasma electrolytic deposition (CPED) process, Al2O3 coatings were fabricated via CPED technique on prepared TiAl alloy in Al(NO3)3 electrolyte with different time. Microstructure, morphology and chemical compositions of coatings were analyzed by scanning electron microscopy (SEM) with energy–dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X–ray diffraction (XRD), and heat–resistance of TiAl alloy substrate and CPED coating were tested. The results indicated that the growth process was divided into three stages, the initial stage, the medium stage and the late stage. The barrier layer was broken down and the coating grew slower in the initial stage, and with uniform and good combination with the substrate. The CPED coating grew stable and more rapid in the medium stage, and the crystallinity of the CPED coating was improved. In the late stage, contained 87.5 % α–Al2O3, the main phase and 12.5 % γ–Al2O3, the CPED coating grew slower, and the loose and porous outer coating was fabricated.

Abstract:Based on the in-phase thermomechanical fatigue experiments under stress control, the damage mechanism of in-phase thermomechanical fatigue of DD6 superalloy is investigated. Based on the continuous damage mechanics, a lifetime prediction model reflecting the coupling of creep damage and fatigue damage is established, and the damage parameters in the model are obtained through using the experiment data of creep and fatigue. Furthermore, the experiments of in-phase thermomechanical fatigue with dwells of DD6 superalloy are conducted in this paper, and the experiment lifetimes are within a factor of 2.3 of the prediction lifetimes based on the coupling damage, which indicates that the coupling damage based lifetime prediction model developed in this paper could accurately predict the in-phase thermomechanical fatigue lifetimes of DD6 superalloy and provide a basis for the lifetime prediction of DD6 superalloy structure in engineering applications.

Abstract:The grain orientation and grain shape changes of commercial pure titanium after low rolling deformation were observed using scanning electron microscope and backscattering electron diffraction techniques, and the actual active slip systems during deformation process were analyzed statistically. The mechanical interaction between grains and its influence on the slip systems selection and orientation evolution of grains were studied. The active slip systems and orientations’ evolution of rolling grains were simulated using Sachs model and Reaction Stress model. The results show that the plastic strain of grains in the polycrystalline commercial pure titanium is not in accordance with Taylor deformation principle, and the Sachs model can partly reveal the mechanism of slip and trend of the orientation evolution. The plastic deformation of grains depend not only on the external stress, but also on the reaction stress between grains, which influence the selection of slip systems. The Reaction Stress model based on the grains’ interaction can be used to reveal the activation process of the slip systems more comprehensively, and to predict the grain orientation after deformation more accurately. The reaction stress between grains is influenced by many factors, among which, the orientation of grains and its relation to the grain orientation have important impact on the magnitude of the reaction stress.

Abstract:The high temperature flow behavior of TC31 titanium alloy at the 840~960℃ 0.0001~0.1s_^-1. and the effects of the deformation temperature, strain rate and strain on flow stress and microstructure were studied, respectively. the Arrhenius constitutive equation of TC31 titanium alloy was established and verified. The results show that when the temperature is lower than 880°C and the strain rate is higher than 0.01s_^-1, the material exhibits obvious dynamic softening. When the temperature is higher than 920°C and the strain rate is lower than 0.001s_^-1, the coarse grain leads to flow stress increasing. It is indicated that the strain, strain rate and deformation temperature all have different effects on grain size, shape and phase content. In addition, it is confirmed that the strain-corrected Arrhenius constitutive equation has higher prediction accuracy, and its MSE is 2.443 and the R value is 0.9675.

Abstract:The porous metal structure represented by aluminum foam and metal lattice has wide application potential in the field of lightweight structural load due to its light weight, high strength (specific strength, specific stiffness, energy absorption rate) and multifunctional design. However, the low strength of foamed aluminum and the buckling failure of metal lattices have largely limited their engineering applications. In this paper, three kinds of double-layer metal lattice structures with different geometries are obtained by laser welding, and then a closed-cell aluminum foam is filled into the pores to obtain a new foam aluminum-filled double-layer lattice structure. The experimental and finite element simulation methods are used to study the bearing capacity, energy absorption characteristics, mechanism and deformation failure mode under quasi-static surface compression load. The results show that the filling of aluminum foam can effectively change the post-buckling behavior of the hollow lattice structure, improve the buckling stability of the lattice core unit, and have obvious coupling enhancement effect, which is manifested in the significant increase of bearing capacity and energy absorption efficiency.

