Marine engineering equipment serves long-term under multi-factor coupled environments featuring high salinity, high humidity, high hydrostatic pressure, as well as corrosion, fatigue, erosion and cavitation. Such service conditions raise higher requirements for lightweight design, corrosion resistance, strength-toughness, dimensional stability and long-life reliability of critical components. Titanium alloys possess advantages including low density, high specific strength, outstanding seawater corrosion resistance, favorable low-temperature toughness and non-magnetism, and thus act as vital structural materials for marine engineering equipment. As deep-sea facilities, marine power systems, seawater conveying systems and propulsion systems develop toward larger scale, integration and complication, the demands for large and complex titanium alloy castings such as pump-valve housings, impellers, propeller blades, deep-sea connectors and gas turbine casings keep growing. Centered on the main line of "marine service demand-cast titanium alloy materials-large complex castings-manufacturing equipment-quality assurance", this review summarizes the material systems, typical castings and upscaling tendency of titanium alloys applied in marine environments. It emphatically analyzes the development status and application features of key facilities, such as vacuum arc remelting, induction skull melting, vacuum induction levitation melting, vacuum consumable electrode skull melting and casting, hot isostatic pressing and vacuum heat treatment. From the aspects of composition matching, melt purification, mold filling and solidification, defect control, post-casting densification and dimensional stability, the requirements on equipment capacity and process control for manufacturing large complex titanium alloy castings are discussed. Finally, it is pointed out that the stable, controllable and industrial-scale development of domestic large complex titanium alloy castings for marine engineering shall be promoted via developing novel cast titanium alloys, constructing high-capacity high-cleanliness melting-casting equipment, implementing coordinated heat treatment control, and establishing supporting non-destructive testing and service evaluation systems.

China ranks first in the world in both shipbuilding completion volume and hand-held order backlog, emerging as a veritable major and powerful shipbuilding country. Shipbuilding steel serves as the main structural material for ships and offshore engineering platforms, and its performance and technical level exert a decisive influence on the overall service performance of ships. This paper conducts a systematic literature investigation on ultra-high strength shipbuilding steels at home and abroad, and analyzes the future development trends of this field on this basis. The research results show that with the continuous improvement of China's shipbuilding steel technical system, domestically produced shipbuilding steels with standard strength and high strength have achieved full independence and can meet the manufacturing demands of conventional domestic ships. However, compared with the top technologies of maritime powers including Europe, America, Japan and South Korea, the research, development and manufacturing capabilities of China's ultra-high strength shipbuilding steels are still insufficient. Obvious gaps exist in strength grades, thickness specifications and other aspects, which cannot match the rapid development demands of China's marine economy. Four types of ultra-high strength shipbuilding steels, namely GPa-grade ultra-high yield strength steel, low-carbon medium-manganese steel, low yield ratio steel and corrosion-resistant steel, are the core development directions of the industry in the future. Nano-precipitate strengthening acts as the core key technology to break the performance bottleneck of ultra-high strength steels. Niobium, vanadium and titanium carbonitrides as well as copper-rich nanoclusters and precipitates are widely applied in the research, development and industrial production of ultra-high strength hull steels, forming a relatively mature technical system. Latest studies confirm that NiAl intermetallic compounds with a B2 structure possess superior strengthening efficiency and great application potential. Nevertheless, their precipitation behavior and strengthening and toughening mechanisms in the multi-component alloy system of shipbuilding steels remain unclear and need further in-depth exploration. Therefore, intensive research and development of second-phase precipitate strengthening technology to promote the quality improvement and upgrading of ultra-high strength shipbuilding steels has important practical and strategic significance for making up for the shortcomings of China's shipbuilding materials, improving the international competitiveness of the shipbuilding industry, and consolidating the development foundation of the marine economy.

Copper alloys with excellent comprehensive properties play an irreplaceable role in key marine service components. Nevertheless, their wider application is severely restricted by single corrosion damage, coupled corrosion-wear damage and coupled corrosion-fatigue damage. Deep insight into the intrinsic correlation between damage behaviors and microstructures, as well as the synergistic interaction mechanisms between corrosion and wear, and between corrosion and fatigue, bears important research value. This review systematically summarizes the research advances regarding corrosion, corrosion-wear and corrosion-fatigue behaviors of typical precipitation-hardened and solid-solution-hardened Cu-Al and Cu-Ni base alloys, and clarifies the influences of diverse microstructural features (crystal structures, precipitated phases, annealing twins and precipitate-free zones) on damage mechanisms. Focusing on coupling interaction mechanisms, it reveals the nonlinear synergistic damage behaviors of copper alloys under multi-field coupling conditions from the perspectives of microstructural characteristics, tribological parameters (load, sliding velocity, testing duration, sliding distance) and fatigue parameters (strain amplitude, stress amplitude). In addition, this paper systematically evaluates mainstream process strategies for improving the resistance of copper alloys against corrosion, corrosion-wear and corrosion-fatigue, including alloying, heat treatment and plastic deformation. The advantages and drawbacks of each strategy are analyzed, which provides theoretical foundations and technical routes for the design and development of novel damage-resistant copper alloys. Finally, this work sorts out common characterization techniques and research methodologies adopted in coupled damage investigations, and prospects the potential and existing challenges of integrating these technologies to clarify the complex coupled damage evolution of copper alloys.

