Abstract:Brazing filler metals are widely applied, which serve as an industrial adhesive in the joining of dissimilar structures. With the continuous emergence of new structures and materials, the demand for novel brazing filler metals is ever-increasing. It is of great significance to investigate the optimized composition design methods and to establish systematic design guidelines for brazing filler metals. This study elucidated the fundamental rules for the composition design of brazing filler metals from a three-dimensional perspective encompassing the basic properties of applied brazing filler metals, formability and processability, and overall cost. The basic properties of brazing filler metals refer to their mechanical properties, physicochemical properties, electromagnetic properties, corrosion resistance, and the wettability and fluidity during brazing. The formability and processability of brazing filler metals include the processes of smelting and casting, extrusion, rolling, drawing and ring-making, as well as the processes of granulation, powder production, and the molding of amorphous and microcrystalline structures. The cost of brazing filler metals corresponds to the sum of materials value and manufacturing cost. Improving the comprehensive properties of brazing filler metals requires a comprehensive and systematic consideration of design indicators. Highlighting the unique characteristics of brazing filler metals should focus on relevant technical indicators. Binary or ternary eutectic structures can effectively enhance the flow spreading ability of brazing filler metals, and solid solution structures contribute to the formability. By employing the proposed design guidelines, typical Ag based, Cu based, Zn based brazing filler metals, and Sn based solders were designed and successfully applied in major scientific and engineering projects.

Abstract:Dopamine polymerization reaction and hydrothermal method were used to prepare nickel coated Al₂O₃ reinforcement phase (Ni/Al₂O₃). Ni/Al₂O₃ reinforced Sn1.0Ag0.5Cu (SAC105) composite solder was prepared using traditional casting method. The result shows that the nickel coating layer is continuous with uneven thickness. The interface between nickel and aluminum oxide exhibits a metallurgical bonding with coherent interface relationship. The strength, toughness and wettability of the SAC105 solder on the substrate are improved, while the conductivity is not decreased significantly. The fracture mode of composites transitions from a mixed toughness-brittleness mode to a purely toughness-dominated mode, characterized by many dimples. The prepared composite brazing material was made into solder paste for copper plate lap joint experiments. The maximum shear strength is achieved when the doping amount was 0.3wt%. The growth index of intermetallic compound at the brazing interface of Ni/Al₂O₃ reinforced SAC105 composite solder is linearly fitted to n=0.39, demonstrating that the growth of intermetallic compound at the interface is a combined effect of grain boundary diffusion and bulk diffusion.

Abstract:A static corrosion experiment of 347H stainless steel alloyed with elements Cu and Mo was carried out in a nitrate molten salt (60% NaNO₃+40% KNO₃) at 565 °C for 720 h. The effects of elements Cu and Mo on the corrosion resistance of 347H stainless steel in molten salt were investigated by analyzing the phase composition, microstructure and chemical composition of the corrosion products. The results show that the grain refinement induced by element Mo imparts the stainless steel with optimal corrosion resistance at a medium grain size. Furthermore, the formation of MoC significantly enhances the intergranular corrosion resistance of the stainless steel. The stainless steel exhibits uniform corrosion in the nitrate solution. The corrosion layer displays a dual-layer structure, and the corrosion products protecting matrix are present in both the inner and outer layers. The outer layer consists of a mixture of Fe oxides (Fe₂O₃, Fe₃O₄), NaFeO₂, and a minor amount of MgFe₂O₄. Conversely, the inner layer is primarily composed of a spinel layer (FeCr₂O₄, MgCr₂O₄) and a thin Cu₂O layer. The oxidation of Cu in the inner layer leads to the formation of a dense Cu₂O layer, effectively impeding O^2- plasma infiltration into the matrix.

