Search Results (514)

In this study, C/N-containing Fe₃O₄ oxide films over an inner nitride layer were fabricated on 45# steel by oxynitriding to improve corrosion resistance in chloride-containing environments. The films exhibited a dense polyhedral structure, with nanoscale Fe₃O₄ precipitates at grain boundaries. Nitrogen and carbon were uniformly distributed within the oxide grains, inducing lattice expansion and modifying the Fe-O bonding environment. First-principles calculations based on C/N substitution models suggested that C/N incorporation may increase the unit cell volume, strengthen lattice bonding, and enhance the theoretical hardness of Fe₃O₄. The optimally doped films exhibited outstanding corrosion resistance, with a corrosion potential of 0.115 V_SCE, a corrosion current density of 3.16 × 10⁻¹⁰ A/cm² in 3.5 wt.% NaCl solution, and a corrosion-free lifetime of up to 3600 h in neutral salt spray testing. This superior performance is attributed to the synergistic effects of the compact single-phase magnetite layer, grain boundary precipitates, and modified electronic structure, which collectively inhibit chloride ingress and convert localized electrochemical attack into uniform corrosion. The experimental results are consistent with first-principles predictions, which clarified the mechanism of nitrogen doping in material corrosion protection from a mechanistic perspective. Full article

In this paper, the supersaturated solid solution of Al-Cu3-Si-Mg alloy prepared by molten metal die forging (MMDF) was used as the research object. The formation and evolution of precipitates during aging treatment were investigated through experiments at different temperatures and times, and the precipitation mechanisms and sequences of various precipitates were analyzed. The main precipitated phases formed in the supersaturated solid solution of the Al-Cu3-Si-Mg alloy after aging treatment are θ(Al₂Cu), θ′(Al_3.6Cu₂), γ′(Al_0.63Mg_0.37), and η′(Cu, Si). Based on XRD and TEM analysis under different aging treatment conditions, the precipitation sequence is determined as follows: SSS → GP₀ → GP₀ + γ′ → GP₀ + (γ′ + γ) + θ″ + η′ → (γ′ + γ) + (θ″ + θ′) + (η′ + η) → (γ′ + γ) + (θ + θ′) + (η′ + η) → (γ′ + γ) + (θ + θ′) + η → γ + θ + η. After aging treatment at 165–185 °C for 4 h, chain-like θ(Al₂Cu) precipitates are discontinuously distributed at the α-Al grain boundaries, and disc-shaped θ′(Al_3.6Cu₂) and θ″(Al₂Cu) phases mainly precipitate within the grains. When the temperature exceeds 185 °C, the chain-like θ(Al₂Cu) precipitates at the grain boundaries gradually become continuous, and the fraction increase from 1.5% to 15.2%. The amount of the θ(Al₂Cu) phase in the grains increases from 2 to 6, and the size of θ′(Al_3.6Cu₂) decreases obviously. After aging treatment at 185 °C for 5–6 h, the chain-like θ(Al₂Cu) precipitates become more continuous, and the fraction continues to increase from 32.1% to 52.6%. The effect of chain-like precipitates at grain boundaries on the mechanical properties of the matrix is opposite to the strengthening contribution of dispersed intragranular precipitates. When the aging condition exceeds 185 °C × 5 h, the excessive formation of chain-like grain boundary precipitates causes both the strength and hardness of the alloy to show a decreasing trend. Full article

In this study, 310S austenitic stainless-steel was welded using a laser with varying amounts of Nb to systematically investigate the effect of Nb on solidification cracking susceptibility, mechanical properties, and corrosion resistance of the weld. Under the present experimental conditions, the critical restraint width was higher for the 0.58 wt.% Nb and 1.45 wt.% Nb welds than for the Nb-free and 2.3 wt.% Nb welds, indicating that Nb addition affected the solidification cracking response of the weld. At low-to-moderate Nb contents, Nb can aggravate compositional segregation and increase the presence of low-melting-point liquid films, thereby increasing cracking susceptibility. At higher Nb contents, the reduced cracking susceptibility was accompanied by microstructural refinement and changes in the distribution of Nb-rich constituents during solidification. With increasing Nb content, the number of precipitated phases in the weld increases, mainly distributed at the austenite grain boundaries in granular, elongated, and chain-like forms. The introduction of Nb generally increases the microhardness and tensile strength of the welded joint, attributed to grain refinement strengthening and solid-solution strengthening. The reduction in area first increased and then decreased, suggesting that excessive Nb addition may reduce ductility because of the increased amount of grain-boundary precipitates and local strengthening heterogeneity. With increasing Nb content, the Ir/Ia ratio decreased from 67.6% to 52.2%, suggesting improved intergranular corrosion resistance. This improvement is likely related to the preferential reaction of Nb with carbon, which may suppress the formation of Cr-depleted zones at grain boundaries. Overall, Nb addition improved the corrosion resistance and increased the hardness and tensile strength of the weld; however, its effect on solidification cracking susceptibility was non-monotonic, indicating that careful control of Nb content is required to balance cracking susceptibility, mechanical properties, and corrosion resistance. Full article

