Transactions of Nonferrous Metals Society of China最新文献

A new method of repeated upsetting−extrusion (RUE) deformation was presented. The effects of different deformation temperatures on the microstructure and texture of RUE deformed AZ31 magnesium (Mg) alloy were studied using optical microscopy (OM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and Vickers hardness tester. The results show that the microstructure of the AZ31 Mg alloy is locally refined to 0.8 μm after RUE deformation, and the main refinement mechanism is the continuous dynamic recrystallization (CDRX). As the temperature increases, the texture of basal plane parallel to shearing plane is formed firstly, followed by texture with weaker intensity, and finally and textures. The texture split is caused by the dislocation slip of basal 〈a〉 and pyramidal 〈a〉. The maximum hardness of the AZ31 Mg alloy after RUE deformation is increased by 42.4% compared with that at the initial state, which is mainly due to the fine-grained strengthening and the dislocation strengthening.

Al coating was fabricated on the surface of Mg−Zn−Al−Sn−Mn alloy by cold spraying process to enhance its corrosion resistance. The effects of temperature and pressure of working gas on the microstructure and corrosion behaviors of Al coating were investigated. The results show that as gas temperature and pressure increase, the densification and thickness of Al coating obviously increase. High gas temperature declines the critical velocity of powder deposition and increases the deposition efficiency. High gas pressure not only improves the quality of mechanical bonding, but also provides strong tamping effect on the deposited coating. Due to the reduction of porosity, the corrosion resistance of Al coating is improved with the increase of gas temperature and pressure. The corrosion products of Al(OH)₃ and Al₂O₃ can block the pores in Al coating and act as physical barriers to corrosion, which effectively protects Mg alloy from being corroded.

The differences in the sulfidization mechanism of cerussite (PbCO₃) and smithsonite (ZnCO₃) were comparatively studied by the density functional theory (DFT) method. Pb−S/Zn−S layers over PbCO₃/ZnCO₃ surfaces are constructed to simulate the sulfidization structure. The results of layer distance and Mulliken charge suggest that Pb−S layer formed over PbCO₃ is stable but Zn−S layer formed over ZnCO₃ is unstable. Because of the high covalency of Pb−S layer and the high ionicity of Zn−S layer, the sulfidization−xanthate method is effective for cerussite but ineffective for smithsonite flotation. To recover smithsonite, strong ionicity collectors are required, and hence the sulfidization−amine method is applied more to smithsonite flotation. These differences in the sulfide layer structure and flotation behavior of PbCO₃ and ZnCO₃ surfaces are mainly attributed to the different polarizabilities of the metal ions (Pb²⁺ and Zn²⁺) and the ligands (O ligand and S ligand) and the inertness of Zn²⁺ 3d¹⁰ orbital.

The effects of initial textures on the microstructures and mechanical properties of rolled Mg−4.7Gd− 3.4Y−1.2Zn−0.5Zr (wt.%) plates were studied by optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and tensile test. In the plate with an initial texture of 〈0001〉//RD (rolling direction), relatively coarse grains and bimodal basal texture are developed. However, in the plate with an initial texture of 〈0001〉//TD (transverse direction), finer grain size and strong 〈0001〉//ND basal textures are obtained due to the higher degree of dynamic recrystallization during rolling. Additionally, the irregular-shaped long-period stacking ordered (LPSO) phases are elongated along RD in both plates, which results in anisotropic load-bearing strengthening behavior, and further leads to a certain degree of yield anisotropy. The analysis of the strengthening mechanism shows that grain boundary strengthening is the most effective strengthening method in the plates. Because of the finer grain size and the strengthening caused by the elongated irregular-shaped LPSO phases, the rolled plate with an initial 〈0001〉//TD texture achieves the highest yield strength along RD.

Simple two-parameter models were proposed for correlating grain size to process conditions and phase diagram variables. However, these models have not been fully examined using data obtained from well controlled experiments. This work intended to clarify these models with selected experimental data obtained in dilute hypoeutectic alloys. Criteria for data selected were proposed. The selected experimental data were fitted by these models to examine curve fitting quality and the applicability. Models that fit experimental data better were identified. Mechanisms by which grain size is reduced under the influence of a solute element were examined based on the data analysis. Results suggest that there is a clear dependence of grain size on the solidification interval of an alloy, which can be expressed as the P variable. Such a clear dependence of grain size on solidification interval indicates that mechanisms that are associated with dendrite fragmentation are the dominant operating mechanisms governing grain refinement by solute element in ingots and castings where convection in the molten alloy exists during its mold filling and solidification.

