Tungsten (W) plays an important role in the defense industry, aerospace and nuclear industry due to its excellent properties such as high melting point (3410 ℃), high density (19.35 g/cm³), high hardness, high elastic modulus, high thermal conductivity, low expansion coefficient and low vapor pressure. However, its disadvantages, such as low temperature brittleness (ductile brittle transition temperature usually above 400 ℃), low tensile strength, recrystallization embrittlement, high thermal load induced cracking and irradiation embrittlement, affected seriously its processing and servicing performance. Focusing on these problems, carbides/oxide dispersion strengthened W alloys were studied widely. The mechanical properties and other service properties of W were significantly improved by nano scale carbide/oxide dispersion strengthening and microstructure optimization. This article mainly reviews carbide and oxide dispersion strengthening design and the corresponding W-based materials preparation, microstructure and properties of regulation and service performance evaluation, introduces the latest progress of the research and development of the authors' team, and looks forward to the future development trend and the problems to be solved.

Undercooling is an important parameter to characterize the process of solidification and the physical properties of the melt. However, the traditional experimental conditions do not provide mature technical conditions and experimental platforms for the study of this subject. Molecular dynamics simulation method can not only study the experimental process and the organization structure, but also break through the limited conditions of the laboratory, and provide advanced prediction for scientific research. In order to study the influences of superheated temperature and cooling rate on the undercooling of the homogeneous nucleation and the solidified structure, the solidification of undercooled Ti melt was studied by molecular dynamics simulation in this work; and the solidified structure was then analyzed by the radial analysis, the H-A key type analysis and the largest groups of cluster analysis. The results show that, the nucleation undercooling of Ti melt increases with the rise of superheated temperature. In the undercooling vs temperature curve there are two inflection points at 2100 K (T₁) and 2490 K (T₂), which correspond to the breaking-start temperature and breaking-end temperature for bond pair of nucleation cluster. In this temperature range, the number of nucleation clusters decreases with rise of temperature. When the superheated temperature is higher than T₂, the nucleation undercooling approaches a constant. On the other hand, the nucleation undercooling of Ti melt increases with the accelerate of cooling rate until an anomalous structure is formed, and in the numbers of the bonds of the structure vs different cooling rate curves, the number of 1541, 1551 and 1431 bond types gradually adds with cooling rate going up. In addition, when the cooling rate is less than 1.0×10¹¹ K/s, the hcp and bcc inlaid crystalline structures are obtained after the solidification of Ti melt. When the cooling rate is greater than or equal to 1.0×10¹³ K/s, two kinds of crystalline structure are reduced, and the microstructures are mainly amorphous. When the cooling rate ranges between 1.0×10¹¹ K/s and 1.0×10¹³ K/s, its structure is a mixture of crystalline and amorphous. From the results of radial distribution, H-A bond type and atomic cluster analysis, it was found that the critical cooling rate for amorphous structure is determined as 1.0×10¹³ K/s.

High carbon steel is prone to produce macroscopic/semi-macroscopic segregation, and C segregation along casting direction will have a serious impact on the quality of billet. Based on qualitative analysis (macroscopic quality rating and elemental macro content analysis) and relatively simple quantitative analysis (segregation index, mean square error), the existing technologies have judged the degree of segregation of billet in different levels, but with requirement of stricter quality stability standard, especially for the typical raw material of high-level rod-wire steel (e.g. 70 high carbon steel), it is very necessary to introduce a new method to measure the fluctuation characteristics more effectively. In this work, the ARMA (auto regressive moving average) model in the time series is firstly used to study the segregation degree of high carbon steel billet at different positions from the aspects of inherent period and damping rate in terms of the fluctuation characteristics (period and extremum) of C element along the casting direction. The size of the billet is 170 mm×170 mm and the sampling location is in the center longitudinal plane of the billet. The experiment is conducted by considering the effect of cooling strength (conventional cooling and strong cooling) on the fluctuation characteristics. Firstly, it is shown that the inherent period and the damping rate can quantitatively describe the inherent characteristics (periodic and extreme characteristics) of the C element along the casting direction. Secondly, strong cooling makes the average inherent period and the average damping rate of C element time series in the columnar grain region smaller, thus increasing the fluctuation degree and the segregation degree. However, strong cooling makes the average inherent period and the average damping rate of the C element time series in the equiaxed grain region larger, thus decreasing the fluctuation degree and the segregation degree. Finally, the period and the damping rate of the columnar grain region are respectively affected by the dendritic spacing and the temperature gradient, and the period and the damping rate of the equiaxed grain zone are respectively affected by the liquid flow in the solidification process (V-shaped segregation) and local cooling rate. By this research, a new theoretical basis may be supplied for delicacy control of element segregation and related quality fluctuation phenomenon.

