Laser powder bed fusion (LPBF) offers high forming precision and the ability to manufacture complex structures, providing a new paradigm for the microstructural design of nickel-based superalloys. However, the rapid cooling and interlayer reheating inherent to LPBF lead to significant deviations in the evolution of secondary phases compared with traditional processes. This study systematically reviews the formation and evolution mechanisms of various secondary phases (γ′ phase, γ″ phase, topologically close-packed (TCP) phase, MC/M23C6 carbides, and dispersed oxides) in LPBF-fabricated nickel-based alloys, and establishes a causal framework linking process parameters, melt pool behavior, solidification substructure, and precipitation. The study reveals that nonequilibrium segregation and cyclic thermal history induce dissolution, reprecipitation, coarsening sequences, and a precipitation gradient along the build direction. By integrating experimental, heat treatment, and multiscale simulation results, a heat treatment window is identified to stabilize γ′/γ″ phase and suppress Laves/δ/TCP phases. The contributions of carbides and oxides to grain boundary pinning and high-temperature stability, coupled with the risks of embrittlement associated with excessive coarsening, are discussed. The synergistic secondary effects of γ′/γ″ phase on creep and fatigue performance, along with the service risks arising from the transformation of γ″ phase into δ phase, are also elucidated. Furthermore, the cooperative effects of hot isostatic pressing, aging, and scanning strategies on precipitation homogenization and defect mitigation are examined. Finally, key challenges and future development paths in multiphase cooperative control, multiscale prediction, and integrated process-microstructure-property design are proposed.
Driven by the “dual-carbon” strategy, the lightweighting of rail transit equipment has become a dominant industry trend. Among the key components, the brake disc—critical to the braking system—faces particularly stringent weight-reduction requirements. SiCp-reinforced aluminum-based composites (SiCp/Al), known for their low density, high thermal conductivity, and superior wear resistance, are now regarded as the prime candidate to replace conventional cast-iron discs. In recent years, SiCp/Al brake discs have been implemented in urban rail transit vehicles operating at 120 km/h. However, their high-temperature stability remains a technical challenge, and no breakthroughs have been achieved for operation on suburban railways at speed ≥ 140 km/h. This study systematically reviews the research progress and engineering applications of SiCp/Al brake discs in rail transit, focusing on its fabrication methods, core performance, in-service behavior, and evaluation standards. Future development directions are discussed to guide technological innovation and the industrial deployment of advanced rail transit equipment.
Austenitic stainless steels are utilized in the in-core and out-of-core structural components of various Generation-IV nuclear reactors owing to their excellent comprehensive properties, mature manufacturing processes, and decades of service experience in pressurized water reactors. However, after prolonged exposure to high temperature and high-intensity irradiation, these steels gradually exhibit some performance limitations, including lower creep strength, inadequate microstructural stabilities, and higher radiation embrittlement sensitivities. Recently, alloying with Nb has been proven to effectively regulate the precipitation behavior of secondary phases and the formation and evolution of irradiation defects in austenitic stainless steels, promising simultaneous improvements in creep strength and irradiation tolerance. Therefore, Nb alloying has emerged as a key pathway for the development of next-generation austenitic stainless steels with enhanced high-temperature performance, improved irradiation resistance, and extended service life. This paper systematically reviews the research progress of Nb-stabilized austenitic stainless steels for Generation-IV nuclear power applications, including the compositional design and optimization to improve the creep properties and irradiation resistance, the effects of Nb addition on austenite stability, the formation mechanisms of primary NbC, and the precipitation behavior of Nb-bearing secondary phases. Additionally, the mechanisms by which Nb alloying influences the strength-ductility balance, creep strength, high-temperature microstructural stability, and irradiation-induced defect evolution are discussed. Finally, some future research prospects of Nb-stabilized austenitic stainless steels for Generation-IV nuclear reactors are discussed.
Ultra-high-strength (≥ 3 GPa) and ultra-fine (diameter ≤ 16 μm) stainless steel wires are critical components in solar photovoltaic manufacturing, yet their domestic production poses distinct technical challenges. Countries such as Japan and Germany currently dominate this field, having developed the capability to manufacture wires with tensile strengths of ≥ 3.5 GPa and diameters of ≤ 11 μm. Localizing ultra-high-strength and ultra-fine stainless steel wire technology has therefore become an urgent priority. The extreme true strain and fine diameter attained through cold drawing produce a complex three-dimensional lamellar and heterogeneous microstructure that conventional two-dimensional characterization techniques cannot fully capture. Moreover, the absence of systematic three-dimensional characterization methodologies and standards further impedes localization efforts. This study presents a comprehensive overview of the characterization techniques currently employed for ultra-fine metallic wires and critically evaluates existing approaches for analyzing microstructure, mechanical properties, and performance, along with their respective limitations and strengths. The study also identifies opportunities for developing multiscale, three-dimensional in situ characterization techniques that would provide essential technical support and a theoretical foundation for optimizing wire performance and advancing localization.
