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Hydrogen embrittlement of structural materials, such as nickel-based alloys, is often characterized by enhanced dislocation processes as well as grain boundary decohesion leading to macroscale intergranular fracture. Nanoindentation and scanning probe microscopy (SPM) were used to characterize slip transfer across random grain boundaries and Σ3 recrystallization twins in annealed Ni-201. Thermal hydrogen charging leads to an increase in slip step width within pileups produced by nanoindentation along grain boundaries. The likelihood of slip transmission in the presence of hydrogen depends on the ease of slip within adjacent grains as well as on the misorientation of the grain boundary between them. The observed changes suggest that hydrogen limits dislocation cross-slip while increasing overall dislocation mobility. Coupled nanoindentation and SPM investigations provide a unique, local method for analyzing hydrogen effects on dislocation plasticity, which will be useful in developing grain-boundary-engineered materials.
INTRODUCTION
Degradation of metallic systems exposed to hydrogen, especially those used in pressure vessels, tanks, and pipelines, has led to unexpected catastrophic failures. Hydrogen has been linked to a decrease in ductility, fracture strength, and fracture toughness in many metals, i.e., the well-known phenomenon of hydrogen embrittlement, and it has resulted in loss of life in engineering failures in the energy industry. Thus, understanding the microscopic mechanisms that ultimately lead to macroscale failures is an important step in mitigating hydrogen embrittlement, thereby enabling effective transport and storage of hydrogen. The two microscale mechanisms commonly associated with hydrogen degradation of Ni are decohesion, where hydrogen at interfaces lowers cohesive strength, and hydrogen-enhanced localized plasticity, where hydrogen impacts local instabilities associated with plastic flow.1
Hydrogen embrittlement of many structural materials, such as high-strength steels and nickelbased alloys, is characterized by decohesion on grain boundaries or interfaces, resulting in low-toughness intergranular fracture in materials that would normally fail in a ductile manner; embrittlement occurs without significant macroscopic plastic deformation.2-6 As a result, solute hydrogen is thought to decrease the cohesive force and corresponding surface formation energy required to separate atomic bonds along a crystallographic plane, grain boundary, or particle/matrix interface. This postulate, called the hydrogen enhanced decohesion (HEDE) mechanism, suggests that dislocation motion is limited to maintain atomically sharp crack tips (although dislocation motion may work to increase stress at decohesion sites by strain gradient hardening7,8 or similar...