稀有金属材料与工程最新文献

The precipitation process of Ni₇₅Al₁₅Ti₁₀ alloy was simulated by a microscopic phase-field kinetics model. The microscopic morphology evolution was studied, which indicates that the precipitation process can be divided into two stages: the first is the transformation from L1₀ phase to L1₂ phase, and the second is the formation of the stoichiometric L1₂ phase. The effect of single aging temperature on the microstructure, atomic occupation probability and atomic diffusion was investigated. The result shows that the shape of γ′ precipitates transforms from irregular shape to regular cuboid and the degree of orientation growth increases with increasing aging temperature. But the higher the aging temperature, the lower the occupation probability and the order degree. Meanwhile, the path of diffusion of Al atom is from the interface of the γ′ phase to the internal, and the diffusion of Ti atom is the opposite. Furthermore, we studied the dual aging to improve occupation probability of γ′ precipitates and to discuss their influence on the γ′ phase. Dual aging can not only obtain a stable γ′ phase with regular shape, but also increase the occupation probability of Al and Ti and inhibit the formation of anti-site defects.

The g-C₃N₄/MoS₂ nanosheet heterojunction was prepared via a facile ball milling method. The microstructure and morphology of the composite were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy. The photocatalytic activities were evaluated by the degradation of organic Rhodamine B (RhB) under visible light irradiation. The results indicate that MoS₂ nanosheets are successfully coupled into g-C₃N₄ to form a C₃N₄/MoS₂ heterojunction. The kinetic constant of RhB degradation with g-C₃N₄/MoS₂ nanosheets-2 wt% heterojunction (0.0368 min⁻¹) is about 4.3 times as high as that of the bulk g-C₃N₄ (0.00840 min⁻¹). The enhanced photocatalytic activities can be mainly ascribed to the efficient separation and transportation of photo-induced electron-hole pairs. The possible photocatalytic mechanism of composites was proposed according to the light trapping experiment.

The thermodynamic assessment of the Co-Nb-W ternary system has been carried out by applying the CALPHAD method. A slight modification of the Co-W binary system has been performed to unify the model of μ phase in Co-Nb-W system. The three published isothermal sections at 1273, 1373 and 1473 K have been critically examined and used to optimize the model parameters. The solution phases, including αCo and bcc were modeled as substitutional solutions; the λ, χ, and Co₃Nb phases have been assessed using two-sublattice models. The Co₇W₆ and Co₇Nb₆ phases were described as (Co, Nb)₁(Co, Nb, W)₂(Co, Nb, W)₄(Co)₆. Calculated isothermal sections and vertical sections agreed well with the experimental data.

Calcium phosphate (Ca-P) coatings were deposited on AZ31 magnesium by hydrothermal treatment at various pH values to improve the corrosion resistance and biocompatibility of magnesium alloys. The crystal phase, morphology and composition of the coatings were investigated by XRD, SEM and EDS, respectively. Electrochemical measurements and immersion tests were performed in Hank's solution to examine the bio-corrosion behaviors of the coated specimens. The results show that the pH value of solutions influences the phase composition and microstructure of the coating. And the degree of protection provided by the Ca-P coating varies with its microstructure. The deposited Ca-P coating treated at pH=6 is composed of OCP (Ca₈H₂(PO₄)₆·5H₂O). When the pH value increases to 8 and 10, only the HA is formed. The HA coating with nano-scale and rod-like crystals prepared at pH=10 can prevent the penetration of solution more effectively than the OCP coating at pH=6 and the honeycomb-like HA coating at pH=8.

