Deformation Behavior of Nickel-Based Superalloys with Bimodal Γ′ Size Distribution Studied by In-Situ Neutron Diffraction Combined with Evpsc Modeling

Abstract

Nickel-based superalloys with multimodal γ′ size distribution microstructure have usually been widely used in many extreme engineering applications due to their excellent mechanical properties at high temperatures. Understanding the underlying mechanisms is vital to provide some insight into improving the performance. Up to now, a well understanding of the deformation mechanisms of unimodal microstructures has been obtained. However, it remains mysterious for superalloys with multimodal microstructures due to the complex size and distribution of γ′ particles. In this study, bimodal superalloys of GH4738 and GH4720Li alloys were selected. In GH4738 alloy, the fine and coarse γ′ particles are 32 nm and 221 nm, respectively, in which the coarse γ′ particles are all intragranular. In GH4720Li alloy, the fine and coarse γ′ particles are 37 nm and 1800 nm, respectively, in which most of the coarse γ′ particles are intergranular. 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 γ′ particles in different locations on the partitioning of interphase stress and deformation behaviour have been revealed through comparing with other unimodal superalloys. The results indicate that load partitioning between γ and γ′ phases is distinct between the unimodal and bimodal superalloys. On the one hand, premature load partitioning is observed before microscopic yielding, indicating that γ phase starts to deform plastically in elastic region. It could be attributed to the much lower critical stress of Orowan looping for intragranular coarse γ′ particles or the dislocations emitted by the straight interface plane of intergranular coarse γ′ particles in the bimodal superalloys. On the other hand, the extent of load partitioning at 15% strain for GH4738 and GH4720Li alloys is much lower than that of the unimodal superalloys with coarse γ′ particles, demonstrating that the effect of coarse γ′ particles on load partition is weakened by the shearing effect of fine γ′ particles. Those findings could contribute to a better understanding of deformation mechanisms in superalloys with multimodal γ′ size distributions and shine a spotlight on material design to modulate intergranular and interphase stresses.

Keywords: Nickel-based superalloys, Bimodal γ′ size distribution, In situ neutron diffraction, Elastic-viscoplastic self-consistent modeling, Deformation mechanism