Microstructure, hardness and wear performance of quaternary CrNbTiZr refractory medium-entropy alloy coating fabricated on a commercial Zr alloy by pulsed laser cladding

To attain enhanced surface hardness and wear resistance, a quaternary CrNbTiZr refractory medium-entropy alloy (RMEA) coating was fabricated on a commercial Zr alloy (Zr-702) by a pulsed laser cladding technique. Phase constitution, microstructural feature, hardness and wear performance of the RMEA coating were systematically investigated. The specific influence of Zr diluted from the substrate on the coating was explored. The results reveal that the cladding zone is mainly featured by a BCC solid-solution phase and net-like Cr2Zr intermetallics. Such microstructural features agree with those predicted based on atomic radius difference, Allen electronegativity and valence electron concentration. Allotropic solid phase transformation occurs in the heat-affected zone, resulting in a mixed microstructure of blocky grains (α-Zr) and martensitic laths. The hardness of the RMEA coating (637 ± 46 HV) is ∼3.3 times that of the Zr-702 substrate, which results from joint contribution from solid-solution, low-angle grain boundary and second-phase strengthening. The RMEA coating exhibits a wear rate ∼56% lower than the substrate, demonstrating significantly improved wear resistance. Both oxidation and adhesive wear are determined to be dominant wear mechanisms of the RMEA coating, accounting for its enhanced wear performance.