Microstructure and hardness of NbTiZr and NbTaTiZr refractory medium-entropy alloy coatings on Zr alloy by laser cladding

Abstract

Two refractory medium-entropy alloy (RMEA) coatings (NbTiZr and NbTaTiZr) with BCC solid-solution phase as their main constitutional phase were successfully fabricated on Zr702 alloy by utilizing pulsed laser cladding. Phase constitutions, chemical compositions, crystallographic orientations and hardnesses of the coatings were carefully characterized and analyzed. Results show that both the RMEA coatings correspond to cladding zones (CZs) that are composed of irregular-shaped bulk grains (BCC solid-solution phase) with scattered orientations and 370–380 μm in thickness. The addition of Ta is able to promote grain refinement and produce higher micro strains in the coatings. Theoretical analyses show that the phase constitution in the RMEA coatings is consistent with that predicted according to several thermodynamic or structural parameters. Below the CZs, there exist heat-affected zones (HAZs) consisting of a mixture of bulk α-Zr grains and fine martensite plates. Average thicknesses of the HAZs in both the specimens are 180–190 μm. Hardnesses of the NbTiZr and the NbTaTiZr coatings are 510 ± 22 HV and 550 ± 26 HV, respectively, significantly higher than that of the substrate (196 ± 4 HV). Such high hardnesses are attributed to combined effects of solid-solution strengthening, grain refinement strengthening and increased contents of low angle boundaries.