As a core clean energy for global energy transition, liquefied natural gas (LNG) storage and transportation equipment serves long-term at an ultra-low temperature of -163 ℃, which puts forward stringent requirements for the low-temperature strength and toughness, microstructural stability and formability of the core applied austenitic stainless steel. In this work, 304N austenitic stainless steel was taken as the research object, and three experimental steels marked as 8Ni, 9Ni and 10Ni were prepared by adjusting nickel mass fraction. The effects of Ni mass fractions (approximately 8%, 9%, 10%) on the microstructure and room-temperature mechanical properties of experimental steels were analyzed. The influence of pre-strain on the microstructure and mechanical properties at room temperature and -196 ℃ ultra-low temperature of 10Ni steel was further investigated. Combined with electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) characterization and fracture analysis, the internal action mechanism was revealed. The results show that the increase of Ni content significantly improves the strength of experimental steels through solid solution strengthening, and refines austenite grains and increases stacking fault energy simultaneously. Pre-strain induces massive proliferation and entanglement of dislocations, forming dislocation walls and subgrain structures. It remarkably improves the room-temperature and low-temperature strength of materials via work hardening, but consumes plastic reserve and leads to a sharp decline in elongation. At ultra-low temperature, the strengthening effect of pre-strain on yield strength is significantly weaker than that at room temperature, while the variation laws of tensile strength and ductility are basically consistent with those at room temperature. In addition, an improved Hollomon model considering the influence of strain is established based on the energy dissipation variable, which can accurately predict the mechanical properties of strain-hardened austenitic stainless steels for LNG storage and transportation. The research results provide important theoretical basis and data support for the composition design, processing parameter optimization and service safety evaluation of ultra-low temperature austenitic stainless steels for LNG storage and transportation.

Nitrogen, as an efficient and economical alloying element, is one of the element to regulate the microstructural stability and mechanical properties of duplex stainless steels. However, studies on the influence of nitrogen content variation on the low-cycle fatigue life of S32750 duplex stainless steel and the micro-deformation mechanisms of the two phases under cyclic loading are still relatively insufficient. In this regard, three experimental steels with different nitrogen contents were designed and prepared with nitrogen content as the core variable. Tensile tests and low-cycle fatigue tests were carried out to systematically study the effects of nitrogen content on the strength and fatigue properties of the experimental steels. Thermo-Calc calculations, optical microscopy (OM) and transmission electron microscopy (TEM) were combined to characterize the microstructure and the evolution of dislocation morphologies of two phases in the experimental steels. The results show that the strength of experimental steels is co-affected by multiple factors including nitrogen solid-solution strengthening, heat treatment temperature and ferrite content. Under the synergistic effect of multiple factors, the yield strength difference of experimental steels with nitrogen mass fraction ranging from 0.23% to 0.34% is controlled within 10 MPa. The low-cycle fatigue life of experimental steel with 0.23% nitrogen mass fraction is about 1.5 times that of experimental steels with 0.29% and 0.34% nitrogen mass fraction. The main reason is that the increase in nitrogen content raises the stacking-fault energy and intensifies the lattice distortion of austenite, which promotes the formation of dislocation pile-up and deformation twins. The deformation structure of the austenite phase gradually evolves from a single slip band to a composite structure of slip bands, microbands and deformation twins. In experimental steels with high nitrogen content (0.29% and 0.34% mass fraction), the massive formation of deformation twins in austenite accelerates local plastic dissipation, which is more likely to induce fatigue crack initiation and eventually lead to fracture failure. This study clarifies the influence of the nitrogen content on the low-cycle fatigue properties and micro-deformation mechanisms of S32750 duplex stainless steel, which can provide a theoretical basis for composition optimization and process design of high-life duplex stainless steels.

To meet the demand for ultra-high-strength steels for next-generation deep-sea pressure hulls, the effects of prior austenite deformation (ausforming)o n the martensitic microstructure and mechanical properties of HY180 steel were investigated. A Ni-Co-based HY180 steel was used as the experimental material. The specimens were heated to 900 ℃ and held for 300 s in a thermomechanical simulator, then cooled to 600, 700, and 800 ℃, respectively, and subjected to single-pass hot compression with a reduction of 50%. Specimens directly quenched from 900 ℃ were prepared as the reference. The microstructures and mechanical properties were characterized using optical microscopy, scanning electron microscopy, electron backscatter diffraction, microhardness testing, and sub-size tensile testing. The results show that decreasing the deformation temperature increases the flow stress and work-hardening capacity. Although the microstructure remains predominantly lath martensite after hot deformation, both the prior austenite grains and martensitic microstructure are significantly refined. The prior austenite grain size decreases from 7.32 μm to approximately 5.00 μm, while the effective martensite grain size decreases from 1.06 μm to 0.76- 0.87 μm. The increased kernel average misorientation (KAM)value indicates enhanced local orientation gradients and dislocation storage. For the specimen deformed at 600 ℃, the microhardness increases from 449HV to 507HV, the yield strength increases from 1 192 MPa to 1 368 MPa, and the ultimate tensile strength increases from 1 363 MPa to 1 451 MPa, while no evident reduction in total elongation is observed. These results demonstrate that, without altering the chemical composition, prior austenite deformation can effectively enhance the strength of HY180 steel through microstructural refinement and increased dislocation storage. This study provides an experimental basis for the thermomechanical microstructure control of Ni-Co-based martensitic ultra-high-strength steels. Further investigations involving multiple deformation passes, smaller reductions per pass, and continuous cooling conditions are required to evaluate the engineering applicability of this processing route.

