Double-Edge Effect of Dislocation Cell Structures on the Recrystallization of Additive Manufactured Commercially Pure Nickel

Abstract

Recrystallizing additive manufactured alloys, such as stainless steels or nickel-base superalloys, requires high homologous temperatures and longer annealing times compared with their wrought counterpart. Subject to thermal cycling during printing, additive manufactured alloys retain thermal strain equivalent to a few percent stored under the form of dislocation cell structures, an inherent driving force to recrystallization. Opposing effects such as solute drag and grain boundary particle pinning have been proposed to explain the sluggish recrystallization kinetics. Here, we decouple these antagonist effects by characterizing the recrystallization mechanisms of commercially pure nickel fabricated by laser powder bed fusion. We observe sluggish recrystallization and rationalize this finding via multi-scale electron microscopy on interrupted annealing treatments. While cellular structures promote the necessary driving force for nucleation and strain-induced boundary migration, they simultaneously hinder the migration of the front by forming stable arrangements during annealing. The recrystallized microstructure presents relatively large twin-related domains with remnant crystallographic texture, drastically differing from wrought FCC alloys. Annealing twinning actively participates in boundary migration but is limited because of unfavorable curvature at the recrystallization front as it is pinned by dislocation cell structures. These findings can reasonably be generalized to most FCC alloys fabricated via AM.