Journal of Materials Science & Technology最新文献

Overcoming the tradeoff between mechanical strength and electrical conductivity is a long-standing challenge in developing advanced copper alloys for industrial applications. Herein, we report a new strategy to obtain high strength and good conductivity of Cu-Ni-Si-Ca alloy by introducing and regulating the discontinuous precipitation (DP) and continuous precipitation (CP) behaviors. The DP process combined with thermomechanical treatment was exploited to expedite the precipitation kinetics, whilst the competition between DP and CP was utilized to inhibit the nucleation and growth of continuous precipitation phase (CPP). The resultant copper alloy exhibits superior comprehensive properties with a yield strength of 956 MPa, fracture strength of 989 MPa, and electrical conductivity of 34.1% IACS. The improved electrical conductivity is attributed to the heterogeneous-nucleation dominant DP, while the high strength stems from the combination of strain hardening and precipitation strengthening of δ-Ni₂Si and t-Ni₃Si precipitates. Notably, the precipitation strengthening arises from both the dislocation passing and cutting mechanisms, with the strongly ordered DO₂₂-type (t-Ni₃Si) phase contributing approximately 202 MPa to the overall strength through the cutting mechanism. This work offers a new pathway for alloy design of high-strength and high-electrical-conductivity copper alloys, by regulating coherent ordered nanoprecipitates through DP and CP.

Laser additively manufactured (LAM) Ni-based superalloys commonly exhibit low strength and high residual stress in the as-built state, requiring post-heat treatment to improve mechanical properties. We propose a modified heat treatment (MHT) process that only involves a single-step aging at 650°C for 4 h to achieve high strength, high ductility, and low residual stress simultaneously in a laser powder bed fusion (LPBF)-processed Inconel 718 (IN718) alloy. The MHT treated alloy exhibits comparable tensile strength (1368 MPa) to the conventional solution plus two-step aging (SA) treated alloy (1398 MPa), while the tensile elongation (∼21.7% for MHT treated alloy and 13.4% for SA treated alloy) is 60% higher and the residual stress (∼195 MPa) is 20% lower than the SA treated alloy. The balanced high performance of the MHT IN718 alloy was mainly attributed to the precipitation of abundant γ" phase with a size of ∼5 nm, while the original nano-sized Laves precipitates and dislocation cells were mostly retained. The finer size and higher fraction of γ" of the MHT sample mainly result from the dislocation structure and compositional variations in the as-built IN718, which promotes precipitation during aging. The retention of Laves phase, and cellular dislocation network in the MHT alloy also contributes to work hardening during tension and suspends the occurrence of necking. This study unveils a unique strengthening and toughening mechanism in the Ni-based superalloy produced by LAM with the presence of abundant Laves precipitates and provides a simple, low energy-consumption and cost-effective heat treatment route for achieving desirable mechanical properties.

Secondary dendrite orientation and wall thickness considerably affect the stress rupture life of thin-walled samples. However, the effect of the secondary dendrite orientation on the thickness debit effect of nickel-based single-crystal superalloys has not been thoroughly investigated until now. Owing to geometrical constraints, typical sheet samples cannot reveal the mechanism responsible for the thickness debit effect in turbine blades. This study examined the effect of secondary dendrite orientation on the thickness debit effect of nickel-based single-crystal superalloys at 1100°C/137 MPa in tubular samples. As the wall thickness decreased from 1.5 mm to 0.3 mm, the stress rupture life decreased from approximately 170 h to 64 h, demonstrating a noticeable thickness debit effect. Among the different secondary dendrite orientation areas, the variation in plastic deformation increased from 7% (1.5 mm) to 45% (0.5 mm) and subsequently decreased to 4% (0.3 mm). In thinner samples, the thickness contraction and microstructure evolution were more pronounced in the [100] areas than that in the [110] and [210] areas. The theoretical calculation quantitatively indicated that as the effective stress increased, the contribution of plastic deformation (45%) was slightly lower than that of oxidation (55%) in 0.3 mm samples; nevertheless, plastic deformation played a prominent role in 0.5, 0.8, 1, and 1.5 mm samples and increased from 61% (0.5 mm samples) to 85% (1.5 mm samples). In thinner samples, increased plastic deformation in the secondary dendrite orientation of the [100] areas and oxidation increased the effective stress, resulting in a shorter rupture life. These findings are conducive to the structural optimization and performance improvement of turbine blades.

