The additive manufacturing of nitrogen-containing steels encounters significant challenges, including nitrogen loss and the formation of nitrogen pores, both of which detrimentally impact mechanical properties. This study fabricated FeCrMnNi nitrogen-containing stainless steel specimens using selective laser melting (SLM) technology. The effects of different process parameters (including laser power and scanning speed) on the nitrogen behavior, microstructure and mechanical properties of the specimens were systematically investigated. The results indicate that the nitrogen content in the as-deposited specimens was reduced compared to the original powder (0.32 %). The increase in energy density led to a gradual increase in nitrogen loss and a decrease in the porosity of the as-deposited specimens, which can be attributed to the combined effects of changes in melt pool size and solidification rate. A balanced performance was achieved at a laser power of 280 W and a scanning speed of 1200 mm/s, with 0.025 % porosity, 773.5 MPa tensile strength, and 37.25 % elongation. Less loss of nitrogen content and allowing the presence of fewer holes seem to result in better mechanical properties.

In this study, a NiCoCrW (K447A) superalloy was fabricated using selective laser melting (SLM). The hot-corrosion behaviour of the SLM-fabricated NiCoCrW (K447A) was investigated in Na2SO4 at 900 °C. Microstructural analysis and hot-corrosion testing revealed its anisotropic behaviour in different build directions. The corrosion resistance along the build direction (XOY plane) was superior to that perpendicular to the build direction (YOZ plane). The stability of grain boundaries in the XOY plane is enhanced by the fine grains and a larger carbide area fraction, effectively hindering the diffusion of S and O2 at grain boundaries. Furthermore, the more uniform distribution of carbides in the XOY plane reduces the local stress concentration, contributing to the stability of the protective oxide film. The enhanced hot-corrosion resistance is attributed to the formation of a more continuous and dense corrosion layer on the XOY surface.

Metastable austenite plays an important role in improving the cryogenic impact toughness of maraging steels by strain-induced martensitic transformations. However, a significant volume fraction of austenite decreases the yield strength of the material. In this study, to overcome the strength-toughness issue in maraging steels, austenite with a compositional core-shell structure and specific volume fraction are designed in Fe-Cr-Ni-Co-Mo maraging stainless steel. The core-shell compositional structure comprises retained austenite with bulk content in the core region and high Ni content reverted austenite layers in the shell region. During tensile testing, a step transformation of austenite with a continuous lattice is observed; herein, the shell regions are preserved. In contrast, the core regions transform to martensite. Close to the impact fracture, the thin austenite shell layers are retained from the martensitic transformation, which further contribute to the impact toughness. Under the combined influence of austenite and nanoprecipitates (Laves phase), the investigated alloy reaches a yield strength >1200 MPa at room temperature with good cryogenic impact toughness (77 K, >90 J), thereby out-performing conventional maraging steels and several high-entropy alloys. The current study demonstrates that the chemical heterogeneity within metastable austenite may create unique mechanical properties in structural materials.

Meltflow-VAR software was employed to simulate the vacuum arc remelting process of ϕ660 mm large-scale ingot of M54 ultra-high strength steel， six processing curves with steady-state melting rates of 3.6， 3.9， 4.2， 4.5， 4.8， and 5.1 kg/min were selected for the study. Calculation results showed that as the melting rate increased， the depth and volume of the molten pool increased. The molten pool was completely in contact with the crucible at the melting rate of 4.8 kg/min or above， resulting improved cooling effect. The primary dendrite spacing monotonically increased with the increase of melting rate， while the local solidification time and secondary dendrite spacing decreased with the increase of melting rate in the range of 3.6-4.8 kg/min， whereas no significant change in the range of 4.8-5.1 kg/min. To verify the simulation results， a melting rate of 4.2 kg/min was chosen for industrial trial production. The shape of the molten pool was in good agreement with the calculated results. The degree of macro-segregation in steel ingots was relatively low， while the degree of micro-segregation （dendrite segregation） could reach over 30%. Micro-segregation was positively correlated with the spacing between secondary dendrites in the presence of secondary dendrite structures， while positively correlated with the spacing between primary dendrites in the absence of secondary dendrite structures. Taking into account the influence of melting rate on the shape of the molten pool， dendrite spacing and element segregation， it is recommended to optimize the steady-state melting rate to 4.8 kg/min.

In this study, systematic heat treatment routes including solution (980 °C to 1130 °C) and aging (675 °C to 775 °C) processes were used to tailor the microstructural evolution and mechanical performance of a Fe-Ni-Cr based austenitic alloy, strengthened by a γ ′ phase (Ni _3 (Ti, Al)). Grain growth was observed with increasing solution treatment temperature from nearly 90 μ m (980 °C) to nearly 200 μ m (1130 °C). Grain refinement during solution heat processes was found to ascribe to the solute drag effect and the pinning effect of nickel-titanium-enriched segregates and precipitates. During aging, the precipitation behavior of γ ′ is found to be almost independent of solution treatment temperatures. Interestingly, abnormal grain growth during aging was observed. The motion of grain boundaries was ascribed to the formation of nickel-titanium-enriched γ ′ in the austenitic matrix, thereby contributing to the dissolution of the previously enriched segregates and precipitates from grain boundaries. The studied alloy shows a wide range of mechanical properties, from tensile strength of 1131 MPa with ductility at 36%, to tensile strength of 880 MPa with ductility at 50%. The current study demonstrates that grain refinement of the alloy at solution treatment may not benefit the final mechanical properties.