In the temperature range of 760 °C to 1100 °C, the transverse tensile properties of a nickel-based third generation single crystal superalloy were studied. The microstructures and fracture surfaces were observed by optical microscopy (OM), field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM). The results show that the tensile strength of the alloy decreases as the temperature increases, while the tensile elongation of the alloy increases with the temperature increasing. The fracture surfaces of the tensile ruptured specimens are characterized by quasi-cleavage features at 760 and 850 °C. In the temperature range of 980 °C to 1100 °C, dendrites characteristics exhibiting the solidification direction are observed on the fracture surfaces and the proportion of dendrites characteristic on the fracture surfaces increases with the temperature increasing. The fracture surface displays mixed quasi-cleavage and dimple features at 980 °C. The fracture surfaces are characterized by dimples at 1070 and 1100 °C. As the temperature increases, more slip systems tend to be activated during the plastic deformation, resulting in different dislocation configurations. At 760°C, high density a/2<110> dislocations are found to be of a roughly parallel distribution with each other in the tensile ruptured specimens. The dislocations are observed to be tangled at 980 °C and dislocation networks have formed at 1100 °C.

Microstructures and mechanical properties of the Mg-6Gd-4Y (wt%) alloys with and without 1wt% Zn additions were investigated. The results show that the as-cast microstructure of the Mg-6Gd-4Y alloy consist of α-Mg matrix and Mg₂₄(GdY)₅ secondary phase. However, the as-cast microstructure of the Zn-containing Mg-6Gd-4Y-1Zn alloy consist of α-Mg matrix, Mg₂₄(GdY)₅ secondary phase and Mg₁₂Y₁Zn₁ phase which has a 18R long period staking ordered (18R-LPSO) structure. After extrusion a 14H-LPSO phase is found in the Zn-containing alloy which is distributed between the 18R-LPSO strips in the as extruded microstructure. The formation mechanism of the 14H-LPSO phase is precipitation, and the reaction can be expressed as ‘α-Mg′ → α-Mg+14H’. Zn content has no obvious effect on the precipitation of β series. Aged (T6 and T5 treatment) conducted on both the Mg-6Gd-4Y alloy and the Mg-6Gd-4Y-1Zn alloy causes the formation of β′ precipitates. The T6-aged Mg-6Gd-4Y-1Zn alloy exhibits high tensile strength combined with good ductility, and the values of the yield strength (YS), ultimate tensile strength (UTS) and elongation are 309 MPa, 438 MPa and 6.8%, respectively. It is due to the coexistence of 18R-LPSO phase and finely dispersed distribution of the 14H-LPSO phase and the β′ precipitates in the microstructure.

Inconel718 superalloy samples with V-grooves were experimentally repaired by electromagnetic stirring assisted laser repairing (EMS-LR) under different magnetic field currents. The effects of the magnetic field current on morphologies of single pass repaired zone (RZ), microstructure and mechanical properties of multilayer RZ were experimentally investigated. Microstructure observations show that metallurgical bonding is obtained between the RZ and the substrate when the optimized process parameter is used. The microstructure in RZ is coarse columnar crystal when no electromagnetic stirring is used, which grows epitaxially along the deposition direction. With the increase of magnetic field current, the convection of liquid metals prompts the transformation from coarse columnar to fine equiaxed grains. The strong scour of liquid metal convection can affect the interdendritic γ+Laves eutectic reaction and prohibit the growth of Laves phase. Electromagnetic stirring can improve the spreading of liquid metal in a certain extent. The width and deposition height of the single trace were measured and it is found that the width and deposition height ratio changes from 3.26 when no electromagnetic stirring is used to 3.33, 4.14 and 5.14 when the applied magnetic field current is 20, 40 and 60 A, respectively, and the penetration is found to be decreased inversely. The tensile strengths of repaired components increase from 487 MPa to 510, 673 and 770 MPa for magnetic field currents of 0, 20, 40 and 60 A, respectively.

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal were analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B₀ and Cauchy pressure parameters C₁₂-C₄₄ values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.

Sn is a kind of low melting point metal and can be used as a liquid coolant due to its higher thermal conductivity. Compared with the currently used sodium liquid coolant, Sn is more chemically stable and unfeasible to fire or explore when contacting with air or water. The chemical reactions between Sn and 304 stainless steel that is widely used as primary circuit pipeline materials in fast reactor were researched, and the effect of temperature on the corrosion behavior between 304 stainless steel and liquid Sn was discussed. The results show that pitting happens when the temperature is lower than 823 K, and dissolution happens when the temperature is higher than 823 K.