Selective laser melting (SLM) or laser powder bed fusion (LPBF) technologies are known to introduce a wide spectrum of both, macro- and microscopic defects in final products, dominantly due to repeated and rapid melting/solidification processes. Recently, radiotracer diffusion measurements discovered the existence of so-called non-equilibrium grain boundaries (GBs) in SLM-produced CoCrFeMnNi high-entropy alloy (HEA). The corresponding GB diffusion rates exceed those of general high-angle GBs as they are present in cast and homogenized HEAs by orders of magnitude. Astonishingly, Ni GB diffusion in as-prepared (SLM) HEA is similar to the diffusion rates reported previously for severely plastic deformed Ni. The diffusion enhancement disappears after low-temperature pre-annealing which confirms the existence of non-equilibrium GBs in SLM-produced alloy. The non-equilibrium state of GBs is characterized by non-equilibrium segregation of Mn (and partially of Ni) to the high- and low-angle GBs.

In the present study, the influence of laser scan speed on microstructure, mechanical property and GB diffusion in SLM-produced CoCrFeMnNi HEAs is investigated. The scan speed is found to affect the size and morphology of the grains, but not the diffusivity through the non-equilibrium GBs. The Ni GB diffusion coefficients are measured at 500 K, where the microstructural evolution is negligible and the non-equilibrium state of GBs is not relaxed. Excepting the case of 1200 mm/s laser scan speed, where massive pore and lack of fusion are prominent, the GB diffusivity does not practically depend on the laser scan speed with a prominent plateau around 600 mm/s, suggesting that the metastable energy state of non-equilibrium GBs is well-defined property attained as GB response on the external stimulus. The correlation of microstructure, mechanical (nano-indentation) and diffusion properties of the material is discussed in detail.