Characterization and Modeling of NbNiTaTiW and NbNiTaTiW-Al Refractory High-Entropy Alloys

High-entropy alloys (HEAs) represent a new class of material that exhibit unique materials properties and complex microstructure. Little work to date has been done on refractory HEAs (RHEAs), which can loosely be defined as an HEA with the majority of the alloy being refractory metals. In this study, the authors examined two unique RHEA systems: (1) Nb20Ni20Ta20Ti20W20 and (2) Nb18Ni18Ta18Ti18W18Al10. The RHEAs were characterized in the as-cast and heat-treated condition using scanning electron microscopy equipped with energy dispersive spectroscopy to observe the microstructural evolution and elemental segregation and X-ray diffraction to determine the number of phases present and their crystal structure. The RHEA mechanical properties were tested via Vickers hardness measurements. Thermodynamic simulations of the solidification process were performed using a CALPHAD approach involving two basic models: equilibrium lever rule and non-equilibrium Scheil-Gulliver. The simulations were compared with the experimental data.