A strong and ductile medium-entropy alloy with a thermally stable cellular heterostructure by additive manufacturing

Microstructural thermal stability usually defines the upper limit of the mechanical properties of an alloy. Many complex alloys such as high- and medium-entropy alloys (HEAs and MEAs) often suffer from low microstructural thermal stability. For instance, the cellular structures formed in many additively manufactured HEAs/MEAs, which are beneficial to both strength and ductility, could be eliminated in a post-process heat treatment or thermal exposure while brittle topologically closed-packed (TCP) phases such as σ phase can form during this process. In this study, we design a novel MEA based on calculation of phase diagram, (CoNi)₇₁Cr₂₀Ti₃Al₅Mo_0.5Ta_0.5 (at.%), which upon selective laser melting and heat treatment forms no TCP phase but a cellular heterostructure that can remain up to 1000 °C, demonstrating extraordinary thermal stability. The cellular heterostructure comprising clusters of large γ′ precipitates at the cell boundaries and ultrafine γ′ particles in the cell interior, induces effective precipitate strengthening and back stress strengthening during deformation. The clusters of large γ′ particles decorated cell boundaries effectively impede dislocation motion while the gaps between the clusters allow straight and uniform slip across cellular structures. Consequently, the cellular heterostructure imparts the material with an excellent combination of strength and ductility.