Unveiling microstructure characteristics and cryogenic mechanical properties of friction stir additive manufactured CoCrFeNiMn high-entropy alloy

Abstract

Additive manufacturing of fully-dense, fine-grained equiatomic CoCrFeNiMn high-entropy alloy (HEA) with exceptional cryogenic mechanical properties is of great scientific and industrial interest, yet remains highly challenging. Herein, we successfully fabricated large-scale, three-dimensional, fine-grained CoCrFeNiMn HEA component using friction stir additive manufacturing (FSAM), achieving significantly enhanced mechanical performance. The microstructure, mechanical behavior from room temperature to cryogenic conditions, and fracture characteristics of the FSAM component were systematically examined. The results reveal that FSAM-induced severe thermoplastic deformation markedly refines the grains, achieving an average grain size much smaller than that of the base material (BM). Moreover, the repeated thermal cycling along the build direction induces a gradient microstructure, characterized by increased grain size and decreased dislocation density from the top to the bottom regions. Fracture analyses reveal that dislocation–twin interactions govern the dominant failure mechanisms at both 298 K and 77 K, contributing to the superior strength of the FSAM component compared with the BM. Specifically, the yield strength (YS) increases by 42% to 470 MPa at 298 K and by 27% to 752 MPa at 77 K, while the ultimate tensile strength (UTS) rises by 17% to 617 MPa and by 8.5% to 1092 MPa, respectively. Although ductility is somewhat reduced, an excellent strength–ductility balance is maintained. These results establish FSAM as a viable pathway for producing bulk CoCrFeNiMn HEA component and potentially extendable to other FCC-based medium- and high-entropy alloy systems for demanding cryogenic applications.