Additively manufactured high strength and ductility CrCoNi medium entropy alloy with hierarchical microstructure

The CrCoNi medium entropy alloy (MEA) is a technologically intriguing material showing superior cryogenic mechanical properties. In this work, we fabricated near full-dense hierarchical CrCoNi MEA via selective laser melting (SLM) and achieved superior yield strength of 860 MPa at 77K that surpasses that of conventionally fabricated counterparts, in addition to reasonable ductility. The strength and ductility increase as temperature drops, reaching a 1340 MPa ultimate tensile strength and a 47% elongation at 77K. EBSD, TEM, and in-situ synchrotron X-ray diffraction (SXRD) tensile tests disclose that the high strength stems from the hierarchical microstructure composed of high-density dislocations-formed cellular structures and low-angle grain boundaries (LAGB) within the complex heterogeneous columnar grains, and further validated by theoretical calculation. In-situ SXRD tensile tests and post-deformation TEM and EBSD reveal that nano-twinning response in the SLM-built CrCoNi MEA is suppressed by both high-density LAGB and near <100> growth texture. The results demonstrate that SLM is a viable technique for fabricating dense hierarchical CrCoNi MEA and suggest a design strategy to improve mechanical properties further.