Abstract:The microstructures and corrosion behavior of 1.0wt% Gd-containing neutron-absorbing duplex stainless steel annealed at different temperatures were studied. Results reveal that the content of Gd-containing secondary phase increases with increasing the annealing temperatures to 1080 ℃, and then decreases. In the sample annealed at 1080 ℃, M-Gd (M=Fe, Cr, Ni) intermetallic with M₃Gd as the core phase and M₁₂Gd as the shell is the primary secondary phase. In the sample annealed at 1140 ℃, M₃Gd phase is dominant. The corrosion behavior of the two annealed steel samples were analyzed in NaCl, HCl and H₃BO₃ solutions. It is found that the sample annealed at 1140 ℃ has lower corrosion rate. M₃Gd is more electrochemically active than M₁₂Gd when the sample is immersed in NaCl and HCl solutions, but more noble in H₃BO₃ solution.

Abstract:The microstructure, micro-hardness, and tensile properties of interface between hot isostatic pressing densified low alloy steel and Inconel 690 cladding were investigated during the aging process at 600 ℃. The results show that the interface region can be divided into four zones from base metal to deposited metal: carbon-depleted zone (CDZ), partial melting zone (PMZ), planar growth zone (PGZ), and brownish feature zone (BFZ). Dimensions of these zones do not significantly change during aging. However, type I carbides noticeably increase in size in the PMZ, and precipitates clearly occur in the PGZ. The main reason for their growth and occurrence is continuous carbon migration. The highest micro-hardness appears in the PGZ and BFZ regions, which is related to carbon accumulation and precipitates in these regions. Tensile failure occurs on the base metal side due to the high strength mismatch between these two materials. The CDZ, composed of only ferrite, has lower strength and fractures at the boundary between CDZ and base metal. The ultimate tensile strength decreases by only 50 MPa after aging for 1500 h, and the interface region maintains high strength without significant deformation.

Abstract:Ti(C, N)-Mo₂C-Ni cermet as alternative materials was explored for use in alkaline conditions, replacing the WC-Co cemented carbides, since Co is classified as a potentially carcinogenic substance and there is potential hazard of “hard metal disease” under the exposure to cobalt dust. The changes in microstructure, corrosion rate and volumetric loss rate of the two materials were compared under electrochemical corrosion and erosion-corrosion in alkaline environment. The results demonstrates that Ti(C, N)-Mo₂C-Ni cermet undergoes passivation when exposed to electrochemical corrosion of NaOH solution, resulting in a significant increase in oxygen content on the corroded surface. The corrosion rate of cermet is approximately one order of magnitude lower than that of the cemented carbide. Under the erosion-corrosion of an alkaline sand-water mixture, both the cermet and cemented carbide experience a gradual increase in volumetric loss rate with prolonging the erosion time. During erosion, the rim phase in cermet is fragile, so cracks easily penetrate it while the core phase remains intact. The medium-grained cemented carbide commonly demonstrates transgranular fracture mode, while in the fine-grained cemented carbide, cracks tend to propagate along phase boundaries. The erosive wear and damage caused by sand particles play a predominant role in the erosion-corrosion process of alkaline sand-water mixtures. This process represents an accelerated destructive phenomenon influenced and intensified by the combined effects of corrosion and erosion. It is confirmed that using cermet as an alternative anti-wear material to cemented carbides is feasible under alkaline conditions, and even better.

Abstract:The hot compression curves and deformed microstructures were investigated under various hot deformation conditions in three states: hot isostatic pressing (HIP, A1), HIP+hot extrusion at 1100 ℃ (A2), and HIP+hot extrusion at 1150 ℃ (A3). The results show that A2 sample, extruded at 1100 ℃ with uniform γ+γ′ duplex microstructures, demonstrates excellent hot deformation behavior at both 1050 and 1100 ℃. The true stress-true strain curves of A2 sample maintain a hardening-softening equilibrium over a larger strain range, with post-deformation average grain size of 5 μm. The as-HIPed A1 sample and 1150 ℃ extruded A3 sample exhibit a softening region in deformation curves at 1050 ℃, and the grain microstructures reflect an incomplete recrystallized state, i.e. combination of fine recrystallized grains and initial larger grains, characterized by a necklace-like microstructure. The predominant recrystallization mechanism for these samples is strain-induced boundary migration. At 1150 ℃ with a strain rate of 0.001 s^-1, the influence of the initial microstructure on hot deformation behavior and resultant microstructure is relatively less pronounced, and post-deformation microstructures are fully recrystallized grains. Fine-grained microstructures are conducive to maximizing the hot deformation potential of alloy. By judiciously adjusting deformation regimes, a fine and uniform deformed microstructure can be obtained.