This study focuses on Ni-saving 5.5Ni steel for LNG storage and transportation and aims to clarify the effect of tempering temperature during the QLT (quenching-lamellarizing-tempering) process on its microstructure and mechanical properties. The correlations among lamellar microstructural features, reversed-transformation austenite, and crack propagation behavior were systematically investigated at different tempering temperatures. Combined with tensile tests, impact tests and characterizations including SEM, EBSD and XRD, the lamellar characteristics of microstructure, dislocation density, Taylor factor, texture evolution and reversed-transformation austenite were quantitatively analyzed. The results show that after tempering at various temperatures, the 5.5Ni steel exhibits a microstructure with alternating distribution of lath martensite and quasi-polygonal ferrite. As the tempering temperature decreases, the strength of the tested steel remains essentially stable, while the longitudinal impact toughness decreases. The elongation is influenced by the content and stability of reversed-transformation austenite, which initially increases and then decreases. Since this study employed longitudinal cryogenic toughness as the assessment criterion, the optimal tempering temperature was determined to be 610 ℃. At this temperature, the tested steel achieves the optimal balance of strength, ductility and toughness, with a yield strength of 670.70 MPa, a tensile strength of 754.40 MPa, an elongation of 20.40%, and a longitudinal impact energy of 145.97 J at -196 ℃. Higher tempering temperatures promote the formation of a distinct martensite/ferrite lamellar structure. While imparting excellent longitudinal cryogenic toughness to the material, this structure also introduces structural anisotropy, thereby reducing the transverse impact energy. During longitudinal impact, back stresses and strain gradients generated at the heterogeneous interface effectively deflect the main crack path, and the highly stable reversed-transformation austenite effectively blunts the crack tip. During transverse impact, the main crack tends to propagate along parallel lamellar interfaces, which weakens the crack resistance and leads to the reduction of transverse impact energy. In summary, this study provides a theoretical basis for microstructure control and heat treatment process optimization of low-Ni steels.

Longitudinal profiled (LP) steel plates can optimize material distribution according to bearing requirements and serve as essential materials for the lightweight design of heavy equipment. Quenched and tempered heat treatment is a critical technical route for LP steel plates to achieve strength grades above 400 MPa in future engineering applications. Aiming at the technical problems of gravity deformation and composition adaptation encountered during the heat treatment of 590 MPa grade high-strength marine LP steel plates, this study carried out systematic research including numerical simulation, theoretical calculation and industrial trial production. Through Deform finite element simulation, the gravity deformation law of LP steel plates at a high temperature of 910 ℃ was revealed, the coupling influence mechanism of wedge length and base thickness on deformation was clarified, and the process and dimension windows for controllable deformation were determined. On this basis, combined with material hardenability analysis and Thermo-Calc thermodynamic calculation, the effects of alloy design and copper precipitation behavior on the property matching of thick and thin specifications were analyzed, and the Mn-Ni-Mo-Cu alloy system was optimized. The results show that the deformation degree of LP steel plates increases with the extension of wedge segment length and the reduction of base thickness. When the base thickness is 50 mm, favorable shape stability can be maintained even with a wedge length of 1.5 m. The optimized composition design realizes the uniform grain size control (grain grade of 10.0-10.5) of LP steel plates within the thickness range of 27-35 mm, and the microstructure is dominated by tempered lath martensite and bainite. The industrially trial-produced steel plates possess a yield strength of no less than 650 MPa and a tensile strength of no less than 715 MPa. The Charpy impact energy exceeds 200 J at both -40 ℃ and -84 ℃, with excellent mechanical property uniformity between thick and thin ends. This study realizes the first industrial production of 590 MPa grade heat-treated LP steel plates, providing a theoretical basis and technical support for the engineering application of high-performance LP steel plates.

