Behavior of High-Entropy NbTaMoW Alloy under Low-Temperature Plasma Conditions

Currently, a substantial amount of research is related to high-entropy alloys (HEAs) based on refractory metals (Hf, Ta, Mo, Nb, V, W, Cr, Zr, Ti). The inclusion of refractory elements contributes to an increase in the melting point (Tm) of the alloy and, thus, makes it a potential candidate for use at high temperatures, along with existing superalloys. One of the promising technologies for improving the characteristics of HEAs is plasma activation (modification) of the surface, as a result of which the resistance to fatigue deformation and abrasion increases, hardness, tensile strength and corrosion resistance increase. When studying complex multifunctional structures, it is necessary to conduct preliminary thermodynamic studies that have predictive potential for various properties of materials, as well as significantly reduce the amount of experimental research and, thereby, reduce time and material costs. In this work, thermodynamic modeling of the behavior of a high–entropy NbTaMoW alloy of equimolar composition under conditions of a low-temperature argon plasma is carried out: the temperature range is 300–10 000 K, the total pressure is P = 105 Pa. The simulated system consists of a condensed phase (an ideal NbTaMoW solution) and a gas phase above it. The temperature dependences of the equilibrium composition of the condensed and gaseous phases formed by heating this alloy with an argon plasma stream, as well as the thermodynamic characteristics (enthalpy, entropy, and Gibbs energy) of the simulated system are calculated. It is shown that the temperature dependences of the thermodynamic characteristics are not monotonous, but have kinks due to phase transformations that occur during equilibrium heating.