Influence of grain refinement via high-pressure torsion on corrosion behavior of medium- and high-entropy alloys CoCrFeNi and CoCrFeMnNi with various chromium contents

The influence of grain refinement by high-pressure torsion (HPT) on the corrosion behavior of CoCrFeNi alloys with varied Cr content was investigated in aqueous 0.5 M H2SO4 and 3.5% NaCl solutions. The results were compared with CoCrFeMnNi alloys. Both the alloys showed a single fcc phase after vacuum melting and HPT. Protective passivation capability and resistance to general corrosion in H2SO4 was higher than that of Fe-Cr alloys and became higher with increasing Cr contents, indicating that high corrosion resistance of Cr-containing HEAs is attributed to the incorporation of Cr with other supporting elements. However, the impact of the nanocrystalline structure by HPT on the general corrosion behavior in the H2SO4 solution was negligibly small while the resistance to the local attack as pitting in NaCl was improved for the CoCrFeNi alloy. This is contrasted with the CoCrFeMnNi alloy, which exhibited negligibly small change by HPT in both NaCl and H2SO4 solutions. X-ray photoelectron spectroscopy indicated the Cr enrichment in passive films, but its degree is smaller regardless of grain size and Mn content, as compared with that of Fe-Cr alloys reported in the literature. The small change in the Cr enrichment in the passive film and the resulting corrosion behavior by grain refinement through HPT could be attributed to an intrinsic nature of HEAs such as sluggish diffusion and intrinsic lattice distortion. The improved resistance to pitting corrosion in CoCrFeNi alloys could be explained by the homogenization effect of the ultrahigh strain deformation, which may not take effect in alloys containing the solute Mn.