This study adopted a high-alloy 02Cr20Ni18Mo5Mn6N austenitic stainless steel as the research object and systematically analyzed the precipitation behavior of secondary phases in the tested steel under different aging temperatures and aging durations. Thermo-Calc thermodynamic calculation, scanning electron microscopy, transmission electron microscopy and other experimental characterization methods were combined in the research to support the application research of composite pipes serving in harsh service environments. Thermodynamic calculation results show that the single-phase austenite region of the tested steel ranges from 1 080 ℃ to 1 330 ℃. Due to the high contents of Cr, Mo, Ni and N elements, the tested steel exhibits complex precipitation behavior in the temperature range of 700 ℃ to 1 080 ℃, where Cr₂N, Cr₂₃C₆, Laves phase and σ phase are precipitated. After solution treatment at 1 150 ℃ for 1 h, the alloying elements in the tested steel are fully dissolved. The microstructure consists of completely recrystallized and uniformly distributed equiaxed austenite grains with a large number of annealing twins. As the aging temperature increases from 700 ℃ to 1 100 ℃, the type, morphology and distribution of secondary phases in the tested steel change gradually. The Cr-rich σ phase at grain boundaries is coarsened gradually and transforms into Cr-Mo composite σ phase. In the grain interior, the single nanoscale Cr₂N evolves into multiple Mo-rich phases with diverse morphologies including Cr₂N, Cr₂₃C₆, the Laves phase and the σ/χ phase, and all precipitates finally dissolve back into the matrix at high temperatures. The temperature range of 800 ℃ to 900 ℃ is the sensitive temperature interval, where the tested steel presents the most complex precipitation behavior and the most abundant types of precipitates. According to the observation results of aging treatment at 900 ℃ with the duration varying from 10 min to 48 h, the precipitates preferentially nucleate and grow at grain boundaries, and then all precipitates at grain boundaries and grain interiors undergo rapid growth and coarsening comprehensively. The JMA kinetic model can accurately predict the precipitation behavior of the tested steel during the aging process.

Aiming at the challenge that metastable austenitic stainless steel fails to achieve a coordinated balance between strength and toughness during low-temperature service, particularly that plastic deformation tends to degrade its impact properties, this work takes hot-rolled solid-solution (SS) metastable austenitic stainless steel as the research object. A 20% cold-rolled deformation (CR20) process was applied for microstructure regulation, to investigate the influence mechanism of plastic deformation on room-temperature mechanical properties and cryogenic toughness at -196 ℃. With the SS state as the control group, room-temperature tensile tests and -196 ℃ Charpy impact tests were conducted on CR20 specimens. The fracture morphology was observed by scanning electron microscopy (SEM); the phase composition, grain orientation and grain boundaries were analyzed via electron backscatter diffraction (EBSD); the evolution of dislocations, stacking faults and substructures was characterized through transmission electron microscopy (TEM). Accordingly, the differences in microstructure and mechanical properties between the two groups of specimens were systematically compared. The results show that the SS specimen possesses a uniform single-phase austenite microstructure. Whereas, as for CR20, strain-induced phase transformation takes place in the matrix, forming a γ+ε+α' three-phase composite structure. Meanwhile, high-density dislocations, dislocation cells, Lomer-Cottrell (LC) dislocation locks and massive stacking faults are formed inside the grains, building up a multi-level strengthening-toughening system. In terms of mechanical properties, compared with the SS state, the yield strength of the CR20 state increases from 313 MPa to 759 MPa, and the tensile strength rises from 727 MPa to 914 MPa, with the elongation after fracture maintained at 36%. Besides, its -196 ℃ impact energy reaches 65 J, satisfying the low-temperature service requirements of ship structures. Fracture mechanism analysis reveals that at -196 ℃, driven by stress-induced martensitic transformation, the fracture mode of the SS state shifts from ductile dimple fracture to a mixed dimple-quasi-cleavage mode. By contrast, the CR20 state synergistically retards crack propagation relying on retained austenite and stable phase interfaces. Its cryogenic fracture is dominated by quasi-cleavage morphology, and the extent of toughness deterioration is limited. In summary, 20% cold-rolled deformation effectively achieves favorable matching between high strength and cryogenic toughness of metastable stainless steel, which provides a theoretical reference for the deformation strengthening and toughening design of stainless steel for cryogenic engineering. Further optimization research can be carried out focusing on deformation gradient and heat treatment processes in follow-up work.

LNG cargo tanks require safe and stable service at an ultra-low temperature of -162 ℃, which puts forward extremely stringent requirements for the strength and cryogenic toughness of structural materials. However, the widely applied 9Ni steel currently suffers from increased cost and welding magnetic arc blow defects due to its high nickel content. To reduce the nickel content of 9Ni steel for LNG carriers, cut down production cost and lower the dependence on high-nickel materials, this work adopted the TMCP-LT (thermo-mechanical controlled process+lamellarizing+tempering) process. The effects of lamellarizing temperatures ranging from 650 ℃ to 720 ℃ and tempering temperatures ranging from 500 ℃ to 600 ℃ on the microstructure, room-temperature tensile properties, and -196 ℃ impact absorbed energy of the 5.5Ni-0.25Mo tested steel were systematically investigated, and the corresponding strengthening and toughening mechanism was clarified. The results of multi-scale characterizations including OM, SEM, XRD and TEM, as well as room-temperature tensile tests and Charpy impact tests show that when the lamellarizing temperature increases from 650 ℃ to 700 ℃, clear interfaces are observed, grains are gradually refined, the volume fractions of intercritical ferrite and retained austenite decrease, the content of tempered martensite increases, and the strength of the steel decreases. A large number of fine blocky tempered martensite structures form and retained austenite disappears completely at a relatively high lamellarizing temperature of 720 ℃. With the increase of tempering temperature, the volume fraction of retained austenite rises, the dislocation density decreases, and the strength of the tested steel is reduced. The -196 ℃ impact absorbed energy of the tested steel under different processes is far higher than the minimum requirement of 100 J specified in the national standard GB/T 713.4—2023. The excellent cryogenic toughness is attributed to the matrix structure formed by high-degree recovery and the extremely low retained austenite volume fraction with a minimum value of 2.7%. The optimal TMCP-LT process is determined as lamellarizing at 680 ℃ combined with tempering at 500 ℃ to 530 ℃. The tested steel exhibits a yield strength of 610-632 MPa, a tensile strength of 696-721 MPa, a total elongation of higher than 19.5%, and a -196 ℃ impact absorbed energy of 183-219 J. All the performance indicators meet the requirements for 9Ni steel specified in GB/T 713.4—2023, which provides a theoretical basis for the industrial production of Ni-saving cryogenic steel for LNG carriers.

