Microstructure and Phase Equilibria in BCC-B2 Nb-Ti-Ru Refractory Superalloys

Refractory superalloys (RSAs) are promising candidates for high-temperature, high-strength applications. Two-phase RSAs containing body-centered cubic (BCC) and ordered B2 phases are among the more promising candidates. Systems containing Ru-based B2 precipitates exhibit stable two-phase microstructures at temperatures in excess of 1600 °C. The present study experimentally investigated one potential foundational ternary system for these alloys, Nb-Ti-Ru. Two alloys, (Nb₃Ti)_0.85Ru_0.15 and (Nb₄Ti)_0.85Ru_0.15, were studied to determine phase equilibria and properties at temperatures between 900 °C and 1300 °C. The B2 phase was found to be dominated by RuTi ordering, although considerable Nb solubility was observed up to 18 mol %. The Nb-rich BCC matrix contained up to 15 mol % Ru and 20 mol % Ti. Although a two-phase microstructure of B2 precipitates in a BCC matrix was confirmed, the distribution of elements in the two phases resulted in a larger lattice misfit than expected. The results obtained in this investigation provide valuable information for the future development of RSAs utilizing Ru-based B2 strengthening precipitates.