Medium Mn steel is composed of sub-micron grained ferrite and austenite, the unstable austenite may transform to martensite during plastic straining. Although the mechanical properties of medium Mn steel could be easily tested by tensile test, it is quite difficult to directly measure the influences of different constituent phases on the tensile and work hardening behavior. Thus, at the present work, EBSD, TEM, XRD and a constitutive model based on dislocation density have been used to study the effects of intercritical annealing (IA) temperature on the tensile properties and work hardening behavior of a newly designed medium Mn steel, Fe-7%Mn-0.3%C-2%Al (mass fraction). Experimental results showed that with the increase of IA temperature, the mechanic stability of reverted austenite decreased gradually and the kinetics of strain induced martensite rose rapidly. The stability of the reverted austenite was moderate when intercritically annealed at 700 ℃, this led to the best plasticity and the optimal mechanical properties. Simulated results exhibited that the mechanic stability of austenite has a decisive influence on the tensile behavior of the material. The austenite stability will be too high if the IA temperature is lower, and this will lead to the lower work hardening rate and uniform elongation; when the IA temperature is moderate, the stability of austenite will be optimum, consequently strain-induced martensite would be progressively produced during straining and result in the higher work hardening rate and prolonged uniform elongation; the stability of austenite will be too lower if the IA temperature is higher, thus larger volume fraction of strain-induced martensite would be formed in a short period, and this would result in the higher tensile strength but the inferior uniform elongation.

Three-dimensional characterization of grains and grain boundaries is significant to study the microstructure of polycrystalline materials, and is the key to advance the subject of three-dimensional materials science (3DMS). In this work, the technique of serial sectioning by mechanical polishing coupled with 3D electron backscatter diffraction (3D-EBSD) mapping was used to measure the microstructure of a 316L stainless steel in 3D. Volume of the collected 3D-EBSD microstructure is 600 μm×600 μm×257.5 μm, which is quite large to study the 3D microstructure of structural materials with conventional grain size (20~60 μm). Dream3D and in-house developed Matlab programs were used to process the 3D-EBSD data, and subsequently ParaView was used to visualize the grains and grain boundaries in 3D. Combined usage of these tools and in-house programs make the possibility that not only 3D grains but also 3D grain boundaries can be studied in both morphology and quantification. In total, 1840 grains and 9177 grain boundaries are included in the measured 3D-EBSD microstructure. The 3D morphological characteristics and size distributions of grains and grain boundaries in the 316L stainless steel were investigated, including 3D grain size, grain surface area, boundary quantity per grain, grain boundary size and the average boundary size per grain, as well as relationships between these morphological parameters were discussed. Results showed that distributions of all of these morphological parameters of 3D grains and grain boundaries in the polycrystalline 316L steel can be well represented by log-normal distribution, and all relationships of these parameters versus grain size can be well represented by power function. Additionally, the 3D morphologies of most grains in the 316L stainless steel deviate from the ideal equiaxed grains, having complex shapes due to existing of twins, such as semi-sphere shaped, plate shaped and some very complex grains. In many ways, the larger grains have more complex morphology with greater number of faces, larger surface area and larger deviation from equiaxed grains.

