Structural and mechanical properties of refractory Nb30X25Ti25Al15V5 (X = Zr, Mo) high-entropy alloys

High-entropy alloys (HEAs) represent a significant breakthrough in materials science, offering numerous compositional possibilities for alloy design and abroad range of systems with remarkable property combinations. This study aims to enhance our understanding of HEAs through experimental and first-principles computational characterization of the structural and mechanical properties of two Nb-based refractory high-entropy alloys (RHEAs), specifically Nb30X25Ti25Al15V5 (X = Zr and Mo), identified as promising materials for aerospace applications. Our experimental results confirmed a single-phase solid solution for both alloys, with theoretical and experimental predictions indicating a cubic B2 structure for X = Zr and a body-centered cubic (bcc) structure for X = Mo. The alloys exhibit an estimated liquidus temperature above 1800 degrees C and densities lower than those of conventional Ni superalloys, with experimental values below 7.1 g cm-3, accurately predicted by theoretical calculations. All polycrystalline elastic constants and Vickers' hardness increased when Zr was replaced with Mo in the Nb30X25Ti25Al15V5 alloy, which also changed the plastic behavior from ductile to completely brittle. Both alloys exhibited significant lattice distortion, with V inducing the largest deviation from the ideal average bond distances expected in a perfect crystal. From an electronic standpoint, both systems exhibit metallic characteristics, possessing a significant density of states at the Fermi level. Moreover, an analysis of the local density of states revealed that the alloy containing Zr exhibits a predominance of Nb states at the Fermi level, in contrast to the Mo system, where Ti states are predominant.