High power impulse magnetron sputtering (HiPIMS) is of great significance for improving the quality of sputtered films because of its high ionization degree of sputtered particles and high ion fluxes. Therefore, it has been widely studied by researchers. However, the conventional HiPIMS shows a significantly low deposition rate, which greatly limits the industrial applications of HiPIMS. In this work, a novel high power impulse magnetron sputtering is proposed to enhance the low deposition rate encountered in conventional HiPIMS. The novel technology is based on dual pulses discharge mode, in which a pulsed high voltage with short duration is utilized to high-current discharge and produce initial high density plasma and a subsequent work-pulse of low voltage with long duration is employed to sustain the high-current discharge. Consequently the re-adsorption effect by magnetron target may be weakened. The influence of ignition pulse voltage discharge characteristics of Cr target and microstructure of CrN films were investigated. The discharge characteristics of Cr target and the structure characteristics of CrN coatings were characterized by digital oscilloscope, spectrometer, focused ion beam/electron beam dual-beam microscope and X-ray diffraction. The results show that the discharge of Cr target is ignited rapidly and the discharge current is substantially large with the ignition voltage applied to the target. In contrast, the pulse current gradually rises for the conventional HiPIMS meaning a weak discharge. Compared with the conventional HiPIMS, the dual-pulse HiPIMS produce a higher substrate current integral value and more amount of Ar⁺ and Cr⁰ with the same input power. With ignition pulse voltage of 590 V, the deposition rate at unit power for CrN coating is 2.52 μm/(h·kW) for dual-pulse HiPIMS, which is nearly three times higher than that of conventional HiPIMS. With the increase of the ignition pulse voltage, the CrN films prepared by dual-pulse HiPIMS possess denser structure with smaller grain size.

Tantalum coating attracts increasing attention in heat, corrosion and wear resistant applications today because of its high melting point, immunity to chemical attack and high toughness. Recently, tantalum has been considered a desirable candidate to replace electrodeposited (ED) chromium coating which is often used as protective coating against corrosion and wear. However, the wastes associated with ED chromium contain a well-known carcinogen, i.e. hexavalent chromium, which is a hazard to environment. In comparison, thick Ta coating is regarded as a more environmental and beneficial replacement. Tantalum coating is usually obtained by magnetron sputtering. However, tantalum exhibits two distinct crystalline phases. The body-centered cubic α-phase is the common phase in bulk metal and thermodynamically stable. α-Ta with good ductility and excellent mechanical properties is welcomed in most fields. β-Ta is a metastable phase with tetragonal crystalline lattice structure. The properties of β-Ta are not as advantageous as α-Ta because it is hard and brittle. The existence of β-Ta may compromise tantalum coating in adhesion, corrosion and wear resistance, hence, finding appropriate deposition conditions to obtain pure α-phase Ta coating has attracted a lot of interests. In previous work, pure α-phase Ta coating has been deposited by direct current magnetron sputtering when substrates were located in negative glow space. In this work, nitrogen was mixed in sputtering gases to deposit Ta coating with N interstitially dissolved on stainless steel. Effect of N on microstructure, mechanical and tribological performance of Ta coating was studied. Results indicated that when no nitrogen or very low flux of N₂ (l mL/s) were introduced in gas mixtures, α-phase Ta coating with coarse grains grew and revealed strong reflections of (211) and (110) diffraction peaks. When N₂ flow rate reached to 5 mL/s, Ta coating with N interstitially dissolved was obtained and revealed grain refinement and (110) preferred orientation of TaN_0.1 phase. Compared to α-phase Ta coating, N-doped tantalum coatings displayed excellent wear resistance for their high hardness and H ³/E ² ratio (H—hardness, E—elastic modulus). The wear mechanism for α-Ta coating was abrasive wear, while that of N-doped Ta coating switched to adhesive wear.

