Effect of nickel on hydride formation and hydrogen embrittlement in NiCrFe alloys

Hydride formation and hydrogen embrittlement in NiCrFe alloys were investigated with particular emphasis on the effect of nickel content. In all the alloys containing 30–60 wt.% Ni, γ hydride formed during cathodic hydrogen charging but, in the 30 wt.% Ni alloy, the formation of β hydride was also observed. During the decomposition of the hydride after the cessation of charging, surface cracks appeared along both the grain boundaries and the interfaces of {111} microtwins which were induced by the volume expansion due to the hydride formation. In the lower nickel alloys the formation of microtwins and the subsequent surface cracks along these interfaces were more pronounced. This probably arose from the decrease in stacking fault energy with decreasing nickel content. The ductility loss caused by cathodic hydrogen charging was much enhanced by increasing the nickel content, and the fracture mode changed from transgranular to intergranular. This change was quite similar to that observed in surface cracks and is thought to be related to the frequency of microtwin formation. Aging at 673 K suppressed hydrogen embrittlement, changed the fracture mode from intergranular to transgranular and reduced the amount of absorbed hydrogen. This can be explained in terms of the site competition mechanism due to the segregation of carbon atoms to the grain boundaries.