High-strength steel has the advantages of high strength, low cost and good hot and cold workability, etc., which is widely used in various fields of national economy as engineering steel, such as bridge, vehicle, ship, pressure vessel and so on. As increasing strength, the plasticity and toughness of high strength steel have not meet the demand in some industrial areas, especially the low temperature impact toughness. Recently, a Fe-Cr-Ni-Mo steel with high-strength and high-toughness has been deve-loped and has been successfully used to prepare high pressure vessels. In this work, metal active gas (MAG) welding with multi-pass welding was used to join a Fe-Cr-Ni-Mo high-strength and high-toughness steel. The microstructure and fracture morphologies of welded joint are investigated by SEM, EPMA and TEM and the micro-hardness, tensile strength and Charpy impact energy are tested as well. The results show that the morphologies of welded metal (WM) consist of columnar crystal (CC) and equiaxed crystal (EC), where the upper WM is predominantly CC and the proportion of EC increases in the lower WM. The microstructure of upper WM is tempered martensite for the faster cooling rate. Because the higher content of alloying elements in lower WM improves the hardening tendencies, the lower WM is granular bainite. The heat affected zone near WM is coarsen martensite and has the highest hardness (621 HV), which is significantly higher than that of the base metal (BM) (410 HV). The hardness of the upper WM is 365 HV, which is lower than that of BM and the lower WM has higher hardness (450 HV). Therefore, the upper tensile sample of welded joint was broken in the WM and the fracture strength is 1109 MPa and lower than that of BM (1190 MPa). While the fracture position of lower tensile sample is in the BM and the strength is about 1183 MPa. The welded joint of experimental Fe-Cr-Ni-Mo steel has higher strength and the welding factor is not lower than 0.93. Moreover, the impact energy of WM is 53 J.

Fe-Cr-Ni-Mo steel is widely used in various industrial fields, such as water turbine in hydroelectric power station, pressure vessel and shipbuilding section etc. due to its excellent performance in strength and impact toughness. In order to fulfill the needs of high-strength and good toughness, the quenching and following tempering are often used for this kind of Fe-Cr-Ni-Mo steel. In particular, the carbide precipitation in the tempering process is the key to determine the strength and toughness. In this work, TEM and SEM were used to investigate the effect of tempering time (10, 20, 40 and 120 min) on carbide evolution and mechanical properties of Fe-Cr-Ni-Mo steel with different V contents (0, 0.08% and 0.14%, mass fraction) after quenched at 860 ℃ and following tempering at 610 ℃. The results show that some M₇C₃ type carbides precipitated along martensite lath boundaries in quenched 0V steel, but no carbide in the quenched 008V and 014V steels. As a result, the strength of 0V steel (2060 MPa) is higher than 008V and 014V (1906 and 1857 MPa, respectively). After tempering for 20 min, a small amount of M₃C type carbides were found on the lath boundaries in 0V steel. With tempering time increasing, M₃C will transform into M₂₃C₆ carbide gradually. Both M₃C and M₂₃C₆ type carbides exhibited a large size in range from 150 nm to 300 nm which were unfavorable to strength. As a result, the tensile strength of 0V steel decreases from 1197 MPa to 1088 MPa when tempering time increases from 20 min to 120 min. As for the 008V and 014V steels tempered for 20min, there are not only M₃C type carbides precipitated in the grain boundary, but also M₂C type carbides found inside the grains. The size of both carbides is no larger than 80 nm. With increasing tempering time, the M₃C will dissolve gradually and there will precipitate much more M₆C and MC. Compared with coarse M₃C, the finer M₂C, M₆C and MC have better precipitation strengthening effect and less deterioration of ductility and toughness. Therefore, with increasing tempering time the strengthes of 008V and 014V steels keep stable and the elongation and impact toughnesses increase gradually. This indicates that the excellent combination of strength and impact toughness can be obtained in 008V and 014V steels.

