Search Results (44)

Current limiting turns the closed loop of a grid-forming converter into a mode-switching system. For projection-based (feedback-optimization) limiting, this paper shows that conventional linearization fails at the constraint boundary in two distinct ways, and that dynamic mode decomposition (DMD) of trajectories of a nonlinear averaged-dq model recovers the physically consistent spectra. At unconstrained operating points, the Jacobian of the smooth primal–dual field is evaluated where that field is not stationary, and it contains a spurious lightly damped mode belonging to neither the projected nor the unprojected formulation. At constrained operating points, the dual flow pins the constraint whenever the multiplier is active and, because the current predictor is exact in steady state, a backstop limiter sharing the projection limit places the raw current reference on the limiter kink; the constrained equilibria are then non-isolated and path-dependent, and the two well-conditioned one-sided Jacobians differ by an order-one amount. Audited against branch-consistent and one-sided linearizations and certified by out-of-sample reconstruction, DMD recovers the correct spectra over a two-parameter operating map and flags the non-linearizable points. A matched comparison with saturation and virtual-impedance baselines attributes the loss of the network resonance to hard clipping of the bridge current, which instead exposes a lightly damped filter resonance; a backstop above the projection limit restores unique, linearizable equilibria at the price of retaining the resonance. Sensitivities to identification settings, grid strength, controller gains, control delay, and measurement noise are quantified, and a three-converter microgrid study shows a sustained oscillation where a shared threshold admits no equilibrium. Full article

Oxide dispersion strengthened (ODS) austenitic steels exhibit superior high-temperature stability and mechanical properties, which are attributed to the strong pinning effect of nano-oxide particles on dislocations and grain boundaries. The microstructure and properties of ODS steels are intrinsically governed by their fabrication techniques. While powder metallurgy (PM) serves as the predominant synthesis route, the fabrication of austenitic ODS steels faces distinct technical challenges compared to their ferritic counterparts, primarily due to issues such as powder sticking to the milling media. Conversely, although traditional melting processes often lead to oxide agglomeration, coarsening, or flotation, which makes it difficult to achieve a uniform dispersion, they remain of significant interest because of their scalability and cost-effectiveness. Furthermore, emerging technologies such as additive manufacturing (AM) have also been employed for the preparation of ODS steels. This review provides a systematic and comprehensive overview of the recent progress in the fabrication technologies of ODS austenitic steels. The scope encompasses a critical analysis of the merits and limitations of techniques including PM, in situ internal oxidation, melting, and AM, alongside an examination of the potential impacts of adding process control agents (PCA) to mitigate powder sticking during mechanical alloying. Additionally, the microstructural characteristics resulting from different processing routes are discussed in detail, followed by a consolidated evaluation of their mechanical properties. Full article

Quasi in situ observations of the early stage of secondary recrystallization in Fe-3.25 wt.% Si high-permeability grain-oriented silicon steel indicate that large matrix grains locally impede the migration of secondary recrystallized grain boundaries. A critical grain-size criterion for identifying potential pinning grains is proposed, and a kinetic model incorporating the matrix grain-size distribution is developed. For the abnormal grain tracked at 1000 °C, the measured grain-size increment over 100 s was approximately 60 μm; the proposed pinning model predicted 47 μm, whereas the weighted-average model predicted 171 μm. Model calculations further suggest that matrix grain-size dispersion, pinning force, grain size, and relative grain-boundary energy jointly affect the early-stage growth of Goss grains. These results provide a framework for interpreting early secondary-recrystallization kinetics, while broader experimental validation is still required before the predicted parameter combinations can be used for process control. Full article

Non-uniform heat flux in high-power electronic devices can induce local hotspots. To address hotspot thermal management, a hybrid microchannel heat sink featuring micro pin-fins with secondary channels (MPSCs) is proposed. Straight microchannels are used in the low-heat-flux region, while staggered pin-fins are arranged in the central hotspot region. Five models are considered: solid pin-fins without secondary channels (M1) and pin-fins with one-, three-, four-, and six-directional secondary channels (M2–M5). Over Re = 200–800, the thermal resistance of M3 is reduced by up to 43.3% compared with the SPMS, and the maximum PEC reaches 1.178. MPSC structures achieve better cooling at lower pumping power. At similar pumping power, the thermal resistance of M3 is 36.8% lower than the minimum value attainable by the SPMS. However, the marginal reduction in thermal resistance gradually weakens as the pumping power increases. The effects of hotspot heat flux and relative pin-fin height are investigated. MPSCs can withstand higher heat loads. Under a hotspot heat flux of 800 W/cm², the maximum temperature of M3 remains below the safety threshold. Relative pin-fin height has a coupled effect on the thermal–hydraulic performance. At Re = 800, the PEC of M3 at RPH = 0.9 is about 2.8% higher than that at RPH = 1.0, and a PEC fitting equation is developed. The results show that the hybrid micro pin-fin heat sink with secondary channels disrupts the thermal boundary layer through internal flow diversion and local disturbance, thereby balancing hotspot suppression and pressure drop control. This design provides a reference for hotspot thermal management and thermal–hydraulic optimization of microchannel heat sinks. Full article