Accurate individual tree crown (ITC) segmentation from unmanned aerial vehicle (UAV) imagery is important for fine-scale forest inventory, plantation management, and ecological monitoring. However, delineating ITCs in dense plantation environments remains difficult because crowns are strongly adjacent, canopy structures are highly homogeneous, and crown boundaries are often blurred, making it hard for existing methods to preserve both regional integrity and boundary continuity. This study proposes the Perceptual Segment-Anything Model with Multi-head Cross-Parallel Attention (Per-SAM-MCPA), a lightweight and effective framework for fine-grained ITC segmentation in dense plantation scenes. Based on a compact ResNet-50 backbone, the framework integrates perceptual target-aware representation, multi-scale detail enhancement, global contextual modeling, and semantic-boundary collaborative refinement to improve crown discrimination and structural consistency. A perceptual relation module is used to strengthen pixel-level semantic dependency modeling, and a Multi-head Cross-Parallel Attention (MCPA) mechanism is designed to capture long-range contextual interactions through orthogonally decomposed spatial attention, improving global geometric consistency with limited computational overhead. A Composite Constraint Loss (CCL) that combines a weighted cross-entropy loss, a structural similarity loss, and a boundary term based on Hausdorff distance is introduced to jointly optimize region-level segmentation quality and boundary fidelity. Experiments on the Catalpa bungei UAV dataset show that the proposed method achieves an intersection over union (IoU) of 87.3% and an F1-score of 91.0%, outperforming representative baseline methods such as SAM and Mask R-CNN while maintaining an inference speed of 35.7 FPS on a single GPU. These results indicate that Per-SAM-MCPA offers an accurate, efficient, and practical solution for ITC segmentation in dense plantation environments. Full article

This investigation elucidates the elevated-temperature (650 °C) monotonic mechanical response and very-high-cycle fatigue (VHCF) characteristics of Inconel 718 superalloys additively manufactured via selective laser melting (SLM), with a comparative assessment between the as-built and post-process heat-treated states. The results indicate that mechanical performance improves after heat treatment, primarily due to the formation of γ′ and γ″ precipitates, which interact with dislocations to strengthen the alloy. Relative to the as-built specimens, the fatigue strength of the specimen after heat treatment has increased by more than twice. For the as-built specimen, fatigue cracks nucleate at the specimen surface. However, in the high stress range, crack initiation in the heat-treated specimens consistently occurs at the free surface, whereas under low stress conditions, the crack initiation site transitions to the subsurface region encompassing internal defects. Post heat treatment, the fatigue crack trajectory adopts a markedly ductile and tortuous morphology, engendered by the concerted influence of grain-boundary (Laves/δ) precipitates that enforce repeated crack deflection, matrix-strengthening phases that homogenize plastic strain and the attendant reduction in local strain accumulation under the effect of cyclic load. Full article

Accurate extraction of mountaintop areas from grid digital elevation models (DEMs) is essential for terrain analysis, geomorphological research, hydrological modeling, natural disaster monitoring, and emergency communication site selection. However, existing deep-learning-based methods often suffer from inadequate representation of local details and limited global contextual awareness, leading to blurred boundaries and reduced segmentation accuracy in complex mountainous terrains. To address these limitations, this study proposes a dual-encoder and global-context-enhanced TransUNet framework, named DEGC-TransUNet, for automated mountaintop delineation. The architecture integrates a convolutional encoder to capture fine-grained local terrain features and a MaxViT-based encoder to model multi-scale global context by encoding low-dimensional topographic attributes such as slope and curvature. A dedicated feature fusion module harmonizes complementary representations from both encoding paths, while a BiFormer-based strategy is introduced at the bottleneck to strengthen long-range dependencies and enhance convergence. The experimental results demonstrate that DEGC-TransUNet significantly outperforms baseline models such as TransUNet, DE-TransUNet, and GC-TransUNet, with relative improvements of 19.8% in Intersection over Union (IoU), 10.4% in overall accuracy (ACC), and 10.9% in F1-score. These findings provide a robust solution for mountaintop extraction, with significant potential in analyzing geomorphological evolution, simulating soil erosion, modeling species distribution in “sky island” ecosystems, and optimizing strategic placements for communication base stations and wind energy infrastructures. Full article