The solid-state reaction behavior of MgO−V₂O₅ mixtures with different molar ratios was explored. The solubility of the solid-state reaction products (magnesium vanadates) in water at 25−55 °C was measured using the isothermal solution saturation method. The dissolution behavior and kinetics of the magnesium vanadates in dilute sulfuric acid were also investigated. The results showed that the molar ratio of MgO to V₂O₅ and roasting temperature significantly influenced the phase transformation of the solid-state reaction product. MgV₂O₆ exhibits the highest solubility in water, followed by Mg₂V₂O₇ and Mg₃V₂O₈. The dissolution rate of magnesium vanadates in dilute sulfuric was significantly increased with the decrease of pH from 4.0 to 2.5 and the temperature increase from 30 to 70 °C. The dissolution of magnesium vanadate can be described using a second-order pseudo-homogeneous reaction model.

To reveal how the performance of LiNi_xCo_yMn_zO₂ (NCM) changes as a function of transition metal (TM) composition, the effect of TM ratio on the structure, morphology, electrochemical performance and thermal behavior of different NCMs was systematically investigated. Increasing Ni content leads to higher reversible capacity but worse rate capability and cycling stability. Inhibited kinetics of Li⁺ and poor electronic conductivity cause major capacity loss in Ni-rich NCMs. Comparison of thermal behaviors was carried out via in-situ and ex-situ micro-calorimetry. With the decrease of Ni content, the thermal stability is significantly improved. The oxygen release related to phase transitions was evaluated based on Li residuals in delithiated NCMs, which verifies that Ni-rich materials exhibit severer structural deterioration, lower onset temperature and more heat release. Comprehensive characterization identifies that LiNi_0.5Co_0.3Mn_0.2O₂ strikes a well-balanced combination of electrochemical performance and safety features.

The corrosion behavior of WE43 alloy with different rolling reductions (0%, 25%, 45%, and 80%) at 500 °C was investigated in 3.5 wt.% NaCl solution. The microstructure evolution of the rolled WE43 alloy was characterized in detail by means of optical microscopy (OM), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). The results show that with increasing reductions, basal texture is gradually enhanced and grain size gradually decreases. Only the alloy with 45% reduction exhibits dispersed dynamic precipitates. Immersion and electrochemical measurements demonstrate the decreasing order of the corrosion resistance of the rolled WE43 alloy: 45% reduction > 80% reduction > 25% reduction > 0 reduction. The best corrosion resistance of the WE43 alloy with 45% reduction is mainly related to a large number of finely dispersed precipitates, which are capable of promoting the formation of a compact corrosion product layer. The increased corrosion rate of the sample with 80% reduction is induced by the re-solution of the majority of precipitates into the matrix. The grain size, basal texture and deformation twins have a limited effect on the corrosion resistance of rolled WE43 alloy compared with the precipitates.

A range of annealing treatments were implemented to regulate the microstructure and mechanical properties of Ti−6Al−4V forging. The results show that both the as-forged and as-annealed microstructures exhibit a bimodal structure composed of equiaxed primary α phase (α_p) and transformed β phase (β_t). With the increase of annealing temperature, the secondary α (α_s) phase precipitates from the β phase. There is no obvious texture and the crystal orientation is uniformly distributed. Meanwhile, strength initially increases and then decreases, whereas ductility remains largely unaffected. The alloy annealed at 860 °C has an excellent combination of strength and ductility, due to the larger content of the fine α_s phase. All fracture surfaces contain massive dimples, which is a typical ductile fracture feature. The characteristics of microcracks, microvoids, and kinked α_p/α_l (equiaxed α_p and lamellar α phase) phases are found near the fracture.

Heterogeneous microstructure with nano-precipitate and bimodal grain size distribution was obtained in a FeCoCrNiMn high-entropy alloy (HEA) doped with 0.5 at.% C by a controlled thermo-mechanical treatment. The coarse M₂₃C₆ carbides tend to aggregate along the fine grain boundaries. Compared to the sample with homogenous microstructure, the heterostructure FeCoCrNiMn−0.5at.%C HEA has approximately the same average grain size of 4.8 μm. However, it shows bimodal grain size distribution and higher volume fraction of the fine grains (<3 μm), resulting in the increase of yield strength from 552 to 632 MPa. The sample with heterostructure presents different mechanical responses and deformed microstructures in different regions, accounting for significant strain localization and high density of the geometrically necessary dislocations during tensile deformation. These deformation characteristics are beneficial to the enhancement of strain hardening capacity, thereby promoting strength−ductility synergy.