Carbon fiber-reinforced aluminum matrix composite has been considering as an ideal structural material for aerospace and automotive industries due to its high specific strength, high specific modulus, high thermal and electric conductivity as well as low coefficient of thermal expansion. However, for the fabrication of carbon fiber-reinforced aluminum matrix composites, the critical issues are the poor wettability and chemical reaction between carbon fibers and aluminum matrix. In order to improve the wetting behavior and prevent the chemical reaction between carbon fibers and the aluminum matrix, the electroplating technology assisted with ultra-sonic vibration dispersion method was applied to fabricate the copper interphase on the carbon fibers. It was found that a smooth, continuous copper interphase with homogeneous thickness could be deposited on carbon fibers. The carbon fiber-reinforced aluminum matrix composite (C_f/Al) was fabricated by the melt-infiltration process under pressure and vacuum conditions. The microstructure observations found that the carbon fibers homogeneously dispersed in the aluminum matrix by the introduction of copper interphase. There was no obvious carbon fiber damage caused by the reaction between carbon fibers and Al matrix. When the volume fraction of carbon fibers was 8%, the density of C_f/Al was about 2.70 g/cm³. Compared with pure Al, the mean tensile stress of C_f/Al composite was increased from 59.1 MPa to 144.9 MPa, which increased by 143%. The observation of fracture surfaces revealed the occurrence of the sliding and pull out of carbon fibers under tensile stress. The sliding and pull-out of carbon fibers can refrain the crack initiation and propagation of micro-cracks in the Al matrix. Therefore, the tensile strength of C_f/Al composite was improved significantly.

In ferroelectrics, the presence of domain structures and switchable polarization plays an important role in ferroelectric performance and the design of future electronic devices. Understanding domain behaviors is crucial for ferroelectrics promising applications, particularly in nonvolatile memory, microwave ceramics, electromechanical sensors and actuators. As a convenient, nondestructive and high-resolution technique, the piezoresponse force microscopy (PFM) provides a powerful method for observing domain structures and their dynamic behavior at the micron and nanometer scales. In this work, PFM has been used to study the domain structures and their dynamic behavior of 0.8PbTiO₃-0.2Bi(Mg_0.5Ti_0.5)O₃ thin film. Both the a domain and the c domain coexist in the ferroelectric thin film nanometer grains. Under the tip-bias-induced electric field, the domain switching follows the two 90° steps of 180° domain switching, showing the domain polarization change from c to a to c. A remarkable effect of temperature on the domain configurations and domain dynamic response in 0.8PbTiO₃-0.2Bi(Mg_0.5Ti_0.5)O₃ thin film was found by PFM. Under the tip bias voltage of 5 V, domain evolution was more rapid with a higher temperature at 70 ℃. The surface charge is related with c domain polarization. At high temperature, the surface charge induced effective electric filed increases, allowing for the easier domain motion.

The volume fraction and stability of retained austenite play an important role in the performance of low carbon steels, while the C and Mn elements have a stabilizing effect on the thermal stability and mechanical stability of retained austenite. Therefore, the C and Mn elementals partitioning was promoted by intercritical annealing. As a result, the mechanical properties of the low carbon steels are improved. The microstructure of quenching and partitioning bainitic steels and retained austenite characteristics were studied by means of SEM, TEM and XRD. The partitioning and content of C and Mn elements in retained austenite at different locations were characterized by EPMA, EBSD and nanoindentation. The effect of C and Mn elements on the stability of retained austenite at different locations and phase change law of retained austenite were investigated by combining the tensile stress-strain curves under the treatment of intercritical annealing (partial austenitizing)-quenching and partitioning in the bainitic region process (IQ&PB). In the process of tensile deformation, the transformation induced plasticity (TRIP) effect occurs, the volume of retained austenite decreases, the transformation takes place preferentially in the ferrite grain boundary, and finally occurs between the bainite laths. C and Mn elements have a stabilizing effect on the retained austenite, which make retained austenite is not prone to phase change. The stress at the tensile fracture is concentrated, and the retained austenite is completely transformed into martensite. The volume fraction of retained austenite is 3.12% and 5.03% at 2 mm and 4 mm distances away from the fracture. Film-like retained austenite is more stable than blocky retained austenite, and the retained austenite of the <111>_γ crystal orientation is unstable and easily transforms into martensite.

Cast steel is an important metal material that is widely used in civil engineering due to its strength and ductility. However, a variety of casting defects such as micro/meso pores are usually present in the as-cast components and can lead to the degradation of mechanical properties. In this work, the initial micro/meso pores in the G20Mn5N low-alloy cast steel were investigated based on high resolution 3D X-ray tomography technology. Based on their formation mechanism and characteristics, pores were classified into gas, gas-shrinkage and shrinkage pores, and the parameters such as the number, size and sphericity of three types of pores have been counted and analyzed. Then the evolutionary behavior of micro/meso pores in G20Mn5N low-alloy cast steel specimens under monotonic tensile loading has also been studied. The results showed that the volume of gas pore was small and its sphericity coefficients were high. Compared with the gas pore, the shrinkage pore had large volume and more complex shape in space. The volume and sphericity coefficients of gas-shrinkage pore were between the gas pore and the shrinkage. Damage evolution to metallic materials can be divided into void nucleation, growth and coalescence. The void nucleation and growth law were investigated by statistical analysis, which showed that the evolution of the void density could be modeled by an empirical function, and the evolution of void average radius was not only related to void growth but also affected by void nucleation.

