Stability and mechanical properties of various Hf-H phases: A density-functional theory study

We performe first-principles density functional theory calculations to investigate the stability and mechanical properties of various Hf Hx(0 ≤ x ≤ 1) phases. For pure Hf phases, the calculated results show that the HCP and FCC phases are mechanically stable, while the BCC phase is unstable at 0 K. Also, as for various Hf Hx phases, we find that H location and concentration could have a significant effect on their stability and mechanical properties. When 0 ≤ x ≤ 0.25, the HCP phases with H at(tetrahedral) T sites are energetically most stable among various phases. The FCC and BCC phases with H at T sites turn to be relatively more favorable than the HCP phase when H concentration is higher than 0.25. Furthermore, our calculated results indicate that the H solution in Hf can largely affect their mechanical properties such as the bulk moduli(B) and shear moduli(G).     

Effects of the Be_(22)W phase formation on hydrogen retention and blistering in mixed Be/W systems

We have performed first-principles density functional theory calculations to investigate the retention and migration of hydrogen in Be_(22) W, a stable low-W intermetallic compound. The solution energy of interstitial H in Be_(22) W is found to be 0.49 eV lower, while the diffusion barrier, on the other hand, is higher by 0.13 eV compared to those in pure hcp-Be. The higher solubility and lower diffusivity for H atoms make Be_(22) W a potential beneficial secondary phase in hcp-Be to impede the accumulation of H atoms, and hence better resist H blistering. We also find that in Be_(22) W, the attraction between an interstitial H and a beryllium vacancy ranges from 0.34 eV to 1.08 eV, which indicates a weaker trapping for hydrogen than in pure Be. Our calculated results suggest that small size Be_(22) W particles in hcp-Be might serve as the hydrogen trapping centers, hinder hydrogen bubble growth, and improve the resistance to irradiation void swelling, just as dispersed oxide particles in steel do.