To explore the temper brittleness behavior of deposited metal for ultra-high strength marine steel, this study investigates the effects of different cooling schedules of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of deposited metal of a 1 000 MPa-grade welding wire matched with deep-sea marine steel. The results show that the as-welded deposited metal presents a typical dendritic morphology with obvious element segregation. The inter-dendritic (ID) zone consists of martensite while the dendritic core (DC) zone consists of bainite. The deposited metal was heat-treated at 550 ℃-2 h and then treated by air cooling or furnace cooling separately. After PWHT, the ID zone transforms into tempered sorbite and the DC zone transforms into tempered troostite, which improves the microhardness uniformity of the deposited metal. The average ultimate tensile strength of the heat-treated deposited metal rises slightly with an increment from 30 MPa to 92 MPa. Its elongation remains stable between 15% and 17%, and the tensile fracture surface shows typical ductile fracture features. Nevertheless, the PWHT deteriorates the impact toughness of the deposited metal. At room temperature (25 ℃), the average impact energy of the as-welded deposited metal decreases from 125 J to 91 J (air cooling) and to 74 J (furnace cooling). At low temperature (-40 ℃), the impact energy reduces from 84 J to 65 J (air cooling) and to 33 J (furnace cooling). Slow cooling after PWHT causes coarse carbides to precipitate along grain boundaries and weakens the bonding strength of grain boundaries. Meanwhile, the fraction of high-angle grain boundaries (HAGBs) decreases. The barrier effect against crack propagation is weakened, which finally leads to an obvious deterioration of low-temperature impact toughness.

With the further advancement of China's strategies including the Maritime Power initiative and the Belt and Road Initiative, the demand for seawater corrosion-resistant steels for various offshore platforms continues to grow. However, the corrosion behavior of seawater corrosion-resistant steels in harsh tropical marine environments with high temperature, high humidity and high salt spray has not been fully studied. The lack of field exposure test data severely restricts their engineering application. In this work, field exposure tests in the tropical marine splash zone were carried out. Combined with scanning electron microscopy, micro-Raman spectroscopy, X-ray diffractometry and electrochemical tests, the differences in corrosion behaviors including corrosion weight loss and corrosion product composition between Al-Sn-Cr-Cu series seawater corrosion-resistant steel and Q355 low-alloy high-strength steel were investigated. The results show that obvious brownish corrosion product layers form on the surfaces of the two types of steels after four months of field exposure in the tropical marine splash zone. The corrosion products are composed of α-FeOOH, γ-FeOOH, β-FeOOH and Fe₃O₄. The enrichment of alloying elements such as Al, Cr and Cu in corrosion products makes the corrosion product layer of the seawater corrosion-resistant steel denser. By contrast, the corrosion product layer of Q355 steel is loose and porous due to its higher content of β-FeOOH. Electrochemical test results show that the corrosion product layer on the seawater corrosion-resistant steel possesses higher charge transfer resistance and lower corrosion current density, presenting better protective performance than that of Q355 steel. After the removal of the surface corrosion product layers, the pits on the seawater corrosion-resistant steel are mostly wide and shallow, while those on Q355 steel are mainly deep and narrow. Therefore, alloying with Al, Cr and Cu elements improves the corrosion resistance of seawater corrosion-resistant steel in the marine splash zone, which ensures the service safety of offshore platforms.

