Acta Metallurgica Sinica-English Letters最新文献

Nickel-based alloys applied in marine environments often face multiple challenges of stress, corrosion and wear. In this work, heterostructured NiCrTi alloy was prepared by spark plasma sintering coarse Ni20Cr and ultrafine Ti powders. Apart that some are dissolved into the nickel alloy, Ti powders react in situ with Ni20Cr during sintering to form hard intermetallic Ni₃Ti. It builds up a typical heterostructure that endows NiCrTi alloy with well-balanced mechanical strength and plasticity, e.g. high yield strength of 1321 MPa, compressive strength of 2470 MPa, and compressive strain of 20%. On tribocorrosion, the hard shell enriched with Ti transforms to connected protrusion and form in situ surface texture. Oxides or wear debris are trapped at the textured surface and compacted to form a stable tribofilm. Thus negative synergy between corrosion and wear is observed for NiCrTi and high tribocorrosion resistance is achieved. At a potential of + 0.3 V, the tribocorrosion rate of NiCrTi is reduced by an order of magnitude to 1.87 × 10^− 5 mm³/(Nm) in comparison to the alloy Ni20Cr.

The spray-deposition was used to produce billets of Mg-4Al-1.5Zn-3Ca-1Nd (A alloy) and Mg-13Al-3Zn-3Ca-1Nd (B alloy), and evolution of deformation substructure and Mg_xZn_yCa_z metastable phase in fine-grained (3 μm) Mg alloys was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD). It was found that different dislocation configurations were formed in A and B alloys. Redundant free dislocations (RFDs) and dislocation tangles were the ways to form deformation substructure in A alloy, no RFDs except dislocation tangles were found in B alloy. The interaction between nano-scale second phase particles (nano-scale C15 and β-Mg₁₇(Al, Zn)₁₂ phase) and different dislocation configurations had a significant effect on the deformation substructures formation. The mass transfer of Mg_xZn_yCa_z metastable phases and the stacking order of stacking faults were conducive to the Mg-Nd-Zn typed long period stacking ordered (LPSO) phases formation. Nano-scale C15 phases, Mg-Nd-Zn typed LPSO phases, c/a ratio, β-Mg₁₇(Al, Zn)₁₂ phases were the key factors influencing the formation of textures. Different textures and grain boundary features (GB features) had a significant effect on k-value. The non-basal textures were the main factor affecting k-value in A alloy, while the high-angle grain boundary (HAGB) was the main factor affecting k-value in B alloy.

9Cr ferritic/martensitic (9Cr F/M) steels are considered ideal structural materials for various nuclear energy systems. However, δ-ferrite (δ), as a controlled phase, may occur in its welds. Three deposited metals with different carbon contents (0.04, 0.07, and 0.10 wt%) were investigated using experimental and finite element simulation methods. The results showed that the incomplete peritectic reaction, the incomplete δ to austenite phase transition, and the segregation of ferrite-stabilized elements led to the residual δ. The amount and morphology of δ significantly influence the mechanical properties. After increasing the carbon content, the increase in strength comes mainly from precipitation strengthening and dislocation strengthening, the presence of δ will reduce the strength. During the impact process, δ affects the absorbed energy for the stable crack growth through its morphology, and M₂₃C₆ affects the crack formation energy through its quantity. By decreasing the carbon content to a certain extent, the reduction of M₂₃C₆ content and the generation of large polygonal δ can effectively improve the toughness of 9Cr-steel deposited metals.

Due to their high theoretical capacity and abundant resources, transition metal sulfides are regarded as a prospering alternative to replace the commercial graphite anode in lithium-ion batteries (LIBs), particularly for large-scale energy storage and conversion applications. Nonetheless, low conductivity, easy agglomeration and obvious volume change greatly impede their practical application. In this work, a novel crystalline/non-crystalline carbon co-modified strategy is proposed to fabricate N, S co-doped carbon (NSC) layer wrapped Fe_0.95S_1.05/carbon nanotubes (CNTs) composite (Fe_0.95S_1.05/CNTs@NSC) through a simple Fenton reaction followed by a sulfurization process. Systematical characterizations and analyses reveal that this strategy well combines the advantages of crystalline CNTs and non-crystalline NSC, ensuring good conductivity and a high contribution to capacity from the carbon matrix. Meanwhile, the joint encapsulation of Fe_0.95S_1.05 by both CNTs and NSC can significantly mitigate the agglomeration and volume change of Fe_0.95S_1.05 during the continuous charge/discharge process. Benefiting from these advantageous features, the resultant Fe_0.95S_1.05/CNTs@NSC composite displays much improved cycling stability and rate capability when compared to the counterparts. Clearly, our research offers a distinct and innovative approach to design and construct advanced transition metal sulfides/carbon composite anodes for LIBs.

