A direct observation has been made of dislocation behavior and the effect of the solute hydrogen on the distribution of dislocation in a single crystal nickel-base superalloy during deformation in an electron microscope. It has been found that the dislocations are emitted from the crack tip during tensile deformation and are driven out of the crack tip region, leaving behind a dislocation-free zone (DFZ). A series of experimental results reveal that the dislocation emitted from the crack tip can interact strongly with interfaces in specimen. The γ'/γ interface just right in front of the crack tip, as an obstacle to motion of dislocation, can change the distribution of dislocations in the plastic zone leading the dimination of the DFZ's length.
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