Local chemical fluctuation promoted formation of nano CrN to achieve superhardness in FeCoCrNiMn high entropy alloy

High entropy alloys (HEAs) with nitriding process have been widely regarded as potential materials to manufacture machinery parts use in extremely environment such as high temperature, cryogenic temperatures, and corrosion conditions. The multi-equal elements induced local chemical fluctuation (LCF) revealed significant effects in diffusion process which is the fundament for excellent thermal and chemical stability. However, the influence of LCF on the absorption stage of chemical surface process such as nitriding has remained uncertain so far, which greatly limited the application of chemical process in enhancing the surface hardness of HEAs. In this work, atomic-scale observations and first-principle calculations are combined to reveal that the presence of LCF determined the detailed effects of non-nitride forming element on nitrogen absorption and diffusion of nitride forming elements. In particular, Ni preferred locates closer with Cr while Co reveals equal space with Cr and Mn. This LCF promoted the absorption and diffusion of nitrogen in Cr lattice, arising from great decreased absorption energy and diffusion energy barrier (approximate to 91.5 %). Thereby, the formation of nano CrN grains is enhanced via the cocontribution of accelerated absorption resulted from LCF and boundary diffusion via grain refinement. These findings provide profound understanding for HEA nitriding and promising strategy to improve the nitriding rate and hardness of HEAs.