Abstract:Compared with Cu/Al₂O₃ composites, high-strength Cu/Al₂O₃ composites usually exhibit obviously deteriorated electrical conductivity. A chemical and mechanical alloying-based strategy was adopted to fabricate ultrafine composite powders with low-content reinforcement and constructed a combined structure of Cu ultrafine powders covered with in-situ Al₂O₃ nanoparticles. After consolidation at a relatively lower sintering temperature of 550 ℃, high-volume-fraction ultrafine grains were introduced into the Cu/Al₂O₃ composite, and many in-situ Al₂O₃ nanoparticles with an average size of 11.7±7.5 nm were dispersed homogeneously in the Cu grain. Results show that the composite demonstrates an excellent balance of high tensile strength (654±1 MPa) and high electrical conductivity (84.5±0.1% IACS), which is ascribed to the synergistic strengthening effect of ultrafine grains, dislocations, and in-situ Al₂O₃ nanoparticles. This approach, which utilizes ultrafine composite powder with low-content reinforcement as a precursor and employs low-temperature and high-pressure sintering subsequently, may hold promising potential for large-scale industrial production of high-performance oxide dispersion strengthened alloys.

Abstract:The hot deformation behavior of electrolytic copper was investigated using a Gleeble-3500 thermal simulation testing machine at temperatures ranging from 500 °C to 800 °C and strain rates ranging from 0.01 s^-1 to 10 s^-1, under 70% deformation conditions. The true stress-true strain curves were analyzed and a constitutive equation was established at a strain of 0.5. Based on the dynamic material model proposed by Prasad, processing maps were developed under different strain conditions. Microstructure of compressed sample was observed by electron backscatter diffraction. The results reveal that the electrolytic copper demonstrates high sensitivity to deformation temperature and strain rate during high-temperature plastic deformation. The flow stress decreases gradually with raising the temperature and reducing the strain rate. According to the established processing map, the optimal processing conditions are determined as follows: deformation temperatures of 600–650 °C and strain rates of 5–10 s^-1. Discontinuous dynamic recrystallization of electrolytic copper occurs during high-temperature plastic deformation, and the grains are significantly refined at low temperature and high strain rate conditions.

Abstract:Laser beam welding was used to join a near-β titanium alloy (Ti-3Al-6Mo-2Fe-2Zr), followed by aging treatments. The relations among aging temperature, microstructure, and tensile properties of joints were revealed. For as-welded joints, the fusion zone features primarily single β phase. It is attributed to the high Mo equivalency of this alloy and the fast cooling rate in laser beam welding. After aging treatments, many α precipitates form in the fusion zone and heat affected zone. The rising aging temperature coarsens α precipitates and reduces the volume fraction of α precipitates. Compared with the as-welded joints, the aging treated joints'' tensile strength and elongation are improved. The increasing aging temperature weakens the strengthening effect because of the decreasing volume fraction of α precipitates. After the aging treatment at 500 °C for 8 h, the joints obtain the optimal match between strength and plasticity. The fracture mode of joints changes from quasi-cleavage fracture in as-welded condition to microvoid coalescence fracture after heat treatments.

Abstract:To investigate the original as-cast microstructure of 27Cr44Ni5W3Al+MA ethylene cracking furnace tube manufactured by centrifugal casting, the phase composition and microstructure of the as-cast furnace tube were analyzed using XRD, OM, SEM and TEM. The results show that the original as-cast microstructure of the furnace tube is mainly composed of austenite matrix (γ phase) and fishbone-like multi-phase carbides at grain boundaries; the inside of the multi-phase carbides is lamellar M₇C₃, while the edge is blocky M₂₃C₆. In addition, two shapes of Ni₃Al (γ′ phase) are observed, namely the granular phase distributed near the junction between M₂₃C₆ and matrix, and the blocky phase adjacent to M₂₃C₆. Both of M₂₃C₆ and granular γ′ have the cube-on-cube orientation relationship with the γ matrix ( , , ; , , . , , ; , , ). In addition, a large number of dislocations are observed in the as-cast microstructure, with sparse distribution away from M₂₃C₆ and dense distribution near M₂₃C₆. The precipitation mechanism of γ′ and M₂₃C₆ during centrifugal casting was also discussed. The W element reduces the lattice misfit between M₂₃C₆ and γ phase, thereby promoting the precipitation of M₂₃C₆. The chemical composition of M₂₃C₆ and γ′ phase exhibit complementarity, and the precipitation of M₂₃C₆ further facilitates the formation of γ′ phase.

