Interaction and competition between continuous and geometric dynamic recrystallization in high-strain-rate deformation of nickel-based superalloys

High-strain-rate shear deformation of advanced alloys is the bases of a wide range of processing methods (e.g. cutting, forming, shot peening) for highly engineered components used in a wide range of industries (e.g. aerospace, nuclear, automotive). When such shear deformations occur, layers of very fine equiaxed grains have been widely reported which are commonly explained via a continuous dynamic recrystallization (CDRX) mechanism. However, employing a cutting operation to induce shear deformations at high strain rates (10⁴–10⁵ s^-1) in a Ni-based superalloy we found features that cannot be explained by this classical approach. Here we quickly stopped the shear deformation process so that the phenomena leading to grain refinement can be inferred by examining the deformation zones in a time successive manner. Our analysis using Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscopy (TEM), we prove that the grain refinement is much more complex than previously reported as this is the result of a bi-modal mechanism where Geometric Dynamic Recrystallization (GDRX) combines with CDRX leading to unique microstructural features. We further supported the proposed bi-modal grain refinement mechanism by showing differences in mechanical properties by performing micro-pillar compression tests within targeted deformation zones (i.e. dominated by CDRX and GDRX+CDRX). These findings highlight new mechanisms of dynamic recrystallization caused by high-strain-rate shear deformations which have pivotal importance on how to conduct key manufacturing processes so that the properties of resultant recrystallized layers can be controlled.