Lattice defects are unavoidable structural units in materials and play an important role in determining material properties.Compared with the periodic structure of crystals,the atomic configurations of the lattice defects are determined by the coordinates of a large number of atoms,making it difficult to experimentally investigate them.In computational materials science,multiparameter optimization is also a difficult problem and experimental verification is usually required to determine the possibility of obtaining the structure and properties predicted by calculations.Using our recent studies on oxide surfaces as examples,we introduce the method of integrated aberration-corrected electron microscopy and the first-principles calculations to analyze the atomic structure of lattice defects.The atomic configurations of defects were measured using quantitative high-resolution electron microscopy at subangstrom resolution and picometer precision,and then the electronic structure and dynamic behavior of materials can be studied at the atomic scale using the firstprinciples calculations.The two methods complement each other and can be combined to increase the understanding of the atomic structure of materials in both the time and space dimensions,which will benefit materials design at the atomic scale.
Single-crystal elastic constants and mechanical hardness of covalent and ionic crystals have been studied using first-principles calculations.The results show that the hardness is dominated by the softest elastic mode,not by the averaged elastic moduli as generally assumed.It reveals that the mechanical stability and anisotropy play an important role in determining the hardness of materials.The concept is then employed in designing hard alloys.By strengthening the softest elastic mode of tungsten carbide,which is the primary component in industrial hard alloys,we show that the carbide can be made even harder by alloying with nitrogen or rhenium via Fermi-level tuning.
In this study,the three-windows method,the jump-ratio method and the R-map method in energy-filtered transmission electron microscopy(EFTEM) have been applied to mapping carbon distribution in 35SiMn steel after a quenching-partitioning treatment.The carbon contamination is successfully suppressed by using EFTEM and plasma-cleaning TEM samples.Compared to the three-windows method and the jump-ratio method,the R-map method provides carbon distribution with less noises,and is insensitive to changes in sample thickness.We have demonstrated that the R-map method is a better way for carbon mapping in middle-carbon steel without the influence of carbon contamination.
Nickel-based single-crystal superalloys are the key materials for the manufacturing and development of advanced aeroengines. Rhenium is a crucial alloying element in the advanced nickel-based single-crystal superalloys for its special strengthening effects. The addition of Re could effectively enhance the creep properties of the single-crystal superalloys; thus, the content of Re is considered as one of the characteristics in different-generation single-crystal superalloys. Owing to the fundamental importance of rhenium to nickel-based single-crystal superalloys, much progress has been made on understanding of the effect of rhenium in the single-crystal superalloys. While the effect of Re doping on the nickelbased superalloys is well documented, the origins of the socalled rhenium effect are still under debate. In this paper,the effect of Re doping on the single-crystal superalloys and progress in understanding the rhenium effect are reviewed. The characteristics of the d-states occupancy in the electronic structure of Re make it the slowest diffusion elements in the single-crystal superalloys, which is undoubtedly responsible for the rhenium effect, while the postulates of Re cluster and the enrichment of Re at the c/c0 interface are still under debate, and the synergistic action of Re with other alloying elements should be further studied.Additionally, the interaction of Re with interfacial dislocations seems to be a promising explanation for the rhenium effect. Finally, the addition of Ru could help suppress topologically close-packed（TCP） phase formation and strengthen the Re doping single-crystal superalloys.Understanding the mechanism of rhenium effect will be beneficial for the effective utilization of Re and the design of low-cost single-crystal superalloys.
