Phase Evolution and Nanomechanical Properties of Phases in Nb-Ta-B Ternary Alloys after Long-Term Annealing at 1323 K

This study investigates the relationships between chemical composition, microstructure, and mechanical properties in the Nb–Ta–B ternary alloys over a wide composition range after long-term heat treatment at 1323 K for approximately 2900 h. In particular, the phase composition and resulting microstructures were analyzed, and nanomechanical properties, namely indentation hardness and indentation modulus, of the individual phases and two-phase microstructural constituents were evaluated. Microstructural and phase analyses performed using scanning electron microscopy and X-ray diffraction revealed single-phase, two-phase, and three-phase microstructures depending on the composition. The presence of the BCC + MB eutectic microstructure confirms the extension of the binary eutectic reaction, L → BCC + MB, into the ternary compositional range. In the Ta-rich alloy, the Ta₂B boride was observed, despite being stable only at substantially higher temperatures in the corresponding binary system. Within the investigated alloy compositions, no ternary phase was identified, and all observed borides correspond to binary borides exhibiting solubility of the third element. Nanomechanical measurements were performed on the MB, MB₂, and M₃B₄ phases, as well as on the BCC + MB, BCC + Ta₂B, and M₃B₄ + M₅B₆ regions. The eutectic BCC + MB structure exhibited the lowest hardness and indentation modulus, whereas the MB boride in the Ta-rich alloy showed the highest values. The M₃B₄ and M₅B₆ phases exhibited similar hardness values. Overall, the results provide new experimental insights into the phase constitution and nanomechanical behavior of the investigated Nb–Ta–B alloys and establish an experimental basis for future thermodynamic modeling of the B–Nb–Ta ternary system.