The quenching and partitioning (Q&P) process is a promising method to create novel martensitic steels with improved balance of strength and ductility by retaining considerable amount of austenite in martensitic matrix. This kind of microstructure provides suitable condition to study wear and abrasion mechanism since the effect of retained austenite on the wear property of martensitic steel is still controversial by now. Selecting traditional quenching and tempering (Q&T) sample with identical composition Fe-0.4C-1.5Mn-1.5Si as reference, the dry sliding wear property of Q&P samples with different austenitization temperatures was studied. The results show that the volume fraction of retained austenite in Q&P samples with full austenitization at 860 or 1000 °C respectively is nearly the same (about 14.37% in the former and about 13.79% in the later), and the corresponding carbon concentration (mass fraction) in retained austenite is relatively high (1.37% in the former and 1.38% in the later). Under the conditions of low loading (50 N) and slide speed (40 mm/s), it is not easy to induce martensitic transformation because of very strong mechanical stability, leading to the low friction and wear resistance of samples. The slight better wear resistance of samples with low austenitization temperature can be attributed to microstructural refinement. When the austenitization temperature was 800 ℃, the intercritical Q&P samples were obtained. Microstructure analysis indicates there exist the highest volume fraction of retained austenite (about 22.28%) and a small volume fraction of ferrite (about 6.75%) in martensitic matrix, which results in the lowest microhardness among present four kinds of samples. However, the mechanical stability of retained austenite in this kind of sample is weak due to the low carbon concentration (about 1.06%). The obvious martensitic transformation accompanying sliding wear contributes to extra hardening and provides additional compressive stress on the touching surface caused by volume expansion. Therefore, the intercritical Q&P samples exhibit the best wear resistance. Based on the experimental results, it is true that the mechanical stability instead of the amount of retained austenite in martensitic steel plays a critical role in improving wear resistance.

Nanoscale co-precipitation strengthening in steels has attracted increasing attention in recent years and has become a new cornerstone for the development of advanced high performance steels with superior combination of strength and ductility. Rolling process, finishing temperature, cooling rate and coiling temperature are the main factors which affect the mechanical properties of microalloyed steels by changing the volume fraction and particle size of precipitates. Nevertheless, the influence of cooling rate on microstructure evolution, precipitates and mechanical properties of complex microalloyed ferritic Ti-V-Mo steel has been rarely reported. In this work, the precipitation law of (Ti, V, Mo)C carbides at different cooling rates and its effect on microstructue evolution and mechanical properties of Ti-V-Mo complex miroalloyed steel were studied by OM, EBSD, HRTEM and Vickers-hardness test. The results indicated that the hardness first increased quickly and then increased slowly as the cooling rate increased (lower than 20 ℃/s); the mean size of (Ti, V, Mo)C particles decreased from 13.2 nm to 6.9 nm and the average size of ferrite grain reduced from 5.06 μm to 2.97 μm; the hardness of Ti-V-Mo steel was improved by the means of grain refinement hardening and precipitation hardening. However, when the cooling rate increased from 20 ℃/s to 30 ℃/s, its effects on grain refinement hardening and precipitation hardening has become saturated, so the hardness was kept flat and achieved a maximum vlaue of 410 HV, and the yield strength reached as high as 1090 MPa. The hardness y of Ti-V-Mo microalloyed steel and cooling rates x accord with a exponential decay relationship: y=-229exp(-x/5)+412.