Marine engineering equipment operates permanently in a multi-factor coupled corrosive environment characterized by high hydrostatic pressure, low dissolved oxygen, low temperature and microbial activity. Traditional single-principal-element alloys have approached their performance limits due to the single component, insufficient stability and high localized corrosion susceptibility of their passive films. The multi-principal-element design of FCC high-entropy alloys (HEAs) endows passive films with multi-component synergistic characteristics, and high configurational entropy stabilizes the solid solution matrix. The synergistic effect of the above two features enables FCC HEAs to exhibit promising corrosion resistance application potential in complex marine working conditions. This paper systematically reviews the corrosion behavior and corrosion resistance mechanism of FCC HEAs in marine environments. First, the constituent elements of FCC HEAs are classified into five categories, including matrix alloy elements, low-melting-point metal elements, refractory metal elements, non-metallic elements and rare earth elements. The differential influence rules of each type of element on the composition and stability of passive films, pitting resistance and interphase galvanic corrosion susceptibility are summarized. Second, the effects of precipitated phases and microstructures on corrosion resistance are discussed, and the strengthening mechanism of nano-scale coherent precipitated phases that simultaneously improve the strength-plasticity synergy, localized corrosion resistance and hydrogen embrittlement resistance is concluded. Subsequently, based on the process methods including additive manufacturing, synergistic processing deformation and heat treatment, as well as shot peening, this paper summarizes the research progress of these processes in regulating the formation and repair capability of passive films by adjusting grain size, elemental segregation and residual stress distribution. On this basis, focusing on three typical extreme working conditions of corrosion-wear coupling, deep-sea multi-factor coupling and hydrogen-induced damage, the failure mechanisms of FCC HEAs under multi-field coupled environments are summarized. It is indicated that the corrosion failure rules of FCC HEAs under the above working conditions differ from those in single static corrosion environments, and the composition design needs differentiated trade-offs according to targeted service conditions. Finally, the application status of machine learning in the design of corrosion-resistant high-entropy alloys is reviewed. Existing models are mostly based on data obtained from single media under normal pressure, which cannot accurately predict the corrosion behavior of FCC HEAs under deep-sea multi-field coupling. Future research needs to optimize feature engineering combined with physical cognition to improve the generalization ability of models. This paper holds that for the long-term service of marine engineering, FCC HEAs urgently need to solve the matching problem between composition and service stability under extreme working conditions, and accelerate the construction of multi-factor coupled performance databases to promote the engineering transformation of laboratory achievements.

NiCrMo alloys serve as critical materials for marine engineering, and their corrosion resistance is strongly dependent on the characteristics of the surface passive film. The Ni/Mo ratio (molar ratio)acts as a key factor affecting the passivation behavior of such alloys, while its internal mechanism remains unclear. To solve the above problem, this paper systematically investigated the effect of the Ni/Mo ratio on the corrosion resistance and passivation behavior of NiCrMoAl alloys in a 3.5% NaCl solution at room temperature. Five tapes of Ni_74.8-yCrMo_yAl (y=0.5, 1.8, 3.1, 4.4, 5.7) alloys with different Ni/Mo ratios were prepared via vacuum arc melting. Electrochemical tests, including open-circuit potential measurement, potentiodynamic polarization, electrochemical impedance spectroscopy, and Mott-Schottky analysis, combined with X-ray photoelectron spectroscopy (XPS) characterization, were performed to explore the corrosion behavior and passive film characteristics of NiCrMoAl alloys under as-cast conditions and after short-time heat treatment at 800 ℃ for 10 min. Results show that all as-cast alloys consist of a single face-centered cubic phase, and no new phases precipitate with the variation of the Ni/Mo ratio. Among the as-cast alloys, Ni_74.3CrMo_0.5Al exhibits the highest charge transfer resistance of 11.89×10⁵ Ω·cm² and the lowest corrosion current density of 0.81×10^-7 A/cm². This phenomenon is attributed to the low defect concentration and the highest Cr₂O₃ content in its passive film, which significantly improve the compactness and ion barrier capability of the passive film and further enhance its protective performance. Notably, the Ni_71.7CrMo_3.1Al alloy can form a passive film with an extremely low defect density under anodic polarization, whereas it presents high electrochemical activity at open-circuit potential, leading to poor overall corrosion resistance. After short-term heat treatment at 800 ℃, a NiO oxide layer forms on the alloy surface. The corrosion current density decreases by one order of magnitude, while the pitting potential drops significantly. The results indicate that heat treatment improves the uniform corrosion resistance of the alloy but increases its pitting corrosion susceptibility, which is mainly caused by the local depletion of Cr and Mo elements on the alloy surface due to oxidation. The research findings provide experimental evidence and theoretical guidance for the development of high-performance NiCrMoAl alloys for marine applications.