Irradiation assisted stress corrosion cracking (IASCC) of austenitic stainless steel core components is one major concern for maintenance of nuclear power plants. Previous studies on the IASCC had mainly focused on the effect of irradiation on changes in deformation modes and interaction of dislocation channels with grain boundary. The role of corrosion in IASCC, however, has not received sufficient attentions. In the process of stress corrosion cracking (SCC), corrosion occurs simultaneously with localized deformation in the vicinity of the crack tip. This indicates that corrosion is one of the potential contributors to IASCC. In this work, IASCC of proton-irradiated nuclear grade 304 stainless steel (304SS) was investigated. The IASCC tests were conducted by interrupted slow strain rate tensile (SSRT) tests at 320 ℃ in simulated primary water of pressurized water reactor containing 1200 mg/L B as H₃BO₃ and 2.3 mg/L Li as LiOH·H₂O, with a dissolved hydrogen concentration of 2.6 mg/L. Following the SSRT tests, the localized deformation, corrosion and IASCC of the specimens were characterized. The results revealed that increasing the irradiation dose promoted residual strain accumulation at slip steps and grain boundaries of nuclear grade 304SS. Since the slip step usually transmitted or terminated at the grain boundary, it eventually promoted localized deformation at the grain boundary. Specially, the slip step transmitted at grain boundary led to slip continuity at the grain boundary. In contrast, a slip discontinuity was observed at the grain boundary where the slip step terminated, which caused a much higher strain accumulation by feeding dislocations to the grain boundary region. Further, formation of the slip discontinuity was related to the Schmidt factor pair type of the adjacent grains. The irradiation resulted in a depletion of Cr and an enrichment of Ni at grain boundary, while the magnitude of Cr depletion and Ni enrichment increased with increasing the irradiation dose. Following the SSRT tests, intergranular cracking was observed on surfaces of the irradiated specimens, while the number of the cracks was increased by a higher irradiation dose and applied strain. This suggested a higher IASCC susceptibility of nuclear grade 304SS in the primary water. Meanwhile, significant intergranular oxidation ahead of the crack tip was observed, while both the width and length of the oxide were larger at a higher irradiation dose. The synergic effect of irradiation-promoted deformation and intergranular corrosion was the primary cause for the IASCC of the irradiated steel.

Grain-oriented silicon steel is a key material used for iron cores of transformers because grain-oriented is most desirable for magnetic cores. With the rapid development of modern power industry, the requirement for grain-oriented silicon steel with lower magnetostriction and iron loss have exigent. Although the application of laser-scribed technology can effectively reduce iron loss by refining main magnetic domain in high permeability grain-oriented silicon steels, the influence of laser-scribing on the magnetostriction of grain-oriented silicon steel is still controversial due to the complex magnetic domain structure led by interaction among crystal orientation, surface tension and scribing parameters. In this work, a magnetostriction model for laser-scribed grain-oriented silicon steel is proposed based on the relationship between magnetostrictive coefficient and two kinds of 90° magnetic domain, stress closure domain and transverse domain, and the interaction effects of laser scribing parameters and orientation deviation angle (tilt angle of [001] easy axis out of sheet surface) are analyzed. The orientation deviation angle determines which 90° domain structure of either transverse domain or stress closure domain acts as the dominant factor for magnetostrictive behavior. The stress closure domain and stray magnetic field introduced by laser scribing can reduce the magnetostriction coefficient originated from orientation deviation angle. The theoretical calculation on the effects of laser-scribed energy density and laser-scribed spacing on magnetostriction coefficient is in agreement with the direct experimental measurement. The proposed model concerning the interaction between laser-scribing parameters and orientation deviation angle can provide the theoretical basis to reduce the noise of laser-scribed grain-oriented silicon steel.