Grain-oriented silicon steel is mainly used as the core material in electrical transformers, and its magnetic properties are closely related to the sharpness of Goss texture ({110}<001>) formed by secondary recrystallization during high-temperature annealing. However, the mechanism of the abnormal growth of Goss oriented grains is still disputed in the literatures. It is well know that microstructure characterization is important to study the relevant mechanism and improve the properties of materials. Usually, the microstructures are characterized only using a single two-dimensional plane of polished or thin foil specimen. Much information on the morphologies is lost owing to the fact that a large part of microstructures is embedded beneath the polished surface, or removed during specimen preparation. Recently, computer-aided three-dimensional morphologies have been developed which can visualize microstructures in metals. The three-dimensional visualization promotes a better understanding of the actual information of polycrystalline materials, especially when the grain morphologies and size are required in three dimensions. In this work, the three-dimensional morphologies of abnormally grown Goss oriented grains in Hi-B steel during secondary recrystallization annealing were investigated by a combination of serial sectioning, computer-aided reconstruction and visualization, and electron back-scattered diffraction technique, then the rules and features of abnormal grown Goss oriented grains were also discussed. The results show that the abnormally grown Goss oriented grains have a pancake-shape grain structure in three-dimensional scale, and follow corresponding grain growth behavior. That is, the secondary recrystallization nuclei of the Goss oriented grains in the subsurface grow quickly into the center layer with a grain size advantage, and further extend back to the surface, then continue to grow abnormally along the plate plane direction with the help of surface energy. Finally, the dimension in the plate plane direction is much larger than that of the thickness direction. During abnormal growth of Goss oriented grains, some large-size grains will prevent Goss grains growth, and temporarily retain in the grains to become 'island' grains. On the other hand, the growth front is quite ragged because Goss oriented grains growth is blocked in some directions, showing typical anisotropic growth features, and there are three possible reasons to account for this phenomenon. One reason is that Goss oriented grains may encounter some large-size grains due to inhomogeneity of matrix grains size, and these large-size grains will block the growth. Another is that two abnormally grown Goss oriented grains which meet together may cause the stagnation of grains growth in some directions, and at the same time some matrix grains are encapsulated to form 'island' or 'peninsula' grains. Furthermore, it is also possible that there are obvious differences in grain boundary mobilities between different oriented grains and Goss oriented grains.

High voltage direct current transmission (HVDC) systems develop fast in China recently. The ground electrodes of HVDC systems can inject/absorb large amount of DC current into/from soil, introducing DC interference to nearby pipelines. Then the pipe-to-soil potential shifts positively and high corrosion risk may appear. In this work, indoor HVDC simulation experiments were designed and carried out based on the field test results. Under high voltages, the variation regularity of DCdensity and the corrosion behavior of X80 steel in Guangdong soil were studied. The result showed that under 50, 100, 200 and 300 V DC voltages, the DC density of the coupons had the same trend and could be divided into 3 stages. Firstly, the DC density climbed to peak sharply in several seconds. Then, the DC density decreased gradually to steady value in hundreds of seconds. Lastly, the DC density stayed at that level for the rest of time. The local environment was monitored. The results indicated the variation of the DC density was mainly related to the local soil temperature increment, water content decrement and the substantially growth of the soil spread resistance. After the interference, the corrosion rates were measured to be 5.56, 7.85, 10.63 and 7.78 μm/h, respectively. The variation regularity of the corrosion rates was same with the steady values of DC density, but different from the peak values. Furthermore, 3 methods of calculating corrosion rates were studied. The theoretical corrosion rates calculated by integration of DC density curve had the smallest errors compared with the measured values. The method of using steady DC density had bigger errors and using peak DC density led to the biggest errors. Based on the results, the method of predicting HVDC corrosion rate was proposed.

