Ultra-ductile and high-strength CoCrNi medium entropy alloy with directional grains via laser melting deposition

Laser melting deposition (LMD) involves large temperature gradients and rapid cooling, which enables microstructural refinement at the nanoscale to achieve high strength. However, LMD provides a high level of strength, often at the expense of ductility due to the refined microstructure and the introduced high density of dislocations. Herein, directional growth along the deposition direction (DD) was obtained by adjusting the process parameters of LMD to improve the strength and ductility of CoCrNi medium-entropy alloy (MEA). In comparison to the as-cast sample, the simultaneous increment in both strength and ductility is achieved by LMD along the DD. Especially, the ductility of the LMD sample tensile along the DD (ultimate tensile strain ε_U = 91 %) was superior to those of other additively manufactured MEAs and the state-of-the-art steels, which was mainly attributed to the reduced transverse grain boundary density and simplified orientation ([001]) of columnar grain, and the enhancement of work-hardening capacity due to the activation of a large number of stacking faults and twins during the deformation. This work provides a new possibility to achieve high-strength and ultra-ductile complex-shaped metallic parts via designing directional grains by LMD.