Acta Metallurgica Sinica-English Letters最新文献

In this paper, the isothermal oxidation experiments were used to study the effect of Ag on the high-temperature oxidation behavior of Mg–6.5Gd–5.6Y–0.1Nd–0.01Ce–0.4Zr (wt%) alloy oxidized at 350 °C, 400 °C and 450 °C for 120 h. The results show that the oxidation weight gain of the alloy mainly occurs in the early oxidation stage (0–20 h). This reason attributes to the lack of protective oxide film and the rapid inward diffusion of oxygen through the macroscopic defects of the incomplete oxide film. When dense oxide films such as Y₂O₃, Gd₂O₃, and ZrO₂ form, they hinder the inward transport of oxygen ions and improve the high-temperature oxidation resistance of the alloy. In addition, the role of the Ag element at three temperatures is different. The addition of Ag mainly promotes the formation of eutectic phases such as Mg₃Gd, Mg₂₄Y₅, and Ag₂Gd, which reduces the content of Gd and Y elements in the alloy matrix, resulting in a decrease in the diffusion rate of Gd and Y elements during the oxidation process at 350 °C and 400 °C, and weakens the oxidation resistance of Ag-containing alloys. However, in the oxidation experiment at 450 °C, a large amount of eutectic phase is solid dissolved into the matrix, reducing the difference in element content. At this time, it is detected that the Ag element promoted the outward diffusion of Gd and Y elements, accelerating the formation of the oxide film. The oxidation resistance of Ag-containing alloys is improved.

Detwinning behavior of a pre-twinned magnesium alloy AZ31 at cryogenic temperature was investigated, also with a focus on the annealing hardening behavior of samples with different fractions of pre-twins. Pre-compression along the transverse direction with strains of 1.7%, 3.0%, and 6.0% was applied to generated (left{ {10overline{1}2} right}) twins. Mechanical behavior, microstructure, and texture evolution during subsequent tension were examined. Our results show that low temperature did not change the fact that detwinning still pre-dominated in the pre-twinned samples under reverse loading. However, a relatively harder migration of twin boundaries was found at cryogenic temperature. An annealing hardening of 27-40 MPa was observed in the pre-twinned samples, and such a hardening effect shows a close relation with the fraction of pre-twins or the level of pre-strains. The annealing hardening effect disappeared if the matrix was consumed by twins along with the increased pre-compression strains. The corresponding reasons for the annealing hardening behavior were discussed.

Enhancing saturation magnetic flux density (B_s) while reducing high-frequency core loss in Finemet-type nanocrystalline alloys is of great significance in achieving the miniaturization, high-frequency, and energy-saving of modern power electronic devices. In this work, we first designed a high-B_s Fe_77.2Si₁₁B_8.5Cu_0.8Nb_2.5 alloy by appropriately reducing the non-magnetic elements in typical Finemet nanocrystalline alloys, and subsequently alloyed 2 at% Co, Al, and Mo, respectively. The effects of alloying elements on structure and static and high-frequency magnetic properties were studied. The results reveal that, alloying Al or Mo reduces the average α-Fe grain size (D_α-Fe) in the nanocrystalline alloys, while Co exhibits a slight influence. The added Al or Mo results in decreases in both the B_s and coercivity (H_c) of the nanocrystalline alloys, whereas Co increases the B_s without changing H_c, and meanwhile, all alloying elements show minimal effects on effective permeability (μ_e). Furthermore, the addition of Co, Al, or Mo lowers the core loss (P_cv) at 0.2 T/100 kHz of the based nanocrystalline alloy with reductions of 10.9%, 29.6%, and 26.8%, respectively. A Fe_75.2Si₁₁B_8.5Cu_0.8Nb_2.5Al₂ nanocrystalline alloy exhibits outstanding soft magnetic properties with B_s, H_c, μ_e at 10 kHz and 100 kHz, and P_cv at 0.2 T/100 kHz of 1.34 T, 0.8 A/m, 27,400, 18,000, and 350 kW/m³, respectively. The reduction in P_cv is primarily attributed to the decreased eddy current losses, originating from the increased electrical resistivity by elements alloying.