In order to improve the steam oxidation resistance of G115 steel (9Cr3W3CoVNbCuBN) at 650 ℃, pre-oxidation treatment was carried out in argon environment with low oxygen partial pressure. The oxidation behaviors of the pre-oxidized and untreated samples were simultaneously investigated by a cyclic oxidation experiment. Weight gains of samples were measured by analytical balance, phases of oxide products were identified by XRD and EDS, morphology and structure of scales were characterized by SEM and EDS. The result showed that pre-oxidation treatment significantly decrease oxidation weight gains in 1800 h. After pre-oxidation treatment, the oxidation kinetics transformed from cubic into linear form, and the scale structures transformed from duplex layers into triple layers. In the scale of pre-oxidized samples, the outermost layer was enriched in Fe, the middle layer was enriched in Cr, and the innermost layer was transformed from the matrix metal. The middle layer had chromium content as high as 46% (mass fraction) and was considered to be conformed of chromite (FeCr₂O₄). This layer was the most protective layer due to its highest Cr content, and the diffusion of O and Fe though it was the main controlling process of the whole oxidation. It suggested that the stable structure of the middle layer improved the oxidation resistance of pre-oxidation samples. The thickness of the middle layer nearly kept constant during the whole oxidation process, which was the main reason why the pre-oxidized sample had linear oxidation kinetics. The long term effect of the pre-oxidation treatment was evaluated based on the scale structure and oxidation mechanism.

High entropy alloys (HEAs) origin from a new alloy design concept with multi-principal elements, which have attracted significant interests in the past decade. The high configurational entropy in HEAs results in simple solid solutions with fcc and bcc structures. Especially, the single solid solution CoCrFeNi alloy exhibits excellent properties in many aspects, such as mechanical properties, thermal stability, radiation resistance and corrosion resistance. The excellent corrosion resistance of CoCrFeNi alloy is ascribed to the single-phase structure and uniform element distribution coupled with much higher Cr content than stainless steel. The single-phase structure and uniform element distribution can prevent the occurrence of localized corrosion, and higher Cr content can protect the alloy surface better with the form of oxidation film. Moreover, the corrosion resistance of CoCrFeNi-based HEAs, such as CoCrFeNiAl_x, CoCrFeNiCu_x, CoCrFeNiTi_x, have also been extensively investigated. In most CoCrFeNi-based HEAs, the elements of Co, Cr, Fe and Ni are with equal-atomic ratio. However, the equal-atomic ratio is not necessary to obtain satisfactory properties and to ensure the single fcc structure in Co-Cr-Fe-Ni system. Accordingly, it is essential to further consider the effect of alloying elements on the corrosion resistance in Co-Cr-Fe-Ni HEA. In this work, the effect of Co, Fe and Ni elements on the corrosion resistance of single fcc Co-Cr-Fe-Ni system with concentrated constitution but different atomic ratios in 3.5%NaCl solution are investigated by using LSCM and EIS. The potentiodynamic polarization results indicate that the increase of Fe and the decrease of Ni will decrease the passivation current density of the alloys when the Co and Cr contents are equal. With the increase of Co and the decrease of Ni, the alloys show smaller passivation current density and better corrosion resistance when the Fe and Cr contents are equal. With the decrease of Co and the increase of Fe and Ni, the alloys show higher corrosion potential and smaller corrosion tendency when the Cr content is constant. These results will be helpful for the design of corrosion resistant HEAs in NaCl aqueous solution.

