Yongtang Li

Li₂TiO₃ is one of the most significant breeder materials and has potential applications in future fusion reactors. Defect models with three types of lithium vacancies were considered to study the diffusion behavior of tritium in Li₂TiO₃ by the density functional theory calculations. The possible tritium adsorption sites inside the lithium vacancy were examined and analyzed. The energy barrier of all diffusion paths between different adsorption sites was calculated and the minimum energy barrier is about 0.45 eV, which indicates that the tritium atom diffuses freely inside the lithium vacancy; when a tritium diffuses across the crystal in the typical three directions, our results reveal that the tritium atom prefers to move along the [010] direction. Furthermore, we found that the minimum energy barrier for the tritium atom to escape the trap of Li vacancy is 0.76 eV. After the tritium jumping out of the Li vacancy, the minimum energy barrier is 0.5 eV for the tritium atom diffusing in the crystal. Therefore, we predict that tritium can easily escape from the trap of the Li vacancy and then diffuse across the crystal. Such results are beneficial to the tritium release process in Li₂TiO₃ and could provide theoretical guidance for the future applications of the Li₂TiO₃ materials.

Electro‐codeposition was used to prepare the Co and Co‐Mn₃O₄ precursor coatings on ferritic stainless steel E‐brite. Plated samples were exposed at 800°C in air for different durations (i.e., 10 min and 4 h) for thermal conversion of the deposited layer to a spinel coating. The converted layer was characterized with scanning electron microscopy/energy‐dispersive spectroscopy (SEM/EDS) and X‐ray diffraction (XRD). The results showed that after 10 min heat treatment, the plated Co layer was not fully oxidized and converted into a double‐layer microstructure with an inner CoO layer and outer Co₃O₄ layer, while for the Co‐Mn₃O₄ layer, the Mn₃O₄ particles were not completely dissolved into the oxide layer. After 4 h of exposure, the surface layer was fully converted into the Co₃O₄ or (Co,Mn)₃O₄ spinel coating. Though the Mn content in (Co,Mn)₃O₄ was relatively low, the Mn‐doped spinel coating was advantageous over the pure cobalt spinel coating, as the thermally grown Cr₂O₃ scale at the coating/substrate interface was more compact and protective. Co diffusion from the deposited layer into the alloy substrate was observed for both coatings.

The solidification microstructure evolution of undercooled Cu‐15 wt.% Fe alloy melt was studied in this study by the combined method of glass fluxing and overheating. The liquidus and peritectic reaction temperatures of Cu‐15 wt.% Fe were experimentally obtained, and the obtained results were consistent with the previous studies. Based on the experimental results and related theories, the solidification process and microstructure evolution of undercooled Cu‐15 wt.% Fe alloy melt were illustrated. The conclusions provide the basis to the further study on the liquid‐phase separation and application of copper‐iron alloy.

The spline tooth of ASTM 1045 was fabricated by high‐speed cold roll‐beating (HSCRB) process at room temperature. Microhardness of the spline tooth was examined by a nanoindenter. The grains and misorientation angle distributions were measured by electron backscatter diffraction (EBSD). The results showed that the microhardness was improved up to 1280 μm deep from the surface of the spine tooth. The microhardness and the grain sizes gradually decreased in the direction away from the surface. On the surface, the fraction of ultrafine grains increased up to about 90%, and the average grain diameter (which was ∼0.56 µm) decreased by 71.4%. The model of grain evolution during HSCRB process is proposed in this work. New grains appear first on the boundaries of the elongated grains within numerous subgrains. The elongated grains are refined as a result of subgrain rotation. By analyzing the HSCRB technical principle, we concluded that the process parameters affect the refinement degree of studied steel by determining beating pass, beating pass interval time, and strain rate.

The constitutive models of as-cast P91 alloy steel were investigated by unidirectional thermal compression tests, which were conducted over a wide range of temperature and strain rate on Gleeble-3500 thermal simulation machine. The constitutive models were coupled with finite element method to simulate the hot extrusion process. Furthermore, the industrial hot extrusion experiment on actual pipe was conducted on 500 MN vertical extrusion machine and the load-displacement data during the extrusion were recorded. Comparisons between the numerical simulation and industrial experimental results were carried out and confirmed that the developed constitutive models can be used in the simulation and industrial research of compact hot extrusion process.

To investigate dynamic recrystallization (DRX) of as-cast 25Mn steel, the hot compression experiments corresponding to a height reduction of 70% was conducted on Gleeble-3500 simulator in the temperature range from 900 to 1200 ℃ at an interval of 100 ℃ and the stain rate of 0.1, 0.5, 1 and 2 s-1. By analyzing and calculating the true stress-strain data, the deformation activation energy, characteristics strain model, DRX kinetic model and grain size model were obtained. The experimental results revealed that strain hardening and flow softening characterized the flow behavior. The higher temperature and the lower strain rate made DRX of as-cast 25Mn easier. The best final forging temperature is no less than 900 ℃ and strain rate is no more than 2 s-1. In addition, through the detailed comparison between predicted and experimental fraction of DRX, the predicted showed a very good agreement with the actual experimental.

Research on compact manufacturing technology for shape and performance controllability of metallic components can realize the simplification and high-reliability of manufacturing process on the premise of satisfying the requirement of macro/micro-structure. It is not only the key paths in improving performance, saving material and energy, and green manufacturing of components used in major equipments, but also the challenging subjects in frontiers of advanced plastic forming. To provide a novel horizon for the manufacturing in the critical components is significant. Focused on the high-performance large-scale components such as bearing rings, flanges, railway wheels, thick-walled pipes, etc, the conventional processes and their developing situations are summarized. The existing problems including multi-pass heating, wasting material and energy, high cost and high-emission are discussed, and the present study unable to meet the manufacturing in high-quality components is also pointed out. Thus, the new techniques related to casting-rolling compound precise forming of rings, compact manufacturing for duplex-metal composite rings, compact manufacturing for railway wheels, and casting-extruding continuous forming of thick-walled pipes are introduced in detail, respectively. The corresponding research contents, such as casting ring blank, hot ring rolling, near solid-state pressure forming, hot extruding, are elaborated. Some findings in through-thickness microstructure evolution and mechanical properties are also presented. The components produced by the new techniques are mainly characterized by fine and homogeneous grains. Moreover, the possible directions for further development of those techniques are suggested. Finally, the key scientific problems are first proposed. All of these results and conclusions have reference value and guiding significance for the integrated control of shape and performance in advanced compact manufacturing.