The theory of grain boundary segregation was introduced, and three classical models of equilibrium segregation were summarized, while the theory related with grain boundary segregation engineering and the influence of grain boundary segregation on mechanical properties of materials were also briefly introduced. The relationship between grain boundary segregation and interface phase was discussed. The interfacial phases can be divided into six types according to the structural characteristics of interfaces in atomic scale, and the interfacial phase transitions determined by grain boundary thermodynamics were introduced. The interfacial phase transformation leads to the formation of new structures at grain boundaries, which may either improve the properties of materials or have adverse effects on them. The type VI interfacial phase at grain boundary (such as amorphous intergranular film) inhibits the nucleation of crack and reduces the damage of grain boundary, however, the type II and type III interfacial phases that weaken the atomic bond strength at the grain boundary (such as the bi-atomic interfacial phase at the grain boundary of Ni alloy with Bi component) produce grain boundary embrittlement. At the same time, nanocrystalline metal materials have high strength but poor thermal stability and plasticity, which has always been the focus of research. The interfacial phase can significantly reduce grain boundary energy and pin grain boundaries rather than segregates at grain boundaries. Therefore, the interfacial phase can significantly improve the thermal stability of nanocrystalline metallic materials. As the sites for nucleation and absorption of dislocations, the amorphous intercrystalline film (VI interface phase) can improve the ductility of nanomaterials. Whilst, amorphous intercrystalline films can improve the shear resistance of grain boundaries and inhibit grain sliding and rotation of nanocrystalline metallic materials, thus further improving the plasticity of nanocrystalline metallic materials. Finally, the effects of grain boundary segregation and interface on material properties were also summarized and the future development was prospected.

A novel three-dimensional porous self-supporting electrode material for electrochemical catalytic oxygen evolution were prepared by chemical etching method from a bulk high-entropy alloy Mn₅₀Fe_12.5Co_12.5Ni_12.5Cr_12.5. The electrochemical test results show that the overpotential of the prepared electrode material is only 281 mV at the current of 10 mA·cm^-2 and the Tafel slope is 63 mV/dec in an alkaline solution of 1 mol/L KOH, which is better than that of commercial RuO₂. At the same time, the working voltage does not increase significantly after continuous operation for 50 h at the current density of 50 mA·cm^-2, which reflects the excellent stability during electrocatalytic oxygen evolution process of the Mn-rich high-entropy porous alloy as electrocatalytic electrode material. The Nyquist plots show that the free-standing structure of the bulk HEA catalyst has outstanding electron transfer ability compared with the ordinary supported catalyst.

The effect of tempering temperature on the microstructure and impact toughness of two high-strength leaf spring steels 50CrMnSiVNb and 50CrMnMoVNb for automobile was comparatively studied by means of optical microscope, scanning electron microscope, transmission electron microscope and impact tester. The results show that compared with those of 50CrMnSiVNb steel, there are more segregation bands along with a larger proportion of large-angle grain boundaries in the microstructure of 50CrMnMoVNb steel, while the later steel shows less temper brittleness. When comparing the impact toughness of the two leaf spring steels, it is found that being quenched and then tempered in the range of 150~400℃ for the two steels, the 50CrMnSiVNb steel presents better impact toughness. The impact toughness of the steel tempered in this range is mainly affected by the degree of banded segregation, which is more prone to cleavage fracture and leads to a straighter impact crack propagation path; In the contrast, after the two steels were tempered in the range of 400~500℃, the 50CrMnMoVNb steel shows better impact toughness, and the impact toughness in this region is mainly affected by the tempering brittleness and the proportion of large-angle grain boundary. The tempering brittleness caused by the thin-film like carbides at the interface of the laths during tempering greatly worsens the impact toughness, while the large angle grain boundary has a stronger barrier effect to crack propagation and consumes more energy, leading to the improvement of the impact toughness.

The impact of building dimensions on the corrosion performance of the selective laser melting (SLMed) 304L stainless steel with different thickness and width was investigated by changing the scanning track (T) and depositing layers (L). The results show that the coarsening of grain size and the accumulation of the residual stress, as well as the number of pits and the pitting area all increased with the increasing of sample size. Accordingly, the preliminary relationship between dimension design, microstructure morphology, corrosion performance and residual stress were established.

