In response to the increasing demands for weight reduction, enhanced passenger safety and reduced manufacturing cost, advanced high strength steels (AHSS) have gained considerable attention as crucial structural materials in automobile industry. Medium-Mn steels (MMnS) stand out as the most promising candidate for the new generation of AHSS, due to their advantages such as cost-effectiveness, superior strength-elongation balance, pronounced work-hardening capacity, and excellent wear resistance. Given the high hardenability of MMnS, a softening annealing is typically required prior to cold rolling to alleviate internal stresses. Temperature variations of softening annealing significantly affect the microstructure evolution during subsequent heat treatment. In this study, treatments of subcritical and intercritical softening annealing are conducted on MMnS, and the effect of microstructures resulted from distinct softening annealing processes on the static recrystallization and phase transformation of cold-rolled MMnS is investigated.Results indicate that subcritical softening annealing yields a microstructure composed of tempered martensite matrix embedded with cementite. In contrast, intercritical softening annealing produces a dual-phase lamellar microstructure comprising martensite and ferrite. After cold rolling, both softening annealed microstructures suffer severe plastic deformation, leading to lath-shaped structures with higher defect densities. Compared to tempered martensite, the martensite lamellae resulted from intercritical softening annealing and cold rolling exhibit a tendency for lamellar collapse and spheroidization during subsequent heat treatment, significantly promoting the static recrystallization of martensite. The rapid martensite recrystallization induces widespread and dispersed nucleation of austenite in the early stage of intercritical annealing, thereby improving the microstructure homogeneity and refining grains. Furthermore, intercritical softening annealing effectively inhibits the massive nucleation of austenite during rapid heating, thus expanding the design window for heat treatment processing for cold-rolled medium-Mn steels.
To meet the growing demands of the wind power industry, enhancing the strength and toughness of weld joints of steels for wind turbine tower has become a key research focus. Herein, the performance of 500 MPa grade wind power steel was optimized by means of thermo-mechanical control process (TMCP) routes combined with intercritical annealing (IA). Namely, Q500 steel plates, a wind power of 500 MPa grade were subjected to 1200 oC solid solution for 2 h, followed by 7 passes of rough rolling and 4 passes of fine rolling to acquire the so called original plates. These plates were then heated to 720 oC, 750 oC and 800 oC respectively, for 15 min, and water cooling. The original plate and the three heat-treated plates were respectively welded by V-shaped groove submerged arc welding for butt joints. Then the weld joints were characterized by microstructure examination, microhardness tester, tensile test, and crack tip opening displacement (CTOD) tests at -20 oC etc. The variations in microstructure, mechanical properties, and fracture mechanisms were systematically analyzed. The results indicate that the microstructure of the heat treated steels is mainly composed of ferrite and martensite. The heat-affected zone of their weld joints is predominantly composed of ferrite and granular bainite, while the weld seam features as lath ferrite and martensite/austenite (M/A). Compared with the weld joints for the original plate, the comprehensive mechanical performance of the weld joint for the steel plate after 800 oC intercritical annealing is significantly improved, which may be ascribed to that the ferrite enhances plastic deformation capacity, while the fine blocky M/A constituents at grain boundaries increase crack propagation resistance, thereby contributing to an overall improvement in strength and toughness. Specifically, the yield strength and tensile strength increase by 12.6% and 8.4%, respectively, elongation improves by 9.6%, and the maximum CTOD fracture value reaches 0.722 mm, representing an enhancement of over 110%.