The compression behavior during semi-solid state is a fundamental basis for the following rheoforming or thixoforming. Coexist of solid/liquid phase leads to the unique deformation behavior. The chemical composition at each phase is different from conventional forming process. Deformation behavior and microstructure evolution are determined by various effects such as initial state, heating, cooling, etc. In this work, the semi-solid compression tests of 9Cr18 as hot-rolled material and semi-solid billet were conducted, respectively. Microstructure evolution during heating, semi-solid state, deformation and cooling was investigated by OM and SEM. Solid/liquid flow behavior and the relationship of stress-strain were analyzed. The results showed the preparation of semi-solid billet is essential for the uniformity of solid particle and liquid phase, which would help to demonstrate the flow behavior. Only heating the as hot-rolled material to semi-solid led to the banded precipitation of liquid phase. The banded melting of as hot-rolled material made it hard for liquid phase to connect with each other. Liquid flow only happened in partial area and plastic deformation of solid particles was the main deformation behavior. The stress increased at the final stage. As for semi-solid billet, solid particles and liquid film coexisted uniformly. Macro separation of solid/liquid occurred as deformation came into thixotropic stage. Liquid flew towards outside and solid particles rotated, thus leading to the decrease of stress. Microstructure evolution at semi-solid state was different from conventional heat treatment. Solid austenite particles at semi-solid state could dissolve more alloying elements than normal austenization (1050 ℃). This phenomenon would help to improve the stability of austenite and over-saturated meta-austenite was obtained after cooling. The special microstructure evolution during semi-solid state might provide a possible way to design a new heat treatment procedure.

Radiation-induced damage, especially the effect of He, has always been one of the crucial issues in future fusion reactors. It is thus essential to further understand the formation of He bubbles and hardening characteristics for future development of fusion application materials, for instance bcc-Fe as a simple model. Behaviors of crack propagation have been investigated in two different orientated cracks (001)[010] and (121)[111] of bcc-Fe models under different densities of He at 300 K by molecular dynamics simulation. The results show that these behaviors are tailored by crack orientations on the condition of non-He atoms: (001)[010] orientated crack can be divided into elastic deformation, phase transformation and cleavage fracture of crack tip along phase transformation zone; however, (121)[111] orientated crack is elastic deformation, stacking twin and after that formation and coalescence of voids to rupture. Furthermore, the yield stress and strain of (121)[111] orientated crack are higher than (001)[010] orientated crack, therefore (121)[111] orientated crack has stronger ability to resist crack propagation. In addition, it is revealed that the influence of He density on the crack propagation exhibits two major aspects: when the density of He is lower (0.9%, atomic fraction), He can reduce the efficiency of phase or twin transformation and decrease the rate of crack propagation; when the density of He is higher (6.0%, atomic fraction), a large number of He clusters contribute to promote micro-voids nucleation, fracture mechanism for both crack models is the transformation of He clusters to voids, then voids coalescence, accelerating the occurrence of fracture. There is no twin or phase transformation in higher density of He.

Mn18Cr18N austenitic stainless steel with excellent mechanical properties and corrosion resistance is widely used in nuclear industries, power plants and medicine field. However, precipitation of the second phases during hot deformation deteriorates the mechanical properties and hot formability. In order to clarify the precipitation behavior of this steel, the precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel with different nitrogen contents were investigated by JmatPro software, OM, SEM and TEM analytical methods. The results indicate that precipitation phases consist of Cr₂N and a few M₂₃C₆, in which Cr₂N preferentially precipitates along grain bound aries and then grows up to the interior of austenite grain by discontinuous cellular. With increasing of ageing temperature, the precipitation of Cr₂N became more sensitive. When the nitrogen content increases to 0.7%, the most sensitive precipitation temperature of Cr₂N is 750 ℃ with an incubation period of 10 min. However, M₂₃C₆ mainly precipitates by granular at austenitic grain boundaries and maintains cube-on-cube orientation relationship with adjacent austenite grain. The results of mechanical property test indicate that the precipitation of Cr₂N has a negligible effect on strength and obvious deterioration on plasticity of Mn18Cr18N austenitic stainless steel. The precipitation of Cr₂N after ageing treatment leads to remarkable decrease in elongation and reduction of area, and the elongation reduced from 52.9% to 27.7%. Meanwhile, fracture mode also transformed from ductile fracture to intergranular fracture and transgranular fracture with the increasing of Cr₂N. TEM analysis shows that solution treatment sample reveals good plastic deformation ability and coordinates deformation by slip and twinning, simultaneously. Nevertheless, dislocations slipped, propagated and eventually piled up between lamellas of Cr₂N and around granular M₂₃C₆ after ageing treatment, which induce the degeneration of the plastic deformation capacity of Mn18Cr18N austenitic stainless steel.