Abstract:The dilatometry behavior and microstructure of the α+β→β phase transformation in the Ti6Al4V-0.55Fe alloy were investigated during continuous heating process. Four heating rates (1, 5, 10, 15 K/min) are been applied for the dilatometry behavior research. For the α+β→β phase transformation curve, the S-shaped pattern under the different heating rates indicates that the α+β→β phase transformation is a nucleation-growth-controlled process. The mean phase transition activation energy E was obtained by the Kissinger-Akahira-Sunose (KAS) method equals to 200 kJ/mol. By calculating the corresponding Avrami index n with the increase of the β phase volume under the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model, we can divide the Avrami index n into three stages which including the initial phase transformation stage (0

Abstract:In this paper, the oxidation and combustion characteristics of Mg-Al-Gd alloy with the 2/8 mass ration of Mg/Al have been studied. The oxidation and combustion process as well as the reacted and unreacted particles of the alloy have been analyzed by using TG-DSC, SEM, XRD, EDS and high-speed camera. The results show that the ignition temperature of Mg-Al-Gd alloy in air is about 487.6 ℃, which is lower than the ignition temperature of Mg-Al alloy. Oxidation of the Mg-Al-Gd alloy in TG-DSC experiments is a staged process：The first stage is the oxidation of Mg and the second stage is the formation of MgAl2O4. When burning in air, a thin compact oxide layer will be formed firstly on the particle surface. This layer can hinder the solid or liquid phase oxidation and promote the gas phase combustion of the alloy. The combustion products of the Mg-Al-Gd alloy in air mainly contain MgO and AlN, MgAl2O4 has not been detected in the products.

Abstract:The special two-phase micro-structure of nickel-based single crystal superalloy makes it anisotropic. Creep tests of three different orientations of the single crystal superalloy DD6 were carried out under 980℃, indicated that the creep failure of superalloy is the initiation of micropores and the propagation of microcracks, which is caused by dislocation motion. Transmission electron microscopy (TEM) was used to observe the dislocation morphology of monocrystals in [001], [111] and [011] orientation at the initial stage of creep, found matching the characteristics of the octahedral sliding system activated, the hexahedral sliding system activated and the simultaneous motion of the two sliding systems respectively. Based on the crystal plasticity theory, the creep constitutive model and creep damage model?under variational stress conditions were established with Orowan effect and dislocation blocking effect considered, meanwhile, the model parameters were fitted according to the creep curve obtained from the test. Moreover, the finite element simulation results of the model and creep fracture morphology of monocrystalline materials mutually confirm and explain the anisotropic behavior of monocrystalline creep.

Abstract:The hot compression deformation of GH79 superalloy were investigated systematically on a Gleeble-3800 thermal simulating tester. Based on the hot compression test behavior and mechanism on high temperature deformation behavior and microstructure evolution, the variation of strain rate sensitivity exponent m, activation energy Q and grain size exponent P values at different strain rates and different temperatures were obtained. The dynamic DMM hot processing map based on different instability criteria was drawn while the deformation mechanism maps incorporating dislocations the inside grains were obtained. The processing map was used to analyze the processing instability regime and appropriate processing regime. With the deformation mechanism maps, the Burgers vector compensated grain size, the dislocation evolution rule at modulus compensated flow stress and the modulus compensated stress and the dislocation quantities of GH79 superalloy at different temperatures, the hot deformation mechanisms were elucidated.

Abstract:The ignition temperature and the diffusion combustion conditions of TC4 titanium alloy were studied under the simulated compressor environment by the friction tests. The temperature and distribution characteristics of the combustion process were calculated by the finite element method. It was shown that rubbing rings of rotors were first burned, which dependedSon the heat accumulation of stators. For the first time, the ignition temperature(1266.47℃) of TC4 titanium alloy under the simulated compressor environment was obtained, and the error between the measured temperature and the ignition temperature(1210℃) calculated by the finite element method was just 3.6%. The stator began to burn when its total heating areas of cross section reached 25% and its areas of high temperature between 932℃ and 1210℃ were 3.06%.

Abstract:With the uniaxial cyclic loading/unloading test at room temperature, data of the plastic deformation, elastic recovery deformation and inelastic recovery deformation of polycrystalline beryllium samples with different elongation was obtained. Investigation on mathematical characteristics of the inelastic recovery was carried out, and corresponding expression functions were obtained. The result indicates that: after completely unloading, the relationship between inelastic recovery strain and plastic strain of beryllium can be described by the power function; even being incomplete unloading, their relationship can also be described by the power function; all the two function parameters are related to the unloading stress level in analytic form with the square polynomials,and the linear correlativity is presented between the two parameters；in the same unloading cycle, the relationship between inelastic recovery strain and plastic strain can be described by the square polynomials, at least two function parameters show the universal mathematical characteristics beyond specific samples; the modulus of elasticity (also maybe the elongation) is closely relevant to trends of change of all mathematical model parameters..

