In-situ transmission electron microscopy study of dislocation processes at precipitate-free zones in a γ′-strengthened superalloy

In precipitation-strengthened polycrystals, precipitate-free zones (PFZs) often form along grain boundaries. These PFZs lower the yield strength. In this investigation, thin foils of the commercial gamma'-strengthened nickel-based superalloy Nimonic PE16 have been strained inside a transmission electron microscope and the relevant dislocation processes in the PFZs and in the gamma'-strengthened material next to them have been observed under load. Since the PFZs are only solid solution strengthened, they are softer than the interior of the gamma'-strengthened grains. Many different slip systems are activated in the PFZs even at relatively low external stresses. Multiple slip allows for compatible deformation of neighbouring grains. Extensive cross-slip and double cross-slip in the PFZs lead to a high dislocation multiplication rate. Easy creation of dislocations in the PFZs and pile-ups at the border between the PFZs and the gamma'-strengthened interior of the grains enhance the propagation of slip across grain boundaries and thus lower the yield strength of the material.