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

Abstract

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.