Microstructure evolution and performance enhancement of NbTaTiV-(Cr, Zr, W) single-phase refractory high-entropy alloy coatings: Role of additional elements

Refractory high entropy alloys (RHEAs) with single-phase structure hold far great potential in the field of surface protection application under extreme environment such as high temperature, abrasion and corrosion. In this work, the effects of Cr, Zr and W on the microstructure evolution and properties of a single-phase body-centered-cubic (BCC) NbTaTiV RHEA coating have been investigated. Experimentally, in addition to a small amount of Laves phase introduced by Cr element, NbTaTiV-(Cr, Zr, W) RHEA coatings basically maintain the single BCC structure. The addition of different elements affects segregation degrees in dendrite microstructure, and Zr shows nanoscale chemical heterogeneity. Notably, grain refinement, elevated geometrically necessary dislocation density and high-density low angle grain boundaries lead to a dramatically increase of micro-hardness due to elements addition. The wear rate of NbTaTiVCr and NbTaTiVW RHEA coatings reduce by 74.98 % and 70.79 % compared with NbTaTiV coating. The wide passive passivation zones close to 1000 mV obtained by potentiodynamic polarization exhibit excellent corrosion resistance. In addition, the formation mechanism of oxide layers is studied by thermodynamic parameters and oxidation behavior of simple oxides. The current work provides theoretical support for the application of strengthening elements in a single-phase BCC RHEA coating.