Microstructure evolution and properties of NiTiCrNbTax refractory high-entropy alloy coatings with variable Ta content.

• The formation of solid solution phase is promoted by increasing Ta content. • The microstructure evolution mechanism with varied Ta content is investigated. • The RHEA coatings possess the good structure stability below 900 °C. • The stability of passive film is improved by increasing Ta content. [Display omitted] In this work, the novel NiTiCrNbTa x (x = 0.1, 0.3, 0.5, 1) refractory high-entropy alloy (RHEA) coatings were successfully synthesized on Ti6Al4V by laser cladding technology. The effect of Ta content on microstructure, thermal stability, micro-hardness and corrosion resistance of RHEA coatings was investigated in detail. The results showed that NiTiCrNbTa x RHEA coatings were composed of BCC, NiTi 2 -Laves and Cr 2 Nb-Laves phases. With Ta content increasing, the formation of BCC phase was promoted while the precipitate of Cr 2 Nb-Laves phase was affected. The average micro-hardness values of Ta 0.1 , Ta 0.3 , Ta 0.5 and Ta 1.0 RHEA coatings were calculated as ~922.8 HV 0.3 , ~985.1 HV 0.3 , ~851.3 HV 0.3 and ~825.4 HV 0.3 , respectively. The decrease of micro-hardness value might be ascribed to the reduction of hard Laves phase. After annealing at 900 °C for 3 h, the phase compositions of all RHEA coatings remained unchanged but the grains of Cr 2 Nb-Laves phase were refined obviously. Besides, RHEA coatings exhibited excellent corrosion resistance and the minimum corrosion current density was 1.08 × 10−7 A/cm2. Moreover, the secondary passivation phenomenon emerged in Ta 0.5 and Ta 1.0 coatings, and the passive film mainly consisted of the oxidation state of Ti, Cr, Nb, Ta and the sub-oxide state of Nb, Ta. [ABSTRACT FROM AUTHOR]