稀有金属材料与工程最新文献

(TiC+TiB)/Ti6Al4V composites with different TiC and TiB contents were prepared by in situ synthesis. The influence of load on the dry sliding friction and wear performance of in-situ (TiC+TiB)/Ti6Al4V composites (TMC) was studied by HT-1000 friction and wear testing machine, and the wear behavior of the composites was also investigated by scanning electron microscopy (SEM) and Bruker 3D profilometer. The results show that the wear resistance of TMC is improved by the formation of TiC and TiB phases compared to the Ti6Al4V matrix. For the composites with different volume fractions of reinforcing phases, the wear rate and wear depth increase with the increase of the applied load, and the friction coefficient decreases and fluctuates within a small range. Under low loads, the worn surface is covered with grooves and a small amount of wear debris; under heavy loads, the worn surface is covered with narrow and shallow grooves and a large amount of wear debris. The wear mechanism is abrasive wear and oxidation wear. As the load increases, the size of the debris increases.

Atomization is an effective approach to obtain fine and spherical alloy powders. Undercooling is an important factor that can affect the property of powders during atomization. The effects of powder size and cooling rate on the undercooling and microstructure of powders were investigated, and the relation between powder size, cooling rate and undercooling was obtained through DSC. The results show that a small powder size and a low cooling rate will result in a large undercooling. Meanwhile, it is found that when the undercooling is large, the dendrite arm spacing of the particles decreases. The smaller the powder size, the greater the proportion of cellular grains, and the finer the grain size of the powders.

Mg-5Li-1Al magnesium alloy sheets processed by hot rolling were annealed at 150 and 300 °C for 30 min. The microstructure, mechanical properties, and texture of the rolled and annealed Mg-5Li-1Al alloy sheet were investigated. The plastic deformation mechanism was also discussed. The results show that the annealed sheet possesses weak basal texture and homogenous recrystallized structure with grain size of ∼15 μm. Increasing annealing temperature results in an increase in tensile elongation and a decrease in tensile strength, yield strength and yield ratio, which generally favor the improvement of sheet formability. The rolled LA51 sheet after annealing at 300 °C exhibits improved mechanical properties due to Li addition and high annealing temperature.

Boron-doped nanodiamond was prepared by a high-temperature vacuum-diffusion method. Thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy were used to characterize the prepared material. Results show that the product mainly contains C, O, and B in mass fractions of 92.08%, 7.14%, and 0.78%, respectively. In addition to diamond (111)_D and (220)_D diffraction peaks, hexagonal diamond (100)_D diffraction peaks are also observed in the XRD pattern of the boron-doped product. The introduction of B atoms increases the defect content in the nanodiamond and causes the Raman G peak to move to 1620 cm⁻¹. B atoms are mainly present in two forms in the diamond lattice: substitutional carbon atoms in C-B bonds, and being bonded with impurity elements (such as B-O). The shape and morphology of the boron-doped nanodiamond particles (particle size of detonation nanodiamond, 2∼10 nm) exhibit no obvious changes compared to the pristine nanodiamond. However, a small amount of cubic diamond is observed. In conclusion, the initial oxidation temperature of the boron-doped nanodiamond increases by 175 °C, the oxidation rate is slower, and the thermal stability is improved.

A novel magnetic-field-driving approach was proposed and used to efficiently enhance the mechanical properties of selective laser melting (SLM) Ti-6Al-4V. The microstructures of the as-built and the SLM specimens annealed at 400, 800 °C below the β transus, and 1200 °C above the β transus for 30 min in the high magnetic field of 7 T were comprehensively characterized in terms of X-ray diffraction, optical microscope, scanning electron microscope, and atomic force microscope. Lattice distortions induced by Al and V atoms were characterized by bonding charge density, providing an insight into the atomic and electronic basis for the solid solution strengthening mechanism and the martensitic transformation mechanism. Referring to the as-built specimens, the ultimate tensile strength and the elongation of annealed specimens at 400 and 1200 °C in 7 T high magnetic field increase due to the short annealing time. Based on the coupling effect of force field induced by the heat and magnetic, it is expected that the microstructures of SLM Ti-6Al-4V would be conventionally optimized through changing the phase transformation thermodynamics. The validation of this hypothesis will pave a path to develop a novel magnetic-field-driving approach efficiently enhancing the mechanical properties of additive manufactured materials.