The magnetic properties of materials similar to Nd-Fe-B permanent magnets are highly sensitive to microstructure. Using Hybrid Monte Carlo micromagnetics simulations, we systematically investigate how grain boundary (GB) and grain crystallographic orientation affect coercivity (H_c) and remanence (M_r). A polycrystalline model with independently adjustable microstructural parameters is constructed via Voronoi tessellation. Our results show that increasing GB width from 2 nm to 10 nm reduces Hc by 32% and M_r by 16%. Grain boundary acts as both a nucleation site and pinning center: a wider GB facilitates reverse domain nucleation, especially at the triple junctions. However, domain wall propagation is underpinned by GB during the propagation process. For a thick GB, H_c decreases with increasing GB saturation magnetization (M_s′), because the thick weakly magnetic layer weakens exchange coupling between adjacent grains, shifting the reversal behavior from collective switching to more localized nucleation. Increasing the average easy-axis tilt angle reduces H_c, as the misalignment lowers the effective anisotropy component along the applied field direction, facilitating magnetization reversal. These findings confirm the importance of GB and texture control in optimizing the magnetic performance of Nd-Fe-B permanent magnets, offering references for experimental investigations. Full article

The microstructure and mechanical behavior during 100 °C warm rolling of the high-Zn-content Al₈₀Zn₁₄Li₂Mg₂Cu₂ alloy were investigated. The alloy plate was warm-rolled to reductions of 40%, 60%, and 80%. Hardness and tensile strength decreased continuously with increased rolling up to 60%, demonstrating work softening, followed by a slight increase at 80% reduction, indicating work hardening. Systematic characterization revealed that this non-monotonic mechanical response arises from a competition between particle-stimulated nucleation (PSN)-assisted recrystallization and dislocation-driven hardening. The multi-scale intermetallic particles in this alloy play a dual role: coarse Al₅CuLi₃ particles generate high-strain particle deformation zones (PDZs) that serve as potent PSN sites, while fine nano particles pin the recrystallized grain boundaries and restrict their growth. The unusually low PSN activation temperature is attributed to the synergistic effects of the high PDZ storage energy and the progressive subgrain rotation mechanism within the PDZ. The ability to control PSN via micro- and nano-scale intermetallics presents a viable pathway for achieving grain refinement in Al-based alloys and enhancing the machinability of high-Zn-content Al alloys. Full article

We developed an arbitrary Lagrangian–Eulerian (ALE)-based multiphysics model for evaporation from a contact-line-pinned sessile drop of neat water subject to a vertically oriented sinusoidal alternating current (AC) electric field applied across parallel-plate electrodes. The framework fully couples electrostatics, incompressible flow, heat transfer with evaporative cooling, and transient vapour transport in air, and includes an instantaneous, voltage-controlled electrowetting contact-angle response under constant-contact-radius conditions. Validation against published data shows that the model captures both pinned-droplet evaporation and electrically induced deformation. Because Maxwell traction scales with the squared electric-field magnitude, droplet height and contact angle exhibit a robust 2:1 frequency-doubled response, producing two peak–trough events per voltage period. The resulting periodic deformation drives oscillatory interfacial shear and internal recirculation, yielding a synchronous double-peaked evaporative-flux waveform. Gas-side analysis quantifies a time-varying diffusion-layer thickness via a characteristic diffusion length; two thinning events per period coincide with flux maxima, indicating that AC enhancement is dominated by periodic compression of the vapour boundary layer and reduced gas-side mass-transfer resistance. Increasing voltage amplitude (0–60 kV) strongly accelerates volume loss, while frequency has a secondary effect: the cycle-averaged flux rises from 1 to 10 Hz but decreases slightly at 20 Hz due to phase lag and weaker boundary-layer modulation. Full article

The microstructural evolution and tensile behavior of Inconel 617 welded joints produced by gas tungsten arc welding (GTAW) with ERNiCrCoMo-1 filler were systematically investigated. Detailed microstructural characterization revealed that Cr-rich M₂₃C₆ and Ti-rich MC carbides are the dominant precipitates, while Mo-rich M₆C forms locally along grain boundaries after thermal exposure. The fusion and weld zones exhibit fine dendritic morphologies with uniformly distributed precipitates, resulting in significant strengthening through precipitation and dislocation–pinning mechanisms. Owing to the low heat input and compositional compatibility between the weld and base metals, the heat-affected zone remains extremely narrow and free of compositional transitions. The welded joint attains tensile strengths of 920 MPa at room temperature and 605.5 MPa at 750 °C, corresponding to joint efficiencies of 117% and 121%, respectively, with fracture consistently occurring in the base metal. Deformation analysis shows that plasticity at room temperature is governed by planar slip and dislocation entanglement, whereas deformation twinning predominates at elevated temperatures owing to the reduced stacking-fault energy and the pinning effect of M₂₃C₆ carbides. These results provide key insights into the deformation and strengthening mechanisms controlling the high-temperature performance of GTAW-welded Inconel 617 joints and offer guidance for their application in advanced nuclear and high-temperature energy systems. Full article

