First-principles Investigations Of Hydrogen Embrittlement In The PFMs

As the ideal energy of21st century, the nuclear fusion has attracted considerable attentions all over the world. In nuclear industry, there usually exists a material problem, which urgently requires to be resolved:Plasma-facing materials (PFMs) in fusion reaction will be exposed to extremely high fluxes of H isotope ions, which leads to blister formation, surface sputtering erosion, and then hydrogen embrittlement in materials, finally destroy the mechanics and thermal properties, shortens the life of materials for service. At present, W and W alloy are considered as a promising candidate for plasma facing material (PFM) of International Thermonuclear Experimental Reactor (ITER) because of their high thermal conductivity, high melting and low sputtering erosion. Under plasma irradiation at high temperature and high flux, the properties of the surface of PFM are possibly changed because of being easily mixed with different elements. It may cause the retention of hydrogen and change the mechanics and thermal properties of materials. One part of this thesis it to study the effect of W/C mixed surface on the retention of H in W. Moreover, the high performance ceramic to metal transition materials, Ti3SiC2, possesses high temperature thermostability, thermal conduction, machinability and plasticity, nice connectivity, low activation characteristics and certain anti-irradiation properties, which makes it hopefully applied in nuclear fusion and fission reactions of the next generation. So, the other part of work is to study the behavior of H in Ti3SiC2by analysis of chemical bonding in order to explore the possibility of Ti3SiC2as PFMs in nuclear industry.The calculations have been performed with the first-principles that based on Density Functional Theory (DFT). The major contents include two parts:Carbon effect on the behaviors of hydrogen in bcc tungsten and the doping of H on the bonding properties of Ti3SiC2material. The main conclusions are:1) We found that in W, H atom prefers to occupy the tetrahedral interstitial site (TIS) with the solution energy of-2.46eV, while the octahedral interstitial site (OIS) is the favorable position for C atom with solution energy of0.78eV. Because of the electronegativity of C and H is greater than W, both C and H can all capture the electrons from W atoms around, resulting in negatively charge of-0.26eV for H and-0.68eV for C. Therefore, because of the Coulombic repulsion interaction between C-H, C cannot efficiently capture H in W in the absence of other defect. On the other hand, the Coulombic repulsion between negative changed H and C will greatly increase the immigrate barrier of H in bcc tungsten, as a consequence, W/C mixed surface layers impede the incoming H to escape from the W matrix and to go back to vacuum vessel, and H atoms are forced to diffuse into W. This will make a large number of H atoms aggregate in W far from the surface, and increase the concentration of H by increasing the possibility of H trapping via intrinsic defects, such as vacancies, and advantages the H blister formation.2) According to the crystal symmetry of Ti3SiC2, there are three possible interstitial sites for H atoms. It was found that positions with the lower solution energies and smaller distortion are those that H atoms occupy the interstitial sites of Si plane (I-SiTi) and nearby (I-SiC). Strongly covalent bonds of Si-Ti-C-Ti-C chains in Ti3SiC2based on p-d hybridizations attribute to the excellent properties of Ti3SiC2, such as high modulus of elasticity, high temperature thermostability. Using partial density of states (PDOS) analysis, it was showed that the doping of H atom generally hybridizes just with s state of Si and sp hybridized states of C. The feature of chemical bonding in Ti3SiC2mainly associated with p-d hybridization is not substantially disturbed by doping of hydrogen (This is different from the behavior of hydrogen-doping in transition metals, where hybridization occurs between H s and metallic atom d state, which consequently reduces the cohesive strength of inter-atomic bonds in metals). The results can imply that Ti3SiC2may possess the properties to some extent of resisting the hydrogen embrittlement from the point of view of lattice decohesion model.The main innovative points of this paper are:the calculations gave a good explanation to the experimental phenomenon about the effect of doping of carbon on the retention of H in tungsten; Hydrogen-doping behavior in Ti3SiC2is different from that in transition metals. The embrittlement phenomenon coming from latice decohesion model may not occur in Ti3SiC2.