The oxygen evolution reaction (OER) is regarded as the bottleneck of electrolytic water splitting. Thus, developing robust earth-abundant electrocatalysts for efficient OER has received a great deal of attention and it is an ongoing scientific challenge. Herein, hierarchical hollow nanorods assembled with ultrathin mesoporous cobalt silicate hydroxide nanosheets (denoted as CoSi) were successfully fabricated, using the silica nanotube derived from halloysite as a sacrificial template, via a simple hydrothermal method. The resulting cobalt silicate hydroxide nanosheets stack with thicknesses ∼10 nm, as confirmed by transmission electron microscopy. The elaborated nanoarchitecture possesses a high specific surface area (SSA) allowing good exposure to the cobalt active centers exhibiting superior catalytic activity vs analogs synthesized using sodium silicate. Among all as-prepared CoSi samples, those synthesized at 150°C (CoSi-150) exhibited the minimum overpotential of ∼347 mV at a current density of 10 mA cm^–2. In addition, CoSi-150 also exhibited superior performance against typical cobalt-based catalysts, and its surface hydroxyl groups were beneficial for the enhancement of OER performance. Furthermore, the CoSi-150 showed excellent durability and stability after the 10⁵ s chronopotentiometry test in 1 M KOH. This design concept provides a new strategy for the low-cost preparation of high-quality cobalt water splitting electrocatalysts.

Aramid papers (AP), made of aramid fibers, demonstrate superiority in electrical insulation applications. Unfortunately, the strength and electrical insulating properties of AP remain suboptimal, primarily due to the smooth surface and chemical inertness of aramid fibers. Herein, AP are modified via the nacre-mimetic structure composed of aramid nanofibers (ANF) and carbonylated basalt nanosheets (CBSNs). This is achieved by impregnating AP into an ANF-CBSNs (A-C) suspension containing a 3D ANF framework as the matrix and 2D CBSNs as fillers. The resultant biomimetic composite papers (AP/A-C composite papers) exhibit a layered “brick-and-mortar” structure, demonstrating superior mechanical and electrical insulating properties. Notably, the tensile strength and breakdown strength of AP/A-C5 composite papers reach 39.69 MPa and 22.04 kV·mm⁻¹, respectively, representing a 155% and 85% increase compared to those of the control AP. These impressive properties are accompanied with excellent volume resistivity, exceptional dielectric properties, impressive folding endurance, outstanding heat insulation, and remarkable flame retardance. The nacre-inspired strategy offers an effective approach for producing highly promising electrical insulating papers for advanced electrical equipment.

Powder bed fusion-laser beam with metals (PBF-LB/M) can be used to manufacture intricate NiTi components. However, the ductility of NiTi alloys printed by PBF-LB/M is generally ∼20% less than those made via conventional processes. Although many heat treatment methods have been proposed, solving this issue has been proven difficult. An intractable problem is the brittleness of PBF-LB/M-manufactured NiTi after solid solution treatment at 1000°C. By investigating the microstructural and fractography change after heat treatment in the range of 100-1000°C, this study found that this ductile-to-brittle transition stems from abnormal oxygen-containing Ti-rich precipitates being generated in the PBF-LB/M fabricated Ni-rich NiTi. We identified laser processing-induced local oxygen segregation and tiny TiO₂(B) particles at the fusion and grain boundaries. During the heat treatment at temperatures above 700°C, these oxides decompose due to their low thermal stability. After this decomposition, most oxygen diffuses into the matrix, with titanium remaining in local regions. This process enriches titanium in the interfaces, forming a brittle oxygen-rich Ti₂Ni network that is known to hinder the recrystallization process in heat treatment. Furthermore, when subjected to external loading, these precipitates can induce high misfit levels and local distortion, resulting in brittle fractures along the interfaces. Based on these results, we also propose approaches to avoid high-temperature-induced embrittlement in Ni-rich NiTi.

High entropy alloys (HEAs) have attracted much attention for their excellent mechanical properties stemming from diverse deformation mechanisms. Particularly, face-centered cubic (FCC) to body-centered cubic (BCC) martensitic transformation is crucial for enhancing the strength and plasticity of HEAs, particularly at cryogenic temperatures. However, the fundamental atomic mechanism underlying martensitic transformation remains elusive, and the impact of martensitic transformation on the mechanical properties of HEAs at room temperature is unknown. Here, we report in situ atomic-scale observations of a reversible martensitic transformation from FCC to body-centered tetragonal (BCT) and ultimately back to FCC in nanostructured CrMnFeCoNi HEA at room temperature under deformation. This martensitic transformation is completed by the synergistic action of 90° partial dislocations slip on (111)_FCC plane and atom shuffling, involving the periodic arrangement and slip of two 90° half Shockley partial dislocations a/12$\left[\overline{1}\overline{1}2\right]$$\left[\overline{1}\overline{1}2\right]$(111) and one 90° Shockley partial dislocation –a/6$\left[\overline{1}\overline{1}2\right]$$\left[\overline{1}\overline{1}2\right]$(111) on three successive (111)_FCC atomic planes. Additionally, the reversible phase transformation induced by high stress dissipates strain energies and hinders crack propagation, thereby enhancing the fracture toughness of HEAs. Our findings contribute to a deeper comprehension of the martensitic transformation mechanisms in HEAs, offering valuable insights for improving their mechanical properties.