Abstract:In this paper, the high temperature oxidation resistance of YAG/8YSZ double ceramic and 8YSZ single ceramic thermal barrier coatings system were analyzed. The bond-coat (NiCoCrAlY) was deposited on a 310S heat-resistant stainless steel substrate by detonation gun spraying, and YAG/8YSZ double ceramic and 8YSZ single ceramic thermal barrier coatings were deposited on the bond-coat by air plasma spraying (APS), respectively. The cross-section and surface characteristics of the coating were analyzed by SEM and XRD before and after oxidation. The oxidative weight gain kinetics, microstructure, phase of the YAG ceramic layer, and the TGO growth process and growth kinetics of the two thermal barrier coating systems were comparatively investigated after isothermal oxidation at 1100 °C. The results showed that the YAG ceramic layer shows no obvious phase transformation after isothermal oxidation at 1100 °C for 200 h, and the porosity was slightly reduced. The oxidation weight gain rate and TGO growth rate of YAG/8YSZ double ceramic layers system was 1.7 times and 1.4 times lower than that of 8YSZ single ceramic layer system, respectively. The double ceramic layer system showed a low the β-NiAl phase consumption rate and the island oxide growth rate, which resulted in a better high temperature oxidation resistance.

Abstract:The morphology of submicron copper crystals was controlled by polyvinylpyrrolidone (PVP). The microscopic properties of copper crystals were characterized by SEM, XRD and laser particle size analyzer. The Materials Studio was used to combine Gibbs-Wulff"s law (crystal growth balance theory). It is speculated that the adsorption behavior of PVP molecules on the crystal faces of Cu and the morphology of copper crystals show that the copper crystals are transformed from truncated octahedron to nearly spherical after PVP modification. PVP mainly undergoes chemisorption on the crystal faces of copper. The order of adsorption is: (111)>(200)>(220). The morphology control mechanism of PVP on submicron copper crystals is that PVP forms a covalently unstable coordinating chelate with Cu, inhibiting the growth of (111), (200) and (220) crystal planes, revealing more crystal planes result in the Cu crystals becoming closer to a sphere.

Abstract:Because of its light weight, high strength and excellent corrosion resistance, titanium and its alloys have been currently used as a brand-new functional material in ships, aviation and other fields. However, it yet exists some defects such as poor surface weldability and wear resistance, especially under the dry-sliding conditions without solid lubricants, so it is essential to enhance its surface properties using surface modification techniques. Herein, this study focused on surface electroplating of nanocrystalline Ni coating on the anodized nanoporous surface of Ti substrate, as well as to achieve a metallurgical bonding interface. In view of this, the objectives of this study are to explore the detailed correlation between growth textures and wear resistance of Ni deposits by means of adjusting ultrasonic frequencies (shorted as “UF”) values. The textural directions and microstructures of the as-deposited Ni nanocrystals were characterized using XRD, FE-SEM and TEM. According to Nanoindentation and wear tests, the synergistic effects from ultrasonic oscillations on both grain refinement and crystallographic texture of Ni nanocrystals were evaluated. Besides the underlying mechanism involved in effects by ultrasonic oscillation on dynamic re-crystallization of Ni textures during the electroplating process, and the relationship between different textural directions of Ni crystals and surface properties were disclosed. The survey results manifested that the pre-adsorbed Pd atoms onto the as-anodized porous Ti surface were acted as nucleation sites for inducing the subsequent Ni growth, having an effective interfacial strength of Ti/Ni composites. Based on Nanoindentation results, it significantly improved its surface toughness by depositing Ni coatings onto the as-anodized Ti matrix, and the hardness and elastic modulus reached 15.6 and 197.2 GPa. Meanwhile its friction coefficient of Ti matrix within Ni coating at UF values of 45+80kHz was much lower, which was reduced by about 2 times relative to that of bare Ti matrix. Ultimately, the wear mechanism was transformed from the originally cutting wear behavior of Ti matrix into the sightly adhesive wear together with grinding abrasion for Ni deposits, effectively compensating the shortcomings of poor wear resistance for Ti alloys used in engineering applications.