Rapid solidification by melt-spinning produces aluminum alloys with extremely refined microstructures but also introduces strong structural gradients across the ribbon thickness. In this work, the microstructural evolution of a rapidly solidified Al-Cu-Li-Mg-Sc-Zr alloy was investigated during model homogenization using in situ STEM heating experiments and correlated with bulk electrical-resistivity measurements. The as-cast ribbons exhibit two distinct solidification zones: a near-contact region consisting of columnar cells containing fine Cu-rich spherical precipitates, and a central region composed of larger eutectic cells enriched in Al₂Cu and Al₇Cu₂Fe phases. Stepwise in situ STEM annealing between 200 °C and 500 °C reveals a sequence of transformations, including matrix depletion due to precipitation of strengthening phases, coarsening of primary phases, and formation of Al₃(Sc,Zr) dispersoids. Above 500 °C, rapid dissolution of Cu-rich primary phases occurs, leaving only a limited number of stable grain-boundary particles of the Al₇Cu₂Fe phase, eliminating the original two-zone structure, and resulting in a fully homogenized ribbon. Ex situ annealing confirms that the resulting microstructure is uniform across the ribbon thickness and enables consistent precipitation strengthening during artificial aging. The proposed annealing treatment is based on numerical models for homogenization of eutectic systems. The final annealing step combines homogenization and solution treatment at 530 °C for periods close to 5 min—two orders of magnitude shorter than standard holding times. Microhardness measurements from both ribbon surfaces reveal an identical peak-aged hardness of 135 HV, validating the effectiveness of the short-time homogenization strategy for rapidly solidified Al-Cu-Li-Mg-based alloys. Full article

In this study, the effects of friction stir welding (FSW) parameters on the mechanical properties and formability of pre-hardened (PH) 2219 aluminum alloy welds were systematically investigated through tensile testing and Erichsen tests. Energy dispersive spectrometry (EDS), electron back scatter diffraction (EBSD), and a transmission electron microscope (TEM) were employed to characterize the microstructure of the PH alloy weld joints, revealing the strength–ductility synergy mechanism of the PH welded sheets. Experimental results indicated that with respect to mechanical properties, when the welding rotational speed was fixed at 1000 rpm, increasing the forward speed from 50 mm/min to 150 mm/min reduced the ultimate tensile strength (UTS) by 6.3% and decreased the EL by 21.4%. When the forward speed was fixed at 50 mm/min, increasing the rotational speed from 500 rpm to 1500 rpm resulted in only a 0.4% variation in UTS and maintained a stable EL, demonstrating that forward speed is the dominant parameter affecting mechanical properties. In terms of formability, at a lower forward speed (50 mm/min), the Erichsen value exhibited a single-peak trend with increasing rotational speeds. At higher forward speeds (100 or 150 mm/min), the Erichsen value was insensitive to changes in rotational speed. When the rotational speed was fixed at 1500 rpm, increasing the forward speed from 50 mm/min to 150 mm/min reduced the Erichsen value by 21.3%. Microstructural strengthening mechanism: In the weld zone, the cooperative precipitation of the θ″ and θ′ phases effectively hindered dislocation motion. Simultaneously, the high geometric compatibility factor promoted the activation of multiple slip systems, and dislocation rearrangement subsequently led to the formation of sub-grain boundaries, thereby achieving strength–ductility cooperation. These findings provide theoretical support for the performance-driven welding process design of high-strength aluminum alloy components in aerospace applications. Full article

This study systematically investigates the effects of solution temperature ranging from 1060 to 1150 °C on grain growth kinetics, microstructural evolution, and tensile properties of GH4698 superalloys. The results indicate that grain size coarsens parabolically with increasing solution temperature. Based on the Sellars model, the grain growth time exponent n is determined to be 3.4 and the activation energy Q is 478.7 kJ·mol⁻¹. This confirms that the grain growth process is significantly influenced by both MC carbide pinning and alloying element drag effects. Additionally, due to the coarsening of grains, the precipitation density of M₂₃C₆ carbides per unit grain boundary length increased from 0.26 μm⁻¹ to 0.39 μm⁻¹. The ultimate tensile strength at room temperature decreased from 1268 MPa to 1226 MPa, and the yield strength decreased from 840 MPa to 807 MPa, while the elongation remained at 28–32%. At 700 °C, the ultimate tensile strength decreases from 974 MPa to 904 MPa, and the yield strength decreases from 755 MPa to 696 MPa, with the elongation remaining at ~6%. Quantitative analysis reveals that the decrease in strength is primarily due to the weakening of grain boundary strengthening caused by grain coarsening. At 700 °C, the deformation mechanism transitions from dislocation shearing at room temperature to stacking fault shearing. This not only leads to a reduction in strength but also, accompanied by grain boundary weakening, results in a decrease in elongation. Full article