Magnesium alloys with excellent degradability and biocompatibility are promising materials for biomedical implants, saving patients the burden of second surgeries. However, their mechanical properties and corrosion resistance are significantly below the requirements for implant applications. This study aims to improve the mechanical and corrosion resistance properties of Mg-3Zn-0.2Ca alloy by pre-torsion treatment, and find out the optimal shear strain. The rod-shaped Mg-3Zn-0.2Ca alloy specimens were pre-torsion treated at different torsion angles. The effect of free-end torsion on Mg-3Zn-0.2Ca alloy was characterized through microstructure analysis, mechanical testing, and corrosion testing. Pre-torsion treatment can refine grain and induce twins and dislocations in Mg-3Zn-0.2Ca alloy. The surface hardness increases with the increase in torsion angle. Tensile yield strength and ultimate strength initially increase and then decrease with increasing torsion angle, while ductility decreases with increasing torsion angle. Specimens with a shear strain of 30% exhibit the highest tensile strength, reaching 284.78 ± 10.62 MPa, with an elongation of 19.37 ± 1.66%. Furthermore, they show a significant improvement in fatigue lives both before and after pre-corrosion. When the stress amplitude is 120 MPa, the fatigue lives for specimens without pre-torsion treatment are 54,275 cycles and 4324 cycles before and after pre-corrosion, while they increased significantly to 92,015 cycles and 5050 cycles with the 30% shear strain, respectively. Additionally, the 30% shear strain specimens show a significant reduction in corrosion rates. In conclusion, pre-torsion treatment can effectively modify the microstructure of Mg-3Zn-0.2Ca alloy, enhancing both mechanical properties and corrosion resistance. The optimal shear strain for this improvement is 30%. This study provides a practical method to enhance the mechanical properties and corrosion resistance of Mg-3Zn-0.2Ca alloy, making it more suitable for biomedical implant applications.

This work systematically investigates the densification, microstructure evolution and the attainment of high strength-ductility in Ti-13Nb-13Zr-2Ta alloy processed by laser powder bed fusion (LPBF). A narrow and viable process window (P_{laser power} = 175 W, v_{scanning speed} = 1000 mm/s, h_{scanning distance} = 0.1 mm and d_{layer thickness} = 0.03 mm) was accordingly determined and the relative density of Ti-13Nb-13Zr-2Ta alloy reaches 99.76%. The depth of molten pool increases gradually with the increase of energy density, and the relationship between the depth of molten pool and energy density has been quantitatively described. Three types of α′ martensites with average grain width less than 3 μm can be observed in the LPBF-fabricated Ti-13Nb-13Zr-2Ta alloys, attributed to the significantly high cooling rate and remelting process. The fine grain size, high density dislocations, nanotwins, ordered oxygen complexes and α + α″ heterostructure all contributed to the high strength (1037.75 ± 25.18 MPa) and ductility (20.32% ± 1.39%) of LPBF-fabricated Ti-13Nb-13Zr-2Ta alloy in this work.

Laser powder bed fusion (LPBF) technology offers a promising solution to the fabricability challenges of titanium alloys; however, it introduces defects such as porosity and cracking. Here, we evaluated the effectiveness of hot isostatic pressing (HIP) in eliminating defects and enhancing the overall properties of LPBF Ti-6Al-4V alloy. Our findings indicated that LPBF Ti-6Al-4V alloy after HIP established better corrosion resistance and ductility. These improvements could be related to the decomposition of αʹ phase and the elimination of internal defects within alloy matrix. Furthermore, the application prospect of LPBF Ti-6Al-4V alloy in spent fuel reprocessing environment was expounded.

The effect of carbon content on the microstructures and stress rupture properties of a newly developed polycrystalline Ni-based superalloy with high Cr content has been studied. It was observed that both grain size and the number of carbides increased with an increase in carbon content. After heat treatment, granular M₂₃C₆ carbides were dispersed around MC carbides along grain boundaries and inside grains. The quantity of granular M₂₃C₆ carbides increased while their sizes decreased. These findings can be verified with the results of thermodynamic calculation and differential scanning calorimetry analysis. The stress rupture times (975 ℃/225 MPa) increased from 13.3 to 25.5 h with the carbon content increased from 0.1 to 0.2 wt.%. The improvement can be attributed to two primary factors. Firstly, grain boundary is typically weak region during deformation process and the grain size increased as carbon content increased in the alloy. Secondly, carbides act as hindrances to impede dislocation movement, leading to dislocation entanglement. As carbon content rose, the quantity of carbides in interdendritic regions and grain boundaries increased, providing a certain degree of strengthening effect and resulting in a longer stress rupture time.

In this study, the effects of ultrasonic coaxial assisted TIG welding (U-TIG) on the formation, microstructure, and mechanical properties of TC4 titanium alloy welded joints were investigated. The results indicated that at optimal ultrasonic power and welding current settings, remarkable improvements of welded joint were observed. Specifically, the weld depth-to-width ratio increased by 267% compared to conventional TIG welding, reaching 0.22. Furthermore, the microhardness of the welded seam increased by 53%, reaching 550.1 HV, while the tensile strength of the welded joint rose to 1197.7 MPa, representing a 12.2% enhancement. The compressing effect of ultrasound concentrates the energy of the arc, thereby improving the penetration depth of the welded seam. In addition, ultrasonic cavitation plays a crucial role in refining the grain structure of the weld seam, consequently enhancing its strength.

As a representative of the third generation advanced high-strength steels (AHSSs), medium Mn steels (MMS) have broad development prospects in the field of automobile manufacturing. MMS with typical austenite reversion treatment have a soft duplex microstructure, i.e. ferrite + austenite, presenting a high ductility but a low yield strength. Here we show that a flash heating and cooling after austenite reversion treatment can replace the ferrite with strong martensite, which greatly enhances the yield strength of a 0.25C-4Mn steel by about 461–886 MPa. By adjusting the reversion temperature before the flash treatment, the C and Mn concentrations of reverted austenite can be altered, which determine the fraction of reverted austenite surviving the flash treatment. In addition, the mechanical stability of final retained austenite is also linked to the reversion temperature, resulting different work hardening behaviors due to transformation induced plasticity (TRIP) effect. By tweaking the reversion temperature before the flash treatment, an optimized combination of strength and ductility can be achieved. The micromechanical differences caused by the replacement of the matrix are also investigated via in-situ digital image correlation method.