Nb-TiCo multiphase alloys: The significant impact of Ti/Co ratio on solidification path, microstructure and hydrogen permeability

Nb-TiCo multiphase alloys are prospective membrane materials for hydrogen permeation. Their composition, such as the initial Nb content, has a great influence on their hydrogen permeability (Phi). What the research hasn't actually mentioned, however, is whether other elements (e.g. Ti or Co) could affect their hydrogen permeability. To address this question, the effects of the Ti/Co ratio on their solidification path, microstructure and hydrogen permeability have been comprehensively investigated. The results show that the Ti/Co ratio has a significant effect on the solidification direction along the eutectic valley but has little effect on the solidification structure. As the Ti/Co ratio decreases, the volume fraction of the BCC-Nb phase decreases whereas the eutectic phases increase. With these changes, the Phi value increases gradually, which is closely connected to the increasing hydrogen diffusion coefficient. Typically, the Phi value of Nb45Ti25Co30 is 1.03 (-1.08) times higher than that of Nb45Ti27.5Co27.5 (Nb45Ti30Co25). This can be attributed to a decrease in the hydrogen diffusion activation barrier and a smaller hydrogen adsorption energy. The present work demonstrates that Nb-TiCo multiphase alloys with higher Phi values can still be exploited by a proper optimum design for the composition, such as the Ti/Co ratio.