Abstract:The laminated Ti/TiNb composites with diffusion layers were fabricated by spark plasma sintering combined with hot rolling. In-situ tensile test monitored by SEM was applied to analyze the crack initiation and propagation behaviour of the composites in different states, so as to understand the effects of Ti component thickness and diffusion layer microstructure on the fracture behavior of the composites. The results show that the length of microcracks can be effectively controlled by reducing the thickness of Ti component, thus delaying the extension of shear bands in adjacent TiNb layers. The microstructure of diffusion layer has a significant effect on the fracture behavior of the composite. Compared with the composite with “hard” diffusion layer, the composite with “soft” diffusion layer has more cracks and shear bands before tensile fracture, and shows a more tortuous crack propagation path after the overall fracture.

Abstract:Through the surface modification of graphene nanoparticles (GNPs) by ethyl cellulose (EC), combined with solution ultrasonic dispersion and wet ball milling, GNPs and aluminum matrix were uniformly mixed and GNPs damage was suppressed. Then, highly wear-resistant GNPs/AlSi10Mg composites were prepared by the suppression of interfacial reaction through rapid sintering with discharge plasma. The microstructure and wear-resistant properties of GNPs/AlSi10Mg composites were characterized and analyzed by scanning electron microscope, transmission electron microscope and friction testing machine. The results show that moderate addition of GNPs can effectively improve the mechanical properties of the composites. When the addition amount of GNPs is 0.5wt%, the wear rate and coefficient of friction of the composites are the lowest, which are 7.8×10^-4 mm³/(N·m) and 0.417, respectively. The wear rate is reduced by 28.4% compared with that of the matrix material (10.9×10^-4 mm³·N^-1·m^-1). The wear mechanism of GNPs/AlSi10Mg composites is mainly abrasive wear, accompanied by slight oxidative wear and adhesive wear. During the friction process, when the composite specimen is in contact with the dyad, GNPs emerge on the surface to form a thin film under the action of shear force, which can be used as a lubricant to reduce the contact point between the dyad and the matrix, preventing excessive spalling and delamination and thereby protecting the matrix.

Abstract:This research focused on two candidate materials (alloy A and alloy B) for superheater and reheater in high-parameter advanced ultra-supercritical power plants. It systematically analyzed the phenomenon of local grain boundary widening, the kinetics, and the mechanism of grain boundary widening of two kinds of Ni-based superalloys during long-term aging. The results show that the main precipitates of two alloys in the widening grain boundary region are M₂₃C₆ carbide and grain boundary γ′ phase during high-temperature and long-term aging. The evolution of grain boundary widening of two alloys with aging time follows the JMAK equation. The formation process of grain boundary widening consists of three stages. In the first stage, the M₂₃C₆ carbides near the grain boundaries are treated by meltback, inducing the coarsening of M₂₃C₆ carbides at grain boundary and the grain boundary migration. In the second stage, new M₂₃C₆ carbides are precipitated after grain boundary migrating, which makes M₂₃C₆ carbides in the grain boundary area arrange in multiple layers, and the width of grain boundary increase. In the third stage, the γ′ phase will precipitate at the M₂₃C₆/γ interface with the decreased coarsening rate of carbides, and the short-circuit diffusion of grain boundary makes the γ′ phase grow and the width of grain boundary further increase.

