Energetics and kinetics of hydrogen at the grain boundary of the Ni alloys: A first-principles study

Materials design at atomic scale to manipulate the behavior of hydrogen is one of the key strategies to improve the hydrogen-embrittlement (HE)resistance of the materials. In the present work, tuning the energetic and kinetics of hydrogen at the Ʃ5(210)[001]GB of Ni by the alloying elements are studied using a comprehensive calculation. Firstly, the metallic elements that tend to stay at the GB are screened from 12 normally used alloying elements. It is found that as the size of the alloying element increases to 30% larger than Ni, its segregation tendency at the GB increases dramatically. The GB-segregated elements reconstruct the energy-landscape of the GB for hydrogen, significantly affecting the segregation and diffusion of hydrogen. Based on the calculations of the attempt frequency and energy barrier, we find that at room temperature the jump rate of hydrogen for the rate-determining step in the long-distance diffusion can be decreased by 5–10 orders (direction dependent), as Al, Ta, W or Zr is alloyed. Meanwhile, the pathway of the rate-limiting jump of H can be diverted in the presence of the alloying element. Thus the alloyed GB can act as an obstacle for the diffusion of hydrogen rather than a fast diffusion channel as in the case of a clean GB. The theoretical results are compared to the available experimental data. The present work provides valuable guidelines for improving the HE resistance of Ni alloys via grain boundary engineering.