Carbon steels as common structural material have been widely used for basic facilities with the development of the city. In these service environments, carbon steel would inevitably encounter the atmospheric corrosion. Especially, the corrosion of carbon steels exposed to coastal-industrial atmosphere is very outstanding. However, the initial corrosion mechanism of carbon steel subjected to coastal-industrial environment still need to be clarified, which would be vital for predicating the subsequent corrosion process. In addition, although many scholars studied the synergism of SO₂ and Cl^-, which obviously accelerates the corrosion of steel and reduces its service life, there is few research about the effect of the synergism of SO₂ and Cl^- (in different proportion) on the early corrosion behavior of the carbon steel. Therefore, it is of great importance to investigate the initial corrosion mechanism of carbon steel and the effect of the synergism of SO₂ and Cl^- (in different proportion) in the coastal-industrial atmosphere. In present work, the initial corrosion behavior of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere has been studied by weight loss, XRD, SEM and electrochemical measurements. Also, the effect of the synergism of SO₂ and Cl^- (in different proportion) on the early corrosion behavior of Q235 car bon steel has been investigated. The results indicate that the initial corrosion behavior of carbon steel exposed to a simulated coastal-industrial atmosphere presented a transition from corrosion acceleration to deceleration, and the kinetics of accelerated corrosion process followed the empirical equation D=Atⁿ. A double-layered corrosion product was formed on the surface of carbon steel after 24 h: the loose outer layer and relative dense inner layer; the synergistic effect between SO₂ and Cl^- accelerated the corrosion of carbon steel. However, the change in the ratio of SO₂ and Cl^- had no significant effect on the corrosion loss of carbon steel, and had not changed the composition of corrosion products formed on carbon steel surface. SO₂ caused the corrosion morphology of carbon steel to tend to uniform corrosion.

Semiconductor-based photocatalytic technology, using abundant and renewable sunlight as an induced light source represents an emerging successful technology to solve the global energy and environmental challenges. Considerable efforts have been paid to develop novel photocatalysts with good response to sunlight and high quantum conversion efficiency. In this work, single ZnNCN microparticles have been prepared by the ligand exchange reaction between zinc salt, ammonia and cyanamide. And Ag₂NCN/ZnNCN hetero structure has been also fabricated using the same ligand exchange process but mixing the silver salt with zinc salt together. Some means, such as XRD, SEM, infrared spectroscopy (FT-IR) and ultraviolet visible spectrometer (UV-Vis) were used to characterize the samples. The results showed that the single ZnNCN was flower-like particles with wide band gap (E_g=4.71 eV). Compared with single ZnNCN, the Ag₂NCN/ZnNCN composite particles presented different morphology with rough surface, and physical interaction was existed between two kinds of metal cyanamide for Ag₂NCN/ZnNCN composites. Because of the heterostructure, the light response spectrum for Ag₂NCN/ZnNCN composite particles was extended to the visible light region, and the band gap was changed to 2.05 eV. The photocatalytic activity of Ag₂NCN/ZnNCN composite particles in the degradation of Rhodamine B under Xenon irradiation was investigated, meanwhile, single ZnNCN and the mixture of Ag₂NCN and ZnNCN was also applied in the photocatalysis under same conditions for comparison. The apparently enhanced photocatalytic activity of Ag₂NCN/ZnNCN heterostructure was observed, and a first-order kinetic was discussed.