The main purpose of the present work is to explore the influence of the Ti addition on crystal structure, phase stability, magnetic properties and electronic structures of the Ni₈Mn_4-xGa₄Ti_x (x is the number of Ti atoms in a unit cell, x=0, 0.5, 1, 1.5 and 2) alloys by first-principles calculations, aiming at providing the theoretical data and directions for developing high performance ferromagnetic shape memory alloys (FSMAs) in this alloy system. The formation energy results indicate that the doped Ti preferentially occupies the Mn sites in Ni₂MnGa alloy. With the increase of Ti content, the optimized lattice parameter of the ferromagnetic austenite increases regularly. For the martensitic phase, the lattice parameter a increases while c decreases, leading to a decreased c/a ratio. The paramagnetic and ferromagnetic austenitic phases both become stable because their formation energies (E_form) gradually decrease with the increasing amount of Ti. The experimentally reported decrease in the Curie temperature with increasing Ti content is derived from the decrease of the total energy difference between the paramagnetic and the ferromagnetic austenite. The total magnetic moment is mainly contributed by Mn, while the magnetic moments of Ga and Ti are nearly zero. The total magnetic moment decreases notably when Mn is gradually substituted by Ti because the atomic magnetic moment of Ti is much less than that of Mn, which is in fair consistent with the experimental observations. The intensity of up-spin total density of state (DOS) decreased dramatically with the increase of the Ti content; whereas the change of the down-spin part below E_F is not obvious. This feature gives rise to the decrease of the total magnetic moments in these alloys. The results of present work are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.

In recent years, the welding of dissimilar metals such as steels and aluminum alloys has attracted much more attentions due to weight reduction, especially in automobile and railway vehicle manufacturing industry. However, many challenges and problems need to be addressed in order to obtain high quality welding joints between steels and aluminum alloys resulting from their differences of thermal-physical properties. The formation of intermetallic compounds (IMCs) in the course of welding will lower the mechanical properties of the joints. Up to now, a few techniques have been tried to weld aluminum alloys and steels, including solid welding and fusion welding. In this work, dissimilar metals of 5182-O and HC260YD+Z were welded by cold metal transfer (CMT) arc-brazing using AlSi₅ as filler metal. The macro and micro morphologies of the overlap joint were investigated using OM, XRD, SEM and EDS analyses. The hardness and shear strength of the joints were tested. Results show that welding line energy can affect the thickness of IMCs existing on the brazing interface and thus depress the combination properties because of the different fracture modes. When the welding speed and wire feed speed are 9 mm/s and 5 m/min respectively, the IMCs thickness is about 6 μm, and the shear strength of the jonts can reach to 160 MPa. Two typical fracture modes of fusion interface fracture and brazing interface fracture were observed. The fracture mode of the position near arc striking is "fusion interface". With the increasing of welding energy, the thickness of IMCs is increased and the fracture mode near arc extinguishing is changed from "fusion interface" to "brazing interface". When the output power of CMT equipment is 150~210 J/mm at welding beam length, the IMCs thickness is less than 9 μm, which benefits the shear strength performance of the joints, and the fracture mode of "fusion interface" can be easily obtained.

Mg-Zn-Er casting magnesium alloys have good properties, such as high specific strength, high specific stiffness and remarkable temperature creep properties. Current researches mainly focused on the phases and mechanical properties at room and high temperatures. However, the effect of Er on hot cracking susceptibility of Mg-5Zn-xEr magnesium alloys was barely studied. In this work, a modified RDG (Rappaz-Drezet-Gremaud) model for predicting the hot cracking susceptibility of Mg-5Zn-xEr (x=0.83, 1.25, 2.5, 5, mass fraction, %) ternary alloys was proposed, which considered the effects of phase and solidification temperature range on the hot cracking susceptibility of the multiphase alloys. And, the hot cracking susceptibility was evaluated by the experiment of constrained rod casting (CRC). The results indicated that the modified RDG model could accurately predict the hot cracking susceptibility of Mg-5Zn-xEr magnesium alloys. The hot cracking susceptibility increased with the addition of Er up to 2.5%, and Mg-5Zn-2.5Er alloy showed the maximal hot cracking susceptibility; when the addition of Er increased to 5.0%, Mg-5Zn-5Er alloy exhibited the minimal hot cracking susceptibility. The calculated results were consistent with the experimental ones. Further analysis on the casting solidification curves, phases and microstructures showed that I-phase precipitated by peritectic reaction during solidification of Mg-5Zn-2.5Er alloy depleted liquid phases and extended the solidification temperature range of the alloy, leading to the hot cracking susceptibility increasing. The Mg-5Zn-5Er alloy underwent eutectic reaction of L→α-Mg+W during solidification, which reduced the solidification temperature range. Meanwhile, this process was beneficial to feeding the interdendritic hot cracking in the terminal period of solidification, which significantly decreased the hot cracking susceptibility of Mg-5Zn-5Er alloy.

