Changes in microstructure, solidification path and hydrogen permeability of Nb-Hf-Co alloy by adjusting Hf/Co ratio

The effect of Hf/Co ratio on microstructure, solidification path and hydrogen permeability of as-cast Nb40Hf30+xCo30−x and Nb35Hf32.5+xCo32.5−x (x = −2, 0, 2, 4) alloys is investigated. The above two series of alloys mainly consist of the primary BCC-(Nb, Hf) and the eutectic {BCC-(Nb, Hf) + Bf-HfCo} phases, while Nb40Hf34Co26 alloy additionally contains small fractions of the Hf2Co phase that results from a ternary eutectic reaction. Increase of Hf/Co ratio induces higher fraction of primary BCC-(Nb, Hf) and Hf2Co phases. Two kinds of solidification paths are found in this two series of alloys, particularly [L→BCC-(Nb,Hf)]→[L→BCC-(Nb,Hf) + Bf-HfCo] and [L→BCC-(Nb,Hf)]→[L→BCC-(Nb, Hf)+Bf-HfCo]→[L→BCC-(Nb, Hf) + Bf-HfCo + Hf2Co]. The permeability increases with increasing Hf/Co ratio and increasing volume fraction of the primary BCC-(Nb, Hf) phase. Nb40Hf32Co28 (x = 2) exhibits a high hydrogen permeability of 5.21 × 10−8 mol H2 m−1s−1Pa−1/2 at 673 K and excellent durability in hydrogen permeation (∼72 h), which becomes superior to that of the Nb40Hf30Co30 previously reported to be the best in the Nb-Hf-Co alloy series. The present work demonstrates that the hydrogen permeability of this alloy series can be optimized by appropriately adjusting the Hf/Co ratio for a given niobium content.