As a key structural material for aviation, aerospace, and major equipment, the lightweight and high-performance development of ultra-high strength (UHS) steel is essential for the advancement of related fields. However, traditional UHS steels have long faced critical challenges: the difficulty in achieving a synergistic combination of high strength, high toughness, good weldability, and corrosion resistance, along with the constraint of high alloy costs that limit their widespread application. To address these issues, we systematically review the development history, composition, microstructure, and performance characteristics of traditional UHS steels, including low-alloy UHS, secondary hardening, maraging, and precipitation-hardening stainless steels. We also review the research progress in developing novel UHS steels through two main pathways: the regulation of composite nanoprecipitates and the design of multiphase composite microstructures. Special attention is paid to exploring the potential and preliminary practices of the “hybrid” design concept, which breaks the boundaries of traditional steel classifications and integrates multiple strengthening mechanisms to develop cost-effective, high-performance, and easily weldable “all-round” UHS steels. The current research status regarding the evolution laws of precipitates and strengthening mechanisms in UHS steels is detailed. Finally, the future development directions of UHS steels are proposed, particularly regarding the synergistic control of complex precipitates, high-temperature stability, and industrial application.
Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing high-performance 304L austenitic stainless steel, which is widely used in the aerospace and automotive industries and in biomedical engineering because of its excellent mechanical properties, corrosion resistance, and high-temperature stability. However, the mechanical properties of LPBF-fabricated materials are influenced by the complex interplay of various process parameters, including laser power, scanning speed, layer thickness, and hatch spacing. Traditional trial-and-error methods for process optimization are costly and time-consuming, and they often fail to precisely control the material's microstructure, resulting in suboptimal performance. Therefore, there is a pressing need to adopt advanced approaches to systematically optimize the LPBF process and ensure reliable, high-performance outcomes. This study proposes an innovative hybrid intelligent framework that integrates Stacking-ensemble learning, interpretable machine learning techniques (Shapley additive explanation (SHAP) method), and multiobjective optimization (nondominated sorting genetic algorithm II-technique for order preference by similarity to ideal solution (NSGA-II-TOPSIS)). The primary objective is to develop an all-encompassing framework for the simultaneous prediction and optimization of ultimate tensile strength, yield strength, and elongation in LPBF-fabricated 304L stainless steel. The framework is designed not only to improve the accuracy of predicting the mechanical properties but also to provide a clear understanding of the influence of process parameters on material behavior. The Stacking-ensemble model demonstrates superior performance in terms of accuracy, generalization, and stability compared to individual machine learning models, such as random forest (RF), gradient boosting decision tree (GBDT), and extreme gradient boosting (XGBoost). SHAP analysis has revealed that laser power plays a critical role in determining the mechanical properties of the material, making it the most important factor to consider in the optimization process. The multiobjective optimization approach facilitates the identification of the optimal process parameters, resulting in a balanced strength-ductility trade-off that is crucial for practical applications. Experimental validation was conducted to confirm the effectiveness of the proposed framework. The optimized LPBF samples exhibited refined, uniform cellular substructures, increased dislocation density, and the formation of twin boundaries, which significantly improved the material's mechanical properties.
The CrCoNi medium-entropy alloy is known to demonstrate superior comprehensive mechanical properties relative to most fcc multiple-principal-elemental alloys. To ensure the long-term stability and safety of this alloy in practical applications, its fatigue characteristics and damage mechanisms must be thoroughly explored. Grain refinement is a crucial method for strengthening this alloy, and the effect of grain size on twin-boundary (TB) fatigue cracking must be clarified to reduce the intergranular fatigue cracks and improve fatigue performance. By conducting SEM measurements, the fatigue cracking behaviors at TBs in a CrCoNi medium-entropy alloy with two different grain sizes were systematically studied employing the slipping morphology method under low-cycle fatigue tests. The surface roughness around the TBs of the fatigued samples was characterized via laser confocal microscopy and white-light interference microscopy to quantify variations in the surface damage levels. Irrespective of grain size, the transition from slip-band cracking to TB cracking was observed to vary with the increasing difference in the Schmid factors (DSF) between the matrix and twin. Further, the propensity for TB cracking was facilitated by the escalation of DSF. Moreover, the magnitude of the required DSF for TB cracking was influenced by the grain size. The requirement of DSF for TB cracking decreased with increasing grain size. With the increase in grain size, even the minimal DSF is expected to result in the significant pilling up of dislocations near the TBs, thereby worsening the damage and rendering these boundaries favorable sites for fatigue cracking.