Abstract:The thermal conductivity of germanene is calculated using the equilibrium and the non-equilibrium molecular dynamics simulations. Firstly, the thermal conductivity of germanene is simulated by the equilibrium method, and the components of thermal conductivity decomposition are further calculated. Unlike graphene, the thermal conductivity of germanene is small and the component is dominant. Secondly, the non-equilibrium method is used to simulate and calculate the thermal conductivity of a series of lengths of germanene. It is obtained that the thermal conductivity of the convergence with the non-dependent length by fitting. Finally, it is found that not only on the numerical results are consistent by comparing both the equilibrium and the non-equilibrium methods, and the simulation data of the equilibrium can be transformed into a length dependent relation by fitting the phonon group velocity, which can also be overlapped with the non-equilibrium data points. Therefore, we have determined that the thermal conductivity of germanene are effective and equivalent, which are calculated using the GPUMD package based on both the equilibrium and the non-equilibrium methods.

Abstract:Ferrite and carbonyl iron power are the major absorbing components of conventional radar absorbing materials (RAM). However, conventional RAM made of the aforementioned single-absorbing components cannot meet the comprehensive requirements of “thin, wide, light, and strong.” By analyzing the complementary relationship among ferrite and carbonyl iron power, a carbonyl iron coated core-shell nano composite absorbent system is established. In addition, by changing the deposition temperature to adjust the morphologies and absorbing properties of powders, excellent performance of the new core-shell structure absorbent is obtained. The nano carbonyl iron (CI) shell in situ grown on Ni_0.4Zn_0.2Mn_0.4Ce_0.06Fe_1.94O₄ (NZMCF) surface by metal organic chemical vapor deposition (MOCVD) for using as electromagnetic wave (EMW) absorbing materials. X-ray diffraction, scanning electron microscope and energy dispersive spectrometer analyses show that NZMCF-CI composites successfully prepared with a core-shell structure. Optimizing the weight ratio of CI would likely cause the composites to attain the EM parameters necessary in EMW absorbing materials, a minimum reflection loss (RL) was -39.9 dB to the corresponding thickness was 1.8mm, and the RL exceeds -10 dB from 3.8 to 18.0 GHz. The RL exceeds -20 dB from 3.2 to 18 GHz and -10 dB from 2.5 to 18 GHz for the absorber thickness from 0.8 to 2.6 mm, which covers almost the whole 2 to 18.0 GHz.

Abstract:In this paper, two morphological high-entropy alloy particles (Al0.25Cu0.75FeCoNi) were prepared by mechanical alloying. One is ellipsoidal particles (average particle size 53μm, no control agent), the other is flake particles (average particle size 15μm, with control agent). High-entropy alloy particles reinforced cast aluminum alloy (volume 5 vol.%) were prepared by squeeze casting. The influence of different reinforcement morphology on the microstructure and mechanical properties in composite was analyzed. The results show that in preparation for precast block, ellipsoidal ones are easily mixed uniformly with aluminum powder, while the flake are prone to agglomeration. The tensile strength of the ellipsoidal and the flake particles reinforced composites reached 162 MPa and 174 MPa, respectively, which was 12.5% and 20.8% higher than that of matrix alloy, but the elongation was significantly lower. Fracture analysis showed that the fracture of ellipsoidal particles reinforced composite was dominated by the tear of the matrix, while the flake particles reinforced composite was dominated by the rupture of agglomeration.

Abstract:Carbon fiber reinforced aluminum matrix composite is a very attractive material for the aerospace technologies. However, the poor wetting ability and the chemical reaction between the carbon fibers and aluminum make some difficulties for the fabrication of such composite. In order to improve the wetting behavior and refrain the chemical reaction, to prepare an interphase between carbon fiber and aluminum matrix is essential. In this paper, conventional electroplating equipment was improved to deposit copper interphase on the carbon fibers. It was found the improved equipment could effectively prepare a homogeneous interphase on carbon fibers with the help of plating additives. On this basis, the influence of fiber pretreatment, PH value of electroplating solution and plating time on the quality of copper interphase was investigated. The scanning electron microscopy and X-ray diffractometer were applied to characterize the interphase.

