Enhancing corrosion resistance of laser powder bed fusion CoCrNi medium-entropy alloys by microstructure regulation

To resolve the issue of local dislocation enrichment in the non-equilibrium solidification of CoCrNi medium-entropy alloys, the microstructures of laser powder bed fusion CoCrNi medium-entropy alloys under different heat treatments were investigated by electron back scattering diffraction and transmission electron microscopy, and the interrelationships between microstructure, corrosion behavior, and passivation film characteristics were investigated by electrochemical experiments and X-ray photoelectron spectroscopy analysis. The results show that the additive manufactured CoCrNi medium-entropy alloys have the distinct columnar crystal structure and dislocation cellular structure. As the heat treatment temperature increases from 900 ℃ to 1200 ℃, the dislocation cellular structure gradually disappears, and the recrystallization gradually occurs. The rapidly solidified columnar crystals transform into the equiaxial crystals during additive manufacturing, the grain size decreases, and the precipitated phases gradually coarser. After heat treatment at 900 ℃, the corrosion potential is the highest, the passivation current density and corrosion current density are the smallest, and the corrosion resistance performance is the best. As the heat treatment temperature rises to 1050~1200 ℃, the corrosion resistance decreases.