Abstract:To improve the ablative properties of ZrC-SiC ceramics, a series of ZrC-SiC multiphase ceramics modified with different contents of La₂O₃ and LaB₆ were prepared by discharge plasma sintering method at 1600 ℃ and 50 MPa. The ablative properties of the materials were tested under oxygen-acetylene flame with a heat flux of 2380 kW/m². The results show that the addition of La₂O₃ and LaB₆ not only improves the sintering performance and density of ZrC-SiC, but also improves the thermal shock resistance. With the increase in addition amount, the surface oxide layer of the samples gradually becomes intact from fracture after ablation. In contrast, the ZrO₂-La₂Zr₂O₇ solid solution layer formed on the ablative surface after La₂O₃ modification still has more holes and cracks, and the combination with internal ceramics is poor, resulting in the poor protection effect. With the addition of LaB₆, the volatilization of low melting-point borides during ablative process can reduce the ablative temperature of the material by nearly 300 ℃. La₂Zr₂O₇-LaBO₃-ZrO₂ outer oxide layer with good thermal stability and lanthanum-rich Zr-O-La-B inner oxide layer with good adhesion are formed on the surface of the ablation center of the sample. The double-layer protective film becomes a dense barrier to prevent the oxidation air from entering the inside of the material, which effectively improves the ablation resistance of the material. The ZrC-SiC ceramic with 20vol% LaB₆ has the best comprehensive ablative performance. After oxygen-acetylene ablation for 60 s, its mass and linear ablative rate are 7.5×10^-4 g/s and 2.1×10^-3 mm/s, respectively.

Abstract:In this work, medium and high entropy alloy coating was prepared on the surface of 38CrMoAl by laser cladding technique. The effects of adding elements such as Al, Si, Fe, and Nb to CoCrNi series alloys on the phase, microstructure, and element distribution of the alloy coating were studied. The hardness, wear resistance, and electrochemical properties of the coating were analyzed and characterized. The results indicate that CoCrNi alloy has a face-centered cubic (fcc) crystal structure, and the addition of Al and Fe promotes the formation of body-centered cubic (bcc) phase. After the addition of Nb and Si elements, a Laves/bcc eutectic+Nb-riched composite phase is formed in the septenary-element-alloy coating, and the microstructure is significantly refined. The comprehensive performance of CoCrNi-based medium and high entropy alloy coatings is superior to that of 38CrMoAl substrate. Compared with CoCrNi and AlCoCrFeNi alloy coatings, the hardness, wear resistance, and corrosion resistance of AlSiCoCrFeNiNb coatings have been significantly improved: the surface hardness is 713.3 HV_0.1, which is 3.24 times higher than that of the substrate. The wear mechanism is mainly slight abrasive wear and adhesive wear, with an average coefficient of friction of 0.52 and a wear rate of 115.73×10^–12 mm³/(N·m), reduced by 64.4% compared with that of the substrate. The self corrosion potential (E_corr) is –0.3392 V, and self corrosion current density (I_corr) is 0.472 μA·cm^-2.

Abstract:In this research, based on the concept of surface engineering and composite multi-component structure design, the titanium-doped diamond-like carbon (DLC) films were prepared on the surface of titanium alloy threaded fasteners by microwave plasma enhanced magnetron sputtering technique. Titanium-doped DLC composite films with different structures and properties were prepared by regulating acetylene flow. The microscopic morphology of titanium-doped DLC films were analyzed by transmission electron microscope. The microstructure, residual stress, nano-hardness, adhesive strength and wear properties were studied by XRD, Raman spectrometer, profilometry technique, nanoindenter and friction test machine. The results show that TiC crystalline phase is formed in titanium-doped DLC films, and the residual stress of titanium-doped DLC film can be effectively reduced with appropriate acetylene gas flow. The sp³ hybrid bond content is decreased gradually with the increase in acetylene flow. The titanium-doped DLC film with acetylene gas flow of 0.025 L/min has higher hardness, elastic modulus, toughness and the largest H/E and H³/E² ratios, which can resist the scratch of the indenter. Therefore, the film maintains good adhesion in the scratch, and has the best wear resistance, which can effectively improve the service life of titanium alloy fasteners.