Two types of ЭП741 pre-alloyed powder were prepared via vacuum induction melting gas atomization (VIGA) technique and plasma rotating electrode process (PREP) respectively. Then with the pre-alloyed powders, ЭП741 alloy was prepared via hot isostatic pressing (HIP) by 140 MPa at 1200 oC for 3 h. In the obtained ЭП741 pre-alloyed powders, there were defects such as inclusions and prior particle boundaries (PPBs) etc., as well as changes in the mechanical properties of the HIPed alloy caused by these defects. Hence, the above matters were systematically assessed by mean of scanning electron microscopy (SEM), while taking the deformed powder metallurgy alloy (PM billet + hot deformation) as a calibration. Results show that the PREP powder exhibited higher purity and fewer inclusions, and the mechanical properties of the HIPed alloy with PREP powder are better than those of the HIPed alloy with VIGA powder. It follows that the PPBs may primarily contribute to the differences in mechanical properties between the HIPed alloys and the deformed PM alloy. Furthermore, PPBs may deteriorate the mechanical properties of HIPed alloys both at room temperature and 650 oC, and the tensile properties of HIPed ЭП741 alloys are lower than those of deformed PM alloy. Notably, at cryogenic temperature (-196 oC), the PPBs can extend the crack propagation paths, and thus the effect of PPBs on mechanical properties is weakened. In summary, the average levels of tensile strength and elongation of the HIPed alloys are better than those of the deformed PM alloy.
As a typical representative steel of the third generation of advanced high-strength steels (AHSS), medium-Mn steel has a great strength-ductility synergy. It enables effective achievement of safety and lightweighting in design and manufacture for vehicles. In order to understand the deformation-induced plasticity effect of medium-Mn steels, herein, the influence of Mn-content on the heterogeneous deformation behavior at the mesoscale during the deformation of medium-Mn steels was studied via calculating with a crystal plasticity model coupled with deformation-induced martensitic transformation. Furthermore, the partition of stress-strain between austenite and ferrite during uniaxial tension, as well as the martensitic phase transformation process triggered by the deformation of austenite were calculated, whilst, the changes in Mn distribution caused by the difference in austenite composition during the critical zone heat treatment of two typical Mn-steels (0.2C-7Mn and 0.2C-5Mn) were also taken into account. Besides, the impact of austenite mechanical stability on the mechanical properties of the medium-Mn steels was also studied in terms of the austenite stacking fault energy (SFE). The results show that the enhanced mechanical stability of the 7Mn steel may be ascribed to the high stacking fault energy of the austenite in the 7Mn steel. Besides, the martensitic transformation occurs within a small strain range for the 5Mn steel, while for the 7Mn steel, the deformation-induced transformation occurs at applied strains higher than that for the 5Mn steel, and the initial nuclei of martensite are more numerous and more dispersed for the 7Mn steel. It follows that the differences in the deformation-induced phase transformation of these two steels may result in different of phase transformation induced plasticity (TRIP) effect and also lead to differences in their mechanical properties.
Cu is widely used due to its excellent electrical and thermal conductivity properties. However, its corrosion resistance in chlorine-containing environments (such as seawater or salt fog) is poor, which limits its long-term application in harsh conditions. To address this challenge, herein block composite of Cu/reduced graphene oxide (Cu/rGO) was fabricated via processes as follows: billets of which were prepared by vacuum hot pressing with mixture of Cu powder and rGO as raw material, and then they were successively subjected to hot-forging and -rolling. Further, the corrosion behavior of Cu/rGO composite and pure Cu in 3.5%NaCl aqueous solution and salt fog was comparatively assessed. Results show that the corrosion rate of Cu/rGO composite in NaCl aqueous solution was 34.8% lower than that of pure Cu, this may be mainly attributed to the physical barrier effect of the lamellar rGO, which effectively delayed the penetration of chloride ions and the charge transfer process. The Cu/rGO had a higher charge transfer resistance and higher electrochemical impedance, indicating that the interface reaction was more difficult to occur and the corrosion product film was compact. In the salt spray testing conditions, the corrosion rate of Cu/rGO was only 0.625 × 10-5 g·cm-2·h-1, which was 66.67% lower than that of pure Cu (1.875 × 10-5 g·cm-2·h-1). The number and size of corrosion pits on the surface of Cu/rGO were small and few, and the decrease in yield strength after corrosion (3 MPa) was significantly lower than that of pure Cu (18 MPa). The findings revealed the mechanism of rGO in enhancing the corrosion resistance of copper-based composites from multiple perspectives, including electrochemical behavior, microscopic morphology, and mechanical properties. Which may provide meaningful reference for the application of composite of Cu/rGO in marine engineering and high-humidity, high-salt environments.
