Dwell-Fatigue of Ni-Based Superalloys with Serrated and Planar Grain Boundary Morphologies: The Role of the γ′ Phase on Strain Accumulation and Cavitation

The effect of grain boundary (GB) morphology on the cavitation behavior in a Ni-based superalloy, RR1000, was studied during elevated temperature dwell-fatigue at 700 °C. Following a solution heat treatment, the material was control cooled at two different rates, resulting in high angle GB morphologies that were tailored as either serrated or planar. The resulting γ′ precipitate structures were characterized near GBs and within grains. Along serrated GBs coarsened and elongated γ′ precipitates formed and consequently created adjacent regions that were denuded of γ′ precipitates. Cyclic dwell-fatigue experiments were performed at low and high stress amplitudes to vary the amount of imparted strain on the specimens. A combination of electron backscatter diffraction and digital image correlation were used to resolve strain localization relative to the GBs, in which strain accumulation was found to precede cavity formation. Additionally, the regions denuded of the γ′ precipitates were observed to localize strain and to be initial sites of cavitation. These results present a quantitative strain analysis between two variants of an RR1000 alloy, which provides the micromechanical rationale to assess the increased proclivity for serrated GBs to form cavities.