Nb-content-dependent dual-phase evolution and cavitation erosion resistance of laser-cladded Fe2CrNiNbx medium-entropy alloy coatings

ABSTRACT In this study, Fe 2 CrNiNb x ( x = 0 – 1.0) medium-entropy alloy (MEA) coatings were fabricated by laser cladding to systematically investigate the effect of Nb content on phase evolution, microstructural refinement, and cavitation erosion resistance. X-ray diffraction (XRD) Rietveld refinement revealed that Nb addition promoted the transition from a single- face-centered-cubic (FCC) solid solution to an FCC + Laves-type dual-phase structure, with the Laves-type phase fraction increasing from 12.4% for Fe 2 CrNiNb 0.4 to 25.7% for Fe 2 CrNiNb 1.0 . EBSD analysis demonstrated significant grain refinement with increasing Nb content, as evidenced by a reduction in average grain size from 65.79 μm in the Nb-free coating to 23.80 μm in the Fe 2 CrNiNb 1.0 coating, corresponding to an approximately 64% decrease. This microstructural refinement is associated with rapid solidification, Nb-assisted heterogeneous nucleation, and grain-boundary pinning during laser cladding. Consequently, the average microhardness increased monotonically from 177.8 HV in the Nb-free coating to 554.3 HV in the Fe 2 CrNiNb 1.0 coating. Nanoindentation results further showed that the Fe 2 CrNiNb 1.0 coating exhibited the highest H 3 / E 2 value of 7.05 × 10 -5 GPa, suggesting improved resistance to localized plastic deformation. Ultrasonic cavitation tests demonstrated that the cumulative mass loss after 12 h decreased substantially from 73.9 mg for the 316L stainless-steel substrate to 4.7 mg for the Fe 2 CrNiNb 1.0 coating, whereas the erosion rate stabilized at 0.79 mg/h. Moreover, the surface roughness (R a ) after cavitation erosion decreased from 3.2478 μm for the Nb-free coating to 0.2830 μm for the Fe 2 CrNiNb 1.0 coating, confirming the suppression of surface roughening and local spallation. The enhanced cavitation erosion resistance was attributed to the synergistic strengthening-toughening effect of grain refinement, increased hardness, and the FCC + Laves-type dual-phase architecture. Within this architecture, Nb-rich Laves-type regions constrain localized plastic flow and crack propagation, whereas the FCC matrix provided strain accommodation. These findings suggest that Nb-mediated dual-phase microstructural engineering is an effective strategy for improving the surface stability of Fe–Cr–Ni-based MEA coatings under fluid-impact conditions.