Al-Bi alloy is a kind of bearing material with self-lubricating performance. Whereas, in such immiscible alloys macrosegregation occurs generally due to the miscibility gap in solidification process. This study is designed to produce CeBi₂ compound particles in Al-Bi melt through the addition of rare earth element Ce. The solidification experiment is processed with liquid quenching and natural cooling respectively, and the effect of dispersive solid particles on liquid-liquid separation is compared at different cooling rate. The dispersive solid particles act as nucleation site and improve the nucleation rate of Bi-rich droplets in liquid-liquid solidification, leading to the size refinement and dispersive distribution of Bi-rich phase, which promote the bearing performance of Al-Bi alloy. The behavior of Bi-rich droplets is simulated by using of discrete multi-particle approach, considering the movement, growth and collision. The results show that Stokes motion is the main cause of macrosegregation, and the natural convection affects the distribution of droplets. The natural convection prolongs the suspension time of droplets, and reduces the macrosegregation. In the case of Ce addition, CeBi₂ acts as nucleation site to enhance the amount of Bi-rich particles in each time step of simulation and refine the droplet size, which lead to a reduction of macrosegregation. Compared with the Ce free specimens, Al-Bi-Ce alloy has a slower segregation rate and relatively less macrosegregation. Both the simulation and experiment results show a multimodal particle size distribution, and their peak value decrease with increasing particle size. The simulated results is in good accordance with the experiment.

Iron ore direct reduction is an attractive alternative route for effective use of low-grade complex symbiotic iron ore resources as well as reducing CO₂ emissions from steel making. In this process, solid iron ore pellets are converted to so-called direct reduced iron with a reduction gas such as CO and H₂. However, the slow reduction rate of iron oxides at lower temperatures has restricted the productivity of direct reduced iron. We have been studying the use of a magnetic field to enhance the direct reduction process, and investigating the influences of the magnetic field on the reduction of iron oxides and morphology of direct reduced iron. Iron ores are rich in iron oxides and also contain other oxides. The presence of other oxides, for instance CaO, is likely to interact with iron oxides during reduction so as to enhance the reduction rate by varying the lattice structure of solid iron oxides and gas/solid mass transport. In the present work, the effects of magnetic field on the reduction of CaO-containing iron oxides were studied. Isothermal reduction of compact samples of pure Fe₂O₃ and 2.5%CaO (mass fraction) containing Fe₂O₃ was carried out at 1073 K under a reaction atmosphere 75%CO+25%CO₂ (volume fraction). A constant magnetic field (B=1.02 T) was applied during reduction to compare with the reaction under a normal condition without a magnetic field applied. The results showed that the magnetic field accelerates the reaction rate of Fe₂O₃ reduction to metallic Fe, and there are no phase compositions change during reduction in a constant magnetic field. The magnetic field promotes the diffusion of Ca in Fe₂O₃+2.5%CaO, and the reduced sample obtained in a magnetic field appears loose and porous. Thermodynamic calculation indicated that the Gibbs free energy of Fe₂O₃ reduction and CaFe₅O₇ phase decomposition is decreased with an interaction of magnetic field, resulting in an increase of the reaction equilibrium constant thus making the reduction of Fe₂O₃ and decomposition of intermediate phase CaFe₅O₇ occur more readily.

GH4065 alloy is a new type of high-alloyed wrought superalloy, in which freckle defect is extremely prone to form in large ingot. In the present research, the freckle of GH4065 alloy bar with the diameter of 280 mm produced by vacuum induction melting (VIM)+electroslag remelting (ESR)+vacuum arc remelting (VAR) triple smelting was studied. The macrostructure, secondary phases and grain structure of the freckle were investigated, the influences of solute elements on the freckle were analyzed, and both the mechanism and control methods were also discussed. The results show that the freckle in GH4065 alloy is caused by channel segregation with the low-density Ti and Nb-rich melt flows. Additionally, lots of lath-like η-phases, block M₃B₂ borides and MC carbides are formed in the forged condition. It is confirmed by the thermodynamic calculations that the η-phases, M₃B₂ borides and MC carbides are much easier forming in the freckle than that in matrix. After heat treatment, compared with matrix, the lathy η-phases are still existed in the freckle; the size and quantity of primary γ′ phases increase significantly while the size and morphology of the secondary γ′ phase are basically identical, only with less quantity. It has been found that due to the high content of γ′ phase, the γ′ dissolution temperature in the freckle is higher than that in the matrix. This induces an impeded recrystallization process caused by the coarsened γ′ phases during forging process and the grain size of the freckle region is significantly smaller than that of matrix. Based on this study, the formation of freckle can be effectively controlled by meticulous controlling of the previous smelting process, releasing of electrode residual stress, suitably reducing VAR melting rate, and accelerating VAR cooling.