Grain Boundary Segregation Suppresses Local Short‐Range Ordering in Nanocrystalline High‐Entropy Alloys

Abstract Multi‐principal‐element alloys like high‐entropy alloys (HEAs) have potential applications in many engineering fields due to their unique mechanical/functional properties. While HEAs are generally considered random solid solutions, recent studies revealed that they are prone to short‐range‐ordering (SRO) due to the complex multi‐pair‐wise interactions among the constituent elements. Meanwhile, SROs' evolution can sometimes be deleterious, and it is necessary to have control over their evolution. Examining the AlCoCrFe‐Zr model alloy, long‐range ordering occurs following the expectation of enthalpic predictions. Advanced characterization techniques—transmission electron microscopy, high‐energy synchrotron X‐ray diffraction/pair distribution function, and atom probe tomography, reveal that SRO is suppressed in as‐milled and GB‐decorated NC‐(AlCoCrFe)100‐xZrx (x = 0–1.5 atomic %). Warren‐Cowley coefficient calculations are further used to validate the suppression of SRO. Besides the low segregation enthalpies of Cr, Fe, and Zr, and the high‐mixing enthalpy of Cr and Fe, the short diffusion path to GBs due to high‐GB density in the NC‐HEAs and the higher energy state of the GBs than the matrix promotes GB‐segregation that further alters the matrix chemistry and consequently disfavors SRO formation within the matrix. Despite the GB‐segregation of Cr, Fe, and Zr, the matrices and GBs remain in a random solid solution.