Abstract:Large-diameter Ti6321 alloy seamless tube with Φ450 mm×20 mm was prepared using forging billet by cross piercing and hot rolling process, and the effect of annealing temperature on microstructure and mechanical properties of seamless tube were investigated. The results show that the microstructure of as-rolled tube is mainly composed of α phase and transformed β phase. Equiaxial structure is obtained after annealing at 940 ℃, duplex structure is obtained after annealing at 970 ℃, and Widmanstatten structure is obtained after annealing at 1020 ℃. With the increase in annealing temperature, the yield strength and tensile strength at room temperature of the tube are gradually decreased. The plasticity of the tube does not change much below the transformation point, but decreases sharply above the transformation point. The impact toughness first increases and then decreases. According to comprehensive analysis, the suitable annealing temperature for the prepared large-diameter Ti6321 alloy seamless tube is about 970 ℃, when the tube has the best impact performance and impact energy is 62 J. The average yield strength, tensile strength, and elongation of the tube after annealing at 970 ℃ are 786 MPa, 878 MPa, and 16.25%, respectively.

Abstract:This research investigated the growth characteristics of TiO₂ nanoparticles in the instantaneous high-temperature and high-pressure gaseous detonation reaction. Computational fluid dynamics was used to simulate the flame propagation process and the temperature-time relationship of the gas explosion in the detonation tube, and it was introduced into the particle growth model. The model was modified through experiments. The results show that the reaction temperature and time are the main factors affecting the particle growth in the gaseous detonation reaction. A particle size correction coefficient k is proposed to improve the classical particle growth model. The improved numerical model can accurately predict the growth characteristics of TiO₂ nanoparticles, which provides effective theoretical support for the controllable synthesis of TiO₂ nanoparticles.

Abstract:The effects of alloying elements on the phase precipitation behavior of a new high-boron Ni-based superalloy were studied using JMatPro thermodynamic software and compared with the actual cast microstructure. The results indicate that the as-cast microstructure of the new high-boron Ni-based superalloy exhibits a typical dendritic morphology, mainly composed of γ, γ′, carbides, borides, and (γ+γ′) eutectic (about 15.5vol%). The segregation of Hf and Ta elements is obvious during the solidification process. Thermodynamic calculations indicate that Ti, Ta, Hf, Al, and B elements have great influence on the melting temperature of alloys. The initial precipitation temperature of γ′ phase and its precipitation amount at 900 ℃ are increased with the increase in Al content, while the influence of Ti element is relatively small. In addition, the Ta and Hf elements will promote the precipitation of MC-type carbide, and the influence of Cr element on the precipitation amount of M₂₃C₆ carbide is greater than that of M₆C carbide. The precipitation of borides is mainly influenced by Cr and W elements, while Mo element has a significant impact on the precipitation temperature of M₃B₂ borides. As the content of Co, Cr, W, and Mo elements increases, the precipitation amount and precipitation temperature of μ phase both show an increasing trend.

Abstract:To investigate microscopic microstructural damage of porous W/Zr-based metallic glass composite, the microscopic-scale finite element model of the composite was established based on their scanning electron microscope images, and the quasi-static compression process of the composite was numerically simulated in conjunction with the cohesive zone model. The effects of interface stiffness, interface strength and fracture energy on the mechanical properties of the composite were investigated, and the values of cohesive zone model parameters were determined by comparing them with quasi-static compression experimental data. Results show that there are three damage modes of porous W/Zr-based metallic glass composite during compression process, which are cleavage fracture of the W particle, shear band fracture within the Zr-based metallic glass and interfacial crack between the two phases. The cohesive zone model parameters have a great influence on the simulation curve: the greater the interface stiffness, the higher the slope of the simulation curve; the greater the interface strength, the higher the yield point of the simulation curve; the larger the fracture energy, the shorter the plastic stage of the simulation curve. As the interface stiffness, interface strength and fracture energy are taken as 10 000 GPa/μm³, 500 MPa and 0.055 J/m², respectively, the simulation results are well consistent with the experimental results, and the simulation model is able to accurately describe the mechanical behavior of porous W/Zr-based metallic glass composite.