Ni-based supperalloys are widely applied in manufacturing of compressor and turbine discs and polycrystal turbine blades in the hot section of aero-engines, since they possessed excellent mechanical strength and creep resistance at high temperatures. Generally, hot working is an effective way for shaping metals and alloys. Lots of typical metallurgical behaviors occurred, which were related to the hot working parameters, including deformation temperature, strain rate and strain. And BP-ANN (artificial neural network based on the error-back propagation) as well as Arrhenius types models were the two of most acknowledged constitutive models to determine the relationship between the flow behavior and hot deformation parameters of various metals and alloys, at present. In order to investigate the relationship between deformation parameters and flow stress behavior, and precisely simulate the flow behavior during hot deformation processes of GH4720Li alloy, the hot compressive tests of GH4720Li alloy were conducted at the deformation temperature range of 1060~1140 ℃ and strain rate range of 0.001~1 s^-1 on Gleeble 3500D thermal simulation testing machine in this work. The relationship between microstructure and hot deformation conditions was identified. The influence of hot processing parameters on flow stress behavior was analyzed. The temperature sensitivity of the flow stress decreased with increasing temperature at a strain rate of 0.1 s^-1. The peak stress increased 23 MPa when the deformation temperature decreased from 1100 ℃ to 1080 ℃, only increased 7 MPa when decreased from 1140 ℃ to 1120 ℃. In addition, the Arrhenius model as well as BP artificial neural network model was established according to the true stress-strain curves. It shows that the established BP artificial neural network model can well exhibit the flow stress behavior of GH4720Li alloy compared with the Arrhenius model during hot deformation. The correlation coefficient between experimental findings and predicted flow stress determined by ANN model and Arrhenius model is 0.998 and 0.949, respectively. In addition, the dynamic recrystallization mechanism of the studied alloy was identified according to the deformed microstructure. Microstructure observation of the samples deformed at 1140 ℃ indicated that the discontinuous dynamic recrystallization was the main nucleation mechanism and newly grain nuclei distributed along the deformed grain boundaries. The dynamic recrystallization grain size of GH4720Li alloy decreases with the increase of strain rate when the samples deformed at 1140 ℃ and a strain of 0.8.

High Nb containing TiAl alloys (TiAl-Nb) are a new generation of materials for high temperature structural applications because of their low density, high strength and corrosion resistance at elevated temperatures. The alloys can be processed by powder metallurgy (PM) which have more advantages including low cost-effectiveness, near net forming for complex parts with fine grain size and uniform microstructure. However, the alloy powders are difficult to achieve full densification due to their lower sintering activity, which impairs the mechanical performance of sintered parts. The present work focuses on the densification performance of TiAl-Nb alloy powders with 1%Sn (atomic fraction) as sintering aids.The effects of Sn addition on the sintering densification process, microstructure and mechanical properties of sintered alloys were investigated. The results show that 1%Sn addition can significantly reduce the sintering densification temperature of alloy powders, and increase the relative density and linear shrinkage of sintered parts. This contributes to control microstructure grain size and improve the comprehensive properties. Sintered with 1500 ℃ for 2 h, 1%Sn containing TiAl-Nb base alloys show the best densification performance, with the relative density of 99.1% and linear shrinkage of 9.3%. The alloy samples exhibit fine and uniform full lamellar microstructure, and α₂/γ lamellar colonies are sized about 40~60 μm. Sn mainly dissolved into γ phase, leading to the enhancement of axial ratio c/a and unit cell volume. The sintered TiAl-Nb-1Sn samples have been found to possess superior room-temperature mechanical properties, with a Rockwell hardness of 50.1 HRC, a compressive strength of 2938 MPa, a yield strength of 680 MPa, and a compression ratio of 29.1%, which is obviously higher than those of TiAl-Nb alloys.

7000 series Al alloy has been widely used in aeronautical structural materials because of its high strength, high stress corrosion cracking resistance and good fatigue resistance when treated by retrogression and re-ageing (RRA). Al-8Zn-2Mg-2Cu (mass fraction, %) thick plate is supposed to manufacture the aircraft wing in Chinese big plane project. Due to the non-isothermal environment in the process of heat treatment for aluminum alloy thick plate, the influence of coupling effect of non-isothermal retrogression temperature field and time on the microstructure and properties of Al-8Zn-2Mg-2Cu alloy thick plate was investigated by the non-isothermal kinetic model, mechanical properties tests, electrical conductivity test and TEM observations. The results show that, the electrical conductivity increases while the hardness and strength decrease with the non-isothermal retrogression time increasing. The optimized non-isothermal retrogression and re-ageing (NRRA) treatment makes the size distribution range of MgZn₂ phase wider. Therefore,the logical matching between the dislocation cutting off mechanism and the dislocation by-passing mechanism effectively reduces the loss of hardness. Meanwhile, the electrical conductivity is significantly improved. After the treatment of 105 ℃, 24 h (pre-ageing) and non-isothermal regression (120 min) with slow heating rate and 120 ℃, 24 h re-ageing, the Al-8Zn-2Mg-2Cu alloy thick plate possesses an excellent comprehensive performance than those of T6 and T73 states. The tensile strength, yield strength and electrical conductivity are 620 MPa, 593 MPa and 21.1 MS/m, respectively. The NRRA treatment with slow heating rate is more suitable for the ageing treatment of thick plate.

