Hydrogen effects on the evolution of plastic deformation in polycrystalline nickel: A mechanism for intergranular failure

Hydrogen embrittlement;Dislocation;Grain boundary;Intergranular;Nickel;Hydrogen-enhanced localized plasticity (HELP)

Bertsch, Kaila Morgen

关键词: Hydrogen embrittlement;    Dislocation;    Grain boundary;    Intergranular;    Nickel;    Hydrogen-enhanced localized plasticity (HELP);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99354/BERTSCH-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Hydrogen embrittlement is a significant and pervasive phenomenon in many structural metals applications, but the underlying physical mechanism is not yet fully understood. The mechanism for hydrogen-induced intergranular failure of polycrystalline nickel was investigated in a series of uniaxial slow strain rate tensile tests in hydrogen-charged and uncharged specimens. The evolution of deformation microstructures was compared in the presence and absence of hydrogen at different length scales to evaluate changes in mechanical response. Dislocation microstructures were systematically interrogated utilizing the focused ion beam (FIB) liftout technique for extraction of site-specific specimens for transmission electron microscopy (TEM). Mesoscale and macroscale deformation responses were analyzed on specimen surfaces using electron backscatter diffraction (EBSD) both before and after straining.The first set of experiments compared the deformation microstructures which developed across grain boundaries in uncharged and hydrogen-charged specimens strained to failure. It was found that hydrogen accelerated the evolution of the dislocation microstructure and reduced the formation of steps on grain boundaries. It was determined that hydrogen altered the ability of grain interiors to respond to constraints imposed by grain boundaries and accelerated the evolution of deformation microstructures even in regions far removed from the fracture surface.The second set of experiments compared the deformation of hydrogen-charged and uncharged specimens strained to equivalent bulk elongations. Microstructures of selected regions on the free surface were mapped in EBSD before and after straining, and dislocation structures in grain interiors were compared quantitatively via TEM. It was found that in addition to reducing dislocation cell size and increasing dislocation density, hydrogen increased orientation deviations that developed within grains, decreased rotation of the grain average orientation with respect to the loading direction for individual grains, decreased texture development, decreased changes to grain morphology, and decreased elongation of grains along the tensile loading direction when compared to uncharged specimens at the same bulk strain level. Consequently, in the presence of hydrogen, the enhanced plasticity at the microscale became increasingly difficult to detect via measurements made at larger length scales such that at the macroscale, ductility appeared to be reduced. This was observed despite observations that hydrogen acted definitively to enhance plasticity at the microscale from the initial stages of deformation microstructure development.It was determined that this occurred because hydrogen enhanced local dislocation activity within grain interiors, particularly in response to local constraints, allowing for significantly increased work-hardening in the grain matrix and less homogeneous response within grain interiors to the applied stress. Dislocation transport of hydrogen to the grain boundaries weakened the boundaries, and the internal hardening of grains decreased their ability to maintain compatibility with adjacent grains, such that fracture was initiated. The number of microcracks that formed upon initiation of failure was correlated with the strain developed at the onset of failure, demonstrating that plastic deformation was correlated with developing local conditions favorable for damage nucleation. The results were interpreted based on known effects of hydrogen on plasticity and micromechanical processes.