Abstract:To analyze the carburizing resistance of Fe-Cr-Ni alloy cracking furnace tube, solid carburizing agent with particle size of 1.5–3 mm was used to conduct carburizing test on two kinds of traditional furnace tubes (25Cr35NiNb+MA and 35Cr45NiNb+MA) as well as two kinds of aluminum-added alloy furnace tubes with prolonged coke cleaning period (27Cr44Ni5W3Al+MA and 29Cr44Ni4Al+MA). The test was carried out at 1075 ℃ for 50–200 h. The composition, microstructure and properties of the furnace tubes after carburizing test were analyzed by optical emission spectrometer, scanning electron microscope, X-ray diffractometer and Vickers hardness tester, and the carburizing kinetics and microstructure transformation rules were studied. The results show that the thickness of the carburized layer of four kinds of materials is increased with the prolongation of carburizing time. After carburizing at 1075 ℃ for 200 h, the thicknesses of the carburized layers of the furnace tubes are about 2.0, 1.8, 1.0 and 0.7 mm, and the average carburizing rates are about 0.01, 0.009, 0.005 and 0.0035 mm/h. The carburizing resistance of the two aluminum-added alloy tubes is better than that of the traditional tubes. The microstructure in the aged zone of 27Cr44Ni5W3Al+MA furnace tube is composed of austenite, blocky M₂₃C₆ and fishbone-like M₇C₃. The "M" in carbides contains elements such as Cr and W. In the carburized zone, blocky M₂₃C₆ is transformed into blocky M₇C₃ and WC, and fishbone-like M₇C₃ coarsens. The microstructure in the aged zone of 29Cr44Ni4Al+MA furnace tube is composed of austenite, blocky M₂₃C₆ and NbTiC. The "M" in carbides is mainly Cr element. In carburized zone, blocky M₂₃C₆ is transformed into blocky M₇C₃, and NbTiC changes from blocky to granular. The inner walls of two aluminum-added alloy furnace tubes form a dense and stable Al₂O₃ film. Compared with the Cr₂O₃ film on the inner walls of traditional furnace tubes, the Al₂O₃ film can more effectively block the penetration of carbon atoms into the substrate, thereby enhancing the anti-carburization performance.

Abstract:The impact toughness and sliding wear behavior with GH5605 alloy at room temperature of solution-and solution+aging-treated Stellite 6B alloy were evaluated. The relevant microstructure, impact fracture surface, worn surface and dissected section were observed and analyzed by Thermal-Calc software, OM, SEM and TEM. The results show that the wear rate of Stellite 6B alloy is significantly reduced by aging treatment after solution, and consequently the wear amount is reduced by approximately 70%. However, the impact toughness of the aging-treated alloy is only 30% of that of the solution-treated alloy. Further analyses indicate that the wear mechanism of solution-treated Stellite 6B alloy is mainly adhesive wear, and adhesive bonding layer is cut off in the matrix. Aging treatment promotes the martensitic transformation and significantly improves the resistance to adhesive wear. The wear mechanism is a small amount of adhesive wear as well as long-term fatigue wear. Since carbide at grain boundary is the main factor affecting the impact toughness of aging alloy, increasing martensitic transformation tendency of the matrix, reducing the total amount of carbides and inhibiting the precipitation of secondary carbide along grain boundary are the process and component optimization direction to comprehensively improve the adhesion wear resistance and impact toughness of Stellite 6B alloy.

Abstract:Directionally solidified superalloys are widely used in turbine blades of advanced power propulsion systems such as industrial gas turbines due to their excellent high-temperature strength, creep resistance, corrosion and oxidation resistance, as well as good structural stability and casting properties. Directionally solidified superalloys for gas turbines have been developed from the first generation to the fourth generation by adjusting the proportions of different solid solution strengthening, precipitation strengthening and grain boundary strengthening elements. The intragranular structures are mainly composed of γ phase and γ? phase. There are carbides, borides and other precipitates at the grain boundaries that can pin the grain boundaries. Under the joint influence of these strengthening phases, nickel-based directionally solidified superalloys have better tensile and creep properties that can change with temperature. Starting with the composition characteristics and microstructure characteristics, this article combines the current application status of directionally solidified superalloys in gas turbines, and further analyzes its performance characteristics. Finally, it looks forward to future research on directionally solidified superalloys.