Crown-level tree species semantic segmentation enables fine-grained forest inventory and management. Current high-precision tree species classification typically relies on multi-source remote sensing data, the acquisition and processing of which remain costly for large-area applications, making low-cost unmanned aerial vehicle (UAV) RGB imagery an attractive option for large-scale forest mapping. However, in heterogeneous forests, complex canopy structures and the limited spectral discriminability of low-cost UAV RGB imagery make 2D appearance cues alone insufficient for reliable species discrimination, crown delineation, and accurate separation of adjacent crowns. This often leads to inter-class confusion, blurred crown boundaries, and poor recognition of small crowns. To address these limitations, this paper proposes MonoCrown (MCrown), which strengthens geometric and contextual representation for distinguishing visually similar species and delineating crowns from single-temporal UAV RGB imagery. To compensate for the insufficiency of appearance cues, MCrown introduces monocular depth inferred offline from the same RGB image as a frozen geometric prior, and integrates cross-window global–local attention (CW-GLA), bidirectional cross-modal attention (BiCoAttn), and depth-adaptive injection (DAI) to capture long-range dependencies and promote complementary use of appearance and geometric features, especially for small crowns with similar visual patterns in complex scenes. To validate the method’s effectiveness, a crown-level UAV RGB dataset covering approximately 40 km² was constructed. Systematic comparative experiments were conducted on the proposed dataset and on public benchmarks, supporting the effectiveness of the proposed approach across ten dominant classes, especially for small crowns and visually similar categories. Its mean Intersection over Union (mIoU) and overall accuracy (OA) reached 74.1% and 87.3%, respectively. The method achieves high-precision crown-level tree species semantic segmentation using single-temporal UAV RGB as the sole acquired modality, while monocular depth inferred from the same RGB image serves only as a frozen geometric prior, without requiring multispectral, multi-temporal, or active-sensor acquisitions. This offers a practical solution for crown-level tree species mapping in heterogeneous forests. Full article

Cracking represents a critical issue in γ’-strengthened Ni-based superalloys processed via laser powder bed fusion. This study systematically investigated the influence of scan speed (800–1200 mm/s) on the crack elimination mechanism, microstructural evolution, and mechanical properties of LPBF-processed IN738LC alloy. Near-defect-free IN738LC parts were successfully produced with a relative density of 99.6% and a crack density of only 0.025%. The results indicate that as the scan speed increased from 800 mm/s to 1100 mm/s, a flatter melt pool (S4) was obtained, which reduced the proportion of high-angle grain boundaries. The cooling rate also increased from 13.68 K/μs to 15.96 K/μs, promoting grain refinement and the dispersion precipitation of MC carbides. The refined grains effectively suppressed stress concentration and inhibited crack propagation along grain boundaries. The optimized process (1100 mm/s) achieved optimal comprehensive mechanical properties. Compared to a scan speed of 800 mm/s, the ultimate tensile strength, yield strength, and elongation at room-temperature increased from 1075 MPa, 820 MPa, and 13.2% to 1179 MPa, 871 MPa, and 21.1%, respectively, while hardness increased from 365 HV_1.0 to 387 HV_1.0. This study demonstrated that the microstructure and mechanical properties of LPBF-processed IN738LC alloy can be tailored via controlling the thermal history of the melt pool, providing a foundation for processing high-crack-sensitivity alloys utilizing laser powder bed fusion. Full article

In order to verify the feasibility of in situ repair of underwater local dry laser welding (ULDLW) on nuclear power reactor components, this work investigates the microstructure and mechanical properties of 304L austenitic stainless steel repaired by ULDLW using ER308L filler metal. Comprehensive comparison would be made between the ULDLW and conventional in-air laser welding to evaluate their applicability. The results demonstrate that the rapid cooling rate inherent to the underwater environment significantly influences solidification behavior and microstructural evolution. The weld metal (WM) solidifies in the ferritic–austenitic (FA) mode, with an increased proportion of lathy δ-ferrite at the expense of skeletal morphology compared to the in-air welds. Electron backscatter diffraction (EBSD) analysis reveals the substantial grain refinement in underwater welds, with average grain sizes of 39.4 μm versus 47.3 μm for in-air weld bead, accompanied by a higher fraction of low-angle grain boundaries (LAGBs). These microstructural modifications yield superior mechanical properties: underwater weld bead exhibits ultimate tensile strength (UTS) of 685.6 MPa, elongation of 57.5%, and impact toughness of 22.6 J, significantly exceeding the corresponding values for in-air welds (663.9 MPa, 51.8%, and 18.6 J, respectively). Fractographic analysis confirms ductile fracture mechanisms in both conditions. The enhanced performance is attributed to grain refinement strengthening via the Hall–Petch relationship and the increased LAGBs fraction, which impedes dislocation motion and crack propagation. Full article

