Employing a first-principles method based on the density function theory, we systematically investigate the structures, stability and diffusion of self-interstitial atoms （SIAs） in tungsten （W）. The （111 〉 dumbbell is shown to be the most stable SIA defect configuration with the formation energy of -9.43 eV. The on-site rotation modes can be described by a quite soft floating mechanism and a down-hill ＂drift＂ diffusion process from （110） dumbbell to 〈111〉 dumbbell and from （001） dumbbell to 〈110〉 dumbbell, respectively. Among different SIA configurations jumping to near neighboring site, the 〈111 〉 dumbbell is more preferable to migrate directly to first-nearest-neighboring site with a much lower energy barrier of 0.004 eV. These resuits provide a useful reference for W as a candidate plasma facing material in fusion Tokamak.
Computer simulation plays a critical role in connecting microscopic structure and macroscopic mechanical properties of structural material,which is a key factor that needs to be considered in design of such kind of material.Via the quantum mechanics first-principles calculations,one can gain structure,elastic constant,energetics,and stress of selected material system,based on which one is able to predict the mechanical properties or provide useful insights for the mechanical properties of the materials.This can be done either directly or in combination with the empirical criterions.This paper reviews the recent research advances on the attempts to predict the mechanical properties of structural materials from first principles.
Stability and diffusion of chromium （Cr） in vanadium （V）, the interaction of Cr with vacancies, and the ideal me- chanical properties of V are investigated by first-principles calculations. A single Cr atom is energetically favored in the substitution site. Vacancy plays a key role in the trapping of Cr in V. A very strong binding exists between a single Cr atom and the vacancy with a binding energy of 5.03 eV. The first-principles computational tensile test （FPCTT） shows that the ideal tensile strength is 19.1 GPa at the strain of 18% along the [100] direction for the ideal V single crystal, while it decreases to 16.4 GPa at a strain of 12% when one impurity Cr atom is introduced in a 128-atom V supercell. For shear deformation along the most preferable { 110} （111） slip system in V, we found that one substitutional Cr atom can decrease the cleavage energy （7cl） and simultaneously increase the unstable stacking fault energy （]＇us） in comparison with the ideal V case. The reduced ratio of ]＇cl/]＇us in comparison with pure V suggests that the presence of Cr can decrease the ductility of V.
We have investigated site occupancy and mechanical properties of a vanadium (V) Σ 5(310)/[001] grain boundary (GB) with hydrogen (H) using a first-principles method. The segregation energy is calculated to be 0.29 eV for the energetically favora- ble V GB interstitial site, indicating that H energetically prefers to segregate into the V GB. We demonstrate that H can largely affect the mechanical properties of the V GB. The tensile strength and the Griffith fracture energy are reduced by approximately 13% (to 18.42 GPa) and 10% (to 1.74 J/m2) because of H segregation in comparison with that of the clean V GB, respectively. Our total energy calculations show that H acts as an embrittler to the V GB based on the Rice-Wang model. The atomic configurations and charge transfer analysis show that the segregated H weakens the surrounding interfacial V-V bonds, leading to the V GB mechanical properties degradation.