Carbon fiber-reinforced ceramic-metal matrix composites, represented by C/SiC-Cu, show potential applications in thermal protection systems of high-speed aircraft due to their exceptional long-term oxidation and ablation resistance at ultra-high temperatures. Hexagonal boron nitride (h-BN) characterized by its hexagonal lamellar structure is an ideal interfacial material, given that it is one of the few materials that does not wet with liquid silicon and possesses excellent oxidation resistance. However, there is currently no public literature on the wetting behavior of Cu-Si alloys to h-BN material. In this study, h-BN powder was synthesized in nitrogen atmosphere with boric acid and urea as raw materials, and the wetting behavior of molten Cu-Si alloy to h-BN matrix, as well as the influence of its crystallinity on the wettability were investigated. The results indicate that among others, the h-BN synthesized at 1400 oC with the molar ratio of boric acid to urea is 1:2 presents the highest purity and crystallinity. While the varying crystallinities of the synthesized h-BNs show little influence on the wetting behavior of Cu-Si alloy. This can be attributed to the absence of dangling bonds on h-BN surfaces, namely, the extremely low surface energy of h-BN, and the similar intrinsic surface properties of h-BN with different crystallinity. The findings may provide a significant reference for the preparation of BN interfacial layers on carbon fiber and the regulation of properties of carbon fiber-reinforced ceramic (-metal) matrix composites.
The turbine blades of heavy-duty gas turbines require materials with excellent mechanical properties and hot corrosion resistance. DD10, a hot-corrosion-resistant single-crystal superalloy is the candidate material for manufacturing F-class gas turbine blades, which are usually fabricated via precision investment casting and then brazing to seal the core removal process remaining holes on the blade tip. However, there are relatively few studies on the brazing process for DD10 alloy. Herein, it is attempted to conduct brazing on DD10 single-crystal superalloy with a Co-based alloy as brazing filler metal. The microstructure of the brazed joints was analyzed using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Mechanical properties of the joints were evaluated, and the underlying mechanisms were explored. The results indicate that in conditions of brazing followed by heat treatment, phases such as M5B3, M3B2, MC, and Ni3Ti precipitated in the weld zone. Additionally, boron (B) diffusion led to the formation of a molten pool-like structure in the diffusion-affected zone. After heat treatment, the rupture strength of the brazed joints reached 80% and 70% of the base metal's strength at room temperature and high temperature, respectively.
FeCrV-based alloy FeCrVTa0.1W0.1Ti0.1C0.17 was prepared by vacuum arc melting technique, and subjected to vacuum heat treatment. Then the effect of heat treatment temperatures (800 oC, 900 oC, 1000 oC) on its microstructure and mechanical properties were investigated viacompression tester, hardness tester, scanning electron microscopy, electron probe microanalyzer and X-ray diffractometer. It reveals that heat treatments at 800 and 900 oC may significantly enhance the alloy strength (yield strength up to 1501 MPa) through precipitation strengthening by fine intragranular Laves phases. However, as temperature increases, the Fe-rich phase at grain boundaries evolves from isolated dendrites into a continuous network structure, acting as a preferential path for crack propagation and leading to reduced ductility. At 1000 oC, coarsening of Laves phases severely weakens their strengthening effect, while the Fe-rich phase transforms into isolated blocky particles, alleviating grain boundary embrittlement and restoring ductility to 28.9%, with strength returning to the as-cast level. The results demonstrate that the morphology (connectivity) of the Fe-rich phase is the key factor governing ductility, while the size of Laves phases dominates the evolution of strength. This finding provides a reference for balancing the strength-ductility of FeCrV-based alloys of multi-components.