Abstract:In this work, the effects of adding trace Sr elements on microstructure, mechanical properties and corrosion properties of Mg-0.2Zn-0.1Mn-xSr(x=0, 0.1, 0.2, 0.3 wt. %) alloys were investigated. The result of microstructure observation reveal that the grain size of the alloy decreases with the increase of Sr. The granular Mg17Sr2 phase is uniformly dispersed in the magnesium matrix. While, the second phase grows up as the Sr increases. The result of mechanical properties investigated by tensile test at room temperature indicated that micro Sr can improve the yield strength and tensile strength. But the elogations decreased with the increase of Sr. Degradation was studied using immersion tests in Kokubo solution. The corrosion rates were faster and the pitting was more likely to occur when the Sr increased. The average corrosion rates of Mg-0.2Zn-0.1Mn-xSr(x=0, 0.1, 0.2, 0.3 wt. %) that measured by weight loss were 6.85→6.01→6.80→7.52mm/a. Trace Sr can improve the corrosion resistance of magnesium alloys. However, with the increase of Sr content, the magnesium alloys are more prone to pitting and intergranular corrosion, which in turn reduces the corrosion resistance of magnesium alloys. The bio-corrosion behaviors can be attributed to the grain refinement and the diffused second phase, which can promote the formation of corrosion product film. However, the bigger second phase in the matrix will accelerate the local corrosion，which will reduce the corrosion resistance of magnesium alloy. The results show that Mg-0.2Zn-0.1Mn-0.1Sr has the best mechanical properties and corrosion resistance.

Abstract:Pulsed electron beam welding is an advanced welding technique in which the beam current is modulated into a pulsed square wave. Compared with the conventional continuous electron beam welding, pulsed electron beam welding can increase the evaporation rate of the welded metal, thereby improving the welding efficiency and increasing the weld depth-to-width ratio. In this paper, the welding experiment of 1.2 mm TC4 titanium alloy sheets were carried out by conventional continuous electron beam welding and pulsed electron beam welding with different frequency. Then, the microstructure and mechanical properties of the welded joint were tested. Results show that pulsed electron beam welding can form welds without fusion defects. As the frequency increases, the weld undercut and backside weld?reinforcement decrease and the surface formation is improved. Due to the change of welding thermal cycle, the pulsed beam flow can accelerate the cooling rate of the molten pool and refine the microstructure grains. Both continious and pulsed electron beam welded joints are broken in the base metal zone during tensile process, so the tensile strength is higher than that of the base metal. The high-frequency pulsed electron beam can improve the plasticity of the welded joint and the micro-hardness of the weld zone and the heat-affected zone. When the frequency is 10 kHz, the elongation after fracture of the welded joint can reach 14.9%, which is about 80% of the base metal, and the microhardness of the welding and heat affected zone are 375 HV and 368 HV, respectively.

Abstract:The fourth-generation 45GHz ECR source(FECR) is under developing in the institute of modern physics(IMP).The one core of the FECR is the sextupole Nb3Sn coil. To test the performance of the individual sextupole coil after the process of winding ,heat treatment and vacuum-impregnated,a Mirror structure based on a Bladder and Key technology during the magnet assembly is devised.In order to simulate the assembly procedure from bladder and key pre-stress at room temperature through cool-down and verity the proposed method with Ansys ,a dummy coil according to the size of the sextupole Nb3Sn coil is got.Test results of the strain in the middle of shell and dummy coil exhibits very good agreement with expected results calculated with the structural analysis program ansys,and the maximum error is less than 10%.The result verifies the rationality of the Mirror structure design and the credibility of simulation results.

Abstract:Al-Mg-Ga-Sn alloys with different Ga contents were prepared by electric furnace melting. Their microstructure morphology and compositions were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD); their degradation rates were measured in pure water at 30 °C,40 °C, 70° C, and 90° C; and their electrochemical performance at room temperature was tested using the electrochemical workstation. The results revealed that in case that Mg+Sn is 10 wt.%, Al-Mg-Ga-Sn alloys with 0 wt.%, 4 wt.%, 8 wt.%, 12 wt.%, and 16 wt.% Ga, respectively, all contain Al matrix phase and Mg<sub>2</sub>Sn phase. Moreover, Ga<sub>5</sub>Mg<sub>2</sub> phase appears with the increase in Ga content. The degradability of Al-Mg-Ga-Sn alloys primarily present start-up with pitting corrosion in aluminum matrix phase, and acceleration with intergranular corrosion between Mg<sub>2</sub>Sn and Ga<sub>5</sub>Mg<sub>2</sub> compound phases. The initial degradation temperatures of alloys with different Ga content depend on the content of low melting point elements (Ga+Sn) solved in the aluminum matrix; the alloy with the same Ga content rapidly degrade with elevating temperature, and the degradation reaction kinetics follows Arrhenius. Uz formula. Electrochemical analysis at room temperature indicated that as Ga content raises in Al-Mg-Ga-Sn alloy, the corresponding corrosion potential negatively shifts to different extent, while the corrosion current gradually improves.