Abstract:Hot isostatic pressing combined with mold control technique can achieve near-net shaping of complex high-performance components. The mold core is crucial for controlling the internal structural accuracy of the formed parts. Presently, mold cores predominantly employ metallic materials. However, these metallic cores are susceptible to substantial deformation under elevated temperatures and pressures. The removal of acid-induced corrosion is not only inefficient but also environmentally unsound. The diffusion of foreign elements from the metal mold cores results in contamination of parts. Additionally, issues such as embedding of forming powder into the surface lead to poor surface quality of the parts. These problems hinder the development of hot isostatic pressing to the forming of complex internal cavity parts. Ceramic mold cores exhibit low chemical reactivity and minimal interdiffusion with metal elements. Its high temperature hardness and stiffness confer resistance to deformation, and its core removal rate is high under alkaline conditions. The above advantages offer a potential solution to issues caused by metal cores. Based on representative literature and research advancements in the field of ceramic mold cores for casting, this paper focuses on analyzing the synergistic relationship between the mechanical and dissolution properties of ceramic mold cores used in hot isostatic pressing. This paper provides a detailed introduction and comparison of the optimization strategies for mechanical properties, dissolution performance, and moisture resistance of silicon oxide, aluminum oxide, calcium oxide, and magnesium oxide-based ceramics used in hot isostatic pressing cores. This paper also explores complex high-precision structural formation, sintering, and post-processing methods. Additionally, it anticipates challenges and future directions for the application of ceramic cores in the near-net shaping hot isostatic pressing process.

Abstract:Zirconium alloy coating can improve the accident resistance of zirconium alloy cladding without changing the present fuel system, which is one of the hot research directions to improve the accident-tolerant ability of nuclear fuel assemblies. Cr-based coating is the most widely concerned coating material at the current stage. The development progress and design ideas from Cr coatings to various Cr-based coatings after Fukushima nuclear accident were systematically reviewed in this paper. The selection basis and high-temperature oxidation failure mechanism of Cr-based coating were introduced. The solution ideas and research progress were discussed from two aspects of composition design and structure design. Finally, the development prospect of Cr-based coating on zirconium alloy in the future was proposed. The review has important reference significance for the development and application of the new generation of accident tolerant fuel coating technique in the future.

Abstract:Ti and Ti alloys and ZrO₂ ceramics are extensively used in high-precision fields due to their excellent characteristics, playing an increasingly important role in modern industry. The reliable connection of Ti and Ti alloys with ZrO₂ ceramics is crucial to complete the complementary properties of dissimilar materials and expand their range of applications. However, there are some problems to be solved in the welding process of dissimilar materials, which are mainly to improve the wettability of filler metal on the surface of base metal and to alleviate the interface stress. This paper focuses on the analysis of the connection difficulties of the two materials, reviews the research progress of brazing of related ceramics and metals, expounds the influence of different pretreatment methods on the joint performance, and finally puts forward the prospect of dissimilar material connection.

Abstract:ZrCo alloy has been initially applied in nuclear fusion experimental reactor due to its high hydrogen isotope storage capacity, low equilibrium pressure at room temperature and no radioactivity. However, ZrCo alloy has some issues, such as long activation time, poor kinetic properties and easy disproportionation, which restrict its engineering process. Therefore, it is of great significance to improve the hydrogen storage performance of ZrCo alloy and realize the synchronous improvement of dynamic characteristics, cycle stability and anti-disproportionation performance, which is crucial for revealing hydrogen storage mechanism of ZrCo alloy and promoting its engineering application. This review summarizes recent progress on nanostructural ZrCo alloys, especially the fact that nanostructural ZrCo alloy particles can shorten the activation time to less than 10 s, effectively improve the hydrogen absorption kinetics, and enhance the anti-disproportionation ability by more than 50% at 500 ℃, significantly improving the comprehensive hydrogen storage performance of ZrCo alloys. In this paper, the latest research trends of nanostructural ZrCo alloys are systematically summarized, and the mechanism of nanostructure on improving hydrogen storage performance is emphatically elaborated. The future research and application prospects of nanostructural ZrCo-based hydrogen storage isotope alloys are prospected.