On the interaction between γ precipitates and dislocation microstructures in Nb containing single crystal nickel-base alloys

The precipitation of the metastable gamma'' phase has been studied in a Nb containing nickel-based alloy in single crystalline state, with a composition close to the commercially available superalloy IN718. Solution heat treatment and isothermal aging treatment are applied to undeformed single crystalline samples. We focus on the investigation of heterogeneous gamma'' precipitation on dislocation sites, without the interference of grain boundaries or co-precipitation with other phases. The gamma'' precipitation microstructure after aging is found to be strongly related to the dislocation microstructure before aging. The dislocations found in the undeformed specimens have been generated during the dendritic solidification or the water quenching after solution heat treatment. Two specific patterns dominate the precipitation microstructure after aging. First, some gamma'' particles precipitate along curved lines formed initially by the dislocations during solution heat treatment along the dendritic boundary. Second, gamma'' particles are found to be arranged along the dislocation in slip planes, which can be seen as largely extended straight lines parallel to the < 011 > directions within the (001) plane, similar to a basket weave structure. Such an aligned arrangement of gamma'' particles has been previously observed in polycrystalline IN718 within individual grains (Choi and Choi, 1972). In contrast, the bands of numerous gamma'' particles can span the whole sample of millimeter size in single crystals, cutting several hundred dendrite cores. All gamma'' particles are in the shape of an oblate spheroid, where the normals of the plates are pointing in < 001 > directions.