Mapping the relationship among composition, stacking fault energy and ductility in Nb alloys: A first-principles study

Transition metals (TMs) are extensively used to improve the mechanical properties of niobium based alloy, one of the most promising high-temperature materials. Yet the microscopic mechanism of the alloying effects of these transition metals on the mechanical properties is unclear. In this study, we have mapped out the composition-SFE-ductility relationship for TM-alloyed Nb systems by comprehensively investigating the unstable stacking fault energies (SFEs), γ_us, and the ductility in binary and ternary Nb alloys using the first-principles calculations. It is found that the valence electron concentration can be used as the key descriptor to evaluate the SFE of Nb matrix, which is applicable to both binary and ternary alloys. The microscopic mechanism arises from the electron redistribution in the local stacking fault area. Moreover, for ternary Nb-Ti based alloys, the interaction between Ti and the third alloying elements has negligible effect on the SFE of the systems, and the valence-electron rule still dominates. The alloying effects on the ductility are further illustrated based on the ratio between surface energies and SFEs. The composition-SFE-ductility map obtained by our theoretical calculations is calibrated by available experimental data.