Multi-effects of Mo on enhancement of wear and corrosion resistances of FeCoNiCrMo x high entropy alloys coatings prepared by laser powder directed energy deposition

Currently, it is significant to investigate the effects of Mo on the wear and corrosion resistance of laser powder directed energy deposition (LP-DED) high entropy alloys (HEAs) coatings. In this work, FeCoCrNiMo x ( x  = 0.1, 0.3, 0.5, at.% ) HEAs coatings were prepared by LP-DED. New face-centered cubic (FCC2) phase separation occurred in Mo 0.3 and Mo 0.5 coatings due to the supersaturation of Mo and the repeated annealing effects. The cyclic thermal activation effect of LP-DED resulted in the formation of a large number of small sub-grains. The solute micro-segregation effect of dislocation walls promoted the formation of σ phases at sub-grain boundaries. The microhardness increased with increasing Mo content owing to the precipitation of hard σ phase. The hardness of Mo 0.5 coating was comparable to that of the as-cast HEAs mainly being composed of body-centered cubic (BCC) phase. The increase of phase led to abrasive wear and oxidation wear instead of common adhesive wear mechanism. Corrosion resistances of the LP-DED HEAs coatings were superior to that of LP-DED SS316L. The corrosion resistance by electrochemical tests showed Mo 0.3  > Mo 0.5  > Mo 0.1. The corrosion competing effects between sub-grains led to a layer-by-layer uniform corrosion due to the unique microstructure, which prevented the occurrence of large-sized and deep localized pitting corrosion. σ phase can act as a stabilizer to improve significantly the inter-subgranular corrosion resistance at trans-passivated state. This study provides an insight on the multi-effects of Mo in HEAs for fine tuning of sub-granular micro-structures and enhancement of wear and corrosion resistances.