The mechanical response of a single crystal titanium sample against (0001)_α surface impact was investigated using molecular dynamics simulation. Remarkably, non-uniform plastic deformation was observed in the sample. At high strain rates, amorphization occurred near the edge of the contact region where severe shear strain induced a large number of stacking faults (SFs) and dislocations. In contrast, the central part of the contact region underwent less deformation with significantly fewer dislocations. Moreover, instead of amorphization by consuming SFs and dislocations, there was a gradual increase in the density of dislocations and SFs during the process of amorphization. These local amorphous regions eventually grew into shear bands.

In this work, the impact of extrusion and post-extrusion heat treatment (T6) on the microstructure and mechanical properties of the Al-1.2Mg-0.8Si-0.5Mn alloy with different Cu contents (0, 0.6, 1.3 and 2.0 wt%) was studied. Microstructure characterization showed that all extruded alloys exhibited elongated grain structure with an average grain size of ~ 4.8 μm. The dominant texture components were deformation texture (A*, Copper and P texture), while the proportion of random texture initially increased and then decreased with increasing Cu content. After T6 treatment, the grain size of the four alloys increased significantly, but the growth trend decreased with increasing Cu content, and the textures transformed into recrystallized textures (Cube, A and Goss texture). Tensile testing revealed that the designed T6 alloys with 2.0% Cu content exhibited an excellent strength-ductility balance, i.e., a yield strength of 342.9 MPa, an ultimate tensile strength of 424.8 MPa and an elongation of 15.9%. The enhanced strength was mainly attributed to fine grain strengthening, solid solution strengthening and aging strengthening mechanisms. The superior ductility was due to the pinning effect of fine precipitates and high dislocation accommodation capacity caused by heat treatment.

The transpassivation and pitting corrosion behavior of a high-nitrogen stainless steel (HNS), Fe18Cr15Mn3Mo0.92N, were systematically investigated by electrochemical analysis, morphology observation, and X-ray photoelectron spectroscopy surface analysis. It was surprisingly found that no pitting corrosion occurred in the transpassivation region of HNS. This electrochemical corrosion behavior is untypical for stainless steels, i.e., the traditional critical pitting potential method was invalid for HNS. Both N and Cr enrichments in the transpassivation film on HNS were found extremely higher than those in the passivation film. The N existed in the form of [CrN] complex, which could stabilize the above both films. Besides, the corrosion product of N was detected as NH₃ that exhibited an effective corrosion inhibition effect. On this basis, although the transition of Cr from 3-valent to 6-valent was confirmed, the transpassivation film on HNS still maintained it high stability and no pitting was found to occur. Therefore, the real pitting resistance of HNS should be higher than the expected before. And the stable transpassivation film played an important role in its untypical pitting corrosion resistance.

The continued existence of high-energy radiation in nuclear reactors at high temperatures results in the formation of radiation-induced voids, which will further lead to inevitable swellings of polycrystalline structural components and thus premature failures. A deep understanding of the effect of temperature and grain boundary on void evolution in irradiated copper is significant for preventing this kind of failures. Here, the phase-field method was employed to study void evolution in irradiated copper under different temperatures and grain sizes. The results show that, due to the different sensitivities of point defect production rate and vacancy diffusion rate to temperature changes, both the nucleation-growth rate and the coarsening rate during void evolution increase first and then decrease with increasing temperature; moreover, the nucleation mechanism exhibits site-saturated nucleation at low temperatures while continuous nucleation at high temperatures. The presence of grain boundary can accelerate the emergence of void because grain boundaries can absorb more interstitials than vacancies. The finer the grain size, the stronger inhibitory effect of grain boundaries on the growth rate of void, due to the formation of void denuded zone near grain boundaries. At high temperatures, the growth rate of void in fine grains is significantly reduced due to the increase of vacancy diffusion rate and the enhancement of sink effect of grain boundary on vacancy.