In recent years, Zr is widely used as an important additive element in magnesium alloys containing rare earth (RE), to improve the mechanical properties of Mg-RE alloys such as strength, ductility, creep resistance and corrosion resistance property. Heterogeneous nucleation mechanism and peritectic reaction mechanism are recognized as the main grain refining mechanisms. Whereas, during the solidification process, the melt wetting angle and nucleation energy are important factors which influence the nucleation. In this work, the effect of Zr on the solidification microstructure of the Mg-Gd-Er alloy was analyzed by using OM and EBSD; the undercooling of alloy melts was tested by using DSC; and the Mg/Zr interface relationship and interfacial energy were investigated by using HRTEM. Moreover, the effects of Zr on the wetting angle and nucleation activation energy of the Mg-11Gd-2Er and Mg-11Gd-2Er-0.4Zr alloys were investigated; the refinement mechanism of Zr on the alloys was discussed. The results indicates that the addition of Zr element can significantly refine the grain, and the grain size decreased from 1000 μm to 50 μm. Compared with the Zr-free alloy, the nucleation wetting angle of the present alloy melt decreased from 18.3° to 11.1°, and the activation energy of nucleation decreased by 44.4%. The (1010) plane of Mg was completely coherent with the (1100) plane of Zr, reducing the interfacial energy between the (1010)_Mg and the (1100)_Zr. The grain refinement of Mg-Gd-Er alloy was ascribed to the decrease of melt wetting angle and the fully coherent interface relationship between Mg and Zr.

Due to the great advantage in manufacturing component with complex structures, additive manufacturing (3D print), essentially the rapid solidification of tiny metallic molten pool (hemisphere like with diameter ranging from dozens of microns to several millimeters) has become an important formation technique. Using powder laser melting, the effect of transverse static magnetic field on the solidified structure of additive manufactured Al-12%Si alloy was studied. The macrostructure was formed by white band (mainly primary α-Al phase) and dark grey area (mainly eutectic phase) and no obvious influence was presented with or without static transverse magnetic field of 0.35 T. However, for the microstructure, the primary α-Al in dark grey area formed as columnar structure without magnetic field was found to transform to dendritic like with developed dendrite arms when under a static transverse magnetic field. The analysis on thermoelectricity and dimensionless Hartman parameter which used to characterize the restriction of static magnetic field on molten flows show that under a static transverse magnetic field of 0.35 T, the thermoelectric magnetic force can be as high as a magnitude of 10⁵ N/m³, and Hartman values is far more than 10. The results indicate that the Marigoni and thermosolutal convection in laser melting pool was restricted. The transform from columnar to equiaxed dendrite of primary α-Al in dark grey area under static magnetic field was attributed to the fragmentation by thermoelectric magnetic force (10⁵ N/m³) in solid phase. In addition, the formation of high order dendrite arms was supposed to be caused by the restriction of static magnetic field on the melt.

The low kinetic energy and low ionization rate of deposited particle of traditional magnetron sputtering led to low density and poor adhesion of TiN film. The peak current density between cathodic target and anodic chamber was increased several times through the adoption of pulsed power supply mode with low duty cycle, which further enhanced kinetic energy and ionization rate of deposited particle. But the average deposition rate of thin film was significantly reduced. Therefore, a design concept of dual pulsed electric field mode was proposed, which allowed to adjust duration time and target peak current density of the dual pulses. It not only enhanced kinetic energy and ionization rate of deposited particle to satisfy the demand of fabrication of high performance film, but also increased the duration time of pulse to achieve high average deposition rate. In the manuscript, TiN films were deposited by dual pulsed power magnetron sputtering with different target peak current densities of the second pulse stage. The microstructure and mechanical properties of TiN films were characterized using XRD, SEM, nanoindentation and microscratch test. It was found that the TiN film deposited under target peak current density of 0.87 A/cm² exhibited finely dense microstructure with average grain size of 17 nm. Additionally, the hardness and film-substrate adhesion of such film were as high as 29.5 GPa and 30.0 N, respectively.