Abstract:The effects of Nd on the microstructure and mechanical properties of Mg-13Gd-0.5Zr alloy were studied by X-ray diffraction, optical microscope, scanning electron microscope, energy dispersive spectrometer and electronic tensile testing machine. The mechanism of grain refinement is discussed based on the mismatch theory and the change rule of dislocation density. The strengthening mechanism is also discussed from fine-grained strengthening and precipitation strengthening. It is found that the main constituent phases of Mg-13Gd-0.5Zr alloy are α-Mg and Mg₅Gd. The addition of Nd forms a new phase Mg₄₁Nd₅ in the alloy and fines the grain of the alloy. The addition of Nd significantly improved the mechanical properties of Mg-13Gd-0.5Zr alloy at room temperature and high temperature. When the addition of Nd is 2%, the mechanical properties of the alloy reach the maximum values of 279 (room temperature) and 319 MPa (250 C) at room temperature and high temperature. The improvement of mechanical properties of the alloy is mainly attributed to the dual effects of precipitation strengthening and fine grain strengthening of Mg₅Gd and Mg₄₁Nd₅ phases. Brittle fracture is the main fracture mode of Mg-13Gd-2Nd-0.5Zr alloy at different temperatures. As the stretching temperature increases, the alloy changes from brittle fracture to ductile fracture.

Abstract:The effects of unidirectional and reversal hot rolling on the microstructure and mechanical properties of TC4 titanium alloy ingot melted by electron beam cold bed furnace were studied. The results show that the microstructure of TC4 titanium alloy slab is Widmannstatten structure. After hot rolling plastic deformation, the grain boundary is fully broken, and the α phase bundle is twisted, deformed and broken, and it is criss-crossed and distributed in a basket shape. With the increase of the number of reversing hot rolling, the lath-like α phase cross-section is more obvious, which reduces the anisotropy in the rolling direction and the transverse direction, and the comprehensive performance of the reversing secondary hot-rolled sheet is optimal. The fracture transformation forms of TC4 titanium alloy are unidirectional hot-rolled RD ductile fracture, TD ductile + quasi-cleavage mixed fracture reversed one-time hot-rolled RD ductile + brittle mixed fracture, TD ductile + quasi-cleavage mixed fracture reversed two-time hot-rolled ductile fracture. The hot-rolled sheet is mainly a matrix phase of α-Ti and β-Ti. The grain orientation of the unidirectional hot-rolled sheet has a preferred orientation in the <0001> direction; the grain orientation of the reversing once hot-rolled sheet is mainly between <0001> and<"1" ?21 ?0>crystal orientation; the grain orientation of the sheet moves toward <01"1" ?0>, and its grain orientation is between <0001> and <01"1" ?0>. Key words: Electron beam cold bed furnace; TC4 titanium alloy; reversing hot rolling; microstructure; mechanical properties

Abstract:Ti-6Al-4V alloy was nitrided in nitrogen atmosphere using diode laser, in order to study the influence of line energy, laser power and scanning speed on the surface morphology, molten pool geometry, width, the penetration depth and the forming coefficient of the nitrided layers, the surface and cross-section pictures of the nitrided layers were taken by optical and scanning electron microscopy, and the hardness was measured by microhardness tester. The results show that the surface morphology are composed of the smooth and rough surface, When the surface temperature of the molten pool is low, increasing the interaction time has little effect on the surface morphology. With the increase of laser power under the same line energy, the melting depth and width of the nitrided layer increases, and the forming coefficient decreases. As the scanning speed decreases under the same laser power, the penetration depth and width increases, and the forming coefficient reduces; When the forming coefficient is very small, the nitrided layers will produce longitudinally distributed crack. The convection form and the strength result in the formation of a hemispherical interface, a finger interface and a W-shaped interface and so on, besides, multiple inflections happen in the slope of the boundaries of molten pool.