Variations in the Mn / N ratio greatly affect the formation of reformed austenite and precipitation and two-phase ratio in duplex stainless steel (DSS) during the welding thermal cycle. Therefore, investigating the influence of the Mn / N ratio on the multipass welding heat-affected zone (HAZ) microstructure is beneficial for enhancing the comprehensive mechanical properties of the HAZ of low-nickel-type DSS thick plates. This study comparatively investigated the effect of Mn / N ratio on the microstructure evolution and mechanical properties of DSS in the solution-treated and multi-pass welded HAZ, with 2205 DSS as a reference. The higher nitrogen content (Mn / N ratio = 3.77) resulted in substantial partial transformed austenite formation with a high volume fraction of 58.7%, yielding a tensile strength of 836 MPa and an elongation of 39.5%, indicating its high strength and plasticity. As the Mn / N ratio increases to 17.80, an increase in the Widmanstätten austenite (WA) amount caused cellular Cr2N formation at the δ/γ phase interfaces and dislocation walls in ferrite for HAZ, considerably decreasing elongation compared to the solid solution state. For DSS with a high Mn / N ratio of 65.91, the grain boundary austenite (GBA) amount initially increased and then decreased, while the intragranular austenite (IGA) and WA amounts increased in HAZ, inhibiting Cr2N precipitation and reducing ductility and strength. Compared with the 2205 DSS (Mn / N ratio = 3.06), the fracture surfaces of DSS with lower Mn / N ratios exhibited brittle fracture. The combined strengthening effect of the high-nitrogen solid solution and Cr2N precipitation on the austenite phase increased the hardness difference between the two phases in the HAZ of the DSS with Mn / N ratio of 3.77. In addition, the formation of some large (Cr, Mn)O inclusions promoted crack initiation to some extent, lowering the impact energy to 24.2 J. High Mn / N ratio led to more GBA and WA segmented ferrite refinement, as well as the formation of small amounts of IGA, which hindered crack propagation. Moreover, the precipitation of a small amount of the σ phase increased the hardness of ferrite, reduced the hardness difference between the two phases, and increased the impact energy to 134.8 J.
To address the critical issue of microstructural degradation and dynamic failure of Cu-Ni alloys subjected to multiple stress couplings in flowing seawater, this study systematically investigates the microstructural evolution and failure characteristics of the alloy in a complex service environment. Focusing on the widely used B10 Cu-Ni alloy in pipeline systems, a combined experimental and computational simulation approach was employed. A molecular dynamics method was used to construct an alloy/seawater solvation model, with emphasis on the effects of seawater flow rate, pressure, and their coupled interactions on microstructural evolution and corrosion kinetics. In flowing seawater, the strain level of the alloy increases markedly, accompanied by a rise in microstructural defects. The results reveal that corrosion of Cu-Ni alloys proceeds through a multi-step coupled mechanism governed by the migration-dissolution energy barrier of Cu atoms. Density functional theory calculations show that as seawater pressure increases from 0.1 MPa to 12 MPa, the migration-dissolution energy barrier decreases from 1.76 eV to 1.54 eV, significantly accelerating the corrosion rate. Furthermore, increasing seawater flow rate induces atomic-level axial elastic tensile strain on the (111) crystal plane of the alloy, further reducing the migration-dissolution energy barrier. A critical flow rate of 4 m/s has been identified as exacerbating corrosion. Under the coupled influence of flow rate and pressure, the migration-dissolution energy barrier is further reduced, the work function decreases, and corrosion kinetics are significantly accelerated. The corrosion rate constants obtained from simulations exhibit highly consistent trends with experimentally measured corrosion rates, enabling reliable assessment and prediction of the corrosion tendency of the B10 Cu-Ni alloy during dynamic failure under real operational conditions.