Bismuth and its alloys exhibit a number of peculiarities and mysterious features due to its three-dimensional (3D) hexagonal crystal, and have attracted the interest of many researchers for many years. Currently, the trivial-to-topological and semimetal-semiconductor transitions have been focused, as the result of its semi-metallic and large spin-orbit coupling. The binary compounds of Bi₂M₃ and binary alloys Bi_xM_1-x (M=Se, Sb and Te) are found to be 3D topological insulators, as the result of small band gap and large spin-orbit coupling in Bi crystals and Bi compounds, which make these crystals topologically important. In the case of Bi films, strong spin-orbit (SO) coupling interaction is also a fundamental mechanism to induce the Z₂ topology. Recently, ultrathin Bi films have also been theoretically predicted to be an elemental two-dimensional topological insulator. And, all the ultrathin Bi(111) films are characterized by a nontrivial Z₂ number independent of the film thickness. In the past few years, ultrathin films of Bi with a thickness down to several BLs (bilayers) on Si substrate have been prepared in experiments, finding that thicknesses have an effect on the properties of Bi films. However, the effect of thickness on films had not be studied for microscopic mechanism experimentally in detail. In this work, the effects of thickness on the surface and electronic properties of (00Ɩ) and (012) oriented films of Bi using the first-principles method were studied. With the increase of thickness, (00Ɩ) oriented Bi films became more stable, and the film of the even-numbered layers was more stable than that of the odd-numbered layer. However, the (012) oriented Bi films presented totally different behavior comparing with the (00Ɩ) oriented Bi film. The stabilities of (012) oriented film became less stable as the thickness increased, and possessed the approximated surface energy of even-numbered layers (00Ɩ) oriented Bi films when their layer numbers were closed to four. Further analysis of the cohesive energy, geometry structure and electronic band structures showed that, all the thin films presented the transition from semi-conductors to semi-metal or metal as the thickness increases.

Currently, metallic biomaterials used in orthopedics are normally bioinert which is hard to integrate with the bone tissue inducing aseptic loosening and easy to get infection, which is the main reason of implantation failure. Mg base metals are considered to be a new generation of revolutionary metallic biomaterials due to its similar density and mechanical properties with natural bone, good biocompatibility, degradability in the body as well as the biological functional ability to promote new bone tissue formation. In addition, the degradation of Mg may increase the local pH which can inhibit the growth of bacteria. In this work, pure Mg coating was deposited on Ti6Al4V substrate by arc ion plating. The effects of different working pressures on the surface quality and properties of Mg coating were investigated. The degradation, antibacterial and biosafety properties were studyied. The results showed that the pure Mg coating can be deposited on the surface of Ti6Al4V substrate and the coating was uniform and smooth. The immersion test in vitro showed that the degradation was very fast because of galvanic corrosion, and the whole process was finished in about one week. The results of antimicrobial experiments showed that the Mg coating can kill staphylococcus aureus and showed good antibacterial function. The results of cytotoxicity test showed that Mg coating promoted rabbit bone marrow mesenchymal stem cells (rBMSCs) growth and proliferation.

As an important parameter, the Schmid factor has been widely applied to analyze the deformation modes in metals. In order to analyze the deformation mechanisms of magnesium alloys under high strain rate, the Schmid factors of four slip modes (basal, prismatic, pyramidal <a> and pyramidal <c+a> slips) and two twinning systems ({10\stackrel{\text{?}}{1}2} tension and {10\stackrel{\text{?}}{1}1} contraction twinnings) were systematically calculated in this work. The experimental values of Schmid factor of as-received AZ31 rolling magnesium alloy sheets were obtained by electron backscatter diffraction (EBSD) technique, and then the theoretical calculated values were compared with those values. The high strain rate compression test of AZ31 rolling magnesium sheets was conducted by using split Hopkinson pressure bar at the strain rate of 1600 s^-1, and the microstructures after compression were observed by optical microscopy. The Schmid factors and microstructures are combined to discuss the predominant deformation mechanisms for different orientation samples under different loading directions. The results showed that the theoretical calculated values of Schmid factors are in good agreement with their experimental values. Therefore, the Schmid factor, owing to its simplicity and conveniene, could be used to analyze the predominant deformation mechanism and interpret the unique characteristics of "true stress-true strain" curves in magnesium alloys. Furthermore, since the Schmid factor and its variation trend associated with deformation behavior in magnesium alloys are related, the calculation result of Schmid factor can provide a theoretical analytic approach to understand anisotropic phenomena caused by strong texture in magnesium alloys.