Abstract:The Cr-12Nb-4.4 Ni alloy, prepared by mechanical alloying and hot pressing, was thermally exposed at 1200℃ for 30, 50 and 100 h, respectively. Then, the microstructure and properties of the thermally exposed Cr-12Nb-4.4Ni alloy were studied. The results show that the phase constituent is stable during the thermal exposure process. It consists of Cr solid solution (Crss) and NbCr2, and Ni is mainly presented in NbCr2. Grain size of Crss increases with the thermal exposure time, while grain size of NbCr2 particle has no obvious change. The NbCr2 structure becomes loose due to the replacement of Cr by Ni. The increase of compressive stress in the phase interface of Cr/NbCr2 promotes the emergence of dislocations in Cr matrix and the increase of stacking fault/ twinning fault in NbCr2 particles. With the thermal exposure time, the compressive strength, yield strength and plastic strain of the Cr-12Nb-4.4 Ni alloy decrease slightly while the alloy still remains relatively high room temperature strength and good plastic during the thermal exposure process. After thermal exposure for 100h, the compressive strength, yield strength and plastic strain are 2170MPa, 1406MPa and 9.5%, respectively. The Cr-12Nb-4.4 Ni alloy can keep good fracture toughness after thermal exposure for 100h.

Abstract:It is studied that effect of temperature, passes, thinning rate and feed ratio for alien piece with large aspect ratio during spinning formed, though processing test and analysis of microstructure and mechanical properties. It is respectively optimized that characteristic temperature is 370~420 ℃, deformation passes is 10~12 passes, thinning rate both every pass is 20~25 % and whole process is 35~50 %, and feed ratio of ordinary spinning is 2.0~2.5 mm/r, and feed ratio of strong spinning is 1.3~1.8 mm/r. It is achieved that alien piece with large aspect ratio spinning formed directly from sheet material which is undergoed only one time card loading. The process aboved not only improved production efficiency significantly, but also simultaneously fufil requirements of controling shape and properties. The wall thickness difference of alien piece with large aspect ratio is no greater than 0.2 mm, and the unilateral gap between inner profile and theoretical model is not higher than 0.1 mm. Relatived to the original plate, the ultra tensile strengrh and elongation of alien piece with large aspect ratio basically unchanged, and the yield strength increased 10.1 %.

Abstract:In recent years, due to the unique physical and chemical effects of ultrasonic waves, it has been widely used in the welding process. High-quality joints were obtained with the ultrasonic vibration was applied during the resistance brazing process. To date, however, little studies have been reported on the microstructure evolution mechanism of joints. In the present study, a Zn-Al alloy was used as filler metal to braze 6063 aluminum alloy using ultrasonic-assisted electrical resistance-brazing technology. The effects of ultrasonic field and electric field on the microstructure evolution of the joints were studied, and its mechanism was also analyzed. The results showed that applying ultrasonic vibration to substrate during brazing process could effectively promote the effective connection between the filler metal and base metal, reduce the defects, improved homogeneity and have obtained the joints with good metallurgical bonding. In addition, both ultrasonic power and electrical current intensity have significant effect on the dissolution during the brazing process. With the increment of ultrasonic power, the dissolution of the base metal was intensified, the Al content in the brazing seam increased, and the eutectoid α-Al phase increased. As the electrical current increases, the primary α-Al phase increases.

Abstract:The PEDOT:PSS/Bi0.5Sb1.5Te3 powder was prepared by a solution phase process and grounding in liquid N2. Then the powder was densified by hot press. The polymer composite powder was reformed to polymer bulk. The XRD and FESEM were used to check the phase structure and microstructure. The electrical transport properties and thermal conductivity were measured. The results showed that 90 wt% Bi0.5Sb1.5Te3 with PEDOT: PSS specimen got the highest ZT value of 0.1 at 75 ℃ which is 40 times higher than that of pristine PEDOT: PSS specimen. The Seebeck coefficient was greatly improved with the increase of Bi0.5Sb1.5Te3. The continuous distribution of inorganic phases is the key to improve Seebeck coefficient of composites.

Abstract:Characteristics of microstructure, inclusions, Prior Particle Boundary , Thermally Induced Porosity , defects and several large size disk′s tensile properties, stress rupture life ,low cycle fatigue of a P/M superalloy FGH97, which was made by Argon Atomization (AA) + Hot Isostatic Pressing (HIP)process, were investigated. The results reveal that the grain size of FGH97 material prepared by AA + HIP +Heat Treatment is ASTM 8-9. No continuous network PPB is found, and there is a very small amount of TIP. The size of mostly square γ' is about 200 nm, and grain boundary γ' is 1.5 to 2 μm. No coarse γ' phase and coarse carbides are observed. The average stress rupture time of 650 °C / 980MPa is 449 h. The maximum low cycle fatigue life is 258,909 cycles at 650 ℃, the average cycle reaches 19,014 cycles. The room temperature, 650℃, 750 ℃tensile properties, stress reputure performance at 650 ℃ / 980 MPa and the fracture life of Low cycle fatigue at 650 ℃ meet the FGH97 alloy technical requirements.