Pyrochemical reprocessing utilizing a molten LiCl–KCl eutectic medium is regarded as the most promising approach for recovering uranium and transuranic elements from spent metallic nuclear fuels. However, the harsh corrosiveness of molten chloride poses a significant challenge to the durability of structural materials. Herein, we report the corrosion behavior of 304 SS, 316H SS and Inconel 800H in LiCl–KCl eutectic salt at 550 °C for 100 h under an argon atmosphere. Experimental results indicate that all three materials can form a rather continuous Cr₂O₃-based scale through oxidation reaction at the beginning, but only the scale developed on 800H maintains excellent protection against corrosion throughout the entire exposure period. In contrast, both 304 SS and 316H SS experience considerable active dissolution on the bare substrate under a detached scale. We suggest that the primary reasons for the outstanding resistance of 800H to molten salt corrosion are the high concentration of noble Ni in the system, which lowers the inclination for active dissolution, and the beneficial addition of Al, which accelerates the formation of a less defective Cr₂O₃-based scale. Our work offers an in-depth understanding on the corrosion performance of high-Cr alloys in molten chloride, insights critical for the selection and subsequent development of structural materials for pyrochemical reprocessing applications.

Indirect additive manufacturing (AM) methods have recently attracted attention from researchers thanks to their great potential for cheap, straightforward, and small-scale production of metallic components. Atomic diffusion additive manufacturing (ADAM), a variant of indirect AM methods, is a layer-wise indirect AM process recently developed based on fused deposition modeling and metal injection molding. However, there is still limited knowledge of the process conditions and material properties fabricated through this process, where sintering plays a crucial role in the final consolidation of parts. Therefore, this research, for the first time, systematically investigates the impact of various sintering conditions on the shrinkage, relative density, microstructure, and hardness of the 17-4PH ADAM samples. For this reason, as-washed samples were sintered under different time–temperature combinations. The sample density was evaluated using Archimedes, computed tomography, and image analysis methods. The outcomes revealed that sintering variables significantly impacted the density of brown 17-4PH Stainless Steel samples. The results indicated more than 99% relative densities, higher than the value reported by Markforged Inc. (~ 96%). Based on parallel porosities observed in the computed tomography results, it can be suggested that by modifying the infill pattern during printing, it would be possible to increase the final relative density. The microhardness of the sintered samples in this study was higher than that of the standard sample provided by Markforged Inc. Sintering at 1330 °C for 4 h increased the density of the printed sample without compromising its mechanical properties. According to X-ray diffraction analysis, the standard sample provided by Markforged Inc. and “1330 °C—4 h” one had similar stable phases, although copper-rich intermetallics were more abundant in the microstructure of reference samples. This study is expected to facilitate the adoption of indirect metal AM methods by different sectors, thanks to the high achievable relative densities reported here.

B₄C/Al composites are widely utilized as neutron absorbing materials for the storage and transportation of spent nuclear fuel. In order to improve the high-temperature mechanical properties of B₄C/Al composites, in-situ nano-Al₂O₃ was introduced utilizing oxide on Al powder surface. In this study, the Al₂O₃ content was adjusted by utilizing spheroid Al powder with varying diameters, thereby investigating the impact of Al₂O₃ content on the tensile properties of (B₄C + Al₂O₃)/Al composites. It was found that the pinning effect of Al₂O₃ on the grain boundaries could hinder the recovery of dislocations and lead to dislocation accumulation at high temperature. As the result, with the increase in Al₂O₃ content and the decrease in grain size, the high-temperature strength of the composites increased significantly. The finest Al powder used in this investigation had a diameter of 1.4 μm, whereas the resultant composite exhibited a maximum strength of 251 MPa at room temperature and 133 MPa at 350 °C, surpassing that of traditional B₄C/Al composites.