The influence of temperature and pressure on the electronic, elastic, structural, and thermodynamic properties of Ni₃Al alloy was investigated by performing a first-principles study. The calculated elastic constants, equilibrium lattice constants, and elastic modulus agree well with the recorded theoretical and experimental data. The calculated elastic constants indicate that C₁₁ is more sensitive than C₁₂ and C₄₄ to pressure. The Young’s modulus, bulk modulus, and shear modulus increase with the increase of pressure. The ratio of bulk to shear modulus (B/G) and anisotropy factor A were also analyzed. The Debye temperature was obtained by calculating the elastic constants, and it changes with the change in the pressure. The thermal expansion coefficient, normalized volume, heat capacity, bulk modulus, and Debye temperature Θ_D were determined and analyzed using the quasi-harmonic Debye model at pressures of 0∼60 GPa and temperatures of 0∼1600 K. Finally, the density of states and Mulliken population were investigated and the effect of pressure on these was analyzed.

Numerical simulation and analysis of the influence of electrode shrinkage cavity on electroslag remelting (ESR) process of IN718 alloy ingot with a diameter of 430 mm were carried out using the self-developed ESR process model. Electromagnetic fields of ESR system were simulated by a specially designed shrinkage cavity with different shapes and sizes in the electrode. The results show that the contacting area changes between the electrode and the slag owing to the cavity, which plays a predominant role in the distribution of the Joule heat and electromagnetic force in the slag, while the effect of axial dimension change of the shrinkage cavity is negligible. Constant melt rate ESR processes were simulated for different radius cavity situations. It is shown that, at a constant melt rate, the shrinkage cavity has no effect on the ESR process as its radius is less than 0.025 m, and only a small influence on the slag flow when the radius reaches 0.05 m. As the radius increases over 0.05 m, an increasingly evident influence on the slag zone appears via the weakened center downward flow and the increased temperature. Nevertheless, the cavity has no obvious influence on the ingot including the melt pool and mush zone. There is a nonlinear relation between the shrinkage cavity radius and the ESR melting parameters such as current and power, and the critical value of the radius is approximately 0.05 m. Below the critical value, faint or even no change in the parameters appears, while, above the value, the parameters of power and current increase rapidly in an approximately linear manner. From the standpoint of process control stability, the shrinkage cavity radius should be controlled below 0.05 m.

Gas burner is the key component of coal-water slurry gasifier system. However, due to the sulfidation at high temperature, gas burner often fails in advance, which affects the stable operation of the system. In this work, Al-Mo coatings were deposited by plasma spraying with a Mo bonding layer. The sulfidation and oxidation behavior of the Al-Mo coatings was studied and compared with those of Mo coatings and Inconel 600 alloy at 973, 1073 and 1173 K. Results show that both the high-temperature sulfidation resistance and the high-temperature oxidation resistance of Al-Mo coatings are superior to those of Mo coatings. The high temperature sulfidation resistance of coatings is better than that of Inconel 600 alloy. Plasma-sprayed Al-Mo coatings with a Mo bonding layer are promising candidates for the corrosion protection of steels in a sulfur/oxide-rich atmosphere.

The use of carbon fiber-reinforced polymer (CFRP) in aviation, automobile, marine and offshore, etc. increases sharply. The metals such as aluminum alloy are still widely used in these industries. Then, the joining technology of CFRP-metal is one of the key problems urgently to be solved and developed in these industries, especially in aviation industries. Thus, this paper gives a review on the joining processes of carbon fiber-reinforced polymer and metal. The implementation processes and joining material types of adhesive bonding, bolt connection, riveting, welding, “z-pin” (pin inserts), and joining (such as self-pierce riveting, hot riveting, mechanical clinching, friction welding) by plastic deformation, were summarized.

The saturated vapor oxidation behavior of GH2984 alloys at 750 °C were investigated by oxidation weight increasing method, X-ray diffraction and scanning electronic microscopy in different dissolved oxygen concentrations. The results show that the oxide film of GH2984 alloys in different dissolved oxygen concentrations is of a single layer consisting of continuous Cr₂O₃. The oxidation mass gain of GH2984 alloys slightly increases with the increase of the dissolved oxygen concentrations in water vapor. Oxygenated treatment could accelerate the internal oxidation phenomenon, enhance the oxidation film thickness and make the scattered nodular bulges of Fe₂O₃ disappear and more Cr-rich nodular bulges form. Moreover, high dissolved oxygen concentrations do not affect the stabilization of Cr₂O₃ but contribute to the formation of a little Al₂O₃ and TiO₂ which are oxidized internally.