Interfacial coupling between CoFe₂O₄ (CFO) nanoparticles and oxidatively functionalized multi-walled carbon nanotubes (MWCNTs) enables controlled modulation of structural, optical, and spin dynamic properties in CFO–MWCNT nanocomposites. The solvothermal synthesis promotes nucleation of CFO on –COOH/–OH functional groups, ensuring uniform anchoring along the nanotube surface. X-ray diffraction confirms a cubic spinel phase with lattice expansion from 8.385 Å to 8.410 Å and crystallite growth from 18 nm to 25 nm, reflecting strain transfer and partial nanoparticle coalescence at the carbon interface. The observed bandgap narrowing from 2.72 eV to 2.50 eV, confirmed via Tauc plot analysis, is attributed to localized defect states induced by charge delocalization and orbital hybridization at the interface of the CFO–MWCNT boundary. DC magnetometry reveals a reduction in saturation magnetization from 46 emu/g to 35 emu/g due to diamagnetic dilution and interfacial spin canting, while coercivity decreases from 852 Oe to 841 Oe, indicating modified pinning and domain-wall dynamics associated with exchange-coupled interfaces. Ferromagnetic resonance measurements show a resonance field shift from 3495 G to 3500 G and an increase in the Landé g-factor from 1.97 to 2.00, signifying altered spin–orbit coupling and enhanced local magnetic perturbations. The spin–lattice relaxation time increases from 1.41 ns to 1.59 ns, demonstrating suppressed phonon-mediated relaxation and improved spin coherence across the hybrid network. Spin density rises from 3.72 × 10²² to 4.58 × 10²² spins/g, confirming an increase in unpaired electrons generated by orbital asymmetry at the interface. The anisotropy field and effective magnetocrystalline anisotropy constant exhibit pronounced modulation, evidencing strengthened exchange stiffness and altered Co²⁺/Fe³⁺ superexchange pathways. These results establish CFO-MWCNT nanocomposites as tuneable platforms for spintronic logic elements, high-frequency microwave attenuation, and magneto-optical device architectures. Full article

This paper presents a breakthrough in activating the skin effect at conventional broaching speeds (1–8 m/min) by using laser defocus gradient modification to induce surface embrittlement in martensitic stainless steel Z10C13. Through controlled defocusing, a 50 μm gradient remelting layer was created, which features ultrafine grains (0.8 μm) and a high-density geometrically necessary dislocation (GND) zone (ρGND = 2.27 μm⁻³). The quasi-cleavage fracture was triggered via dislocation pinning by non-oriented low-angle grain boundaries (28.4% LAGBs). Multiscale characterization confirms that this microstructural transformation enhances surface hardness by 12.95% (reaching 31.4 HRC), reduces cutting force by 34.07%, and improves surface roughness by 63.74% (Sz = 28.80 μm). Simultaneously, a parallel crack-deflection mechanism restricts subsurface damage propagation, resulting in a crack-free subsurface zone. These results demonstrate the effectiveness of the embrittlement–toughening dichotomy for precision machining of difficult-to-cut materials under low-speed constraints. Full article

Friction Stir Processing (FSP) has emerged as an advanced solid-state surface engineering technique for tailoring high-performance surface architectures in metal matrix composites (MMCs). By combining localized thermo-mechanical deformation with controlled material flow, FSP enables grain refinement, homogeneous dispersion of reinforcement, and strong interfacial bonding without melting or altering bulk properties. This review critically examines the role of FSP in enhancing the mechanical, tribological, and corrosion performance of composites, with emphasis on process–structure–property relationships. Key strengthening mechanisms, including grain boundary strengthening, load transfer, particle pinning, and defect elimination, are systematically discussed, along with their implications for wear resistance, fatigue life, and durability. Special attention is given to corrosion and tribo-corrosion behavior, highlighting electrochemical mechanisms such as micro-galvanic interactions, passive film stability, and interfacial chemistry. Furthermore, the eco-efficiency, industrial viability, and sustainability advantages of FSP are evaluated in comparison with conventional surface modification techniques. The review concludes by identifying critical challenges and outlining future research directions for the scalable, multifunctional, and sustainable design of composite surfaces. Full article

