Surface Grain Refinement of CoCrFeMnNi High‐Entropy Alloy via Combined Ultrasonic Impact Treatment and High‐Temperature Annealing

This study investigates the effects of ultrasonic impact treatment (UIT) and subsequent high‐temperature exposure on the grain size, wear, and corrosion behavior of a CoCrFeMnNi high‐entropy alloy (HEA). High‐coverage UIT produced a severe plastically deformed surface layer with refined grains, which significantly increased surface hardness and reduced the volumetric wear rate by 94%. Electrochemical and immersion tests in NaCl and HCl solutions revealed that UIT markedly improved corrosion resistance. Brief annealing after UIT promoted recrystallization and grain coarsening and induced significant relaxation of residual compressive stress, leading to a partial decline in both hardness and corrosion performance, with the extent of degradation scaling with annealing temperature and time. The enhanced corrosion resistance is attributed to the high‐density grain boundary network resulting from grain refinement, which facilitates rapid formation and repair of a Cr‐rich passive film and acts as a barrier against corrosion propagation. These results demonstrate that high‐coverage UIT is an effective surface engineering approach for concurrently improving the hardness, wear resistance, and corrosion resistance of CoCrFeMnNi HEA, with the caveat that these enhanced properties show limited thermal stability.