The effect of δ phase on the microplasticity evolution of a heat-treated nickel base superalloy

The micromechanical deformation behaviour of the commercial Ni-base polycrystalline Ni-base superalloy GH4169 has been investigated in terms of the volume fraction and grain boundary distribution of the δ (Ni₃Nb) phase. The main objective of this work was to gain an understanding about the mechanisms of deformation of the superalloy at the grain scale. To that purpose, experimental and microstructural observations are relied upon to construct realistic models of the polycrystalline superalloy microstructure using crystal plasticity concepts. It was found that a volume fraction of the grain boundary δ phase in excess of approximately 3.0% results in a weakening of the room temperature strength, and the detrimental effect increases with increasing the δ phase volume fraction. This study also revealed that the grain boundary δ phase, being stronger than the surrounding grains, impedes the transmission of plastic slip through the grain boundaries, thus enhancing the localisation of plastic slip within the grains. However, the strengthening effect associated with such slip blocking mechanism by the δ phase is considerably lower than the weakening of the deformation resistance of the grains due to the depletion of the meta-stable γ″ precipitates within the grains resulting from the stable δ phase formation.