A pinning boundary control strategy that can achieve the exponentially clustered synchronization of a specific class of complex networks is developed. Firstly, the studied model captures the essential features of networks, including spatial dependence, stochastic switching, noise perturbation, and time delays. Secondly, the proposed control algorithm can save the implementation cost and overcome environmental constraint by acting on the boundary of a few nodes. Thirdly, an average state related to the directed topology of the nodes in the same cluster is calculated as the target network. Finally, nonlinear simulations show that the proposed controller can solve the cluster synchronization of a directed coupled reaction–diffusion neural network with Markovian switching, stochastic noise and time delay. Full article

This study proposes a novel framework for quantifying curved grain boundaries that overcomes key limitations of existing methods. Unlike Fourier-based approaches that require labor-intensive sequential analysis of individual boundaries and selectively represent only high-amplitude regions, or spline-based methods that demand complex parameter selection for interpolation points, the proposed framework integrates curvature variance filtering with U-chord curvature calculation to enable automated, comprehensive, and noise-resistant characterization of grain boundary morphology. The curvature variance filtering adaptively determines smoothing parameters based on local curve properties, while the U-chord curvature method ensures rotational invariance and robustness against digitization errors. Four heat treatment processes were applied to GH4169 alloy, producing distinct grain boundary morphologies with mean curvature (MC) values ranging from 0.0625 to 0.1252. Controlled cooling alone (Process A) yielded predominantly straight boundaries (91.06% straight, 0.12% serrated), while re-dissolution treatment (Process D) produced the highest serration degree (58.81% straight, 3.53% serrated). The quantitative analysis reveals that dispersed δ-phase precipitation creates discrete pinning points, forming serrated boundaries with sharp curvature peaks, whereas dense, parallel δ-phase arrays at specific angles produce coordinated wavy undulations. This framework provides a reliable quantitative tool for optimizing heat treatment protocols to achieve target grain boundary configurations in nickel-based superalloys. Full article

Crystal grain size control in steel is critical for achieving mechanical properties. This study investigates the effectiveness of microalloying with titanium, niobium, zirconium, and yttrium to inhibit grain growth with the pinning effect. The comparison of selected microalloying elements in the exact same conditions is crucial for understanding their effect and is novel. Hot-rolled samples were annealed across a wide range of temperatures (1050 to 1200 °C) for up to eight hours. Microstructural analysis confirmed the presence of stable precipitates and non-metallic inclusions such as Nb(C,N), Ti(C,N), ZrO₂, and Y₂O₃ acting as obstacles to grain boundary migration. All microalloying elements significantly outperformed the reference steel, but their effectiveness was highly dependent on the annealing temperature. Titanium was the most effective inhibitor at lower temperatures (1050 °C), while zirconium maintained control up to 1150 °C. Critically, at the highest temperature of 1200 °C, only the yttrium-alloyed steel retained a fine-grain structure, demonstrating superior thermal stability. Niobium, conversely, only showed a minimal effect at 1050 °C, though this grade also exhibited the highest hardness (up to 165 HB) due to precipitation hardening. The kinetics of grain growth were successfully modeled using the Arrhenius-type Sellars–Whiteman equation, accurately describing the behavior for up to four hours of annealing. The findings provide critical insight for selecting optimal microalloying strategies based on maximum operating temperature. Full article

This study systematically investigates the influence of Zr additions (0–0.24 wt.%) on the microstructure evolution and mechanical properties of Al–4.0Cu–0.5Mg–0.5Zn–0.5Mn–0.4Ag alloys under peak-aged conditions. Alloys were subjected to homogenization (420 °C/8 h + 510 °C/16 h), solution treatment (510 °C/1.5 h), and aging (190 °C/3 h). Microstructural characterization via OM, SEM, EBSD, and TEM revealed that Zr refines grains and enhances recrystallization resistance through coherent Al₃Zr precipitates, which pin grain boundaries and dislocations. However, excessive Zr (0.24 wt.%) induces heterogeneous grain size distribution and significant Schmid factor variations, promoting stress concentration and premature intergranular cracking. Crucially, Al₃Zr particles act as heterogeneous nucleation sites for Ω-phase precipitates, accelerating their nucleation near grain boundaries, refining precipitates, and narrowing precipitate-free zones (PFZs). Mechanical testing demonstrated that the Al–4.0Cu–0.5Mg–0.5Zn–0.5Mn–0.4Ag alloy exhibits optimal properties: peak tensile strength of 368.8 MPa and 79.8% tensile strength retention at 200 °C. These improvements are attributed to synergistic microstructural modifications driven by controlled Zr addition, establishing Al–4.0Cu–0.5Mg–0.5Zn–0.5Mn–0.4Ag–0.16Zr as a promising candidate for high-temperature aerospace applications. Full article