Deformation behavior of nickel-based superalloys with bimodal γ′ size distribution studied by in-situ neutron diffraction combined with EVPSC modeling

Nickel-based superalloys with multimodal gamma ' size distribution microstructure have been designed to employ in extreme environments due to their excellent mechanical performance. Whilst the effect of gamma ' size on deformation mechanism has been studied before, little has been done to assess the deformation sequence and contribution to the load partition for different types of gamma ' precipitates. In order to address this gap, this study focused on bimodal microstructure superalloys of GH4738 (containing the secondary and tertiary gamma ' precipitates) and GH4720Li (containing the primary and tertiary gamma ' precipitates) alloys. In-situ neutron diffraction assisted by a 2-site elastic-viscoplastic self-consistent (EVPSC) model was employed to elucidate the deformation mechanisms. The effects of different types of gamma ' precipitates on the partitioning of interphase stress and the underlying mechanisms have been revealed. The findings indicated that bimodal superalloys exhibited distinct deformation behaviours compared to unimodal ones. Notably, premature load partitioning was observed in the microplasticity stage, attributable to Orowan looping for secondary gamma ' precipitates or the emission of dislocations from the straight interface of primary gamma ' precipitates. In the macroplasticity stage, considerable dislocations shear three types of gamma ' precipitates, especially for the tertiary gamma ' precipitates, which results in the decrease in the increment rate of interphase stress. These findings could contribute to a better understanding of deformation mechanisms in superalloys with multimodal gamma ' size distributions and shine a spotlight on material design to modulate intergranular and interphase stresses.