To reveal the pitting initiation mechanism of Al-Li alloys under simulated seawater immersion and clarify the electrochemical heterogeneity and interfacial coupling effects inside multiphase particles, a systematic investigation was carried out on the pitting behavior and corresponding influence mechanism of Al-Li alloys immersed in simulated seawater. This work aimed to accurately identify the phase compositions, potential differences, surface activity and atomic-scale electronic structural characteristics of intermetallic compounds in Al-Li alloys, establish the correlation laws between multiphase microstructures and localized corrosion susceptibility, and provide a theoretical basis and data support for the composition design, heat treatment process optimization and corrosion protection technologies of Al-Li alloys. Electron backscatter diffraction phase mapping was adopted to precisely characterize the phase compositions of intermetallic compounds in Al-Li alloys. A simulated coupling system composed of intermetallic compound particles and a pure Al matrix was fabricated to achieve precise regulation of interfacial structures and eliminate interference induced by other precipitated phases and defects. Scanning electron microscopy combined with energy dispersive spectroscopy was utilized to observe pitting morphologies and localized corrosion characteristics. Scanning Kelvin probe, scanning vibrating electrode technique and electrochemical impedance spectroscopy(EIS) were employed to test potential distribution, local current density and interfacial charge-transfer performances. First-principles calculations based on density functional theory were implemented to analyze surface energy, adsorption energies of oxygen and chloride ions, electron localization function and charge density difference, and reveal the differentiated corrosion mechanism of each phase at the atomic and electronic scales. The results show that intermetallic compounds in Al-Li alloys consist of three phases, including Al₄Cu₉, Cu₃Fe₁₇ and Fe₁₉Mn. The electrode potential of Al₄Cu₉ is approximately 300 mV higher than that of the aluminum matrix. This phase acts as a cathode and accelerates preferential dissolution of the surrounding matrix to form annular trenches. The potentials of the Cu₃Fe₁₇ and Fe₁₉Mn are roughly 300 mV lower than that of the aluminum matrix, and these two anodic phases dissolve preferentially to generate pitting pits. EIS test results indicate that the charge transfer resistance of the Al₄Cu₉ is markedly higher than those of the other two phases, which endow this phase with better corrosion resistance. First-principles calculations confirm that the Al₄Cu₉ (330) crystal plane possesses the lowest surface energy at 1.293×10²¹ eV/m² (12.93 eV/Ų). The phase exhibits the strongest structural stability. O₂ presents low adsorption energies on the Cu₃Fe₁₇ (110) crystal plane and the Fe₁₉Mn (310) crystal plane, which promotes the proceeding of oxygen reduction reactions. Analyses of electron localization function and charge density difference demonstrate that the Al₄Cu₉ phase has stronger interatomic bonding and interfacial electron enrichment. The other two phases display weak interatomic bonding and obvious electron loss, corresponding to higher corrosion activity. This work illustrates the multi-galvanic coupling corrosion mechanism within intermetallic compounds and verifies that electrochemical heterogeneity of internal phases serves as the core factor triggering pitting corrosion of Al-Li alloys. The research findings establish the intrinsic correlations among phase structure, surface activity, electronic characteristics and corrosion behavior. The results can provide theoretical support for regulating precipitated phases and enhancing the pitting resistance of Al-Li alloys and offer references for durability design and service life prediction of Al-Li alloy structural components applied in marine environments.

Novel Al-Mg-Zn(-Cu) alloys exhibit broad application prospects to satisfy the urgent demand for lightweight and high-performance aluminum alloys in shipbuilding and rail transit fields. Nevertheless, a systematic understanding of the synergistic regulation mechanism of Cu content on strength and corrosion resistance is still lacking. In this work, Al-4.0Mg-3.0Zn-xCu alloys (x=0-1.5%, mass fraction) were taken as the research object. Hardness measurements, tensile tests, intergranular corrosion (IGC) tests and transmission electron microscopy (TEM) characterizations were conducted to systematically reveal the coupled effect of Cu content on age precipitation behavior, mechanical properties and intergranular corrosion resistance. The results show that increasing Cu content remarkably accelerates the age-hardening response, and the peak-aged hardness rises from 99.0HV to 159.8HV. When the Cu mass fraction reaches 1.5%, the alloy obtains a yield strength of 386 MPa while maintaining an elongation of 20.6%, realizing the synergistic optimization of high strength and high ductility. Its comprehensive mechanical properties outperform existing commercial aluminum alloys for ship and rail transit service. In terms of corrosion resistance, raising Cu mass fraction to 1.5% changes grain boundary precipitates into discontinuous distribution and narrows the width of precipitate-free zone (PFZ), which significantly improves the resistance to intergranular corrosion. The underlying reason is that high Cu addition reduces the formation energy and critical nucleation radius of precipitates. It facilitates the nucleation and precipitation of T″ phase and effectively inhibits the segregation of solute atoms at grain boundaries. These results provide a theoretical basis for composition design and microstructure control of high-performance Al-Mg-Zn(-Cu) alloys applied in ships and rail transit.

