Grain size effect on corrosion behaviour of TiZrNb medium-entropy alloys

The effects of grain size on the corrosion resistance of TiZrNb medium-entropy alloys (MEAs) in physiological saline (0.9 wt% NaCl solution) were investigated. MEAs generally consist of three principal elements with similar atomic fractions, and their mixing entropy lies between that of conventional alloys and high-entropy alloys. The results reveal that the corrosion resistance of TiZrNb declines as the grain size decreases. The deterioration in corrosion resistance can be attributed to the synergistic effect between the inevitable defect accumulation and grain refinement induced by surface mechanical attrition treatment (SMAT), which collectively aggravate localized corrosion propagation. The passive film on the TiZrNb MEAs consists of TiO₂, ZrO₂, Nb₂O₅, and hydroxides with a bilayer structure. The inner layer is denser and dominates the corrosion resistance of the alloy. However, the SMAT-treated alloy exhibits substantial localized corrosion, accompanied by a discontinuous passive film. SMAT-induced defects accelerate selective dissolution of matrix elements, resulting in Nb depletion within passive films and consequently elevating pitting corrosion susceptibility of TiZrNb alloys. This paper offers new insights into the corrosion behaviour of TiZrNb MEAs with different grain sizes.