Atomistic simulation study of the helium effects on the deformation behavior in nickel bicrystals

Experimental and theoretical studies have shown that helium impurities in nickel tend to precipitate into nano-sized bubbles. The formation of helium bubbles has been found to have detrimental effects on the mechanical properties of nickel and its alloys. However, the mechanisms that lead to the observed helium-induced embrittlement are still unclear. In the present work, we perform molecular dynamics simulations to study helium effects on the deformation behavior of nickel systems. The role of symmetric tilt grain boundary (STGB) structures in nickel is investigated for various helium concentration levels. The deformation behavior is investigated in relation to the STGB properties, the helium distribution, and the evolution of lattice defects in the nickel matrix. In pure nickel bicrystals, it is found that the tensile strength and failure strain depend on the misorientation angle and the GB energy, with the lowest-energy STGB exhibiting the highest strength. In bicrystals with helium, we observe that the emission of dislocation loops nucleated at helium clusters significantly reduces the strength. The mechanical properties were found to be dependent on the GB misorientation angle, with the GB energy having no significant effect. Reduction of the material strength, with the presence of helium, was found to be associated with heterogeneous nucleation of dislocation loops at helium bubbles, which was not limited to GB region.