Special wettability includes superhydrophobic, superhydrophilic, superoleophobic and superoleophilic etc. The superhydrophobic surfaces are governed by the surface chemistry and unique micro/nanostructures. Up to now, numerous methods have been reported in constructing superhydrophobic surfaces including chemical vapor deposition, chemical etching, hydrothermal, sol-gel and so on. Preparing superhydrophobic films on metal surfaces is an effective way to improve the anti-corrosion property of metal substrates. In addition, superhydrophobic films can be used to oil-water separation. In this work, a one-step electrodeposition was applied to prepare superhydrophobic surfaces on copper meshes. The morphology, wettability and chemical composition of the prepared films were characterized by SEM, optical contact angle meter, EDS, FTIR and XPS. The results showed that the surface on copper meshes obtained at 30 V with 10 min was uniformly covered by microcells aggregated by nanosheets. The surfaces of the copper meshes were composed of copper myristate (Cu[CH₃(CH₂)₁₂COO]₂) and reach the maximum contact angle of 156.2° with the rolling angle as low as 1°. The potentiodynamic polarization curves were utilized to analyze the corrosion resistance, which demonstrated that corrosion current densities of the superhydrophobic film was 4.77×10^-9 A/cm², decreased by more than 3 orders of magnitude, and the corrosion potential was 0.036 V more positive compared with the copper substrate. Moreover,the oil-water separation tests showed that the separation efficiency of the film after reused for 5 times maintained above 95%, exhibiting excellent oil-water property and recycle capability.

Macrosegregation, or compositional heterogeneity, is a very common and serious defect in large steel ingots, which is hard to remove in the following processing procedures. It not only decides the final properties of the product, but also restricts downstream hot working processing severely. This compositional heterogeneity occurs due to the relative motion between the liquid and solid phases during solidification. Therefore, it is necessary to develop an effective method to manufacture large ingots with less macrosegregation. In this work, a novel casting method was proposed to alleviate macrosegregation of large ingots, i.e., layer casting (LC). With this method, alloy melt will be poured into the mould step by step, so that the melt could be solidified layer by layer and the macrosegregation will be alleviated. Both experimental and numerical studies were carried out to verify the feasibility and effectiveness of LC method. Two small Al-4.0%Cu (mass fraction) ingots were cast using two casting methods, conventional casting method, in which melt was cast into mould in one stage, and LC method, in which melt was cast into mould in several stages. Each ingot was sectioned into two parts along the center line, and then the specimens were measured by optical emission spectrometry to obtain the compositional distribution of Cu. Both severe bottom negative segregation and top positive segregation zones were observed in the ingot fabricated by conventional casting method. More homogeneity of compositional distribution was observed in the ingot fabricated by LC method, and the max negative and positive macrosegregation along the center line decreased by 24.6% and 77.2%, respectively. At the same time, a mixed three-phase (equiaxed, columnar and liquid) solidification model was employed to study the solidification processing in large ingots. The macrosegregation formation processes of 100 t and 13 t steel ingots fabricated by both conventional casting and LC methods were numerically simulated. The simulation results indicated that LC method had the capability of alleviating macrosegregation of large steel ingots significantly, compared with conventional casting method. With the increment of ingot size and amount of ladles, LC method had more significant effect on the alleviation of macrosegregation in large ingots. The mechanism of macrosegregation alleviation of LC method was analyzed.