With the growing demand for the development of high-performance shipbuilding steel, the accumulation of industrial data and the rapid development of information and communication technologies, data-driven methods provide a new technical approach for the transformation of composition-process design of high-strength ship plate steel from empirical trial and error to efficient collaborative optimization. Aiming at the problems of long trial production cycle, difficulty in collaboratively meeting multiple performance indicators and insufficient consideration of manufacturing adaptability in traditional design, a data-driven optimization design method for composition-process of high-strength ship plate steel under multi-performance constraints based on industrial data is proposed in this paper. Based on the industrial production data of low-alloy steel from the medium and heavy plate production line of a large domestic iron and steel enterprise, 5 617 valid samples were obtained after data cleaning. An input system containing 24 core decision variables was constructed, and XGBoost algorithm was adopted to establish prediction models for yield strength, tensile strength, elongation after fracture and impact energy respectively. The coefficient of determination R² values of the test sets of the four performance models are 0.925, 0.949, 0.798 and 0.855, with the corresponding mean absolute errors (E_MA) of 12.147 MPa, 8.115 MPa, 1.379% and 15.065 J, respectively, which indicates that the models can well characterize the nonlinear mapping relationship among composition, process and performance. On this basis, taking the yield strength of 420 MPa, tensile strength of 520 MPa, elongation after fracture of 30% and impact energy of 120 J as the optimization targets, the performance errors and cold crack susceptibility index P_cm were incorporated into the NSGA-Ⅲ multi-objective optimization framework, and 495 groups of Pareto non-dominated solutions were acquired. A total of 68 groups of quasi-compliant candidate schemes were screened out and representative schemes including S418 were determined via the strategy of quasi-compliance screening and manufacturability priority ranking. For the S418 scheme, the predicted performance values are 420.48 MPa (yield strength), 520.67 MPa (tensile strength), 29.75% (elongation after fracture) and 182.36 J (impact energy), with a P_cm value of 0.137. The verification test with 10 kg-grade test steel showed that after hot rolling the 35 mm rolled piece to 7 mm, the measured yield strength, tensile strength, elongation after fracture and impact energy were 435 MPa, 516 MPa, 30.3% and 149.1 J, respectively. The results show that the proposed method can effectively narrow the composition-process design space of high-strength ship plate steel, and provides technical support for the digital development and multi-performance collaborative optimization of low-alloy high-strength ship plate steel.

Aluminum alloys feature light weight, high specific strength, favorable corrosion resistance and outstanding fatigue properties, and possess wide application prospects in marine equipment. Accurate prediction of fatigue crack growth rate (FCGR) at various crack propagation stages is crucial to evaluate the service life and structural integrity of key load-bearing components of marine equipment. Conventional physics-based theoretical models and empirical equations generally fail to accurately capture the complicated nonlinear relationship between fatigue crack growth rate (da/dN) and stress intensity factor range (ΔK). Furthermore, the influences of stress ratio (R) and temperature (t) on the FCGR of aluminum alloys are remarkably affected by material composition and manufacturing processes, which imposes challenges to the generality of existing crack growth rate and life prediction models. In this work, fatigue crack growth rate tests were firstly performed on 7B05 aluminum alloy. Three-stage FCGR curves under different t and R were acquired, and the effects of temperature and stress ratio on crack growth rate were investigated. Published aluminum alloy FCGR data from literatures were further collected and fused with the above experimental data to establish a dataset of aluminum alloy fatigue crack growth rate. The effectiveness of Extreme Gradient Boosting (XGBoost) and Random Forest (RF) models for predicting FCGR covering all three stages was compared respectively. The results show that prediction outputs of the two machine learning models are unsatisfactory when trained merely with literature data. Nevertheless, the introduction of experimental data of the target material greatly improves prediction accuracy, which highlights the significance of such data for aluminum alloys applied in marine engineering. As the XGBoost algorithm achieves optimal prediction performance, a wider range of independent variables were incorporated and hyperparameters were optimized. The optimized model realizes high accuracy in predicting FCGR under various t and R conditions, so as to reduce dependence on extensive and expensive experimental programs. These findings not only deepen the understanding of fatigue crack propagation of this specific aluminum alloy, but also offer a more universal method to predict crack growth of other materials subjected to extreme ambient temperatures and complex stress ratios.

To address the high trial-and-error cost and the difficulty in synergistically optimizing multiple properties within a wide composition space of the Cu-Al-Ni-Fe-Mn quinary copper alloy system, this study developed a composition design framework for copper alloys based on the differential evolution algorithm. A total of 214 sample data were collected by integrating literature data and industrial data. A 117-dimensional feature vector was constructed based on 58 elemental physicochemical properties and discrete encoding of process states, and the feature dimension was reduced through a feature selection strategy. Weighted ensemble prediction models combining random forest and multilayer perceptron algorithms were established separately for three target properties, including ultimate tensile strength, elongation, and corrosion rate. The differential evolution algorithm was embedded into the ensemble models as an evaluator of the comprehensive objective function to minimize the normalized weighted deviation of multiple performances in the constrained composition space. The results show that under the target conditions of ultimate tensile strength no less than 690 MPa, elongation no less than 17%, corrosion rate no more than 0.011 mm/a, and annealed process state, the algorithm achieves stable convergence at approximately the 60th generation. The optimized composition in mass fraction is determined as Cu 80.2%, Al 9.70%, Ni 4.78%, Fe 2.91%, Mn 2.14%, Sn 0.055%, Si 0.134%, Zn 0.067%, and Pb 0.015%. The ensemble model predicts that the corresponding ultimate tensile strength, elongation, and corrosion rate reach 690.000 2 MPa, 17.5%, and 0.014 4 mm/a, respectively. Ingots prepared with the optimized composition were annealed at 675 ℃ for 2 h. The measured average values of ultimate tensile strength, elongation, and corrosion rate are 689 MPa, 16.75%, and 0.012 mm/a, respectively, which fully satisfy the specified performance requirements. The deviations of ultimate tensile strength and elongation are both within 2%. The proposed framework exhibits engineering applicability for the multi-objective synergistic optimization of copper alloys, which provides a feasible technical approach for the intelligent composition design of high-performance copper alloys.