Effects of helium and vacancy in Ni symmetric tilt grain boundaries by first-principles

• Eight representative high-angle low-Σ symmetric tilt grain boundaries(STGBs) atomic models were constructed. • Vacancies are energetically favorable to diffuse to the STGBs from Ni matrix and enhance the GB binding. • Interstitial helium defects can initiate severe GB cracking comparing to vacancy-bind-helium. • The polarized charge transfer between Ni-d and He-p states results in the experimentally observed helium-induced GB embrittlement. The vacancy and helium effects on eight low-Σ symmetric tilt grain boundaries (STGBs) of Ni were investigated by first-principles calculations. The simulations demonstrate that vacancies in Ni matrix could diffuse easily to the grain boundary and helium atoms are quite stable at the grain boundary vacancy sites. Vacancy accumulation at high-energy STGBs could enhance the binding strength while He defects are generally detrimental to grain boundary binding and interstitial helium defects can initiate more severe grain boundary cracking comparing to vacancy-bind-helium. Further electron charge analysis suggested the influence of grain boundary binding greatly relies on the grain boundary atomic structure and the interaction between Ni-d and He-p states. The polarized charge transfer induced by He helium occupation was predicted to be detrimental to the binding strength, which explains the experimentally-observed helium bubbles and helium-induced GB embrittlement.