Laser-welded Ti-6Al-4V is prone to severe residual stresses, microstructural variation, and structural defects which are known detrimental to the mechanical properties of weld joints. Residual stress removal is typically applied to weld joints for engineering purposes via heat treatment, in order to avoid premature failure and performance degradation. In the present work, we found that proper welding residual stresses in laser-welded Ti-6Al-4V sheets can maintain better ductility during uniaxial tension, as opposed to the stress-relieved counterparts. A detailed experimental investigation has been performed on the deformation behaviours of Ti-6Al-4V butt welds, including residual stress distribution characterizations by focused ion beam ring-coring coupled with digital image correlation (FIB-DIC), X-ray computerized tomography (CT) for internal voids, and in-situ DIC analysis of the subregional strain evolutions. It was found that the pores preferentially distributed near the fusion zone (FZ) boundary, where the compressive residual stress was up to -330 MPa. The removal of residual stress resulted in a changed failure initiation site from the base material to the FZ boundary, the former with ductile and the latter with brittle fracture characteristics under tensile deformation. The combined effects of residual stresses, microstructures, and internal pores on the mechanical responses are discussed in detail. This work highlights the importance of inevitable residual stress and pores in laser weld pieces, leading to key insights for post-welding treatment and service performance evaluations.

Laser powder bed fusion (LPBF) has been extensively investigated owing to its high geometry formation accuracy and excellent mechanical properties. However, the LPBFed Haynes 230 parts typically display poor tensile and wear properties due to internal porosity. In this work, the ultrasonic impact treatment (UIT) was applied as a post-treatment to the LPBFed Haynes 230 alloy, and porosity and microstructure modulation were performed to improve the strength properties and wear resistance. The pore closure and microstructure were studied by numerical simulations and experiments, and the mechanisms of increasing densification and strength were discussed. Results show that UIT can effectively close pores and reduce porosity, the internal porosity of the ultrasonic impacted layer for one, two, and three times decreases by 63.64%, 71.25%, and 81.97%, respectively. Pore closure is attributed to the residual compressive stress and shear stress introduced by UIT. Besides, the UIT weakened texture strength and refined grains, especially promoting the formation of fine grains. Meanwhile, it also promotes the formation of a high dislocation density and improves the phase structure distribution. Furthermore, the ultimate tensile and yield strengths of the optimal impact process increased by 9.63% and 34.56%, respectively. The improvement in strength was attributed to dislocation, grain boundary, and promoting densification strengthening. The average friction coefficient reduces by 4.90%–14.59% by refining the surface grains and increasing dislocation density. This work has verified the feasibility of improving the mechanical properties and pore closure of the LPBFed Haynes 230 alloy by UIT.

Control of the columnar to equiaxed transition (CET) is a major challenge in additively manufactured β titanium alloys. In this work, the promotion of CET was successfully achieved through in-situ fabrication of Ti-5Cu (wt.%) alloys with additions of 5, 15, and 25 wt.% Nb using elemental Ti, Cu, and Nb powders by employing laser powder bed fusion (LPBF). The alloy containing 5 wt.% Nb consisted of α lamellae, Ti₂Cu precipitates, and unmelted β-Nb inclusions, whereas the 25 wt.% Nb alloy consisted of equiaxed β grains, ω precipitates, and Ti₂Cu precipitates at the grain boundaries. In terms of mechanical properties, despite the presence of Nb inclusions and liquation cracks in the 5 wt.% Nb alloy, it showed a yield strength of 1051 ± 40 MPa and an elongation of 5.2% ± 1.3%. Both the strength and ductility decreased with increasing Nb content, e.g., the 25 wt.% Nb alloy exhibited a yield strength of 808 ± 53 MPa and an elongation of 1.6% ± 0.2%. As the Nb content increased from 5 to 25 wt.%, the Young's modulus decreased from 110 to 65 GPa. The 25 wt.% Nb alloy showed a high ratio of hardness to Young's modulus (H/E) and yield pressure (H³/E²). However, due to its brittle nature, the material manifested high wear rates. These findings provide a basis for the future development of novel low-modulus isotropic β-titanium alloys using LPBF.