Effect of Tantalum Content on Structural Evolution and Nanomechanical Properties of TiZrVNb_1-xTa_x (x= 0.1; 0.15; 0.2) High-Entropy Alloys

Refractory high-entropy alloys (RHEAs) based on Ti, Zr, V, Nb, and Ta have attracted considerable attention for advanced structural applications due to their combination of high strength, thermal stability, and resistance to degradation under extreme conditions. In this work, the influence of tantalum concentration on the microstructure, phase constitution, density, and nanomechanical behavior of Ti–Zr–V–Nb–Ta high-entropy alloys was systematically investigated. Three alloys containing approximately 10, 15, and 20 at.% Ta were synthesized by arc melting and characterized using energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), density measurements, and nanoindentation. EDS elemental mapping revealed dendritic solidification accompanied by microsegregation in all alloys. Increasing Ta content reduced the extent of elemental partitioning and promoted a more homogeneous chemical distribution. XRD analysis showed that all compositions predominantly formed a body-centered cubic (BCC) solid-solution phase with a minor hexagonal close-packed (HCP) phase associated with Ti- and Zr-rich regions. The BCC structure remained stable across the investigated compositional range, confirming the strong BCC-stabilizing effect of tantalum. Lattice parameters obtained by Rietveld refinement were consistently lower than values predicted by Vegard’s rule, indicating significant lattice distortion and non-ideal atomic interactions. Nanoindentation measurements revealed a progressive improvement in mechanical performance with increasing Ta content. Hardness increased from 6.21 to 7.43 GPa, while the elastic modulus increased from 118.7 to 146.1 GPa. All alloys exhibited low indentation creep (1.44–1.51%) and distinct pop-in events associated with the onset of localized plastic deformation. Furthermore, the resistance-to-plastic-deformation parameter (H³/E²) increased with Ta addition, while the H/E ratio remained nearly constant. These results demonstrate that tantalum effectively enhances chemical homogeneity, phase stability, and local mechanical performance, providing valuable guidance for the compositional design of high-performance refractory high-entropy alloys.