The effect of Mn on the solidification microstructure, tensile properties and corrosion behavior of Mg_95.5Y₃Cu_1.5 alloy was investigated. The results show that with the increase of Mn content, the primary α-Mg phase in the alloy is gradually refined, and it also changes from dendritic to equiaxed dendritic morphology, while the morphology and volume fraction of LPSO phase do not change significantly. The addition of Mn improves the tensile properties of the alloy. When the Mn content is 0.9% (atom fraction) the yield strength, ultimate tensile strength and elongation are increased by 20.7%, 17.6% and 41.0% respectively compared with the alloy without Mn addition. In addition, the corrosion performance of the alloy is also improved. With the increase of Mn content, the volume of hydrogen evolution rate, mass loss rate and corrosion current density of the alloys gradually decrease, and the corrosion potential of the alloys, meanwhile, gradually moves towards positive direction.

The microstructure and deformation mechanism of two "replacement of Re by W" type low-cost second-generation nickel based single crystal superalloys after fracture at 982℃/248 MPa and 1070℃/137 MPa were investigated by SEM and TEM. The results show that the stress rupture properties of two alloys both reached the level of the second-generation single crystal superalloys; After fracture γ′ phases connected and combined to form "N-type" rafted structure, and the degree of γ′ phases distortion increased with the distance from the fracture. Under the same conditions, the raft degree of γ′ phases in 8.5W+1.0Re alloy was lower than that in 8.0W+1.5Re alloy; At 1070℃/137 MPa, the interfacial dislocation networks of the two alloys became denser; However, the dislocation networks of 8.0W+1.5Re alloy were denser than that of 8.5W+1.0Re alloy, a<010> superdislocations shearing into the γ′ phases were observed after fracture in 8.5W+1.0Re alloy under both conditions; The unstable fracture of the two alloys was mainly ascribed to a/2<110> dislocations in γ matrix shearing into the rafted γ′ phases, which intensify the deformation of rafted γ′ phases, and results in initiation and propagation of microcracks at the γ/γ′ interface, eventually leading to the fracture of the alloy; The interfacial dislocation networks and a<010> superdislocations could both improve the creep resistance of the two alloys.

Nitrogen enhanced low carbon 316LN stainless steel, as a single-phase austenitic stainless steel, is selected as a candidate material of primary pipe for the third-generation pressurized water reactors. In the present work, the solution annealed 316LN was thermal aged at 400℃ for 400, 1000, 5000 and 10000 hours, respectively. The microstructures of the solution annealed and thermal aged steels were compared by optical microscope, scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The effect of thermal aging on mechanical properties of the steels was evaluated by using small punch test, nanoindentation and microhardness tester. The intergranular corrosion resistance of the steels was measured by using electrolytic etching in oxalic acid solution, and the effect of thermal aging on the corrosion resistance of various types of grain boundaries were characterized by using EBSD and atomic force microscope (AFM). The results show that thermal aging at 400℃ for 10000 h will not cause microstructural changes in the micron level of 316LN stainless steel. No change was observed in grain size, grain boundary morphology and characteristics of grain boundary distribution, and no new phase was formed during thermal aging. However, after thermal aging, the lattice parameter becomes smaller, which may be ascribed to that the interstitial atoms and dislocations originally dissolved in grains diffused and migrated towards the grain boundaries. This may result in changes of the mechanical properties of the 316LN, including the increase of strength and hardness, and the lower of plasticity and intergranular corrosion resistance. The intergranular corrosion susceptibilities of all types of grain boundaries increase with the extension of thermal aging time, but the susceptibility of coincidence site lattice (CSL) grain boundaries is always lower than that of random grain boundaries. Hence grain boundary engineering, which is a thermomechanical process and could produce high fraction of low-∑CSL grain boundaries in materials, could be applied to mitigate the intergranular corrosion of 316LN.

The selective laser melting 316L stainless steel (SLM-316L) was prepared with preferred process parameters, and then the effect of high strain rate compression on the plastic deformation behavior of SLM 316L stainless steel by high strain rates (1000, 2000 and 3000 s^-1) was assessed by means of split Hopkinson pressure bar, scanning electron microscope and backscattered electron diffractometer in terms of the microstructure and microscopic deformation such as dislocation slip and twinning etc. Results show that SLM-316L stainless steel exhibits a significant strain rate strengthening effect by high strain-rate loading, and its microstructure is composed of closely packed columnar grains with irregular polygonal cross section. High strain rate loading decreases the degree of preferred orientation of grains and increases the number of small-angle grain boundaries and twin boundaries, and the twin boundaries are densely emerge in the cross-twisting region of small-angle grain boundaries. The plastic deformation process of the SLM-316L stainless steel is accompanied by dislocation slip and twinning behavior.