Abstract:In this paper, TiAl alloys with a nominal composition of Ti-45Al-8Nb (at.%) were prepared by spark plasma sintering (SPS) process. The effect of sintering temperatures on microstructure and mechanical properties of Ti-45Al-8Nb alloy has been studied. The results show that the microstructures of Ti-45Al-8Nb alloy sintered at different temperatures all exhibit full lamellae structure. With the increasing of sintered temperature, the amount of γ phase in the alloy increases and α₂ phase decreases. There is no B2 Phase when sintered at 1250℃. As the sintering temperature increases, the B2 phase precipitates at the grain boundary and the amount of B2 phase increases. The main reason is that the alloy may melt at local areas with the increasing of sintering temperature; the β phase would order to B2 phase at high cooling rates. The tensile strength and compression ratio decreases with the sintering temperature increasing from 1250℃ to 1300℃. The maximum tensile strength and compression ratio reached 2084.20MPa and 33.10% at 1250℃ respectively.

Abstract:The plastic deformation behavior of mechanical alloying powder at different ball milling times, microstructure and mechanical properties of hot press sintering Nb-35Ti-6Al-5Cr-8V alloys were investigated using powder metallurgy method. The results show that the Nb-35Ti-6Al-5Cr-8V powders with good plasticity were deformed into large-sized flakes powder firstly, and then broken into flocculent powder during continues process hardening as the increasing of ball milling time. The Nb-35Ti-6Al-5Cr-8V alloy with controllable microstructure and fine grain can be prepared using hot pressing sintering technology, and the sintered materials consist of single Nbss phase. The solid solution element of Ti, Al, Cr and V dissolving into Nb crystal causes the reduction of lattice size about 0.0685 ?. The Vickers hardness and the compression strength at room-temperature were increased significantly for grain refinement caused by ball milling time increasing, and this trend was in accordance with the Hall-Petch law of hardness and strength for material. It can be firmed that multi-mechanical properties of Nb-35Ti-6Al-5Cr-8V alloy prepared by powder metallurgy technology are obviously better than these prepared by fusion casting method.

Abstract:In present paper, NbTi/Cu wire the low copper-to-superconductor ratio of 1.3 and filament number of 630 was prepared using high homogenous Nb47Ti alloy bar and oxygen-free copper firstly. New type of aging heat treatment (“405℃/3h + 405℃/3h + 420℃/20h + 420℃/40h + 420℃/80h”) was proposed. Comparing with the traditional heat treatment, this new type heat treatment can largely improve the critical current density of NbTi superconducting wire to 3208A/mm2, with an increment of 17.5%. Only when the pre-heat treatment times is equal to or more than 2 times, huge of nucleation can be formed and critical current density can be improved obviously. In addition, Critical current densities of wire under three pre-strain conditions (4.64, 5.35 and 6.25) all get the peak value when final strain is between 5.0~5.2.

Abstract:NiTi shape memory alloys have received extensive attention due to their excellent functional properties, biocompatibility, damping properties, low stiffness, and corrosion resistance. However, the processability and machinability of NiTi shape memory alloys are of great difficulties. In the past 20 years, the additive manufacturing technology has been developed sharply, resulting in the direct fabrication for complex structure of NiTi shape memory alloys, which has a great potential value in the aerospace, medical equipment and other fields. This paper makes a comprehensive review of the key issues and solutions in the domestic and foreign NiTi shape memory alloys laser additive manufacturing researches, including comparison the traditional process manufacturing and additive manufacturing of NiTi shape memory alloys. Effect of laser additive manufacturing process parameters and post-heat treatment parameters on the microstructure, the mechanical properties and the functional properties of NiTi shape memory alloys. Based on research results, it is predicted that the future direction of NiTi shape memory alloys laser additive manufacturing.

Abstract:The concept of material genome engineering has attracted extensive attention from researchers in the field of materials at home and abroad. As an important part of material genome engineering, high-throughput material synthesis and characterization technology become the focus of research, and which can significantly improve the speed of materials research and development. In this work, the related concepts of material genome engineering are briefly introduced. A series of representative high-throughput synthesis and characterization techniques are introduced,and the research progresses in this field are reviewed. The main developing trend of high-throughput material synthesis and characterization technology is discussed in order to provide new ideas for the development of this field, as well as providing references for further development of material genome engineering technology.