Modeling the behavior of high entropy alloys (HEAs) at high temperatures

High Entropy Alloys (HEA) belong to a group of multicomponent metal alloys, in which the formation of solid solutions is preferable over the formation of intermetallic compounds. Since the solid solution form has a high mixing entropy, its thermodynamic stability greatly increases with temperature. Due to this unique trend, HEAs may possess superior mechanical properties, namely high temperature strength and creep resistance, which may be successfully utilized in the aerospace industry for the production of high-temperature airborne critical parts, such as engine blades.

Among the major refractory elements comprising the refractory-based HEAs, there are no stable binary intermetallic compounds, or in other words, the solid solution form of an alloy comprising these elements alone is the only viable option. On the other hand, additions of the quasi-refractory elements V, Cr and Ti may lead to the formation of several stable binary intermetallides with the refractory elements, such as TaV₂, NbCr₂, TaCr₂ and ZrMo₂. The addition of Ti may also cause the precipitation of β-Ti particles out of solution.

The topic of the presented work is the thermodynamic modeling of the microstructure of HEAs at high temperatures, as well as the reactive diffusion causing the microstructural changes. The formulated thermodynamic model considers the overall Gibbs free energy of the alloy as the sum of the free energy values of each phase. It enabled to determine that some elemental modifications of HEAs may cause thermodynamic preference of the ordered multi-intermetallide form over the solid solution form, specifically at low temperatures.

To correlate and compare the model with the actual behavior of HEAs, the following characterization techniques were employed: XRD, SEM, HR-SEM and DSC.