Abstract:
New insights are provided into the role of vacancy-hydrogen (VaH) complexes,
compared to the hydrogen atoms alone, on hydrogen embrittlement of nickel.
The effect of the concentration of hydrogen atoms and VaH complexes is investigated in different crystal orientations on dislocation emission and propagation
in single crystal of nickel using atomistic simulations. At first, embrittlement
is studied on the basis of unstable and stable stacking fault energies as well
as fracture energy to quantify the embrittlement ratio (unstable stacking fault
energy/fracture energy). It is found that VaH complexes lead to high embrittlement compared to H atoms alone. Next, dislocation emission and propagation
at pre-cracked single crystal crack-tip are investigated under Mode-I loading. Depending upon the elastic interaction energy and misfit volume, high
local concentrations at the crack front lead to the formation of nickel-hydride
and nickel-hydride with vacancies phases. These phases are shown to cause
softening due to earlier and increased dislocation emission from the interface
region. On the other hand, dislocation propagation under the random distribution of hydrogen atoms and VaH complexes at the crack front or along the
slip plane shows that VaH complexes lead to hardening that corroborates well
with the increased shear stresses observed along the slip plane. Further, VaH
complexes lead to the disintegration of partial dislocation and a decrease in dislocation travel distance with respect to time. The softening during emission and
