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Hydrogen effects on small-volume plasticity and elastic stiffness constants are investigated with nanoindentation of Ni-201 and sonic velocity measurements of bulk Ni single crystals. Elastic modulus of Ni-201, calculated from indentation data, decreases ~ 22% after hydrogen charging. This substantial decrease is independently confirmed by sonic velocity measurements of Ni single crystals; c44 decreases ~ 20% after hydrogen exposure. Furthermore, clear hydrogen-deformation interactions are observed. The maximum shear stress required to nucleate dislocations in hydrogen-charged Ni-201 is markedly lower than in as-annealed material, driven by hydrogen-reduced shear modulus. Additionally, a larger number of depth excursions are detected prior to general yielding in hydrogen-charged material, suggesting cross-slip restriction. Together, these data reveal a direct correlation between hydrogen-affected elastic properties and plastic deformation in Ni alloys.
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INTRODUCTION
Hydrogen-assisted damage of structural metals such as steels, nickel, titanium, and refractory alloys is characterized by loss of ductility and toughness. These symptoms are manifest during exposure to both gaseous hydrogen and hydrogenproducing environments, such as corrosive electrolytes. Extensive research has resulted in a number of proposed degradation mechanisms focusing on different aspects of hydrogen-material interactions.1-5 Of these, three mechanisms attributed to metals containing internal hydrogen that do not readily form hydrides are particularly relevant to the current investigation. The hydrogen-enhanced localized plasticity mechanism suggests that solute hydrogen atmospheres along the elastic strain field of a dislocation interact with obstacles (e.g., neighboring dislocations), reducing the repulsive force between them and enhancing dislocation mobility.3,6 Enhanced dislocation mobility can lead to localized plasticity causing macroscopic brittle fracture via locally ductile processes.3,6 In contrast, observations of low-ductility fracture, occurring without significant plasticity, in hydrogen-exposed systems4,7,8 promulgated the hydrogen-enhanced decohesion (HEDE) mechanism. HEDE assumes that dissolved hydrogen weakens interatomic bonds at grain boundaries or other interfaces resulting in brittle failure at low applied stresses, including intergranular fracture in some systems.7-10 Additionally, recent investigations, such as those by Nagumo,5 point to the importance of vacancies in hydrogen-assisted degradation processes. The hydrogen-enhanced strain-induced vacancy (HESIV) theory suggests that hydrogen stabilizes vacancy formation and agglomeration during deformation.
The objective of the current work is to investigate the link between hydrogen effects on elastic properties and hydrogen effects on plasticity measured in small material volumes. The combination of sound speed measurements in...