The precipitation characteristics and grain-boundary structure of Al–Cu–Mg alloys strongly affect their corrosion behavior, whereas the roles of Si and Ag microalloying in low Cu/Mg ratio systems are not yet fully understood. In this work, the effects of Si and Ag additions on age-hardening response, precipitation characteristics, and corrosion performance were systematically investigated by combining transmission electron microscopy with electrochemical and corrosion measurements. Si addition significantly accelerated the age-hardening kinetics, enabling the alloy to reach a hardness of 147 HV after only 6 h of aging, whereas the base alloy required 24 h to reach a similar level. This accelerated response was accompanied by refined S-phase precipitation and a markedly narrowed precipitation-free zone along grain boundaries. Further Ag addition introduced coherent Ω precipitates and a more complex multi-phase precipitation structure, which increased microstructural heterogeneity. As a result, the Al–Cu–Mg–Si alloy exhibited the lowest corrosion current density and the shallowest corrosion depth, whereas the Al–Cu–Mg–Si–Ag alloy showed deteriorated corrosion resistance. These results indicate that Si microalloying alone can simultaneously accelerate aging and improve corrosion resistance, while further Ag addition enhances precipitation complexity and strengthening potential but increases susceptibility to localized corrosion. Full article

This study investigates the effects of tempering temperature on the microstructural evolution and mechanical properties of Cr-Ni-Mo-V steel designed for pressure vessel applications. The microstructure was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM, Thermo Fisher Talos F200X), electron backscatter diffraction (EBSD), and physicochemical phase analysis. Mechanical performance was evaluated through tensile and impact tests, followed by a detailed discussion of the underlying strengthening mechanisms. The results demonstrate that the microstructure after tempering is fully tempered martensite. Samples tempered between 425 °C and 525 °C exhibit significant tempering resistance, maintaining a tensile strength of approximately 1300 MPa. This is primarily attributed to the synergistic effect of dislocation strengthening and the precipitation of MC-type carbides. As the tempering temperature increases to 625 °C, the dislocation density decreases sharply from 3.71 × 10¹¹ cm⁻² to 1.18 × 10¹¹ cm⁻², leading to a decline in strength. Concurrently, the impact energy increases significantly from 71 J to 132 J. The improvement in toughness is mainly attributed to the significant elevation of the crack initiation threshold, which is dominated by the reduction in matrix dislocation density, the coarsening and spheroidization of carbides, and the alleviation of local stress concentration. The relative proportion of high-angle grain boundaries (HAGBs, misorientation > 15°) increases from 51.9% to 57.7% during tempering, which is a result of the massive elimination of low-angle grain boundaries rather than an increase in the absolute length per unit area of HAGBs. Full article

Molecular dynamics (MD) simulations were performed to investigate the dynamic deposition behavior, growth mechanism, and mechanical properties of nickel–tungsten (Ni-W) alloy films on single-crystal Al(001) substrates. The results demonstrate that the incorporation of W atoms lowers the Ehrlich–Schwoebel (ES) barrier for Ni adatoms, facilitating downhill diffusion and effectively suppressing Volmer–Weber (VW) mode, thereby improving surface morphology and reducing film roughness. Additionally, W atoms exhibit a tendency to segregate at grain boundaries, inducing lattice distortion and structural disorder. With increasing W content (≥15 at%), the films undergo a transition from a nanocrystalline to an amorphous structure. Nanoindentation simulations reveal that film hardness increases with W content, with the strengthening mechanism being composition-dependent: dislocation pinning dominates at low W concentrations (≤5 at%), while the formation of an amorphous structure emerges as the primary strengthening mechanism at higher W contents (≥15 at%). This work elucidates the growth regulation and strengthening mechanisms of Ni-W films from an atomic-scale perspective, providing a theoretical foundation and simulation-driven guidance for the design and optimization of high-performance, environmentally benign Ni-W coatings. Full article