Exceptional strength–ductility combination of CoCrFeMnNi high-entropy alloy with fully recrystallized structure by selective laser melting after post-deformation annealing

Abstract

The low yield strength of the equiatomic CoCrFeMnNi high-entropy alloy (HEA) at ambient temperature severely limits its future application as engineering materials. Here, a CoCrFeMnNi HEA with a fully recrystallized structure and an excellent strength-ductility combination was prepared by post-deformation annealing (PDA) process (i.e., 773–1173 K for 2 h) after selective laser melting. The evolution of the microstructures and mechanical properties of the as-printed and PDA-processed samples were systemically studied, while the strengthening mechanism of PDA-processed samples was clarified. The high density of dislocations and anisotropic columnar grains in the as-printed samples improved the rate of stored deformation energy, thus effectively promoting grain refinement during the PDA process. Compared to the average grain size of 21.26 μm of the as-printed sample, the average grain size of the PDA-873 sample was decreased to 1.20 μm after full recrystallization. Especially in the PDA-873 sample, we found a large amount of high angle grain boundaries (HAGBs) (∼98.3%) and twin boundaries (TBs) (∼46.6%). Besides, some Cr and Mn-rich oxide particles also precipitated in the PDA-processed samples. The main strengthening mechanisms of PDA-processed samples can be attributed to grain boundary strengthening. The PDA-873 sample exhibits a high yield strength, which is twice as large as that of the as-printed samples, without sacrificing ductility significantly. The present work provides an effective way to manipulate grain size and precipitates in HEAs.