In this work, based on the first principles calculation of density functional theory (DFT), we studied the band structure changes of monolayer ZnO and ZnO/WSe2 before and after vacancy generation, and systematically studied the vacancy formation energy, band structure, density of states, electronic density difference and optical properties of ZnO/WSe2 heterostructure before and after vacancy generation. The results show that the band structures of ZnO, WSe2, and ZnO/WSe2 heterostructure are changed after the formation of Zn, O, W, and Se vacancies. The bandgap of the ZnO/WSe2 heterostructure can be effectively controlled, the transition from direct to indirect bandgap semiconductor will occur, and the heterostructure will show metallic properties. The optical properties of heterostructure have also changed significantly, and the absorption capacity of heterostructure to infrared light has been greatly increased with red shift and blue shift respectively. The generation of vacancy changes the electrical and optical properties of ZnO/WSe2 heterostructure, which provides a feasible strategy for adjusting the photoelectric properties of two-dimensional optoelectronic nano devices and has good potential and broad application prospects.

Ultra high-strength M54 steel blocks were fabricated by laser metal deposition. The microstructure and mechanical behavior of the material were investigated systematically. The microstructure of the as-deposited M54 steel is anisotropic; the cross-section (XOY plane) has a cellular structure, whereas the longitudinal section (XOZ and YOZ planes) shows a mixture of alternating cellular and columnar forms. Compositional segregation is present at the cell walls (interdendritic regions) in the as-deposited state, resulting in retained austenite at the cell walls. The cross-sectional XOY plane contains 10.08% austenite, whereas the XOZ and YOZ planes contain 24.59% and 22.4% austenite, respectively. The retained austenite at the cell wall (interdendritic region) has low thermal and mechanical stability and disappears after the cryogenic treatment or is transformed into martensite during a tensile test. The as-deposited samples show anisotropic mechanical properties. The transverse samples exhibit stronger transformation-induced plasticity (TRIP) and work hardenability with a lower yield strength of 662 MPa and higher ultimate strength of 1982 MPa, corresponding to a higher amount of retained austenite in this direction. The longitudinal ultimate strength and yield strength are 1832 MPa and 997 MPa, respectively. The ductility and toughness are also largely anisotropic, and their reduction in the transverse direction is only 1/3 of that in the longitudinal direction. The Vickers hardness of the microstructure increases slightly from the bottom to the middle and upper part of the sample due to less thermal cycling in the upper part.

In this study, FeCrNiCoMo maraging stainless steel was fabricated by laser melting deposition. The initial nonequilibrium microstructure exhibited cellular segregation of Cr and Mo with a 10-20 mu m cell size. We investigated the microstructural evolution, particularly during aging with different time(1 h, 3 h, 5 h), for three post heat treatments: direct aging (500 degrees C) and low-temperature solution treatment (750 degrees C) and aging without and with additional prior homogenization (1000 degrees C). Microsegregation profoundly affected the austenite reversion behavior. The segregation at the cell walls promoted the nucleation and growth of reverted austenite, which eventually linked to form a net-like structure after prolonged aging. The net-like austenite in the direct-aged samples exhibited low thermal stability and result in brittle fracture upon impact at the temperature of liquid nitrogen (-196 degrees C). Net-like austenite and abundant lath austenite in the cell interior were found in the low-temperature solution-treated samples, which exhibited well-matched strength, toughness, and annealing temperature. Prior homogenization eliminated the microsegregation-induced net-like austenite. The measured mechanical properties indicate that the segregation-induced net-like austenite failed to benefit both the strength and toughness of the studied steel.

The effect of aging on transformation behavior of reverted austenite and impact toughness in Co-free maraging stainless steel were investigated via thermodynamic calculation, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Excluding the film-shaped austenite growing along the phase interface, other shapes of reverted austenite are evolved from the growth and aggregation of acicular austenite in the range of 300-600 °C. Under N-W orientation relationship, {111}γ grows inside the martensite lath along <100>α, and austenite merges in <100>α and <110>α simultaneously. Under K-S orientation relationship, the growth direction of {111}γ is 60° or parallel to <112>α. From 300 to 500 °C, Ni prefers to diffuse into η-Ni3Ti and matrix. The precipitation of Ni3Ti hinders the formation of reverted austenite and significantly deteriorates the toughness. Above 500 °C, due to the coarsening of Ni3Ti and the recovery of matrix, the resistance to the formation of austenite is obviously weakened, and then austenite plays a leading role in the improvement of toughness. When the aging temperature reaches 600 °C, the dissolution of Ni3Ti promotes the formation of austenite and η-Ni3Ti changes to γ'-Ni3Ti. The interaction between Ni3Ti and reverted austenite essentially depends on the diffusion behavior of Ni.