[SY-C4] Hydrogen consequences on cyclic behaviour of <001> nickel single crystals : a multi-scale approach.
Invited
A multi-scale study on the influence of hydrogen on cyclically strained <001> oriented nickel single crystals is conducted in order to understand the impact of the solute on the microstructure and the fatigue properties of nickel single crystal. At macroscopic scale, uniaxial cyclic tests are performed to evaluate the impact of pre-charged hydrogen on the work hardening of the material. Then, at microscopic scale, a mechanical database is provided and associated with the dislocation features, which are explored by transmission electronic microscopy (TEM) observations and correlated with the mechanical behaviour of nickel single crystal with hydrogen. In addition, at atomistic scale, molecular dynamics simulations are carried out to quantify the influence of hydrogen on the dipole sizes and the dislocation organisations, which are partially connected to the elastic properties of nickel. Therefore, we have finally performed a comprehensive study on the impact of hydrogen on the elastic properties of nickel single crystal using a theoretical formalism based on Density Functional Theory and uniaxial tensile tests on <001> oriented nickel single crystals with different concentrations of hydrogen.
This multi-scale approach allows to question the main effect of hydrogen on cyclic plasticity mechanisms. The impact of hydrogen on cyclic stress-strain curves highlights a softening behaviour for <001> multi slips orientation in a similar way as previously related for single slip orientation. This result is firstly discussed in term of dislocation organisations, dipole sizes and distribution but also internal stresses states. Hydrogen impacts the density and the distribution of dislocations and consequently, modifies the internal stress state. Moreover, according to TEM observations, hydrogen ingress promotes the formation of vacancy clusters, in agreement with the super-abundant vacancies (SAV) model. The main consequence of this result is a decrease of the elastic properties of the material, which depends mainly on the formation of the defect induced by the incorporation of hydrogen than a direct effect of the solute itself.
This multi-scale approach allows to question the main effect of hydrogen on cyclic plasticity mechanisms. The impact of hydrogen on cyclic stress-strain curves highlights a softening behaviour for <001> multi slips orientation in a similar way as previously related for single slip orientation. This result is firstly discussed in term of dislocation organisations, dipole sizes and distribution but also internal stresses states. Hydrogen impacts the density and the distribution of dislocations and consequently, modifies the internal stress state. Moreover, according to TEM observations, hydrogen ingress promotes the formation of vacancy clusters, in agreement with the super-abundant vacancies (SAV) model. The main consequence of this result is a decrease of the elastic properties of the material, which depends mainly on the formation of the defect induced by the incorporation of hydrogen than a direct effect of the solute itself.