Vacancy-enhanced Hydrogen Degradation of Ni-201
Samantha Lawrence, Purdue University
Hydrogen degradation of ductile structural metals used in energy industries, such as Ni alloys, is often characterized by enhanced strain localization prior to failure. Flow localization may generate “free volume” in the form of excess vacancies, which can facilitate hydrogen-induced damage by coalescing into voids. Positron annihilation spectroscopy (PAS) and thermal desorption spectroscopy (TDS) were employed to investigate applied stress, deformation temperature, and grain size effects on hydrogen-induced free volume in hydrogen-charged and strained commercially pure Ni-201.
Plastically deformed hydrogen-charged nickel exhibits higher average positron lifetime compared to deformed hydrogen-free material, demonstrating hydrogen-enhanced vacancy clustering. This effect is most pronounced for large-grained (1 mm) material strained 10 percent in tension at 77 K and minimized for large-grained samples strained 10 percent at 300 K. Increasing lifetimes for hydrogen-charged specimens during measurement indicate continuous nucleation and/or growth of vacancy clusters. TDS measurements substantiate PAS results. The main peak is associated with desorption of solute hydrogen from the crystal lattice, while a small secondary peak is attributed to hydrogen de-trapping from vacancy clusters/voids induced by deformation in the presence of hydrogen.
The results indicate that hydrogen-enhanced free volume contributes to increased work hardening rates and impacts the degree of intergranular failure in hydrogen-charged Ni-201. Comparisons between coarse- and fine-grained samples highlight the complex interactions between dislocations and initial microstructure during deformation of hydrogen-containing metals. In light of complementary nanoindentation and diffraction data revealing hydrogen-induced cross-slip restriction and dislocation-grain boundary interactions, the current results will aid in developing a model for vacancy-enhanced hydrogen degradation of structural metals.
Abstract Author(s): Samantha K. Lawrence, Yuriy Yagodzinskyy, Hannu Hänninen, Esa Korhonen, Filip Tuomisto, Zachary D. Harris, Brian P. Somerday