In high-quality steel production, the final stage of secondary refining typically involves a ladle holding period to facilitate the flotation and removal of inclusions. This improves the cleanliness of the molten steel. This study investigated the evolution of inclusion number density and area fraction during the ladle holding period by conducting industrial trial on composition adjustment via the sealed argon bubbling refining process of steel plate hot rolled commercial (SPHC) low-carbon steel. The results indicated that inclusions would be removed by floating during the ladle holding period. The inclusion number density and area fraction decreased from 52.8 mm-2 and 279 × 10-6 at the start of the holding period to 22.1 mm-2 and 148 × 10-6 after 15 min, respectively. To clarify the movement and removal of inclusions, a numerical model was developed based on a discrete phase model, incorporating multiphase flow and heat transfer during the ladle holding period. The calculations revealed the following conclusions: (1) inclusions smaller than 10 μm primarily followed the molten steel's fluid flow, with removal rates of 51.6% and 66.7% after 900 and 1800 s, respectively; (2) inclusions with a diameter of 100 μm were influenced by the molten steel's fluid flow and their buoyancy, achieving removal rates of 70.2% and 89.2% after 900 and 1800 s, respectively; (3) inclusions with a diameter of 1000 μm primarily underwent self-floating, reaching an approximately 100% removal rate after 120 s.
Nano-multilayers composed of immiscible metals have been widely investigated over the past decades due to their exceptional microstructural stability. The Cu/Ta system, which is also immiscible, is particularly notable because Ta offers higher melting point, strength, and superior wear and corrosion resistance compared to metals like Nb. Cu/Ta nano-bilayer films are extensively used in the electronics industry for semiconductors, microelectronic devices, optical systems, and magnetic applications. However, the presence of interfaces and the distinct deformation responses of the Cu and Ta layers during processing, fabrication, and service conditions make mechanical deformation and subsequent failure inevitable, potentially compromising the performance of microscale devices. Therefore, understanding the deformation mechanisms and enhancing the mechanical strength of Cu/Ta nano-bilayer films at the microscopic scale is essential. In this study, molecular dynamics simulations were employed to investigate the tensile behavior and deformation mechanisms of Cu/Ta nano-bilayer films. In addition, the effects of Al and W doping in the Cu layer on the deformation behavior were analyzed. The results indicate that tensile loading direction significantly influences the plastic deformation mode. When the bilayer films were stretched parallel to the interface, both Cu and Ta layers exhibited sequential plastic deformation. In contrast, when the loading was applied perpendicular to the interface, only the Cu layer deformed plastically, while the Ta layer remained elastically constrained throughout. Doping with Al or W atoms enhanced the overall hardness and yield strength of the nano-bilayer films. Furthermore, W doping induced stacking faults in both loading directions when stretched parallel to the interface, and a martensitic transformation from fcc to bcc structure occurred in the Cu layer under perpendicular tension.
The novel all-d-metal Ni-Mn-Ti Heusler alloys have various application prospects in the field of functional materials owing to their excellent elastocaloric effect and mechanical properties. However, the noteworthy magnetocaloric effect in conventional Ni-Mn-based Heusler alloys is not reflected in Ni-Mn-Ti alloys because of the weak magnetism of both its austenitic parent and martensitic phases. To overcome this limitation, this study explored the effect of Co and Si codoping on the martensitic transformation and magnetic and mechanical properties of Ni-Mn-Ti alloys based on first-principles calculation. The codoping of Co and Si effectively modulated the comprehensive properties of the Ni-Mn-Ti alloys. Specifically, Si atoms tended to directly occupy the Mn sublattice, whereas Co atoms tended to directly occupy the Ni sublattice. Further, the codoped Co and Si atoms exhibited a tendency of aggregated distribution in the alloy. In the Ni-(Co)-Mn-(Si)-Ti alloy, the austenite transformed from an antiferromagnetic state to a ferromagnetic state with increasing Co content. In contrast, the martensite remained in an antiferromagnetic state. This resulted in the observation of a magneto-structural transition at specific compositions, which facilitated the realization of a significant magnetocaloric effect. Furthermore, the Si-doped Ni-Mn-Ti alloy significantly increased its strength while decreasing its toughness. Whereas, the Co and Si-codoped Ni-Mn-Ti alloy exhibited better overall mechanical properties than the ternary. Finally, the study also explored the density of electronic states (DOS) of the alloys to elucidate the physical mechanisms underlying the experimentally observed martensitic transformation and the changes in the magnetic properties.
