Alternating columnar-equiaxed heterostructure for superior strength and ductility in laser additive manufactured Co-based alloys

The strength-ductility trade-off presents a significant challenge for applying Stellite 6 cobalt-based alloy as a structural material. This study addresses this issue by fabricating a heterostructure (HS) with alternating columnar and equiaxed grain layers in a Stellite 6 alloy via laser additive manufacturing (AM) through precise heat input control. The microstructure, tensile properties, and deformation mechanisms of this heterostructure were systematically investigated and compared with a coarse columnar (CC) structure. Results demonstrate that the HS sample achieved a superior ultimate tensile strength of 1458 MPa and a yield strength of 884 MPa, significantly outperforming the CC sample while maintaining comparable ductility. Load-unload-reload (LUR) tests and microstructural analysis reveal that the enhanced performance is primarily attributed to hetero-deformation induced (HDI) strengthening. Significant HDI stress, generated from the strain incompatibility between the soft columnar and hard equiaxed zones, promotes the accumulation of geometrically necessary dislocations (GNDs) at their interfaces, thereby enhancing the work hardening capability and ultimate strength. This work provides a novel microstructural design strategy for developing high-performance additively manufactured cobalt-based alloys.